Method for improving size precision and surface quality of Nb521 alloy formed through electron beam selective melting

Abstract

The invention discloses a method for improving size precision and surface quality of Nb521 alloy formed through electron beam selective melting. The method comprises the steps that firstly, a three-dimensional model of the Nb521 alloy is stretched in the height direction, slicing and layering are conducted, each single slicing layer is divided into an inner area and an outer area, and data of each slicing layer are obtained; secondly, spherical Nb521 pre-alloyed powder is added, and data of all cutting layers are imported; 3, after powder spreading, scanning and melting in different areas to form a first single-layer solid sheet layer; and 4, powder spreading and melting processes are repeated. According to the method, through height stretching pre-compensation cooling shrinkage of the Nb521 alloy three-dimensional model, a single cutting layer is divided into a surface area and an internal area, double-concentric-square-shaped contour scanning is conducted on the surface area, snakelike continuous scanning, negative defocusing and corner dynamic adjustment are conducted on the internal area, the forming quality and efficiency are guaranteed, and the method is suitable for large-scale production. And the size precision and the surface quality of the Nb521 alloy part are improved, and the method is suitable for manufacturing Nb521 alloy complex structural parts.

Description

Technical Field

[0001] This invention belongs to the field of refractory metal materials and their preparation, specifically relating to a method for improving the dimensional accuracy and surface quality of Nb521 alloy formed by electron beam selective melting. Background Technology

[0002] Nb521 alloy is a typical refractory niobium-based alloy with excellent high-temperature strength, oxidation resistance, and corrosion resistance. It can operate for extended periods at high temperatures of 1200℃ to 1500℃, making it a key material for manufacturing high-temperature core components for aerospace applications. Electron beam selective melting, as an important branch of additive manufacturing, has become one of the mainstream forming methods for complex Nb521 alloy components due to its advantages of high energy density and a clean vacuum forming environment.

[0003] However, there are still two major problems when electron beam selective melting is used to form Nb521 alloy: First, the dimensional accuracy is difficult to control. The dimensional error of Nb521 alloy after high temperature melting often exceeds ±0.7mm. Second, the surface quality is poor. Traditional methods use a single scanning path and process parameters. The surface area is prone to burrs and depressions due to uneven melting. Energy concentration at the scanning corners of the internal area is prone to edge warping. The surface roughness Ra is often higher than 25μm, which cannot meet the usage requirements of some parts. The above dimensional and surface quality problems mainly stem from: (1) Melt pool size and control problem: The electron beam has a high energy density and is prone to forming a large melt pool, which causes the part contour to "collapse" or "stick powder", making the actual formed contour deviate from the design model. Fine features such as sharp corners, thin walls, and peaks are difficult to retain. (2) Heat accumulation and scanning strategy: Large-area continuous scanning will lead to heat accumulation, causing local overheating, splashing and spheroidization, deteriorating the surface quality and affecting the uniformity of the internal structure. (3) Z-axis shrinkage and dimensional distortion: After the metal powder melts and solidifies, it will cause the part to shrink in the vertical direction. If a fixed layer thickness is used and this shrinkage is not taken into account, the final part height will be smaller than the design value, the feature layer position will be distorted, and the assembly and use performance of the part will be seriously affected.

[0004] Existing technologies typically employ uniform scanning strategies and process parameters for electron beam melting forming, making it difficult to simultaneously achieve both internal densification efficiency and external contour accuracy. For example, to obtain high density, higher energy input and continuous scanning are often used, but this sacrifices surface quality; conversely, reducing energy input to improve surface quality may lead to internal incomplete fusion defects. Therefore, developing a selective electron beam melting forming method for niobium-tungsten alloys that can precisely control dimensions and surface quality has become a pressing technical challenge in this field. Summary of the Invention

[0005] The technical problem this invention aims to solve is to address the shortcomings of the prior art by providing a method for improving the dimensional accuracy and surface quality of Nb521 alloy formed by electron beam selective melting. This invention utilizes a three-dimensional model with high-stretch pre-compensation for cooling shrinkage to divide a single layer into surface and internal regions. The surface region undergoes a "double-loop contour scan," while the internal region is subjected to a "serpentine continuous scan + negative defocus + corner dynamic adjustment." This ensures both forming quality and efficiency while simultaneously improving the dimensional accuracy and surface quality of Nb521 alloy parts, thus resolving the problems of large dimensional deviations and poor surface quality during electron beam selective melting of Nb521 alloy.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: a method for improving the dimensional accuracy and surface quality of Nb521 alloy formed by electron beam selective melting, characterized in that the method includes the following steps: Step 1: Stretch the three-dimensional model of the target product Nb521 alloy in the height direction, and then slice and layer the stretched three-dimensional model of Nb521 alloy to obtain several layers. Divide the cross-sectional area in each individual layer into an internal area and a surface area. The surface area is the area in the cross-sectional area that is D away from the outermost edge, and the other areas are the internal areas. Obtain the data of each layer. Step 2: Add spherical Nb521 pre-alloyed powder into the electron beam selective melting and forming equipment, and import the three-dimensional model after slicing and layering in Step 1, as well as the data of each slice. Step 3: Spread the spherical Nb521 pre-alloyed powder added to the electron beam selective melting forming equipment in Step 2 to form a powder layer, and scan and melt the powder layer according to the first layer data in the layer data imported in Step 2: First, perform a back-shaped scan on the surface area of ​​the first layer to form the inner contour, then perform a serpentine continuous scan on the internal area of ​​the first layer, and dynamically adjust the current reduction and scanning speed increase at the corners of the scanning path, and finally perform a back-shaped scan on the surface area of ​​the first layer to form the outer contour, forming the first single-layer solid sheet, with the distance between the outer contour and the inner contour being d, the surface contour indentation of the outermost edge of the outer contour being H1, and the indentation of the scanning line of the internal area from the boundary line of the internal area and the surface area being H2; Step 4: Lower the forming cavity of the electron beam selective melting forming equipment by one layer thickness, and complete the preparation of the second single-layer solid sheet according to the powder spreading and scanning melting process in Step 3. Continue to repeat the forming cavity lowering, powder spreading, and scanning melting process so that each single-layer solid sheet is formed and accumulated layer by layer to obtain Nb521 alloy.

[0007] In the research process of this invention, the functions and requirements of different regions of the target product, Nb521 alloy, were decomposed: the core requirements of the surface region (contour) are dimensional accuracy, geometric contour accuracy, and surface finish; the core requirements of the internal region (core) are forming efficiency, densification, and internal metallurgical quality. Based on this, this invention adopts different scanning strategies and process parameters, namely "double-circle contour scanning". On the one hand, precise contour forming is achieved by controlling the heat input and the size of the molten pool. "Double-circle" means that at least two scans are performed at the boundary: the first circle (inner circle) serves as the main melting zone, ensuring good bonding with the internal molten pool and fixing the edge metal powder; the second circle (outer circle) typically uses lower energy to "refine" the contour formed by the first circle, melting or remelting flash and spheroidization defects formed by heat conduction into the powder, making the molten pool boundary smoother and closer to the theoretical contour line. In addition, by precisely controlling the parameters of the inner and outer contour scanning, a small molten pool with stable dimensions and controllable depth can be formed, avoiding energy penetration to the outside of the model and causing dimensional expansion, thereby ensuring the dimensional accuracy of the Nb521 alloy. On the other hand, surface metallurgical quality is improved by adjusting thermal cycling and residual stress. The second scan performs localized "heat treatment" on the area scanned in the first scan, using part of the heat input from the second scan to alleviate the thermal stress formed in the first scan. Although it cannot completely eliminate the stress, it can optimize the stress distribution in the surface area, reducing the risk of cracking and subsequent deformation.

[0008] The method described above for improving the dimensional accuracy and surface quality of Nb521 alloy formed by selective electron beam melting is characterized in that the stretching factor in the height direction in step one is 1.02 to 1.04.

[0009] The aforementioned method for improving the dimensional accuracy and surface quality of Nb521 alloy formed by electron beam selective melting is characterized in that, in step one, the thickness of each slice after slicing is 0.05mm~0.08mm, and D = 0.12mm~0.35mm. D is equivalent to the surface area width in the cross-sectional region. By controlling the surface area to be the region in the cross-sectional region with a distance of 0.12mm~0.35mm from the outermost edge (i.e., surface area width D = 0.12mm~0.35mm), the cross-sectional region within each individual slice is divided into an internal region and a surface region, laying the foundation for subsequent "differential scanning of internal and external regions".

[0010] The method described above for improving the dimensional accuracy and surface quality of Nb521 alloy formed by selective electron beam melting is characterized in that the loose packing density of the spherical Nb521 pre-alloy powder in step two is 45% to 55% of the target product Nb521 alloy. By limiting the loose packing density of the spherical Nb521 pre-alloy powder, both the uniformity of powder spreading and the avoidance of voids during electron beam melting are ensured, which is beneficial to improving the dimensional accuracy of the Nb521 alloy.

[0011] The method described above for improving the dimensional accuracy and surface quality of Nb521 alloy formed by selective electron beam melting is characterized in that, in step three, the distance d between the outer contour and the inner contour is 0.02mm~0.05mm, the surface contour indentation H1 of the outermost edge of the outer contour is 0.1mm~0.3mm, and the indentation H2 of the scanning line of the inner region from the boundary line between the inner region and the surface region is 0.02mm~0.05mm. Through the dual contour design of inner and outer contours and the limitation of each distance and indentation, the final melted outer contour utilizes a finer beam than the inner contour to perform minor remelting correction on the surface, further reducing surface burrs.

[0012] The method described above for improving the dimensional accuracy and surface quality of Nb521 alloy formed by selective electron beam melting is characterized in that the process parameters for forming the inner contour by a single zigzag scan in step three are: electron beam current 5mA~8mA, electron beam scanning speed 0.5m / s~0.6m / s; the process parameters for forming the outer contour by a single zigzag scan are: electron beam current 3mA~5mA, electron beam scanning speed 0.2m / s~0.3m / s; and the process parameters for performing a serpentine continuous scan of the inner region are: electron beam current 10mA~12mA, electron beam scanning speed 0.7m / s~0.8m / s.

[0013] The aforementioned method for improving the dimensional accuracy and surface quality of Nb521 alloy formed by selective electron beam melting is characterized in that the dynamic adjustment of current reduction and scanning speed increase at the scanning path corner in step three specifically involves: when the scanning path direction change angle is greater than 60°, the electron beam current is instantaneously reduced by 15% to 25% starting 0.5 mm before the corner point, while the scanning speed is instantaneously increased by 10% to 20%. This dynamic adjustment of current reduction and scanning speed increase at the scanning path corner avoids energy accumulation and warping at the corner. Typically, "instantaneous" means that the electron beam current and scanning speed are directly adjusted and changed 0.5 mm before the corner point.

[0014] The aforementioned method for improving the dimensional accuracy and surface quality of Nb521 alloy formed by selective electron beam melting is characterized in that, in step three, the scanning of the inner and outer contours of the surface region both employ normal focusing mode, while the scanning of the inner region employs negative defocusing focusing. Normal focusing mode refers to the focusing plane being on the surface of the powder layer, while negative defocusing focusing refers to the focusing plane being below the surface of the powder layer. This invention ensures surface melting accuracy by controlling the use of normal focusing mode during surface region scanning, and then employs negative defocusing focusing during the serpentine continuous scanning of the inner region to achieve more uniform energy and reduce internal defects. Finally, by reducing the electron beam current and slowing down the scanning speed during the outer contour scanning of the surface region, the surface of the resulting Nb521 alloy is smoother.

[0015] In this invention, the "serpentine continuous scanning + negative defocusing + corner dynamic adjustment" method in the internal region achieves a balance between efficiency and internal quality. Specifically, by employing a higher electron beam current and a moderate electron beam scanning speed, combined with a negative defocusing focusing method, deep penetration welding is achieved, ensuring both penetration depth and efficiency. However, at the corners of the focusing method, the scanning direction and speed of the electron beam change drastically. If the high current is not controlled, it will lead to heat accumulation, and the electron beam will stay at the corner for a relatively long time, resulting in excessive heat input in that area. Therefore, dynamic adjustment at the corners is also used to compensate for the increase in effective energy density caused by the change in direction, ensuring that the heat input is basically consistent throughout the entire scanning path (including straight lines and corners).

[0016] Compared with the prior art, the present invention has the following advantages: 1. This invention stretches the three-dimensional model of the target product Nb521 alloy in the height direction to actively compensate for the cooling and shrinkage in the vertical direction after the raw material powder melts, solidifies and densifies, thereby controlling the dimensional accuracy of Nb521 alloy from the source.

[0017] 2. This invention divides the single-layer cross-sectional area of ​​the three-dimensional model of the target product, Nb521 alloy, into an internal region and a surface region. The surface region undergoes a "double-loop contour scan," while the internal region is subjected to a "serpentine continuous scan + negative defocus + corner dynamic adjustment." By controlling the scanning process parameters and focusing mode of each region, this invention effectively balances surface quality and forming efficiency. 3. This invention solves the problem of warping caused by concentrated energy accumulation at corners in traditional methods by dynamically adjusting the current reduction and scanning speed increase at the corners of the scanning path in the internal region, thus achieving "warping-free, high-precision" forming.

[0018] 4. The method of the present invention does not require complex equipment modification. Through process optimization alone, it can reduce the dimensional error of Nb521 alloy parts from more than ±0.7mm to within ±0.3mm and the surface roughness from more than Ra25μm to less than Ra15μm. At the same time, it also avoids the problem of warping. It is suitable for the manufacture of complex structural parts of Nb521 alloy and has strong practicality.

[0019] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the region division and scanning melting within a single slice in this invention. Detailed Implementation

[0021] Example 1 This embodiment includes the following steps: Step 1: Extrude the 3D model of the target Nb521 alloy part (length × width × height) by 1.02 times in the height direction. Then, slice the extruded Nb521 alloy 3D model to obtain several layers with a thickness of 0.05mm. Divide the cross-sectional area within each individual layer into an internal region and a surface region. The surface region is the area in the cross-sectional area at a distance of D=0.12mm from the outermost edge. The other regions are the internal regions. Figure 1 As shown, the data for each slice layer is obtained; Step 2: Add spherical Nb521 pre-alloy powder to the electron beam selective melting and forming equipment. The loose packing density is 45% of the target product Nb521 alloy part. Then import the three-dimensional model after slicing and layering in Step 1 and the data of each slice. Step 3: Spread the spherical Nb521 pre-alloyed powder added to the electron beam selective melting and forming equipment in Step 2 to form a powder layer, and melt the powder layer according to the first layer cutting data in the cutting data imported in Step 2, such as... Figure 1 As shown: First, a loop scan is performed on the surface area of ​​the first layer to form the inner contour. The electron beam current is 5mA, the electron beam scanning speed is 0.5m / s, and the normal focusing mode is used. Then, a serpentine continuous scan is performed on the internal region of the first slice layer, with an electron beam current of 10mA and an electron beam scanning speed of 0.7m / s. A negative defocusing focusing method is adopted, and dynamic adjustment is implemented at the corners of the scanning path to reduce the current and increase the scanning speed: when the angle of change of the scanning path direction is greater than 60°, the electron beam current is instantaneously reduced by 15% 0.5mm before the corner point, while the scanning speed is instantaneously increased by 10%. Finally, a back-scan is performed on the surface area of ​​the first slice to form the outer contour. The electron beam current is 3mA, the electron beam scanning speed is 0.2m / s, and the normal focusing mode is used to form the first single-layer solid sheet. Among them, the distance between the outer contour and the inner contour ( Figure 1 The abbreviation for the inner and outer contour spacing is d=0.02mm, and the surface contour indentation of the outermost edge of the outer contour ( Figure 1 The abbreviation for surface contour indentation is H1=0.1mm, which is the amount of indentation between the scan line of the inner region and the boundary line between the inner region and the surface region. Figure 1 The internal melt line indentation (abbreviated as H2 = 0.02 mm) is 0.02 mm. Step 4: Lower the forming cavity of the electron beam selective melting forming equipment by 0.05 mm. Following the powder spreading and melting process in Step 3, complete the preparation of the second single-layer solid sheet. Continue to repeat the forming cavity lowering, powder spreading, and melting process to form and accumulate each single-layer solid sheet layer by layer to obtain the Nb521 alloy part.

[0022] Testing revealed that the Nb521 alloy parts prepared in this embodiment had a dimensional error of ±0.3 mm, a surface roughness Ra=14 μm, no obvious burrs or warping, and exhibited high dimensional accuracy and excellent surface quality.

[0023] Example 2 This embodiment includes the following steps: Step 1: Extrude the 3D model of the target Nb521 alloy part (length × width × height) by 1.03 times in the height direction. Then, slice the extruded Nb521 alloy 3D model to obtain several layers with a thickness of 0.06mm. Divide the cross-sectional area within each individual layer into an internal region and a surface region. The surface region is the area in the cross-sectional area at a distance of D=0.2mm from the outermost edge. The other regions are the internal regions. Figure 1 As shown, the data for each slice layer is obtained; Step 2: Add spherical Nb521 pre-alloy powder to the electron beam selective melting and forming equipment. The loose packing density is 50% of the target product Nb521 alloy part. Then import the three-dimensional model after slicing and layering in Step 1 and the data of each slice. Step 3: Spread the spherical Nb521 pre-alloyed powder added to the electron beam selective melting and forming equipment in Step 2 to form a powder layer, and melt the powder layer according to the first layer cutting data in the cutting data imported in Step 2, such as... Figure 1 As shown: First, a loop scan is performed on the surface area of ​​the first layer to form the inner contour. The electron beam current is 6mA, the electron beam scanning speed is 0.55m / s, and the normal focusing mode is used. Then, a serpentine continuous scan is performed on the internal region of the first slice layer, with an electron beam current of 11mA and an electron beam scanning speed of 0.75m / s. A negative defocusing focusing method is adopted, and dynamic adjustment is implemented at the corners of the scanning path to reduce the current and increase the scanning speed: when the angle of change of the scanning path direction is greater than 60°, the electron beam current is instantaneously reduced by 20% 0.5mm before the corner point, while the scanning speed is instantaneously increased by 15%. Finally, a back-scan is performed on the surface area of ​​the first layer to form the outer contour. The electron beam current is 4mA, the electron beam scanning speed is 0.25m / s, and the normal focusing mode is used to form the first single-layer solid sheet. Among them, the distance between the outer contour and the inner contour ( Figure 1 The abbreviation for the inner and outer contour spacing is d=0.03mm, and the surface contour indentation of the outermost edge of the outer contour ( Figure 1 The abbreviation for surface contour indentation is H1=0.2mm, which is the amount of indentation between the scan line of the inner region and the boundary line between the inner region and the surface region. Figure 1 The internal melt line indentation (abbreviated as H2 = 0.03 mm) is 0.03 mm. Step 4: Lower the forming cavity of the electron beam selective melting forming equipment by 0.06 mm. Following the powder spreading and melting process in Step 3, complete the preparation of the second single-layer solid sheet. Continue to repeat the forming cavity lowering, powder spreading, and melting process to form and accumulate each single-layer solid sheet layer by layer to obtain the Nb521 alloy part.

[0024] Testing revealed that the Nb521 alloy parts prepared in this embodiment had a dimensional error of ±0.2 mm, a surface roughness Ra=12.40 μm, no obvious burrs or warping, and exhibited high dimensional accuracy and excellent surface quality. Furthermore, the forming quality was superior to that of Example 1.

[0025] Example 3 This embodiment includes the following steps: Step 1: Extrude the 3D model of the target Nb521 alloy part (length × width × height) by 1.04 times in the height direction. Then, slice the extruded Nb521 alloy 3D model to obtain several layers with a thickness of 0.08mm. Divide the cross-sectional area within each individual layer into an internal region and a surface region. The surface region is the area in the cross-sectional area at a distance D = 0.35mm from the outermost edge. The other regions are the internal regions. Figure 1 As shown, the data for each slice layer is obtained; Step 2: Add spherical Nb521 pre-alloy powder to the electron beam selective melting and forming equipment. The loose packing density is 55% of the target product Nb521 alloy part. Then import the three-dimensional model after slicing and layering in Step 1 and the data of each slice. Step 3: Spread the spherical Nb521 pre-alloyed powder added to the electron beam selective melting and forming equipment in Step 2 to form a powder layer, and melt the powder layer according to the first layer cutting data in the cutting data imported in Step 2, such as... Figure 1 As shown: First, a loop scan is performed on the surface area of ​​the first layer to form the inner contour. The electron beam current is 8mA, the electron beam scanning speed is 0.6m / s, and the normal focusing mode is used. Then, a serpentine continuous scan is performed on the internal region of the first slice layer, with an electron beam current of 12mA and an electron beam scanning speed of 0.8m / s. A negative defocusing focusing method is adopted, and dynamic adjustment is implemented at the corners of the scanning path to reduce the current and increase the scanning speed: when the angle of change of the scanning path direction is greater than 60°, the electron beam current is instantaneously reduced by 25% 0.5mm before the corner point, while the scanning speed is instantaneously increased by 20%. Finally, a back-scan is performed on the surface area of ​​the first layer to form the outer contour. The electron beam current is 5mA, the electron beam scanning speed is 0.3m / s, and the normal focusing mode is used to form the first single-layer solid sheet. Among them, the distance between the outer contour and the inner contour ( Figure 1 The abbreviation for the inner and outer contour spacing is d=0.05mm, and the surface contour indentation of the outermost edge of the outer contour ( Figure 1 The abbreviation for surface contour indentation is H1=0.3mm, which is the amount of indentation between the scan line of the inner region and the boundary line between the inner region and the surface region. Figure 1 The internal melt line indentation (abbreviated as H2 = 0.05 mm) is 0.05 mm. Step 4: Lower the forming cavity of the electron beam selective melting forming equipment by 0.08 mm. Following the powder spreading and melting process in Step 3, complete the preparation of the second single-layer solid sheet. Continue to repeat the forming cavity lowering, powder spreading, and melting process to form each single-layer solid sheet and accumulate them layer by layer to obtain the Nb521 alloy part.

[0026] Testing revealed that the Nb521 alloy parts prepared in this embodiment had a dimensional error of ±0.25 mm, a surface roughness Ra=11.02 μm, no obvious burrs or warping, and exhibited high dimensional accuracy and excellent surface quality.

[0027] Comparative Example 1 The difference between this comparative example and Example 3 is that the three-dimensional model stretching in step one, the regional division within each individual slice, and the partitioned melting scanning and corner dynamic process in step three were not performed. Instead, continuous scanning with a single parameter, namely electron beam current of 8mA, electron beam scanning speed of 0.6m / s, and normal focusing mode, was performed directly.

[0028] Testing revealed that the Nb521 alloy parts prepared in this comparative example had a dimensional error of ±0.7 mm, a surface roughness Ra=25 μm, two warping points at the corners, and obvious burrs on the surface, indicating low dimensional accuracy and poor surface quality.

[0029] Comparing Example 3 of the present invention with Comparative Example 1, it can be seen that the present invention improves the dimensional accuracy and surface quality of Nb521 alloy parts simultaneously through designs such as high tensile pre-compensation of the three-dimensional model of Nb521 alloy, partitioned scanning of a single cut layer, and dynamic adjustment of corner areas.

[0030] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any way. Any simple modifications, alterations, and equivalent changes made to the above embodiments based on the inventive essence shall still fall within the protection scope of the present invention.

Claims

1. A method for improving the dimensional accuracy and surface quality of Nb521 alloy formed by electron beam selective melting, characterized in that, The method includes the following steps: Step 1: Stretch the three-dimensional model of the target product Nb521 alloy in the height direction, and then slice and layer the stretched three-dimensional model of Nb521 alloy to obtain several layers. Divide the cross-sectional area in each individual layer into an internal area and a surface area. The surface area is the area in the cross-sectional area that is D away from the outermost edge, and the other areas are the internal areas. Obtain the data of each layer. Step 2: Add spherical Nb521 pre-alloyed powder into the electron beam selective melting and forming equipment, and import the three-dimensional model after slicing and layering in Step 1, as well as the data of each slice. Step 3: Spread the spherical Nb521 pre-alloyed powder added to the electron beam selective melting forming equipment in Step 2 to form a powder layer, and scan and melt the powder layer according to the first layer data in the layer data imported in Step 2: First, perform a back-shaped scan on the surface area of ​​the first layer to form the inner contour, then perform a serpentine continuous scan on the internal area of ​​the first layer, and dynamically adjust the current reduction and scanning speed increase at the corners of the scanning path, and finally perform a back-shaped scan on the surface area of ​​the first layer to form the outer contour, forming the first single-layer solid sheet, with the distance between the outer contour and the inner contour being d, the surface contour indentation of the outermost edge of the outer contour being H1, and the indentation of the scanning line of the internal area from the boundary line of the internal area and the surface area being H2; Step 4: Lower the forming cavity of the electron beam selective melting forming equipment by one layer thickness, and complete the preparation of the second single-layer solid sheet according to the powder spreading and scanning melting process in Step 3. Continue to repeat the forming cavity lowering, powder spreading, and scanning melting process so that each single-layer solid sheet is formed and accumulated layer by layer to obtain Nb521 alloy.

2. The method for improving the dimensional accuracy and surface quality of Nb521 alloy by electron beam selective melting and forming according to claim 1, characterized in that, The stretching factor in the height direction mentioned in step one is 1.02 to 1.

04.

3. The method for improving the dimensional accuracy and surface quality of Nb521 alloy formed by selective electron beam melting according to claim 1, characterized in that, In step one, the thickness of each slice after slicing is 0.05mm to 0.08mm, and D is 0.12mm to 0.35mm.

4. The method for improving the dimensional accuracy and surface quality of Nb521 alloy by electron beam selective melting and forming according to claim 1, characterized in that, The loose packing density of the spherical Nb521 pre-alloyed powder in step two is 45% to 55% of the target product Nb521 alloy.

5. A method for improving the dimensional accuracy and surface quality of Nb521 alloy formed by selective electron beam melting according to claim 1, characterized in that, In step three, the distance between the outer contour and the inner contour is d = 0.02mm~0.05mm, the surface contour indentation of the outermost edge of the outer contour is H1 = 0.1mm~0.3mm, and the indentation of the scan line of the inner region from the boundary line between the inner region and the surface region is H2 = 0.02mm~0.05mm.

6. A method for improving the dimensional accuracy and surface quality of Nb521 alloy formed by selective electron beam melting according to claim 1, characterized in that, The process parameters for forming the inner contour in step three are: electron beam current 5mA~8mA, electron beam scanning speed 0.5m / s~0.6m / s; the process parameters for forming the outer contour in step three are: electron beam current 3mA~5mA, electron beam scanning speed 0.2m / s~0.3m / s; the process parameters for performing serpentine continuous scanning of the inner region are: electron beam current 10mA~12mA, electron beam scanning speed 0.7m / s~0.8m / s.

7. A method for improving the dimensional accuracy and surface quality of Nb521 alloy formed by selective electron beam melting according to claim 1, characterized in that, The dynamic adjustment of reducing the current and increasing the scanning speed at the corner of the scanning path described in step three is as follows: when the angle of change of the scanning path direction is greater than 60°, the electron beam current is instantaneously reduced by 15% to 25% starting 0.5mm before the corner point, while the scanning speed is instantaneously increased by 10% to 20%.

8. A method for improving the dimensional accuracy and surface quality of Nb521 alloy formed by selective electron beam melting according to claim 1, characterized in that, In step three, the scanning of the contour within and outside the surface area both adopts the normal focusing mode, while the scanning of the internal area adopts the negative defocus focusing mode.

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