U.S. patent application number 10/638192 was filed with the patent office on 2004-02-19 for methods for salvaging a cast article.
Invention is credited to Arnold, James E., Blake, Wayne C..
Application Number | 20040031140 10/638192 |
Document ID | / |
Family ID | 31721905 |
Filed Date | 2004-02-19 |
United States Patent
Application |
20040031140 |
Kind Code |
A1 |
Arnold, James E. ; et
al. |
February 19, 2004 |
Methods for salvaging a cast article
Abstract
A method of salvaging a cast article by correcting the
dimensional characteristics of the cast article and forming a
protective coating. The dimensional differences are determined
between pre-repair cast article dimensions and desired post repair
cast article dimensions to correct a casting defect in the article.
The determination may be made by determining the location and
approximate volume of a void in the surface of the article. The
determination may also be made by determining an amount of buildup
volume required to make at least a portion of the surface of the
cast article built up to the desired post repair dimensions. The
article is coated in at least an area of the casting defect with a
high-density coating material capable of forming a diffusion
boundary between the coating material and the article. The coating
material may comprise an alloy with substantially no oxide forming
constituents so as to avoid the formation of oxide inclusions in
the coating material. The coating material may be applied using a
coating process that is effective to create a coating on the
surface of the article that will be diffusion bonded to the article
after the hot isostatic heat treatment. The hot isostatic heat
treatment process is performed to form the diffusion boundary
between the coating material and the article
Inventors: |
Arnold, James E.; (New
Haven, CT) ; Blake, Wayne C.; (Wallingford,
CT) |
Correspondence
Address: |
John J. Daniels, Esq
511 Foot Hills Road
Higganum
CT
06441
US
|
Family ID: |
31721905 |
Appl. No.: |
10/638192 |
Filed: |
August 11, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10638192 |
Aug 11, 2003 |
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10423722 |
Apr 28, 2003 |
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10423722 |
Apr 28, 2003 |
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10241854 |
Sep 13, 2002 |
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10241854 |
Sep 13, 2002 |
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09505803 |
Feb 17, 2000 |
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09505803 |
Feb 17, 2000 |
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09143643 |
Sep 3, 1998 |
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6049978 |
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09143643 |
Sep 3, 1998 |
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08993116 |
Dec 18, 1997 |
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5956845 |
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60033858 |
Dec 23, 1996 |
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Current U.S.
Class: |
29/526.4 |
Current CPC
Class: |
B23P 2700/06 20130101;
F01D 5/288 20130101; C23C 4/18 20130101; Y02T 50/60 20130101; Y10T
29/49975 20150115; Y02T 50/672 20130101; B23P 15/02 20130101; C23C
4/00 20130101; B23P 6/007 20130101; C23C 4/01 20160101; B23K 20/021
20130101; F01D 5/005 20130101 |
Class at
Publication: |
29/526.4 |
International
Class: |
B21B 001/46; B22D
011/126 |
Claims
1) A method of correcting the dimensional characteristics of a cast
article, comprising the steps of: Determining dimensional
differences between pre-repair cast article dimensions and desired
post repair cast article dimensions to correct a casting defect in
the article; Coating the article in at least an area of the casting
defect with a high-density coating material capable of forming a
diffusion boundary between the coating material and the article;
and Performing a hot isostatic heat treatment process to form the
diffusion boundary between the coating material and the
article.
2) A method of correcting the dimensional characteristics of a cast
article according to claim 1; further comprising the step of
removing material in an area of the casting defect before the step
of coating the article.
3) A method of correcting the dimensional characteristics of a cast
article according to claim 1; wherein the casting defect being
caused by at least one of an inclusion at a surface of the article,
an air bubble at the surface of the article, undercasting, a void
and shrinkage
4) A method of correcting the dimensional characteristics of a cast
article according to claim 1; further comprising the step of
performing a sintering heat treatment before the step of performing
the hot isostatic heat treatment to limit the occurrence bubbles on
the surface of the coating material after an isostatic heat
treatment.
5) A method of correcting the dimensional characteristics of a cast
article according to claim 4; wherein the sintering heat treatment
is performed at a temperature substantially the same as the
temperature of the hot isostatic heat treatment.
6) A method of correcting the dimensional characteristics of a cast
article according to claim 1; wherein the coating material
comprises an alloy with substantially no oxide forming constituents
so as to avoid the formation of oxide inclusions in the coating
material.
7) A method of correcting the dimensional characteristics of a cast
article according to claim 6; wherein the coating material
comprises:
7 Element Percentage Nickel Balance Chromium 9.0 Cobalt 10.0 Carbon
0.14 Molybdenum 8.6 Tungsten 12.5 Boron 0.015 Columbium 1.0
8) A method of applying a protective coating to a metal article,
comprising the steps of: Providing a metal article; Coating the
metal with a high-density coating material capable of forming a
diffusion boundary between the coating material and the article,
the coating material comprising an alloy with substantially no
oxide forming constituents so as to avoid the formation of oxide
inclusions in the coating material; and Performing a hot isostatic
heat treatment process to form the diffusion boundary between the
coating material and the article whereby the substantially oxide
free coating and the diffusion boundary provide a protective
coating to protect the article from damage.
9) A method of applying a protective coating to a metal article
according to claim 8; further comprising the step of performing a
sintering heat treatment before the step of performing the hot
isostatic heat treatment to limit the occurrence bubbles on the
surface of the coating material after an isostatic heat
treatment.
10) A method of applying a protective coating to a metal article
according to claim 9; wherein the sintering heat treatment is
performed at a temperature substantially the same as the
temperature of the hot isostatic heat treatment.
11) A method of applying a protective coating to a metal article
according to claim 8; wherein the coating material comprises an
alloy with substantially no oxide forming constituents so as to
avoid the formation of oxide inclusions in the coating
material.
12) A method of applying a protective coating to a metal article
according to claim 11; wherein the coating material comprises:
8 Element Percentage Nickel Balance Chromium 9.0 Cobalt 10.0 Carbon
0.14 Molybdenum 8.6 Tungsten 12.5 Boron 0.015 Columbium 1.0
13) A method of repairing a turbine engine airfoil part, comprising
the steps of: Determining dimensional differences between
pre-repair airfoil dimensions of a turbine engine airfoil part
substrate and desired post repair airfoil dimensions of the turbine
engine airfoil part substrate, the pre-repair airfoil dimensions
having different airfoil characteristics than the post-repair
aifoil dimensions, the turbine engine airfoil part being comprised
of a metal alloy; Coating the engine airfoil part with a coating
capable of forming a diffusion boundary with the turbine engine
airfoil part substrate, the coating material comprising an alloy
with substantially no oxide forming constituents so as to avoid the
formation of oxide inclusions in the coating material; and
Performing a hot isostatic heat treatment process to obtain a
post-repair turbine engine airfoil part having the desired
post-repair dimensions and having a substantially oxide free
coating and diffusion bonding between the coating material and the
turbine engine airfoil part substrate to provide a protective
coating to protect the article from damage.
14) A method of repairing a turbine engine airfoil part according
to claim 13; further comprising the step of performing a sintering
heat treatment before the step of performing the hot isostatic heat
treatment to limit the occurrence bubbles on the surface of the
coating material after an isostatic heat treatment.
15) A method of repairing a turbine engine airfoil part according
to claim 14; wherein the sintering heat treatment is performed at a
temperature substantially the same as the temperature of the hot
isostatic heat treatment.
16) A method of applying a protective coating to a metal article
according to claim 13; wherein the coating material comprises:
9 Element Percentage Nickel Balance Chromium 9.0 Cobalt 10.0 Carbon
0.14 Molybdenum 8.6 Tungsten 12.5 Boron 0.015 Columbium 1.0
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a Continuation-in-Part of
application Ser. No. 10/423,722, filed Apr. 28, 2003, which is a
Continuation-in-Part of application Ser. No. 10/241,854, filed Sep.
13, 2002, which is a Continuation-in-Part of application Ser. No.
09/505,803, filed Feb. 17, 2000, which is a Continuation-in-Part of
application Ser. No. 09/143,643, filed Sep. 3, 1998, now U.S. Pat.
No. 6,049,978, which is a Continuation-in-Part of application Ser.
No. 08/993,116, now U.S. Pat. No. 5,956,845, which is the utility
patent application of a U.S. provisional application Serial No.
60/033,858, filed Dec. 23, 1996; and relates to an invention
disclosed in an Invention Disclosure Document accepted under the
Disclosure Document program on or about Nov. 5, 1996 and assigned
Disclosure Document No. 407616.
BACKGROUND OF THE INVENTION
[0002] The present invention pertains to methods for salvaging cast
articles, such as turbine engine airfoil parts. More particularly,
the present invention pertains to a method for correcting the
dimensional characteristics of a cast article; a method for
applying a protective coating to a metal article; and a method for
repairing a turbine engine airfoil part.
[0003] Airfoil parts, such as blades and vanes, are critical
components in the gas turbine engines that are used to power jet
aircraft or for the generation of electricity. Each airfoil part is
an individual unit having a root or attachment section and an
airfoil section. The airfoil section has specific cordal and length
dimensions that define the airfoil characteristics of the part. The
root section is engaged with and held by a housing member. A
plurality of the airfoil parts are thus assembled with the housing
member to form a disc or ring. Blades, which during operation are
rotating part, are assembled into and disc. Vane, which remain
stationary, are assembled into a nozzle or vane ring. In the
operating gas turbine engine the assembled rings and discs,
determine the path of the intake, combustion and exhaust gasses
that flow through the engine.
[0004] The airfoil part may be either a rotating component or a
non-rotating component of the gas turbine engine. If the part is a
rotating component, during operation of the turbine engine the part
is subjected to centrifugal forces that exert deforming stresses.
These deforming stresses cause creep rupture and fatigue problems
that can result in the failure of the part. Non-rotating
components, such as vanes, are not subjected to centrifugal forces
that exert deforming stresses. However, like the rotating parts,
these parts are subjected to other deformation such as from hot gas
erosion and/or foreign particle strikes. This deformation results
in the alteration of the dimensions of the airfoil section. The
alteration of the dimensions of the airfoil section can
detrimentally modify the airflow through the gas turbine engine
which is critical to the engine's performance.
[0005] An example of a non-rotating airfoil part is the 2nd stage
vane of the Pratt & Whitney JT8D model 1 through 17R gas
turbine engine. This part is manufactured by the "lost wax" or
"investment casting" process. The vane is cast from one of several
highly alloyed nickel or cobalt-base materials. As a new part in a
new gas turbine engine, or as a new spare part in an overhauled
engine, it begins its life cycle with a protective diffusion
coating on its airfoil surfaces and a wear coating on surfaces
known to have excessive wear patterns.
[0006] When the gas turbine engine is operating, the vane will see
temperatures of about 1500 degree F. Since the vane does not rotate
and thus is not subject to creep rupture, its demise is most often
influenced by the number of times it is repaired. The reason for
this is the repair process itself.
[0007] The repair process consists of the following operations:
[0008] 1). degrease, wash to remove engine carbon, etc.
[0009] 2.) grit blast to remove wear coatings, and any sulfidation
which is present
[0010] 3.) chemically remove the diffusion coating
[0011] 4.) blend to remove nicks, dents, etc.
[0012] 5.) weld, grind, polish etc.
[0013] The repair operations that remove metal by chemical
stripping, grit blasting, blending and polishing shorten the life
cycle of the vane. The coating removal is a major contributor
because it is diffused into the parent metal. When certain minimum
airfoil dimensions cannot be met the part is deemed non-repairable
and must be retired from service. Thus, there is a need for a
method for repairing gas turbine engine airfoil parts that
effectively and efficiently restores the airfoil dimensions of the
part.
[0014] On another front, during the manufacture of metal components
a coating operation is performed to provide a coating material
layer on the surface of a component substrate. The coating material
layer is formed to build-up the metal component to desired finished
dimensions and to provide the finished product with various surface
attributes. For example, an oxide layer may be formed to provide a
smooth, corrosion resistant surface. Also, a wear resistant
coating, such as Carbide, Cobalt, or TiN is often formed on cutting
tools to provide wear resistance.
[0015] Chemical Vapor Deposition is typically used to deposit a
thin film wear resistant coating on a cutting tool substrate. For
example, to increase the service life of a drill bit, chemical
vapor deposition can be used to form a wear resistant coating of
Cobalt on a high speed steel (HSS) cutting tool substrate. The bond
between the substrate and coating occurs primarily through
mechanical adhesion within a narrow bonding interface. During use,
the coating at the cutting surface of the cutting tool is subjected
to shearing forces resulting in flaking of the coating off the tool
substrate. The failure is likely to occur at the narrow bonding
interface.
[0016] FIG. 12(a) is a side view of a prior art tool bit coated
with a wear resistant coating. In this case, the wear resistant
coating may be applied by the Chemical Vapor Deposition method so
that the entire tool bit substrate receives an even thin film of a
relatively hard material, such as Carbide, Cobalt or TiN. Since the
coating adheres to the tool bit substrate mostly via a mechanical
bond located at a boundary interface, flaking and chipping off the
coating off of the substrate is likely to occur during use,
limiting the service life of the tool bit. FIG. 12(b) is a side
view of a prior art tool bit having a fixed wear resistant cutting
tip. In this case, a relatively hard metal cutting tip is fixed to
the relatively soft tool bit substrate. The metal cutting tip,
which is typically comprised of a Carbide or Cobalt alloy, is fixed
to the tool bit substrate by brazing. During extended use the tool
bit is likely to fail at the relatively brittle brazed interface
between the metal cutting tip and the tool substrate, and again,
the useful service life of the tool bit is limited.
[0017] Another coating method, known as Conventional Plasma Spray
uses a super heated inert gas to generate a plasma. Powder
feedstock is introduced and carried to the workpiece by the plasma
stream. Conventional plasma spray coating methods deposit the
coating material at relatively low velocity, resulting in voids
being formed within the coating and in a coating density typically
having a porosity of about 5.0%. Again, the bond between the
substrate and the coating occurs primarily through mechanical
adhesion at a bonding interface, and if the coating is subjected to
sufficient shearing forces it will flake off of the workpiece
substrate.
[0018] Another coating method, known as the Hyper Velocity Oxyfuel
(HVOF) plasma thermal spray process is used to produce coatings
that are nearly absent of voids. In fact, coatings can be produced
nearly 100% dense, with a porosity of less than 0.5%. In HVOF
thermal spraying, a fuel gas and oxygen are used to create a
combustion flame at 2500 to 3100.degree. C. The combustion takes
place at a very high chamber pressure and a supersonic gas stream
forces the coating material through a small-diameter barrel at very
high particle velocities. The HVOF process results in extremely
dense, well-bonded coatings. Typically, HVOF coatings can be formed
nearly 100% dense, with a porosity of <0.5%. The high particle
velocities obtained using the HVOF process results in relatively
better bonding between the coating material and the substrate, as
compared with other coating methods such as the Conventional Plasma
spray method or the Chemical Vapor Deposition method. However, the
HVOF process also forms a bond between the coating material and the
substrate that occurs primarily through mechanical adhesion at a
bonding interface.
[0019] Detonation Gun coating is another method that produces a
relatively dense coating. Suspended powder is fed into a long tube
along with oxygen and fuel gas. The mixture is ignited in a
controlled explosion. High temperature and pressure is thus created
to blast particles out of the end of the tube and toward the
substrate to be coated.
[0020] An example of using HVOF or Detonation Gun coating
techniques is disclosed in U.S. Pat. No. 5,584,663, issued to
Schell. This reference discloses that the tips of turbine blades
can be formed by melting and fusing a powder alloy. Preferably, the
blade tip is generated by depositing molten metal alloy powder in
multiple passes. Squealers at the perimeter of the blade tip may be
formed using methods such as Detonation Gun or HVOF spray methods.
The forming step may be used to generate a near-net shaped blade
tip, and a subsequent machining step may be employed to generate
the final or preferred shape of the blade tip.
[0021] Casting is a known method for forming metal components.
Typically, a substrate blank is cast to near-finished dimensions.
Various machining operations, such as cutting, sanding and
polishing are performed on the cast substrate blank to eventually
obtain the metal component at desired finished dimensions. A cast
metal component will typically have a number of imperfections
caused by voids and contaminants in the cast surface structure. The
imperfections may be removed by machining away the surface layer of
the component, and/or by applying a surface coating.
[0022] The manufacture of metal components often entails costly
operations to produce products with the desired surface texture,
material properties and dimensional tolerances. For example, a
known process for manufacturing a metal component requires, among
other steps, making a casting of the metal component, treating the
metal component using a Hot Isostatic Pressing (HIP) treatment
process, and then machining the metal component to remove surface
imperfections and obtain the desired dimensional tolerances.
[0023] HIP treatment is used in the densification of cast metal
components and as a diffusion bonding technique for consolidating
powder metals. In the HIP treatment process, a part to be treated
is raised to a high temperature and isostatic pressure. Typically,
the part is heated to 0.6-0.8 times the melting point of the
material comprising the part, and subjected to pressures on the
order of 0.2 to 0.5 times the yield strength of the material.
Pressurization is achieved by pumping an inert gas, such as Argon,
into a pressure vessel. Within the pressure vessel is a high
temperature furnace, which heats the gas to the desired
temperature. The temperature and pressure are held for a set length
of time, and then the gas is cooled and vented.
[0024] The HIP treatment process is used to produce near-net shaped
components, reducing or eliminating the need for subsequent
machining operations. Further, by precise control of the
temperature, pressure and time of a HIP treatment schedule a
particular microstructure for the treated part can be obtained.
[0025] All casting processes must deal with problems that the
wrought processes do not encounter. Major among those are porosity
and shrinkage that are minimized by elaborate gating techniques and
other methods that increase cost and sometimes lower yield.
However, the ability to produce a near-net or net shape is the
motivating factor. In some cases, it is more cost effective to
intentionally cast the part not using elaborate and costly gating
techniques and HIP treat the part to eliminate the sub-surface
porosity. The surface of the part is then machined until the dense
substrate is reached.
[0026] U.S. Pat. No. 5,156,321, issued to Liburdi et al and U.S.
Pat. No. 5,071,054, issued to Dzugan et al. are examples of methods
that employ the HIP treatment process. Liburdi et al. discloses a
technique to repair or join sections of a superalloy article. A
powder matching the superalloy composition is sintered in its solid
state to form a porous structure in an area to be repaired or
joined. A layer of matching powder, modified to incorporate melting
point depressants, is added to the surface of the sintered region.
Liburdi discloses that the joint is raised to a temperature where
the modified layer melts while the sintered layer and base metal
remain solid. The modified material flows into the sintered layer
by capillary action resulting in a dense joint with properties
approaching those of the base metal. This reference discloses that
HIPing can be used as part of the heat treatment to close any minor
interior defects. Dzugan et al. discloses fabricating a superalloy
article by casting, and then refurbishing primary defects in the
surface of the cast piece. The defects are removed by grinding. The
affected portions of the surface are first filled with a material
that is the same composition as the cast article. Then, a cladding
powder is applied to the surface through the use of a binder coat
to obtain a smooth surface. The article is then heated to melt the
cladding powder, and then cooled to solidify. Finally, the article
is HIPed to achieve final closure of the surface defects.
[0027] Metal alloy components, such as gas turbine parts such as
blades and vanes, are often damaged during use. During operation,
gas turbine parts are subjected to considerable degradation from
high pressure and centrifugal force in a hot corrosive atmosphere.
The gas turbine parts also sustain considerable damage due to
impacts from foreign particles. This degradation results in a
limited service life for these parts. Since they are costly to
produce, various repair methods are employed to refurbish damaged
gas turbine blades and vanes.
[0028] Some examples of methods employed to repair gas turbine
blades and vanes include U.S. Pat. No. 4,291,448, issued to
Cretella et al.; U.S. Pat. No. 4,028,787, issued to Cretella et
al.; U.S. Pat. No. 4,866,828, issued to Fraser; and U.S. Pat. No.
4,837,389, issued to Shankar et al.
[0029] Cretella '448 discloses a process to restore turbine blade
shrouds that have lost their original dimensions due to wear while
in service. This reference discloses using the known process of TIG
welding worn portions of a part with a weld wire of similar
chemistry as the part substrate, followed by finish grinding. The
part is then plasma sprayed with a material of similar chemistry to
a net shape requiring little or no finishing. The part is then
sintered in an argon atmosphere. The plasma spray process used in
accordance with Cretella '448 results in a coating porosity of
about 5.0%. Even after sintering the coating remains attached to
the substrate and weld material only be a mechanical bond at an
interface bonding layer making the finished piece prone to chipping
and flaking.
[0030] Cretella '787 discloses a process for restoring turbine
vanes that have lost their original dimensions due to wear while in
service. Again, a conventional plasma spray process is used to
build up worn areas of the vane before performing a sintering
operation in a vacuum or hydrogen furnace. The porosity of the
coating, and the interface bonding layer, results in a structure
that is prone to chipping and flaking.
[0031] Fraser discloses a process to repair steam turbine blades or
vanes that utilize some method of connecting them together (i.e.
lacing wire). In accordance with the method disclosed by Fraser,
the area of a part that has been distressed is removed and a new
piece of like metal is welded to the part. The lacing holes of the
part are plug welded. The part is then subjected to hot striking to
return it to its original contour, and the lacing holes are
re-drilled.
[0032] Shankar et al. disclose a process for repairing gas turbine
blades that are distressed due to engine operation. A low-pressure
plasma spray coating is applied to the vanes and the part is
re-contoured by grinding. A coating of aluminum is then applied
using a diffusion coating process. Again, the conventional
low-pressure plasma spray process forms a mechanical bond at an
interface boundary between the coating and the substrate, resulting
in a structure that is prone to failure due to chipping and
flaking.
[0033] Other examples of methods for repairing or improving the
characteristics of turbine engine airfoil parts include U.S. Pat.
No. 5,451,142 issued to Cetel et al.; U.S. Pat. No. 4,921,405,
issued to Wilson; U.S. Pat. No. 4,145,481 issued to Gupta et al.;
and U.S. Pat. No. 5,732,467 issued to White et al.
[0034] Cetel discloses a turbine engine blade having a blade root
with a surface having a thin zone of fine grains. A plasma spray
technique is used to form a thin layer of material on the root or
fir tree portion of the blade. The blade is then HIPed. After the
HIP process, the blade is solution heat treated and then machined.
This reference is directed to a process for modifying the root
section of a turbine blade to improve the mechanical properties of
this area of the part. The root section is serrated and is attached
to the disc by inserting the root serrations into matching
serrations of the disc. The blade is normally produced, as relating
to chemistry and microstructure, to maximize the creep rupture and
high cycle fatigue properties of the airfoil which is exposed to
the hot gas path. The root section of the part thus has those same
properties as the airfoil section. However, the root section of the
blade is exposed to stress of a type different than the airfoil
section, usually referred to as low cycle fatigue. The root section
experiences colder operating temperatures than the airfoil section
and is not directly in the path of the hot gasses that flow through
the engine. Also, the root section is subjected to metal to metal
stress during rotation resulting in low cycle fatigue cracking.
Cetal is concerned with treating only the fir tree or root portion
of the blade to improve its mechanical properties. The root portion
or a new or refurbished blade is treated with a plasma spray
process, HIPing, and a heat treatment and then machined. The blade
is machined to remove material from a high stress portion of the
blade root. The material removed by the machining operation is
replaced by a zone of fine grains by a plasma spray technique. The
part is processed through a HIP cycle to densify the deposit, and
then a heat treatment cycle to enhance its properties. Finally, the
root is machined back to the desired blueprint dimensions and the
part returned to service.
[0035] Wilson discloses a turbine engine blade having a single
crystal body having an airfoil section and an attachment or root
section. A layer of polycrystalline superalloy is applied to the
attachment section, preferably by plasma spraying. The coated blade
is HIPed and then solution heat-treated to optimize the
polycrystalline microstructure.
[0036] Grupta discloses a process for producing high temperature
corrosion resistant metal articles. A ductile metallic overlay is
formed on the surface of an article substrate, and an outer layer
is applied over the overlay. The article is then subjected to a HIP
treatment to eliminate porosity and create an inter-diffusion
between the outer layer the overlay and the substrate.
[0037] None of these prior attempts provide for the effective and
efficient restoration of the critical airfoil dimensions of a gas
turbine engine airfoil part. Typically, an airfoil part will have
to be discarded after it has gone through a certain number of
repair cycles. The stripping of the protective coating on the part
during the repair process is a major contributing factor resulting
in the discarding of the part. After a number of repair cycles the
part simply does not have the minimum dimensional characteristics
necessary for it to perform it intended function. Therefore, there
is a need for a method for repairing gas turbine engine airfoil
parts that effectively and efficiently restores the critical
airfoil dimensions of the part.
[0038] Turbine engine airfoil parts, such as vanes, are
manufactured to precise tolerances that determine the airflow
characteristics for the part. The class of a turbine vane is the
angular relationship between the airfoil section and the inner and
outer buttresses of the vane. This angular relationship has a
direct bearing on the angle of attack of the airfoil section during
the operation of the gas turbine engine. Over time, the angular
relationship between the airfoil section and the inner and outer
buttresses of the vane may become altered due to, for example,
deformation of the airfoil section from engine operation and repair
processes and the like. Or, the particular angular relationship of
the airfoil section and the inner and outer buttresses as
originally manufactured may need to be changed to improve engine
performance. In any event, there is a need for a method of
restoring or reclassifying a gas turbine engine airfoil part.
SUMMARY OF THE INVENTION
[0039] The present invention overcomes the drawbacks of the
conventional. It is an object of the present invention to provide a
method for correcting the dimensional characteristics of a cast
article. It is another object of the present invention to provide a
method for applying a protective coating to a metal article. It is
another object of the present invention to provide a method is
provided for repairing a turbine engine airfoil part.
[0040] In accordance with the present invention, a method of
correcting the dimensional characteristics of a cast article is
provided. The dimensional differences are determined between
pre-repair cast article dimensions and desired post repair cast
article dimensions to correct a casting defect in the article. The
determination may be made by determining the location and
approximate volume of a void in the surface of the article. The
determination may also be made by determining an amount of buildup
volume required to make at least a portion of the surface of the
cast article built up to the desired post repair dimensions. The
article is coated in at least an area of the casting defect with a
high-density coating material. Depending on the coating process,
the coating can be formed in a vacuum, inert atmosphere or under
ambient conditions. How ever it is applied, in accordance with the
invention, the coating must be capable of forming a diffusion
boundary between the coating material and the article. A hot
isostatic heat treatment process is performed to form the diffusion
boundary between the coating material and the article.
[0041] Depending on the type of casting defect, material in an area
of the casting defect may be removed before the step of coating the
article. For example, if the casting defect is an inclusion of an
undesired composition, such as an oxide or dirt particle, the
inclusion and some of the base article material can be removed by a
machining or other operation. The area of the casting defect is
enlarged, and may be contoured to create a better surface for
holding the coating material. The casting defect may be caused, for
example, by at least one of an inclusion at a surface of the
article, an air bubble at the surface of the article, undercasting,
a void and shrinkage. A sintering heat treatment can be performed
before the step of performing the hot isostatic heat treatment to
limit the occurrence bubbles on the surface of the coating material
after an isostatic heat treatment. The sintering heat treatment may
be performed at a temperature substantially the same as the
temperature of the hot isostatic heat treatment.
[0042] In accordance with the present invention, the coating
material may comprise an alloy with substantially no oxide forming
constituents so as to avoid the formation of oxide inclusions in
the coating material. In this case, the coating material may be
applied using a coating process that is effective to create a
coating on the surface of the article that will be diffusion bonded
to the article after the hot isostatic heat treatment.
[0043] In accordance with the present invention, a method is
provided for applying a protective coating to a metal article. A
metal article is provided and coated with a high-density coating
material capable of forming a diffusion boundary between the
coating material and the article. In accordance with this aspect of
the invention, the coating material comprises an alloy with
substantially no oxide forming constituents so as to avoid the
formation of oxide inclusions in the coating material. Applicants
have discovered that the oxides in the coating may form crack
initiation sites, and cracks formed due to the oxides may propagate
through the diffusion boundary and into the article substrate. By
limiting the formation of oxides in the coating, these crack
initiation sites are reduced or eliminated, thereby enabling the
coating material to act as a protective coating. Depending on the
coating process, the coating may be applied in a vacuum, under an
inert atmosphere or under ambient conditions. In the case of
ambient conditions in which oxygen may be present, the coating
material may be composed of constituents that substantially avoid
the formation of oxide particles, even when oxygen is present.
Stated otherwise, the coating material has a chemistry that does
not result in crack producing elements, such as oxides, located in
the coating and in the diffusion boundary between the coating and
the substrate.
[0044] The hot isostatic heat treatment process is performed to
form the diffusion boundary between the coating material and the
article. Thus, in accordance with this aspect of the invention, the
substantially oxide free coating and the diffusion boundary provide
a protective coating to protect the article from damage.
[0045] A sintering heat treatment can be performed before the step
of performing the hot isostatic heat treatment to limit the
occurrence bubbles on the surface of the coating material after an
isostatic heat treatment. The sintering heat treatment may be
performed at a temperature substantially the same as the
temperature of the hot isostatic heat treatment. The sintering heat
treatment increases the production yield by significantly reducing
the formation of bubbles on the surface of the coating due to the
hot isostatic heat treatment, etc.
[0046] In accordance with another aspect of the invention, a method
is provided for repairing a turbine engine airfoil part. The
dimensional differences are determined between pre-repair airfoil
dimensions of a turbine engine airfoil part substrate and desired
post repair airfoil dimensions of the turbine engine airfoil part
substrate. The pre-repair airfoil dimensions having different
airfoil characteristics than the post-repair airfoil dimensions.
The turbine engine airfoil part being comprised of a metal alloy.
The engine airfoil part is coated with a coating capable of forming
a diffusion boundary with the turbine engine airfoil part
substrate. The coating material comprises an alloy with
substantially no oxide forming constituents so as to avoid the
formation of oxide inclusions in the coating material. A hot
isostatic heat treatment process is performed to obtain a
post-repair turbine engine airfoil part having the desired
post-repair dimensions and having a substantially oxide free
coating and diffusion bonding between the coating material and the
turbine engine airfoil part substrate. The substantially oxide-free
coating provides a protective coating to protect the article from
damage. A sintering heat treatment can be performed before the step
of performing the hot isostatic heat treatment to limit the
occurrence bubbles on the surface of the coating material after an
isostatic heat treatment. The sintering heat treatment may be
performed at a temperature substantially the same as the
temperature of the hot isostatic heat treatment.
[0047] In accordance with the present invention, a method is
provided for forming a diffusion coating on the surface of a
workpiece. A workpiece substrate is provided. A coating is formed
on at least selected portions of the workpiece substrate. The
coating material is capable of forming a diffusion bond with the
workpiece substrate. The diffusion bond is a metallurgical bond
between the workpiece and the coating that does not have an
interface boundary. This diffusion bond creates a secure attachment
between the coating and the substrate, much stronger than the
mechanical bond that is originally formed between the coating and
the substrate. A sintering heat treatment is first performed to
expel trapped gas from the coating material. Applicant has found
that the entrapped gas is problematic because it results in a
weaker, bubbled surface with an inconsistent diffusion bond between
the coating and the substrate. The sintering heat treatment removes
the entrapped gas and prevents outgassing of the trapped gas during
a hot isostatic pressing treatment. This preventive treatment has
been experimentally proven to greatly reduces the formation of
bubbles on the surface of the coated workpiece after the hot
isostatic pressing treatment. After the entrapped gas is removed by
the sintering heat treatment, the hot isostatic pressing treatment
is then performed to drive the coating material into the workpiece
substrate. The hot isostatic pressing treatment results in the
formation of the diffusion bond so that the metallurgical bond
between the workpiece and the coating is formed.
[0048] A method of correcting defects in a metal workpeice. A
location of a defect in a workpiece is determined. The defect
comprising a void or an inclusion in a workpiece substrate. The
workpiece substrate is comprised of a metal alloy. Material of the
workpiece substrate at the location of the defect is removed to
form cleaned area in the workpiece substrate. The cleaned area in
the workpiece substrate is coated with a high-density coating. A
sintering heat treatment is performed on the coated workpiece
substrate to remove entrapped gas from the coating material prior
to a step of hot isostatic pressing treating. Then, hot isostatic
pressing treating is performed on the coated workpiece to produce
diffusion bonding between the workpiece substrate and the
high-density coating. The material can be removed by techniques
such as sandblasting or grinding. A high-density coating process
such as hyper-velocity oxy-fuel thermal spray process or a
detonation gun process is used to apply the high-density coating to
the substrate at the location of the cleaned area. The high-density
coating may have the same metal alloy composition as the metal
alloy substrate. The metal alloy substrate may comprise a nickel or
cobalt-based superalloy, and the high-density coating may have the
same nickel or cobalt-based super alloy composition as the metal
alloy substrate.
[0049] The workpiece substrate is prepared for a high-density
coating process. The preparation may include cleaning, blasting,
machining, masking or other like operations. Once the workpiece
substrate has been prepared, a high-density coating process is
performed to coat the workpiece substrate. The coating material is
built-up to a thickness that is effective to obtain desired
finished dimensions after performing a hot isostatic pressing
treatment (described below). The high-density coating process may
comprise performing a hyper velocity oxy-fuel thermal spray
process. In the case of HVOF, a fuel gas and oxygen are used to
create a combustion flame at 2500 to 3100.degree. C. The combustion
takes place at a very high chamber pressure and a supersonic gas
stream forces the coating material through a small-diameter barrel
at very high particle velocities. The HVOF process results in
extremely dense, well-bonded coatings. Typically, HVOF coatings can
be formed nearly 100% dense, with at a porosity of about 0.5%. The
high particle velocities obtained using the HVOF process results in
relatively better bonding between the coating material and the
substrate, as compared with other coating methods such as the
conventional plasma spray method or the chemical vapor deposition
method. However, the HVOF process forms a bond between the coating
material and the substrate that occurs primarily through mechanical
adhesion at a bonding interface. As will be described below, in
accordance with the present invention this mechanical bond is
converted to a metallurgical bond by creating a diffusion bond
between the coating material and the workpiece substrate. This
diffusion bond does not have the interface boundary which is
usually the site of failure.
[0050] The diffusion bond is created by subjecting the coated
workpiece substrate (or, in the case of the inventive repair
method, the coated airfoil part) to a hot isostatic pressing (HIP)
treatment. The appropriate hot isostatic pressing treatment
parameters are selected depending on the coating, the workpiece
substrate and the final attributes that are desired. The hot
isostatic pressing treatment is performed on the coated workpiece
substrate to obtain a metal product having the desired finished
dimensions and diffusion bonding between the coating material and
the workpiece substrate.
[0051] HIP treatment is conventionally used in the densification of
cast metal components and as a diffusion bonding technique for
consolidating powder metals. In the HIP treatment process, a part
to be treated is raised to a high temperature and isostatic
pressure. Typically, the part is heated to 0.6-0.8 times the
melting point of the material comprising the part, and subjected to
pressures on the order of 0.2 to 0.5 times the yield strength of
the material. Pressurization is achieved by pumping an inert gas,
such as Argon, into a pressure vessel. Within the pressure vessel
is a high temperature furnace, which heats the gas to the desired
temperature. The temperature and pressure is held for a set length
of time, and then the gas is cooled and vented.
[0052] In accordance with the present invention, the HIP treatment
process is performed on a HVOF coated substrate to convert the
adhesion bond, which is merely a mechanical bond, to a diffusion
bond, which is a metallurgical bond. In accordance with the present
invention, an HVOF coating process is used to apply the coating
material having sufficient density to effectively undergo the
densification changes that occur during the HIP process. After the
HVOF spray material is applied, a sintering heat treatment process
can be performed to further densify the coating to prevent gas
entrapment of the coating material and/or the diffusion bonding
area during the hot isostatic pressing process. If the coating
material and the workpiece substrate are comprised of the same
metal composition, then the diffusion bonding results in a
particularly seamless transition between the substrate and the
coating.
[0053] The inventive method can be used for forming a metal product
having a wear resistant surface. This method can be employed to
produce, for example, a long lasting cutting tool from a relatively
inexpensive cutting tool substrate. In accordance with this aspect
of the invention, a workpiece substrate is formed to near-finished
dimensions. A high-density coating process, such as a hyper
velocity oxy-fuel thermal spray process, is performed to coat the
workpiece substrate with a wear resistant coating material. The
coating material is built-up to a thickness that is effective to
obtain desired finished dimensions after performing a hot isostatic
pressing treatment. A sintering heat treatment step may be
performed improve the density of the coating material and prevent
gas entrapment during the hot isostatic pressing treatment. The hot
isostatic pressing treatment is performed on the coated workpiece
substrate to obtain a metal product having the desired finished
dimensions and diffusion bonding between the coating material and
the workpiece substrate.
[0054] The inventive method can also be used for forming a cast
metal product. This method can be employed to produce, for example,
a cast part having a hard and/or smooth surface. In accordance with
the present invention, a part is cast to dimensions to less than
the finished dimensions, or a cast part is machined to less than
the finished dimensions. The cast part is then coated using the
HVOF coating method as described herein. The HVOF coating is
applied to a thickness sufficient to bring the part to its finished
dimensions. The HVOF coated, cast part is then HIP treated as
described herein to obtain a finished part having desired
dimensions and surface characteristics.
[0055] In accordance with this aspect of the invention, a cast
metal workpiece is provided. The cast metal workpiece may be formed
from any conventional casting method such as: investment, sand and
resin shell casting.
[0056] The cast metal workpiece is machined, if necessary, to
near-finished dimensions. A high-density coating process, such as a
hyper velocity oxy-fuel thermal spray process (HVOF), is performed
to coat the workpiece substrate with a coating material. The
coating material is built-up to a thickness effective to obtain
desired finished dimensions after performing a hot isostatic
pressing treatment. A sintering heat treatment step may be
performed improve the density of the coating material and prevent
gas entrapment during the hot isostatic pressing treatment. The hot
isostatic pressing treatment is performed on the coated workpiece
substrate to obtain a metal product having the desired finished
dimensions and diffusion bonding between the coating material and
the workpiece substrate.
[0057] In accordance with another aspect of the present invention,
the reclassification of a gas turbine engine airfoil part is
obtained. The dimensional differences between pre-reclassified
dimensions of the buttresses of a turbine engine airfoil part and
desired post-reclassified dimensions of the buttresses are
determined. That is, the change in shape of the inner buttress and
outer buttress necessary to obtained a desired angular relationship
between the airfoil section and the buttresses is determined.
Build-up thickness of coating material required to obtain the
desired post-reclassified dimensions of the buttresses is
determined. A high-density coating process, such as HVOF, is used
to coat the buttresses of the turbine engine airfoil part with a
coating material. The portions of the part that are not to be built
up, such as the airfoil section and parts of the buttresses, may be
masked before applying the high-density coating. Also, some of the
coated surfaces of the part may need to be built up more than
others. The coating material is applied to the determined build-up
thickness of coating material effective to obtain the desired
post-reclassification dimensions after performing a hot isostatic
pressing treatment, and after the selective removal of some of the
original buttress material and some of the built up coating
material. A sintering heat treatment may be performed before the
hot isostatic pressing treatment.
[0058] As discussed herein, the coating material comprises a metal
alloy capable of forming a diffusion bond with the substrate of the
turbine engine airfoil part. After the coating material is applied,
the sintering heat treatment process may be performed to prevent
gas entrapment of the coating material and/or the diffusion bonding
area during the hot isostatic pressing process. Then, the hot
isostatic pressing (HIP) process is performed so that the
buttresses of the turbine engine airfoil part have a robust
diffusion bonding between the coating material and the original
material of the buttresses. Having built up the appropriate
dimensions of the inner buttress and outer buttress, the
reclassification of the part is obtained by selectively removing
the original buttress material and, if necessary, some of the built
up material until the angular relationship between the airfoil
section and the inner and outer buttresses is obtained. The
material can be removed through milling, grinding, or other
suitable and well known machining operations. Further, to
facilitate obtaining the correct dimensions the centerline position
of the airfoil part can be located and held by mounting the part in
a suitable holding fixture when machining the buttresses.
[0059] The fixture may be so constructed so that a vane that has at
least a minimum amount of material built up on its buttresses can
be machined and reclassified. In this case, it may not be necessary
to determine the dimensional differences or the required build-up
thickness. Rather, the inventive high density coating and HIPing
process (and, if needed sintering) can be performed to build up at
least the minimum amount of material diffusion bonded to the
buttresses. Then, the vane is placed in the fixture and the excess
material (both original buttress material and the built-up
material) is machined until the buttresses have been reshaped and
the vane reclassified as intended.
BRIEF DESCRIPTION OF THE DRAWINGS
[0060] FIG. 1(a) is a flow chart showing the steps of the inventive
method for repairing a gas turbine engine airfoil part;
[0061] FIG. 1(b) is a flow chart showing the steps of the inventive
method of forming metal products and metal components having a wear
resistant coating;
[0062] FIG. 1(c) is a flow chart showing the steps of the inventive
method for correcting defects in a workpiece;
[0063] FIG. 2(a) is a schematic view of a tool substrate provided
in accordance with the inventive method of forming metal components
having a wear resistant coating;
[0064] FIG. 2(b) is a schematic view of the tool substrate having a
wear resistant coating applied using an HVOF thermal spray process
in accordance with the inventive method of treating metal
components having a wear resistant coating;
[0065] FIG. 2(c) is a schematic view of the HVOF spray coated tool
substrate undergoing a HIP treatment process in a HIP vessel in
accordance with the inventive method of forming metal components
having a wear resistant coating;
[0066] FIG. 2(d) is a schematic view of the final HVOF spray coated
and HIP treated tool having a wear resistant coating layer
diffusion bonded to the tool substrate in accordance with the
inventive method of forming metal components having a wear
resistant coating;
[0067] FIG. 3(a) is a schematic perspective view of a cast metal
component undergoing a machining operation in accordance with the
inventive method of forming a metal product;
[0068] FIG. 3(b) is a schematic perspective view of the machined
cast metal component in accordance with the inventive method of
forming a metal product;
[0069] FIG. 3(c) is a schematic perspective view of the machined
cast metal component having a coating applied using an HVOF thermal
spray process in accordance with the inventive method of forming a
metal product;
[0070] FIG. 3(d) is a schematic perspective view of the HVOF spray
coated machined cast metal component undergoing a HIP treatment
process in a HIP vessel in accordance with the inventive method of
forming a metal product;
[0071] FIG. 3(e) is a schematic perspective view of the final HVOF
spray coated and HIP treated machined cast metal product having a
coating layer diffusion bonded to the machined cast metal component
in accordance with the inventive method of forming a metal
product;
[0072] FIG. 4 is a flow chart showing the steps of the inventive
method of repairing a turbine engine part;
[0073] FIG. 5(a) is a schematic side view of a worn turbine engine
part before undergoing the inventive method of repairing a turbine
engine part;
[0074] FIG. 5(b) is a schematic cross-sectional view of the worn
turbine engine part before undergoing the inventive method of
repairing a turbine engine part;
[0075] FIG. 6(a) is a schematic side view of the worn turbine
engine part showing the worn areas to be repaired using the
inventive method of repairing a turbine engine part;
[0076] FIG. 6(b) is a schematic cross-sectional view of the worn
turbine engine part showing the worn areas to be repaired using the
inventive method of repairing a turbine engine part;
[0077] FIG. 7(a) is a schematic side view of the worn turbine
engine part showing the worn areas filled in with similar weld
material in accordance with the inventive method of repairing a
turbine engine part;
[0078] FIG. 7(b) is a schematic cross-sectional view of the worn
turbine engine part showing the worn areas filled in with similar
weld material in accordance with the inventive method of repairing
a turbine engine part;
[0079] FIG. 8(a) is a schematic side view of the welded turbine
engine part showing areas to be built up with similar coating
material using an HVOF spray coating process in accordance with the
inventive method of repairing a turbine engine part;
[0080] FIG. 8(b) is a schematic cross-sectional view of the welded
turbine engine part showing areas to be built up with similar
coating material using an HVOF spray coating process in accordance
with the inventive method of repairing a turbine engine part;
[0081] FIG. 9(a) is a schematic side view of the HVOF built up,
welded turbine engine part showing an area masked before performing
the HVOF spray coating process in accordance with the inventive
method of repairing a turbine engine part;
[0082] FIG. 9(b) is a schematic cross-sectional view of the HVOF
built up, welded turbine engine part in accordance with the
inventive method of repairing a turbine engine part;
[0083] FIG. 10 is a schematic view of the HVOF built up, welded
turbine engine part undergoing a HIP treatment process in a HIP
vessel in accordance with the inventive method of repairing a
turbine engine part;
[0084] FIG. 11(a) is a schematic side view of the final HVOF spray
coated and HIP repaired turbine engine part having a similar metal
coating layer diffusion bonded to the original parent substrate and
welded portions in accordance with the inventive method of
repairing a turbine engine part;
[0085] FIG. 11(b) is a schematic cross-sectional view of the final
HVOF spray coated and HIP repaired turbine engine part having a
similar metal coating layer diffusion bonded to the original parent
substrate and welded portions in accordance with the inventive
method of repairing a turbine engine part;
[0086] FIG. 12(a) is a side view of a prior art tool bit coated
with a wear resistant coating;
[0087] FIG. 12(b) is a side view of a prior art tool bit having a
fixed wear resistant cutting tip;
[0088] FIG. 13 is a flow chart showing the steps of the inventive
method for reclassifying a gas turbine engine airfoil part;
[0089] FIG. 14(a) is a front view of a vane from a gas turbine
engine showing the airfoil section, the outer buttress and the
inner buttress;
[0090] FIG. 14(b) is a partial top view of the vane shown in FIG.
14(a) showing the outer buttress and angle .alpha. indicating the
angular relationship between the airfoil and the outer
buttress;
[0091] FIG. 14(c) is a partial bottom view of the vane shown in
FIG. 14(a) showing the inner buttress and angle .alpha.' indicating
the angular relationship between the airfoil and the inner
buttress;
[0092] FIG. 14(d) is a partial left-side view of the vane shown in
FIG. 14(a) showing the leading edge foot of the inner buttress and
the outer foot front face of a buttress rail of the outer
buttress;
[0093] FIG. 14(e) is a partial right-side view of the vane shown in
FIG. 14(a) showing the trailing edge foot of the inner diameter
buttress and the other buttress rail of the outer diameter
buttress;
[0094] FIG. 15(a) is a flowchart showing the steps of the inventive
method for repairing a workpiece with an electroplated coating
diffusion bonded to the workpiece;
[0095] FIG. 15(b) is a flow chart showing the steps of the
inventive method for repairing a gas turbine engine airfoil part
with an electroplated coating diffusion bonded to the airfoil
substrate;
[0096] FIG. 15(c) is a flow chart showing the steps of the
inventive method for correcting defects in a workpiece with an
electroplated coating diffusion bonded to the workpiece;
[0097] FIG. 15(d) is a flow chart showing the steps of the
inventive method for reclassifying a gas turbine engine airfoil
part with an electroplated coating diffusion bonded to the airfoil
part;
[0098] FIG. 16(a) shows an airfoil part prepared for
electroplating;
[0099] FIG. 16(b) shows the prepared airfoil part being
electroplated;
[0100] FIG. 16(c) shows the electroplated airfoil part undergoing a
sintering heat treatment;
[0101] FIG. 16(d) shows the sintered electroplated airfoil part
undergoing a hot isostatic heat treatment;
[0102] FIG. 16(e) shows the finished airfoil part having a
diffusion bond between the electroplated areas and the airfoil
substrate;
[0103] FIG. 17 illustrates the steps of correcting the dimensional
characteristics of a cast article;
[0104] FIG. 18 is a flow chart showing the steps of the inventive
method of correcting the dimensional characteristics of a cast
article;
[0105] FIG. 19 schematically illustrates a coated substrate wherein
the coating material is diffusion bonded to the substrate and
includes an oxide inclusion;
[0106] FIG. 20 schematically illustrates the coated substrate shown
in FIG. 19 wherein a crack is forming at the site of the oxide
inclusion;
[0107] FIG. 21 schematically illustrates the coated substrate shown
in FIG. 19 wherein the crack formed at the site of the oxide
inclusion propagates through the diffusion boundary and into the
substrate;
[0108] FIG. 22(a) is a schematic perspective view of a cast turbine
engine airfoil part showing a casting defect;
[0109] FIG. 22(b) is a schematic perspective view of the cast
turbine engine airfoil part having the area of the casting defect
being machined;
[0110] FIG. 22(c) is a schematic perspective view of the cast
turbine engine airfoil part after the area of the casting defect
has been machined;
[0111] FIG. 23(d) is a schematic perspective view of the cast
turbine engine airfoil part having the area of the casting defect
being filled with a coating material;
[0112] FIG. 22(e) is a schematic perspective view of the coated
cast turbine engine airfoil part being subjected to a hot isostatic
pressing treatment; and
[0113] FIG. 22(f) is a schematic perspective view of the repaired
cast turbine engine airfoil part.
DETAILED DESCRIPTION OF THE INVENTION
[0114] For purposes of promoting an understanding of the principles
of the invention, reference will now be made to the embodiments
illustrated in the drawings and specific language will be used to
describe the same. It will nevertheless be understood that no
limitation of the scope of the invention is thereby intended, there
being contemplated such alterations and modifications of the
illustrated device, and such further applications of the principles
of the invention as disclosed herein, as would normally occur to
one skilled in the art to which the invention pertains.
[0115] Referring to FIG. 1(a), in accordance with the present
invention, the dimensional differences between pre-repaired
dimensions of a turbine engine airfoil part and desired post-repair
dimensions of the turbine engine airfoil part are determined (Step
One-B). The turbine engine airfoil part has a substrate comprised
of a superalloy. A build-up thickness of coating material required
to obtain the desired post-repair dimensions of the turbine engine
airfoil part is determined (Step Two). A high-density coating
process, such as HVOF, is used to coat the turbine engine airfoil
part with a coating material to the determined build-up thickness
of coating material effective to obtain the desired post-repair
dimensions after performing a sintering heat treatment and a hot
isostatic pressing treatment (Step Three). The coating material
comprises a metal alloy capable of forming a diffusion bond with
the substrate of the turbine engine airfoil part. After the coating
material is applied, a sintering heat treatment process is
performed to prevent gas entrapment of the coating material and/or
the diffusion bonding area during the hot isostatic pressing
process (Step Four). Then, the hot isostatic pressing process is
performed to obtain a post-repair turbine engine airfoil part
having the desired post-repair dimensions and having diffusion
bonding between the coating material and the turbine engine airfoil
substrate (Step Five).
[0116] In accordance with the present invention, a protective
coating must be first removed from the turbine engine airfoil part
prior to performing the high-density coating process (Step One-A).
After performing the hot isostatic pressing process, a protective
coating may be re-applied (Step Six). In this case, the build-up
thickness may determined in Step Two to take into consideration the
additional thickness of the post-repaired part due to the addition
of the protective coating.
[0117] Typically, this protective coating is present on an airfoil
part to protect it from the hot corrosive environment it
experiences during service. This protective coating must be removed
during the inspection and/or repair process. After undergoing a
number of inspection and/or repair cycles, the airfoil part was
conventionally discarded simply because the airfoil dimensions of
the part were too deformed for the part to be usable. However, in
accordance with the present inventive repair method, the airfoil
dimensions are restored and a robust repaired airfoil part is
obtained
[0118] In the typical application of the inventive method, the
metal alloy substrate of the turbine engine airfoil part will
comprise a nickel or cobalt-base superalloy. The step of performing
the high-density coating process (Step Three) may thus include
performing a high-density coating process such as a hyper velocity
oxy-fuel thermal spray process or a detonation gun process to apply
a high-density coating having the same nickel or cobalt-base
superalloy composition as the metal alloy substrate.
[0119] In an embodiment of the invention in which the coating
material and the substrate alloy comprise INCO713C nickel or
cobalt-base superalloy, the sintering heat treatment (Step Four)
comprises sintering at a temperature at or about 2150 degrees F.
for about 2 hours, which has been found to effectively prevent gas
entrapment of the applied high-density coating during the hot
isostatic pressing process. The range at which the sintering heat
treatment may be performed is about 1900 to 2300 degrees F. In the
case of the nickel or cobalt-base superalloy substrate, an
effective hot isostatic pressing treatment (Step Five) can be
performed at a temperature of about 2200F in about 15 KSI argon for
about 4 hours. The inventive process may be used with alloys of
other metals, such as titanium or aluminum. The parameters of the
hot isostatic pressing treatment typically call for heating the
engine part to a temperature that is substantially 80% of the
melting point of the metal alloy; and pressurizing the engine part
to a pressure substantially between 20 and 50 percent of the yield
strength of the metal alloy in an inert gas atmosphere.
[0120] The dimensional differences between the pre-repaired
dimensions of the turbine engine airfoil part and the desired
post-repair dimensions of the turbine engine airfoil part are
measured from at least one of the cordal and length dimensions of
the airfoil part (Step One-B). By performing the inventive method
for repairing a gas turbine engine airfoil part, the post-repair
dimensions are equal to the dimensions necessary for effectively
returning the part to active service. The obtained diffusion
bonding between the coating material and the substrate ensures that
the repaired airfoil part is robust enough to withstand the highly
demanding environmental conditions present in an operating gas
turbine engine. Thus, the present invention offers substantial cost
savings over having to replace a turbine gas engine airfoil part
which otherwise might have been discarded.
[0121] The present invention can be used as a process for restoring
critical gas path area dimensions in cast nickel or cobalt-base
superalloy vane components. These dimensions may become altered due
to erosion or particle strikes during the service life of the part,
and/or may become altered during an inspection or repair process
wherein a protective coating is stripped from the part.
[0122] The inventive process, referred to herein as "recast",
briefly consists of applying a pre-alloyed metal powder,
compositionally identical to the superalloy used in the original
manufacture of the vane being repaired, directly on dimensionally
discrepant surfaces, densifying the metal powder coating, and
causing it to bond to the affected surface.
[0123] More specifically, in the preferred embodiment of the
invention candidate recast surfaces are abrasively clean, thermal
sprayed using high velocity oxy fuel processes (HVOF), sintered,
and hot isostatically pressed (HIPed).
[0124] Thermal splay metal powders, produced by a vacuum/inert gas
atomization processes, are applied directly to the dimensionally
discrepant surfaces of a turbine engine airfoil part using robotic
HVOF processes carefully controlled to produce dense coatings while
minimizing thermal gradients and oxidative solute losses.
[0125] Properly applied HVOF coatings are dense but sometimes
contain interconnected micropores. In accordance with the present
invention, such "porous" HVOF coatings are more fully densified by
sintering and subsequently diffusion-bonded to substrate surfaces
by HIPing at temperatures and pressures commensurate with the
nickel or cobalt-base alloy under consideration.
[0126] Recast surfaces are compositionally identical to, but
microstructurally different from, original or "as-cast" substrates.
As-cast substrates are defined herein as a substrate formed by a
conventional casting process, such as the lost wax or investment
casting process described above. The microstructures of cast nickel
or cobalt-base superalloy substrate materials such as used in the
manufacture of gas turbine vanes generally consist of relatively
large amount of an intermetallic precipitate referred to as "gamma
prime" within, and networks of carbides and borides within and
around, large "gamma" matrix grains. The amount and morphology of
gamma prime, carbides, and borides are determined by composition,
processing history, and heat treatment.
[0127] Recast microstructures similarly consist of gamma prime,
carbides, and borides precipitated in and around gamma matrix
grains; but, recast matrix grains are considerably smaller than
as-cast grains. Recast gamma prime, carbide and boride precipitates
are similarly finer than as-cast. In addition, some of the more
reactive solutes (e.g., aluminum) in the thermal spray powders
oxidize during the HVOF spray process to form oxide particles which
become randomly dispersed in the recast deposit.
[0128] Articles repaired by recast are best described as bimetallic
composites comprised of recast coatings bonded to as-cast
substrates. The mechanical properties of such repaired articles
vary depending on the relative volume fraction of the recast
coating, the specific alloy(s) under consideration, and processing
history.
[0129] Example of Recast INCO713C/Cast INCO713C Composite
Mechanical Properties Obtained in Accordance with the Present
Invention:
[0130] Representative tensile and stress-rupture properties of
recast INCO713C/cast INCO713C composite test specimens were
measured to more fully elucidate the recast process.
[0131] INCO713C was selected as the base nickel or cobalt-base
superalloy for measurement because it is specified by a large
number of engine manufactures for gas turbine component
applications, and is bill-of-material for JT8D second-stage vanes,
a candidate component for the inventive recast repair method.
[0132] Near cast-to-size INCO713C test bars were machined into ASTM
proportioned mechanical test specimens with tapered (approximately
three percent) gauge lengths. The average minimum gauge length
diameter was 0.2137 inches.
[0133] The machined test specimens were grit-blasted with silicon
carbide, ultrasonically cleaned, and robotically sprayed with
INCO713C powder using Diamond Jet HVOF processes. The composition
of the INCO713C powder used in these evaluations is shown in Table
I.
1TABLE I Certified Compositions of INCO713C Atomized Powder and
Cast-To-Size Test Bars Cast-To-Size Test Bars Element EMS 55079
Atomized Powder (Heat # 8616) Nickel Balance Balance Balance
Chromium 11.0 to 13.0 13.6 13.67 Aluminum 5.5 to 6.5 5.86 5.61
Molybdenum 3.8 to 5.2 4.39 4.06 Columbium 1.5 to 2.5 2.1 2.08
Titanium 0.4 to 1.0 0.9 0.84 Zirconium 0.05 to 0.15 0.07 0.05
Carbon 0.05 to 0.07 0.1 0.13 Boron 0.005 to 0.015 0.01 0.008 Cobalt
1.00 max. <0.01 <0.05 Silicon 0.50 max. 0.09 <0.05 Copper
0.05 max. 0.04 <0.05 Iron 0.25 max. 0.18 <0.05 Manganese 0.25
max. 0.01 <0.05 Sulfur 0.015 max. 0.002 <0.05 Phosphorus
0.015 max.
[0134] Sufficient HVOF coating was applied to increase the
composite specimen gauge length diameter to approximately 0.250
inches. The sprayed test bars were then sintered at 2150F for 2
hours in vacuum, HIPed at 2200F in 15 KSI argon for 4 hours in a
standard commercial HIP toll cycle, and tested for room temperature
tensile and elevated-temperature stress-rupture.
[0135] The composite test specimens used for these measurements
were nominally comprised of 28 percent recast INCO713C and 72
percent as-cast INCO713C. The recast INCO713C percentage varied,
however, from 25.5 to 30.9 percent depending on precise machined
and sprayed specimen dimensions.
[0136] Mechanical Properties:
[0137] The room temperature tensile and 1800F stress-rupture
properties of the as-cast INCO713C core material used in these
measurements are summarized in Table II.
2TABLE II INCO713C Heat # 8616 Qualification Tests 1. Room
Temperature Tensile a. 0.2% Y.S. 108 KSI UTS 126 KSI Elongation
6.0% b. 0.2% Y.S. 112.2 KSI 111.0 KSI UTS 126 KSI 135.7 KSI
Elongation 6.3% 6.7% 2. Stress-Rupture Temperature Stress Rupture
Life Elongation a. 1800 F. 22 KSI 30.0 hours 1800 F. 24 KSI 14.8
hours 14.0% b. 1800 F. 22 KSI 55.3 hours 9.1% 1800 F. 22 KSI 58.2
hours 10.3%
[0138] The room-temperature tensile and 1800F stress-rupture
properties of the 28 percent recast INCO713C composite test
specimens are summarized in Table III.
3TABLE III Measured Tensile and Stress-Rupture Properties of
Composite Cast/Recast INCO713C Test Specimens 1. Room Temperature
Tensile Properties Specimen 0.2 YS UTS Elongation #1 123.3 KSI
150.3 KSI 5.6% #2 122.0 KSI 151.5 KSI 6.6% #3 122.4 KSI 148.1 KSI
6.7% Average 122.4 KSI 150.0 KSI 6.3% 2. Stress-Rupture Properties
(stress calculated on cast INCO713C cross-section only) Reduction
in Specimen Rupture Life Elongation Area@ 1800 F/22 KSI #4 60.9
hrs. 10.7% 21.1% #5 55.9 hrs. 6.3% 17.8% #6 60.9 hrs. 7.1% 16.8% @
1600 F/42 KSI (stress calculated on cast INCO713C cross-section
only) #5 202.5 hrs. 6.9% 12.2% #6 >212.5 hrs. 4.9% 8.6%
[0139] The room temperature yield and ultimate tensile strengths of
the 28 percent recast INCO713C composite test specimens were
approximately 11 percent higher than those of as-cast INCO713C core
material. The room temperature ductility of the 28 percent recast
INCO713C composite test specimens was virtually identical to that
of the as-cast INCO713C core material.
[0140] The as-cast INCO713C core material and the 28 percent recast
INCO713C composite test specimens were tested for stress-rupture at
1800F under "constant load" conditions to experimentally assess the
effect of the recast process on the sustained, high-temperature,
load-bearing capacity of as-cast INCO713C.
[0141] The approximate time to rupture as-cast INCO713C at 1800F/22
KSI, as estimated from available "Larsen-Miller" correlations, is
48 hours. The time to rupture the as-cast INCO713C core material
test bars at 1800F/22 KSI was 30.0 hours. The average time to
rupture machined as-cast INCO713C test specimens at 1800F/22 KSI
was 56.5 hours. The average as-cast INCO713C 1800F/22 KSI
stress-rupture life was 45 hours, plus or minus 15 hours.
[0142] The 28 percent recast INCO713C composite test specimens were
tested at 1800F under loads sufficient to produce 22 KSI stress
based on as-cast INCO713C substrate dimensions rather than
composite test specimen dimensions. Test loads ranged from 795 to
799 pounds (797 pounds average) depending on precise as-cast
INCO713C machined diameters. Corresponding composite specimen
stresses ranged from 15 to 16 KSI.
[0143] The average time to rupture the 28 percent INCO713C
composite test specimens under such "constant load" test conditions
was 60.9 hours at 1800F.
[0144] Data Analyses:
[0145] The data summarized in Table III show that the recast
process augments the room temperature tensile properties of as-cast
INCO713C.
[0146] Assuming the room temperature tensile properties of the
as-cast INCO713C substrate remain unchanged by the thermal
treatments associated with the recast process, "rule of mixture"
analyses of the room temperature 28 percent recast INCO713C
composite tensile data summarized in Table III indicate that the
recast INCO713C portion of the composite has the following room
temperature tensile properties:
4 150 KSI 0.2% yield strength 190 KSI ultimate tensile strength
5.8% elongation
[0147] The data summarized in Table III similarly show that the
recast process augments the sustained high-temperature,
load-bearing capacity of as-cast INCO713C.
[0148] "Load partitioning analysis", for lack of a better
description, were used to distinguish the stress-rupture strength
properties of the recast INCO713C coating from those of the as-cast
INCO713C substrate.
[0149] "Larsen-Miller" stress-rupture data correlation's suggest
that the stress required to increase the 1800F rupture life of an
as-cast INCO713C substrate specimen to 60.9 hours is only 21 KSI.
The load required to develop a stress of 21 KSI, based on an
average 0.2145 inch as-cast INCO713C substrate diameter, is 759
pounds. Since 797 pounds were applied to the 28 percent recast
INCO713C composite specimens tested at 1800F/16 KSI, it follows
that the balance of the load (39 pounds) was accommodated by the
recast INCO713C coating.
[0150] Since the cross-sectional area of the recast INCO713C
coating in the 28 percent recast INCO713C composite specimens was
0.0161 square inches, the recast INCO713C coating stress was 2.4
KSI. The 1800F/60.9 hour stress-rupture strength of recast INCO713C
is, therefore, approximately 2.4 KSI.
[0151] Two 28 percent recast INCO713C composite test specimens were
similarly tested in stress-rupture at 1600F under loads calculated
to develop a stress of 42 KSI based on as-cast INCO713C substrate
dimensions.
[0152] One of the 28 percent recast INCO713C composite test
specimens ruptured in 202.5 hours at 1600F/42 KSI (based on as-cast
substrate dimensions) while the other was arbitrarily terminated
without rupture after 212.5 hours. An as-cast INCO713C test
specimen might be expected to rupture in approximately 100 hours at
1600F/42 KSI.
[0153] "Load-partitioning analyses" of these 1600F stress-rupture
test results suggest that the 1600F/200 hour stress-rupture
strength of the recast INCO713C coating is greater than 8 KSI.
[0154] The stress-rupture properties of the recast INCO713C
coating, as inferred from "load partitioning analyses", generally
correspond to those of wrought nickel or cobalt-base levels through
post HIP heat treatments.
[0155] The experimental data discussed above indicate that recast
INCO713C coating:
[0156] 1. have intrinsically higher room temperature tensile
strength than as-cast INCO713C; and,
[0157] 2. have intrinsic stress-rupture strengths approximately
equivalent to wrought nickel or cobalt-base alloys.
[0158] More importantly, the experimental data presented and
discussed in this study convincingly demonstrate that the recast
process augments the room-temperature tensile and sustained
high-temperature, load-bearing capacities of as-cast INCO713C.
[0159] In accordance with another aspect of the present invention,
a method of forming metal products and components having a durable
wear resistant coating is provided. FIG. 1(b) is a flow chart
showing the steps of the inventive method of forming metal products
and metal components having a wear resistant coating. This method
obtains a metal product having robust diffusion bonding occurring
between a metal substrate and an applied coating. The first step of
the inventive method is to determine the attributes of a final
workpiece product (Step One). For example, if the final workpiece
product is a cutting tool the attributes include a wear resistant
surface formed on a relatively inexpensive tool substrate 10. If
the final workpiece is a cast metal component, a decorative, smooth
final surface may be desired on a cast substrate 16.
[0160] An appropriate substrate composition is then determined
(Step Two) depending on the selected attributes. In the example of
a cutting tool, the substrate composition may be high speed steel,
which is relatively inexpensive to form but durable enough for its
intended purpose. In the case of a cast metal component, the cast
workpiece substrate can be formed from cast iron or aluminum (or
other cast metal or metal alloy). A workpiece substrate is formed
to near-finished dimensions (Step Three), using known processes
such as casting, extruding, molding, machining, etc. An appropriate
coating material 12 composition is determined depending on the
selected attributes (Step Four). Again, in the example of a cutting
tool the coating material 12 could be selected from a number of
relatively hard and durable metals and alloys such as Cobalt,
Carbide, TiN, etc. In the example of the cast metal component,
aluminum oxide may be chosen to provide both a decorative and
corrosion resistant surface. The selection of both the substrate
and coating composition also depends on their metallurgical
compatibility with each other.
[0161] The workpiece substrate is prepared for a high-density
coating process (Step Five). The preparation may include cleaning,
blasting, machining, masking or other like operations. Once the
workpiece substrate has been prepared, a high-density coating
process is performed to coat the workpiece substrate (Step Six).
The coating material 12 is built-up to a thickness that is
effective to obtain desired finished dimensions after performing a
hot isostatic pressing treatment (described below). The
high-density coating process may comprise performing a hyper
velocity oxy-fuel thermal spray process. In the case of HVOF, a
fuel gas and oxygen are used to create a combustion flame at 2500
to 3100.degree. C. The combustion takes place at a very high
chamber pressure and a supersonic gas stream forces the coating
material 12 through a small-diameter barrel at very high particle
velocities. The HVOF process results in extremely dense,
well-bonded coatings. Typically, HVOF coatings can be formed nearly
100% dense, with at a porosity of about 0.5%.
[0162] The high particle velocities obtained using the HVOF process
results in relatively better bonding between the coating material
12 and the substrate, as compared with other coating methods such
as the Conventional Plasma spray method or the Chemical Vapor
Deposition method. However, the HVOF process also forms a bond
between the coating material 12 and the substrate that occurs
primarily through mechanical adhesion at a bonding interface. As
will be described below, in accordance with the present invention
this mechanical bond is converted to a metallurgical bond by
creating a diffusion bond between the coating material 12 and the
workpiece substrate. The diffusion bond does not have the interface
boundary which is usually the site of failure.
[0163] The diffusion bond is created by subjecting the coated
workpiece substrate to a hot isostatic pressing (HIP) treatment.
The appropriate hot isostatic pressing treatment parameters are
selected depending on the coating, the workpiece substrate and the
final attributes that are desired (Step Seven). The hot isostatic
pressing treatment is performed on the coated workpiece substrate
to obtain a metal product having the desired finished dimensions
and diffusion bonding between the coating material 12 and the
workpiece substrate (Step Eight).
[0164] By proper formation of the workpiece substrate, the final
dimensions of the finished workpiece product can be accurately
achieved through the precise control of the build up of coating
material 12 when the HVOF plasma spray process is performed.
Alternatively, the HIP treated and HVOF coated workpiece substrate
may be machined to final dimensions as necessary (Step Nine).
[0165] HIP treatment is conventionally used in the densification of
cast metal components and as a diffusion bonding technique for
consolidating powder metals. In the HIP treatment process, a part
to be treated is raised to a high temperature and isostatic
pressure. Typically, the part is heated to 0.6-0.8 times the
melting point of the material comprising the part, and subjected to
pressures on the order of 0.2 to 0.5 times the yield strength of
the material. Pressurization is achieved by pumping an inert gas,
such as Argon, into a pressure vessel 14. Within the pressure
vessel 14 is a high temperature furnace, which heats the gas to the
desired temperature. The temperature and pressure is held for a set
length of time, and then the gas is cooled and vented.
[0166] The HIP treatment process is used to produce near-net shaped
components, reducing or eliminating the need for subsequent
machining operations. Further, by precise control of the
temperature, pressure and time of a HIP treatment schedule a
particular microstructure for the treated part can be obtained.
[0167] In accordance with the present invention, the HIP treatment
process is performed on a HVOF coated substrate to convert the
adhesion bond, which is merely a relatively weaker mechanical bond,
to a diffusion bond, which is a relatively stronger metallurgical
bond. In accordance with the present invention, an HVOF coating
process is used to apply the coating material 12 having sufficient
density to effectively undergo the densification changes that occur
during the HIP process. A sintering heat treatment step may be
performed improve the density of the coating material and prevent
gas entrapment during the hot isostatic pressing treatment. If the
coating material 12 and the workpiece substrate are comprised of
the same metal composition, then the diffusion bonding results in a
particularly seamless transition between the substrate and the
coating.
[0168] FIG. 1(c) is a flow chart showing the steps of the inventive
method for correcting defects in a workpiece. A location of a
defect in a workpiece is determined (Step one). The defect
comprises, for example, a void or an inclusion in a workpiece
substrate. For example, an oxide or dirt might be introduced or
formed in the workpiece during a manufacturing process. The
workpiece substrate is comprised of a metal alloy. Material of the
workpiece substrate at the location of the defect is removed to
form cleaned area in the workpiece substrate (Step two). The
cleaned area may be formed by sand or grit blasting, machining,
grinding, or the like. The cleaned area in the workpiece substrate
is coated with a high-density coating (Step three). A sintering
heat treatment is performed on the coated workpiece substrate to
remove entrapped gas from the coating material prior to a step of
hot isostatic pressing treating (Step four). Then, hot isostatic
pressing treating is performed on the coated workpiece to produce
diffusion bonding between the workpiece substrate and the
high-density coating (Step five). If necessary, after the HIP
process is complete, the coated workpeice may be machined to the
desired dimensions (Step six). A high-density coating process such
as hyper-velocity oxy-fuel thermal spray process or a detonation
gun process is used to apply the high-density coating to the
substrate at the location of the cleaned area. The high-density
coating may have the same metal alloy composition as the metal
alloy substrate. The metal alloy substrate may comprise a nickel or
cobalt-based superalloy, and the high-density coating may have the
same nickel or cobalt-based super alloy composition as the metal
alloy substrate.
[0169] As shown in FIGS. 2(a) through 2(d), the inventive method
can be used for forming a metal product having a wear resistant
surface. FIG. 2(a) is a schematic view showing a tool substrate 10
provided in accordance with the inventive method of forming metal
components having a wear resistant coating. The inventive method
can be employed to produce, for example, a long lasting cutting
tool from a relatively inexpensive cutting tool substrate 10.
[0170] In accordance with this aspect of the invention, a workpiece
substrate is formed to near-finished dimensions. The tool substrate
10 may be a drill bit, end mill, lathe tool bit, saw blade, planer
knifes, cutting tool inserts, or other cutting tool part. The
substrate may, alternatively, be something other than a tool. For
example, ice skate blades and snow ski edges may be treated in
accordance with the present invention to obtain a long wearing
edge. Kitchen knives may be treated in accordance with the present
invention to reduce or even eliminate the need for constant
sharpening. Further, products such as pen tips and fishing hooks
may be treated in accordance with the present invention so as to
benefit from long lasting durability. Nearly any metal component
that could benefit from a longer wearing, dense surface structure
might be a candidate from the present invention. For example, steam
turbine erosion shields, fly ash fan blades, power plant conveyors,
are all subjected to wear and/or surface erosion forces. The
present invention can be used to provide the protective surface
characteristics, as described herein, that enhance the
effectiveness of products such as these.
[0171] FIG. 2(b) is a schematic view of the tool substrate 10
having a wear resistant coating applied using an HVOF thermal spray
process in accordance with the inventive method. A high-density
coating process, such as a hyper velocity oxy-fuel thermal spray
process, is performed to coat the workpiece substrate 10 with a
wear resistant coating material 12 using, for example, an HVOF
nozzle. The coating material 12 is built-up to a thickness that is
effective to obtain desired finished dimensions after performing a
hot isostatic pressing treatment.
[0172] FIG. 2(c) is a schematic view of the HVOF spray coated tool
substrate 10 undergoing a HIP treatment process in a HIP vessel 14.
The hot isostatic pressing treatment is performed on the coated
workpiece substrate to obtain a metal product having the desired
finished dimensions and diffusion bonding between the coating
material 12 and the workpiece substrate.
[0173] FIG. 2(d) is a schematic view of the final HVOF spray coated
and HIP treated tool having a wear resistant coating layer
diffusion bonded to the tool substrate 10. In accordance with the
present invention the mechanical bond formed between the parent
substrate and the applied coating is converted to a metallurgical
bond by creating a diffusion bond between the coating material 12
and the parent substrate. The diffusion bond does not have the
interface boundary which is usually the site of failure, thus a
superior product is obtained that has desired surface properties,
such as wear resistance, color, smoothness, texture, etc. These
surface properties do not end abruptly at a bonding interface (as
is the case of conventional coated or brazed products), but rather
remain present to a continuously varying degree from the product
surface to the parent metal. A cutting edge can be put on the tool
surface by conventional sharpening techniques taking care not to
remove more of the diffusion bonded coating than is necessary.
[0174] FIGS. 3(a) through 3(e) illustrate the present inventive
method employed for forming a cast metal product having
predetermined dimensions and surface characteristics. FIG. 3(a) is
a schematic perspective view of a cast metal workpiece substrate
undergoing a machining operation. As shown in FIG. 3(a), the cast
metal workpiece is machined, if necessary, to near-finished
dimensions. FIG. 3(b) is a schematic perspective view of the
machined cast metal component.
[0175] A high-density coating process, such as a hyper velocity
oxy-fuel thermal spray process, is performed to coat the workpiece
substrate with a coating material 12. FIG. 3(c) is a schematic
perspective view of the machined cast metal component having a
coating applied using an HVOF thermal spray process. The coating
material 12 is built-up to a thickness effective to obtain desired
finished dimensions after performing a hot isostatic pressing
treatment. FIG. 3(d) is a schematic perspective view of the HVOF
spray coated machined cast metal component undergoing a HIP
treatment process in a HIP vessel 14. The hot isostatic pressing
treatment is performed on the coated workpiece substrate to obtain
a metal product having the desired finished dimensions and
diffusion bonding between the coating material 12 and the workpiece
substrate. FIG. 3(e) is a schematic perspective view of the final
HVOF spray coated and HIP treated machined cast metal product
having a coating layer diffusion bonded to the machined cast metal
component.
[0176] FIG. 4 is a flow chart showing the steps of the inventive
method of repairing a turbine engine part. The present inventive
method can be used for repairing a turbine engine part 18, such as
a blade or vane. In accordance with this aspect of the invention a
turbine engine part 18, which is comprised of a metal or metal
alloy, is first cleaned (Step One). If necessary, eroded portions
of the turbine engine part 18 are welded using a weld material
comprised of the same metal or metal alloy as the parent or
original metal engine part (Step Two). The welding operation is
performed to build up heavily damaged or eroded portions of the
turbine engine part 18. If the part is not heavily damaged, the
welding operation may be obviated.
[0177] The welding operation will typically produce weld witness
lines. The weld witness lines are ground flush to prevent blast
material from becoming entrapped in the weld witness lines (Step
Three). Portions of the engine part that are not to be HVOF sprayed
are masked (Step Four), and the engine part is again cleaned in
preparation for HVOF spraying (Step Five). HVOF plasma spraying of
the unmasked portions of the engine part is performed (Step Six).
The HVOF plasma spray material (coating material 12) is comprised
of the same metal alloy as the parent or original metal engine
part. The HVOF plasma spray material is applied so as to build up a
cordal dimension of the engine part to a thickness greater than the
thickness of an original cordal dimension of the engine part. A
sintering heat treatment process may be performed to further
densify the coating material. A hot isostatic pressing (HIP)
treatment if performed on the coated engine part to densify the
coating material 12, to create a diffusion bond between the coating
material 12 and the parent and weld material, and to eliminate
voids between the turbine engine part 18, the weld material and the
coated material (Step Seven). Finally, the engine part is machined,
ground and/or polished to the original cordal dimension (Step
Eight).
[0178] FIG. 5(a) is a schematic side view and FIG. 5(b) is a
schematic cross-sectional view of a worn turbine engine part 18
before undergoing the inventive method of repairing a turbine
engine part 18. Metal alloy components, such as gas turbine parts
such as blades and vanes, are often damaged during use. During
operation, gas turbine parts are subjected to considerable
degradation from high pressure and, in the case of rotating
components such as blades, centrifugal force in a hot corrosive
atmosphere. The gas turbine parts also sustain considerable damage
due to impacts from foreign particles. Further, during inspection
and/or repair operations the engine parts are stripped of a
protective diffusion coating, which usually results in the
reduction of some of the substrate thickness. This degradation
results in a limited service life for these parts. Since they are
costly to produce, various conventional repair methods are employed
to refurbish damaged gas turbine blades and vanes. However, these
conventional repair methods generally require labor intensive
machining and welding operations that often subject the part to
damaging stress. Also, these conventional repair methods typically
utilize low pressure plasma spray for the application of a coating
material 12. Conventional plasma spray coating methods deposit the
coating material 12 at relatively low velocity, resulting in voids
being formed within the coating and in a coating density typically
having a porosity of about 5.0%. Again, the bond between the
substrate and the coating occurs primarily through mechanical
adhesion at a bonding interface, and if the coating is subjected to
sufficient shearing forces it will flake off of the workpiece
substrate. Further, the high porosity of the coating obtained
through conventional plasma spray coating make them inadequate
candidates for diffusion bonding through the HIP treating process
described herein.
[0179] FIG. 6(a) is a schematic side view and FIG. 6(b) is a
schematic cross-sectional view of the worn turbine engine part 18
showing the worn areas 20 to be repaired using the inventive method
of repairing a turbine engine part 18. The area enclosed by the
dashed lines represent the material that has been erode or
otherwise lost from the original turbine engine part 18. In
accordance with the present invention, this area is reconstituted
using the same material as the original blade and using the
inventive metal treatment process. The worn turbine engine part 18
(in this case, a turbine blade) is first cleaned to prepare the
worn surfaces for welding (see Step One, FIG. 4).
[0180] FIG. 7(a) is a schematic side view and FIG. 7(b) is a
schematic cross-sectional view of the worn turbine engine part 18
showing the worn areas filled in with similar weld material 22 in
accordance with the inventive method of repairing a turbine engine
part 18 (see Step Two, FIG. 4). In accordance with the present
invention, the weld material is the same as the original blade
material making the bond between the weld and the substrate
exceptionally strong.
[0181] FIG. 8(a) is a schematic side view and FIG. 8(b) is a
schematic cross-sectional view of the welded turbine engine part 25
showing areas 24 to be built up with similar coating material 12
using an HVOF spray coating process in accordance with the
inventive method of repairing a turbine engine part. In accordance
with the present invention, the coating material 12 is the same as
the original blade material, again making the bond between the weld
and the substrate exceptionally strong.
[0182] FIG. 9(a) is a schematic side view and FIG. 9(b) is a
schematic cross-sectional view of the HVOF built up, welded turbine
engine part 27 showing an area, such as the vane or blade root,
masked 26 before performing the HVOF spray coating process in
accordance with the inventive method of repairing a turbine engine
part. The coating material 12 is built-up to a thickness that is
effective to obtain desired finished dimensions after performing a
hot isostatic pressing treatment (described below).
[0183] The high-density coating process may comprise performing a
hyper velocity oxy-fuel thermal spray process. In the case of HVOF,
a fuel gas and oxygen are used to create a combustion flame at 2500
to 3100.degree. C. The combustion takes place at a very high
chamber pressure and a supersonic gas stream forces the coating
material 12 through a small-diameter barrel at very high particle
velocities. The HVOF process results in extremely dense,
well-bonded coatings. Typically, HVOF coatings can be formed nearly
100% dense, with at a porosity of about 0.5%. The high particle
velocities obtained using the HVOF process results in relatively
better bonding between the coating material 12 and the substrate,
as compared with other coating methods such as the conventional
plasma spray method or the chemical vapor deposition method.
However, the HVOF process forms the bond between the coating
material 12 and the substrate that occurs primarily through
mechanical adhesion at a bonding interface. As will be described
below, in accordance with the present invention this mechanical
bond is converted to a metallurgical bond by creating a diffusion
bond between the coating material 12 and the workpiece substrate.
The diffusion bond does not have the interface boundary which is
usually the site of failure.
[0184] The diffusion bond is created by subjecting the coated
workpiece substrate to a hot isostatic pressing (HIP) treatment.
The appropriate hot isostatic pressing treatment parameters are
selected depending on the coating, the workpiece substrate and the
final attributes that are desired. The hot isostatic pressing
treatment is performed on the coated workpiece substrate to obtain
a metal product having the desired finished dimensions and
diffusion bonding between the coating material 12 and the workpiece
substrate.
[0185] FIG. 10 is a schematic view of the HVOF built up, welded
turbine engine part 27 undergoing a HIP treatment process in a HIP
vessel 14 in accordance with the inventive method of repairing a
turbine engine part.
[0186] HIP treatment is conventionally used in the densification of
cast metal components and as a diffusion bonding technique for
consolidating powder metals. In the HIP treatment process, a part
to be treated is raised to a high temperature and isostatic
pressure. Typically, the part is heated to 0.6-0.8 times the
melting point of the material comprising the part, and subjected to
pressures on the order of 0.2 to 0.5 times the yield strength of
the material. Pressurization is achieved by pumping an inert gas,
such as Argon, into a pressure vessel 14. Within the pressure
vessel 14 is a high temperature furnace, which heats the gas to the
desired temperature. The temperature and pressure is held for a set
length of time, and then the gas is cooled and vented.
[0187] The HIP treatment process is used to produce near-net shaped
components, reducing or eliminating the need for subsequent
machining operations. Further, by precise control of the
temperature, pressure and time of a HIP treatment schedule a
particular microstructure for the treated part can be obtained.
[0188] FIG. 11(a) is a schematic side view and FIG. 11(b) is a
schematic cross-sectional view of the final HVOF spray coated and
HIP repaired turbine engine part 28 having a similar metal coating
layer diffusion bonded to the original parent substrate and welded
portions in accordance with the inventive method of repairing a
turbine engine part. By proper formation of the workpiece
substrate, the final dimensions of the finished workpiece produce
can be accurately achieved through the precise control of the build
up of coating material 12 when the HVOF plasma spray process is
performed. Alternatively, the HIP treated and HVOF coated workpiece
substrate may be machined to final dimensions as necessary (Step
Eight).
[0189] An experimental test piece was prepared in accordance with
the inventive method of treating metal components. Photomicrographs
of the test piece showed the grain structure and diffusion bonding
of the coating material 12 and the substrate after the inventive
method has been performed. The HIP treatment process was performed
on an HVOF coated test substrate to convert the adhesion bond
between the coating and the substrate, which is merely a mechanical
bond, to a diffusion bond, which is a metallurgical bond. In
accordance with the present invention, an HVOF coating process is
used to apply the coating material 12 having sufficient density to
effectively undergo the densification changes that occur during the
HIP process. In the case of the test piece example, the coating
material 12 and the workpiece substrate are comprised of the same
metal composition. The diffusion bonding results in a transition
between the substrate and the coating that has a much stronger
structural integrity and wear characteristics as compared with the
conventional art.
[0190] The test piece was prepared by building up coating material
12 to a thickness of approximately 0.02 inches, and the composition
of the test pieces was determined at seven locations (A-G) across a
cross section of the piece. The composition was found to be
substantially uniform across the cross-section of the test piece,
as shown in the following table.
5 TABLE I Elemental Composition (Weight %) Element A B C D E F G
Aluminum 5.4 5.2 5.5 6.2 6.3 6.4 6.5 Titanium 0.6 0.6 1.0 0.6 1.0
0.6 0.9 Chromium 12.9 13.2 14.5 12.7 11.5 13.7 14.1 Nickel REM REM
REM REM REM REM REM Niobium 1.4 1.5 1.8 2.1 1.7 2.3 2.6 Molybdenum
3.7 4.1 3.6 3.3 3.4 3.9 3.0
[0191] A photomicrograph of the treated workpiece shows the grain
structure and diffusion bonding of the coating material 12 and the
substrate after the inventive method has been performed. In
accordance with the present invention, the HIP treatment process is
performed on a HVOF built up, welded turbine engine part to convert
the adhesion bond, which is merely a mechanical bond, to a
diffusion bond, which is a metallurgical bond.
[0192] In accordance with the present invention, an HVOF coating
process is used to apply the coating material 12 having sufficient
density to effectively undergo the densification changes that occur
during the HIP process. If the coating material 12 and the
workpiece substrate are comprised of the same metal composition,
then the diffusion bonding results in smooth transition between the
substrate and the coating. In contrast, a conventional plasma spray
coating method results in a relatively weak bond between the
coating and the substrate. The bond is primarily due to a
mechanical adhesion bond that occurs relatively locally within a
boundary interface.
[0193] As discussed in detail above, in accordance with the present
inventive method a deformed gas turbine engine airfoil part can be
returned to the dimensions required to place the part back into
useful service. A diffusion bond is created between the coating
material and the substrate of a repaired gas turbine engine airfoil
part. This diffusion bond is extremely robust and results in a
repaired engine part that has the appropriate mechanical properties
that allow the part to be safely returned to service. The inventive
method of repairing a turbine engine airfoil part offers
substantial savings because it provides for the efficient and
effective repairing of expensive engine parts which otherwise might
have been discarded.
[0194] As shown in FIG. 13 in accordance with another aspect of the
present invention, the reclassification of a gas turbine engine
airfoil part is obtained. The dimensional differences between
pre-reclassified dimensions of the buttresses of a turbine engine
airfoil part and desired post-reclassified dimensions of the
buttresses are determined (Step One). That is, the change in shape
of the inner buttress and outer buttress necessary to obtained a
desired angular relationship between the airfoil section and the
buttresses is determined. Build-up thickness of coating material
required to obtain the desired post-reclassified dimensions of the
buttresses is determined (Step Two). A high-density coating
process, such as HVOF, is used to coat the buttresses of the
turbine engine airfoil part with a coating material (Step Three).
The portions of the part that are not to be built up, such as the
airfoil section and parts of the buttresses, may be masked before
applying the high-density coating. Also, some of the coated
surfaces of the part may need to be built up more than others. The
coating material is applied at least to the determined build-up
thickness of coating material effective to obtain the desired
post-reclassification dimensions after performing a hot isostatic
pressing treatment, and after the selective removal of some of the
original buttress material and some of the built up coating
material.
[0195] As discussed herein, the coating material comprises a metal
alloy capable of forming a diffusion bond with the substrate of the
turbine engine airfoil part. After the coating material is applied,
the sintering heat treatment process may be performed (Step Four)
to prevent gas entrapment of the coating material and/or the
diffusion bonding area during the hot isostatic pressing process.
Then, the hot isostatic pressing process is performed so that the
buttresses of the turbine engine airfoil part have a robust
diffusion bonding between the coating material and the original
material of the buttresses (Step Five). Having built up the
appropriate dimensions of the inner buttress and outer buttress,
the reclassification of the part is obtained by selectively
removing the original buttress material and, if necessary, some of
the built up material until the angular relationship between the
airfoil section and the inner and outer buttresses is obtained
(Step Six). The material can be removed through milling, grinding,
or other suitable and well known machining operations. Further, to
facilitate obtaining the correct dimensions the centerline position
of the airfoil part can be located and held by mounting the part in
a suitable holding fixture when machining the buttresses.
[0196] The fixture may be so constructed so that a vane that has at
least a minimum amount of material built up on its buttresses can
be machined and reclassified. In this case, it may not be necessary
to determine the dimensional differences or the required build-up
thickness. Rather, the inventive high density coating and HIPing
process (and, if needed sintering) can be performed to build up at
least the minimum amount of material diffusion bonded to the
buttresses. Then, the vane is placed in the fixture and the excess
material (both original buttress material and the built-up
material) is machined until the buttresses have been reshaped and
the vane reclassified as intended or restored to original.
[0197] The class of a turbine engine vane is defined by the angular
relationship between the airfoil section and the inner and outer
buttresses. The inventive recast process is utilized to change or
restore the original class of a turbine engine airfoil part by
building up sufficient material on the inner buttress and the outer
buttress so that the buttresses can then be machined to create the
desired angles .alpha. and .alpha.' (shown in FIGS. 14(b) and
14(c)) and reclassify the vane.
[0198] All buttresses are dimensionally the same and all airfoils
are dimensionally the same for all classes of vanes. In accordance
with the present invention, the airfoil centerline position is held
by mounting the vane in a fixture, and the buttresses are machined
to obtained to desired reclassification parameters.
[0199] The class of a turbine engine vane 20 is defined by the
angular relationship between the airfoil section 22 and the inner
buttress 24 and outer buttress 26. The inventive recast process is
utilized to change or restore the original class of a turbine
engine airfoil part by building up sufficient material on the inner
buttress 24 and the outer buttress 26 so that the buttresses 24, 26
can then be machined to create the desired angles .alpha. and
.alpha.' (shown in FIGS. 14(b) and 14(c)) and reclassify the vane
20.
[0200] All buttresses 24, 26 are dimensionally the same and all
airfoils are dimensionally the same for all classes of vanes. In
accordance with the present invention, the airfoil centerline
position is held by mounting the vane 20 in a fixture, and the
buttresses 24, 26 are machined to obtained to desired
reclassification parameters.
[0201] FIG. 14(a) is a front view of a vane 20 from a gas turbine
engine showing the airfoil section 22, the outer buttress 26 and
the inner buttress 24. In accordance with this aspect of the
invention, it is first determined what dimensions of the inner
buttress 24 and outer buttress 26 need to be adjusted in order to
obtain the desired reclassification of the vane 20. Having
determined the dimensional differences between the pre-reclassified
buttresses 24, 26 and the post-reclassified buttresses 24, 26, it
is next determine how much material must be added, and where the
material must be added so that the buttresses 24, 26 can be
reshaped.
[0202] FIG. 14(b) is a partial top view showing the outer buttress
26 and angle a indicating the angular relationship between the
airfoil section 22 and the outer buttress 26 and FIG. 14(c) is a
partial bottom view showing the inner buttress 24 and angle
.alpha.' indicating the angular relationship between the airfoil
section 22 and the inner buttress 24. In accordance with the
present invention, the vane 20 is reclassified by changing the
shape of the buttresses 24, 26 so that the angles .alpha. and
.alpha.' are changed resulting in a changed angle of attack of the
airfoil section 22, and thus reclassification of the vane 20.
[0203] FIG. 14(d) is a partial left-side view showing the leading
edge foot 28 of the inner buttress 24 and the outer foot front face
30 of a buttress rail 32 of the outer buttress 26 and FIG. 14(e) is
a partial right-side view showing the trailing edge foot 34 of the
inner buttress 24 and the other buttress rail 32 of the outer
buttress 26. In accordance with the present invention, the surfaces
of the buttresses 24, 26, such as the leading edge foot 28, center
log 36, trailing edge foot 34 (inner buttress 24), and the outer
foot front face 30 and buttress rails 32 (outer buttress 26) are
selectively built up and machined so that the angle of attack of
the airfoil section 22 is adjusted. The build up of material on the
buttresses 24, 26 may be uniform, and then the buttresses 24, 26
machined to selectively remove portions of the original substrate
and portions of the build up material. To reduce machine costs, the
surfaces of the original buttresses 24, 26 that are going to be
machined may be masked before the buildup material is applied. In
this case, the buildup material will not have to be later machined
along with the original substrate to reshape the buttresses 24, 26
24, 26.
[0204] A fixture for holding the vane 20 during the machining
operation(s) may be so constructed so that the vane 20 having at
least a minimum amount of material built up on its buttresses 24,
26 can be machined and reclassified. In this case, it may not be
necessary to determine the dimensional differences or the required
build-up thickness. Rather, the inventive high density coating and
HIPing process (and, if needed sintering and other processes
described herein) can be performed to build up at least the minimum
amount of material diffusion bonded to the buttresses 24, 26 24,
26. Then, the vane 20 is placed in the fixture and the excess
material (both original buttress material and the built-up
material) is machined until the buttresses 24, 26 have been
reshaped and the vane reclassified as intended.
[0205] The resulting reclassified vane has inner and outer
buttresses with the mechanical properties required for safe return
to active service in an operating gas turbine engine. The diffusion
bonding between the applied coating material built up on the
buttresses and the original buttress substrate ensures, as
substantiated by the test results discussed herein, that the
reclassified vane can be safely returned to active service.
[0206] FIG. 15(a) is a flowchart showing the steps of the inventive
method for repairing a workpiece with an electroplated coating
diffusion bonded to the workpiece. A workpiece substrate is
provided and prepared for a coating operation (Step One). The
preparation may include, for example, masking off portions that are
not to be coated, cleaning and machining surfaces to be coated,
etc. A coating is formed on at least selected portions of the
workpiece substrate through an electroplating process (Step Two).
The coating material is capable of forming a diffusion bond with
the workpiece substrate. The diffusion bond is a metallurgical bond
between the workpiece and the coating that does not have an
interface boundary. This diffusion bond creates a secure attachment
between the coating and the substrate, much stronger than the
mechanical bond that is originally formed between the coating and
the substrate. This diffusion bond is formed through the hot
isostatic pressing treatment. The diffusion bond can be formed when
the coating on the substrate is dense. It may be possible to form
this coating by a spray process, such as vacuum spray, detonation
gun, HVOF, or by a solution process such as electroplating. To
ensure a diffusion bond is formed, a sintering heat treatment may
have to be first performed to densify the coating prior to the hot
isostatic heat treament step and, if necessary, to remove entrapped
gas (Step Three). If the coating is not dense enough, it may flake
off of the substrate during the heat and pressure of the hot
isostatic treatment step. Further, applicant has found that
entrapped gas is problematic because it results in a weaker,
bubbled surface with an inconsistent diffusion bond between the
coating and the substrate. The sintering heat treatment densities
the coating and removes entrapped gas and prevents outgassing of
the trapped gas during a hot isostatic pressing treatment.
[0207] This preventive treatment has been experimentally proven to
greatly reduces the formation of bubbles on the surface of the
coated workpiece after the hot isostatic pressing treatment. After
the entrapped gas is removed by the sintering heat treatment, the
hot isostatic pressing treatment is then performed to drive the
coating material into the workpiece substrate (Step Four). The hot
isostatic pressing treatment results in the formation of the
diffusion bond so that the metallurgical bond between the workpiece
and the coating is formed. Further post-HIP treatments can be
performed such as heat treatments, machining operations, removing
masking material, forming a protective coating over the diffusion
bonded coating, etc (Step Five).
[0208] FIG. 15(b) is a flow chart showing the steps of the
inventive method for repairing a gas turbine engine airfoil part
with an electroplated coating diffusion bonded to the airfoil
substrate. In accordance with the present invention, the protective
coating on a turbine engine airfoil part is removed so that the
part can be prepared for the inventive electroplating recast repair
method (Step One). The dimensional differences between pre-repaired
dimensions of a turbine engine airfoil part and desired post-repair
dimensions of the turbine engine airfoil part are determined (Step
Two). The turbine engine airfoil part has a substrate comprised of
a superalloy. A build-up thickness of coating material required to
obtain the desired post-repair dimensions of the turbine engine
airfoil part is determined (Step Three). An electroplating process
is used to coat the turbine engine airfoil part with a coating
material to the determined build-up thickness of coating material
effective to obtain the desired post-repair dimensions after
performing a sintering heat treatment and a hot isostatic pressing
treatment (Step Four). The electroplating process allows the
controlled build up of material even between surfaces and around
angles of the substrate that would be difficult or impossible to
coating using a spray coating process. A spray coating process
requires a straight line from the spary nozzle to the coated
surface. When the surface has contours and/or interior portions it
is difficult or impossible to coat these surfaces using a spray
coating process. Even if the coating material can be sprayed into
the contour or interior portion, it remains difficult or impossible
to apply an even coating thickness. The electroplating process
enables the coating to be applied evenly even within interior
surfaces, around corners or onto contours. The coating material
comprises a metal alloy capable of forming a diffusion bond with
the substrate of the turbine engine airfoil part. After the coating
material is applied, a sintering heat treatment process may be
performed if necessary to densify the electroplated coating prior
to the hot isostatic pressing process (Step Five). The
electroplating process has the advantages of enabling a uniform
coating to be applied to a substrate, even if the substrate has
contours and interior spaces. The electroplating process may not
result in trapped gas, as a spray coating process does. However, it
still may be necessary to perform the sintering heat treatment in
order to densify the coating, so as to prevent the coating from
flaking from the substrate due to the heat and pressure of the hot
isostatic pressing treatment. The hot isostatic pressing process is
performed to obtain a post-repair turbine engine airfoil part
having the desired post-repair dimensions and having diffusion
bonding between the coating material and the turbine engine airfoil
substrate (Step Six). After performing the hot isostatic pressing
process, a protective coating may be re-applied (Step Seven).
Typically, this protective coating is present on an airfoil part to
protect it from the hot corrosive environment it experiences during
service. This protective coating must be removed during the
inspection and/or repair process. After undergoing a number of
inspection and/or repair cycles, the airfoil part was
conventionally discarded simply because the airfoil dimensions of
the part were too deformed for the part to be usable. However, in
accordance with the present inventive repair method, the airfoil
dimensions are restored and a robust repaired airfoil part is
obtained
[0209] In the typical application of the inventive method, the
metal alloy substrate of the turbine engine airfoil part will
comprise a nickel or cobalt-base superalloy. The step of performing
the electroplating coating process (Step Four) may include
performing a the electroplating coating process using an
electroplatable material that is effective to create a diffusion
bond with the airfoil substrate after the sintering and hot
isostatic pressing treatment steps.
[0210] By performing the inventive method for repairing a gas
turbine engine airfoil part, the post-repair dimensions are equal
to the dimensions necessary for effectively returning the part to
active service. The obtained diffusion bonding between the coating
material and the substrate ensures that the repaired airfoil part
is robust enough to withstand the highly demanding environmental
conditions present in an operating gas turbine engine. Thus, the
present invention offers substantial cost savings over having to
replace a turbine gas engine airfoil part which otherwise might
have been discarded. The present invention can be used as a process
for restoring critical gas path area dimensions in cast nickel or
cobalt-base superalloy vane components. These dimensions may become
altered due to erosion or particle strikes during the service life
of the part, and/or may become altered during an inspection or
repair process wherein a protective coating is stripped from the
part.
[0211] FIG. 15(c) is a flow chart showing the steps of the
inventive method for correcting defects in a workpiece with an
electroplated coating diffusion bonded to the workpiece. A location
of a defect in a workpiece is determined (Step one). The defect may
comprise, for example, a void or an inclusion in a workpiece
substrate. For example, a crack or divot may be present in the
workpiece due to manufacturing or service-related problems. Or, an
oxide or dirt might be introduced or formed in the workpiece during
a manufacturing process. Further, a cast workpiece may have casting
flaws such as surface porosity, voids, cracks, or may be undersized
due to shrinkage. The invention method for correcting defects in a
workpiece can be employed to correct such casting defects prior to
finish machining operations. The workpiece substrate is comprised
of a metal alloy. Material of the workpiece substrate at the
location of the defect may be removed, if necessary, to form a
cleaned area in the workpiece substrate (Step two). The cleaned
area may be formed by sand or grit blasting, machining, grinding,
selective etching, or the like. Parts of the workpiece that are not
to be coated may then be masked.
[0212] An electroplating process is used to coat the turbine engine
airfoil part with a coating material to the determined build-up
thickness of coating material effective to obtain the desired
post-repair dimensions after performing a sintering heat treatment
and a hot isostatic pressing treatment. The electroplating process
allows the controlled build up of material even between surfaces
and around angles of the substrate that would be difficult to
coating using a spray process. The coating material comprises a
metal alloy capable of forming a diffusion bond with the substrate
of the turbine engine airfoil part (Step three). A sintering heat
treatment may be performed on the coated workpiece substrate to
densify the coating material prior to a step of hot isostatic
pressing treating (Step four). Then, hot isostatic pressing
treating is performed on the coated workpiece to produce diffusion
bonding between the workpiece substrate and the electroplated
coating (Step five). If necessary, after the HIP process is
complete, the masking may be removed and/or the coated workpiece
may be machined to the desired dimensions (Step six).
[0213] FIG. 15(d) is a flow chart showing the steps of the
inventive method for reclassifying a gas turbine engine airfoil
part with an electroplated coating diffusion bonded to the airfoil
part. The dimensional differences between pre-reclassified
dimensions of the buttresses of a turbine engine airfoil part and
desired post-reclassified dimensions of the buttresses are
determined (Step One). That is, the change in shape of the inner
buttress and outer buttress necessary to obtained a desired angular
relationship between the airfoil section and the buttresses is
determined. Build-up thickness of coating material required to
obtain the desired post-reclassified dimensions of the buttresses
is determined (Step Two).
[0214] An electroplating process is used to coat the turbine engine
airfoil part with a coating material to the determined build-up
thickness of coating material effective to obtain the desired
post-repair dimensions after performing a sintering heat treatment
and a hot isostatic pressing treatment. The electroplating process
allows the controlled build up of material even between surfaces
and around angles of the substrate that would be difficult to coat
using a spray process. The coating material comprises a metal alloy
capable of forming a diffusion bond with the substrate of the
turbine engine airfoil part (Step three). The portions of the part
that are not to be built up, such as the airfoil section and parts
of the buttresses, may be masked before applying the electroplated
coating. Also, some of the coated surfaces of the part may need to
be built up more than others. In this case, the masking can be done
in stages, so that after a build up of electroplated material
occurs, a portion of the built up surface is masked before
additional electroplating build is performed on the unmasked
portions. The coating material is applied at least to the
determined build-up thickness of coating material effective to
obtain the desired post-reclassification dimensions after
performing a hot isostatic pressing treatment, and after the
selective removal of some of the original buttress material and
some of the built up coating material.
[0215] As discussed herein, the coating material comprises a metal
alloy capable of forming a diffusion bond with the substrate of the
turbine engine airfoil part. After the coating material is applied,
the sintering heat treatment process may be performed (Step Four)
to densify the electroplated coating prior to the hot isostatic
pressing process. Then, the hot isostatic pressing process is
performed so that the buttresses of the turbine engine airfoil part
have a robust diffusion bonding between the coating material and
the original material of the buttresses (Step Five). Having built
up the appropriate dimensions of the inner buttress and outer
buttress, the reclassification of the part is obtained by
selectively removing the original buttress material and, if
necessary, some of the built up material until the angular
relationship between the airfoil section and the inner and outer
buttresses is obtained (Step Six). The material can be removed
through milling, grinding, or other suitable and well known
machining operations. Further, to facilitate obtaining the correct
dimensions the centerline position of the airfoil part can be
located and held by mounting the part in a suitable holding fixture
when machining the buttresses.
[0216] FIG. 16(a) shows an airfoil part prepared for
electroplating. A workpiece substrate is provided and prepared for
a coating operation. The preparation may include, for example,
masking off portions that are not to be coated, cleaning and
machining surfaces to be coated, etc
[0217] FIG. 16(b) shows the prepared airfoil part being
electroplated. A coating is formed on at least selected portions of
the workpiece substrate through an electroplating process. The
coating material is capable of forming a diffusion bond with the
workpiece substrate. The diffusion bond is a metallurgical bond
between the workpiece and the coating that does not have an
interface boundary. This diffusion bond creates a secure attachment
between the coating and the substrate, much stronger than the
mechanical bond that is originally formed between the coating and
the substrate.
[0218] FIG. 16(c) shows the electroplated airfoil part undergoing a
sintering heat treatment. A sintering heat treatment may be
performed to densify the coating material (Step Three). The
sintering heat treatment may be necessary to prevent the coating
material from separating from the workpiece substrate under the
temperature and pressure of the hot isostatic heat treatment.
[0219] FIG. 16(d) shows the sintered electroplated airfoil part
undergoing a hot isostatic heat treatment. After the sintering heat
treatment, the hot isostatic pressing treatment is then performed
to drive the coating material into the workpiece substrate (Step
Four). The hot isostatic pressing treatment results in the
formation of the diffusion bond so that the metallurgical bond
between the workpiece and the coating is formed.
[0220] FIG. 16(e) shows the finished airfoil part having a
diffusion bond between the electroplated areas and the airfoil
substrate. Further post-HIP treatments can be performed such as
heat treatments, machining operations, removing masking material,
forming a protective coating over the diffusion bonded coating, etc
(Step Five).
[0221] FIG. 17 illustrates the steps of correcting the dimensional
characteristics of a cast article. In accordance with the present
invention, a method of correcting the dimensional characteristics
of a cast article is provided. As shown in Step One, the
dimensional differences are determined between pre-repair cast
article dimensions and desired post repair cast article dimensions
to correct a casting defect in the article. The determination may
be made by determining the location of a void(s) in the surface of
the article. The determination may also be made by determining an
amount of buildup volume required to make at least a portion of the
surface of the cast article built up to the desired post repair
dimensions. For example, as shown in step one, a defect consisting
of an inclusion can be found on the surface of a cast article. As
shown in step two, the inclusion is removed, and the substrate
material in the immediate area around where the inclusion was has
been removed by a machining operation such as drilling or milling.
As shown in step three, the article is coated in at least an area
of the casting defect with a high-density coating material capable
of forming a diffusion boundary between the coating material and
the article. A sintering heat treatment may be performed to remove
any trapped gas and/or to densify the coating surface to prevent
gas from infiltrating the coating during a hot isostatic pressing
treatment (step four). As shown in step five, the hot isostatic
heat treatment process is performed to form the diffusion boundary
between the coating material and the article. By this method,
casting defects, such as oxide inclusions, surface bubbles or
undercastings can be repaired. The repaired area has filler
material diffusion bonded with the casting substrate, ensuring the
integrity of the repair.
[0222] Depending on the type of casting defect, material in an area
of the casting defect may be removed before the step of coating the
article. For example, if the casting defect is an inclusion of an
undesired composition, such as an oxide or dirt particle, the
inclusion and some of the base article material can be removed by a
machining or other operation (step two). The area of the casting
defect is enlarged, and may be contoured to create a better surface
for holding the coating material. The casting defect may be caused,
for example, by at least one of an inclusion at a surface of the
article, an air bubble at the surface of the article, undercasting,
a void and shrinkage. A sintering heat treatment can be performed
(step four) before the step of performing the hot isostatic heat
treatment (step five) to limit the occurrence bubbles on the
surface of the coating material after an isostatic heat treatment.
The sintering heat treatment may preferably be performed at a
temperature substantially the same as the temperature of the hot
isostatic heat treatment.
[0223] In accordance with the present invention, the coating
material may comprise an alloy with substantially no oxide forming
constituents so as to avoid the formation of oxide inclusions in
the coating material. In this case, the coating material may be
applied using a coating process that is effective to create a
coating on the surface of the article that will be diffusion bonded
to the article after the hot isostatic heat treatment, without the
formation of crack inducing oxides. Applicant has discovered that
by preventing the formation of oxide constituents in the coating,
the ductility and other desirable properties of the coating is
improved. This improved ductility provides a protective barrier
that may effectively prevent the propagation and the formation of
cracks in the coating material, the diffusion boundary and the
substrate. An example of the chemistry of a suitable non-oxide
inclusion forming coating is as follows:
6 Element Percentage Nickel Balance Chromium 9.0 Cobalt 10.0 Carbon
0.14 Molybdenum 8.6 Tungsten 12.5 Boron 0.015 Columbium 1.0
[0224] As shown in step six, the coated surface of the substrate
may be smoothed using a grinding or polishing operation. Thus, in
accordance with the present invention, the dimensional
characteristics of the cast article are corrected.
[0225] FIG. 18 is a flow chart showing the steps of the inventive
method of correcting the dimensional characteristics of a cast
article and for providing a protective coating to a metal article.
A defect (which may be a casting defect or other defect such as
wear and tear) is identified (step one). Material may be removed
from the workpiece substrate as necessary (step two). For example,
correcting a defect may require that the inclusion material and
substrate material surrounding the inclusion be removed. A bubble
may leave a semi-spherical pit which can be drilled for easier
filling with the coating material. A coating material is applied at
least to the area of the defect. The coating material is capable of
forming a diffusion boundary between the coating material and the
article. In accordance with this aspect of the invention, the
coating material comprises an alloy with substantially no oxide
forming constituents so as to avoid the formation of oxide
inclusions in the coating material (step three). Applicants have
discovered that the oxides in the coating may form crack initiation
sites, and cracks formed due to the oxides may propagate through
the diffusion boundary and into the article substrate. By limiting
the formation of oxides in the coating, these crack initiation
sites are reduced or eliminated, thereby enabling the coating
material to act as a protective coating. A sintering heat treatment
can be performed before the step of performing the hot isostatic
heat treatment to limit the occurrence bubbles on the surface of
the coating material after an isostatic heat treatment. The
sintering heat treatment may be performed at a temperature
substantially the same as the temperature of the hot isostatic heat
treatment (step four). The hot isostatic heat treatment process is
performed to form the diffusion boundary between the coating
material and the article (step five). The coated surface of the
substrate may be smoothed using a grinding or polishing operation
(step six). Thus, in accordance with the present invention, the
dimensional characteristics of the cast article are corrected, as
necessary, and the substantially oxide free coating and the
diffusion boundary provide a protective coating to protect the
article from damage.
[0226] FIG. 19 schematically illustrates a coated substrate wherein
the coating material is diffusion bonded to the substrate and
includes an oxide inclusion. FIG. 20 schematically illustrates the
coated substrate shown in FIG. 19 wherein a crack is forming at the
site of the oxide inclusion. FIG. 21 schematically illustrates the
coated substrate shown in FIG. 19 wherein the crack formed at the
site of the oxide inclusion propagates through the diffusion
boundary and into the substrate. These figures schematically
illustrate the propagation of a crack caused by an oxide inclusion
in a diffusion coating formed on a substrate. By removing the oxide
forming materials from the coating composition, such cracks are
reduced or eliminated. Further, the oxide-free coating may act as a
prophylactic preventing the formation of some cracks within the
substrate.
[0227] FIG. 22(a) is a schematic perspective view of a cast turbine
engine airfoil part showing a casting defect. FIG. 22(b) is a
schematic perspective view of the cast turbine engine airfoil part
having the area of the casting defect being machined. FIG. 22(c) is
a schematic perspective view of the cast turbine engine airfoil
part after the area of the casting defect has been machined. FIG.
23(d) is a schematic perspective view of the cast turbine engine
airfoil part having the area of the casting defect being filled
with a coating material. FIG. 22(e) is a schematic perspective view
of the coated cast turbine engine airfoil part being subjected to a
hot isostatic pressing treatment. FIG. 22(f) is a schematic
perspective view of the repaired cast turbine engine airfoil
part.
[0228] As shown in FIGS. 23(a) through 23(f), in accordance with
this aspect of the invention, a method is provided for repairing a
turbine engine airfoil part. The dimensional differences are
determined between pre-repair airfoil dimensions of a turbine
engine airfoil part substrate and desired post repair airfoil
dimensions of the turbine engine airfoil part substrate. The
pre-repair airfoil dimensions having different airfoil
characteristics than the post-repair airfoil dimensions. The
turbine engine airfoil part being comprised of a metal alloy. The
engine airfoil part is coated with a coating capable of forming a
diffusion boundary with the turbine engine airfoil part substrate.
If necessary, the engine airfoil part can be masked so that only
the desired area(s) is coated. The coating material comprises an
alloy with substantially no oxide forming constituents so as to
avoid the formation of oxide inclusions in the coating material. In
addition, or alternatively, the method of coating can be chosen so
as to limit or avoid the formation of oxide inclusions. For
example, the coating can be performed with the airfoil part
shrouded with an inert atmosphere, such as argon gas. Or, the
coating can be performed under vacuum. In any case, in accordance
with this aspect of the invention, the coating material applied to
repair the turbine engine airfoil part is substantially free from
oxide inclusions. A hot isostatic heat treatment process is
performed to obtain a post-repair turbine engine airfoil part
having the desired post-repair dimensions and having a
substantially oxide free coating and diffusion bonding between the
coating material and the turbine engine airfoil part substrate. The
substantially oxide-free coating provides a protective coating to
protect the article from damage. A sintering heat treatment can be
performed before the step of performing the hot isostatic heat
treatment to limit the occurrence bubbles on the surface of the
coating material after an isostatic heat treatment. The sintering
heat treatment may be performed at a temperature substantially the
same as the temperature of the hot isostatic heat treatment.
[0229] With respect to the above description, it is realized that
the optimum dimensional relationships for parts of the invention,
including variations in size, materials, shape, form, function, and
manner of operation, assembly and use, are deemed readily apparent
and obvious to one skilled in the art. All equivalent relationships
to those illustrated in the drawings and described in the
specification are intended to be encompassed by the present
invention. Therefore, the foregoing is considered as illustrative
only of the principles of the invention. Further, since numerous
modifications and changes will readily occur to those skilled in
the art, it is not desired to limit the invention to the exact
construction and operation shown and described. Accordingly, all
suitable modifications and equivalents may be resorted to, falling
within the scope of the invention.
* * * * *