A microstructure refinement method for powder bed electron beam additive manufacturing of niobium tungsten alloy

Abstract

The present invention discloses a microstructure refinement method suitable for powder bed electron beam additive manufacturing of niobium tungsten alloys. By adjusting the distribution of the tiny molten pools formed after the electron beam interacts with the raw niobium tungsten alloy powder, and then adjusting the molten pool morphology, temperature gradient, and solidification rate, the microstructure of the niobium tungsten alloy manufactured by powder bed electron beam additive manufacturing is transformed from a regularly arranged coarse structure to a fine, elongated structure. The present invention uses an electron beam to selectively melt the powder layer, adjusting the molten pool to a discrete distribution, avoiding the thermal influence of continuous molten pools on each other, making the molten pool morphology more stable during the solidification and cooling process, and reducing the temperature gradient, solidification rate, and cooling rate. The microstructure is effectively refined to obtain a niobium tungsten alloy with a fine, elongated structure, thereby ensuring the mechanical properties of the niobium tungsten alloy.

Description

Technical Field

[0001] The present invention belongs to the technical field of refractory metal materials and their preparation, and in particular relates to a microstructure refinement method suitable for powder bed type electron beam additive manufacturing of niobium tungsten alloy. Background Art

[0002] Niobium alloy has the lowest density among refractory metals, and its high-temperature performance surpasses that of the widely used nickel-based superalloys. Niobium-tungsten alloys, which are strengthened by a combination of solid solution and precipitation strengthening, have an operating temperature range of 1200°C to 1650°C and can operate at 2000°C for short periods. They also exhibit excellent weldability and room-temperature ductility, making them ideal high-temperature-resistant materials. Currently, niobium-tungsten alloys developed in my country have been widely applied in aerospace applications such as nozzles, endpieces, and fasteners. Additive manufacturing technology can achieve near-net-shape integration of complex components, facilitating design iteration, shortening product development cycles, and improving structural manufacturability, making it ideal for forming complex structures. However, due to the high melting point of refractory metals, the high energy input and melt pool stability during additive manufacturing are difficult to balance, resulting in a narrow forming window. Consequently, the application of additively manufactured refractory metals is still relatively lagging. Previous research results have found that powder bed electron beam additive manufacturing technology has great advantages in the forming of complex components of refractory metal materials such as niobium tungsten alloy due to its high energy, high forming efficiency, low powder cost, more effective control of forming stress, and the vacuum environment is more conducive to the control of interstitial elements. However, at the same time, due to the high energy input of the heat source, there are problems such as severe crystal epitaxial growth, excessive temperature, and excessively long high-temperature holding time during the accumulation of the molten pool. The problem of coarse microstructure is more significant, which seriously affects the mechanical properties of niobium tungsten alloy. The optimization effect of simply adjusting the melting energy and scanning speed is limited. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to address the deficiencies of the above-mentioned prior art and provide a method for refining the microstructure of niobium-tungsten alloys in powder bed electron beam additive manufacturing. This method selectively melts the powder layer with an electron beam to adjust the molten pool to a discrete distribution, avoiding the thermal influence of continuous molten pools on each other. This makes the molten pool morphology more stable during the solidification and cooling process, reduces the temperature gradient, and increases the solidification and cooling rates, effectively refining the microstructure. The microstructure of niobium-tungsten alloys manufactured in powder bed electron beam additive manufacturing is transformed from a regularly arranged coarse structure to a fine and long strip structure, solving the problem of coarse microstructure of niobium-tungsten alloys caused by continuous melting to form a continuous molten pool in traditional powder bed additive manufacturing, which produces adverse thermal effects.

[0004] In order to solve the above technical problems, the technical solution adopted by the present invention is: a method for refining the microstructure of niobium tungsten alloy manufactured by powder bed electron beam additive manufacturing, which is characterized by adjusting the distribution of the tiny molten pool formed after the electron beam reacts with the raw material niobium tungsten alloy powder, and then adjusting the morphology, temperature gradient and solidification rate of the molten pool, so that the microstructure of the niobium tungsten alloy manufactured by powder bed electron beam additive manufacturing is transformed from a regularly arranged coarse structure to a fine and long strip structure.

[0005] The present invention has found that traditional powder bed additive manufacturing technology mainly uses a high-energy beam to continuously melt metal powder along a specific path. The continuous fine metal molten pool forms a scanning line, which is formed by continuous accumulation point by point, line by line, and surface by surface. Therefore, for some high-melting-point metal materials, the traditional melting method is very likely to form a coarse microstructure, affecting the alloy performance. The present invention addresses the problem of coarse microstructure that is prone to occur when manufacturing refractory metal materials such as niobium tungsten alloy using powder bed electron beam additive manufacturing. The present invention improves the microstructure of niobium tungsten alloy manufactured by additive manufacturing by changing the superposition method of the molten pool. That is, by adjusting the distribution of the tiny molten pool formed after the electron beam interacts with the raw niobium tungsten alloy powder, the morphology, temperature gradient and solidification rate of the molten pool are adjusted. Specifically, the present invention selectively melts a single powder layer locally, sets a single melting point on the scanning path, and performs electron beam melting scanning at each single point, thereby forming a single independent molten pool. Since the heating effect of the subsequent melting point on the previous melting point is limited, the thermal influence between consecutive adjacent molten pools is effectively avoided. Each molten pool completes the melting and solidification process independently, so the molten pool morphology during the molten pool solidification and cooling process is more stable, and the temperature gradient is reduced, the solidification rate and cooling rate are increased, and the microstructure of the niobium tungsten alloy is effectively refined. The melting method of each single powder layer is the same, and the structure of each corresponding single solid layer is refined, thereby transforming the microstructure of the niobium tungsten alloy produced by powder bed electron beam additive manufacturing from a regularly arranged coarse structure to a fine and long strip structure. At the same time, in the traditional continuous melting of metal powder, when the melting area changes, the scanning line length changes significantly, and the cumulative temperature change affects the temperature field balance of the entire melting plane. For niobium tungsten alloys with higher melting points, the imbalance is more significant. The present invention ensures consistent energy within each individual melt pool through discrete distribution, preventing local overmelting during the niobium-tungsten alloy forming process. Furthermore, the present invention achieves uniform, high-quality overlap of the niobium-tungsten alloy melt pools through discrete superposition, avoiding insufficient overlap caused by melt pool spheroidization during continuous scanning and further improving the quality of the niobium-tungsten alloy.

[0006] The above-mentioned microstructure refinement method suitable for powder bed electron beam additive manufacturing of niobium tungsten alloy is characterized in that a three-dimensional model is first established according to the target product niobium tungsten alloy, and the three-dimensional model is layered. Then, the raw material niobium tungsten alloy powder is laid according to the thickness of each layer to form a powder layer. Then, the single layer of powder layer is selectively melted by an electron beam, and a single layer of solid sheet is formed after solidification. The powder laying-melting-solidification process is repeated until each single layer of solid sheet is accumulated to obtain niobium tungsten alloy.

[0007] The above-mentioned microstructure refinement method applicable to powder bed electron beam additive manufacturing of niobium tungsten alloy is characterized in that, in the process of adjusting the distribution of the micro molten pool, the melting process of the single layer of powder includes the following steps:

[0008] Step 1: First, design a scanning path for a single powder layer, and set melting point 1, melting point 2, ..., melting point n in sequence along the scanning path. The melting points from melting point 1 to melting point n are equally spaced, and the spacing is d.

[0009] Then, the electron beam is moved to the melting point 1, and the electron beam is turned on. The electron beam stays at the melting point 1 and melts the niobium-tungsten alloy powder at the melting point 1 to form a small molten pool. After the electron beam is turned off, the small molten pool gradually solidifies to form a solid.

[0010] Continue to move the electron beam to the melting point 2, turn on the electron beam, stay at the melting point 2 and melt the niobium tungsten alloy powder at the melting point 2 to form a small molten pool, and then turn off the electron beam after completion. The small molten pool gradually solidifies to form a solid;

[0011] According to the melting method of melting point 1 and melting point 2, the melting operation is carried out in sequence along the designed scanning path until the niobium tungsten alloy powder at melting point n is completely melted and solidified, at which point the first round of melting and solidification process is completed;

[0012] Step 2: Melting points n+1, n+2, ..., 2n are sequentially set along the scanning path from melting point 1 to melting point n at an interval of m. The intervals between the melting points n+1 to 2n are equal, and the intervals are equal to the intervals between the melting points 1 to n in step 1, which are all d. Then, according to the first round of melting and solidification process in step 1, the niobium tungsten alloy powder at the melting points n+1 to 2n is sequentially melted and solidified using an electron beam. At this time, the second round of melting and solidification process is completed.

[0013] Step 3: Melting point 2n+1, melting point 2n+2, ..., melting point 3n are sequentially set at intervals of m from melting point n+1 to melting point 2n on the scanning path. The intervals between the melting points of melting point 2n+1 to melting point 3n are equal, and the interval distances are equal to the intervals between the melting points of melting point 1 to melting point n in step 1 and the intervals between the melting points of melting point n+1 to melting point 2n in step 2, which are all d. Then, according to the first round of melting process in step 1, the niobium tungsten alloy powder at melting point 2n+1 to melting point 3n is melted and solidified using an electron beam in sequence. At this time, the third round of melting and solidification process is completed.

[0014] Step 4: According to the methods in steps 2 and 3, the melting point is set and the niobium-tungsten alloy powder at the melting point is melted and solidified in sequence until the Nth round of melting and solidification process is completed, and N=d / m+1. At this time, the melting forming process of the single-layer powder layer is completed.

[0015] The present invention sets melting points in sequence along the scanning path and designs the position regularity of the melting points as described above, so that the molten pool is discretely distributed along the scanning path and superimposed in an interval and jump-like manner, thereby effectively controlling the molten pool distribution and facilitating the regulation of the molten pool morphology, temperature gradient, solidification rate and cooling rate during the molten pool solidification and cooling process, thereby achieving control over the degree of microstructure refinement of the niobium tungsten alloy.

[0016] The above method is characterized in that the current during the electron beam melting dwell in steps 1 to 3 is 20 mA to 25 mA, the electron beam dwell time is 0.0001 s to 0.0003 s, and the electron beam movement speed between melting points is 300 mm / s to 1000 mm / s. The present invention selects the above melting current and electron beam dwell time based on the energy input required at the melting points of the niobium tungsten alloy powder to ensure that the niobium tungsten alloy powder at each melting point is smoothly melted to form a small molten pool.

[0017] The above method is characterized in that the scanning path designed in step one is in the shape of a zigzag, that is, the electron beam moves toward the center in the shape of a zigzag along the outer contour of the scanning plane of the single-layer powder layer, and the spacing of the zigzag scanning path is 0.05mm to 0.15mm.

[0018] The above method is characterized in that the distance d between the melting points 1 to n in step 1, the distance d between the melting points n+1 to 2n in step 2, and the distance d between the melting points 2n+1 to 3n in step 3 are all 1 mm to 3 mm.

[0019] The above method is characterized in that the interval m of setting the melting points in step 2 and step 3 is 0.05 mm to 0.15 mm.

[0020] Compared with the prior art, the present invention has the following advantages:

[0021] 1. The present invention adjusts the distribution of tiny molten pools formed after the electron beam interacts with the raw material niobium tungsten alloy powder, so that the molten pools become separate and independent individuals and are discretely distributed, avoiding the thermal influence between continuous adjacent molten pools. The molten pool morphology during the solidification and cooling process is more stable, and the temperature gradient is reduced, the solidification rate and the cooling rate are increased, and the microstructure of the niobium tungsten alloy is effectively refined. As a result, the microstructure of the niobium tungsten alloy manufactured by powder bed electron beam additive manufacturing is transformed from a regularly arranged coarse structure to a fine and long strip structure, thereby improving the mechanical properties of the niobium tungsten alloy.

[0022] 2. The present invention ensures the consistency of energy of a single molten pool by adjusting the molten pool to be discretely distributed, thereby avoiding the uneven temperature field and local overmelting during the forming process caused by the change of the scanning line length formed by traditional continuous melting.

[0023] 3. The present invention achieves uniform and high-quality overlap of the niobium-tungsten alloy molten pool by adjusting the discrete distribution of the molten pool and then superimposing it, avoiding insufficient overlap caused by spheroidization of the molten pool during continuous scanning, ensuring the smooth progress of the additive manufacturing process, and further improving the quality of the niobium-tungsten alloy.

[0024] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Schematic diagram of the melting process of a single powder layer in the present invention.

[0026] Figure 2 This is the microstructure diagram of the niobium tungsten alloy prepared in Example 1 of the present invention.

[0027] Figure 3 This is the microstructure diagram of the niobium tungsten alloy prepared in Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0028] Example 1

[0029] This embodiment includes the following steps: first, a three-dimensional model with a length × width × height of 10 mm × 10 mm × 10 mm is established according to the target product niobium tungsten alloy, and the three-dimensional model is layered, and the thickness of each layer is 0.05 mm. Then, the three-dimensional model is introduced into a powder bed electron beam additive manufacturing device, and the raw material niobium tungsten alloy powder is loaded into the powder bin of the device. Then, the raw material niobium tungsten alloy powder is laid out according to the thickness of each layer to form a powder layer, and the single layer of powder layer is selectively melted by an electron beam to form a single layer of solid sheet after solidification. The powder laying-melting-solidification process is repeated until the single layers of solid sheets are accumulated to obtain a niobium tungsten alloy block.

[0030] like Figure 1 As shown, in the process of adjusting the distribution of the micro-molten pools, the melting process of the single powder layer includes the following steps:

[0031] Step 1: First, design the scanning path of the single-layer powder layer into a U-shaped path. That is, the electron beam moves along the outer contour of the scanning plane of the single-layer powder layer toward the center in a U-shaped path. The spacing of the U-shaped scanning path is 0.05 mm, and the total length is approximately 2020 mm. Melting point 1, melting point 2, ..., melting point 2020 are set in sequence along the scanning path. The melting points of melting point 1 to melting point 2020 are equally spaced, and the spacing is 1 mm.

[0032] Then, the electron beam is moved to the melting point 1, and the electron beam is turned on. The electron beam stays at the melting point 1 and melts the niobium-tungsten alloy powder at the melting point 1 to form a small molten pool. After the electron beam is turned off, the small molten pool gradually solidifies to form a solid.

[0033] Continue to move the electron beam to the melting point 2, turn on the electron beam, stay at the melting point 2 and melt the niobium tungsten alloy powder at the melting point 2 to form a small molten pool, and then turn off the electron beam after completion. The small molten pool gradually solidifies to form a solid;

[0034] According to the melting method of melting point 1 and melting point 2, the melting operation is carried out in sequence along the designed scanning path until the niobium tungsten alloy powder at the melting point 2020 is completely melted and solidified, at which point the first round of melting and solidification process is completed;

[0035] Step 2: Melting points 2021, 2022, ..., 4040 are sequentially set along the scanning path from melting point 1 to melting point 2020 at intervals of 0.05 mm. The melting points 2021 to 4040 are spaced evenly apart and have an interval of 1 mm. Then, according to the first round of melting and solidification process in step 1, the niobium-tungsten alloy powder at melting points 2021 to 4040 is sequentially melted and solidified using an electron beam. At this time, the second round of melting and solidification process is completed.

[0036] Step 3: Melting points 4041, 4042, ..., 6060 are sequentially set at intervals of 0.05 mm along the scanning path from melting point n+1 to melting point 2n. The melting points 4041 to 6060 are spaced evenly apart and have an interval of 1 mm. Then, according to the first round of melting process in step 1, the niobium-tungsten alloy powder at melting points 4041 to 6060 is sequentially melted and solidified using an electron beam. At this point, the third round of melting and solidification process is completed.

[0037] Step 4: According to the methods in Step 2 and Step 3, the melting point is set and the niobium-tungsten alloy powder is melted and solidified at the melting point in sequence until the 21st round of melting and solidification is completed. At this time, the melting forming process of the single powder layer is completed;

[0038] The current during the electron beam melting and dwelling in steps 1 to 3 is 20 mA, the electron beam dwelling time is 0.0001 s, and the moving speed of the electron beam between the melting points is 300 mm / s.

[0039] Comparative Example 1

[0040] The difference between this comparative example and Example 1 is that an electron beam is directly used to continuously melt a single powder layer to form a single solid sheet layer.

[0041] Figure 2 This is the microstructure diagram of the niobium tungsten alloy prepared in Example 1 of the present invention. Figure 3 This is the microstructure of the niobium tungsten alloy prepared in Comparative Example 1 of the present invention. Figure 2 and Figure 3 It can be seen that the microstructure of the niobium tungsten alloy in Comparative Example 1 is a coarse equiaxed structure with a grain size of about 0.25 mm, while the microstructure of the niobium tungsten alloy in Example 1 is transformed into a fine strip-like structure with a strip width of only about 0.05 mm, indicating that the method of the present invention effectively refines the microstructure of the niobium tungsten alloy.

[0042] Example 2

[0043] This embodiment includes the following steps: first, a three-dimensional model with a length × width × height of 10 mm × 10 mm × 10 mm is established according to the target product niobium tungsten alloy, and the three-dimensional model is layered, and the thickness of each layer is 0.05 mm. Then, the three-dimensional model is introduced into a powder bed electron beam additive manufacturing device, and the raw material niobium tungsten alloy powder is loaded into the powder bin of the device. Then, the raw material niobium tungsten alloy powder is laid out according to the thickness of each layer to form a powder layer, and the single layer of powder layer is selectively melted by an electron beam to form a single layer of solid sheet after solidification. The powder laying-melting-solidification process is repeated until the single layers of solid sheets are accumulated to obtain a niobium tungsten alloy block.

[0044] like Figure 1 As shown, in the process of adjusting the distribution of the micro-molten pools, the melting process of the single powder layer includes the following steps:

[0045] Step 1: First, design the scanning path of the single-layer powder layer into a U-shaped path. That is, the electron beam moves along the outer contour of the scanning plane of the single-layer powder layer in a U-shaped path toward the center. The spacing of the U-shaped scanning path is 0.15 mm, and the total length is approximately 2033 mm. Melting points 1, 2, ..., 677 are set in sequence along the scanning path. The melting points of melting points 1 to 677 are equally spaced, and the spacing is 3 mm.

[0046] Then, the electron beam is moved to the melting point 1, and the electron beam is turned on. The electron beam stays at the melting point 1 and melts the niobium-tungsten alloy powder at the melting point 1 to form a small molten pool. After the electron beam is turned off, the small molten pool gradually solidifies to form a solid.

[0047] Continue to move the electron beam to the melting point 2, turn on the electron beam, stay at the melting point 2 and melt the niobium tungsten alloy powder at the melting point 2 to form a small molten pool, and then turn off the electron beam after completion. The small molten pool gradually solidifies to form a solid;

[0048] According to the melting method of melting point 1 and melting point 2, the melting operation is carried out in sequence along the designed scanning path until the niobium tungsten alloy powder at melting point 677 is melted and solidified, at which point the first round of melting and solidification process is completed;

[0049] Step 2: Melting points 678, 679, ..., 1354 are sequentially set along the scanning path from melting point 1 to melting point 677 at intervals of 0.15 mm. The melting points 678 to 1354 are spaced evenly apart and have an interval of 3 mm. Then, according to the first round of melting and solidification process in step 1, the niobium-tungsten alloy powder at melting points 678 to 1354 is sequentially melted and solidified using an electron beam. At this point, the second round of melting and solidification process is completed.

[0050] Step 3: Melting points 1355, 1356, ..., 2031 are sequentially set at intervals of 0.15 mm from melting point 678 to melting point 1354 on the scanning path. The melting points 1355 to 2031 are spaced evenly apart, and the intervals are 3 mm. Then, according to the first round of melting process in step 1, the niobium tungsten alloy powder at melting points 1355 to 2031 is sequentially melted and solidified using an electron beam. At this time, the third round of melting and solidification process is completed.

[0051] Step 4: According to the methods in Step 2 and Step 3, the melting point is set and the niobium-tungsten alloy powder is melted and solidified at the melting point in sequence until the 21st round of melting and solidification is completed. At this time, the melting forming process of the single powder layer is completed;

[0052] The current during the electron beam melting dwell in steps 1 to 3 is 25 mA, the electron beam dwell time is 0.0003 s, and the moving speed of the electron beam between the melting points is 1000 mm / s.

[0053] Example 3

[0054] This embodiment includes the following steps: first, a three-dimensional model with a length × width × height of 10 mm × 10 mm × 10 mm is established according to the target product niobium tungsten alloy, and the three-dimensional model is layered, and the thickness of each layer is 0.05 mm. Then, the three-dimensional model is introduced into a powder bed electron beam additive manufacturing device, and the raw material niobium tungsten alloy powder is loaded into the powder bin of the device. Then, the raw material niobium tungsten alloy powder is laid out according to the thickness of each layer to form a powder layer, and the single layer of powder layer is selectively melted by an electron beam to form a single layer of solid sheet after solidification. The powder laying-melting-solidification process is repeated until the single layers of solid sheets are accumulated to obtain a niobium tungsten alloy block.

[0055] like Figure 1 As shown, in the process of adjusting the distribution of the micro-molten pools, the melting process of the single powder layer includes the following steps:

[0056] Step 1: First, design the scanning path of the single-layer powder layer into a U-shaped path. That is, the electron beam moves along the outer contour of the scanning plane of the single-layer powder layer in a U-shaped path toward the center. The spacing of the U-shaped scanning path is 0.1 mm, and the total length is about 808 mm. Melting points 1, 2, ..., 404 are set in sequence along the scanning path. The melting points of melting points 1 to 404 are equally spaced, and the spacing is 2 mm.

[0057] Then, the electron beam is moved to the melting point 1, and the electron beam is turned on. The electron beam stays at the melting point 1 and melts the niobium-tungsten alloy powder at the melting point 1 to form a small molten pool. After the electron beam is turned off, the small molten pool gradually solidifies to form a solid.

[0058] Continue to move the electron beam to the melting point 2, turn on the electron beam, stay at the melting point 2 and melt the niobium tungsten alloy powder at the melting point 2 to form a small molten pool, and then turn off the electron beam after completion. The small molten pool gradually solidifies to form a solid;

[0059] According to the melting method of melting point 1 and melting point 2, the melting operation is carried out in sequence along the designed scanning path until the niobium tungsten alloy powder at melting point 404 is completely melted and solidified, at which point the first round of melting and solidification process is completed;

[0060] Step 2: Melting points 405, 406, ..., 808 are sequentially set along the scanning path from melting point 1 to melting point 404 at intervals of 0.1 mm. The melting points 404 to 808 are spaced evenly apart and at intervals of 2 mm. Then, according to the first round of melting and solidification process in step 1, the niobium-tungsten alloy powder at melting points 404 to 808 is sequentially melted and solidified using an electron beam. At this point, the second round of melting and solidification process is completed.

[0061] Step 3: Melting points 809, 810, ..., 1212 are sequentially set at intervals of 0.1 mm from melting points 404 to 808 on the scanning path. The melting points 809 to 1212 are spaced evenly apart and have an interval of 2 mm. Then, according to the first round of melting process in step 1, the niobium-tungsten alloy powder at melting points 809 to 1212 is sequentially melted and solidified using an electron beam. At this point, the third round of melting and solidification is completed.

[0062] Step 4: According to the methods in Step 2 and Step 3, the melting point is set and the niobium-tungsten alloy powder is melted and solidified at the melting point in sequence until the 21st round of melting and solidification is completed. At this time, the melting forming process of the single powder layer is completed;

[0063] The current during the electron beam melting dwell in steps 1 to 3 is 21 mA, the electron beam dwell time is 0.0002 s, and the electron beam moving speed between the melting points is 500 mm / s.

[0064] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any way. Any simple modification, change and equivalent variation made to the above embodiment based on the essence of the invention technology shall still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A microstructure refinement method for powder bed electron beam additive manufacturing of niobium tungsten alloy, characterized in that: By adjusting the distribution of the tiny molten pools formed by the electron beam interacting with the raw niobium-tungsten alloy powder, and subsequently adjusting the molten pool morphology, temperature gradient, and solidification rate, the microstructure of the niobium-tungsten alloy produced by powder bed electron beam additive manufacturing is transformed from a regularly arranged coarse structure to a fine, elongated structure. During the process of adjusting the distribution of the tiny molten pools, the melting process of a single powder layer includes the following steps: Step 1: First, design a scanning path for a single powder layer. The designed scanning path is in the shape of a U-shaped triangle, that is, the electron beam moves along the outer contour of the scanning plane of the single powder layer toward the center in the U-shaped triangle. Melting points 1, 2, ..., n are set in sequence along the scanning path. The melting points from melting point 1 to melting point n are equally spaced, and the spacing is d. Then, the electron beam is moved to the melting point 1, and the electron beam is turned on. The electron beam stays at the melting point 1 and melts the niobium tungsten alloy powder at the melting point 1 to form a small molten pool. After the electron beam is turned off, the small molten pool gradually solidifies to form a solid. Continue to move the electron beam to the melting point 2, turn on the electron beam, stay at the melting point 2 and melt the niobium tungsten alloy powder at the melting point 2 to form a small molten pool, and then turn off the electron beam after completion. The small molten pool gradually solidifies to form a solid. According to the melting method of melting point 1 and melting point 2, the melting operation is carried out in sequence along the designed scanning path until the niobium tungsten alloy powder at melting point n is completely melted and solidified, at which point the first round of melting and solidification process is completed; Step 2: Melting points n+1, n+2, ..., 2n are sequentially set along the scanning path from melting point 1 to melting point n at an interval of m. The intervals between the melting points from melting point n+1 to melting point 2n are equal, and the intervals are equal to the intervals between the melting points from melting point 1 to melting point n in step 1, which are all d. Then, according to the first round of melting and solidification process in step 1, the niobium tungsten alloy powder at melting point n+1 to melting point 2n is sequentially melted and solidified using an electron beam. At this time, the second round of melting and solidification process is completed. Step 3: Melting point 2n+1, melting point 2n+2, ..., melting point 3n are sequentially set along the melting point n+1 to melting point 2n on the scanning path at an interval of m. The intervals between the melting points of melting point 2n+1 to melting point 3n are equal, and the interval distances are equal to the intervals between the melting points of melting point 1 to melting point n in step 1 and the intervals between the melting points of melting point n+1 to melting point 2n in step 2, which are all d. Then, according to the first round of melting process in step 1, the niobium tungsten alloy powder at melting point 2n+1 to melting point 3n is sequentially melted and solidified using an electron beam. At this time, the third round of melting and solidification process is completed. Step 4: According to the methods in steps 2 and 3, the melting point is set and the niobium tungsten alloy powder at the melting point is melted and solidified in sequence until the Nth round of melting and solidification process is completed, and N=d / m+1. At this time, the melting forming process of the single layer of powder is completed.

2. A microstructure refinement method for powder bed electron beam additive manufacturing of niobium tungsten alloy according to claim 1, characterized in that: First, a three-dimensional model of the target product, niobium-tungsten alloy, is established and layered. Then, raw niobium-tungsten alloy powder is laid according to the thickness of each layer to form a powder layer. An electron beam is then used to selectively melt the single powder layer, and after solidification, a single solid sheet is formed. The powder laying-melting-solidification process is repeated until each single solid sheet is accumulated to obtain niobium-tungsten alloy.

3. The microstructure refinement method for powder bed electron beam additive manufacturing of niobium tungsten alloy according to claim 1, characterized in that: The current during the electron beam melting and dwelling in steps 1 to 3 is 20 mA to 25 mA, the electron beam dwelling time is 0.0001 s to 0.0003 s, and the moving speed of the electron beam between the melting points is 300 mm / s to 1000 mm / s.

4. The microstructure refinement method for powder bed electron beam additive manufacturing of niobium tungsten alloy according to claim 1, characterized in that: In step 1, the spacing of the zigzag scanning path is 0.05mm~0.15mm.

5. The microstructure refinement method for powder bed electron beam additive manufacturing of niobium tungsten alloy according to claim 1, characterized in that: The distance d between the melting points 1 to n in step 1, the distance d between the melting points n+1 to 2n in step 2, and the distance d between the melting points 2n+1 to 3n in step 3 are all 1 mm to 3 mm.

6. The microstructure refinement method for powder bed electron beam additive manufacturing of niobium tungsten alloy according to claim 1, characterized in that: The interval m between the melting points in step 2 and step 3 is set to 0.05 mm to 0.15 mm.

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