Process for melting and forming high-specific-gravity tungsten alloy and refining columnar crystal by using laser powder bed

Abstract

The invention discloses a process for melting and forming a high-specific-gravity tungsten alloy and refining columnar crystals by using a laser powder bed, which comprises the following steps of: melting composite powder layers layer by layer by using the laser powder bed along the stacking direction to obtain a high-specific-gravity tungsten alloy forming component; the composite powder layer is made of high-specific-gravity tungsten composite powder; and in the process of melting the composite powder layers layer by layer, high-energy laser density and low-energy laser density are adopted to cooperatively regulate and control the melting degree of the composite powder layers, so that partial melting layers and complete melting layers periodically appear in the stacking direction, and the columnar crystal refined high-specific-gravity tungsten alloy is prepared. Therefore, the W particles in the partial melting layer are used for inhibiting the growth of the columnar crystals with relatively strong orientation in the complete melting layer, so that the columnar crystals are refined from a process perspective, the expansion of cracks of the W alloy along the grain boundary of the columnar crystals with relatively strong orientation is further reduced, and a new idea is provided for inhibiting the cracks of the W alloy.

Description

Technical Field

[0001] The invention relates to a process for forming a high-density tungsten alloy by laser powder bed melting and refining columnar crystals, and belongs to the field of laser powder bed melting additive manufacturing. Background Art

[0002] Laser powder bed fusion (LPBF) is an advanced manufacturing process that uses a computer to design a part model and uses a laser beam to selectively melt metal powder in a layered manner to directly form parts with complex structures. Compared with traditional forming methods, LPBF forming technology has obvious advantages such as high freedom in shape design, short process route, and mold-free near-net forming. LPBF has great advantages in the preparation of high-density W alloys. As a type of refractory metal, W alloy melts powder under high energy density input to form a molten pool, which is then accumulated and rapidly solidified into parts through multi-layer molten pool accumulation. However, due to the inherent characteristics of LPBF, the high temperature gradient and cooling rate result in excessive residual stress during the solidification process, which causes many cracks in pure W. See the attached figure. Figure 1 Most of the cracks extend along the grain boundaries of coarse columnar crystals. Therefore, in order to suppress cracks in pure W, the current means include nanoparticle dispersion strengthening and solid solution strengthening by adding alloy elements. The principles of crack suppression are relatively similar, all of which are based on grain refinement and improvement of grain boundary strength. The main purpose of grain refinement is to reduce the epitaxial growth of columnar crystals along the stacking direction. However, even if nanoparticles or alloy elements are added to pure W and the process parameters are optimized, it is difficult to reduce the number of columnar crystals with strong orientation that grow epitaxially along the stacking direction to become coarse columnar crystals. Therefore, it is very necessary to explore a new process method to further refine the columnar crystals based on the solid solution strengthened or dispersion strengthened W alloy. Summary of the invention

[0003] The primary technical purpose of the present invention is to provide a process for laser powder bed melting forming of high-density tungsten alloy and refining columnar crystals, in which a partial melting layer and a complete melting layer appear periodically during the powder melting forming process, and the partial melting layer contains unmelted W particles, incompletely melted W particles and completely melted W particles. The unmelted W particles and incompletely melted W particles in adjacent partially melted layers are used to inhibit the epitaxial growth of columnar crystals with strong orientation in the completely melted layer, thereby refining the columnar crystals with strong orientation in the LPBF-formed high-density W alloy and reducing the tendency of crack propagation.

[0004] In order to achieve the above technical objectives, the present invention will adopt the following technical solutions:

[0005] A process for forming a high-density tungsten alloy by laser powder bed melting and refining columnar crystals, wherein a laser powder bed is used to melt composite powder layers layer by layer along a stacking direction to obtain a high-density tungsten alloy formed component; the composite powder layers are made of high-density tungsten composite powder; in the process of melting the composite powder layers layer by layer, high-energy laser density and low-energy laser density are used to coordinately control the melting degree of the composite powder layers, so that partially melted layers and completely melted layers appear periodically along the stacking direction to form a high-density tungsten alloy with refined columnar crystals;

[0006] The partially melted layer is formed by melting with low energy laser density provided by the laser beam, while the completely melted layer is formed by melting with high energy laser density provided by the laser beam; in the partially melted layer formed, there are incompletely melted W particles, unmelted W particles and completely melted W particles, and the pores between the incompletely melted W particles and the unmelted W particles are filled with completely melted W particles; in the completely melted layer formed, the tungsten element is completely melted W particles.

[0007] Preferably, in the process of laser powder bed fusion composite powder layer forming, the process forming parameters of the partially melted layer and the completely melted layer are determined by the following steps:

[0008] Step 1: Modeling, layered slicing and overall planning of the melting degree of each slice layer:

[0009] A three-dimensional model of the part is created in the modeling software, and then the three-dimensional model of the part is imported into the slicing software for layered slicing and overall planning of the laser melting path of each slice along the stacking direction, so that from the bottom slice to the top slice, each layer of slices is periodically assigned with different laser powers and scanning speeds, so as to finally form a printed component in which a partial melt layer and a complete melt layer appear periodically along the stacking direction;

[0010] Step 2: Determine the process forming parameter range of the partially melted layer and the completely melted layer:

[0011] Determine the laser power range and scanning speed range of the partially melted layer and the completely melted layer, and preliminarily determine the thickness range of each composite powder layer. At the same time, the thickness of the composite powder layer of the completely melted layer meets the following requirements:

[0012]

[0013] Where: H1 represents the thickness of the powder layer of the completely melted layer; H2 represents the height of the columnar crystal growth; k1 represents the correction factor; P1 is the laser power of the completely melted layer; v1 is the scanning speed of the completely melted layer; k s is the thermal conductivity of W alloy;

[0014] Step 3: Preliminarily determine the process forming parameters of the partially melted layer and the completely melted layer:

[0015] Based on the process forming parameter ranges of the partially melted layer and the completely melted layer determined in step 3 and the calculation formula for the thickness of the composite powder layer of the completely melted layer, preliminarily determine the process forming parameters of the partially melted layer and the completely melted layer;

[0016] Step 4: Laser powder bed fusion forming components:

[0017] According to the process forming parameters of the partially melted layer and the completely melted layer preliminarily determined in step 3, based on the laser melting path planned in step 1, a chessboard scanning strategy is adopted to form each slice layer by layer on the printing substrate to obtain a printed component;

[0018] Step 5: Selecting several partial melt layers along the stacking direction of the printed component for metallographic preparation to obtain corresponding metallographic images one by one;

[0019] Step 6: Determine whether there are holes in the obtained metallographic image. If the determination result shows that there are holes in the metallographic image, return to step 3 and re-preliminarily determine the process forming parameters of the partial melt layer and the complete melt layer; otherwise, proceed to step 7.

[0020] Step 7, using an image recognition algorithm to calculate the percentage of the area of ​​unmelted W particles or incompletely melted W particles in each metallographic image to the area of ​​the corresponding metallographic image;

[0021] Step 8: Based on the percentage of the area of ​​the unmelted W particles or the incompletely melted W particles in each metallographic image obtained in step 7 to the area of ​​the corresponding metallographic image, calculate the average percentage of the unmelted W particles or the incompletely melted W particles in the partially melted layer to the entire metallographic mosaic.

[0022] Step 9: Determine the average percentage Is it within the preset percentage threshold range? When the judgment result shows that the average percentage If it is within the preset percentage threshold range, the process forming parameters of the partial melt layer and the complete melt layer preliminarily determined in step three are determined to be the final process forming parameters of the partial melt layer and the complete melt layer. Otherwise, return to step three, re-preliminarily determine the process forming parameters of the partial melt layer and the complete melt layer, continue to optimize the process parameters, and finally narrow the optimal process parameter range and determine the optimal process parameters to complete the forming of high-performance W alloy components.

[0023] Preferably, in step five, the metallographic image obtained is an image formed by stitching together photos at different positions in the corresponding partial melt layer.

[0024] Preferably, in step seven, in the process of calculating the percentage of the area of ​​the unmelted W particles or the incompletely melted W particles in each metallographic image to the area of ​​the corresponding metallographic image, the Hough circle detection algorithm and the contour area screening technology are combined to calculate the area of ​​the unmelted W particles or the incompletely melted W particles in the metallographic image, which specifically includes the following steps:

[0025] Step 71, pre-processing the metallographic image;

[0026] Step 72, performing Hough circle detection on the preprocessed metallographic image and performing Hough circle transformation to identify the areas of unmelted W particles and incompletely melted W particles in the metallographic image;

[0027] Step 73, draw a circle in the metallographic image and calculate the area of ​​the circle, so as to obtain the area of ​​the unmelted W particles or the incompletely melted W particles in the metallographic image.

[0028] Preferably, in the high specific gravity tungsten composite powder, the mass fraction of tungsten is 98wt.%W.

[0029] Preferably, the high specific gravity tungsten composite powder further comprises TiC and Y2O3, and the mass fractions of the components TiC and Y2O3 are 1.5wt.% TiC and 0.5wt.% Y2O3 respectively;

[0030] In step three, the laser power P of the partially melted layer is set in the range of 150W to 200W, the laser power P of the completely melted layer is set in the range of 350W to 400W, the scanning speed v of the partially melted layer is set in the range of 400mm / s to 500mm / s, the scanning speed of the completely melted layer is set in the range of 300mm / s to 400mm / s, and the powder layer thickness of the partially melted layer is in the range of 10μm to 20μm.

[0031] Preferably, in step nine, the preset percentage threshold range is 55±2%.

[0032] Preferably, the final process forming parameters of the partially melted layer and the completely melted layer are: the process parameters of the partially melted layer are P=170W, v=500mm / s, t=20μm, and the process parameters of the completely melted layer are P=350W, v=350mm / s, H1=10μm.

[0033] Preferably, the high energy laser density and the low energy laser density represent the heat input of the laser beam to the composite powder layer, which is calculated by the following formula:

[0034]

[0035] Where P is the laser power; v is the scanning speed; t is the thickness of the composite powder layer; and h is the scanning distance.

[0036] Another technical purpose of the present invention is to provide a laser powder bed fusion-formed high-density tungsten alloy, which is prepared by the above-mentioned laser powder bed fusion-formed high-density tungsten alloy and columnar crystal refinement process.

[0037] Based on the above technical objectives, the present invention has the following advantages over the prior art:

[0038] The process of forming a high-density tungsten alloy and refining columnar crystals by laser powder bed melting described in the present invention adopts high-energy laser density and low-energy laser density to coordinately control the melting degree of the composite powder layer in the process of melting the composite powder layer layer by layer, so that the partially melted layer and the completely melted layer appear periodically along the stacking direction to form a high-density tungsten alloy with refined columnar crystals. Specifically, in the process of melting the composite powder layer layer by layer along the stacking direction by laser powder bed, there is a partially melted layer on both sides of the upper and lower sides of each completely melted layer, so that the growth of the columnar crystals with stronger orientation in the completely melted layer is suppressed by the unmelted W particles and the incompletely melted W particles in the adjacent partially melted layers on both sides thereof, so as to achieve the purpose of refining the columnar crystals. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 The coarse columnar crystals with strong orientation in pure W are prone to cracks at their grain boundaries;

[0040] Figure 2 The diagram shows the mechanism of the partial melting layer inhibiting the growth of the strongly oriented columnar crystals in the complete melting layer.

[0041] Figure 3 This is a diagram of the coordinated regulation mechanism of high energy density and low energy density;

[0042] Figure 4 A flowchart for evaluating the melting degree of W particles in the partially melted layer;

[0043] Figure 5 The optical micrographs of the melt layer in the part with more holes and the inhibition of the growth of columnar crystals are shown in the figure; (a) shows the optical micrograph of the melt layer in the part with more holes, and (b) shows the inhibition of the growth of columnar crystals by the melt layer in the part with more holes;

[0044] Figure 6 The optical micrographs of the dense partial melting layer and its inhibition on the growth of columnar crystals; in the figure, (a) is the optical micrograph of the partial melting layer with more holes, and (b) is the inhibition on the growth of columnar crystals by the partial melting layer with more holes;

[0045] Figure 7It is an optical micrograph of the partial melting layer without holes and its inhibition on the growth of columnar crystals; in the figure, (a) is an optical micrograph of the partial melting layer with more holes, and (b) is an optical micrograph of the partial melting layer with more holes and its inhibition on the growth of columnar crystals. DETAILED DESCRIPTION

[0046] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is by no means any limitation to the present invention and its application or use. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. Unless otherwise specified, the relative arrangement, expressions and numerical values ​​of the components and steps described in these embodiments do not limit the scope of the present invention. The techniques, methods and equipment known to ordinary technicians in the relevant fields may not be discussed in detail, but in appropriate cases, the techniques, methods and equipment should be regarded as part of the specification. In all examples shown and discussed here, any specific value should be interpreted as merely exemplary, rather than as a limitation. Therefore, other examples of the exemplary embodiments may have different values.

[0047] like Figure 2-7 As shown, the process of laser powder bed melting forming high density tungsten alloy and refining columnar crystals described in the present invention adopts a laser powder bed to melt the composite powder layer layer by layer along the stacking direction to obtain a high density tungsten alloy formed component; the composite powder layer targeted by the present invention is paved with high density tungsten composite powder.

[0048] In order to suppress the growth of columnar crystals in high-density tungsten alloy formed components during the forming process, the present invention uses high-energy laser density and low-energy laser density to coordinately control the melting degree of the composite powder layer in the process of melting the composite powder layer layer by layer, so that the partially melted layer and the completely melted layer appear periodically along the stacking direction to form a high-density tungsten alloy with refined columnar crystals. Specifically, the partially melted layer is melted and formed by the low-energy laser density provided by the laser beam, while the completely melted layer is melted and formed by the high-energy laser density provided by the laser beam; in the partially melted layer formed, there are incompletely melted W particles, unmelted W particles and completely melted W particles at the same time, and the pores between the incompletely melted W particles and the unmelted W particles are filled by completely melted W particles; in the completely melted layer formed, the tungsten element is a completely melted W particle.

[0049] In order to ensure the forming quality, the present invention provides a method for determining the process forming parameters of the partially melted layer and the completely melted layer. Figure 4 , including the following steps:

[0050] Step 1: Modeling, layered slicing and overall planning of the melting degree of each slice layer:

[0051] A three-dimensional model of the part is created in the modeling software, and then the three-dimensional model of the part is imported into the slicing software for layered slicing and overall planning of the laser melting path of each slice along the stacking direction, so that from the bottom slice to the top slice, each layer of slice is periodically assigned different laser powers and scanning speeds, so as to finally form a printed component in which partially melted layers and completely melted layers appear periodically along the stacking direction.

[0052] Specifically, the present invention uses low energy laser density when printing the first layer (i.e., the bottom slice) to make it a partially melted layer, and uses high energy laser density when printing the second layer to make it a completely melted layer... and so on until the laser melting of all slices is completed. The energy density distribution in the stacking direction is as follows: Figure 3 As shown, the unmelted W particles and the incompletely melted W particles in the partially melted layer are finally used to inhibit the growth of the columnar crystals with strong orientation in the completely melted layer, so as to achieve the purpose of refining the columnar crystals. The mechanism of the partial melt layer inhibiting the growth of the columnar crystals with strong orientation in the completely melted layer is as follows: Figure 2 shown.

[0053] Step 2: Determine the process forming parameter range of the partially melted layer and the completely melted layer:

[0054] Determine the laser power range and scanning speed range of the partially melted layer and the completely melted layer, and preliminarily determine the thickness range of each composite powder layer. At the same time, the thickness of the composite powder layer of the completely melted layer meets the following requirements:

[0055]

[0056] Where: H1 represents the thickness of the powder layer of the completely melted layer; H2 represents the height of the columnar crystal growth; k1 represents the correction factor; P1 is the laser power of the completely melted layer; v1 is the scanning speed of the completely melted layer; k s is the thermal conductivity of W alloy.

[0057] In specific implementation, the value ranges of the laser power and the scanning speed of the partially melted layer can be determined in advance by forming a 98wt.% W-1.5wt.% TiC-0.5wt.% Y2O3 block gradient process parameter block in a layered printing manner. The optimal process parameters obtained can form a relatively dense partially melted layer with no pores between unmelted W particles or incompletely melted W particles. Specifically, the laser power P of the partially melted layer is set in the range of 150W to 200W, and the scanning speed v of the partially melted layer is set in the range of 400mm / s to 500mm / s. The range of laser power and scanning speed for the completely melted layer is determined in advance by the printing quality of multiple 98wt.%W-1.5wt.%TiC-0.5wt.%Y2O3 dense blocks formed by single laser. The optimal process parameters obtained can form a completely melted layer without cracks. Specifically, the laser power P of the completely melted layer is set to 350W~400W, and the scanning speed of the completely melted layer is set to 300mm / s~400mm / s. In addition, when exploring the optimal process parameters for forming blocks, the thickness of the powder layer used is 10μm~20μm. It can be seen that the range of process forming parameters for the partially melted layer and the completely melted layer determined in this step is related to the specific composition of the composite powder used.

[0058] In addition, based on the composite powder of the new formula, the process forming parameter range of the partial melting layer and the complete melting layer can be obtained by the above method. For some known composite powder forming, the process forming parameter range of the partial melting layer and the complete melting layer can be directly obtained by referring to the known data in the literature.

[0059] Step 3: Preliminarily determine the process forming parameters of the partially melted layer and the completely melted layer:

[0060] Based on the process forming parameter range of the partial melt layer and the complete melt layer determined in step 2 and the calculation formula for the thickness of the composite powder layer of the complete melt layer, the process forming parameters of the partial melt layer and the complete melt layer are preliminarily determined.

[0061] In the present invention, the selection of the process parameters of the partial melting layer and the process parameters of the complete melting layer only needs to satisfy the process forming parameter ranges of the partial melting layer and the complete melting layer determined in the above step 2.

[0062] Step 4: Laser powder bed fusion forming components:

[0063] According to the process forming parameters of the partially melted layer and the completely melted layer preliminarily determined in step three, based on the laser melting path planned in step one, a chessboard scanning strategy is adopted to form each slice layer by layer on the printing substrate to obtain a printed component.

[0064] Specifically, laser powder bed fusion forming includes the following steps:

[0065] Step 4.1, import the part model (i.e., the three-dimensional model of the part to be printed) into the slicing software for slicing, assign the process forming parameters determined in step 3, adopt the chessboard scanning strategy, and finally import the slicing data into the printing device.

[0066] Step 4.2, install the new substrate and manually level it, pour 98wt.% W-1.5wt.% TiC-0.5wt.% composite powder into the powder cylinder, replace the new scraper and debug the powder spreading device to make it have a good powder spreading effect, and then introduce argon gas into the closed forming cavity for gas washing operation. After completing the above preparations, part printing begins.

[0067] Step 4.3: After printing is completed, wait for the part to slowly cool down to room temperature inside the forming chamber, take out the part, and perform subsequent characterization.

[0068] Step 5: Select several partial melt layers along the stacking direction of the printed component for metallographic preparation to obtain corresponding metallographic images one by one.

[0069] Specifically, for the longitudinal surface of the polished and corroded parts, which is parallel to the stacking direction, several (e.g., 10 or other numbers) metallographic photographs of the partially melted layer are taken under a light microscope. Due to the limitation of the field of view of the light microscope image, in order to reflect the melting state of the W particles in the partially melted layer as a whole, the jigsaw function is used to stitch together the photos of different areas of each partially melted layer to obtain a metallographic jigsaw of each partially melted layer. The effect is as follows: Figure 5 As shown in the metallographic diagram.

[0070] Step 6: Determine whether there are holes in the obtained metallographic image. When the judgment result shows that there are holes in the metallographic image, return to step 3 and re-preliminarily determine the process forming parameters of the partial melt layer and the complete melt layer; otherwise, go to step 7.

[0071] Specifically, since the metallographic mosaic is obtained in step five, in this step, it is determined whether there are holes in the metallographic mosaic. If there are many pores between the unmelted W particles and the incompletely melted W particles, return to step three, adjust the subsequent process forming parameters, and appropriately increase the energy density of the partially melted layer. If there are no pores between the unmelted W particles and the completely melted W particles, an image recognition system is used to judge the melting degree of the W particles in the metallographic mosaic.

[0072] Step 7: Use an image recognition algorithm to calculate the percentage of the area of ​​unmelted W particles or incompletely melted W particles in each metallographic image to the area of ​​the corresponding metallographic image.

[0073] In this step, in the process of calculating the percentage of the area of ​​unmelted W particles and incompletely melted W particles in each metallographic image to the area of ​​the corresponding metallographic image, the metallographic puzzles of each partially melted layer are first imported into the image recognition system. The image recognition system first identifies the size of the imported metallographic puzzle and calculates its area G. The size recognition and area calculation of the metallographic puzzle are realized by Python programming. Then, the Hough circle detection algorithm and contour area screening technology are combined to calculate the area of ​​unmelted W particles and incompletely melted W particles in the metallographic image, which specifically includes the following steps:

[0074] Step 71, pre-process the metallographic image; specifically, use the cv2.imread() function in Python to read the metallographic image, then use the cv2.cvtColor() function to convert the image into a grayscale image for subsequent edge detection and Hough circle detection, and finally use the cv2.GaussianBlur() function to perform Gaussian blur processing on the grayscale image to reduce the impact of noise on edge detection.

[0075] Step 72: Perform Hough circle detection on the preprocessed metallographic image and perform Hough circle transformation to identify the areas of unmelted W particles and incompletely melted W particles in the metallographic image.

[0076] Specifically, the cv2.HoughCircles() function in Python is used to perform Hough circle transform to detect circles in the image.

[0077] Step 73, draw a circle in the metallographic image and calculate the area of ​​the circle, so as to obtain the area of ​​the unmelted W particles and the incompletely melted W particles in the metallographic image.

[0078] Step 8: Based on the percentage of the area of ​​the unmelted W particles and the incompletely melted W particles in each metallographic image obtained in step 7 to the area of ​​the corresponding metallographic image, calculate the average percentage of the unmelted W particles and the incompletely melted W particles in the partially melted layer to the entire metallographic mosaic.

[0079] Since there are several metallographic images obtained in step 5, for each metallographic image, there is a set of corresponding metallographic images with a total area G i and the area S of the circular contour included therein i , so that the percentage Y of the area of ​​unmelted W particles and incompletely melted W particles in each metallographic image to the area of ​​the corresponding metallographic image can be calculated i , and then the average percentage of unmelted W particles and incompletely melted W particles in the partially melted layer in the entire metallographic puzzle can be calculated Percent Y i Calculated by the following formula:

[0080]

[0081] In the above formula, n represents the total number of metallographic images.

[0082] Step 9: Determine the average percentage of the melted layer of the printed sample Whether the value is within the preset percentage threshold range, when the judgment result shows that the average percentage If it is within the preset percentage threshold range, the process forming parameters of the partial melting layer and the complete melting layer preliminarily determined in step three are determined to be the final process forming parameters of the partial melting layer and the complete melting layer. Otherwise, return to step three, re-preliminarily determine the process forming parameters of the partial melting layer and the complete melting layer, continue to optimize the process parameters, and finally narrow the optimal process parameter range and determine the optimal process parameters to complete the forming of high-performance W alloy components. It can be seen that the present invention can provide a new idea for inhibiting the growth of columnar crystals in laser powder bed fusion forming of high-density tungsten alloys, and can apply the process method of high energy density and low energy density to coordinately control the melting degree of the powder layer to the forming of other refractory metals, thereby inhibiting their columnar crystal growth.

[0083] In the present invention, the preset percentage threshold is the moderate melting threshold of the partially melted layer. In order to determine the moderate melting threshold of the partially melted layer, the present invention defines that the partially melted layer has three melting states, namely, light melting, moderate melting and heavy melting. In the lightly melted partially melted layer, too many unmelted W particles will cause the top of the columnar crystal growth to have too little contact with the fully melted W particles. During the external force loading process, it is easy for the interface between the partially melted layer and the fully melted layer to become a crack source; in the heavily melted partially melted layer, too few unmelted W particles have a poor inhibitory effect on the growth of columnar crystals. Therefore, the process parameters used in the moderately melted partially melted layer can be regarded as the optimal process parameter range. If the partially melted layer is lightly melted, the subsequent printing needs to adjust the process parameters to increase the energy density. If the partially melted layer is heavily melted, the subsequent printing needs to adjust the process parameters to reduce the energy density.

[0084] Since the forming quality of W alloy is sensitive to process parameters, a large number of experiments are required to determine the optimal range of process parameters within the preset process parameter range. In order to facilitate the measurement of forming quality and adjustment of process parameters with different process parameters, the metallographic mosaics of different process parameters and their corresponding metallographic mosaics indicate the average percentage of unmelted and incompletely melted W particles in the partially melted layer. The value is used to establish the judgment threshold of the melting state of the partially melted layer, which provides an important reference for the precise optimization of subsequent process parameters. Different laser energy densities are used to form the partially melted layer, and then the moderate melting threshold of the partially melted layer is determined.

[0085] The following is a further detailed description of the method for determining the process forming parameters of the partially melted layer and the completely melted layer in the present invention in conjunction with the embodiments.

[0086] Example

[0087] Step 1: Modeling, layered slicing and overall planning of the melting degree of each slice layer:

[0088] A three-dimensional model of the part is created in the modeling software, and then the three-dimensional model of the part is imported into the slicing software for layered slicing and overall planning of the laser melting path of each slice along the stacking direction, so that from the bottom slice to the top slice, each layer of slice is periodically assigned different laser powers and scanning speeds, so as to finally form a printed component in which partially melted layers and completely melted layers appear periodically along the stacking direction.

[0089] Specifically, the present invention uses low energy laser density when printing the first layer (i.e., the bottom slice) to make it a partially melted layer, and uses high energy laser density when printing the second layer to make it a completely melted layer... and so on until the laser melting of all slices is completed. The energy density distribution in the stacking direction is as follows: Figure 3 As shown, the unmelted W particles and the incompletely melted W particles in the partially melted layer are finally used to inhibit the growth of the columnar crystals with strong orientation in the completely melted layer, so as to achieve the purpose of refining the columnar crystals. The mechanism of the partial melt layer inhibiting the growth of the columnar crystals with strong orientation in the completely melted layer is as follows: Figure 2 shown.

[0090] Step 2: Preliminarily determine the process forming parameter range of the partial melting layer and the complete melting layer:

[0091] In this embodiment, the laser powder bed fusion forming of 98wt.% W-1.5wt.% TiC-0.5wt.% Y2O3 block is taken as an example. The process forming parameter ranges of the partial melting layer and the complete melting layer are as follows: the laser power P of the partial melting layer is set in the range of 150W to 200W, the scanning speed v of the partial melting layer is set in the range of 400mm / s to 500mm / s, the laser power P of the complete melting layer is set in the range of 350W to 400W, the scanning speed of the complete melting layer is set in the range of 300mm / s to 400mm / s, and the powder layer thickness is 10μm to 20μm.

[0092] Step 3: Preliminarily determine the process forming parameters of the partially melted layer and the completely melted layer:

[0093] In this embodiment, the process parameters of the partially melted layer are: laser power P = 150 W, scanning speed v = 500 mm / s, and powder layer thickness t = 20 μm; the process parameters of the completely melted layer are: laser power P = 350 W, scanning speed v = 350 mm / s, and powder layer thickness H1 = 10 μm.

[0094] Step 4: Laser powder bed fusion forming components:

[0095] According to the process forming parameters of the partially melted layer and the completely melted layer preliminarily determined in step three, based on the laser melting path planned in step one, a chessboard scanning strategy is adopted to form each slice layer by layer on the printing substrate to obtain a printed component.

[0096] Specifically, laser powder bed fusion forming includes the following steps:

[0097] Step 4.1, import the part model (i.e., the three-dimensional model of the part to be printed) into the slicing software for slicing, assign the process forming parameters determined in step 3, adopt the chessboard scanning strategy, and finally import the slicing data into the printing device.

[0098] Step 4.2, install the new substrate and manually level it, pour 98wt.% W-1.5wt.% TiC-0.5wt.% composite powder into the powder cylinder, replace the new scraper and debug the powder spreading device to make it have a good powder spreading effect, and then introduce argon gas into the closed forming cavity for gas washing operation. After completing the above preparations, part printing begins.

[0099] Step 4.3: After printing is completed, wait for the part to slowly cool down to room temperature inside the forming chamber, take out the part, and perform subsequent characterization.

[0100] Step 5: Select several partial melt layers along the stacking direction of the printed component for metallographic preparation to obtain corresponding metallographic images one by one.

[0101] Specifically, the longitudinal surface of the polished and etched parts is parallel to the deposition direction. Ten metallographic photos of the partially melted layer are taken under a light microscope. Due to the limitation of the field of view of the light microscope image, in order to reflect the melting state of the W particles in the partially melted layer as a whole, the jigsaw function is used to stitch together the photos of different areas of each partially melted layer to obtain the metallographic jigsaw of each partially melted layer. The effect is as follows: Figure 5 shown.

[0102] Step 6: Determine whether there are holes in the obtained metallographic images. Finally, it is found that there are many pores between the unmelted W particles and the incompletely melted W particles in the metallographic mosaic of the partially melted layer, such as Figure 5As shown in the metallographic puzzle, the process parameters of subsequent printing are directly adjusted to appropriately increase the energy density of the partially melted layer. That is, return to step three and re-determine the process forming parameters of the partially melted layer and the completely melted layer: the process parameters of the partially melted layer are P = 180W, v = 400mm / s, t = 20μm, and the process parameters of the completely melted layer are P = 350W, v = 350mm / s, H1 = 10μm. Repeat steps four to six, and determine whether there are holes in the metallographic puzzle in step six. It is found that the unmelted W particles and the incompletely melted W particles in the metallographic puzzle of the partially melted layer are relatively dense, without obvious holes. Then, the image recognition system is used to judge the melting degree of the W particles in the metallographic puzzle, and then enter step seven.

[0103] Step 7: Use an image recognition algorithm to calculate the percentage of the area of ​​unmelted W particles and incompletely melted W particles in each metallographic image to the area of ​​the corresponding metallographic image.

[0104] In this step, in the process of calculating the percentage of the area of ​​unmelted W particles and incompletely melted W particles in each metallographic image to the area of ​​the corresponding metallographic image, the metallographic puzzles of each partially melted layer are first imported into the image recognition system. The image recognition system first identifies the size of the imported metallographic puzzle and calculates its area G. The size recognition and area calculation of the metallographic puzzle are realized by Python programming. Then, the Hough circle detection algorithm and the contour area screening technology are combined to calculate the area of ​​unmelted W particles or incompletely melted W particles in the metallographic image, which specifically includes the following steps:

[0105] Step 71, pre-process the metallographic image; specifically, use the cv2.imread() function in Python to read the metallographic image, then use the cv2.cvtColor() function to convert the image into a grayscale image for subsequent edge detection and Hough circle detection, and finally use the cv2.GaussianBlur() function to perform Gaussian blur processing on the grayscale image to reduce the impact of noise on edge detection.

[0106] Step 72: Perform Hough circle detection on the preprocessed metallographic image and perform Hough circle transformation to identify the areas of unmelted W particles and incompletely melted W particles in the metallographic image.

[0107] Specifically, the cv2.HoughCircles() function in Python is used to perform Hough circle transform to detect circles in the image.

[0108] Step 73, draw a circle in the metallographic image and calculate the area of ​​the circle, so as to obtain the area of ​​the unmelted W particles and the incompletely melted W particles in the metallographic image.

[0109] Step 8: Based on the percentage of the area of ​​the unmelted W particles and the incompletely melted W particles in each metallographic image obtained in step 7 to the area of ​​the corresponding metallographic image, calculate the average percentage of the unmelted W particles and the incompletely melted W particles in the partially melted layer to the entire metallographic mosaic.

[0110] Since there are several metallographic images obtained in step 5, for each metallographic image, there is a set of corresponding metallographic images with a total area G i and the area S of the circular contour included therein i , so that the percentage Y of the area of ​​unmelted W particles and incompletely melted W particles in each metallographic image to the area of ​​the corresponding metallographic image can be calculated i , and then the average percentage of unmelted W particles and incompletely melted W particles in the partially melted layer in the entire metallographic puzzle can be calculated Percent Y i Calculated by the following formula:

[0111]

[0112] In the above formula, n represents the total number of metallographic images.

[0113] Calculate the average percentage of the melt layer

[0114] Step 9: Determine the average percentage of the melted layer of the printed sample The value is within the moderate melting threshold range of the partially melted layer. In the present invention, the average percentage of the unmelted W particles and the incompletely melted W particles in the partially melted layer in the entire partially melted layer is calculated. As the moderate melting threshold of the partially melted layer. If the range is within 55±2%, the process parameters preliminarily determined in step 3 can be used as the optimal process parameter range. If not, return to step 3 and continue to optimize the process parameters. Finally, the optimal process parameter interval is narrowed and the optimal process parameters are determined to complete the forming of high-performance W alloy components.

[0115] Considering the average percentage of the partially melted layer calculated in step 8 It is not within the threshold range of the moderate melting layer set in step 9, and combined with the microstructure morphology of the partial melting layer, such as Figure 6 As shown in Figure 2, it is found that the columnar crystals in the completely melted layer are Figure 1The degree of columnar crystal refinement in pure W is low, so the energy density needs to be appropriately reduced, that is, return to step three and re-preliminarily determine the process forming parameters of the partial melting layer and the complete melting layer: the process parameters of the partial melting layer are P = 170W, v = 500mm / s, t = 20μm, and the process parameters of the complete melting layer are P = 350W, v = 350mm / s, H1 = 10μm. Repeat steps 4 to 6, and when judging whether there are holes in the metallographic puzzle in step 6, it is found that there are both unmelted W particles and incompletely melted W particles in the metallographic puzzle of the partially melted layer, and the particles are relatively dense without obvious holes. Then, the image recognition system is used to judge the degree of melting of W particles in the metallographic puzzle. For details, please refer to the attached Figure 4 . Calculate the average percentage of the melt layer according to steps 7 and 8. If the value is within the threshold range of the medium melting layer set in step 9, refer to the attached Figure 7 It can be seen that the epitaxial growth of columnar crystals is hindered by the round contour of unmelted W particles and the completely melted W particles. On the basis of adding nano-reinforced phase, the columnar crystal size of W alloy is further refined. The compressive strength of the block is measured, which is improved compared with the W alloy with only nano-reinforced phase added.

[0116] It can be seen that the process forming parameters of the present invention for 98wt.% W-1.5wt.% TiC-0.5wt.% Y2O3 are finally determined as: the process parameters of the partial melting layer are P=170W, v=500mm / s, t=20μm, and the process parameters of the complete melting layer are P=350W, v=350mm / s, H1=10μm.

Claims

1. A process for forming a high-density tungsten alloy by laser powder bed melting and refining columnar crystals, wherein a laser powder bed is used to melt composite powder layers layer by layer along a stacking direction to obtain a high-density tungsten alloy formed component; the composite powder layers are made of high-density tungsten composite powder; characterized in that: In the process of melting the composite powder layer layer by layer, the melting degree of the composite powder layer is coordinated and controlled by high-energy laser density and low-energy laser density, so that the partially melted layer and the completely melted layer appear periodically along the stacking direction, so as to produce a high-density tungsten alloy with columnar grain refinement; The partially melted layer is formed by melting with low energy laser density provided by the laser beam, while the completely melted layer is formed by melting with high energy laser density provided by the laser beam; in the partially melted layer formed, there are incompletely melted W particles, unmelted W particles and completely melted W particles, and the pores between the incompletely melted W particles and the unmelted W particles are filled with completely melted W particles; in the completely melted layer formed, the tungsten element is completely melted W particles.

2. The process for laser powder bed melting forming of heavy tungsten alloy and refining columnar crystals according to claim 1, characterized in that: During the laser powder bed fusion composite powder layer forming process, the process forming parameters of the partially melted layer and the completely melted layer are determined by the following steps: Step 1: Modeling, layered slicing and overall planning of the melting degree of each slice layer: A three-dimensional model of the part is created in the modeling software, and then the three-dimensional model of the part is imported into the slicing software for layered slicing and overall planning of the laser melting path of each slice along the stacking direction, so that from the bottom slice to the top slice, each layer of slices is periodically assigned with different laser powers and scanning speeds, so as to finally form a printed component in which a partial melt layer and a complete melt layer appear periodically along the stacking direction; Step 2: Determine the process forming parameter range of the partially melted layer and the completely melted layer: Determine the laser power range and scanning speed range of the partially melted layer and the completely melted layer, and preliminarily determine the thickness range of each composite powder layer. At the same time, the thickness of the composite powder layer of the completely melted layer meets the following requirements: Where: H1 represents the thickness of the powder layer of the completely melted layer; H2 represents the height of the columnar crystal growth; k1 represents the correction factor; P1 is the laser power of the completely melted layer; v1 is the scanning speed of the completely melted layer; k s is the thermal conductivity of W alloy; Step 3: Preliminarily determine the process forming parameters of the partially melted layer and the completely melted layer: Based on the process forming parameter ranges of the partially melted layer and the completely melted layer determined in step 3 and the calculation formula for the thickness of the composite powder layer of the completely melted layer, preliminarily determine the process forming parameters of the partially melted layer and the completely melted layer; Step 4: Laser powder bed fusion forming components: According to the process forming parameters of the partially melted layer and the completely melted layer preliminarily determined in step 3, based on the laser melting path planned in step 1, a chessboard scanning strategy is adopted to form each slice layer by layer on the printing substrate to obtain a printed component; Step 5: Selecting several partial melt layers along the stacking direction of the printed component for metallographic preparation to obtain corresponding metallographic images one by one; Step 6: Determine whether there are holes in the obtained metallographic image. When the determination result shows that there are holes in the metallographic image, return to step 3 and re-preliminarily determine the process forming parameters of the partial melt layer and the complete melt layer. Otherwise, go to step seven; Step 7, using an image recognition algorithm to calculate the percentage of the area of ​​unmelted W particles or incompletely melted W particles in each metallographic image to the area of ​​the corresponding metallographic image; Step 8: Based on the percentage of the area of ​​the unmelted W particles or the incompletely melted W particles in each metallographic image obtained in step 7 to the area of ​​the corresponding metallographic image, calculate the average percentage of the unmelted W particles or the incompletely melted W particles in the partially melted layer to the entire metallographic mosaic. Step 9: Determine the average percentage Is it within the preset percentage threshold range? When the judgment result shows that the average percentage If it is within the preset percentage threshold range, the process forming parameters of the partial melt layer and the complete melt layer preliminarily determined in step three are determined to be the final process forming parameters of the partial melt layer and the complete melt layer. Otherwise, return to step three, re-preliminarily determine the process forming parameters of the partial melt layer and the complete melt layer, continue to optimize the process parameters, and finally narrow the optimal process parameter range and determine the optimal process parameters to complete the forming of high-performance W alloy components.

3. The process for laser powder bed melting forming of heavy tungsten alloy and refining columnar crystals according to claim 2, characterized in that: In step five, the metallographic image obtained is a picture formed by stitching together the photos at different positions in the corresponding part of the melt layer.

4. The process for forming a heavy tungsten alloy by laser powder bed melting and refining columnar crystals according to claim 2, characterized in that: In step 7, in the process of calculating the percentage of the area of ​​unmelted W particles or incompletely melted W particles in each metallographic image to the area of ​​the corresponding metallographic image, the Hough circle detection algorithm and the contour area screening technology are combined. The area calculation of the unmelted W particles or the incompletely melted W particles in the metallographic image includes the following steps: Step 71, pre-processing the metallographic image; Step 72, performing Hough circle detection on the preprocessed metallographic image and performing Hough circle transformation to identify the areas of unmelted W particles and incompletely melted W particles in the metallographic image; Step 73, draw a circle in the metallographic image and calculate the area of ​​the circle, so as to obtain the area of ​​the unmelted W particles or the incompletely melted W particles in the metallographic image.

5. The process for laser powder bed melting forming of heavy tungsten alloy and refining columnar crystals according to claim 2, characterized in that: In the high specific gravity tungsten composite powder, the mass fraction of tungsten is 98wt.%W.

6. The process for forming a heavy tungsten alloy by laser powder bed fusion and refining columnar crystals according to claim 5, characterized in that: The high specific gravity tungsten composite powder also includes TiC and Y2O3, and the mass fractions of the components TiC and Y2O3 are 1.5wt.%TiC and 0.5wt.%Y2O3 respectively; In step three, the laser power P of the partially melted layer is set in the range of 150W to 200W, the laser power P of the completely melted layer is set in the range of 350W to 400W, the scanning speed v of the partially melted layer is set in the range of 400mm / s to 500mm / s, the scanning speed of the completely melted layer is set in the range of 300mm / s to 400mm / s, and the powder layer thickness of the partially melted layer is in the range of 10μm to 20μm.

7. The process for laser powder bed melting forming of high specific gravity tungsten alloy and refining columnar crystals according to claim 6, characterized in that: In step nine, the preset percentage threshold range is 55±2%.

8. The process for forming a heavy tungsten alloy by laser powder bed melting and refining columnar crystals according to claim 7, characterized in that: The final process forming parameters of the partially melted layer and the completely melted layer are: the process parameters of the partially melted layer are P=170W, v=500mm / s, t=20μm, and the process parameters of the completely melted layer are P=350W, v=350mm / s, H1=10μm.

9. The process for forming a heavy tungsten alloy by laser powder bed fusion and refining columnar crystals according to claim 1, characterized in that: High-energy laser density and low-energy laser density represent the amount of heat input from the laser beam to the composite powder layer, which can be calculated using the following formula: Where P is the laser power; v is the scanning speed; t is the thickness of the composite powder layer; and h is the scanning distance.

10. A laser powder bed fusion forming high specific gravity tungsten alloy, characterized in that: The invention is prepared by the process of laser powder bed melting forming a heavy tungsten alloy and refining columnar crystals as described in any one of claims 1 to 9.

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