U.S. patent application number 10/975272 was filed with the patent office on 2006-04-13 for nitrogen-modified titanium and method of producing same.
This patent application is currently assigned to Lockheed Martin Corporation. Invention is credited to Craig A. Brice.
Application Number | 20060075850 10/975272 |
Document ID | / |
Family ID | 36143945 |
Filed Date | 2006-04-13 |
United States Patent
Application |
20060075850 |
Kind Code |
A1 |
Brice; Craig A. |
April 13, 2006 |
Nitrogen-modified titanium and method of producing same
Abstract
A liquid-state reaction between a titanium molten pool and a
fraction of gaseous nitrogen in an inert atmosphere creates an
alloy with increased strength and hardness. A direct manufacturing
technique involving rapid solidification processing is used rather
than conventional casting techniques that require bulk melting of
solid-state nitrided powder. By utilizing rapid solidification
techniques, solubility levels can be increased resulting in alloys
with unique mechanical and physical properties that are
unattainable through conventional processing methods. Laser-powder
deposition of titanium alloys in atmospheres of varying
argon/nitrogen content produce significant strengthening without
cracking in atmosphere concentrations as high as approximately 10%
nitrogen. Very high hardness values indicate that this material has
valuable applications as a hard face coating on titanium structures
and in functionally graded materials.
Inventors: |
Brice; Craig A.; (Keller,
TX) |
Correspondence
Address: |
BRACEWELL & PATTERSON, L.L.P.
Suite 2900
711 Louisiana Street
Houston
TX
77002-2781
US
|
Assignee: |
Lockheed Martin Corporation
|
Family ID: |
36143945 |
Appl. No.: |
10/975272 |
Filed: |
October 28, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60616664 |
Oct 7, 2004 |
|
|
|
Current U.S.
Class: |
75/336 ;
420/417 |
Current CPC
Class: |
B22F 9/14 20130101; B23K
2103/14 20180801; B23K 35/325 20130101; C22C 14/00 20130101; B23K
26/34 20130101; B23K 26/32 20130101; Y02P 10/25 20151101; B22F
10/10 20210101; B23K 35/0244 20130101; B23K 35/383 20130101; B22F
10/20 20210101 |
Class at
Publication: |
075/336 ;
420/417 |
International
Class: |
B22F 9/14 20060101
B22F009/14 |
Claims
1. A method of forming an alloy, comprising: (a) providing a heat
source and a plurality of nozzles; (b) mounting the heat source and
the nozzles to a movable platform; (c) delivering a metallic powder
through the nozzles by entraining the metallic powder in an
atmosphere comprising an inert gas and an gaseous alloying element
for delivery into and through the nozzles; (d) directing the
metallic powder through the nozzles to a point where streams of the
metallic powder converge with the heat source; (e) melting the
metallic powder with the heat source to form a molten pool on a
substrate such that the metallic powder alloys with the gaseous
alloying element; and (f) moving the platform for the heat source
and the nozzles away from the molten pool, such that the molten
pool rapidly cools and solidifies to form a continuous line of
deposited alloy to form a part.
2. The method of claim 1, wherein step (c) comprises providing the
atmosphere as approximately 90 to 99.9% inert gas, and
approximately 0.1% to 10% gaseous alloying element.
3. The method of claim 1, wherein step (c) comprises providing the
gaseous alloying element as nitrogen.
4. The method of claim 1, wherein step (a) comprises providing the
heat source as a laser that is directed by fiber optics.
5. The method of claim 1, wherein step (a) comprises providing the
heat source as an electron beam.
6. The method of claim 1, wherein step (a) comprises providing the
heat source as an arc.
7. The method of claim 1, further comprising the step of
controlling the heat source with optics, the optics also being
mounted to the movable platform, and wherein the movable platform
is computer-controlled to position the heat source and the nozzles
in a desired location for multiple sections and layers of the part
being formed.
8. The method of claim 1, further comprising the step of orienting
the nozzles at 90.degree. increments relative to each other in an
array having a selected radius from, and being centered on the heat
source.
9. The method of claim 1, wherein step (f) 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.
10. An alloy, comprising: titanium; and nitrogen having a weight
percentage of approximately 0.05% to 3.0%.
11. The alloy of claim 10, further comprising a hardness up to 55
HRC, and an ultimate tensile strength as high as 140 ksi.
Description
[0001] This application is based on U.S. Provisional Patent
Application No. 60/616,664, filed Oct. 7, 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 a titanium alloy
and, in particular, to a nitrogen-modified titanium alloy and
method of producing it.
[0004] 2. Description of the Related Art
[0005] Metal objects are currently produced by thermomechanical
processes, which include 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 and molds,
tools, and dies. 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 or extrusion or both, then machining a hollow cylinder and,
separately, 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 cut away 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 cuttings. These methods produce
waste materials, such as metal cuttings, oils, and solvents, which
must be further processed for purposes of reuse or disposal. The
articles produced are contaminated with cutting fluids and metal
chips. They require cutting tools, which wear and must be
periodically reconditioned and ultimately replaced. Fixtures for
use in manufacturing must be designed, fabricated, and manipulated
during production.
[0007] When a part is unusual in shape or has internal features,
machining is more difficult. Choosing the machining operations to
be used and the sequence of 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. Sophisticated machine tools
require a significant capital investment and occupy a good deal of
space. Use of the invention in place of subtractive machining
provides solutions to these problems and 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. Producing parts separately and
joining them requires close tolerance machining of matching parts,
provision of fastening means, such as threaded connections, and
welding together of components. 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 wt. %) 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 are
surface modification techniques only. A hard face coating is a
discrete surface layer 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] To increase the amount of solute levels in the alloys, rapid
solidification processes (RSP) can be used. In these processes, a
rapid quenching is used in freezing the alloy from a molten state,
the solutes remaining in desired phases. After quenching, diffusion
may allow for dispersion throughout the material and agglomeration
at nucleation sites, further improving the 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 titanium 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, method, and
apparatus that uses a liquid-state reaction between a titanium
molten pool and a fraction of gaseous nitrogen in the atmosphere.
This design increases mechanical strength and hardness through
nitrogen alloying. In addition, a direct manufacturing technique
involving rapid solidification processing is used rather than
conventional casting techniques that require bulk melting of
solid-state nitrided powder.
[0014] By utilizing rapid solidification techniques, solubility
levels can be increased resulting in alloys with unique mechanical
and physical properties that are unattainable through conventional
processing methods. The present application documents trials
performed using laser-powder deposition techniques on commercially
pure titanium (Cp-Ti) in atmospheres of varying argon/nitrogen
content. The results show that significant strengthening is
achieved without cracking in atmosphere concentrations as high as
approximately 10% nitrogen. Very high hardness values indicate that
this material has valuable applications as a hard face coating on
titanium structures 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 a portion of a
solid freeform fabrication device.
[0018] FIG. 2 is a schematic front view of the device of FIG. 1
during fabrication of a part.
[0019] FIG. 3 is a flowchart of one embodiment of a method of the
present invention.
[0020] FIG. 4 is a series of optical and electron micrographs
depicting various structures 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 the composition of matter constructed in
accordance with the present invention;
[0022] FIG. 6 is a series of optical micrographs depicting various
structures of a composition of matter constructed in accordance
with the present invention; and
[0023] FIG. 7 is a series of optical micrographs depicting various
structures of a composition of matter constructed in accordance
with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0024] The present invention is directed to a novel composition of
matter comprising a titanium alloy and a method for producing the
alloy. This new alloy is ideally suited for use in applications,
such as aerospace applications, that require a combination of high
strength and low density. To enable formation of this new
composition of matter, one method of producing the alloy utilizes a
solid freeform fabrication (SFF), or direct deposition, device to
achieve rapid cooling and solidification while forming a bulk
part.
[0025] The alloy of the present invention requires 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).
[0026] 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. Other types of feedstock may include wire, for example. The
feedstock is delivered through one or more guide nozzle(s) 15 (four
shown), as depicted at step 305 in FIG. 3. The feedstock or powder
exits nozzles 15 through an outlet 17 at the lower end of each
nozzle 15.
[0027] In one embodiment, the controls for beam 13 or heat source
and nozzles 15 are mounted to a movable platform, as depicted in
step 303 in FIG. 3. In the laser embodiment, the controls utilize
optics to direct the laser beam 13. The platform 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. 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.
[0028] To form a part using the device 11, the feedstock metal is
delivered into and through the nozzles 15. As shown in FIG. 2, when
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 metal 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 or beam 13.
The laser beam 13 melts the metal 19 (step 309, FIG. 3), forming a
molten pool on substrate 21. The metal 19 is simultaneously exposed
to a gaseous alloying element (such as nitrogen). As the platform
for the beam 13 and the nozzles 15 is 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.
[0029] 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.
[0030] 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
[0031] 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
[0032] This solidification behavior is likely valid under
equilibrium conditions and therefore not necessarily valid for
laser deposited structures (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%.
[0033] FIG. 4 shows a micrograph series for the 90/10 and 70/30
mixtures of Ar/N.sub.2. 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 a
grain boundaries in a recrystallized microstructure. The particle
composition was verified using energy dispersive spectroscopy (EDS)
to be Ti.sub.2N.
[0034] 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 a 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).
[0035] 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%
[0036] 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)
[0037] 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.
[0038] 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.
* * * * *