Patent Application
20190161865
The present disclosure relates generally to dynamic cold gas spray processes and, more particularly, to non-equilibrium alloy feedstock powders, cold spray-based processes for additively manufacturing articles with such feedstock powders, and articles produced utilizing such processes.
Abbreviations appearing less frequently in this document are defined upon initial usage, while abbreviations appearing with greater frequency are defined below.
Al—Aluminum;
AM—Additive Manufacturing:
DED—Direct Energy Deposition;
DMLS—Direct Metal Laser Sintering;
GTE—Gas Turbine Engine;
HIP—Host Isostatic Pressing;
NEA—Non-Equilibrium Alloy;
SEM—Scanning Electronic Microscope;
Wt %—Weight percent;
° C.—degrees Celsius; and
° C./s—degrees Celsius per second.
Aerospace components, such as GTE components and airborne valves, are commonly fabricated from alloy powders utilizing powder metallurgy processes. HIP processes, in particular, are often utilized to transform powder bodies into near-net shapes or articles, which are then subject to further processing to complete the desired GTE component. Depending upon the initially-selected alloy powder, it is not uncommon for HIP-based process flows to require a relatively lengthy series of steps including casting, cold compaction, can welding, the HIP process step itself, de-canning, extruding, upset forging, final forging, and machining. Each process steps adds duration, complexity, and tooling cost to the manufacturing process; and, in many instances, incrementally degrades the overall strength, ductility, and other desired properties of the alloy material. For at least these reasons, the aerospace industry has increasingly turned toward DMLS and other DED powder fusion processes for the additive manufacture of many GTE components.
During a generalized DMLS process, a laser or electron beam is scanned across selected regions of a metallic powder bed to fuse layers of metal powder and gradually build-up or compile a component on a layer-by-layer basis. After selected regions of a given metallic layer are fused in this manner, a new powder layer is then applied over the recently-fused layer utilizing a roller-based recoater system. This process of dispensing a metallic powder layer, fusing selected regions of the powder layer, and then applying a fresh metallic powder layer are repeated until completion of the component. As compared to more conventional powder metallurgy processes, such as HIP-based fabrication processes of the type previously described, DMLS fabrication processes generally require fewer processing steps and tooling requirements and, thus, can be carried-out with greater efficiency and at lower costs.
The foregoing benefits notwithstanding, DMLS and other DED powder fusion processes remain limited in multiple respects. For example, while more efficient than other legacy powder metallurgy fabrication techniques, DMLS processes can still be undesirably time consuming; current DMLS processes often require several hours to complete components of relatively modest volumes and complexity. Further, the components fabricated utilizing DMLS processes may have undesirably high porosities. DMLS processes also tend to modify powder microstructures due to the localized fusion temperatures involved. It can consequently be difficult, if not impractical to fabricate components from certain materials utilizing DMLS processes. In particular, DMLS processes may be poorly suited for producing components from certain non-equilibrium alloy powders (described below), while reliably maintaining or creating a desired microstructure throughout the component body; e.g., a microstructure that is substantially free of deleterious phases or participate growth, which can detract from the strength, ductility, and other desired properties of the completed component. As a still further drawback, DMLS and other DED processes often impose undesired limitations on maximum permissible size of the articles produced utilizing such processes.
There thus exists an ongoing demand for methods for fabricating engine components and other metallic articles, which overcome the limitations associated with conventional powder metallurgy manufacturing processes. Ideally, such methods would enable the fabrication of articles with a reduced number of process steps, with reduced tooling requirements, with minimal scrap loss, with relatively low porosities, with reduced constraints on article size, and at reduced manufacturing costs and manufacturing schedules. Further, it would be desirable for such methods to enable fabrication of articles from non-equilibrium alloy powders, while substantially maintaining the desired material properties and microstructure throughout the fabrication process. Embodiments of such methods are set-forth herein, as are other related methods and articles produced in accordance with such methods. Other desirable features and characteristics of embodiments of the present invention will become apparent from the subsequent Detailed Description and the appended Claims, taken in conjunction with the accompanying drawings and the foregoing Background.
Methods for producing NEA feedstock powders utilized in AM cold spray processes are disclosed, as are methods for fabricating articles from such NEA feedstock powders utilizing computer-controlled, AM cold spray processes. In various embodiments, the method includes the step or process of purchasing, producing, or otherwise obtaining an NEA feedstock powder, which is composed of an alloy matrix throughout which a first minority constituent is dispersed. Due to the metastable or non-equilibrium state of the feedstock powder, the first minority constituent precipitates from the alloy matrix when the NEA feedstock powder is exposed to temperatures exceeding a critical temperature threshold (TCRITICAL) for a predetermined time period. An AM cold spray process is carried-out to fabricate a near-net article from the NEA feedstock powder, which is exposed to a maximum temperature (TSPRAY_MAX) during the cold spray process. The near-net article is then further processed to yield a finished article. To preserve the metastable or non-equilibrium state of the feedstock powder, TSPRAY_MAX is maintained below TCRITICAL through the cold spray process.
In other embodiments, the method includes the steps or processes of forming a molten alloy into a solid NEA body, such as an NEA ribbon or other bulk shape, utilizing a casting process having a cooling rate equal to or greater than approximately 1×106° C./s. After casting, the solid NEA body is mechanically converted into a NEA feedstock powder. An annealing process is then performed during which the feedstock powder is exposed to a maximum anneal temperature (TANNEAL_MAX) for a predetermined time period. The NEA feedstock powder is composed of an alloy matrix having a melt point (TALLOY_MP) and throughout which a first minority constituent is dispersed or distributed. The first minority constituent precipitates from the alloy matrix when the NEA feedstock powder is exposed to temperatures exceeding a critical temperature threshold TCRITICAL for a predetermined time period. The NEA body is formulated such that TCRITICAL is less than TALLOY_MP and greater than TANNEAL_MAX.
In yet further embodiments, the method includes the step or process of utilizing an AM cold spray process to fabricate a near-net article from a NEA feedstock powder having a melt point of TALLOY_MP. The NEA feedstock powder contains an aluminum alloy matrix and a non-trace amount of silicon contained in the aluminum alloy matrix and precipitating therefrom when the NEA feedstock powder is exposed to temperatures exceeding a critical temperature threshold (TCRITICAL) for a predetermined time period. The near-net article is further annealed at a maximum annealing temperature (TANNEAL). The method is carried-out such that TANNEAL<TCRITICAL<TALLOY_MP; and, perhaps, such that TANNEAL_MAX is greater than TCRITICAL minus 150° C. and less than TCRITICAL minus 25° C. In certain embodiments, the method further comprises selecting the NEA feedstock powder to further contain a non-trace amount of iron, which is also present in the aluminum alloy matrix and which precipitates therefrom when the NEA feedstock powder is exposed to temperatures exceeding TCRITICAL for the predetermined time period.
The foregoing summaries are provided by way of non-limiting example only. Various additional examples, aspects, and other features of embodiments of the present disclosure are described in more detail below.
At least one example of the present invention will hereinafter be described in conjunction with the following figures, wherein like numerals denote like elements, and:
For simplicity and clarity of illustration, descriptions and details of well-known features and techniques may be omitted to avoid unnecessarily obscuring the exemplary and non-limiting embodiments of the invention described in the subsequent Detailed Description. It should further be understood that features or elements appearing in the accompanying figures are not necessarily drawn to scale unless otherwise stated. For example, the dimensions of certain elements or regions in the figures may be exaggerated relative to other elements or regions to improve understanding of embodiments of the invention.
The following Detailed Description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding Background or the following Detailed Description.
As appearing herein, the term “alloy” refers to a material composed predominately or exclusively of metallic constituents by weight percent (wt %). As further appearing herein, a “non-equilibrium alloy” or “NEA” is defined as a material containing an alloy matrix throughout which one or more minority constituents (metallic or non-metallic constituents present in quantities less than that of the alloy matrix, by wt %) are distributed. Further, the minority constitution(s) exhibit a tendency or propensity, as predicted by established thermodynamic principles, to precipitate from the matrix and agglomerate if the NEA material is heated above a critical temperature threshold (herein, “TCRITICAL”) for an extended period of time on the order of, for example, one minute. Thus, in essence, NEA materials exist in desired metastable states; and, if heated to undesirably high temperatures, the NEA materials will experience undesired precipitate growth detracting from the overall strength, ductility, and, perhaps, other desirable properties of the material. The value of TCRITICAL will vary amongst embodiments depending upon alloy composition, but will generally be less than the melt point of the NEA material and, more specifically, the alloy matrix (herein, “TALLOY_MP”). In certain embodiments, TCRITICAL will range from about 350° C. to about 500° C. or, perhaps, from about 400° C. to about 450° C. In other embodiments, TCRITICAL may be greater than or less than the aforementioned range.
Overview
Methods for producing NEA feedstock powders for usage in cold spray processes are disclosed, as are methods for fabricating articles from NEA feedstock powders utilizing computer-controlled, AM cold spray processes. By virtue of the below-described rapid cool casting processes, the NEA feedstock powders are produced to natively contain desired microstructures, which are metastable in nature and desirably preserved throughout the remainder of the article fabrication process, to the extent practical. Therefore, in an effort to maintain the desired metastable microstructures of the NEA materials throughout the fabrication process, thermal loading of the NEA material is carefully regulated across all pertinent process stages and through the usage of AM cold spray processes in compiling or printing the desired component on a layer-by-layer basis. Accordingly, during a given processing stage, the maximum or peak processing temperatures to which the NEA materials are exposed can be limited to below TCRITICAL, as may be the case during the AM cold spray process. In this manner, the desired metastable microstructures of the NEA material can be substantially preserved to enhance the strength, ductility, and other desired properties of the finished article.
By way of non-limiting example, embodiments of the below-described methods are well-suited for producing or fabricating components utilizing Al-based and nickel-based (Ni-based) NEA feedstock powders; that is, feedstock powders predominately composed of Al or Ni, by wt %. Al-based and Ni-based alloys are presently employed to fabricate certain aerospace articles, such as GTE components, utilizing fundamentally different manufacturing approaches. For example, in one common approach, GTE components (and, more generally, other aerospace and turbomachine components) can be suitably produced from Al-based alloys utilizing HIP-based fabrication techniques of the type previous described. However, as noted in the foregoing section entitled “BACKGROUND,” such HIP-based fabrication techniques commonly require a relatively length series of process steps including casting, cold compaction, HIPing, extrusion, and forging, along with various intervening steps. Many, if not all of the aforementioned process steps require specialized tooling and add cost, complexity, and duration to the manufacturing process. Further, debits to material strength are often experienced at each stage of processing with the possible exception of forging, which may increase component strength in skin region of a metallic article, rather than through an article's bulk. Comparatively, the cold spray-based AM manufacturing processes described herein (when paired with the usage of the specialized NEA feedstock powders) can produce comparable aerospace components with enhanced properties at a small fraction of the cost (due, in substantial part, to decreased tooling requirements) and at abbreviated production schedules on the order of one or more days as opposed to weeks or months.
The cold spray-based AM manufacturing processes may provide other advantages in addition to those listed above. For example, embodiments of the below-described cold spray-based AM manufacturing process can produce highly dense articles have low porosities, such as porosities less than 1% by volume and, perhaps, less than 0.5% by volume. Furthermore, embodiments of the cold spray-based AM manufacturing process can also ease size constraints imposed by other conventional additive powder metallurgy fabrication techniques, such as DED powder fusion processes. Such attributes are highly desirable in the aerospace industry and can also benefit other industries in which metallic components or articles of manufacture are desirably fabricated from NEA materials of the type described herein. Exemplary processes for fabricating aerospace components, as well as various other types of metallic articles, utilizing such cold spray-based AM manufacturing processes are described below in conjunction with
Turning now to
Method 10 commences with obtaining the NEA feedstock powder for usage in the subsequently-performed AM cold spray process (PROCESS BLOCK 24). As indicated above, the NEA feedstock powder may be obtained by purchase from a third party supplier or by independent production; that is, production by the same entity performing STEP 18 of method 10 (the cold spray process itself) and, perhaps, one or more of post-spray STEPS 18, 20, 22. Regardless of the particular entity or entities responsible for feedstock powder production, the NEA feedstock powder can be produced utilizing the following generalized process steps, as set-forth in accordance with an exemplary embodiment of the present disclosure. First, as indicated in
In one approach, molten source material is formed into a desired solid shape utilizing a planar flow casting process (also referred to as a “melt-spin process”) during STEP 12 of method 10. An example of such a planar flow casting process is schematically illustrated in
Immediately following discharge through nozzle 32, molten source material 28 contacts cooled wheel 38, which is rotated at a relatively rapid rate. Wheel 38 is usefully cooled by active water flow or in another manner. As material 28 contacts the outer periphery of wheel 38, molten source material 28 rapidly cools, solidifies, and is quickly cast from wheel 38 as a bulk NEA shape and, specifically, as a melt-spun ribbon 40. The appearance and dimensions of melt-spun ribbon 40 will vary amongst embodiments; however, in many cases, melt-spun ribbon 40 may resemble a metallic foil and have a thickness ranging from about 0.5 to about 5 millimeters (mm) and, perhaps, from about 1 and about 2 mm. The width of ribbon 40 is somewhat arbitrary, but may range from about 10 to about 100 mm in an embodiment. In further embodiments, the thickness and/or width of ribbon 40 may vary with respect to the aforementioned ranges. Due to the relative thinness of melt-spun ribbon 40, the cooled state of rotating wheel 38, and the manner in which ribbon 40 is rapidly ejected from wheel 38, exceptionally high cooling rates are achieved. Such cooling rates can approach or exceed approximately 1×107° C./s in many instances, which enables formation of melt-spun ribbon 40 as an NEA material having a desired metastable microstructure, as described herein. The temperatures to which molten source material 28 is heated and the rotational rate of wheel 38 will vary amongst embodiments of the planar flow casting process; however, by way of example, source material 28 may be heated to temperatures approaching or exceeding 1000° C. in certain implementations, while wheel 38 may be rotated at a rate sufficient to cast-off ribbon 40 at a rate exceeding approximately 300 meters per second.
By virtue of the formulation of molten source material 28 and the rapid solidification thereof, melt-spun ribbon 40 is composed of an NEA material. As previously discussed, the NEA materials exist in metastable states and possess tailored microstructures, which are desirably preserved across all subsequently-performed stages of method 10. The particular composition of the NEA material will vary amongst embodiments, as will the shape of the initially-produced bulk NEA material (here, melt-spun ribbon 40). By definition, the NEA material contains at least one minority constituent or dispersoid (as defined by wt %) having a propensity, as predicted by established thermodynamic principals, to precipitate from the alloy matrix (as formed by the majority metallic constituent(s) of the NEA material) when the NEA material is heated above its particular critical temperature threshold (TCRITICAL) for an extended time period; the term “extended” utilized in a relative sense, noting that a time period of several seconds may be considered “extended” in certain instances. In many cases, the NEA material will contain multiple minority constituents or dispersoids (e.g., silicides or carbides) distributed throughout the alloy matrix and prone to participate growth or agglomeration under such overtemperature conditions.
Further discussion will now be provided in which the NEA material is described as an Al-based NEA material; that is, an NEA material containing Al as its predominate constituent by wt %. Such Al-based NEA material can advantageously provide highly stable microstructure at elevated operating temperatures; e.g., temperatures exceeding 350° C. and, in certain cases, at temperatures approaching or exceeding 425° C. Al-based NEA materials are consequently well-suited for usage in the production of aerospace and engine components including, for example, turbocharger components and components contained within GTEs. It is emphasized, however, that the following description is provided by way of non-limiting example only; and that the NEA material need not be composed of an Al-based alloy in all embodiments. For example, in further embodiment, the NEA material, may be composed of Ni-based alloy or superalloy. Furthermore, and more generally, the fabrication processes described in connection with method 10 (
In an embodiments, molten source material 28 and melt-spun ribbon 40 shown in
With continued reference to
Generally, as the average particle size increases, so too does the oxide content within the cold spray-deposited NEA body due to an increase in the ratio of exposed surface area to volume of the powder particulates. Conversely, as the average particle size decreases, explosivity tends to increase, while (somewhat counter-intuitively) particle velocities tend to decrease during the cold spray process. To balance these competing factors, the bulk NEA body (e.g., melt-spun ribbon 40 shown in
Various different mechanical processing steps can be employed during STEP 14 to convert melt-spun ribbon 40 (
Next, at STEP 16 of method 10, the NEA feedstock powder is annealed. Such an annealing process (herein, a “pre-spray anneal”) is usefully performed to relieve material stress and any work hardening resulting from the mechanical processing steps performed during STEP 14 of method 10. When performed, the pre-spray anneal may be carried-out in accordance with a pre-established heating schedule specific to the NEA material being processed. Generally, pre-spray annealing will entail heating the NEA feedstock powder to a maximum anneal temperature (TANNEAL_MAX) for a predetermined period of time on the order of, for example, approximately one hour. The pre-spray annealing process is controlled such that TANNEAL_MAX is less than TCRITICAL throughout the annealing process. For example, in one embodiment in which TCRITICAL ranges from 400° C. to 450° C., TANNEAL_MAX may range from 350° C. to 400° C. In another embodiment, TANNEAL_MAX may range between a minimum of TCRITICAL minus about 150° C. to a maximum of TCRITICAL minus about 25° C. or, perhaps, between a minimum of TCRITICAL minus about 100° C. to a maximum of TCRITICAL minus about 50° C. By relieving materials stresses and possibly reducing work hardening (particularly in the case of feedstock powders prone to work hardening, such as Al-based NEA feedstock powders), the resulting NEA feedstock powder may be rendered more malleable to enhance adhesion and compaction during the subsequently-performed AM cold spray process. These benefits notwithstanding, the NEA feedstock powder need not be subject to pre-spray annealing in alternative embodiments of method 10.
Following STEP 16 of method 10 and the completion of PROCESS BLOCK 24, the NEA feedstock powder has now been produced. Utilizing the newly-produced NEA feedstock powder, an AM cold spray process can be carried-out to additively manufacture or three dimensionally print articles of manufacture having near-net shapes; that is, shapes encompassing and generally approximately the desired final geometries of the articles desirably produced pursuant to method 10. An exemplary embodiment of a suitable AM cold spray process will now be described in conjunction with
The process parameters governing the AM cold spray process will vary amongst embodiments, providing that the peak temperatures to which the NEA feedstock powder is heated are maintained below TCRITICAL through the cold spray process. In this regard, the cold spray process may be performed such that the NEA feedstock powder is exposed to a maximum processing temperature of TSPRAY_MAX, which is less than TCRITICAL and may be at least 50° C. less than TCRITICAL in an embodiment. For completeness, it is noted that preheater unit 62 may heat the gas flow to temperatures exceeding TCRITICAL in certain instances. Even when this is the case, however, the temperature of the NEA feedstock powder remains below TCRITICAL as the powder particles are entrained in the carrier stream for an extremely brief time period, which prevents complete heat transfer from the carrier gas to the powder particles. Additionally, gas temperatures may decrease rapidly to levels below TCRITICAL by the time the gas reaches cold spray gun 58 and contacts the feedstock powder. Gas temperature will also generally plummet upon discharge from nozzle 60 such that the cooling rate at nozzle 60 may approach or exceed the above-mentioned threshold (e.g., 1×106° C./s) in embodiments. Finally, while a certain amount of thermal input will be reintroduced into the NEA feedstock powder due to the conversion of kinetic energy when contacting the target surface or site-of-deposition, this secondary heating mechanism will also typically be insufficient to heat the NEA feedstock powder (or the resulting compacted body composed of the NEA material) to temperatures exceeding TCRITICAL. Regarding the other process parameters of the AM cold spray process (e.g., deposition rates, particle discharge velocities, carrier gas types, chamber environment conditions, and the like), again such parameters will vary amongst embodiments. In one embodiment, helium is utilized as the carrier gas, which is supplied at a pressure approaching or exceeding approximately 700 pounds per square inch to impart the particles with high velocities and relatively straight trajectories when discharged from cold spray gun 58.
As generically indicated on the right side of
When striking the target surface or site-of-deposition, kinetic energy of the particle impact induces plastic deformation of the cold spray-deposited NEA material to create the desired bond between layers of the deposited NEA feedstock powder. As indicated by the “cold spray,” generally considered, the particles are applied at a temperature well below their melt point such that the kinetic energy of the particles on impact (rather than particle temperature) is the mechanism underlying plastic deformation and bonding of the particle with the target surface. Advantageously, AM cold spray processes are capable of relatively rapid deposition rates often exceeding 1 gram of material per second. As a result, the AM cold spray process carried-out at STEP 18 of method 10 may be capable of producing a part in a highly efficient manner; e.g., the cold spray process may be capable of producing a component of modest volume and complexity in several minutes, while a DMLS process may require several hours to fabricate a comparable component. Furthermore, as each layer of NEA feedstock powder is deposited by cold spray, the newly-applied layer tends to compact and thereby densify the previously-deposited NEA material layers. This, combined with the desirable properties of the NEA feedstock powder, enable the AM cold spray process to fabricate near-net articles having low porosities, which may be less than 1% and, perhaps, less than 0.5% by volume.
After completion of the AM cold spray process, additional process steps may be performed to transform the near-net article(s) into the finished article(s) of manufacture, as indicated in PROCESS BLOCK 26 of
Finally, if any sacrificial bodies or fugacious tooling structures are present within the near net article, such sacrificial tooling may be removed during STEP 22 of method 10. Suitable removal techniques will depend upon the composition of the NEA article produced pursuant to method 10 as compared to the composition of the sacrificial structures. Generally, chemical dissolution (including acid leeching in the case of mild steels), breaking in the case of brittle (e.g., ceramic) materials, and Coefficient of Thermal Expansion (CTE) mismatch techniques can be employed. Collapsible and removable tooling can also be utilized to form desired internal voids, chambers, or flow passages within the additively-manufactured articles. For example, and referring briefly again to
There has thus been provided methods for the production of NEA feedstock powders of the type utilized in cold spray processes, as well as methods for fabricating articles utilizing such NEA feedstock powders and computer-controlled, AM cold spray processes. Embodiments of the above-described methods enable the production of completed parts with desirable NEA microstructures characterized by enhanced material strength, ductilities, low porosities, and other desirable properties. Embodiments of the below-described methods are well-suited for producing or fabricating components utilizing Al-based NEA feedstock powders, although by no means limited to such feedstock powders. Embodiments of the cold spray-based AM manufacturing processes described herein (when paired with the usage of the specialized NEA feedstock powders) can be utilized to produce components with such enhanced properties, while decreasing tooling requirements, in more abbreviated time periods (e.g., on the order of a few days), with fewer processing steps, at lower scrap volumes, and while easing article size constraints imposed by other conventional DED powder fusion processes. The end result is a vastly improved AM production process featuring cost and time savings, which are highly desirable with the aerospace industry and across other industries. Furthermore, parts or articles produced utilizing the AM cold spray processes described herein can be distinguished from other components utilizing known inspection techniques, such as photomicrographs of the NEA material structurer revealing splat boundaries created during the cold spray process.
In one embodiments, the above-described method includes the steps or processes of forming a molten alloy into a solid NEA body, such as an NEA ribbon or other bulk shape, utilizing a casting process having a cooling rate equal to or greater than approximately 1×106° C./s and, perhaps, cooling rate equal to or greater than 1×107° C./s. After casting, the solid NEA body is mechanically converted into a NEA feedstock powder. An annealing process is then performed during which the feedstock powder is exposed to a maximum anneal temperature (TANNEAL_MAX) for a predetermined time period. The NEA feedstock powder is composed of an alloy matrix having a melt point (TALLOY_MP) and throughout which a first minority constituent is dispersed or distributed. The first minority constituent precipitates from the alloy matrix when the NEA feedstock powder is exposed to temperatures exceeding a critical temperature threshold TCRITICAL for a predetermined time period. The NEA body is formulated such that TCRITICAL is less than TALLOY_MP and greater than TANNEAL_MAX. In certain embodiments, the NEA feedstock powder may be formulated such that the NEA feedstock powder is predominately composed of a first material by weight percent, the first material selected from the group consisting of aluminum and nickel; and the first minority constituent forms dispersoids within the NEA feedstock powder, the dispersoids selected from the group consisting of silicide dispersoids and carbide dispersoids.
While at least one exemplary embodiment has been presented in the foregoing Detailed Description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing Detailed Description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention. It being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set-forth in the appended claims.
