U.S. patent application number 11/523333 was filed with the patent office on 2007-01-18 for gas-phase alloying of metallic materials.
This patent application is currently assigned to Lockheed Martin Corporation. Invention is credited to Craig A. Brice.
Application Number | 20070012138 11/523333 |
Document ID | / |
Family ID | 37660458 |
Filed Date | 2007-01-18 |
United States Patent
Application |
20070012138 |
Kind Code |
A1 |
Brice; Craig A. |
January 18, 2007 |
Gas-phase alloying of metallic materials
Abstract
A direct manufacturing technique involving rapid solidification
processing uses a reaction between a metallic molten pool and a
reactant gas in an inert atmosphere to form alloys with improved
desired properties. By utilizing rapid solidification techniques,
solubility levels can be increased resulting in alloys with unique
mechanical and physical properties. Laser deposition of alloys in
atmospheres of varying reactant content produce significant
strengthening without cracking. In addition, these materials have
very high hardness values for hard face coating and functionally
graded materials applications.
Inventors: |
Brice; Craig A.; (Keller,
TX) |
Correspondence
Address: |
BRACEWELL & GIULIANI LLP
P.O. BOX 61389
HOUSTON
TX
77208-1389
US
|
Assignee: |
Lockheed Martin Corporation
|
Family ID: |
37660458 |
Appl. No.: |
11/523333 |
Filed: |
September 19, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10975272 |
Oct 28, 2004 |
|
|
|
11523333 |
Sep 19, 2006 |
|
|
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Current U.S.
Class: |
75/10.13 |
Current CPC
Class: |
C22B 9/223 20130101;
C23C 8/24 20130101; C23C 8/10 20130101; C23C 8/28 20130101; C22B
9/22 20130101 |
Class at
Publication: |
075/010.13 |
International
Class: |
C22C 1/02 20070101
C22C001/02; C22B 9/22 20060101 C22B009/22 |
Claims
1. A method of forming an alloy, comprising: (a) providing a heat
source and a metallic feedstock in a gaseous atmosphere; (b)
delivering a gaseous alloying element proximate to the metallic
feedstock; (c) converging the heat source on the metallic feedstock
and the gaseous alloying element; (d) melting the metallic
feedstock with the heat source to form a molten pool such that the
metallic feedstock alloys with the gaseous alloying element to form
a composition; and (e) cooling and solidifying the composition.
2. The method of claim 1, wherein the gaseous atmosphere is
approximately 70% to 99.9% inert gas, and approximately 0.1% to 30%
gaseous alloying element.
3. The method of claim 1, wherein the gaseous alloying element is
selected from the group consisting of nitrogen and oxygen.
4. The method of claim 1, wherein the heat source is a laser that
is directed by fiber optics.
5. The method of claim 1, wherein the heat source is selected from
the group consisting of an electron beam and an electron arc.
6. The method of claim 1, further comprising the step of
controlling the heat source with optics, the optics also being
mounted to a movable platform, and wherein the movable platform is
computer-controlled to position the heat source and the metallic
feedstock in a desired location for multiple sections and layers of
a part being formed.
7. The method of claim 1, wherein step (e) comprises forming a part
with adjacent, side-by-side layers to form a width of the part, and
adjacent, stacked layers to form a height of the part.
8. A method of forming an alloy, comprising: (a) providing a laser
heat source, a movable platform, and a metallic feedstock in a
gaseous atmosphere; (b) delivering a gaseous alloying element
proximate to the metallic feedstock on the movable platform; (c)
converging the laser heat source on the metallic feedstock and the
gaseous alloying element; (d) melting the metallic feedstock with
the laser heat source to form a molten pool on the movable
platform, such that the metallic feedstock alloys with the gaseous
alloying element to form a composition; and (e) moving the
composition via the movable platform and the heat source relative
to each other, such that the molten pool rapidly cools and
solidifies to form a continuous line of deposited alloy to form a
part.
9. A method according to claim 8, wherein the gaseous atmosphere is
approximately 70% to 99.9% inert gas, and approximately 0.1% to 30%
gaseous alloying element.
10. A method according to claim 8, wherein the gaseous alloying
element is selected from the group consisting of nitrogen and
oxygen.
11. A method according to claim 8, further comprising the step of
controlling the laser heat source with optics, the optics also
being mounted to the movable platform, and wherein the movable
platform is computer-controlled to position the laser heat source
and the metallic feedstock in a desired location for multiple
sections and layers of the part being formed.
12. A method according to claim 8, wherein step (e) comprises
forming the part with adjacent, side-by-side layers to form a width
of the part, and adjacent, stacked layers to form a height of the
part.
Description
[0001] This continuation-in-part patent application is based on and
claims priority to U.S. patent application Ser. No. 10/975,272,
filed Oct. 28, 2004, entitled, Nitrogen-Modified Titanium and
Method of Producing Same, and is incorporated herein by
reference.
BACKGROUND OF THE INVENTION
[0002] 1. Technical Field
[0003] The present invention relates in general to forming metal
alloys and, in particular, to a method for gas-phase alloying of
metallic materials.
[0004] 2. Description of the Related Art
[0005] Many metal objects are produced by thermomechanical
processes including casting, rolling, stamping, forging, extrusion,
machining, and joining operations. Multiple steps are required to
produce a finished article. These conventional operations often
require the use of heavy equipment, molds, tools, dies, etc. For
example, a typical process sequence required to form a small
cylindrical pressure vessel might include casting an ingot, heat
treating and working the casting to homogenize it by forging,
extrusion, or both, machining a hollow cylinder and separate end
caps from the worked ingot and, finally, welding the end caps to
the cylinder.
[0006] Conventional production methods are subtractive in nature in
that material is removed from a starting block of material to
produce a more complex shape. Subtractive machining methods are
deficient in many respects. Large portions of the starting material
are reduced to waste in the form of metal cuttings and the like.
These methods also produce waste materials such as oils and
solvents that must be further processed for purposes of reuse or
disposal. Even the articles produced are contaminated with cutting
fluids and metal chips. The production of such articles also
requires cutting tools, which wear and must be periodically
reconditioned and ultimately replaced. Moreover, fixtures for use
in manufacturing must be designed, fabricated, and manipulated
during production.
[0007] Machining is even more difficult when a part has an unusual
shape or has internal features. Choosing the most appropriate
machining operations and the sequence of such operations requires a
high degree of experience. A number of different machines are
needed to provide capability to perform the variety of operations,
which are often required to produce a single article. In addition,
sophisticated machine tools require a significant capital
investment and occupy a large amount of space. In contrast, using
the present invention instead of subtractive machining provides
improved solutions to these issues and overcomes many
disadvantages.
[0008] Another difficulty with conventional machining techniques is
that many objects must be produced by machining a number of parts
and then joining them together. Separately producing parts and then
joining them requires close-tolerance machining of the
complementary parts, provision of fastening means (e.g., threaded
connections) and welding components together. These operations
involve a significant portion of the cost of producing an article
as they require time for design and production as well as apparatus
for performing them.
[0009] Titanium has been used extensively in aerospace and other
manufacturing applications due to its high strength-to-weight
ratio. To increase the usefulness of titanium, various titanium
alloys have been produced, many being tailored to provide desired
characteristics. However, the equilibrium solute levels (as
measured in weight-percent) in conventionally processed titanium
alloys are below that which maximizes the beneficial effect of the
solute.
[0010] For example, in concentrations over 500 ppm, nitrogen is
typically considered a contaminant in titanium alloys. At levels
higher than 500 ppm, the tensile strength increases greatly with a
corresponding drop in tensile ductility. Additionally,
solidification cracking can be a serious problem at high nitrogen
levels. It is this embrittling effect that prohibits the use of
nitrogen as a significant alloying agent.
[0011] Titanium alloys typically exhibit low wear resistance due to
their low hardness. Under certain circumstances, titanium also can
be subject to chemical corrosion and/or thermal oxidation. Prior
art methods for increasing the hardness of titanium alloys have
been limited to surface modification techniques. For example, a
hard face coating is a discrete surface layer applied to a
substrate and is subject to delamination. Current methods are also
subject to macro and micro cracking of the surface-hardened layer.
For example, U.S. Pat. Nos. 5,252,150 and 5,152,960 disclose
titanium-aluminum-nitrogen alloys. These patents disclose an alloy
that is formed through a solid-state reaction of titanium in a
heated nitrogen atmosphere. The alloy is formed in a melt with
aluminum to create the final alloy product.
[0012] Rapid solidification processes (RSP) also can be used to
increase the amount of solute levels in alloys. In these processes,
a rapid quenching is used in freezing the alloy from a molten state
so that the solutes remain in desired phases. After quenching,
diffusion may allow for dispersion throughout the material and
agglomeration at nucleation sites, which further improves the
desired characteristics of the alloy. While this type of process is
widely used, the resulting product is typically in powder, flake,
or ribbon forms, which are unsuitable for manufacturing
applications requiring material in bulk form. Thus, an improved
metal alloy and process for producing the same would be desirable
for many practical applications.
SUMMARY OF THE INVENTION
[0013] The present invention comprises a system and method that
uses a liquid-state reaction between a metallic molten pool and an
atmosphere having a small fraction of reactive gas. For example,
the invention can increase the mechanical strength and hardness of
a metallic material through gaseous alloying. A direct
manufacturing technique involving rapid solidification processing
is used rather than conventional casting techniques that require
bulk melting of solid-state materials.
[0014] By utilizing rapid solidification techniques, solubility
levels of metallic materials can be increased resulting in alloys
with unique mechanical and physical properties that are
unattainable through conventional processing methods. For example,
laser deposition techniques may be used on commercially pure metals
in atmospheres having various amounts of inert and reactive gases.
In one embodiment, the resultant alloys are significantly
strengthened without cracking in atmospheres having concentrations
of reactive gases of approximately 10%. Very high hardness values
indicate that these types of materials have valuable applications
as hard face coatings and in functionally graded materials.
[0015] The foregoing and other objects and advantages of the
present invention will be apparent to those skilled in the art, in
view of the following detailed description of the present
invention, taken in conjunction with the appended claims and the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] So that the manner in which the features and advantages of
the invention, as well as others which will become apparent, are
attained and can be understood in more detail, more particular
description of the invention briefly summarized above may be had by
reference to the embodiment thereof which is illustrated in the
appended drawings, which drawings form a part of this
specification. It is to be noted, however, that the drawings
illustrate only an embodiment of the invention and therefore are
not to be considered limiting of its scope as the invention may
admit to other equally effective embodiments.
[0017] FIG. 1 is a schematic perspective view of one embodiment of
a portion of a solid freeform fabrication device constructed in
accordance with the present invention;
[0018] FIG. 2 is a schematic front view of the device of FIG. 1
during fabrication of a part, and is constructed in accordance with
the present invention;
[0019] FIG. 3 is a high level flow diagram of one embodiment of a
method constructed in accordance with the present invention;
[0020] FIG. 4 is a series of optical and electron micrographs
depicting various structures of one embodiment of a composition of
matter constructed in accordance with the present invention;
[0021] FIG. 5 is a plot of atmospheric nitrogen versus nitrogen
absorbed and hardness in one embodiment of a composition of matter
constructed in accordance with the present invention;
[0022] FIG. 6 is a series of optical micrographs depicting various
structures of one embodiment of a composition of matter constructed
in accordance with the present invention;
[0023] FIG. 7 is a series of optical micrographs depicting various
structures of one embodiment of a composition of matter constructed
in accordance with the present invention; and
[0024] FIG. 8 is a high level flow diagram of another embodiment of
a method constructed in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0025] The present invention is directed to a method for producing
the novel compositions of matter comprising metal alloys. In one
embodiment, the new alloys are well suited for use in aerospace
applications that require a combination of high strength and low
density. To enable formation of these new compositions of matter,
one method of producing the alloys utilizes a solid freeform
fabrication (SFF), or direct deposition, device to achieve rapid
cooling and solidification while forming a bulk part.
[0026] The alloys of the present invention utilize a rapid
solidification process (RSP) to retain the desired metastable
phases, and a method of direct manufacturing that results in rapid
solidification is shown in the figures. FIG. 1 is a schematic,
perspective view of a portion of a SFF device 11, such as is
available from Optomec Design Company, Albuquerque, N. Mex., and
sold under the trademark LENS.TM. (Laser Engineered Net
Shaping).
[0027] Device 11 comprises a high energy density heat source, such
as a laser beam 13. Other forms of heat sources may include, for
example, electron beams and arcs, as illustrated at step 301 in
FIG. 3. The laser beam 13 may be formed by various laser types and
delivered to the desired location by fixed or fiber optics. Beam 13
acts as the heat source for melting a feedstock, such as a metallic
powder or wire, for example. The feedstock may be simply positioned
for alloying (e.g., on a platform), or delivered through one or
more guide nozzle(s) 15 (four shown), as depicted at step 305 in
FIG. 3. If nozzles are used, the feedstock exits the nozzles
through an outlet 17 at the lower end of each nozzle.
[0028] In one embodiment, the controls for the heat source and
nozzles are mounted to a movable platform, as depicted in step 303
in FIG. 3. In the laser embodiment, the controls may utilize optics
to direct the laser beam 13. The platform also is
computer-controlled to position the beam 13 and nozzles 15 in a
desired location for each section or layer of the part being
formed. These portions of the method are illustrated at step 307 in
FIG. 3. In the illustrated embodiment, device 11 is shown as having
four nozzles 15 located at 90.degree. increments in an array having
a selected radius from, and being centered on, beam 13. Though
shown with four nozzles 15, device 11 may have more or fewer
nozzles 15, and the nozzles 15 may be arranged in various
orientations.
[0029] To form a part using the device 11, the metal or metallic
alloy feedstock is presented, such as by delivery into and through
the nozzles 15. As shown in FIG. 2, when e.g., the powdered metal
19 is used as the feedstock, the metallic powder is entrained in an
inert gas, typically argon, for delivery via the nozzles (step 305,
FIG. 3). The feedstock is carried out of the exit 17 of each nozzle
15 and directed at a point where the stream(s) of the metal 19
converge with the heat source. In one embodiment, the laser beam 13
melts the metal 19 (step 309, FIG. 3), forming a molten pool on the
platform or substrate 21. The metal 19 is simultaneously exposed to
a gaseous alloying element (e.g., nitrogen, oxygen, carbon dioxide,
etc.). As one of or both the platform for the beam 13 and the
nozzles 15 is/are moved (step 311, FIG. 3), the pool rapidly cools
and solidifies as an alloy. When the heat source or beam 13 is
moved away, a continuous line of deposited alloy 19 forms a portion
of part 23. Device 11 is used to form adjacent, side-by-side layers
to form the width of the part, and is used to form adjacent,
stacked layers to create the height of part 23.
[0030] In another embodiment (FIG. 8), one embodiment of the method
starts as indicated at step 801, and comprises providing a heat
source and a metallic feedstock in a gaseous atmosphere (step 803);
delivering a gaseous alloying element proximate to the metallic
feedstock (step 805); converging the heat source on the metallic
feedstock and the gaseous alloying element (step 807); melting the
metallic feedstock with the heat source to form a molten pool such
that the metallic feedstock alloys with the gaseous alloying
element to form a composition (step 809); cooling and solidifying
the composition (step 811); before ending as indicated at step
813.
[0031] In one experiment, five different argon/nitrogen atmospheric
combinations were evaluated in addition to a baseline 100% Ar
CP--Ti. Custom mixed bottles of argon and nitrogen were mixed with
the following ratios (Ar/N.sub.2): 96/4, 93/7, 90/10, 85/15, and
70/30. Cp-Ti specimens were then laser deposited in each gas
composition. Prior to deposition, an amount of the desired
composition was purged through the system to ensure a homogeneous
mixture at the target concentration. Another amount of the desired
composition was used to keep the chamber at operating pressure and
as a carrier gas for the powder delivery system.
[0032] In this embodiment, heat treatments were performed on some
test samples in order to examine microstructural stability and
thermal effects. Microstructural characterization was carried out
using optical and scanning electron microscopy. Under equilibrium
conditions, the solidification sequence for compositions under 1.2%
N, which corresponds to about 7% atmospheric nitrogen, is:
L.fwdarw.L+.beta..fwdarw..beta..fwdarw..beta.+.alpha..fwdarw..alpha.+Ti.s-
ub.2N
[0033] And for equilibrium solidification at compositions greater
than 1.9% N:
L.fwdarw.L+.alpha..fwdarw..alpha.+.beta..fwdarw..alpha..fwdarw..-
alpha.+Ti.sub.2N
[0034] This solidification behavior is likely valid under
equilibrium conditions and therefore not necessarily valid for
laser deposited structures (i.e., due to rapid solidification
characteristics). Rapid solidification tends to increase solid
solubilities, which effectively shifts the phase diagram towards
the solute end, thus favoring metastable phase formation. However,
microstructural analysis is consistent with the above
solidification sequences, though the composition limits may be
uncertain. In one embodiment, the Ti alloy contains a weight
percentage of N of approximately 0.05% to 3.0%.
[0035] FIG. 4 shows a micrograph series for the 90/10 and 70/30
mixtures of Ar/N.sub.2 for one embodiment. For the 90/10 mixture
(FIGS. 4A, 4B, 4C), the macrostructure (FIG. 4A) is typical of what
is seen in conventional Ti alloys (i.e., large prior .beta. grain
boundaries with a martensitic .alpha.' lath basket weave
structure). FIG. 4B shows a backscattered electron SEM image (BSEM)
that reveals compositional contrast and indicates that
Ti.sub.xN.sub.y compounds might exist in the interlath regions.
FIG. 4C shows the 90/10 composition after heat treatment for 1-hour
at 1000.degree. C. Here, the Ti.sub.xN.sub.y particles are clearly
seen pinning .alpha. grain boundaries in a recrystallized
microstructure. The particle composition was verified using energy
dispersive spectroscopy (EDS) to be Ti.sub.2N.
[0036] The 70/30 mixture (FIGS. 4D, 4E, 4F) has a macrostructure
that is quite different from the 90/10 composition. FIG. 4D shows
an optical micrograph of the as-deposited structure clearly showing
dendritic formation of primary .alpha.. Closer look via BSEM (FIG.
4E) shows that the interdendritic region likely contains the
Ti.sub.2N compound. FIG. 4F shows the 70/30 mixture after 1-hour
heat treatment at 1150.degree. C. Here again, the Ti.sub.2N
particles are clearly seen pinning the .alpha. grain boundaries
though the size of the particles is much larger when compared to
those seen in the 90/10 sample (note the micron bars).
[0037] The chemistry results are shown in Table 1. Of interest here
is the nearly linear relationship between atmospheric nitrogen and
dissolved nitrogen in the as-deposited samples. This relationship
is more clearly seen in FIG. 5, as are the plotted superficial
hardness values. Here the relationship seems to follow a power-law
relationship indicating that significant hardening benefits can be
obtained at low concentrations while the effect diminishes at
higher concentrations. TABLE-US-00001 TABLE 1 Element CP-Ti 4% N 7%
N 10% N 15% N 30% N Nominal ASTM B348 C 0.0880% 0.0980% 0.0670%
0.0870% 0.0640% 0.0910% 0.0910% 0.0800% H 0.0050% 0.0018% 0.0020%
0.0012% 0.0037% 0.0038% -- 0.0150% N 0.0200% 0.6700% 1.7300%
1.3300% 1.9400% 3.4500% 0.0080% 0.0300% O 0.1700% 0.1500% 0.1440%
0.1400% 0.1470% 0.1400% 0.1250% 0.1800%
[0038] Table 2 shows results from mechanical testing of the control
CP--Ti specimens and the 96/4 and 90/10 compositions. The samples
above 10% suffered cracking that prevented them from being tested.
A small amount of nitrogen (as little as 0.1%) may result in gains
in ultimate tensile strength on the order of 60% (i.e., as high as
140 ksi), and gains in hardness on the order of 100% (up to 55
HRC). Essentially no ductility was found in any of the
nitrogen-modified samples. TABLE-US-00002 TABLE 2 Comp. ID Test Log
Temp. UTS 0.2% YS % E % RA Mod. Hard. 10% N4 -- -- -- -- -- -- --
55 10% N5 980791 RT 33.3 -- -- -- 18.6 55 10% N6 980792 RT 28.4 --
-- -- 18.5 55 AVG 30.9 18.6 55.0 4% N26 980796 RT 137.9 -- -- --
17.2 46 4% N27 980797 RT 155.5 -- -- -- 17.3 48 4% N28 980789 RT
139 -- -- -- 17 47 AVG 144.1 17.2 47.0 CP N21 980793 RT 88 76.7 6.5
9.5 16.7 100 (23) CP N23 980794 RT 88 74.6 23 31 16.7 97 (18) CP
N24 980795 RT 81 73.2 5.5 13 16.7 98 (19) AVG 85.7 74.8 11.7 17.8
16.7 98.3 (20.0)
[0039] FIGS. 6 and 7 show the effect of heat treatment on the 90/10
composition. FIGS. 6A-6D show a series of optical micrographs of
the sample in the as-deposited condition. Here the layered
deposition structure is clearly seen. This structure is likely due
to local thermal variation resulting in small differences in the
scale of the microstructural features. This inhomogeneity is
detrimental to mechanical properties as it provides a path of least
resistance for defects to propagate. The series of optical
micrographs in FIGS. 7A-7D show the same sample after a .beta.
anneal heat treatment at 1000.degree. C. Here the microstructure
has recrystallized and eliminated the layered structure seen in the
non-heat treated condition. This microstructure might lead to
mechanical property improvement, namely ductility.
[0040] While the invention has been shown or described in only some
of its forms, it should be apparent to those skilled in the art
that it is not so limited, but is susceptible to various changes
without departing from the scope of the invention. For example,
other compositions of materials (e.g., aluminum-oxygen, carbon
dioxide, etc.) may be utilized. Moreover, other alloys having a
mixture range of 0.1 to 30% may be more suitable for other
combinations of materials.
* * * * *