Method for refining 316L stainless steel grains based on selective laser melting technology

Abstract

The invention discloses a method for refining 316L stainless steel grains based on a selective laser melting technology, and belongs to the technical field of metal material additive manufacturing. According to the method, Fe-C-B alloy powder is added, a Fe-B or Fe-B-Cr type compound is formed to serve as a heterogeneous nucleation core when the 316L stainless steel component is manufactured through selective laser melting, columnar crystal epitaxial growth is inhibited, and grain refinement is achieved. By means of the synergistic effect formed by Fe, C and B, the negative influence of the SLM technology on the 316L stainless steel forming process is avoided, the advantages of the SLM technology are fully exerted in the 316L stainless steel component forming process, accordingly, the 316L stainless steel component high in compactness and excellent in comprehensive performance can be formed only through SLM, the method is stable in process, low in crack sensitivity and high in yield, and the production cost is reduced. The method has remarkable advantages in the field of additive manufacturing of high-performance 316L stainless steel components.

Description

Technical Field

[0001] This invention belongs to the field of additive manufacturing technology for metal materials, and specifically designs a method for refining the grains of 316L stainless steel based on selective laser melting technology. Background Technology

[0002] Selective Laser Melting (SLM), also known as Laser Powder Bed Fusion (LPBF), is a metal additive manufacturing technology. Its basic principle is as follows: First, a three-dimensional model of the target part is created, and two-dimensional contour data is obtained through slicing and imported into the equipment. During processing, a layer of raw material powder is first laid on a forming bed. The SLM equipment controls a fiber laser beam according to a preset program to selectively melt the raw material powder within a designated area. After one layer of raw material powder is melted, the forming bed is lowered, and powder is re-laid and melted. Through the continuous melting and accumulation of raw material powder layer by layer, a metal part consistent with the three-dimensional model is finally obtained. This technology can directly form parts with complex geometries, breaking through the limitations of traditional processing methods in terms of part structure and shape. It requires no molds and has advantages such as high design freedom, short forming cycle, high forming accuracy, and high material utilization, and is widely used in the field of complex component forming.

[0003] 316L stainless steel is a low-carbon austenitic stainless steel. Due to its excellent corrosion resistance, high-temperature mechanical properties and radiation damage resistance, it is widely used in high-end fields such as the primary loop of fast neutron breeder reactors, marine engineering, bio-implants and fuel cells.

[0004] With the development of related technologies, the requirements for the geometric complexity and comprehensive performance of 316L stainless steel components are increasing. Therefore, it is of great significance to use SLM for forming 316L stainless steel components.

[0005] However, SLM forming of 316L stainless steel components still faces significant challenges: on the one hand, its high heat input easily leads to the formation of coarse columnar crystals and pronounced grain orientation, resulting in enhanced anisotropy in the microstructure; on the other hand, the rapid solidification characteristics under extremely high cooling rates easily cause uneven solute distribution within the molten pool, forming non-equilibrium solidification structures such as cellular substructures and chemical segregation. These negatively impact the performance of 316L stainless steel components, limiting the potential for improving the mechanical properties of SLM-formed 316L stainless steel components. This makes it difficult to effectively enhance the properties of the inherently low-hardness 316L stainless steel through the SLM forming process, resulting in poor-performing components that often require additional heat treatment for microstructure optimization, increasing process complexity and component manufacturing costs.

[0006] Currently, some technologies attempt to address these issues by increasing scanning speed, microalloying to add Ti and Nb elements, and heat treatment strengthening. However, these methods suffer from drawbacks such as high cost, difficulty in controlling precipitated phases, limited grain refinement effects, and poor compatibility with SLM technology. These limitations hinder SLM from fully leveraging its advantages to achieve uniform and stable ultrafine grain structures while forming complex structures and high-density components.

[0007] Therefore, it is necessary to provide a method for refining the grains of 316L stainless steel based on selective laser melting (SLM) technology, to improve the matching between grain refinement and the SLM forming process, to stably achieve grain refinement during the forming process, to give full play to the advantages of SLM technology, and to obtain 316L stainless steel components with superior comprehensive performance. Summary of the Invention

[0008] To overcome the problems in the prior art, this invention incorporates Fe-CB alloy powder during the SLM forming of 316L stainless steel components. The boron (B) element forms high-melting-point Fe-B or Fe-B-Cr type compounds with Fe and Cr elements in the 316L stainless steel, serving as heterogeneous nucleation sites and effectively increasing the number of nucleation points in the molten pool. Simultaneously, the carbon (C) element forms fine carbides with Fe during solidification, creating a pinning effect on grain boundaries and further inhibiting grain growth. Furthermore, the Fe element maintains good wettability and metallurgical bonding with the 316L stainless steel matrix. Ultimately, this significantly optimizes the grain structure during the SLM forming of 316L stainless steel components. Combined with the inherent advantages of SLM technology, this results in 316L stainless steel components with superior overall performance.

[0009] To achieve the above objectives, the present invention is implemented through the following technical solution: This invention proposes a method for refining the grains of 316L stainless steel based on selective laser melting technology. By adding Fe-CB alloy powder, Fe-B or Fe-B-Cr type compounds are formed during the selective laser melting process of 316L stainless steel components, serving as heterogeneous nucleation cores to suppress the epitaxial growth of columnar crystals and achieve grain refinement.

[0010] Preferably, the method includes the following steps: (1) Under an inert gas protective environment, Fe-CB alloy powder and 316L stainless steel powder are thoroughly mixed to obtain mixed powder.

[0011] (2) Selective laser melting technology is used to shape the mixed powder obtained in step (1) to obtain 316L stainless steel components.

[0012] Preferably, in step (1), the Fe-CB alloy powder composition by mass fraction includes Fe: 95.7%~95.9%, C: 0.3%~0.5%, and B: 3.8%.

[0013] Preferably, in step (1), the Fe-CB alloy powder accounts for 0.3% to 1.0% of the total mass of the mixed powder.

[0014] Preferably, in step (1), Fe-CB alloy powder and 316L stainless steel powder are mixed evenly by dry ball milling.

[0015] Preferably, the ball milling speed is 100~300 r / min, the ball-to-material ratio is 5~10:1, and the milling is paused for 15~25 min after every 20~30 min of milling, with a total milling time of 1~4 h.

[0016] Preferably, the Fe-CB alloy powder has a particle size of no more than 15 μm.

[0017] Preferably, the particle size of the 316L stainless steel powder is 15~53μm.

[0018] Preferably, in step (2), the laser power of the selective laser melting is 80~95W, the laser scanning speed is 1000~1400mm / s, the scanning spacing is 60μm, the scanning strategy is S-shaped orthogonal, the interlayer rotation angle is 67°, the number of times each layer of mixed powder is melted is 1~2 times, and after each layer of mixed powder is formed, the substrate is lowered by 20~40μm.

[0019] Preferably, the 316L stainless steel powder composition, by mass percentage, includes Cr: 16%~18%, Ni: 10%~14%, Mo: 2%~3%, Mn: ≤2%, C: ≤0.03%, Si: ≤0.75%, P: ≤0.025%, S: ≤0.01%, N: ≤0.1%, with the balance being Fe.

[0020] Preferably, the mixed powder obtained in step (1) is vacuum dried.

[0021] Preferably, the drying temperature is 60~80℃ and the drying time is 2~8h.

[0022] The Fe-CB alloy powder of the present invention can be prepared by conventional gas atomization powder preparation method.

[0023] The beneficial effects of this invention are: 1. This invention, by adding Fe-CB alloy powder during the SLM forming of 316L stainless steel components, leverages the high growth limiting factor of boron in iron-based alloys to significantly increase the solute undercooling under rapid cooling conditions of the laser molten pool. This promotes non-equilibrium solidification, reduces crystal growth rate, and forms high-melting-point compounds such as Fe-B or Fe-B-Cr, which act as heterogeneous nucleation nuclei, increasing nucleation point density and interrupting the epitaxial growth of austenite grains in 316L stainless steel. This significantly refines the honeycomb structure and columnar crystals, thereby inhibiting grain growth along the original austenite orientation. Furthermore, it synergistically forms fine carbides with carbon, creating a pinning effect on grain boundaries. The good wettability and metallurgical bonding between Fe and the 316L stainless steel matrix result in a more uniform and finer grain structure during the SLM forming process. While maintaining the good plasticity of 316L stainless steel components, this invention achieves a synergistic improvement in microhardness, yield strength, and wear resistance, providing a reliable solution for SLM forming of high-strength, high-toughness 316L stainless steel components.

[0024] 2. During the SLM forming process, the cooling rate can typically reach 10. 6 K / s, the hard phase in the Fe-CB alloy powder of the present invention is typically 10 K / s. 3 ~10 5 Formed at a cooling rate of K / s, the SLM forming process has a higher cooling rate than the hard phase formation conditions in the Fe-CB alloy powder of the present invention. Therefore, the Fe-CB alloy powder of the present invention has excellent matching with the SLM forming process, and SLM can effectively promote the formation of a stable hard phase.

[0025] 3. The process of this invention is simple, and grain refinement can be achieved during the SLM forming process without the need for complex heat treatment. The process is stable and controllable, and it is suitable for industrial application. Attached Figure Description

[0026] Figure 1 These are SEM images of the Fe-CB alloy powder and 316L stainless steel powder of this invention, wherein... Figure 1 (a) is a SEM image of 316L stainless steel powder. Figure 1 (b) is a SEM image of Fe-CB alloy powder; Figure 2 These are microstructure images of 316L stainless steel samples prepared in the embodiments and comparative examples of the present invention. Figure 2 (a) is a microstructure of the 316L stainless steel sample prepared in Comparative Example 1. Figure 2 (b) is a microstructure image of the 316L stainless steel sample prepared in Example 1. Figure 2 (c) is a microstructure image of the stainless steel sample prepared in Example 3. Figure 2(d) is a microstructure diagram of the stainless steel sample prepared in Example 4; Figure 3 This is a grain size diagram of 316L stainless steel samples prepared in the embodiments and comparative examples of the present invention; Figure 4 The images show the microhardness of stainless steel samples prepared in the embodiments and comparative examples of this invention. Detailed Implementation

[0027] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments, but the scope of protection of the present invention is not limited to the content described.

[0028] The Fe-BC alloy powder used in the embodiments and comparative examples of this invention was prepared by gas atomization powder preparation method.

[0029] Example 1 This embodiment uses the following method to prepare 316L stainless steel samples: (1) Fe-CB alloy powder with a particle size not exceeding 15 μm was screened using 400 mesh, 800 mesh and 1500 mesh sieves (in the alloy powder, the mass fraction of Fe is 95.7%, the mass fraction of C is 0.5% and the mass fraction of B is 3.8%, denoted as Fe). 95.7 C 0.5 B 3.8 Powder) and 316L stainless steel powder with a particle size of 15~53μm (the composition of 316L stainless steel powder by mass percentage is: Cr content 16%~18%, Ni content 10%~14%, Mo content 2%~3%, Mn content ≤2%, C content ≤0.03%, Si content ≤0.75%, P content ≤0.025%, S content ≤0.01%, N content ≤0.1%, and the balance is Fe).

[0030] (2) The Fe obtained by screening 95.7 C 0.5 B 3.8 Powder and 316L stainless steel powder (Fe 95.7 C 0.5 B 3.8 The powder (0.3% of the total mass of the mixed powder) was placed in a ball mill for mechanical mixing to obtain a mixed powder. During the ball milling process, argon gas was introduced into the ball mill jar for protection. The ball milling time was set to 4 hours, and the ball milling speed was controlled at 150 r / min. The milling was paused for 20 minutes every 25 minutes to allow for cooling. The ball-to-powder ratio was 5:1.

[0031] (3) Place the well-mixed powder into a vacuum dryer for drying; evacuate the dryer to a vacuum state, dry for 5 hours, set the drying temperature to 80°C, and after drying, cool the powder to room temperature in a vacuum environment.

[0032] (4) Use UG software to carry out three-dimensional modeling work, import the constructed three-dimensional model into the slicing software for slicing processing, set the slicing thickness to 20μm, save it as an STL file after processing, and then import the file into the SLM device.

[0033] (5) Clean the substrate used for molding with alcohol, then dry it, install the molding substrate in the molding chamber of the SLM equipment, add the dried mixed powder to the powder chamber of the SLM equipment, and introduce argon into the molding chamber to ensure that the oxygen content in the molding chamber is less than 500ppm.

[0034] (6) A layer of mixed powder is laid on the forming substrate using a scraper. The mixed powder is melted using a fiber laser based on the geometry of the two-dimensional slice. The laser scanning process parameters are as follows: laser power is 85W, laser scanning speed is 1000mm / s, laser scanning spacing is 60μm, scanning strategy is S-shaped orthogonal, interlayer rotation angle is 67°, and each layer of mixed powder is melted once.

[0035] (7) After each layer of mixed powder is processed, the substrate is lowered by 20 μm, a new layer of mixed powder is laid, and then it is melted. This cycle is repeated until the sample is formed.

[0036] The sample in this embodiment is designated as SLM-316L-0.3Fe-CB.

[0037] Example 2 This embodiment uses the following method to prepare 316L stainless steel samples: (1) Fe particles with a particle size not exceeding 15 μm were screened using 400-mesh, 800-mesh, and 1500-mesh sieves. 95.7 C 0.5 B 3.8 Powder and 316L stainless steel powder with a particle size of 15~53μm (the composition of 316L stainless steel powder by mass percentage is: Cr content 16%~18%, Ni content 10%~14%, Mo content 2%~3%, Mn content ≤2%, C content ≤0.03%, Si content ≤0.75%, P content ≤0.025%, S content ≤0.01%, N content ≤0.1%, and the balance is Fe).

[0038] (2) The Fe obtained by screening 95.7 C 0.5 B 3.8 Powder and 316L stainless steel powder (Fe 95.7 C 0.5 B 3.8The powder (0.5% of the total mass of the mixed powder) is placed in a ball mill for mechanical mixing to obtain a mixed powder. During the ball milling process, argon gas is introduced into the ball mill jar for protection. The ball milling time is set to 4 hours, and the ball milling speed is controlled at 150 r / min. The milling is paused for 20 minutes every 25 minutes to allow for cooling. The ball-to-powder ratio is 5:1.

[0039] (3) Place the well-mixed powder into a vacuum dryer for drying; evacuate the dryer to a vacuum state, dry for 5 hours, set the drying temperature to 80°C, and after drying, cool the powder to room temperature in a vacuum environment.

[0040] (4) Use UG software to carry out three-dimensional modeling work, import the constructed three-dimensional model into the slicing software for slicing processing, set the slicing thickness to 20μm, save it as an STL file after processing, and then import the file into the SLM device.

[0041] (5) Clean the substrate used for molding with alcohol, then dry it, install the molding substrate in the molding chamber of the SLM equipment, add the dried mixed powder to the powder chamber of the SLM equipment, and introduce argon into the molding chamber to ensure that the oxygen content in the molding chamber is less than 500ppm.

[0042] (6) A layer of mixed powder is laid on the forming substrate using a scraper. The mixed powder is melted using a fiber laser based on the geometry of the two-dimensional slice. The laser scanning process parameters are as follows: laser power is 85W, laser scanning speed is 1000mm / s, laser scanning spacing is 60μm, scanning strategy is S-shaped orthogonal, interlayer rotation angle is 67°, and each layer of mixed powder is melted once.

[0043] (7) After each layer of mixed powder is processed, the substrate is lowered by 20 μm, a new layer of mixed powder is laid, and then it is melted. This cycle is repeated until the sample is formed.

[0044] The sample in this embodiment is designated as SLM-316L-0.5Fe-CB.

[0045] Example 3 This embodiment uses the following method to prepare 316L stainless steel samples: (1) Fe particles with a particle size not exceeding 15 μm were screened using 400-mesh, 800-mesh, and 1500-mesh sieves. 95.7 C 0.5 B 3.8Powder and 316L stainless steel powder with a particle size of 15~53μm (the composition of 316L stainless steel powder by mass percentage is: Cr content 16%~18%, Ni content 10%~14%, Mo content 2%~3%, Mn content ≤2%, C content ≤0.03%, Si content ≤0.75%, P content ≤0.025%, S content ≤0.01%, N content ≤0.1%, and the balance is Fe).

[0046] (2) The Fe obtained by screening 95.7 C 0.5 B 3.8 Powder and 316L stainless steel powder (Fe 95.7 C 0.5 B 3.8 The powder (0.7% of the total mass of the mixed powder) was placed in a ball mill for mechanical mixing to obtain a mixed powder. During the ball milling process, argon gas was introduced into the ball mill jar for protection. The ball milling time was set to 4 hours, and the ball milling speed was controlled at 150 r / min. Every 25 minutes of ball milling, the milling was paused for 20 minutes to allow for cooling. The ball-to-powder ratio was 5:1.

[0047] (3) Place the well-mixed powder into a vacuum dryer for drying; evacuate the dryer to a vacuum state, dry for 5 hours, set the drying temperature to 80°C, and after drying, cool the powder to room temperature in a vacuum environment.

[0048] (4) Use UG software to carry out three-dimensional modeling work, import the constructed three-dimensional model into the slicing software for slicing processing, set the slicing thickness to 20μm, save it as an STL file after processing, and then import the file into the SLM device.

[0049] (5) Clean the substrate used for molding with alcohol, then dry it, install the molding substrate in the molding chamber of the SLM equipment, add the dried mixed powder to the powder chamber of the SLM equipment, and introduce argon into the molding chamber to ensure that the oxygen content in the molding chamber is less than 500ppm.

[0050] (6) A layer of mixed powder is laid on the forming substrate using a scraper. The mixed powder is melted using a fiber laser based on the geometry of the two-dimensional slice. The laser scanning process parameters are as follows: laser power is 85W, laser scanning speed is 1000mm / s, laser scanning spacing is 60μm, scanning strategy is S-shaped orthogonal, interlayer rotation angle is 67°, and each layer of mixed powder is melted once.

[0051] (7) After each layer of mixed powder is processed, the substrate is lowered by 20 μm, a new layer of mixed powder is laid, and then it is melted. This cycle is repeated until the sample is formed.

[0052] The sample in this embodiment is designated as SLM-316L-0.7Fe-CB.

[0053] Example 4 This embodiment uses the following method to prepare 316L stainless steel samples: (1) Fe particles with a particle size not exceeding 15 μm were screened using 400-mesh, 800-mesh, and 1500-mesh sieves. 95.7 C 0.5 B 3.8 Powder and 316L stainless steel powder with a particle size of 15~53μm (the composition of 316L stainless steel powder by mass percentage is: Cr content 16%~18%, Ni content 10%~14%, Mo content 2%~3%, Mn content ≤2%, C content ≤0.03%, Si content ≤0.75%, P content ≤0.025%, S content ≤0.01%, N content ≤0.1%, and the balance is Fe).

[0054] (2) The Fe obtained by screening 95.7 C 0.5 B 3.8 Powder and 316L stainless steel powder (Fe 95.7 C 0.5 B 3.8 The powder (0.9% of the total mass of the mixed powder) was placed in a ball mill for mechanical mixing to obtain a mixed powder. During the ball milling process, argon gas was introduced into the ball mill jar for protection. The ball milling time was set to 4 hours, and the ball milling speed was controlled at 150 r / min. The milling was paused for 20 minutes every 25 minutes to allow for cooling. The ball-to-powder ratio was 5:1.

[0055] (3) Place the well-mixed powder into a vacuum dryer for drying; evacuate the dryer to a vacuum state, dry for 5 hours, set the drying temperature to 80°C, and after drying, cool the powder to room temperature in a vacuum environment.

[0056] (4) Use UG software to carry out three-dimensional modeling work, import the constructed three-dimensional model into the slicing software for slicing processing, set the slicing thickness to 20μm, save it as an STL file after processing, and then import the file into the SLM device.

[0057] (5) Clean the substrate used for molding with alcohol, then dry it, install the molding substrate in the molding chamber of the SLM equipment, add the dried mixed powder to the powder chamber of the SLM equipment, and introduce argon into the molding chamber to ensure that the oxygen content in the molding chamber is less than 500ppm.

[0058] (6) A layer of mixed powder is laid on the forming substrate using a scraper. The mixed powder is melted using a fiber laser based on the geometry of the two-dimensional slice. The laser scanning process parameters are as follows: laser power is 85W, laser scanning speed is 1000mm / s, laser scanning spacing is 60μm, scanning strategy is S-shaped orthogonal, interlayer rotation angle is 67°, and each layer of mixed powder is melted once.

[0059] (7) After each layer of mixed powder is processed, the substrate is lowered by 20 μm, a new layer of mixed powder is laid, and then it is melted. This cycle is repeated until the sample is formed.

[0060] The sample in this embodiment is designated as SLM-316L-0.9Fe-CB.

[0061] Example 5 This embodiment uses the following method to prepare 316L stainless steel samples: (1) Fe particles with a particle size not exceeding 15 μm were screened using 400-mesh, 800-mesh, and 1500-mesh sieves. 95.7 C 0.5 B 3.8 Powder and 316L stainless steel powder with a particle size of 15~53μm (the composition of 316L stainless steel powder by mass percentage is: Cr content 16%~18%, Ni content 10%~14%, Mo content 2%~3%, Mn content ≤2%, C content ≤0.03%, Si content ≤0.75%, P content ≤0.025%, S content ≤0.01%, N content ≤0.1%, and the balance is Fe).

[0062] (2) The Fe obtained by screening 95.7 C 0.5 B 3.8 Powder and 316L stainless steel powder (Fe 95.7 C 0.5 B 3.8 The powder (1% of the total mass of the mixed powder) is placed in a ball mill for mechanical mixing to obtain a mixed powder. During the ball milling process, argon gas is introduced into the ball mill jar for protection. The ball milling time is set to 4 hours, and the ball milling speed is controlled at 150 r / min. Every 25 minutes of ball milling is paused for 20 minutes to allow for cooling. The ball-to-powder ratio is 5:1.

[0063] (3) Place the well-mixed powder into a vacuum dryer for drying; evacuate the dryer to a vacuum state, dry for 5 hours, set the drying temperature to 80°C, and after drying, cool the powder to room temperature in a vacuum environment.

[0064] (4) Use UG software to carry out three-dimensional modeling work, import the constructed three-dimensional model into the slicing software for slicing processing, set the slicing thickness to 20μm, save it as an STL file after processing, and then import the file into the SLM device.

[0065] (5) Clean the substrate used for molding with alcohol, then dry it, install the molding substrate in the molding chamber of the SLM equipment, add the dried mixed powder to the powder chamber of the SLM equipment, and introduce argon into the molding chamber to ensure that the oxygen content in the molding chamber is less than 500ppm.

[0066] (6) A layer of mixed powder is laid on the forming substrate using a scraper. The mixed powder is melted using a fiber laser based on the geometry of the two-dimensional slice. The laser scanning process parameters are as follows: laser power is 85W, laser scanning speed is 1000mm / s, laser scanning spacing is 60μm, scanning strategy is S-shaped orthogonal, interlayer rotation angle is 67°, and each layer of mixed powder is melted once.

[0067] (7) After each layer of mixed powder is processed, the substrate is lowered by 20 μm, a new layer of mixed powder is laid, and then it is melted. This cycle is repeated until the sample is formed.

[0068] The sample in this embodiment is designated as SLM-316L-1Fe-CB.

[0069] Example 6 This embodiment uses the following method to prepare 316L stainless steel samples: (1) Fe particles with a particle size not exceeding 15 μm were screened using 400-mesh, 800-mesh, and 1500-mesh sieves. 95.9 C 0.3 B 3.8 Powder and 316L stainless steel powder with a particle size of 15~53μm (the composition of 316L stainless steel powder by mass percentage is: Cr content 16%~18%, Ni content 10%~14%, Mo content 2%~3%, Mn content ≤2%, C content ≤0.03%, Si content ≤0.75%, P content ≤0.025%, S content ≤0.01%, N content ≤0.1%, and the balance is Fe).

[0070] (2) The Fe obtained by screening 95.9 C 0.3 B 3.8 Powder and 316L stainless steel powder (Fe 95.9 C 0.3 B 3.8The powder (0.3% of the total mass of the mixed powder) is placed in a ball mill for mechanical mixing to obtain a mixed powder. During the ball milling process, argon gas is introduced into the ball mill jar for protection. The ball milling time is set to 1 hour, and the ball milling speed is controlled at 300 r / min. The milling is paused for 15 minutes every 20 minutes to allow for cooling. The ball-to-powder ratio is 10:1.

[0071] (3) Place the well-mixed powder into a vacuum dryer for drying; evacuate the dryer to a vacuum state, dry for 2 hours, set the drying temperature to 70°C, and after drying, cool the powder to room temperature in a vacuum environment.

[0072] (4) Use UG software to carry out three-dimensional modeling work, import the constructed three-dimensional model into the slicing software for slicing processing, set the slicing thickness to 30μm, save it as an STL file after processing, and then import the file into the SLM device.

[0073] (5) Clean the substrate used for molding with alcohol, then dry it, install the molding substrate in the molding chamber of the SLM equipment, add the dried mixed powder to the powder chamber of the SLM equipment, and introduce argon into the molding chamber to ensure that the oxygen content in the molding chamber is less than 500ppm.

[0074] (6) A layer of mixed powder is laid on the forming substrate using a scraper. The mixed powder is melted using a fiber laser based on the geometry of the two-dimensional slice. The laser scanning process parameters are as follows: laser power is 80W, laser scanning speed is 1400mm / s, laser scanning spacing is 60μm, scanning strategy is S-shaped orthogonal, interlayer rotation angle is 67°, and each layer of mixed powder is melted twice.

[0075] (7) After each layer of mixed powder is processed, the substrate is lowered by 30 μm, a new layer of mixed powder is laid, and then it is melted. This cycle is repeated until the sample is formed.

[0076] In this embodiment, the laser scanning speed is slightly increased compared to Example 1, and the surface formability and density of the sample are slightly reduced, but the overall performance of the sample is similar to that of Example 1.

[0077] Example 7 This embodiment uses the following method to prepare 316L stainless steel samples: (1) Fe particles with a particle size not exceeding 15 μm were screened using 400-mesh, 800-mesh, and 1500-mesh sieves. 95.8 C 0.4 B 3.8Powder and 316L stainless steel powder with a particle size of 15~53μm (the composition of 316L stainless steel powder by mass percentage is: Cr content 16%~18%, Ni content 10%~14%, Mo content 2%~3%, Mn content ≤2%, C content ≤0.03%, Si content ≤0.75%, P content ≤0.025%, S content ≤0.01%, N content ≤0.1%, and the balance is Fe).

[0078] (2) The Fe obtained by screening 95.8 C 0.4 B 3.8 Powder and 316L stainless steel powder (Fe 95.8 C 0.4 B 3.8 The powder (0.7% of the total mass of the mixed powder) was placed in a ball mill for mechanical mixing to obtain a mixed powder. During the ball milling process, argon gas was introduced into the ball mill jar for protection. The ball milling time was set to 2.5 hours, and the ball milling speed was controlled at 100 r / min. The milling was paused for 25 minutes every 30 minutes to allow for cooling. The ball-to-powder ratio was 8:1.

[0079] (3) Place the well-mixed powder into a vacuum dryer for drying; evacuate the dryer to a vacuum state, dry for 8 hours, set the drying temperature to 60°C, and after drying, cool the powder to room temperature in a vacuum environment.

[0080] (4) Use UG software to carry out three-dimensional modeling work, import the constructed three-dimensional model into the slicing software for slicing processing, set the slicing thickness to 40μm, save it as an STL file after processing, and then import the file into the SLM device.

[0081] (5) Clean the substrate used for molding with alcohol, then dry it, install the molding substrate in the molding chamber of the SLM equipment, add the dried mixed powder to the powder chamber of the SLM equipment, and introduce argon into the molding chamber to ensure that the oxygen content in the molding chamber is less than 500ppm.

[0082] (6) A layer of mixed powder is laid on the forming substrate using a scraper. The mixed powder is melted using a fiber laser based on the geometry of the two-dimensional slice. The laser scanning process parameters are as follows: laser power is 95W, laser scanning speed is 1200mm / s, laser scanning spacing is 60μm, scanning strategy is S-shaped orthogonal, interlayer rotation angle is 67°, and each layer of mixed powder is melted twice.

[0083] (7) After each layer of mixed powder is processed, the substrate is lowered by 40 μm, a new layer of mixed powder is laid, and then it is melted. This cycle is repeated until the sample is formed.

[0084] In this embodiment, the laser power is slightly increased compared to Example 3, and the surface formability and density of the sample are slightly reduced, but the overall performance of the sample is similar to that of Example 3.

[0085] Comparative Example 1 This comparative example uses the same method as Example 1 to prepare 316L stainless steel samples, the difference being that Fe was not added in this comparative example. 95.7 C 0.5 B 3.8 powder.

[0086] Comparative Example 2 This comparative example uses the same method as Example 1 to prepare 316L stainless steel samples, the difference being that: this comparative example uses Fe... 94.7 C 1.5 B 3.8 Alloy powder replacing Fe 95.7 C 0.5 B 3.8 powder.

[0087] Although the strength of the stainless steel sample in this comparative example was improved, the elongation decreased sharply, from 24.7% to 9.8%. This is because the present invention uses Fe... 95.7 C 0.5 B 3.8 As an alloy powder, the carbon content is relatively low. The boron element in the powder easily forms high-melting-point Fe-B or Fe-B-Cr type compounds with Fe, Cr and other elements in 316L stainless steel, which serve as heterogeneous nucleation nuclei and effectively increase the number of nucleation points in the molten pool. At the same time, the introduction of carbon can form fine carbides with Fe during solidification, which will have a pinning effect on grain boundaries and further inhibit grain growth. The Fe-based composition ensures good wettability and metallurgical bonding with the 316L matrix, avoiding the cracking problem caused by poor wettability in traditional ceramic reinforcing phases.

[0088] Comparative Example 3 This comparative example uses the same method as Example 1 to prepare 316L stainless steel samples, the difference being that: this comparative example uses Fe... 92.2 Ti 3.5 B 3.8 C 0.5 Alloy powder replacing Fe 95.7 C 0.5 B 3.8 powder.

[0089] Because Ti is a highly reactive element, it significantly alters the molten pool behavior, changes the Marangoni convection direction, and reduces fluidity, thereby reducing the compatibility of the alloy powder with the SLM and affecting the quality of the final sample. Furthermore, Ti is a typical strong carbide / boride forming element, and its bonding ability with C and B is much higher than that with Fe (Ti-C and Ti-B bond energies are much higher than Fe-C and Fe-B), directly causing the destruction of the Fe-BC strengthening system designed in this invention.

[0090] Comparative Example 4 This comparative example uses the same method as Example 1 to prepare 316L stainless steel samples, the difference being that: this comparative example uses Fe... 92.195 Ti 3.5 B 3.8 C 0.5 Sr 0.005 RE 0.005 Alloy powder replacing Fe 95.7 C 0.5 B 3.8 Powder, in which RE represents rare earth elements.

[0091] In this comparative example, although the strong affinity of rare earth elements for oxygen and sulfur promotes the formation of stable oxides and sulfides, thus purifying the melt and mitigating the adverse effects of Ti addition, and both Sr and rare earth elements can act as heterogeneous nucleation sites, refining the microstructure and suppressing the coarsening of the titanium-based hard phase, they can also lead to excessive formation of rare earth inclusions and an increase in the amount of brittle phases, thereby increasing the risk of potential crack initiation.

[0092] Comparative Example 5 This comparative example uses the same method as Example 1 to prepare 316L stainless steel samples, the difference being that: in this comparative example, elemental Fe powder, C powder, B powder and 316L stainless steel powder are mixed.

[0093] The stainless steel components prepared in this comparative example exhibited severe compositional segregation, numerous incomplete fusion defects, and uncontrolled metallurgical reactions. Due to the high melting point of boron (B) (2076℃) and the sublimation temperature of carbon (C) exceeding 3500℃, while the melting point of iron (Fe) is only 1538℃, in the SLM molten pool of this comparative example, when the iron powder melts, the high-melting-point B and C particles may only just begin to soften or remain solid. This easily leads to the inability of solid B and C particles to fully alloy with the liquid iron, resulting in inclusions, disrupting the continuity of the matrix, and causing incomplete fusion defects. Simultaneously, because the diffusion of B and C in the molten iron requires time, the ultrafast solidification rate of SLM (up to 10...)... 6K / s will "freeze" these elements that cannot be evenly distributed in time, forming local boron-rich or carbon-rich areas, leading to compositional segregation. Fe-CB powder itself is an iron-based alloy, which has perfect wettability and metallurgical compatibility with molten 316L stainless steel powder. It avoids the risks of compositional segregation, incomplete fusion defects and runaway metallurgical reactions caused by the huge differences in the melting and boiling points of various elements during the SLM ultrafast melting and solidification process, ensuring defect-free bonding at the interface and laying the foundation for obtaining highly dense components.

[0094] Meanwhile, the present invention incorporates alloy powder, which can release boron and carbon in situ in the molten pool in an atomically uniform manner, stably generating fine and dispersed high-melting-point borides as heterogeneous nucleation cores, and ensuring perfect wettability and metallurgical compatibility with the 316L matrix, thereby achieving precise, uniform and stable control of grain structure.

[0095] Comparative Example 6 This comparative example uses the same method as Example 1 to prepare 316L stainless steel samples, the difference being that: this comparative example uses Fe... 95 C 0.5 B 4.5 Alloy powder replacing Fe 95.7 C 0.5 B 3.8 powder.

[0096] In this comparative example, due to the excessively high boron content, coarse Fe-B or Fe-B-Cr type borides were excessively formed during solidification. Especially under the rapid solidification conditions of SLM, the precipitation behavior of borides was difficult to control precisely, easily forming a continuous or semi-continuous brittle phase network at the grain boundaries, severely weakening the grain boundary bonding strength, resulting in a significant decrease in the plasticity of the sample and an increase in crack susceptibility. In addition, excessively high boron content also increases the viscosity of the molten pool, reduces melt fluidity, worsens the uniformity of powder spreading and the quality of interlayer bonding, and increases the risk of metallurgical defects such as voids and lack of fusion, which is not conducive to obtaining high-density, high-performance 316L stainless steel components.

[0097] Comparative Example 7 This comparative example uses the same method as Example 1 to prepare 316L stainless steel samples, the difference being that: this comparative example uses Fe... 96.5 C 0.5 B3 alloy powder replaces Fe 95.7 C 0.5 B 3.8 powder.

[0098] In this comparative example, due to the low B content, the number of high-melting-point Fe-B or Fe-B-Cr heterogeneous nucleation cores formed in the molten pool is reduced, resulting in insufficient nucleation density. This makes it difficult to effectively suppress the epitaxial growth of columnar crystals, significantly weakening the grain refinement effect. Simultaneously, the reduced B content also weakens the solute undercooling effect, decreasing the ability to control the crystal growth rate and making it easier to form coarse columnar crystal structures, leading to increased anisotropy and degraded mechanical properties. Furthermore, insufficient B content also affects the synergistic effect of C during solidification, reducing the grain boundary pinning effect and further weakening the microstructure stability, making it difficult to achieve the comprehensive performance improvement target described in this invention.

[0099] pass Figure 1 (a) and Figure 1 (b) It can be seen that both 316L stainless steel powder and Fe-CB alloy powder exhibit high sphericity, with regular particle morphology and smooth surface, and no obvious agglomeration. Among them, the Fe-CB powder has a smaller particle size than 316L stainless steel powder, which can achieve uniform coating and dispersion during ball milling and mixing, thus providing the basic conditions for full melting and uniform metallurgical bonding in the subsequent selective laser melting process.

[0100] pass Figure 2 It can be seen that the 316L stainless steel sample prepared in Comparative Example 1 exhibits obvious continuous epitaxial growth; while the 316L stainless steel sample prepared in the present invention shows a significant weakening of epitaxial grain continuity, a significant refinement of grain size, and further suppression of epitaxial growth characteristics with increasing addition amount, with grains tending to be equiaxed. This proves that Fe-BC alloy powder can achieve grain refinement by increasing nucleation density and suppressing epitaxial growth during the rapid solidification process of SLM.

[0101] pass Figure 3 It can be seen that with the increase of Fe-CB alloy powder addition, the average grain size of the 316L stainless steel sample gradually decreases. Statistical measurements show that the average grain size of the sample in Comparative Example 1 is 10 μm, while the average grain size of the sample in Example 4 drops to 5.7 μm, a decrease of approximately 43%.

[0102] pass Figure 4 It can be seen that the microhardness of the 316L stainless steel sample in Example 4 is 401 HV, while the microhardness of the 316L stainless steel sample in Comparative Example 1 is 245 HV. The microhardness of the sample in Example 4 is approximately 63% higher than that in Comparative Example 1, and the relative density reaches over 99.8%. This further verifies that Fe-C alloy powder can achieve grain refinement in the SLM rapid solidification environment by increasing the nucleation rate and inhibiting grain growth, thus providing an microstructure basis for improving the mechanical properties of the material.

[0103] In summary, this invention achieves effective grain refinement of 316L stainless steel by adding Fe-BC alloy powder during the SLM preparation process. This is achieved by using Fe-BC alloy powder to provide heterogeneous nucleation sites under rapid solidification conditions, inhibiting grain epitaxial growth, and enhancing solid solution strengthening effects. This significantly improves the material's hardness and overall mechanical properties, thereby expanding the application range of 316L stainless steel in high-strength and high-wear-resistant fields.

[0104] Finally, it should be noted that the above preferred embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail through the above preferred embodiments, those skilled in the art should understand that various changes can be made to it in form and detail without departing from the scope defined by the claims of the present invention.

Claims

1. A method of refining 316L stainless steel grains based on selective laser melting technology, characterized in that: Fe-C-B alloy powder is added to form Fe-B or Fe-B-Cr type compound as heterogeneous nucleation core when 316L stainless steel component is made by selective laser melting, so as to inhibit columnar crystal epitaxial growth and realize grain refinement.

2. The method of claim 1, wherein: The method comprises the following steps: (1) uniformly mixing Fe-C-B alloy powder and 316L stainless steel powder under inert gas protection environment to obtain mixed powder; (2) forming the mixed powder obtained in the step (1) by using selective laser melting technology to obtain 316L stainless steel component.

3. The method of claim 1, wherein: In the step (1), the Fe-C-B alloy powder comprises Fe: 95.7%-95.9%, C: 0.3%-0.5% and B: 3.8% by mass fraction.

4. The method of claim 1, wherein: In the step (1), the Fe-C-B alloy powder accounts for 0.3%-1.0% of the total mass of the mixed powder.

5. The method of claim 1, wherein: In the step (1), the Fe-C-B alloy powder and the 316L stainless steel powder are mixed uniformly by using dry ball milling method.

6. The method of claim 5, wherein: The ball milling rotation speed is 100-300 r / min, the ball-to-material ratio is 5-10:1, the ball milling is paused for 15-25 min after being performed for 20-30 min, and the total ball milling time is 1-4 h.

7. The method of claim 1, wherein: The particle size of the Fe-C-B alloy powder is not more than 15 μm.

8. The method of claim 1, wherein: The particle size of the 316L stainless steel powder is 15-53 μm.

9. The method of claim 1, wherein: In the step (2), the laser power of the selective laser melting is 80-95 W, the laser scanning speed is 1000-1400 mm / s, the scanning interval is 60 μm, the scanning strategy is S-type orthogonal, the layer-to-layer rotation angle is 67°, the number of times of melting each layer of mixed powder is 1-2, and the forming substrate is lowered by 20-40 μm after completing the formation of each layer of mixed powder.

10. The method according to any one of claims 1 to 9, characterized in that: The 316L stainless steel powder comprises Cr: 16%-18%, Ni: 10%-14%, Mo: 2%-3%, Mn: ≤2%, C: ≤0.03%, Si: ≤0.75%, P: ≤0.025%, S: ≤0.01%, N: ≤0.1% and the balance of Fe by mass percentage.

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