A method of laser powder bed fusion forming of a directional columnar grain structure alloy

Abstract

The present application belongs to the technical field of metal additive manufacturing and solidification structure regulation, and particularly relates to a laser powder bed fusion forming method of a directional columnar grain structure alloy. The present application adopts a flat-top beam or a point-ring beam as a forming laser beam, inputs a single-channel three-dimensional transient heat transfer solidification model, obtains a molten pool width, a molten pool depth, a temperature gradient, a solidification rate and a columnar grain-equiaxed grain transition determination result, screens laser power and scanning speed according to the molten pool morphology with a width-depth ratio W / D>3; sets a scanning interval according to the deviation area of the molten pool side temperature gradient vector relative to the preset columnar grain growth direction, so that the overlapping area of adjacent melt channels covers different orientation dendrites or grains on the side; sets the powder layer thickness according to the sensitive area of the columnar grain-equiaxed grain transition at different heights, so that the remelting of the subsequent layer molten pool eliminates the mixed crystals, equiaxed crystals or non-target oriented grains on the top of the previous layer molten pool, and a columnar grain structure metal alloy component is obtained through continuous directional growth across the melt channel and across the layer.

Description

Technical Field

[0001] This invention belongs to the field of metal additive manufacturing and solidification structure control technology, specifically relating to a laser powder bed melting method for an alloy with directional columnar grain structure. Background Technology

[0002] Laser powder bed fusion (LPBF) is a typical metal additive manufacturing technology. This technology typically involves layering metal powder onto a substrate and selectively melting the powder layers using a laser beam along a predetermined path. The molten metal cools and solidifies to form the current layer, and the process of layering, scanning, and solidification is repeated to obtain metal components. LPBF features high forming accuracy, high material utilization, and suitability for near-net-shape forming of complex structures, and has been applied in aerospace, energy and power, mold manufacturing, and high-end equipment industries.

[0003] For materials such as stainless steel, high-temperature alloys, and high-entropy alloys, the solidification morphology, crystallographic orientation, and grain boundary distribution directly affect the strength, plasticity, fatigue performance, creep performance, and service reliability of components. For some load-bearing components and high-temperature service components, if columnar grains can grow continuously along a predetermined direction, transverse grain boundaries and discontinuous grain boundaries can be reduced, the continuity of the microstructure can be improved, and the service performance of the component in a specific direction can be enhanced.

[0004] During LPBF forming, the molten pool morphology, temperature gradient, solidification rate, and heat flow direction are jointly influenced by factors such as the spatial energy distribution of the forming laser beam, laser power, scanning speed, scanning spacing, and powder layer thickness. Traditional Gaussian beams concentrate energy at the center of the spot, easily forming a deep molten pool and complex heat flow distribution. Beams with specific spatial energy distributions, such as flat-top beams and dot-ring beams, can change the energy input mode at the processing plane, thereby adjusting the molten pool aspect ratio, molten pool bottom morphology, temperature gradient direction, and solidification structure evolution.

[0005] The formation of oriented columnar grain structure depends not only on whether the favorable orientation of columnar crystal regions can be preserved, but also on whether the unfavorable structures at the top and sides of the molten pool can be promptly remelted and eliminated. Specifically, the top of the molten pool, being in the late stage of solidification, is prone to entering the columnar-equiaxed crystal transformation sensitive range, forming impurities, equiaxed crystals, or non-target oriented grains. While the sides of the molten pool may still exhibit a columnar morphology, the local temperature gradient vector is prone to deviating from the predetermined columnar crystal growth direction, thus forming dendrites or grains with different orientations.

[0006] If the impurities, equiaxed grains, or non-target oriented grains at the top of the molten pool cannot be effectively remelted by the next layer of molten pool, or if dendrites or grains of different orientations on the side of the molten pool cannot be effectively covered by the overlapping remelting area of ​​the adjacent melt channel, the aforementioned unfavorable structures will remain between layers or between melt channels, hindering the continuous epitaxial growth of columnar grains across layers and across melt channels, and may lead to orientation deviations, competing growth grains, impurity zones, or discontinuous columnar structures.

[0007] Existing technologies include schemes for adjusting the thermal field of the molten pool using homogenized beams, ring beams, composite beams, or multimodal lasers. There are also schemes that utilize thermodynamic calculations, columnar-equiaxed crystal transformation criteria, intermittent deposition, or additional remelting to promote columnar crystal growth. However, these schemes typically focus on one aspect of beam form, heat input adjustment, interlayer cooling, or equiaxed crystal remelting, and have not yet formed a unified process design method based on single-pass molten pool heat transfer and solidification simulation. This makes it difficult to simultaneously achieve coordinated control over molten pool morphology screening, remelting of dendrites or grains with different orientations on the sides of the molten pool, and remelting of impurity crystals at the top of the molten pool. Summary of the Invention

[0008] In view of this, the purpose of the present invention is to provide a laser powder bed melting method for alloys with oriented columnar grain structure. The method provided by the present invention can solve the problems of difficulty in synchronously controlling impurities at the top of the melt pool, dendrites or grains with different orientations on the sides of the melt pool, and insufficient stability of continuous oriented growth of columnar grains across the melt channel and across layers in the existing laser powder bed melting method.

[0009] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a laser powder bed melting method for forming alloys with oriented columnar grain structures, comprising the following steps: A flat-top beam or a dot-ring beam is used as the forming laser beam. Its spatial energy distribution is input into a single-channel three-dimensional transient heat transfer solidification model as a moving heat source. The temperature field and flow state of the molten pool under different laser power, scanning speed, powder layer thickness and spot size are calculated. The molten pool width W, molten pool depth D, solid-liquid interface temperature gradient G, local solidification rate R, temperature gradient vector and columnar crystal to equiaxed crystal transformation curve are obtained. Laser power and scanning speed with a width-to-depth ratio (W / D) greater than 3 are selected based on the molten pool morphology to promote grain growth along the preferred orientation of

[001] . The scanning interval is set according to the deviation area of ​​the local temperature gradient vector on the side of the molten pool from the preset columnar crystal growth direction, so that the overlapping remelting area formed by adjacent melt channels covers the remelting area on the side of the molten pool. The thickness of the powder layer is set according to the columnar crystal to equiaxed crystal transformation curve at different heights, so that the subsequent molten pool enters the interlayer remelting area formed by the solidified area of ​​the previous layer and covers the top of the molten pool to be remelted. Laser powder bed melting is performed on metal alloy powder according to the laser power, scanning speed, scanning spacing and powder layer thickness determined above, to obtain an alloy with directional columnar grain structure that grows continuously across the melting channel and across layers.

[0010] Preferably, the metal alloy powder is one of stainless steel, high-temperature alloy, and high-entropy alloy; the high-temperature alloy includes nickel-based high-temperature alloy or cobalt-based high-temperature alloy; the stainless steel includes 15-5 pH precipitation-hardening martensitic stainless steel; and the high-entropy alloy includes CoCrFeNi-based high-entropy alloy.

[0011] Preferably, the wavelength of the shaped laser beam is 800~1200nm, the power is 300~2000W, and the equivalent size of the spot is 50~500μm.

[0012] Preferably, the power ratio of the central energy region to the annular energy region of the dot-ring beam is 20~40:60~80.

[0013] Preferably, the simulation parameter scanning range of the single-channel three-dimensional transient heat transfer solidification model includes: laser power of 500~2000W, step size of 100W; scanning speed of 200~2000mm / s, step size of 100mm / s; powder layer thickness of 30μm, 50μm, 70μm or 90μm, step size of 20μm.

[0014] Preferably, the deviation angle of the local temperature gradient vector on the side of the molten pool relative to the preset columnar crystal growth direction is... θ G according to It is confirmed that, among them, This represents the local temperature gradient vector at the solid-liquid interface of the molten pool. The unit vector for the preset columnar crystal growth direction; θ G The continuous region with an angle greater than the preset deviation threshold is determined as the remelting area on the side of the molten pool.

[0015] Preferably, the deviation angle threshold of the preset columnar crystal growth direction θ G ≤20°.

[0016] Preferably, if the width of the molten pool is W, the scanning interval is H, and the width of one side of the remelting area on the side is Bs, then the scanning interval H is set such that WH≥Bs, so that the overlapping remelting area formed by adjacent molten channels covers the remelting area on the side of the molten pool.

[0017] Preferably, the scanning interval is 200~400μm.

[0018] Preferably, the columnar crystal to equiaxed crystal transformation curve is obtained by a judgment index Ψ, and the judgment index Ψ satisfies... Where G is the temperature gradient at the solid-liquid interface, R is the local solidification rate, and n is the material-related index determined based on the solidification behavior of the metal alloy to be formed; and K is the critical value of the columnar-equiaxed crystal transformation based on material calibration. CET <Ψ, determine the sensitive region for columnar crystal to equiaxed crystal transformation, that is, the region to be remelted at the top of the molten pool.

[0019] This invention provides a laser powder bed melting method for forming alloys with oriented columnar grain structures, comprising the following steps: A flat-top beam or a dot-ring beam is used as the forming laser beam. Its spatial energy distribution is input into a single-channel three-dimensional transient heat transfer solidification model as a moving heat source. The temperature field and flow state of the molten pool under different laser power, scanning speed, powder layer thickness and spot size are calculated. The molten pool width W, molten pool depth D, solid-liquid interface temperature gradient G, local solidification rate R, temperature gradient vector and columnar crystal to equiaxed crystal transformation curve are obtained. Laser power and scanning speed with a width-to-depth ratio (W / D) greater than 3 are selected based on the molten pool morphology to promote grain growth along the preferred orientation of

[001] . The scanning interval is set according to the deviation area of ​​the local temperature gradient vector on the side of the molten pool from the preset columnar crystal growth direction, so that the overlapping remelting area formed by adjacent melt channels covers the remelting area on the side of the molten pool. The thickness of the powder layer is set according to the columnar crystal to equiaxed crystal transformation curve at different heights, so that the subsequent molten pool enters the interlayer remelting area formed by the solidified area of ​​the previous layer and covers the top of the molten pool to be remelted. Laser powder bed melting is performed on metal alloy powder according to the laser power, scanning speed, scanning spacing and powder layer thickness determined above, to obtain an alloy with directional columnar grain structure that grows continuously across the melting channel and across layers.

[0020] Beneficial effects: This invention introduces the spatial energy distribution of a flat-top beam or a dot-ring beam into the simulation of single-channel molten pool heat transfer and solidification, obtaining the molten pool morphology, the internal temperature gradient distribution characteristics of the molten pool, and the columnar-equiaxed crystal transformation curve, and accordingly determines the laser power, scanning speed, scanning spacing, and powder layer thickness; by overlapping and remelting the molten channels, dendrites or grains with different orientations on the sides of the molten pool are eliminated, and by interlayer remelting, impurities at the top of the molten pool are eliminated, finally obtaining a columnar grain structure metal alloy component with continuous directional growth across molten channels and layers. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the laser powder bed melting forming method for the alloy with oriented columnar grain structure of the present invention; Figure 2 This is a schematic diagram of the spatial energy distribution of a flat-top beam and a dot-ring beam at the machining plane. Figure 3 This diagram illustrates the molten pool width W, molten pool depth D, and molten pool width-to-depth ratio W / D in single-channel molten pool heat transfer solidification simulation and single-channel scanning verification. Figure 4 A schematic diagram and orientation characterization diagram showing how the heat flow direction of the flat molten pool promotes the growth of grains along the preferred orientation of

[001] . Figure 5 A schematic diagram of molten pool overlap remelting based on the temperature gradient distribution characteristics inside the molten pool to determine the remelting area on the side of the molten pool and set the scanning interval. Figure 6 This is a schematic diagram illustrating how the present invention determines the remelting area at the top of the molten pool based on the columnar crystal to equiaxed crystal transformation curves at different heights and controls interlayer remelting by adjusting the thickness of the powder layer. Figure 7 This is a metallographic optical microscope image of the IN738LC alloy processed and formed according to Embodiment 1 of the present invention; Figure 8 This is a metallographic optical microscope image of the MAR-M509 alloy processed and formed in Embodiment 2 of the present invention; Figure 9 This is a metallographic optical microscope image of the 15-5 PH precipitation-hardening martensitic stainless steel alloy processed in Example 3 of the present invention. Figure 10 This is a scanning electron microscope image of the IN738LC alloy processed and formed in Example 4 of the present invention; Figure 11 This is a metallographic optical microscope image of the IN738LC alloy processed and formed in Comparative Example 1 of this invention; Figure 12 This is a metallographic optical microscope image of the 15-5 PH precipitation-hardening martensitic stainless steel alloy processed in Comparative Example 2 of this invention. Detailed Implementation

[0022] This invention provides a laser powder bed melting method for forming alloys with oriented columnar grain structures, comprising the following steps: A flat-top beam or a dot-ring beam is used as the forming laser beam. Its spatial energy distribution is input into a single-channel three-dimensional transient heat transfer solidification model as a moving heat source. The temperature field and flow state of the molten pool under different laser power, scanning speed, powder layer thickness and spot size are calculated. The molten pool width W, molten pool depth D, solid-liquid interface temperature gradient G, local solidification rate R, temperature gradient vector and columnar crystal to equiaxed crystal transformation curve are obtained. Laser power and scanning speed with a width-to-depth ratio (W / D) greater than 3 are selected based on the molten pool morphology to promote grain growth along the preferred orientation of

[001] . The scanning interval is set according to the deviation area of ​​the local temperature gradient vector on the side of the molten pool from the preset columnar crystal growth direction, so that the overlapping remelting area formed by adjacent melt channels covers the remelting area on the side of the molten pool. The thickness of the powder layer is set according to the columnar crystal to equiaxed crystal transformation curve at different heights, so that the subsequent molten pool enters the interlayer remelting area formed by the solidified area of ​​the previous layer and covers the top of the molten pool to be remelted. Laser powder bed melting (LPBF) is performed on the metal alloy powder according to the laser power, scanning speed, scanning spacing and powder layer thickness determined above, to obtain an alloy with directional columnar grain structure that grows continuously across the melt channel and across layers.

[0023] Unless otherwise specified, the present invention does not have special requirements on the source of raw materials used, and commercially available products well known to those skilled in the art can be used.

[0024] In one embodiment, the metal alloy powder is one of stainless steel, high-temperature alloy, and high-entropy alloy; the high-temperature alloy includes nickel-based high-temperature alloy or cobalt-based high-temperature alloy, specifically IN738LC nickel-based high-temperature alloy powder or MAR-M509 cobalt-based high-temperature alloy powder; the stainless steel includes 15-5 PH precipitation-hardening martensitic stainless steel; the high-entropy alloy includes CoCrFeNi-based high-entropy alloy. Before laser powder bed melting (LPBF) forming, the metal alloy powder can be dried and formed in an argon, nitrogen, or other inert atmosphere.

[0025] In one implementation, the spatial energy distribution of the forming laser beam at the processing plane serves as the heat source input for heat transfer and solidification simulation; the flat-top beam refers to a forming laser beam with a continuous platform energy distribution region at the processing plane; the dot-ring beam refers to a forming laser beam with a central energy region and an annular energy region distributed around the central energy region at the processing plane. Figure 2 As shown, the flat-top beam is used to provide a more uniform in-plane energy input, and the point-ring beam is used to form a center-ring composite energy input. The shaped laser beam can be directly output from the laser, or it can be obtained after beam shaping, beam combining, or optical modulation.

[0026] In one embodiment, the wavelength of the forming laser beam is 800~1200nm, specifically 1070nm; the power is 300~2000W, specifically 600~1600W; and the equivalent spot size is 50~500μm, specifically 400~500μm. Specific values ​​can be determined based on the absorptivity of the metal alloy material, powder particle size, powder layer thickness, equipment capacity, and the morphology of the target molten pool.

[0027] In one embodiment, the forming substrate used in the laser powder bed melting forming is one of stainless steel, high-temperature alloy, and high-entropy alloy.

[0028] As one implementation method, a single-channel three-dimensional transient heat transfer solidification model is established. The spatial energy distribution of the forming laser beam at the processing plane is used as the input of the moving heat source. The temperature field and flow state of the molten pool under different laser power, scanning speed, powder layer thickness and spot size are calculated. The molten pool width W, molten pool depth D, solid-liquid interface temperature gradient G, local solidification rate R, temperature gradient vector and columnar crystal to equiaxed crystal transformation curve are extracted.

[0029] As one implementation method, the simulation parameter scanning range of the single-channel three-dimensional transient heat transfer and solidification model includes: laser power of 500~2000W, specifically 600~1600W in this embodiment, with a step size of 100W; scanning speed of 200~2000mm / s, specifically 300~2000mm / s in this embodiment, with a step size of 100mm / s; powder layer thickness of 30μm, 50μm, 70μm or 90μm, with a step size of 20μm; equivalent spot size of 100~500μm, with a step size of 100μm; and the power ratio of the central energy region to the annular energy region of the point-ring beam is 20~40:60~80, with another implementation method being 20:80, 30:70 or 40:60, specifically 30:70 in this embodiment.

[0030] As one implementation method, IN738LC nickel-based superalloy powder and a 400μm flat-top beam were selected. The simulated scanning range was a laser power of 600~1600W, a scanning speed of 300~2000mm / s, and powder layer thicknesses of 30μm, 50μm, 70μm, and 90μm. Based on the combined results of W / D, melt pool stability, temperature gradient direction, and microstructure verification, the usable parameter range for satisfying a W / D greater than 3 was determined to be a laser power of 800~1300W and a scanning speed of 300~800mm / s, with the preferred range being a laser power of 800~1000W and a scanning speed of 400~500mm / s. Using MAR-M509 cobalt-based high-temperature alloy powder and a 400μm flat-top beam, the usable parameter range for a W / D ratio greater than 3 is 900~1300W laser power and 300~800mm / s scanning speed, with a preferred range of 900~1100W laser power and 400~500mm / s scanning speed. Using 15-5PH precipitation-hardening martensitic stainless steel powder and a 400μm flat-top beam, the usable parameter range for a W / D ratio greater than 3 is 900~1500W laser power and 300~800mm / s scanning speed, with a preferred range of 1100~1300W laser power and 400~600mm / s scanning speed.

[0031] As one implementation, the single-channel three-dimensional transient heat transfer solidification model is a three-dimensional transient heat transfer-flow coupling model that considers the powder layer, the forming substrate, the flow of molten metal and the latent heat of phase change, and uses the fluid volume method to track the free surface; the spatial energy distribution of the forming laser beam at the processing plane is input into the model in the form of a surface heat source that moves with the scanning path.

[0032] As one implementation method, the spatial energy distribution of the flat-top beam or dot-ring beam at the processing plane is input into a single-channel three-dimensional transient heat transfer solidification model. This single-channel three-dimensional transient heat transfer solidification model is used to calculate the internal temperature distribution of the molten pool under different laser power, scanning speed, and powder layer thickness conditions, and to obtain the molten pool morphology, internal temperature gradient distribution characteristics, solidification rate, and the columnar-equiaxed crystal transformation curve of the corresponding material.

[0033] This invention determines the laser power, scanning speed, scanning interval, and powder layer thickness based on single-pass molten pool heat transfer solidification simulation. Specifically, it includes: using the spatial energy distribution of the forming laser beam at the processing plane as the heat source input, calculating the internal temperature distribution of the molten pool under different laser power, scanning speed, and powder layer thickness conditions, and obtaining the columnar-equiaxed crystal transformation curve, internal temperature gradient distribution characteristics, and molten pool morphology of the corresponding material.

[0034] As one implementation method, the laser power and scanning speed are determined based on the molten pool morphology, so that the ratio of molten pool width W to molten pool depth D, W / D, is greater than 3 (4~6 in specific embodiments), forming a flat molten pool; the width W and depth D of the single-pass molten pool are obtained through heat transfer solidification simulation, and are calibrated or experimentally verified through the molten pool cross-section formed by single-pass scanning.

[0035] like Figure 3 As shown, the molten pool width W and depth D are determined based on the single-pass molten pool heat transfer and solidification simulation results, and the width-to-depth ratio W / D is calculated. To form a flat molten pool that is conducive to the directional growth of columnar grains, a parameter combination with W / D greater than 3 is selected as the basic forming parameters. The molten pool width W and depth D can be verified by the molten pool cross-section formed by a single-pass scan.

[0036] As one implementation method, for a flat molten pool that satisfies W / D greater than 3, the heat flow direction of the bottom or central region of the molten pool is highly consistent with the construction direction, which is conducive to the preferential orientation growth of grains along

[001] , reduces lateral growth of grains, and suppresses orientation deviation and impurity crystal formation caused by competitive growth.

[0037] like Figure 4As shown, when the molten pool has a flat morphology, the heat flow direction and temperature gradient direction G in the bottom or central region of the molten pool are highly consistent with the construction direction BD, which is conducive to the preferential orientation growth of grains along

[001] . Compared with deep and narrow molten pools or molten pools with obvious lateral heat flow, flat molten pools can reduce laterally grown grains, reduce orientation deviations and the tendency of impurity crystal formation caused by competitive growth, and preserve the target orientation columnar crystal region for subsequent epitaxial growth.

[0038] As one implementation method, the scanning interval is set based on the temperature gradient distribution characteristics inside the molten pool. The scanning interval should be set so that the overlapping remelting area formed by adjacent melt channels covers the area to be remelted on the side of the molten pool. That is, the overlapping remelting area formed by adjacent melt channels covers dendrites or grains of different orientations on the side of the molten pool, while reserving the target orientation columnar crystal area for subsequent epitaxial growth. If the scanning interval is too large, dendrites or grains of different orientations on the side may be left behind; if the scanning interval is too small, the target orientation columnar crystal area may be over-remelted, which is not conducive to stable epitaxial growth.

[0039] In one implementation, the powder layer thickness is set based on the columnar-equiaxed crystal transformation curves at different heights of the molten pool. The powder layer thickness should be set so that the interlayer remelting region of the subsequent molten pool covers the top remelting region of the molten pool. This means that the interlayer remelting region formed by the subsequent molten pool entering the solidified region of the previous layer covers the impurities, equiaxed crystals, or non-target oriented grains at the top of the molten pool, while preserving the stable growth region of columnar crystals located below the impurities at the top of the previous molten pool. Thus, the subsequent molten pool can use the target-oriented columnar crystals retained in the previous layer as the epitaxial growth matrix, promoting continuous cross-layer growth of columnar grains.

[0040] In one embodiment, the powder layer thickness is 50 μm.

[0041] like Figure 6 As shown, the powder layer thickness is set based on the columnar-equiaxed crystal transformation curves at different heights. Specifically, the region at the top of the molten pool that meets the sensitive conditions for the columnar-equiaxed crystal transformation is identified and designated as the remelting region at the top of the molten pool. By setting the powder layer thickness, the interlayer remelting region formed by the solidified region of the previous layer is covered by the subsequent molten pool layer, thereby eliminating impurities, equiaxed crystals, or non-target oriented grains at the top of the previous molten pool layer.

[0042] This invention uses determined laser power, scanning speed, scanning spacing and powder layer thickness for LPBF forming, so that adjacent melt channels remelt dendrites or grains with different orientations on the sides of the melt pool, and the remelting of the subsequent melt pool eliminates impurities, equiaxed crystals or non-target orientation grains at the top of the previous melt pool, thereby obtaining a columnar grain structure metal alloy component with continuous directional growth across melt channels and layers.

[0043] In one implementation, the columnar crystal to equiaxed crystal transformation curve is obtained through a judgment index Ψ, which satisfies... Where G is the temperature gradient at the solid-liquid interface, R is the local solidification rate, and n is a material-related index determined based on the solidification behavior of the metal alloy material to be formed; when the metal alloy powder is IN738LC nickel-based superalloy, n is 3.4, and K... CET 2.7×10 24 K 3.4 ·m -4.4 ·s; When the metal alloy powder is MAR-M509 cobalt-based high-temperature alloy, n is 2.8, K CET Dimensions are 2.0 × 10⁻⁶ 24 K 2.8 ·m -3.8 •s; When the metal alloy powder is 15-5 pH precipitation-hardening martensitic stainless steel, n is 3.02, K CET Dimension 2.5×10 27 K 3.02 ·m -4.02 ·s; Based on the aforementioned judgment index Ψ and the critical value K for the columnar-equiaxed crystal transformation calibrated by material calibration. CET The size relationship between them (K) CET <Ψ), to determine the columnar crystal to equiaxed crystal transformation sensitive region, that is, the remelting region at the top of the molten pool.

[0044] like Figure 5 As shown, after determining the basic forming parameters, the scanning interval is set according to the temperature gradient distribution characteristics inside the molten pool. Specifically, the degree of deviation of the local temperature gradient vector on the side of the molten pool from the preset unit vector of columnar crystal growth direction is identified, and the deviation angle is set accordingly. θ G A continuous region with an angle greater than the preset deviation threshold is defined as the remelting area on the side of the molten pool. By setting the scanning interval, the overlapping remelting area formed by adjacent molten channels covers the remelting area on the side, thereby remelting dendrites or grains with different orientations on the side.

[0045] As one implementation, the deviation angle of the local temperature gradient vector on the side of the molten pool relative to the preset columnar crystal growth direction is... θ G according to It is confirmed that, among them, This represents the local temperature gradient vector at the solid-liquid interface of the molten pool. The unit vector for the preset columnar crystal growth direction; θ G A continuous region with an angle greater than a preset deviation angle threshold is defined as the remelting area on the side of the molten pool, where the preset deviation angle threshold is the columnar crystal growth direction. θ G≤20°, another implementation is 15~20°, and in a specific embodiment it is 15°.

[0046] As one implementation method, let the width of the molten pool be W, the scanning interval be H, and the width of one side of the side area to be remelted be Bs. Then, the scanning interval H is set such that WH≥Bs, so that the overlapping remelting area formed by adjacent molten channels covers the side area to be remelted. The scanning interval is 200~400μm, and in a specific embodiment it is 200μm or 250μm.

[0047] In one implementation, the preset columnar crystal growth direction is the construction direction of additive manufacturing, or the preset stress direction or local service direction of the metal alloy component.

[0048] In one implementation, the present invention verifies the molten pool width W, molten pool depth D, and W / D through a single-pass scanning experiment; verifies whether the overlapping remelting area corresponding to the scanning interval covers the side remelting area of ​​the molten pool through a double-pass overlapping experiment; and verifies whether the interlayer remelting area corresponding to the powder layer thickness covers the top remelting area of ​​the molten pool through a double-layer or few-layer forming experiment.

[0049] In one implementation, at least one of the remelting regions at the top of the molten pool and the remelting regions on the sides of the molten pool is calibrated or verified through microstructural observation, electron backscatter diffraction orientation characterization, or a combination of both. This calibration or verification is used to establish or correct the correspondence between material-beam-process parameters, and is not limited to a production step that must be performed for each layer-by-layer forming of a component. Microstructural observation is used to identify the distribution of impurities, equiaxed crystals, dendrites with different orientations, or non-target orientation grains; electron backscatter diffraction orientation characterization is used to characterize columnar crystal orientation, orientation deviations, high-angle grain boundaries, and continuous growth across melt channels and layers.

[0050] As one implementation method, a block sample or component is printed using a predetermined laser power, scanning speed, scanning spacing, and powder layer thickness. After printing, the microstructure and orientation of the component's longitudinal section, cross section, or characteristic section are analyzed. If the columnar grains grow continuously across the melt channel and across layers along a preset direction, and no continuously retained impurities, equiaxed crystals, or non-target oriented grain regions appear on the top and sides of the melt pool, then it can be determined that the set of process parameters meets the requirements for oriented columnar grain structure formation.

[0051] like Figure 7 As shown, the metal alloy components obtained using the method of this invention can form a columnar grain structure with continuous directional growth across the melt channel and across layers. This result indicates that melt channel overlap remelting can effectively remove dendrites or grains of different orientations on the sides of the melt pool, and interlayer remelting can effectively remove impurities at the top of the previous melt pool. The retained target-oriented columnar grain region can serve as the parent material for subsequent epitaxial growth.

[0052] Through the above embodiments, the present invention uses a flat-top beam or a dot-ring beam as the forming laser beam, determines the morphology of the molten pool, the columnar-equiaxed crystal transformation curve and the temperature gradient distribution characteristics through single-channel molten pool heat transfer and solidification simulation, and controls the remelting and elimination of unfavorable structures on the sides and top of the molten pool by adjusting the scanning interval and the powder layer thickness, respectively, and finally obtains a metal alloy component with a columnar grain structure that grows continuously and directionally across the molten pool and across layers.

[0053] like Figure 1 As shown, the method of the present invention includes: establishing the parameters of the metal alloy material to be formed and the powder; selecting a flat-top beam or a dot-ring beam as the forming laser beam, and using the spatial energy distribution at the processing plane as the heat source input; performing single-pass molten pool heat transfer and solidification simulation to obtain the molten pool morphology, the temperature gradient distribution characteristics inside the molten pool, and the columnar crystal-equiaxed crystal transformation curve; selecting basic forming parameters with a width-to-depth ratio (W / D) greater than 3 based on the molten pool morphology; setting the scanning spacing and powder layer thickness according to the temperature gradient distribution and the columnar crystal-equiaxed crystal transformation curve at different heights; and finally performing LPBF forming.

[0054] Compared with existing technologies, this invention takes single-pass molten pool heat transfer and solidification simulation as the starting point of process design, and incorporates the spatial energy distribution of the forming laser beam, molten pool morphology, temperature gradient distribution characteristics, and columnar crystal to equiaxed crystal transformation curve into the same parameter determination process, reducing the uncertainty of determining the process window by simply relying on experience and trial and error.

[0055] This invention utilizes the characteristic that the heat flow direction of the flat molten pool is relatively consistent with the construction direction to promote the growth of grains along the preferred orientation of

[001] . At the same time, by controlling the scanning spacing, adjacent molten channels are used to remelt dendrites or grains with different orientations on the side of the molten pool. By controlling the thickness of the powder layer, the next layer of molten pool is used to remelt the impurities, equiaxed crystals or non-target orientation grains at the top of the previous layer of molten pool, thereby eliminating unfavorable structures in both the transverse and longitudinal directions.

[0056] This invention can promote the continuous directional growth of targeted columnar grains across melt channels and layers, resulting in metal alloy components with continuously directional columnar grain structures. It is applicable to metal materials requiring directional columnar grain structures, such as stainless steel, high-temperature alloys, and high-entropy alloys, and is particularly suitable for controlling the solidification structure of high-temperature structural materials such as nickel-based and cobalt-based high-temperature alloys. In an example of an IN738LC alloy, the tensile strength, yield strength, and elongation of the continuously directional columnar grain structure sample obtained using the method of this invention were 1418±12 MPa, 972±10 MPa, and 12.0±0.5%, indicating that this invention can maintain a good strength-plasticity balance while controlling the continuous directional growth of columnar grains.

[0057] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments thereof, but they should not be construed as limiting the scope of protection of the present invention.

[0058] The process design sequence of this invention is as follows: establish material and beam parameters; perform single-pass molten pool heat transfer and solidification simulation; select laser power and scanning speed according to molten pool morphology; determine scanning spacing according to the temperature gradient vector distribution on the side of the molten pool; determine powder layer thickness according to the columnar crystal-equiaxed crystal transformation determination result at the top of the molten pool; form a block or component using the determined parameters, and verify it through microstructure and orientation characterization.

[0059] A flat-top beam can be obtained directly from a laser or through beam shaping; a point-ring beam can be formed by combining a central beam and a ring beam. The total power P of the point-ring beam is determined by the power Pcenter in the central region. c and the power P in the ring region r Composition, satisfying P c +P r =P.

[0060] In one specific simulation approach, a three-dimensional transient heat transfer-flow coupling model was established using OpenFOAM. The computational domain included the powder layer and the solid region beneath it used to characterize the substrate or formed layer. Molten metal flow, latent heat of phase change, and free surface evolution were considered, and the free surface was tracked using the fluid volume method. The computational domain size was 1400 μm × 800 μm × 400 μm, the minimum mesh size for the molten pool and the region adjacent to the solid-liquid interface was 2.5 μm, and the time step was 1.0 × 10⁻⁶. -8 s.

[0061] The initial temperature of the model is 298K, and no substrate preheating is performed; the sides use a 298K isothermal boundary, the bottom uses a type II heat transfer boundary, and the convective heat transfer coefficient of the top free surface is 80 W·m. -2 ·K -1 Radiation heat transfer is neglected in the simulation. Material density, specific heat capacity, thermal conductivity, viscosity, and surface tension are expressed as temperature-dependent functions for the respective materials; latent heat of solid-liquid phase transition is accounted for using the enthalpy method.

[0062] The flat-top beam employs a uniformly moving surface heat source, whose heat source term can be expressed as follows: The point-ring beam uses a central heat source superimposed with a ring-shaped heat source; its heat source term can be expressed as follows: Where A is the laser absorptivity, α is the fluid volume fraction, r is the flat-top beam radius, and ra is the laser absorptivity. c r is the radius of the central region in and r out χ represents the inner and outer radii of the annular region, respectively. c and χ rThese are spatial indication functions for the central region and the annular region, respectively; the center of the heat source moves along a preset path as the scanning speed and time increase.

[0063] In the IN738LC simulation, the laser absorptivity is taken as 0.5, the surface emissivity as 0.26, and the density reference value as 8180 kg / m³. 3 The solidus temperature is taken as 1282℃, the liquidus temperature as 1340℃, and the latent heat of fusion as 2.27×10⁻⁶. 5 J / kg; specific heat capacity, thermal conductivity, kinematic viscosity, and surface tension are input using temperature-dependent functions. For MAR-M509 and 15-5PH materials, replace them with the corresponding thermophysical properties and solidification parameters.

[0064] After the simulation is completed, the molten pool boundary is determined by the molten region surrounded by the liquidus temperature, and the molten pool width W and molten pool depth D are extracted on the cross section perpendicular to the scanning direction; the solid-liquid interface temperature gradient G is obtained from the temperature field gradient, and the local solidification rate R is determined by the propagation speed of the solid-liquid interface along the normal direction.

[0065] The parameter scanning matrix used in this embodiment is shown in Table 1.

[0066] Table 1. Single-channel molten pool simulation parameter scanning matrix

[0067] Table 2 presents representative simulation results and single-pass experimental measurement results under different material and beam conditions. The scanning spacing H in the table is a parameter used for subsequent multi-pass or bulk forming and is not involved in the solution of single-pass molten pool W and D.

[0068] Table 2 Representative process parameters and molten pool simulation—experimental results

[0069] As shown in Table 2, parameter combinations that satisfy W / D greater than 3 can form a wide and shallow molten pool. The simulated values ​​and experimental values ​​generally show a consistent trend, which can be used for screening basic forming parameters. For individual materials or parameter combinations, there are certain deviations in the molten pool depth, which can be further calibrated by adjusting temperature-related thermophysical properties, absorptivity, or free surface models.

[0070] Example 1 A laser powder bed melting method for forming an alloy with a directional columnar grain structure, wherein the alloy is IN738LC alloy. IN738LC nickel-based superalloy powder and a flat-top laser beam with an actual spot size of 400 μm (wavelength 1070 nm, power 1000 W) were selected. The simulated scanning range was 600–1600 W laser power, 300–2000 mm / s scanning speed, and powder layer thicknesses of 30 μm, 50 μm, 70 μm, and 90 μm. Based on the combined W / D ratio, melt pool stability, temperature gradient direction, and microstructure verification results, the usable parameter range for a W / D greater than 3 was determined to be 800–1300 W laser power and 300–800 mm / s scanning speed, with an optimal range of 800–1000 W laser power and 400–500 mm / s scanning speed.

[0071] The main implementation parameters were: laser power 1000W, scanning speed 400mm / s, powder layer thickness 50μm, and scanning spacing 250μm. The simulated molten pool width was 830μm, the depth was 171μm, and the W / D ratio was 4.85; the single-pass experiment measured a molten pool width of 806μm and a depth of 165μm, with a W / D ratio of 4.88. The simulation errors for molten pool width and depth were approximately 2.98% and 3.64%, respectively.

[0072] Using the construction direction BD as the preset columnar crystal growth direction, the deviation angle threshold is taken. θ G =15°, the simulated width Bs of the remelting area on the side of the molten pool is 216μm. When the scanning interval H is 250μm, the overlap remelting width WH is 556μm, and the overlap rate of the molten pool calculated by (WH) / W is about 69.0%, which satisfies WH≥Bs.

[0073] The CET determination of IN738LC can be performed using n=3.4 and K. CET =2.7×10 24 K 3.4 ·m -4.4 •s is used as the initial reference value for nickel-based superalloys and is calibrated based on the microstructure and electron backscatter diffraction results of this embodiment. When the powder layer thickness is 50 μm, the interlayer remelting region of the subsequent molten pool covers the top CET-sensitive region of the previous molten pool, while preserving the stable growth region of the target-oriented columnar crystals below.

[0074] The above parameters were used to form a bulk sample. Microstructure and orientation characterization showed that dendrites or grains with different orientations on the side of the molten pool were remelted by adjacent molten channels, and impurities at the top of the previous molten pool were remelted by the next molten pool. Columnar grains grew continuously across molten channels and layers along the construction direction.

[0075] Example 2 A laser powder bed melting method for forming an alloy with a directional columnar grain structure, wherein the alloy is MAR-M509 alloy; MAR-M509 cobalt-based high-temperature alloy powder and a flat-top beam with an actual spot size of 400μm (wavelength of 1070nm and power of 1100W) were selected. The available parameter range that satisfies W / D greater than 3 is laser power of 900~1300W and scanning speed of 300~800mm / s, with the preferred range being laser power of 900~1100W and scanning speed of 400~500mm / s.

[0076] The laser power was 1100W, the scanning speed was 500mm / s, the powder layer thickness was 50μm, and the scanning spacing was 250μm. The simulated molten pool width was 830μm, the depth was 181μm, and the W / D ratio was 4.59; the experimentally measured molten pool width was 812μm, the depth was 164μm, and the W / D ratio was 4.95.

[0077] With θc=15°, the single-sided width Bs of the remelted area on the side of the molten pool is 251 μm. A scanning spacing of 250 μm corresponds to an overlap remelting width of 562 μm, and the weld overlap rate is approximately 69.2%, meeting the coverage requirements of the remelted area on the side. For MAR-M509, n=3.4 and K... CET =2.0×10 24 K 2.8 ·m -3.8 •s is calibrated based on the material's solidification behavior and microstructure characterization results, and does not directly adopt the fixed values ​​of other materials.

[0078] After forming according to the above parameters, microstructure and orientation characterization showed that columnar grains could grow continuously and directionally across the melt channel and across layers, without forming unfavorable grain bands that were continuously retained on the sides and top of the melt pool.

[0079] Example 3 A laser powder bed melting method for forming an alloy with oriented columnar grain structure, wherein the alloy is 15-5PH precipitation-hardening martensitic stainless steel.

[0080] 15-5PH precipitation-hardening martensitic stainless steel powder and a flat-top laser beam with an actual spot size of 400μm (wavelength 1070nm, power 1300W) were selected. The usable parameter range for W / D greater than 3 is laser power 900~1500W and scanning speed 300~800mm / s, with the preferred range being laser power 1100~1300W and scanning speed 400~600mm / s.

[0081] The laser power was 1300W, the scanning speed was 600mm / s, the powder layer thickness was 50μm, and the scanning spacing was 200μm. The simulated molten pool width was 826μm, the depth was 209μm, and the W / D ratio was 3.95; the experimentally measured molten pool width was 814μm, the depth was 202μm, and the W / D ratio was 4.03.

[0082] Under the condition of θc=15°, the single-sided width Bs of the remelting area on the side of the molten pool is 276μm. The overlapping remelting width corresponding to a scanning spacing of 200μm is 614μm, and the molten pool overlap rate is approximately 75.4%, which meets the coverage requirement of the side remelting area. The 15-5PH material has n=3.02 and K... CET =2.5×10 27 K 3.02 ·m -4.02 The ·s value is determined based on the material's solidification behavior, microstructure, and electron backscatter diffraction results.

[0083] After forming according to the above parameters, the unfavorable structure on the sides and top of the molten pool is eliminated by remelting, and the retained target-oriented columnar crystal region serves as the parent material for subsequent epitaxial growth, forming a columnar grain structure that grows continuously across the melt channel and across layers.

[0084] Example 4 A laser powder bed melting method for forming an alloy with a directional columnar grain structure, wherein the alloy is IN738LC. A point-to-ring beam forming method is used; the center / ring power ratio is 30:70.

[0085] IN738LC powder and a point-ring beam with an equivalent spot size of 500 μm (wavelength 1070 nm, power 800 W) were selected, with a power ratio of 30:70 between the central energy region and the ring energy region. The laser power was 800 W, the scanning speed was 400 mm / s, the powder layer thickness was 50 μm, and the scanning spacing was 250 μm. The simulated molten pool width was 784 μm, the depth was 156 μm, and the W / D ratio was 5.03; the experimentally measured molten pool width was 750 μm, the depth was 151 μm, and the W / D ratio was 4.97. These parameters resulted in a stable, flat molten pool, which can be considered the preferred implementation conditions for the point-ring beam.

[0086] Comparative Example 1 A laser powder bed melting forming method for IN738LC alloy, wherein the alloy powder, forming substrate, forming equipment, protective atmosphere, scanning strategy and sample size are the same as those in Example 1.

[0087] The difference from Example 1 is that the scanning spacing in this comparative example is not set according to the width of the remelting area on the side of the molten pool. Specifically, a flat-top beam with a spot diameter of 400 μm is used, the laser power is 1000 W, the scanning speed is 400 mm / s, the powder layer thickness is 50 μm, and the scanning spacing is increased to 650 μm. The remaining forming conditions are the same as in Example 1.

[0088] Comparative Example 2 A laser powder bed melting method for forming 15-5 pH precipitation-hardening martensitic stainless steel alloy, wherein the alloy powder, forming substrate, forming equipment, protective atmosphere, scanning strategy and sample size are the same as those in Example 1.

[0089] The difference from Example 1 is that this comparative example does not set the powder layer thickness according to the columnar crystal to equiaxed crystal transformation sensitive region at different heights of the molten pool.

[0090] Performance testing The alloy parts processed in Examples 1, 2, 3, and 4, and Comparative Examples 1 and 2 were observed using a scanning electron microscope or a metallurgical optical microscope, respectively, and microstructure images were obtained, such as... Figures 7-12 As shown.

[0091] The above results indicate that when the scanning interval is not set according to the remelting area on the side of the molten pool, and the overlap remelting width of adjacent molten channels is less than the width of the remelting area, dendrites or grains with different orientations on the side of the molten pool cannot be fully remelted and eliminated, which is not conducive to the continuous directional growth of the target orientation columnar grains across the molten channel.

[0092] By comparison Figure 7 and Figure 11 It can be seen that the alloy parts processed in Example 1 exhibited continuous growth and a very significant columnar crystal morphology during subsequent forming. Compared with Example 1, the melt channel interface region of Comparative Example 1 still retained dendrites or grains with different orientations, which continued to grow during subsequent forming, thus hindering the continuous growth of the target curve columnar grains across the melt channel. By comparison Figure 9 and Figure 12 It can be seen that the alloy parts processed in Example 3 effectively eliminated impurities, equiaxed crystals, or non-target oriented grains.

[0093] As can be seen from the above embodiments and comparative examples, the method provided by the present invention can effectively obtain oriented columnar grain structure and enable the alloy to grow continuously in the direction of the target orientation columnar grain across the melt channel.

[0094] Although the above embodiments have provided a detailed description of the present invention, they are only some embodiments of the present invention and not all embodiments. People can obtain other embodiments based on these embodiments without creative effort, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A laser powder bed melting method for forming an alloy with a directional columnar grain structure, characterized in that, Includes the following steps: A flat-top beam or a dot-ring beam is used as the forming laser beam. Its spatial energy distribution is input into a single-channel three-dimensional transient heat transfer solidification model as a moving heat source. The temperature field and flow state of the molten pool under different laser power, scanning speed, powder layer thickness and spot size are calculated. The molten pool width W, molten pool depth D, solid-liquid interface temperature gradient G, local solidification rate R, temperature gradient vector and columnar crystal to equiaxed crystal transformation curve are obtained. Laser power and scanning speed with a width-to-depth ratio (W / D) greater than 3 are selected based on the molten pool morphology to promote grain growth along the preferred orientation of [001]. The scanning interval is set according to the deviation area of ​​the local temperature gradient vector on the side of the molten pool from the preset columnar crystal growth direction, so that the overlapping remelting area formed by adjacent melt channels covers the remelting area on the side of the molten pool. The thickness of the powder layer is set according to the columnar crystal to equiaxed crystal transformation curve at different heights, so that the subsequent molten pool enters the interlayer remelting area formed by the solidified area of ​​the previous layer and covers the top of the molten pool to be remelted. Laser powder bed melting is performed on metal alloy powder according to the laser power, scanning speed, scanning spacing and powder layer thickness determined above, to obtain an alloy with directional columnar grain structure that grows continuously across the melting channel and across layers.

2. The laser powder bed melting and forming method according to claim 1, characterized in that, The metal alloy powder is one of stainless steel, high-temperature alloy, and high-entropy alloy; the high-temperature alloy includes nickel-based high-temperature alloy or cobalt-based high-temperature alloy; the stainless steel includes 15-5 pH precipitation-hardening martensitic stainless steel; the high-entropy alloy includes CoCrFeNi series high-entropy alloy.

3. The laser powder bed melting and forming method according to claim 1, characterized in that, The wavelength of the forming laser beam is 800~1200nm, the power is 300~2000W, and the equivalent size of the spot is 50~500μm.

4. The laser powder bed melting and forming method according to claim 1, characterized in that, The power ratio of the central energy region to the annular energy region of the dot-ring beam is 20~40:60~80.

5. The laser powder bed melting and forming method according to claim 1, characterized in that, The simulation parameters of the single-channel three-dimensional transient heat transfer solidification model include: laser power of 500~2000W, step size of 100W; scanning speed of 200~2000mm / s, step size of 100mm / s; and powder layer thickness of 30μm, 50μm, 70μm or 90μm, step size of 20μm.

6. The laser powder bed melting and forming method according to claim 1, characterized in that, The deviation angle of the local temperature gradient vector on the side of the molten pool relative to the preset columnar crystal growth direction. θ G according to It is confirmed that, among them, This represents the local temperature gradient vector at the solid-liquid interface of the molten pool. The unit vector for the preset columnar crystal growth direction; θ G The continuous region with an angle greater than the preset deviation threshold is determined as the remelting area on the side of the molten pool.

7. The laser powder bed melting and forming method according to claim 6, characterized in that, The preset deviation angle threshold of the columnar crystal growth direction θ G ≤20°.

8. The laser powder bed melting and forming method according to claim 1, characterized in that, Let the width of the molten pool be W, the scanning interval be H, and the width of one side of the remelting area on the side be Bs. Then set the scanning interval H such that WH≥Bs, so that the overlapping remelting area formed by adjacent molten channels covers the remelting area on the side of the molten pool.

9. The laser powder bed melting and forming method according to claim 8, characterized in that, The scanning interval is 200~400μm.

10. The laser powder bed melting and forming method according to claim 1, characterized in that, The columnar crystal to equiaxed crystal transformation curve is obtained through a judgment index Ψ, which satisfies... Where G is the temperature gradient at the solid-liquid interface, R is the local solidification rate, and n is the material-related index determined based on the solidification behavior of the metal alloy to be formed; and K is the critical value of the columnar-equiaxed crystal transformation based on material calibration. CET <Ψ, determine the sensitive region for columnar crystal to equiaxed crystal transformation, that is, the region to be remelted at the top of the molten pool.

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