A method for identification and control of metal transition mode in electron beam fusion filament deposition additive manufacturing process

Abstract

The application discloses a kind of identification and control method of metal transition mode in electron beam fuse deposition additive manufacturing process, the size of ideal liquid bridge is studied for ideal liquid bridge transition mode appearing in additive process, and the size is used as the target of control.In actual additive process, the transition state is monitored using machine vision, the transition mode is analyzed by designed control algorithm, the additive height distance that needs to be adjusted is calculated, and the up and down movement of the substrate is controlled to realize the closed-loop control of transition behavior in electron beam fuse deposition additive manufacturing process.The application captures the droplet transition moment by integrating machine vision technology, obtains the current droplet transition state by image processing, develops intelligent control algorithm for different transition states to realize real-time monitoring and adaptive control, solves the existing deposition discontinuity problem, avoids the generation of splash, effectively improves the stability of additive process, ensures the forming precision and quality of component, and improves the additive efficiency.

Description

TECHNICAL FIELD

[0001] The present application relates to a method for identifying and controlling the metal transition mode in the electron beam wire deposition additive manufacturing process, belonging to the technical field of additive manufacturing monitoring and control. BACKGROUND

[0002] Electron beam wire additive manufacturing technology is a kind of high-energy beam additive manufacturing technology, which refers to the use of high-energy electron beams to melt metal wires, and the substrate moves according to the predetermined additive path, and the metal wire is accurately deposited on the substrate under digital control, and the complex three-dimensional components are formed by layer-by-layer accumulation.

[0003] In the field of electron beam wire additive manufacturing, the stability of the droplet transition is the core challenge to ensure the quality of the components and the stability of the manufacturing process. Because the droplet formation and transition speed is extremely fast, and it occurs in a high-temperature and high-energy environment, it is difficult to accurately monitor the droplet transition behavior. At the same time, the immaturity of the control strategy, the instability of the droplet transition and its potential impact on the performance of the components are the main problems currently faced. In order to cope with these challenges, researchers are capturing the transition state of the molten metal liquid through machine vision technology, developing intelligent control algorithms to realize real-time monitoring and adaptive control, and improving the design of the equipment to improve the stability of the transition state. For example, Chinese patent CN109623123 discloses a closed-loop control method for realizing the droplet transition distance, which can control the droplet transition distance within a certain range by collecting droplet transition images and processing. But its disadvantage is that it is only suitable for single additive direction, and the camera setup position is affected by metal vapor, and the camera device is complex. SUMMARY

[0004] The purpose of the present application is to propose a method for identifying and controlling the multi-directional metal transition mode in the electron beam wire deposition additive manufacturing process.

[0005] The present application first studies the size of the ideal liquid bridge transition mode that appears in the additive process, and uses the size as the control target. In the actual additive process, the transition state is monitored by using machine vision, the transition mode is analyzed by using the designed control algorithm, the additive height distance that needs to be adjusted is calculated, and the closed-loop control of the transition behavior in the electron beam wire deposition additive manufacturing process is realized by controlling the up and down movement of the substrate.

[0006] A method for identifying and controlling the multi-directional metal transition mode in the electron beam wire deposition additive manufacturing process, comprising the following specific steps:

[0007] Step 1, first pixel analysis is performed on the picture of the ideal liquid bridge transition state;

[0008] Determine the pixel width size D corresponding to the diameter of the wire material used in the picture;

[0009] The maximum width d of the ideal liquid bridge is determined to be between 1.05D and 1.1D;

[0010] The height between the fuse point and the maximum width d of the liquid bridge is determined as the ideal liquid bridge height h, which is calculated according to the profile of the liquid bridge;

[0011] The d and h are used as the subsequent control parameter targets; the size is adjusted according to different wire diameters and control requirements, and the expected adjustment amount Ah calculated for each transition state is Ah;

[0012] Step 2, emit a circular high-energy electron beam to the to-be-deposited additive workpiece or substrate, and the electron beam will melt the continuously fed wire to form a droplet or a liquid bridge, and the metal liquid will transition to the to-be-deposited additive workpiece to form a molten pool on the surface of the to-be-deposited additive workpiece;

[0013] Step 3, when starting deposition, move the to-be-deposited substrate according to the set trajectory, and perform electron beam wire deposition forming;

[0014] Step 4, at the sampling time t, real-time collect the image of the droplet transition area, process the image of the droplet transition area, identify the current droplet transition mode and extract the droplet transition distance that needs to be adjusted, and control the transition distance by adjusting the height of the additive substrate in the Z direction.

[0015] In particular, the analysis control process in step 4 includes the following steps:

[0016] Step 4-1, obtain the image of the droplet transition area, and perform enhancement and denoising processing on the image, and sequentially perform grayscale, binarization, erosion and expansion processing, and area filtering on the image to obtain a good quality droplet transition image;

[0017] Step 4-2, perform connected component calculation on the image processed in step 4-1; the connected component calculation result is 2, indicating that the droplet and the molten pool exist independently, and it is determined as droplet transition; if the calculation result is 1, it indicates non-droplet transition, and the subsequent processing process is performed;

[0018] Step 4-3, for various judgment methods and control strategies in step 4-2, implement once, and then perform secondary sampling at t+0.5s, and again implement the above control strategy, that is, the frequency of closed-loop control is 0.5s / time; such a cycle can control the metal transition state and the droplet transition distance in the additive process.

[0019] The present application has the following advantages compared with the prior art: the present application captures the droplet transition moment by integrating machine vision technology, obtains the current droplet transition state through image processing, develops intelligent control algorithms for different transition states to realize real-time monitoring and adaptive control, realizes closed-loop control of droplet transition mode and distance by controlling the Z-direction height of the substrate, solves the problems of discontinuous deposition and the like, avoids the generation of splashes, effectively improves the stability of the additive process, ensures the forming precision and quality of the component, and improves the additive efficiency. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 A flow chart of the droplet transition mode recognition and control method in the electron beam wire deposition additive manufacturing process.

[0021] Figure 2 A detection effect diagram of the droplet transition connected domain.

[0022] Figure 3 A diagram showing the difference between the X and Y directions of the molten pool size, (a) Y-direction transition state and h2, W, Wmax parameter diagram, (b) X-direction additive.

[0023] Figure 4 A diagram showing the analysis and closed-loop control calculation method for the intermediate transition, (a) diagram showing the minimum value in the calculation interval (b) diagram showing the calculation of the height difference h3.

[0024] Figure 5 A diagram showing the compensation parameters of the long and short liquid bridges, and a diagram showing the liquid bridge transition h4.

[0025] Figure 6 A diagram showing the Y-direction additive analysis, (a) diagram showing the calculated expected adjustment amount (b) diagram showing the actual single pass morphology.

[0026] Figure 7 A diagram showing the expected adjustment amount calculated for the X-direction additive.

[0027] Figure 8 A diagram showing the additive sample and open-loop comparison results. DETAILED DESCRIPTION

[0028] The present application will be further described below in conjunction with the drawings and examples

[0029] Examples

[0030] The droplet transition mode recognition and control method in the electron beam wire deposition proposed by the present application includes the following steps:

[0031] (1) First, pixel analysis is performed on the image of the ideal liquid bridge transition state. Generally, the diameter of the wire used corresponds to a pixel width dimension D in the image. The liquid bridge is connected to the molten pool. Under normal circumstances, the width of the liquid bridge continuously increases from the molten wire point to the surface of the molten pool. The maximum width d of the ideal liquid bridge is generally between 1.05D and 1.1D, which can be selected according to the specific situation. The height from the molten wire point to the maximum width d of the liquid bridge is set as the ideal liquid bridge height h, which can be calculated based on the contour of the liquid bridge. d and h serve as subsequent control parameter targets and can be adjusted according to different wire diameters and control requirements. The expected adjustment amount calculated for each transition state is Δh.

[0032] (2) A ring-shaped high-energy electron beam is emitted onto the additive workpiece or substrate to be deposited. The electron beam melts the continuously fed wire to form droplets or liquid bridges. The molten metal is transferred to the additive workpiece to be deposited, forming a molten pool on the surface of the additive workpiece.

[0033] (3) When deposition begins, the substrate to be deposited is moved along the set trajectory to perform electron beam filament deposition.

[0034] (4) At sampling time t, images of the droplet transition region are acquired in real time. The images of the droplet transition region are processed to identify the current droplet transition mode and extract the droplet transition distance that needs to be adjusted. The transition distance is controlled by adjusting the height of the additive substrate in the Z direction. The specific values ​​mentioned below are the pixel widths in the image, not the actual widths. The specific analysis and control process includes the following steps:

[0035] (4-1) Obtain an image of the droplet transition region, enhance and denoise the image, and perform grayscale conversion, binarization, erosion and expansion processing and area filtering on the image in sequence to obtain a high-quality droplet transition image.

[0036] (4-2) Perform connected component calculation on the image processed in step (4-1). If the connected component calculation result is 2, it indicates that the droplets and the molten pool exist independently, and it is determined to be a droplet transition; if the calculation result is 1, it indicates that it is not a droplet transition, and the subsequent processing flow is carried out.

[0037] (4-2-1) For the droplet transition case identified in step (4-2), height extraction is performed. In coaxial electron beam fused wire additive manufacturing, the droplet is located directly above the molten pool. The contours of the droplet and the molten pool in the droplet transition state are extracted. The distance h1 between the top of the droplet and the top of the molten pool is extracted from the image. The calculation equation F1 for the expected adjustment amount Δh is:

[0038]

[0039] Wherein, h1 is the difference between the distance of the droplet and the molten pool, h is the height of the liquid bridge to be controlled, Δh is the height to be adjusted in the Z direction of the substrate. a is the control coefficient, considering the reason that the upper and lower parts of the metal liquid meet together, generally take 2, can be determined according to the situation.

[0040] (4-2-2) For the case of judging step (4-2) as non-droplet transition. First, extract the transition profile, calculate the maximum width Wmax of the profile. The transition image in the X direction has no molten pool length profile, but the Y direction image contains the overall molten pool length. Set the XY distinction threshold W, when Wmax is greater than W, judge as Y direction additive, enter the Y direction analysis process; When Wmax is not greater than W, judge as X direction additive, enter the X direction judgment process. W is determined according to the specific field of view, generally more than 5D, while ensuring that it is greater than the maximum molten pool width in the X direction image, it can be used as the judgment standard of the bottom of the liquid bridge.

[0041] (4-2-2-1) For the Y direction additive in step (4-2-2), calculate the profile length from top to bottom, find the boundary line, the upper part of the boundary line is the transition zone, and the lower part is the molten pool. Find the first row with a width greater than W from top to bottom, define this row as the boundary line between the liquid bridge and the molten pool, the distance between the fuse point position and the boundary line is the current liquid bridge height h2, and the expected adjustment amount Δh calculation equation F2:

[0042]

[0043] Wherein, h2 is the calculated liquid bridge height, h is the liquid bridge height to be controlled, Δh is the height to be adjusted in the Z direction of the substrate, and a is the control coefficient. If the Δh calculation result is positive, move Δh height upward, if the Δh calculation result is negative, move |Δh| height downward, and a is the control coefficient.

[0044] (4-2-2-2) For the X direction additive in step (4-2-2), calculate the profile length, and judge whether it is an intermediate transition. The judgment basis is: the liquid bridge transition profile widens from top to bottom, and the intermediate transition profile has necking phenomenon between the liquid bridge and the molten pool. The overall profile width size is first large and then small and then large, so only the middle part needs to be analyzed to distinguish the difference. The judgment method is: analyze the profile from top to bottom, starting from the top, and compare the width of the current row with the width of the next row. For the width at any position, if the width of the current row is always less than or equal to the width of the next row, the profile is judged as a liquid bridge transition. Otherwise, as long as there is a case where the width of the current row is greater than the width of the next row, it is judged as an intermediate transition.

[0045] (4-2-2-2-1) For the intermediate transition of step (4-2-2-2), first find the minimum value in the maximum width interval at the width reduction (Fig. 2 and 1 in the middle of the lower ③), ideally, the minimum value is the boundary line between the liquid bridge and the molten pool. Then search from the minimum value upwards to find the local maximum value in the minimum value interval (Fig. 1 and ③ in the middle of the upper ④). Then search from the minimum value downwards to find the matching width in the minimum and maximum value interval within a certain range of absolute error with the local maximum value (Fig. 1 and ③ in the middle of the lower ⑤), the error starts from 1 and cycles, and the maximum error is set to 5. Finally, calculate the distance h3 between the local maximum value and the matching value (i.e. between ④ and ⑤) according to the image. This design can ensure that the recessed area in the contour can be found, and then the parameters that need to be adjusted can be calculated. The expected adjustment amount Δh calculation equation F3:

[0046]

[0047] In the formula, h3 is the calculated height difference, Δh is the height of the substrate Z that needs to be adjusted, and a is the control coefficient.

[0048] (4-2-2-2-2) For the liquid bridge transition of step (4-2-2-2), find the first row with a width size of d in the liquid bridge transition contour from top to bottom, and the corresponding row height is h4. If there is no equal d, the error can be appropriately increased. In order to reduce the control oscillation, when h4 is less than 0.95h, the liquid bridge is too short; when h4 is greater than 1.05h, the liquid bridge is too long, and the liquid bridge at the intermediate height is not processed. The expected adjustment amount Δh calculation equation F4:

[0049]

[0050] In the formula, h4 is the liquid bridge height corresponding to the ideal liquid bridge width in the current transition state, h is the expected control liquid bridge height, a is the control coefficient, and Δh is the height of the substrate Z that needs to be adjusted. If Δh is positive, it means that the liquid bridge is too long, and the substrate is lifted by Δh height; if Δh is negative, it means that the liquid bridge is too short, and the substrate moves |Δh| height.

[0051] (4-3) For various judgment methods and control strategies in (4-2), after implementation, perform secondary sampling at t+0.5s, and implement the control strategy again, i.e. the frequency of closed-loop control is 0.5s / time. This cycle can control the metal transition state and the molten drop transition distance in the additive process.

[0052] The designed method is tested by specific cases. In the additive process of x and y directions, the closed-loop algorithm is turned on to verify the effectiveness of the expected adjustment. The wire diameter used is 2mm, and the D is analyzed to be 23 combined with the shooting situation, and the d is calculated to be 24, and the h is 23. The width of 1 pixel in the figure corresponds to 0.085mm in reality, and the expected adjustment in the subsequent figures is the specific millimeter size converted according to the scale.

[0053] First, test the Y direction additive, manually adjust the substrate movement in the single pass additive process, form two times of droplet transition, the expected adjustment result given by the algorithm is as follows Figure 6 It can be seen that the algorithm can well reflect the change of the substrate movement, whether it is normal transition or droplet transition, it can give reasonable calculation value. The continuous increase of the expected adjustment means that the substrate needs to be constantly moved up to maintain the ideal liquid bridge transition, and the same reason.

[0054] Then test the X direction additive, manually adjust the substrate movement in the additive process, form various transition states, the expected adjustment result given by the algorithm is as follows Figure 7 The algorithm can also give reasonable expected adjustment for different additive processes.

[0055] Finally, the actual test of closed-loop control is carried out, and the additive sample and the open-loop comparison result are as shown in Figure 8 The closed-loop sample has better surface quality, no collapse, coverage and flow, and the overall height and width are more uniform.

Claims

1. A method for identifying and controlling multi-directional metal transition modes during electron beam filament deposition additive manufacturing, characterized in that, The specific steps include the following: Step 1, first, pixel analysis is performed on the ideal liquid bridge transition state picture; The diameter of the wire used is determined, and the corresponding pixel width size D in the picture is determined; The maximum width d of the ideal liquid bridge is determined to be between 1.05D and 1.1D; The height from the wire melting point position to the maximum width d of the liquid bridge is set as the ideal liquid bridge height h, which is calculated according to the profile of the liquid bridge; d and h are used as the subsequent control parameter target; the size is adjusted according to different wire diameters and control requirements, and the expected adjustment amount Δh calculated for each transition state is Δh; Step 2, a ring-shaped high-energy electron beam is emitted to the additive workpiece or substrate to be deposited, and the electron beam melts the continuously fed wire to form a droplet or a liquid bridge, and the metal liquid transitions to the additive workpiece to be deposited to form a molten pool on the surface of the additive workpiece to be deposited; Step 3, when starting deposition, the substrate to be deposited moves according to the set trajectory, and electron beam wire deposition forming is performed; Step 4, at the sampling time t, the image of the droplet transition region is collected in real time, the image of the droplet transition region is processed, the current droplet transition mode is identified, and the droplet transition distance that needs to be adjusted is extracted, and the transition distance is controlled by adjusting the height of the additive substrate in the Z direction; In step 4, the analysis control process specifically includes the following steps: Step 4-1, the image of the droplet transition region is obtained, and the image is enhanced and denoised, and the image is sequentially subjected to grayscale, binarization, erosion and expansion processing, and area filtering to obtain a good quality droplet transition image; Step 4-2, the image processed in step 4-1 is subjected to connected domain calculation; the connected domain calculation result is 2, indicating that the droplet and the molten pool exist independently, and it is determined to be droplet transition; if the calculation result is 1, it indicates non-droplet transition, and the subsequent processing flow is performed; For the case where step 4-2 is determined to be droplet transition, the height is extracted, in the wire beam coaxial electron beam wire additive process, the droplet is located directly above the molten pool, the profile of the droplet and the molten pool in the droplet transition state is extracted, the distance h1 between the top of the droplet and the top of the molten pool is extracted according to the image, and the calculation equation F1 of the expected adjustment amount Δh is: ； Wherein, h1 is the distance difference between the droplet and the molten pool, h is the expected control liquid bridge height, Δh is the height that needs to be adjusted in the Z direction of the substrate; a is a control coefficient; the control coefficient a is based on the reason that the upper and lower parts of the metal liquid meet together, and is 2; Step 4-2 determines the non-droplet transition case; first, the transition profile is extracted, the maximum width Wmax of the profile is calculated, the transition image in the X direction has no molten pool length profile, but the Y direction image contains the whole molten pool length; set the XY distinction threshold W, when Wmax is greater than W, it is judged to be Y direction additive, and the Y direction analysis process is entered; when Wmax is not greater than W, it is judged to be X direction additive, and the X direction judgment process is entered; Step 4-3, for various judgment methods and control strategies in step 4-2, once the control strategy is implemented, at t+0.5s, secondary sampling is performed, and the above control strategy is again implemented, that is, the frequency of closed loop control is 0.5s / time; thus, the metal transition state and the droplet transition distance in the additive process can be controlled.

2. The method of identifying and controlling multi-directional metal transition patterns in electron beam fusion filament deposition additive manufacturing processes of claim 1, wherein, For Y-direction additive, the length of the profile row is calculated from top to bottom, and the boundary line is found. The area above the boundary line is the transition zone, and the area below the boundary line is the molten pool. The first row with a width greater than W is found from top to bottom, and the row is defined as the boundary line between the liquid bridge and the molten pool. The distance between the molten point position and the boundary line is the current liquid bridge height h2, and the expected adjustment amount Δh is calculated according to equation F2: ； Where h2 is the calculated liquid bridge height, h is the expected control liquid bridge height, Δh is the height of the substrate Z that needs to be adjusted, and a is the control coefficient. If the Δh calculation result is positive, the substrate is moved upward by Δh height. If the Δh calculation result is negative, the substrate is moved downward by |Δh| height.

3. The method for identifying and controlling the multi-directional metal transition mode in the electron beam wire deposition additive manufacturing process according to claim 1, characterized in that, For X-direction additive, the profile length is calculated, and it is judged whether it is an intermediate transition. The judgment basis is that the liquid bridge transition profile gradually increases from top to bottom, while the intermediate transition profile has a necking phenomenon between the liquid bridge and the molten pool. The overall profile width increases first, then decreases, and then increases. Therefore, only the middle part needs to be analyzed to distinguish between different profiles. The judgment method is to analyze the profile from top to bottom. Starting from the top, the width of the current row and the width of the next row are compared. For any position, if the width of the current row is always less than or equal to the width of the next row, the profile is judged to be a liquid bridge transition. Otherwise, as long as there is a situation where the width of the current row is greater than the width of the next row, it is judged to be an intermediate transition.

4. The method for identifying and controlling the multi-directional metal transition mode in the electron beam wire deposition additive manufacturing process according to claim 3, characterized in that, When it is an intermediate transition, first find the minimum value in the width reduction and the maximum width interval. In the ideal state, the minimum value is the boundary line between the liquid bridge and the molten pool. Then, search upward from the minimum value to find the local maximum value in the top and minimum value interval. Then, search downward from the minimum value to find the matching width within the minimum and maximum value interval that has an absolute error within a certain range from the local maximum value. The error starts from 1 and is found in a loop. The maximum error is set to 5. Finally, calculate the distance h3 between the local maximum value and the matching value according to the image. The expected adjustment amount Δh calculation equation F3 is: ； Where h3 is the calculated height difference, Δh is the height of the substrate Z that needs to be adjusted, and a is the control coefficient.

5. The method of identifying and controlling multi-directional metal transition patterns in electron beam fusion filament deposition additive manufacturing processes of claim 3, wherein, When it is a liquid bridge transition, find the first row with a width of d in the liquid bridge transition profile from top to bottom. The row height corresponding to d is h4. If there is no d, the error can be increased appropriately. The expected adjustment amount Δh calculation equation F4 is: ； Where h4 is the liquid bridge height corresponding to the ideal liquid bridge width in the current transition state, h is the expected control liquid bridge height, a is the control coefficient, and Δh is the height of the substrate Z that needs to be adjusted. If Δh is positive, the liquid bridge is too long, and the substrate is lifted upward by Δh height. If Δh is negative, the liquid bridge is too short, and the substrate is moved downward by |Δh| height.

6. The method of identifying and controlling multi-directional metal transition patterns in electron beam fusion filament deposition additive manufacturing processes of claim 5, wherein, In order to reduce the control oscillation, when h4 is less than 0.95h, the liquid bridge is too short. When h4 is greater than 1.05h, the liquid bridge is too long. The liquid bridge at the intermediate height is not processed.

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