The present application claims priority from the Singapore patent application no. 10202007770S, the contents of which are incorporated in entirety by reference.

TECHNICAL FIELD

[0001] The present disclosure relates to additive manufacturing, and more particularly to in-situ alloying using laser powder bed fusion.

BACKGROUND

[0002] Additive manufacturing (3D printing) to form products having an alloy material composition typically involves using a starting material that has been pre-alloyed. Currently, there is a limited range of pre-alloyed materials (e.g., in powder form) suitable for use in 3D printing. In-situ alloying refers to the forming of an alloy at the same time the product is being formed by 3D printing. In laser powder bed fusion (also known as “laser-based powder bed fusion”), this means using a mixture of powders of different materials as the starting materials. The same laser energy that is provided to form the product is also used to simultaneously bring about alloying of the different materials in-situ. It can thus be appreciated that in-situ alloying introduces additional technical challenges to laser powder bed fusion. The porosity-inclusion dilemma that arises when the materials of significantly different melting points are being alloyed together is just one of such issues that need to be addressed.

[0003] The mechanics in the melt pool are complicated by the presence of different materials in the powder bed. Porosities and inclusions are defects commonly found in a product formed by conventional laser powder bed fusion with in-situ alloying. Previous attempts to address these issues mainly focused on the search for an optimal volumetric energy density, and hopefully one at which the laser power is high enough to melt the material of higher melting point while avoiding excessive vaporization of the material of lower melting point. At the same time, when insufficient time is given for diffusion, microsegregation of elements and incomplete melting can occur, which can lead to or promote the formation of inclusions. The porosity-inclusion dilemma is also found to be aggravated under extremely fast thermal cycles. Fast thermal cycles do not allow sufficient time for the elements to diffuse, which leads to micro-segregation and the formation of inclusions.

SUMMARY

[0004] In one aspect, the present disclosure provides a method of in-situ alloying in laser powder bed fusion additive manufacturing, the method comprising: using a laser to scan at a scanning speed along a plurality of scan paths, each of the plurality of scan paths being of a scan path length oriented in a scan direction; and forming a sustained melt pool in a mixture of powders, the sustained melt pool having a net melt pool velocity in a stripe direction, the stripe direction being defined to intersect with each of the plurality of scan paths, wherein the sustained melt pool forms a plurality of ripples distributed along the stripe direction such that each of the plurality of ripples arcuately spans across a corresponding scan path.

[0005] The method above, wherein the sustained melt pool is formed as one melt pool across more than one of the plurality of scan paths.

[0006] The method above, comprising: causing the laser to scan along a first scan path to form a first melt pool such that the first melt pool extends along the first scan path; and causing the laser to scan along one or more subsequent scan paths to form one or more subsequent melt pools, each of the one or more subsequent melt pools extending along a respective subsequent scan path such that the first melt pool and at least one of the one or more subsequent melt pools are indistinct from one another, the at least one of the one or more subsequent melt pools forming at least a part of the sustained melt pool, wherein the first scan path and the one or more subsequent scan paths are spaced apart and selected from the plurality of scan paths.

[0007] The method according to any described above, wherein the scanning speed and the scan path length are configured such that a decrease in a melt pool volume of the sustained melt pool from a partial solidification of the first melt pool is concurrent with a contribution to the melt pool volume of the sustained melt pool by the one or more subsequent melt pools.

[0008] The method according to any described above, wherein the laser is configured to scan at a scanning velocity that intersects the net melt pool velocity at an angle in a range from 70 degrees to 110 degrees.

[0009] The method according to any described above, wherein the scanning velocity and the net melt pool velocity are perpendicular to one another.

[0010] The method according to any described above, wherein the magnitude of the scanning velocity is equal to or greater than the magnitude of the net melt pool velocity.

[0011] The method according to any described above, wherein the sustained melt pool has a melt pool width that is equal to or greater than the scan path length.

[0012] The method according to any described above, wherein the plurality of ripples corresponds to solidification at a trailing boundary of the sustained melt pool.

[0013] The method according to any described above, wherein the sustained melt pool is bounded by the trailing boundary and a leading boundary, the leading boundary and the trailing boundary being spaced apart in the stripe direction, and wherein the laser is proximal to the leading boundary.

[0014] The method according to any described above, further comprising: after one pass of the laser along the first scan path, displacing the laser in the stripe direction by a hatch spacing to scan along one of the subsequent scan paths.

[0015] The method according to any described above, wherein the laser is configured to scan in opposite directions along two adjacent scan paths selected from the plurality of scan paths.

[0016] The method according to any described above, wherein the laser is configured to scan in a same direction along two adjacent scan paths selected from the plurality of scan paths.

[0017] The method according to any described above, wherein the scanning speed is variable along the plurality of scan paths.

[0018] The method according to any described above, wherein the laser is configured with a top-hat profile.

[0019] The method according to any described above, wherein the stripe width is at least two times a diameter of the laser.

[0020] The method according to any described above, wherein the mixture of powders comprises at least two materials, and wherein the sustained melt pool cools to form an alloy of the at least two materials.

[0021] In another aspect, the present disclosure provides a method comprising: forming a product by additively building material on to a previously formed part of the product, the material and the previously formed part being formed by the method of in-situ alloying according to any described above, wherein the material is built up in a stripe extending perpendicularly to a build direction, and wherein a trailing boundary of the sustained melt pool cools to form an alloy on the previously formed part.

[0022] The method according to any described above further comprising: forming a plurality of stripes to form a layer of the product, each of the plurality of stripes being joined to at least one other of the plurality of stripes.

[0023] The method according to any described above, further comprising: configuring the laser to scan at a scanning speed, wherein the stripe width and the scanning speed are configured such that a time period taken by the laser to scan over a distance longer than the scan path length is shorter than a cooling time taken for the first melt pool to completely solidify.

[0024] An apparatus comprising: a powder bed configured to receive a mixture of powders; a laser source configured to provide a laser; one or more lens to focus the laser at a predefined laser diameter in the powder; a scanner operable to direct a laser to the mixture of powders in the powder bed; and a processor coupled to the laser source, the one or more lens, and the scanner, the processor being configured such that the laser source, the one or more lens, and the scanner are controllable by the processor to perform the method of in- situ alloying of the mixture of powders according to any one of the methods described above. [0025] A product comprising an alloy of at least a first material and a second material, wherein the alloy is formed according to any of the methods described above.

[0026] The product according to the above, wherein the alloy comprises no more than 10% of a remnant of any one of the first material and the second material.

[0027] The product according to any of the above, wherein the alloy comprises no more than 1% of a remnant of any one of the first material and the second material.

[0028] The product according to any of the above, wherein the alloy comprises no more than 0.01% of a remnant of any one of the first material and the second material.

[0029] The product according to any of the above, wherein the second material is characterized by a melting point equal to or higher than the first material.

BRIEF DESCRIPTION OF DRAWINGS

[0030] Figs. 1 A and IB illustrate a conventional laser powder bed fusion;

[0031] Figs. 2A and 2B illustrate an in-situ alloying via laser powder bed fusion according to an embodiment of the present disclosure;

[0032] Fig. 3 is a schematic block diagram of an apparatus configured to carry out the method of in-situ alloying;

[0033] Fig. 4 is a schematic flowchart of an embodiment of the method of in-situ alloying;

[0034] Figs. 5A and 5B illustrate examples of forming a sustained melt pool;

[0035] Figs. 6A to 6E illustrate one example of a sustained melt pool;

[0036] Fig. 7 illustrates a convention melt pool;

[0037] Figs. 8A to 8D schematically illustrate examples of scanning strategies;

[0038] Fig. 9 schematically illustrates another aspect of a scanning strategy according to an embodiment of the present disclosure; [0039] Fig. 10 is an intensity plot of a top-hat laser profile; and

[0040] Fig. 11 is a schematic diagram of a scanning strategy according to embodiments of the present disclosure.

DETAILED DESCRIPTION

[0041] 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 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.

[0042] 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.

[0043] 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.

[0044] Fig. 1 A illustrates a conventional laser powder bed fusion method of fabricating a product (100) of Fig. IB. A laser (110) is configured to scan in a scan direction (111) such that powder (102) in its path melts to form a conventional melt pool (120). As the laser continues in its scan direction (111), the conventional melt pool (120) solidifies such that solid material is added to a previously formed part (103) of the product in a direction (101) parallel to the scan direction (111). The solidification of the melt pool (120) leaves behind a pattern of ripples (132) corresponding to a trailing boundary (123) of the conventional melt pool (120). The ripples (132) indicate a melt pool velocity (131) in a direction parallel to the scan direction (111). Since there is a limit to the depth of powder that can be melted by the laser, to increase a height or thickness of the product, a new layer of powder (102) can be provided on top of a previously formed layer (104) such that layers (105) can be added on top of one another in a build direction (1 1).

[0045] Fig. 2A illustrates a method of carrying out in-situ alloying concurrently with laser powder bed fusion additive manufacturing of a product (200) of Fig. 2B, according to one embodiment of the present disclosure. A laser (210) is configured to scan at a scanning speed in a scan direction (211) along a plurality of scan paths (214) such that a sustained melt pool (220) is formed. Each of the plurality of scan paths is defined with a scan path length oriented in a scan direction (211). The scan path length may be the same between two or more of the plurality of scan paths (214), or may vary among the plurality of scan paths (214). In some examples, the scan path length may be approximated by a stripe width (S), although in practice, the scan path length may be longer or shorter than the stripe width (S), or the scan path length may be substantially the same length as the stripe width (S). Although the scan path (214) is represented as a straight line to simplify the appended drawings, each of the plurality of scan paths (214) need not be a straight line. When energy is provided by the laser (210) to the powder (202) deposited in the powder bed, an amount of powder in the scan path (214) melts to form a melt pool around the laser focal spot in the mixture of powders deposited in the powder bed. The laser focal spot in this example is at a position where the laser (210) intersects the layer of powder (202). The sustained melt pool (220) is sustained from the laser scanning along an earlier scanned scan path (214). As the sustained melt pool (220) solidifies, solid material is added to a previously formed part (204) of the product in a stripe direction (201). The stripe direction (201) is non-parallel relative to the scan direction (211). The solidification of the sustained melt pool (220) leaves behind a pattern of ripples (232) corresponding to a trailing boundary (223) of the sustained melt pool (220). The ripples (232) are indicative of a net melt pool velocity (231) in a direction non-parallel to the scan direction (211). In other words, the net melt pool velocity

(231) and the scan direction (211) intersect at a non-zero angle. The plurality of ripples

(232) formed by the sustained melt pool (220) are distributed along the stripe direction (201), with each of the plurality of ripples arcuately spanned across a corresponding scan path (214) traversed by the laser (210).

[0046] Instead of seeking an optimal volumetric energy density as conventional methods generally do, the present disclosure takes a different approach by enhancing the laser induced melt pool. The laser scanning speed (magnitude of the laser scanning velocity) can be selected to be high enough and the scan path can be defined to be short enough, such that in combination, an enhanced melt pool in the form of a sustained melt pool is formed. The present method may thus be referred to as LIME (Laser Induced Melt pool Enhancement).

[0047] To aid understanding, one of many possible embodiments of an apparatus (300) that can be configured to perform the method (400) of LIME will be described with reference to Fig. 3. To avoid obfuscation from reciting excessive details, the apparatus (300) is described as a schematic representative example. A person skilled in the art would understand that the following is a non-limiting example intended to aid understanding, and that the method (400) according to the schematic flow diagram of Fig. 4 is not limited to being performed using an apparatus identical to the schematic of Fig. 3. The apparatus (300) includes a powder bed (302) configured to receive a mixture of powders (202) from a powder delivery system (304). The powders (202) include at least two materials for the purpose of additive manufacturing a product made of an alloyed material, in which the alloyed material is formed in-situ from the powders of the at least two materials. The apparatus (300) includes a laser source (306) operable to provide a laser (210) and one or more lens (308) to focus the laser (210) at a pre-defined laser diameter in the mixture of powders (202). The apparatus (300) further includes a scanner (310) operable to direct the laser (210) to the mixture of powders (202) in the powder bed (302). The scanner (310) enables the laser focal spot to be moved relative to the powder bed (302) and thus melt the mixture of powders at different locations in the powder bed (302). In this document, the term “laser” is used interchangeably with “laser focal spot”, “laser beam”, or “laser radiation”, for the sake of brevity. The apparatus (300) includes a processor (320) that is coupled to the laser source (306), the one or more lens (308), and the scanner (310). The processor (320) may be provided with a memory storing a program, which is executed by the processor to controllably operate the apparatus (300). The laser source (306), the one or more lens (308), and the scanner (310) are controllable by the processor to perform methods of in-situ alloying of the mixture of powders according to any of the embodiments described in the present disclosure. Referring to Fig. 4, one embodiments of the method (400) include using a laser to scan a mixture of powders along scan paths intersecting a stripe direction (402), and forming a sustained melt pool having a net melt pool velocity in the stripe direction (404). The laser source (306) and the one or more lens (308) are configurable to deliver the laser (210) with a top-hat profile (1000) as shown in Fig. 10, with the laser power and the scanning speed configured to be high enough such, for the stripe width (scan path length) defined, a sustained melt pool is formed.

[0048] Embodiments of the present disclosure provide a sustained melt pool (220) in which in-situ alloying takes place concurrently with powder bed fusion additive manufacturing. As illustrated schematically in Fig. 5A, a sustained melt pool (220) is formed as one melt pool across more than one of the plurality of scan paths (2141, 2142) traversed by the laser.

[0049] For example, a first melt pool (2201) is formed by the laser scanning or traversing a first scan path (2141). Before the first melt pool is fully solidified, or before a leading boundary of the first melt pool (2201) solidifies, it is “overlapped” by one or more subsequent melt pools (2202) formed by the laser traversing or scanning along one or more subsequent scan paths (2142). In one aspect, the overlapping first melt pool (2201) and the one or more subsequent melt pools (2202) contribute yet-to-be solidified material to the sustained melt pool (220) such that the sustained melt pool has a larger volume when compared with a conventional melt pool (120) or when compared to the first melt pool (2201). In another aspect, a melt pool is described as a sustained melt pool (220) when there is common material in the melt pool even as the melt pool moves from one scan path to a subsequent scan path. In another aspect, the sustained melt pool (220) can be described as a volume of partially or fully melted material with a generally constant volume with relatively slight fluctuations, and in which the volume is larger than that of a conventional melt pool. In another aspect, the stripe width and the scanning speed may be configured such that a time period taken by the laser to travel (scan) over a distance longer than the scan path length is shorter than a cooling time taken for the first melt pool to completely solidify.

[0050] Another example is illustrated in Fig. 5B, in which a first melt pool (2201) and a subsequent melt pool (2202) do not overlap. In this example, there is sufficient thermal accumulation in the region around the first scan path of the laser (2101), such that when the laser is at the subsequent scan path, a sustained melt pool (220) is formed in which the sustained melt pool (220) is larger than the first melt pool (2201). The subsequent scan path is a distinct from the first scan path such that the laser in traversing the subsequent scan path is not re-scanning the first scan path. The sustained melt pool (220) is larger than a hypothetical second melt pool (2202) of the laser at the subsequent scan path in a situation without the effect of thermal accumulation as mentioned. In some examples, a decrease in a melt pool volume of the sustained melt pool from a partial solidification of the first melt pool is concurrent with a contribution to the melt pool volume of the sustained melt pool by the one or more subsequent melt pools. The term “concurrent” refers to the decrease in melt pool volume (partial solidification of the sustained melt pool) and the contribution to the melt pool volume (melting of new materials) overlapping in time. It is not necessary for the decrease in and/or the contribution to the melt pool volume to start and/or end at the same point in time.

[0051] In some examples, the laser may scan along a first scan path to form a first melt pool such that the first melt pool extends along the first scan path, and the laser may scan along one or more subsequent scan paths to form one or more subsequent melt pools, each of the one or more subsequent melt pools extending along a respective subsequent scan path such that the first melt pool and at least one of the one or more subsequent melt pools are indistinct from one another. At least one of the one or more subsequent melt pools forms at least a part of the sustained melt pool. The first scan path and the one or more subsequent scan paths are spaced apart and selected from the plurality of scan paths.

[0052] Figs. 6A to 6E illustrate a sustained melt pool (220) such as one formed by a method of the present disclosure. The sustained melt pool (220) has an effective melt pool velocity or a net melt pool velocity (231) in the stripe direction (201). The net melt pool velocity (231) is angularly displaced relative to the scan direction (211) of the laser, for at least a period of time over the course of additive manufacturing of the product. The scanning velocity of the laser is characterized by the scan direction (211). Unlike a conventional melt pool (Fig. 7), the net melt pool velocity (231) of the sustained melt pool (220) does not follow the scan direction (211) at all times. Except for the relatively short time intervals (e.g., when the laser is displaced by a hatch spacing or when the laser is moving from one scan track to another scan track), the scan direction (211) and the net melt pool velocity (231) are configured to intersect, i.e., they are configured to be not parallel to one another. The laser may be configured to scan at a scanning velocity (in the scan direction) that intersects the net melt pool velocity at an angle in a range from 70 degrees to 110 degrees. In some instances, the scanning velocity (211) and the net melt pool velocity (231) are configured to be perpendicular to one another. In some instances, the scanning velocity and the net melt pool velocity are substantially perpendicular, e.g., within 10% deviation of an intersection of 90 degrees. In some examples, the magnitude of the scanning velocity is equal to or greater than the magnitude of the net melt pool velocity.

[0053] The sustained melt pool (220) is bounded by a leading boundary (221) and a trailing boundary (223). The leading boundary (221) and the trailing boundary (223) are spaced apart along the stripe direction (201), in which the stripe direction (201) is approximately the same as a direction of the net melt pool velocity (231). The boundary proximal to the laser is described as the leading boundary (221). Relative to the stripe direction (201), the leading boundary (221) leads the trailing boundary (223), i.e., the leading boundary (221) is before the trailing boundary (223). The leading boundary (221) defines the interface between the sustained melt pool (220) and the yet-to-be melted materials or yet-to-be alloyed materials (202). The trailing boundary (223) defines the interface between the sustained melt pool (220) and the alloyed material (204). Fig. 6C is a cross-sectional view of the sustained melt pool (220) of Fig. 6B taken along the line A- A. The sustained melt pool (220) may be described by a melt pool length (L) measured along the stripe direction (231). The melt pool length (L) may be taken as the maximum separation between the leading boundary (221) and the trailing boundary (223). The melt pool length (L) may be the maximum length of the sustained melt pool, measured in a direction parallel to the net melt pool velocity and perpendicular to the build direction. A melt pool slope (0) may be defined as an angle tangential to the melt pool boundary, at a depth where re-melting occurs during a next layer scan. The present method enables the formation of a relatively low melt pool slope (9), which can be useful for promoting the growth of { 100} crystallographic orientation in the build direction, or for facilitating a relatively low crystal growth rate and for epitaxial crystal growth. Fig. 6D is a cross-sectional view of the sustained melt pool (220) of Fig. 6B taken along the line B-B. The sustained melt pool (220) may be described by a melt pool width (W) measured along a direction perpendicular to the stripe direction. Alternatively, the melt pool width (W) can be measured in a direction perpendicular to both the net melt pool velocity (231) and the build direction (241). The melt pool width (W) may be taken as the maximum separation between two boundaries. In some examples, the sustained melt pool has a melt pool width (W) that is equal to or greater than the scan path length. An aspect ratio may be defined by a ratio of a melt pool depth (d) to the melt pool width (W). The sustained melt pool (220) of the present disclosure (Figs. 6A to 6E) is generally characterized by a lower aspect ratio (d/W) than that of a conventional melt pool (120). As shown in Fig. 6E, the trailing boundary (223) also defines the pattern of ripples (232). That is, the ripples (232) correspond to a progressive solidification over time at the trailing boundary (223) of the sustained melt pool (220), and is indicative of the trailing boundary having a net melt pool velocity (231) generally in the stripe direction (201). A comparison between Fig. 6E and Fig. 7 shows that the orientation or the distribution of the ripples is different for a conventional melt pool and in a sustained melt pool, indicating that the net melt pool velocities are differently orientated relative to the respective scan directions.

[0054] A variety of laser scanning strategies may be utilized in conjunction with embodiments of the present disclosure. Figs. 8A to 8D, as well as Fig. 9, illustrate examples of scanning strategies to aid understanding, and these examples are not intended to be limiting or exhaustive. Referring to Fig. 8A, according to an embodiment of the present disclosure, the laser is configured to scan along a plurality of scan paths (2141, 2142). In this example, each scan path is defined with a scan path length that is generally the same as a stripe width (S). After one pass of the laser along a first scan path (2141), the laser is configured to be displaced (215) in the stripe direction (201) by a hatch spacing (H) such that the laser is in a position to start scanning along a subsequent scan path (2142). In some examples, the hatch spacing (H) may be less than two times a diameter of the laser. The laser may be configured to scan (2111) in opposite directions along two adjacent scan paths, in which the adjacent scan paths are selected from a plurality of scan paths (214). Alternatively, as shown in Fig. 8B, the laser may be configured to scan (2111) in a same direction along two adjacent scan paths selected from the plurality of scan paths (214). The laser may be configured with a scanning speed (211) that is constant or variable along the plurality of scan paths (214). The term “subsequent scan path” refers to a scan path (2142) scanned by the laser at a later point in time and is not limited to the scan path immediately following or immediately adjacent to a first scan path (2141). Hence, there may be a plurality of subsequent scan paths (2142), each being scanned at different times after the first scan path (2141) is scanned by the laser. The term “first scan path” is used for the sake of brevity and refers to a scan path selected from the plurality of scan paths. The hatch spacing (H) between adjacent scan paths (214) may be varied on the same layer (205), as illustrated in Fig. 8C. As illustrated in Fig. 8D, the stripes defined on a same layer (205) may vary in orientation (201) and/or in the size of the stripe width (S), and the stripe spacing (spacing between adjacent stripes) may be varied as well. Fig. 8D also schematically illustrates that the length of a scan path (214) may be different from the stripe width (S). The scan path length of at least one of the plurality of scan paths may be longer or shorter than the stripe width (S), or of substantially the same as the stripe width (S). The actual path (214) travelled by the laser may be a continuous serpentine path as illustrated in Fig. 8D, or the actual path travelled by the laser may be a series of disconnected scan paths (214, 215) as shown in Figs. 8A to 8C.

[0055] Embodiments of the present disclosure are not limited in terms of the number of layers (205) formed in the build direction (241). Fig. 9 shows an example in which a plurality of layers (205) forms a product (910), and in which each layer (205) is formed by a plurality of stripes (280). Different stripe directions (201) may be defined for different layers (205), for example, by defining a different orientation (900) for the stripe direction or the net melt pool velocity. The method can thus be used to form a product by additively building material on a previously formed part of the product, in which the material and the previously formed part are formed by the method of in-situ alloying. The material is added on stripe (280) by stripe (280) as a trailing boundary of the sustained melt pool cools to form an alloy on the previously formed part, with each stripe (280) extending in a stripe direction (201). The stripe direction (201) may be defined to extend in a horizontal plane or in a plane normal to the build direction (241). A layer (205) of the product may be formed by a plurality of stripes (280), in which the stripes are parallel, and in which each of the plurality of stripes is joined to at least one other of the plurality of stripes (280).

[0056] The present method may be carried out on a mixture of powders including at least two materials, to form an alloy of the at least two materials. The at least two materials may be selected from any metal, ceramic, polymer, or any combination thereof. The at least two materials may include an alloy, an unalloyed material, and/or a combination thereof. The product formed may be described as including an alloy of at least a first material and a second material. If the second material is characterized by a higher melting point than the first material, the alloy thus formed can be configured to include no more than 50%, 10%, 1% or even 0.01% of a remnant of the second material. In actual practice, the user may of course choose to use the present method to produce an alloy having a relatively high percentage (e.g., 50%) of the intended reactants. A person skilled in the art would appreciate that the present method (LIME) advantageously enables the user greater flexibility and efficiency in using laser powder bed fusion to perform in-situ alloying. LIME also enables a greater range of alloyed material composition using various combinations of unalloyed powders and/or alloyed powders as starting materials (reactants). Advantageously, LIME also enables more efficient in-situ alloying for products with relatively low percentage of internal defects (e.g., remnants). To illustrate, non-limiting examples will be described with reference to in-situ alloying via laser powder bed fusion of a mixture of titanium powder and niobium powder, to form a product of titanium-niobium alloy. Niobium is characterized by a higher melting point than titanium. The quality of the resulting alloy may be measured in terms of the remnant niobium present in the resulting alloy. For the sake of brevity, the term “remnant” is used in the present disclosure to refer generally to a type of internal defects, such as an un-melted amount of any one or more of the starting materials (reactants), an amount of any one or more of the intended reactants, and/or any other undesired inclusions resulting from one or more of the intended reactants.

[0057] In one example, the method (400) of in-situ alloying via laser powder bed fusion includes providing a laser from a laser source and directing the laser to the powder bed during a laser powder bed fusion process to form a sustained melt pool. Fig. 11 illustrates a scanning strategy (1100) characterized by relatively short stripe widths (1102), high scan speed (1104), high laser power (1106), and low intensity laser profile (1108). These aspects of the scanning strategy need not be set in sequence or in any particular order. In particular, the effectiveness of the present method is demonstrated through in-situ alloying of Ti-34Nb (wt%) via laser powder bed fusion using a mixture of elemental powders. The laser may be configured with a laser power greater than 500 W (watt) for titanium-based alloy systems. The laser is preferably one configured with a top-hat profile (such as one of Fig. 10). In this document, the term “top-hat profile” refers to any laser profile with a maximum intensity that is lower than the maximum intensity of a laser with a Gaussian profile, for the same laser power. The laser (210) is controllably made to scan across the powder bed (302) from a first start point to an end point to form a first stripe. The first stripe includes a first group of multiple single scan paths (214). The multiple scan paths may be arranged parallel to one another. The scan paths may be in a serpentine manner or a wave manner. The scan paths may be continuous (with the laser continuing to scan when moving from one scan path to another) or discontinuous (with the laser not interacting with the powder/material when moving from one scan path to another). The laser may be configured with a scanning velocity greater than 500 mm/s (millimeter per second). The scan path length (length of one scan path) can be approximated by the stripe width (S). The scanning strategy and the laser are configured such that a melt pool velocity ratio (V _m , LIME/ V _m , conventional) is smaller than 1. The melt pool velocity ratio is defined by Equation (1) below: and where V _{m LIME} is the effective velocity of a sustained melt pool according to LIME; V _{L LIME} is the laser scanning velocity according to LIME; m, conventional is the velocity of a conventional melt pool;

VL, conventional is the laser scanning velocity according to a conventional method;

H is the hatch spacing; and

S is the stripe width.

[0058] Simulation results demonstrate that the present method enables formation of a sustained melt pool with a melt pool velocity ratio less than 1, in which the melt pool velocity is substantially perpendicular to the scan direction of the laser when the laser is scanning along any one of the scan paths. At laser power of 950 W, a sustained melt pool with a melt pool width of about 1167 pm (micrometer), a melt pool length of about 706 pm, and a melt pool depth of about 180 pm can be formed using the present method. For comparison, a conventional melt pool of similar materials (titanium and niobium) has a width of about 214 pm, a length of about 600 pm, and a depth of about 119 pm. Comparing a sustained melt pool formed at a laser power of 950 W with a conventional melt pool formed at a laser power of 350 W, the temperature gradient in the sustained melt pool is comparatively more gradual.

[0059] In another non-limiting example, a sustained melt pool was successfully formed by a scanning strategy in which a top-hat laser profile with a laser power greater than 650 W was configured with a scanning velocity (scanning speed) of 650 mm/s, and a short stripe width scanning strategy as described above. The stripe width was defined as 1 mm and the scan paths are separated by a hatch spacing of 80 pm. Each layer was configured with a thickness of 50 pm. Energy density was 250 W/mm ³ (Watts per cubic millimeter).

[0060] Traditionally, a high scanning speed is associated with a corresponding high melt pool velocity. The present method counter-intuitively provides a relatively slow net melt pool velocity while causing the laser to scan at a scanning speed that is set as high as the apparatus permits or as high as possible without causing any unwanted internal defects. Internal defects may include voids (e.g., keyhole cavities or other undesired porosities) or remnants (e.g., inclusions), etc. The LIME method can thus enable a product to be built in a significantly shorter time, compared using a conventional laser powder bed fusion method to build the same product with a similar or superior level of alloy quality. The LIME method produces significantly fewer internal defects in a given mass of alloy formed in one pass of the laser, in comparison to a conventional method. For example, the LIME method has experimentally demonstrated the formation of a product including an alloy with no more than 0.01% of a remnant of any one of the intended reactants. For example, the LIME method enables the formation of a product including an alloy with no more than 1% of a remnant of any one of the intended reactants. For example, the LIME method enables the formation of a product including an alloy with no more than 10% of a remnant of any one of the intended reactants. For example, the LIME method enables the formation of a product including an alloy with no more than 50% of a remnant of any one of the intended reactants. The LIME method is therefore a high-speed, high-efficiency process, compared to conventional methods. Scanning electron microscope (SEM) images of the titaniumniobium (Ti-Nb) alloy samples formed using the present method confirmed the absence of lack-of-fusion porosity and the absence of keyhole-induced porosity. At the same time, the amount of inclusions or un-melted elemental materials in the Ti-Nb alloy samples formed is significantly less than that in a conventional sample. For example, in the experiments with laser power at 650 W and at 950 W, the volume % of un-melted Nb is about 0.006% and about 0.002% respectively. This represents a significant improvement over the 0.028% un- melted Nb found in a conventional Ti-Nb sample. This suggests that by providing a sustained melt pool according to embodiments of the present disclosure, it is possible to improve the build rate while getting around issues typically associated with the porosityinclusion dilemma. Using the present method, the melt pool width of the sustained melt pool can be approximately equal to or larger than the stripe width (S). The melt pool width of the sustained melt pool can be several times larger than the width of a conventional melt pool. Due to the relatively higher scanning speed and constant hatch spacing, the build rate can be much higher in the present method. At the same time, a combination of the relatively large volume and low net melt pool velocity of the sustained melt pool promotes homogenization by promoting conditions that do not favor localized melting. In some instances, the present method can be described as one that prevents localized melting. The temperature gradient suggests that the rate of cooling is slower in the sustained melt pool, such that there is sufficient time for the elements to distribute more homogeneously in the sustained melt pool. The present method also enables formation of a sustained melt pool that has a relatively lower aspect ratio (d/W) and lower net melt pool velocity, compared to a conventional melt pool. The relatively lower aspect ratio and net melt pool velocity of the sustained melt pool enables formation of an alloy with significantly fewer or no keyhole induced porosity. The sustained melt pool seems to have keyholes that are more stable, i.e., less prone to collapsing and less likely to form shut pores. At the same time, there is less or minimal pore formation upon solidification of the melt pool.

[0061] In another non-limiting example, such as when the mixture of powders includes magnesium, the laser may be configured with a laser power equal to or less than 500 W, and preferably below 100 W, with a scanning speed of 1000 mm/s. The laser is configured with a profile that has a lower maximum irradiance than a Gaussian profile, for a given laser spot diameter and laser power. For example, the laser profile used may be a super-Gaussian profile. Since the present method can get around the porosity-inclusion dilemma, it enables the alloying of materials with large disparity in melting points.

[0062] Some alloyed materials are not easily atomized into a powder form suitable for use in 3D printing, and this has traditionally limited the range of pre-alloyed powders available for use in 3D printing. Embodiments of the present disclosure circumvent this problem by enabling a significantly wider range of materials to serve as the starting materials (reactants) without compromising on the alloy composition obtained. Besides in-situ alloying, the method may also be used to enable more efficient laser powder bed fusion manufacturing of single crystal or columnar grained components (including highly columnar grains) or compositionally graded components.

[0063] 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.