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.

Description

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates in general to forming metal alloys and, in particular, to a method for gas-phase alloying of metallic materials.

2. Description of the Related Art

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.

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.

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.

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.

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.

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.

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.

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

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.

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.

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

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.

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;

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;

FIG. 3 is a high level flow diagram of one embodiment of a method constructed in accordance with the present invention;

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;

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;

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;

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

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

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.

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™ (Laser Engineered Net Shaping).

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.

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° 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.

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.

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.

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/N₂): 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.

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→L+β→β→β+α→α+Ti₂N

And for equilibrium solidification at compositions greater than 1.9% N: L→L+α→α+β→α→α+Ti₂N

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%.

FIG. 4 shows a micrograph series for the 90/10 and 70/30 mixtures of Ar/N₂ 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 β grain boundaries with a martensitic α′ lath basket weave structure). FIG. 4B shows a backscattered electron SEM image (BSEM) that reveals compositional contrast and indicates that Ti_xN_y compounds might exist in the interlath regions. FIG. 4C shows the 90/10 composition after heat treatment for 1-hour at 1000° C. Here, the Ti_xN_y particles are clearly seen pinning α grain boundaries in a recrystallized microstructure. The particle composition was verified using energy dispersive spectroscopy (EDS) to be Ti₂N.

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 α. Closer look via BSEM (FIG. 4E) shows that the interdendritic region likely contains the Ti₂N compound. FIG. 4F shows the 70/30 mixture after 1-hour heat treatment at 1150° C. Here again, the Ti₂N particles are clearly seen pinning the α 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).

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 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%

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 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)

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 β anneal heat treatment at 1000° 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.

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.

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.