EP4323137A1

EP4323137A1 - Site-specific grain boundary engineering of additively manufactured alloys

Info

Publication number: EP4323137A1
Application number: EP22788564.7A
Authority: EP
Inventors: Matteo Seita; Shubo GAO
Original assignee: Nanyang Technological University
Current assignee: Nanyang Technological University
Priority date: 2021-04-14
Filing date: 2022-04-13
Publication date: 2024-02-21
Also published as: WO2022220752A1; EP4323137A4

Abstract

A method of additively manufacturing a part based on fusion of a metal alloy powder and an article made thereby. In a build stage, additively building at least one first region characterized by a first thermal stability and at least one second region characterized by a second thermal stability. The second thermal stability is configured to be lower than the first thermal stability. The at least one first region and the at least one second region of the as-built part are characterized by a respective microstructure prior to a heat treatment. The at least one first region is not substantially recrystallized and the at least one second region is concurrently substantially recrystallized by the heat treatment. The heat treatment consists essentially of thermal input to the as-built part without mechanical processing.

Description

SITE-SPECIFIC GRAIN BOUNDARY ENGINEERING OF ADDITIVELY

MANUFACTURED ALLOYS

The present application claims priority to the Singapore patent application no. 10202103835Q which is incorporated in its entirety by reference.

TECHNICAL FIELD

[0001] The present disclosure relates to additive manufacturing, and more particularly to a method of grain boundary engineering for use in additive manufacturing and parts made thereby.

BACKGROUND

[0002] Conventional grain boundary engineering relies on the effect of thermo-mechanical processing to improve material properties of alloys. This typically includes subjecting the alloy to a sequence of strain-annealing cycles involving cold work and heat treatments. One of the benefits of additive manufacturing is the ability to produce net-shape or near net- shape parts. Applying conventional grain boundary engineering methods to additive manufacturing would mean subjecting a part (as-built by additive manufacturing) to copious plastic deformation from cold work or other thermo-mechanical processes. The geometry of the as-built part would be substantially changed as a result of conventional grain boundary engineering. It is therefore difficult to apply conventional grain boundary engineering to net-shape or near net-shape additively manufactured parts.

SUMMARY

[0003] In one aspect, the present disclosure provides a method of additively manufacturing a part based on fusion of a metal alloy powder, the method comprising: in a build stage, additively building at least one first region of the part, the at least one first region being characterized by a first thermal stability; in the build stage, additively building at least one second region, the at least one second region being characterized by a second thermal stability, the second thermal stability being configured to be lower than the first thermal stability; and providing the part as an as-built part, the at least one first region and the at least one second region of the as-built part being characterized by a respective microstructure prior to a heat treatment, wherein the respective microstructure of the at least one first region is not substantially recrystallized by the heat treatment, and wherein the respective microstructure of the at least one second region is concurrently substantially recrystallized by the heat treatment, and wherein the heat treatment consists essentially of thermal input to the as-built part without mechanical processing.

[0004] The method may further comprise: subjecting the as-built part to the heat treatment at a heat treatment temperature, wherein the heat treatment temperature is higher than a second critical temperature of the at least one second region and lower than a first critical temperature of the at least one first region to obtain a post-heat treatment part in which the at least one second region is substantially recrystallized by the heat treatment and in which the at least one first region is not substantially recrystallized by the heat treatment.

[0005] The as-built part may be characterized by a higher native geometrically necessary dislocation (GND) density of the at least one second region of the as-built part than a native GND density of the at least one first region of the as-built part. The native GND density of the at least one second region may be higher than a critical GND density. The as-built part may be characterized by a larger median cell size in the at least one second region of the as- built part than in the at least one first region of the as-built part. The as-built part may be characterized by a lower amount of solute segregation in the at least one second region than in the at least one first region.

[0006] The method may further comprise: in the build stage, switching from a first set of additive manufacturing (AM) parameters to build at least a part of the first region to a second set of AM parameters to build at least a part of the second region and/or vice versa, wherein the switching is made in a course of additively building a layer of the part. The second set of AM parameters is preferably configured to enable a greater thermal accumulation in the second region of the as-built part than in the first region of the as-built part. The second set of AM parameters preferably defines a higher number of re-scans than the first set of AM parameters. The second set of AM parameters preferably defines a multiple number of re scans for the at least one second region and wherein the first set of AM parameters defines no re-scanning for the at least one first region. The second set of AM parameters preferably defines a smaller hatch spacing than the first set of AM parameters. The second set of AM parameters preferably defines a slower scan speed than the first set of AM parameters. The second set of AM parameters preferably defines a smaller laser spot size than the first set of AM parameters. The second AM parameters preferably defines a higher laser energy than the first set of AM parameters. The at least one second region of the as-built part preferably contains a higher twin boundary fraction than the at least one first region of the as-built part. The at least one second region of the as-built part preferably contains a higher recrystallization fraction than the at least one first region of the as-built part.

[0007] In another aspect, an article additively manufactured based on fusion of a metal alloy powder, comprises: at least one first region, the at least one first region being characterized by a first thermal stability; and at least one second region, the at least one second region being characterized by a second thermal stability, the second thermal stability being configured to be lower than the first thermal stability, wherein the article is an as-built part, the at least one first region and the at least one second region of the as-built part being characterized by a respective microstructure prior to a heat treatment, wherein the respective microstructure of the at least one first region is not substantially recrystallized by the heat treatment, and wherein the respective microstructure of the at least one second region is concurrently substantially recrystallized by the heat treatment, and wherein the heat treatment consists essentially of thermal input to the as-built part without mechanical processing.

[0008] The at least one second region is configurable to be substantially recrystallized upon the heat treatment if the heat treatment is characterized by a heat treatment temperature that is higher than a second critical temperature of the at least one second region, and wherein the at least one first region is characterized by a first critical temperature that is higher than the heat treatment temperature. A layer of the as-built part may include the at least one first region and the at least one second region. One or both of the at least one first region and the at least one second region may extend across one or more layers of the as-built part.

BRIEF DESCRIPTION OF DRAWINGS

[0009] Fig. 1 A is a schematic diagram of an additive manufacturing system;

[0010] Fig. IB illustrates scan paths traversed by the laser of Fig. 1 A; [0011] Fig. 1C is a non-limiting exemplary illustration of an as-built part of net-shape or near net-shape;

[0012] Fig. 2 illustrates an example of the additive-grain boundary engineering (A-GBE) method;

[0013] Fig. 3A and Fig. 3B illustrate an as-built part before and after heat treatment respectively;

[0014] Fig. 4 shows another example of an as-built part;

[0015] Figs. 5 A to 5D are images and analysis results for an exemplary first region;

[0016] Figs. 6A to 6D are images and analysis results for an exemplary second region;

[0017] Fig. 7 shows the relative GND densities of the first region and the second region;

[0018] Figs. 8 A and 8B shows an example of configuring an A-GBE alloy with the desired material properties;

[0019] Figs. 9A and 9B are images of the microstructure of a first region and a second region of an Inconel alloy; and

[0020] Fig. 10 schematically illustrates processes for obtaining different material properties from identical as-built parts.

DETAILED DESCRIPTION

[0021] Reference throughout this specification to “one embodiment”, “another embodiment” or “an embodiment” (or the like) means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” or the like in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided to give a thorough understanding of embodiments. One skilled in the relevant art will recognize that the various embodiments may be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, some or all known structures, materials, or operations may not be shown or described in detail to avoid obfuscation.

[0022] The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. As used herein, the singular ‘a’ and ‘an’ may be construed as including the plural “one or more” unless apparent from the context to be otherwise.

[0023] Terms such as “first” and “second” are used in the description and claims only for the sake of brevity and clarity, and do not necessarily imply a priority or order, unless required by the context. The terms "about" and "approximately" as applied to a stated numeric value encompasses the exact value and a reasonable variance as will be understood by one of ordinary skill in the art, and the terms “generally” and “substantially” are to be understood in a similar manner, unless otherwise specified.

[0024] In the present disclosure, the terms “substantial recrystallization” and “substantially recrystallized” refer to a microstructure in which the majority of the grains have recrystallized, with only a minor portion of the grains remaining in their original (as-built) nature as formed during the build process. “Substantially recrystallized” includes, but is not limited to, “fully recrystallized” or “complete recrystallization”. The fraction of recrystallization should be understood in the usual manner in the technical field. For example, in the context of a specified material, substantially recrystallized refers to the material having over 80% of its area/volume recrystallized. In another material, “substantially recrystallized” may refer to the material having a volume fraction of over 30% in a recrystallized state, or in other examples, a volume fraction of over 50% in a recrystallized state. In the present disclosure, the term “not substantially recrystallized” will be used to refer to a microstructure that is different or distinct from a substantially recrystallized microstructure. “Not substantially recrystallized” may refer to a microstructure in which a majority of the grains remain in their original (as-built) nature as formed during the build process, with only a minor portion or a negligible portion of the grains undergoing recrystallization or other morphological changes. “Not substantially recrystallized” does not necessarily mean that there are no changes at all in the microstructure, or that there are no features in the microstructure more commonly found in a recrystallized material. It is understood in the technical field that, for a given material, a microstructure with less than 10% recrystallization fraction may be referred to as “incompletely recrystallized” or “non-recry stallized”. Terms which may be used interchangeably with “not substantially recrystallized” include “incompletely recrystallized”, “non-recrystallized”, “slightly recrystallized”, “with only a minor fraction of recrystallization”, etc. The terms “cells” and “grains” are used interchangeably in the present disclosure, and the terms “cell boundary” and “grain boundary”, “cell size” and “grain size”, etc., are to be similarly understood to be interchangeable. The terms “microstructure” and “grain boundary character distribution” (GBCD) are also used interchangeably unless context dictates otherwise.

[0025] The term “conventional grain boundary engineering” as used in the present disclosure refers to the application of thermo-mechanical processes to a polycrystalline material, such as a metal alloy, to modify the structure and properties of the microstructure/grains of the material. The conventional grain boundary engineering process may involve annealing (thermo processes) as well as mechanical processes such as rolling or peening in order to put the material through the large and repeated plastic deformation necessary to produce the desired microstructure and material properties (such as improved ductility and strength). Shaping of the product can take place concurrently with the conventional grain boundary engineering (e.g., in the course of cold work) or after the conventional grain boundary engineering by subtractive methods such as machining.

[0026] The present disclosure introduces a different approach to grain boundary engineering such that it is compatible with additive manufacturing. The proposed additive grain boundary engineering (A-GBE) method is compatible and useful with diverse types of additive manufacturing methods, including but not limited to laser powder bed fusion (LPBF), directed energy deposition (DED), electron-beam melting (EBM), etc. For example, the A-GBE method may be integrated with a fusion-based additive manufacturing technique, a non-limiting example of which is laser powder bed fusion (LPBF).

[0027] One example of the A-GBE method 300 will be described using a non-limiting example of an additive manufacturing system 100 (such as a LPBF system) to show that special or additional hardware is not required to implement the A-GBE method. The LPBF apparatus 100 of Fig. 1 A is illustrated schematically to show a powder bed 102 in which an article can be additively manufactured layer-by-layer. The powder bed 102 may be replenished with more powder from a powder stock 104 with the aid of a roller 106. A laser generator 112 is configurable to emit a laser beam 101. A scanner 114 is configured to direct the laser beam at the powder bed 102 such that powder in the scan path 130 (Fig. IB) is melted by the laser beam 101 to form a pool of melted powder. As the laser beam 101 continues in the scan path 130, the melted powder cools and solidifies to form a new layer of material that is fused to a last formed or a last fused layer of material. The article is built layer-by-layer, with each layer 120 added in a build direction 122 (e.g., along a vertical direction). Prior to building an article, a three-dimensional model of the article can be spliced into corresponding layers to determine a scan path 130 for each layer 120. In the present disclosure, the net-shape or near net-shape article formed directly from fusion and solidification of material, prior to any post-build heat-treatment, will be referred to as the as-built part 200. Fig. 1C shows one non-limiting example of an as-built part 200 which is formed to near net-shape, i.e., with physical dimensions and tolerances substantially similar to the form in which the article is intended for use.

[0028] The laser 100 (referring collectively to the laser generator 112, the scanner 114, and the laser beam 101, for the sake of brevity) is configurable by a plurality of laser parameters, e.g., laser power, scan speed, spot size/laser profile, scan direction, etc. The additive manufacturing process may be described in terms of additive manufacturing (AM) parameters that include, but are not limited to, the laser parameters, build strategy-related parameters such as hatch spacing, materials-related parameters such as material composition, etc. As graphically represented in Fig. 2, according to embodiments of the A- GBE method 300 proposed herein, at least two sets of additive manufacturing (AM) parameters (combinations of a plurality of AM parameters) are determined and applied over the course of a build stage 190 when a plurality of layers 120 are additively formed into a part 200. The part or as-built part 200 is characterized by a first microstructure 321, as shown by an exemplary non-limiting electron backscatter diffraction (EBSD) image. Subjecting the as-built part 200 to a heat treatment 290 at a pre-determined heat treatment temperature triggers recrystallization in site-specific regions 322 and not in other regions 323 of the as- built part 200. The heat-treated part 320 includes one or more regions 323 at which site- specific grain boundary engineering has been implemented or has taken effect, and one or more regions 323 at which the microstructure remains substantially unchanged or remain substantially similar to the microstructure 321 of the (pre-heat treatment) as-built part 200.

[0029] The heat treatment 290 is defined with reference to one heat treatment temperature. For the purpose of the present disclosure, this may refer to subjecting the as-built part 200 to the heat treatment temperature for one or more predetermined periods of time, with intervening intervals during which the as-built part 200 may be allowed to cool down. For the purpose of the present disclosure, this may refer to subjecting the as-built part 200 to one or more heating cycles where the heat treatment temperature defines the highest temperature of the one or more heating cycles.

[0030] Figs. 3A and 3B are schematic cross-sectional diagrams of as-built parts 410, 420 according to embodiments of the present disclosure, in which the layers are depicted simply to aid understanding. In actual additively manufactured products, the different layers may or may not remain distinguishable by unassisted visual inspection.

[0031] Fig. 3 A shows an as-built part 410 built by applying two sets of AM parameters over the course of building the plurality of layers. The as-built part 410 of Fig. 3 A is from a post build and pre-heat treatment stage, that is, the as-built part 410 is one that has been removed from the powder bed 102 and before the as-built part 410 is subject to further processing and/or use. The as-built part 410 includes at least two regions 411, 412 configured with different levels of thermal stability, in which the at least two regions 411, 412 can be found in at least one formed layer 120 of material of the as-built part 410. Each of the at least two regions 411, 412 is characterized by a respective native microstructure, e.g., a first region

411 is characterized by a first native microstructure 321 and a second region 412 is characterized by a second native microstructure 322. Each of the at least two regions 411,

412 is characterized by a respective critical temperature, e.g., the first region 411 is characterized by a first critical temperature and the second region 412 is characterized by a second critical temperature. The respective critical temperatures are different from one another. According to one embodiment of the present disclosure, if the as-built part of Fig. 3 A is put through a course of heat treatment 290 that is characterized by a heat treatment temperature that is lower than all of the respective critical temperatures, the post-heat treatment part retains its respective native microstructures throughout the entire build. That is, in this example, after heat treatment 290, the first region 411 is substantially characterized by the first native microstructure 321 and the second region 412 is substantially characterized by the second native microstructure. Neither the first region 411 nor the second region 412 is substantially recrystallized after the heat treatment 290. This ability to control whether recrystallization takes place is useful because there are applications where the material properties associated with the native microstructure are required, but a post build course of heating or heat treatment is also desired, e.g., to improve fusion of the materials. As understood in the technical field, the critical temperature refers to the minimum temperature to which an alloy must be heated for recrystallization to occur within a certain duration.

[0032] Fig. 3B schematically illustrates a post-heat treatment part 420 resulting from putting the as-built part 410 of Fig. 3 A through a course of heat treatment 290, in which the course of heat treatment 290 is characterized by a heat treatment temperature. In this example, the entire part 410 is subjected to the same heat treatment temperature. The heat treatment temperature is selected to be lower than the first critical temperature and higher than the second critical temperature. No mechanical process is required to trigger the intended site-specific microstructural changes in the second region 412. The heat treatment 290 alone (without mechanical processes) suffices to produce substantial recrystallization in the second region 412. At the end of the heat treatment of the entire part 410, the first region 411 is not substantially recrystallized and the second region 412 is substantially recrystallized. At least two distinct regions with different microstructures can be obtained in the same part 410 after heat treatment, even though the two regions were additively built as one integral net shape or near-net shaped part. The second region 412 of the post-heat treatment part 420 is now characterized by a second microstructure 342 typical of a substantially recrystallized material. The first region 411 of the post-heat treatment part 420 is characterized by a first microstructure 341 that is substantially similar to the first native microstructure 321, i.e., a not substantially recrystallized microstructure. A wide variety of builds can be formed using the proposed A-GBE method 300. As illustrated in Fig. 3B, any layer may be configured to include only one of a plurality of regions, or it may include more than one of a plurality of regions.

[0033] The example of Fig. 3 A and Fig. 3B also illustrate how a plurality of identical metal alloy parts may be mass produced using additive manufacturing to give economies of scale, while the same products may be customized for different applications by subjecting any of the identically shaped additively manufactured parts to heat treatment.

[0034] Fig. 4 schematically illustrates a cross-sectional diagram of another example. The as-built part 500 in Fig. 4 has been heat treated 290 after being additively manufactured from a powder. In this example, in the as-built part 200, there are at least two non-conti guous first regions 502, 504 of the same material and at least one second region 508 also of the same material, which have been concurrently formed by additive manufacturing. The second region 508 is configured to undergo substantial recrystallization upon the entire as- built part 200 being subjected to one course of heat treatment 290, while the first region 502 (external to the second region 508) and the first region 504 (entirely surrounded by the second region 508) is not substantially recrystallized even though it would be subjected to the same course of heat treatment 290. Advantageously, the A-GBE method 300 creates opportunities to design and additively manufactured a wide range of new products.

[0035] For example, at least one layer 506 may be configured to have no more than one region. At least one other layer is preferably configured with more than one region 502, 504, 508. A region may extend across one or more layers as illustrated or it may be configured to one layer. The as-built part 500 includes one or more first regions characterized by a microstructure 341. Each first region is characterized by a comparatively higher thermal stability, i.e., higher than the thermal stability of each second region of the same as-built part 500. The as-built part 200 includes one or more second regions having a second non native microstructure 342. Each first region may be characterized by a first critical temperature within a range of temperatures higher than an intended heat treatment temperature. Each second region may be characterized by a second critical temperature within a range of temperatures lower than the intended heat treatment temperature. The second non-native microstructure is a substantial recrystallized form of a second native microstructure, triggered by the heat treatment temperature (which is selected to be higher than the second critical temperature). The second region is characterized by a comparatively lower thermal stability, i.e., the thermal stability of the second region is lower relative to the thermal stability of the first region(s) of the same as-built part 500. The heat treatment 290 includes subjecting the as-built part 500 to a post-build thermal process (without involving any post-build mechanical process) 290 until substantially or essentially all of the second region has undergone the desired substantial recrystallization to form the non-native second microstructure 342. At the heat treatment temperature selected, there may be a certain amount of recrystallization in the first region, but the first region emerges from the heat treatment with a microstructure that has not been substantially recrystallized. In other words, in some examples, the second region may be substantially recrystallized while concurrently the first region is not substantially recrystallized by the same heat treatment.

[0036] An as-built part 200 obtained with the A-GBE method 300 is different on a microstructural level from an as-built part obtained without applying the A-GBE method 300. To further illustrate, the microstructural differences will be described below using examples of stainless steel 316L (SS316L) or Inconel 725 alloy materials. The description of these two alloys as examples is not intended to be limiting; these materials are used as examples because they are useful in a wide range of practical applications.

[0037] In one example, an as-built part 200 is additively manufactured from one powder such that it is entirely and essentially formed from stainless steel 316L. The as-built part 200 includes a first region 411 and a second region 412. The second region 412 is intentionally architected to include a higher native geometrically necessary dislocation (GND) density relative to the native GND density of the first region 411 or relative to a critical GND density. The GND density is typically defined with reference to a heat treatment temperature, and refers to a value at which the microstructure can be thermally activated at the heat treatment temperature. That is to say, the GND density (also referred to as the critical GND density) of a material may vary according to the heat treatment. For instance, the critical GND density may be around 3 x 10¹⁴ m ² at 1050 °C heat treatment temperature, and around 1.5><10¹⁴ m ² at 1200 °C heat treatment temperature. The critical GND density also varies for different alloy systems. The second region 412 is additionally architected to have a large median cell size (grain size) relative to the media cell size of the second region, such that the amount of solute segregated at the grain boundaries in the second region 412 is low (relative to the amount of solute segregation in the first region 411).

[0038] The scanning electron microscope (SEM) image of Fig. 6A shows an example where the median cell size (median grain size) in the second region is configured to be about 450 nanometers, relative to the median cell size of about 320 nanometers in the first region as shown in Fig. 5A. Comparison of the corresponding scanning transmission electron microscope-energy dispersive X-ray (STEM-EDX) image and analysis of the second region (Figs. 6B and 6C) against those of the first region (Figs. 5B and 5C) show a comparatively lower amount of solute segregation in the second region than in the first region. The AM parameters for forming the second region 412 include a lower scan speed, relative to a higher scan speed to form the first region 411. As shown in the EBSD images of Fig. 5D and Fig. 6D, different grain boundary character distribution can be obtained by using the same heat treatment.

[0039] According to one embodiment of the A-GBE method 300, the laser 100 is configured to make one or more switches between a first set of AM parameters and a second set of AM parameters in the course of additively forming an article, including making such one or more switches in the course of forming a layer and/or between forming immediately adjacent layers. The first set of AM parameters is configured to produce the first region 411 with a first native GND density 802, and the second set of AM parameters is configured to produce the second region 412 with a second native GND density 806. It will be understood that it is possible to define different hatch spacings for the first set of AM parameters and the second set of AM parameters, and achieve the desired result for the purpose of the present embodiment.

[0040] In one example, both the first set of AM parameters and the second set of AM parameters may define the same hatch spacing, but undergo different number of re-scans In one instance, the first set of AM parameters and the second set of AM parameters may differ in that one involves one more re-scan than the other. The additional re-scan is configured to not only slightly increase the native GND density, but also to enable more heat accumulation and facilitate more diffusion of solute segregations.

[0041] In another example, the first set of AM parameters may define a zero number of re- scans, that is, no re-scanning such that the laser 100 traverses a section of the scan path 130 only once if the section of the scan path 130 lies within the first region 411 (or a region intended to be at least part of the first region 411).

[0042] In yet another example, the second set of AM parameters may define a multiple number of re-scans or a smaller hatch spacing, that is, the laser 100 may be configured to traverse/re-melt a section of the scan path 130 more than once if the section of the scan path 130 lies within the second region 412 (or a region intended to be at least part of the second region 412).

[0043] The chart 800 in Fig. 7 shows the estimated native GND density (in units of per square meters), at constant laser power and scan speed, for parts with as-built microstructure 802, 804, 806 formed essentially of stainless steel 316L. The corresponding electron backscatter diffraction (EBSD) images 812, 814, 816 of the respective microstructures after heat treatment are also shown. The EBSD images 812, 814, 816 are similarly oriented in terms of the build direction (BD) and the scan direction (SD). According to one embodiment of the A-GBE method 300, for the same laser power and scan speed, the second set of AM parameters is configured to produce a native GND density that is at least incrementally higher than the native GND associated with the first set of AM parameters, in which the first set of AM parameters includes a zero number of re-scans. The second set of AM parameters preferably define a non-zero number of re-scans such that the resulting native GND density is higher relative to the native GND density with a zero number of re-scans. The second set of AM parameters preferably define multiple re-scans at the same hatch spacing or smaller hatch spacing (relative to the first set of AM parameters) to produce a second region characterized by a GND density higher than the GND density of the first region. A critical GND density (808) may be defined as close to and less than the native GND density of the first region.

[0044] The A-GBE method 300 proposes to manage the thermal stability of the as-built alloy part as a function of both the initial (native) GND density and the solidification structure. As shown, complete recrystallisation of the alloy without any mechanical deformation would be achievable by providing: (i) the native GND density in the as-built alloy to be larger than a critical value, and (ii) a large (median) cell size with a low amount of solute segregated at the cell boundaries. The A-GBE method 300 satisfies both conditions by using selected laser scanning strategies and by employing slow laser scanning speed, respectively.

[0045] The strain energy that accumulates during the build stage 190 derives from the repeated thermal expansion and contraction cycles typical of a layer-by-layer manufacturing process. According to one embodiment of the proposed A-GBE method 300, the number of thermal cycles per unit volume is increased to yield higher GND densities. For example, the hatch spacing distance between two consecutive laser scanning tracks (scan paths) can be reduced and/or re-scanning of each solidified layer can be conducted to re-melt the material one more time. Referring again to the chart of Fig. 7, a comparison of the estimated GND densities between samples produced by LPBF using fine/large hatch spacing and with/without re-melt, shows that it is indeed possible to further increase the GND density of LPBF SS316L, to obtain a GND density that is well above the critical value for recrystallization.

[0046] In another aspect, the proposed A-GBE method 300 includes decreasing the energy barrier for recrystallization by providing higher heat accumulation during the build stage 190. One example is to increase the number of thermal cycles to promote thermal build-up and redistribution of the solute that is kinetically trapped at the cell boundaries. This can be carried out optionally in combination with a slow laser scan speed. The resulting alloy would be able to undergo complete recrystallization without requiring any additional mechanical treatment.

[0047] This example of the A-GBE method 300 was demonstrated on LPBF SS316L. After heat treatment 290, the resulting twin boundary (TB) fraction (which may be as high as 37%) confirms the successful tailoring of the grain boundary character distribution (GBCD) in a way that is entirely compatible with additive manufacturing. Figs. 8A and 8B show an example of site-specific A-GBE in which layers of SS316L are formed using different combinations of LPBF process parameters. Portions of the build which have to undergo recrystallisation are produced by choosing process parameters that are conducive to low thermal stability, such as slow scanning speed, small hatch spacing and re-scanning. By contrast, portions that need to retain the as-built GBCD are produced by employing fast laser scanning speed, large hatch spacing, and no re-scanning. As clearly shown in in the EBSD image of Fig. 8A, recrystallized bands comprise equiaxed grans and copious twin boundaries. At the same time, non-recrystallized bands are made of columnar grains separated by low-angle grain boundaries. These different microstructures exhibit completely different mechanical properties. With reference to the tensile curves in Fig. 8B, a fully recrystallized sample is more ductile, while a non-recrystallized sample shows higher strength. Gaining control over the distribution and arrangement of such microstructural bands and properties can create opportunities to impart superior mechanical behavior to the build. These parts with site-specific microstructures could, for instance, combine both high strength and high ductility.

[0048] Another use of the A-GBE method 300 is in controlling the density of twin boundaries (TB fraction and TB density are used interchangeably) in the recrystallized microstructure by engineering the occurrence and distribution of recrystallisation sites across the build. It is known that TB multiplication is abundant within these recrystallisation nuclei (also referred to as twin-related domains). Using the proposed A-GBE method 300, the density and distribution of these nucleation sites across the build can be manipulated through the choice of AM parameters. For instance, a few specific locations may be selectively re-scanned during the build stage 190 to weaken the solidification segregation. The recrystallized grains would only nucleate in these selected sites and become highly twinned grains as they grow. These twin-related domains will keep growing until they coalesce and produce non-twin-related grain boundaries. The resulting microstructure would consist of a high TB-density with a sparse and disconnected network of high angle grain boundaries (HAGBs). These materials are expected to exhibit properties comparable to those of materials that undergo several strain-annealing cycles following conventional grain boundary engineering processes, but are advantageously produced in a manner that is compatible with the additive manufacturing of net-shape or near net-shape articles.

[0049] Embodiments of the A-GBE method 300 are applicable to other alloy systems, e.g., alloy systems that are characterized by a low stacking fault energy. In another example, an article is additively built using Inconel 725 alloy according to one embodiment of the A- GBE method 300 to form an as-built part 200 with at least one first region and at least one second region. Prior to performing the additive build stage, one or more first regions 411 and one or more second regions 412 are defined based on a model of the article. During the additive build stage, the additive manufacturing system 100 is configured to switch between a first set of AM parameters and a second set of AM parameters as the laser 100 traverses from a first region 411 to a second region 412 or vice versa from a second region 412 to a first region 411. One example is illustrated schematically in Fig. IB, where the first set of AM parameters are in effect for sections 131, 134 of the scan path 130 in the first region 411, and the second set of AM parameters are in effect for sections 132, 133 of the scan path 130 in the second region. The second set of AM parameters includes a smaller laser spot size relative to the laser spot size of the first set of AM parameters. In the experiments, the laser power was kept constant. The resulting second native microstructure has a lower thermal stability relative to the first native microstructure. After a post-build heat treatment 290 at a heat treatment temperature, the first region 411 maintains a microstructure substantially similar to the first native microstructure of the as-built part 200 (as shown by the non-limiting exemplary EBSD image of Fig. 9A). After the same post-build heat treatment 290 at the same heat treatment, the second region has undergone substantial recrystallization as is evident from the annealing twin boundaries formed (as shown by the non-limiting exemplary EBSD image of Fig. 9B).

[0050] In yet another example, an article is additively built using an alloy according to one embodiment of the A-GBE method 300 to form an as-built part 200 with at least one first region and at least one second region. Prior to performing the additive build stage, one or more first regions 411 and one or more second regions 412 are defined based on a model of the article. During the additive build stage, the additive manufacturing system 100 is configured to switch between a first set of AM parameters and a second set of AM parameters as the laser 100 traverses from a first region 411 to a second region 412 or vice versa from a second region 412 to a first region 411. Compared to the first set of AM parameters, the second set of AM parameters includes a higher laser energy. The higher laser energy leads to more heat accumulation and a lower thermal stability in the second region, relative to the first region.

[0051] The various examples given above illustrate how the A-GBE method 300 may be implemented to controllably engineering the propensity of one or more selected regions (second region) to undergo recrystallization relative to one or more other regions (first region) of the same as-built part, such that said propensity of the one or more second regions is sufficiently high for substantial recrystallization to be triggered by input of thermal energy without mechanical processing. Advantageously, the as-built part can be conferred the material properties associated with grain boundary engineering without losing its net shape or near net shape. The recrystallization can be controllably facilitated in a site-specific manner in one article, such that the as-built part can retain the material properties associated with the native microstructure where desired. The proposed A-GBE alloys can be relevant for different applications which involve high-temperature environments and/or highly corrosive environments, in which thermal stability and corrosion resistance are in demand. The proposed A-GBE method enables combination of microstructures with different GBCD and enable improvement of the mechanical performance of alloys. This could in turn enable a broader spectrum of engineering applications for metal alloys.

[0052] In another aspect, embodiments of the A-GBE method create an opportunity to additively manufacture identical as-built parts with the economies of scale of mass production, while enabling the originally identical as-built parts to be customized by a relatively simple course of heat treatment (without involving mechanical processes) for different applications. One example of the new supply chain enabled by the A-GBE method 300 is illustrated schematically in Fig. 10. Economies of scale may be enjoyed by the manufacturer in carrying out the additive build processes 902 to mass produce as-built parts 904. Since the as-built parts 904 are in a sense “standard parts” before heat treatment, the as-built parts 904 can be available as “off-the-shelf’ or catalog parts. The as-built parts 904 can be customized relatively easily by different parties or by the same party, by heat treatment 906, 926, 946 that does not require mechanical processes to produce articles 908, 928, 948 of different material properties, to suit different applications and requirements. For example, for one industrial application, the heat treatment 906 may be characterized by a heat treatment temperature that does not trigger substantial recrystallization of any regions of the as-built part. For another application, the heat treatment 926 may be characterized by a heat treatment temperature to trigger recrystallization in one or more selected regions of the as-built part. For yet another intended use, the heat treatment 928 may be characterized by a heat treatment temperature configured to trigger substantial recrystallization in the entire as-built part. As a result of the proposed A-GBE technology, the manufacturer of the as-built part can look to serving multiple markets with the same as-built part. These are just a few benefits of the proposed A-GBE method 300, and many more will be apparent even to one of ordinary skill in the art.

[0053] All examples described herein, whether of apparatus, methods, materials, or products, are presented for the purpose of illustration and to aid understanding, and are not intended to be limiting or exhaustive. Various changes and modifications may be made by one of ordinary skill in the art without departing from the scope of the invention as claimed.

Claims

1. A method of additively manufacturing a part based on fusion of a metal alloy powder, the method comprising: in a build stage, additively building at least one first region of the part, the at least one first region being characterized by a first thermal stability; in the build stage, additively building at least one second region, the at least one second region being characterized by a second thermal stability, the second thermal stability being configured to be lower than the first thermal stability; and providing the part as an as-built part, the at least one first region and the at least one second region of the as-built part being characterized by a respective microstructure prior to a heat treatment, wherein the respective microstructure of the at least one first region is not substantially recrystallized by the heat treatment, and wherein the respective microstructure of the at least one second region is concurrently substantially recrystallized by the heat treatment, and wherein the heat treatment consists essentially of thermal input to the as-built part without mechanical processing.

2. The method according to claim 1, further comprising: subjecting the as-built part to the heat treatment at a heat treatment temperature, wherein the heat treatment temperature is higher than a second critical temperature of the at least one second region and lower than a first critical temperature of the at least one first region to obtain a post-heat treatment part in which the at least one second region is substantially recrystallized by the heat treatment and in which the at least one first region is not substantially recrystallized by the heat treatment.

3. The method according to claim 1 or claim 2, wherein the as-built part is characterized by a higher native geometrically necessary dislocation (GND) density of the at least one second region of the as-built part than a native GND density of the at least one first region of the as-built part.

4. The method according to claim 3, wherein the native GND density of the at least one second region is higher than a critical GND density.

5. The method according to any one of claims 1 to 4, wherein the as-built part is characterized by a larger median cell size in the at least one second region of the as-built part than in the at least one first region of the as-built part.

6. The method according to any one of claims 1 to 5, wherein the as-built part is characterized by a lower amount of solute segregation in the at least one second region than in the at least one first region.

7. The method according to any one of claims 1 to 6, further comprising: in the build stage, switching from a first set of additive manufacturing (AM) parameters to build at least a part of the first region to a second set of AM parameters to build at least a part of the second region and/or vice versa, wherein the switching is made in a course of additively building a layer of the part.

8. The method according to claim 7, wherein the second set of AM parameters is configured to enable a greater thermal accumulation in the second region of the as-built part than in the first region of the as-built part.

9. The method according to claim 7 or claim 8, wherein the second set of AM parameters defines a higher number of re-scans than the first set of AM parameters.

10. The method according to claim 9, wherein the second set of AM parameters defines a multiple number of re-scans for the at least one second region and wherein the first set of AM parameters defines no re-scanning for the at least one first region.

11. The method according to any one of claims 7 to 10, wherein the second set of AM parameters defines a smaller hatch spacing than the first set of AM parameters.

12. The method according to any one of claims 7 to 11, wherein the second set of AM parameters defines a slower scan speed than the first set of AM parameters.

13. The method according to any one of claims 7 to 12, wherein the second set of AM parameters defines a smaller laser spot size than the first set of AM parameters.

14. The method according to any one of claims 7 to 13, wherein the second set of AM parameters defines a higher laser energy than the first set of AM parameters.

15. The method according to any one of claims 1 to 14, wherein the at least one second region of the as-built part contains a higher twin boundary fraction than the at least one first region of the as-built part.

16. The method according to any one of claims 1 to 15, wherein the at least one second region of the as-built part contains a higher recrystallization fraction than the at least one first region of the as-built part.

17. An article additively manufactured based on fusion of a metal alloy powder, the article comprising: at least one first region, the at least one first region being characterized by a first thermal stability; and at least one second region, the at least one second region being characterized by a second thermal stability, the second thermal stability being configured to be lower than the first thermal stability, wherein the article is an as-built part, the at least one first region and the at least one second region of the as-built part being characterized by a respective microstructure prior to a heat treatment, wherein the respective microstructure of the at least one first region is not substantially recrystallized by the heat treatment, and wherein the respective microstructure of the at least one second region is concurrently substantially recrystallized by the heat treatment, and wherein the heat treatment consists essentially of thermal input to the as-built part without mechanical processing.

18. The article according to claim 17, wherein the at least one second region is configurable to be substantially recrystallized upon the heat treatment if the heat treatment is characterized by a heat treatment temperature that is higher than a second critical temperature of the at least one second region, and wherein the at least one first region is characterized by a first critical temperature that is higher than the heat treatment temperature.

19. The article according to any one of claims 17 to 18, wherein a layer of the as-built part includes the at least one first region and the at least one second region.

20. The article according to any one of claims 17 to 19, wherein one or both of the at least one first region and the at least one second region extends across one or more layers of the as-built part.

21. The article according to any one of claims 17 to 20, wherein the as-built part is characterized by a higher native geometrically necessary dislocation (GND) density of the at least one second region of the as-built part than a native GND density of the at least one first region of the as-built part.

22. The article according to claim 21, wherein the native GND density of the at least one second region is higher than a critical GND density.

23. The article according to any one of claims 17 to 22, wherein the as-built part is characterized by a larger median cell size in the at least one second region of the as-built part than in the at least one first region of the as-built part.

24. The article according to any one of claims 17 to 23, wherein the as-built part is characterized by a lower amount of solute segregation in the at least one second region than in the at least one first region.

25. The article according to any one of claims 17 to 24, wherein the at least one second region of the as-built part contains a higher twin boundary fraction than the at least one first region of the as-built part.

26. The article according to any one of claims 17 to 25 wherein the at least one second region of the as-built part contains a higher recrystallization fraction than the at least one first region of the as-built part.