Method for identifying and controlling metal transition mode in electron beam fused deposition additive manufacturing process

Abstract

The invention discloses a method for identifying and controlling a metal transition mode in an electron beam fused deposition additive manufacturing process, which is characterized in that the size of an ideal liquid bridge is researched aiming at the ideal liquid bridge transition mode appearing in the additive manufacturing process, and the size is used as a control target. In the actual additive manufacturing process, machine vision is used for monitoring the transition state, the transition mode is analyzed through a designed control algorithm, the additive height distance needing to be adjusted is calculated, and closed-loop control over the transition behavior in the electron beam fuse deposition additive manufacturing process is achieved by controlling the substrate to move up and down. According to the invention, a machine vision technology is integrated to capture a molten drop transition moment, a current molten drop transition state is obtained through image processing, and an intelligent control algorithm is developed for different transition states to realize real-time monitoring and self-adaptive control, so that the problems of discontinuous deposition and the like are solved, the generation of splashing is avoided, and the production efficiency is improved. And the stability of the material adding process is effectively improved, the forming precision and quality of the component are ensured, and the material adding efficiency is improved.

Description

Technical Field

[0001] The invention relates to a method for identifying and controlling a metal transition mode in an electron beam fuse deposition additive manufacturing process, and belongs to the technical field of additive manufacturing monitoring and control. Background Art

[0002] Electron beam fuse additive manufacturing technology is a high-energy beam additive manufacturing technology that uses a high-energy electron beam to melt metal wire. The substrate moves along a predetermined additive path, and the metal wire is precisely deposited on the substrate under digital control, stacking layer by layer to form complex three-dimensional components.

[0003] In the field of electron beam fuse additive manufacturing, the stability of droplet transition is the core challenge to ensure component quality and manufacturing process stability. Since the droplet formation and transition speed are extremely fast and occur in a high temperature and high energy environment, it is very 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 component performance are the main problems currently faced. To meet these challenges, researchers are using machine vision technology to capture the transition state of molten metal, developing intelligent control algorithms to achieve real-time monitoring and adaptive control, and improving the stability of the transition state by improving equipment design. 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 and processing the droplet transition image. However, its disadvantage is that it is only applicable to a single additive direction, and the camera installation position is affected by metal vapor, and the camera device is complex. Summary of the invention

[0004] The purpose of the present invention is to provide a method for identifying and controlling multi-directional metal transition modes in an electron beam fuse deposition additive manufacturing process.

[0005] The present invention first studies the size of the ideal liquid bridge in the ideal liquid bridge transition mode that occurs in the additive process, and takes this size as the control target. In the actual additive process, machine vision is used to monitor the transition state, and the transition mode is analyzed through the designed control algorithm to calculate the additive height distance that needs to be adjusted. By controlling the up and down movement of the substrate, closed-loop control of the transition behavior in the electron beam fuse deposition additive manufacturing process is achieved.

[0006] A method for identifying and controlling a multi-directional metal transition mode in an electron beam fuse deposition additive manufacturing process comprises the following specific steps:

[0007] Step 1, firstly, pixel analysis is performed on the ideal liquid bridge transition state image;

[0008] Determine the diameter of the wire used, and the pixel width corresponding to it in the image is D;

[0009] Determine that the maximum width d of the ideal liquid bridge is between 1.05D and 1.1D;

[0010] Determine that the height from the fuse point to the maximum width d of the liquid bridge is set as the ideal liquid bridge height h, which is calculated based on the profile of the liquid bridge;

[0011] Take d and h as the subsequent control parameter targets; adjust the size according to different wire diameters and control requirements, and the expected adjustment amount calculated for each transition state is Δh;

[0012] Step 2, emitting a circular high-energy electron beam onto the additive workpiece or substrate to be deposited, the electron beam melts the continuously fed wire to form a molten droplet or a liquid bridge, the molten metal transitions to the additive workpiece to be deposited, and a molten pool is formed on the surface of the additive workpiece to be deposited;

[0013] Step 3, when the deposition starts, the substrate to be deposited moves along a set trajectory to perform electron beam fuse deposition and formation;

[0014] Step 4, at the sampling time t, the image of the droplet transition area is collected in real time, the image of the droplet transition area is processed, the current droplet transition mode is identified and the droplet transition distance that needs to be adjusted is extracted, and the control of the transition distance is achieved by adjusting the Z-direction height of the additive substrate.

[0015] In the specific step 4, the analysis control process specifically includes the following steps:

[0016] Step 4-1, obtaining an image of the droplet transition area, enhancing and denoising the image, and sequentially performing grayscale, binarization, corrosion expansion, and area filtering on the image to obtain a droplet transition image of good quality;

[0017] Step 4-2, performing connected domain calculation on the image processed in step 4-1; if the connected domain calculation result is 2, it means that the molten droplet and the molten pool exist independently, and it is determined to be a molten droplet transition; if the calculation result is 1, it means it is not a molten droplet transition, and the subsequent processing flow is performed;

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

[0019] Compared with the prior art, the present invention has significant advantages as follows: the present invention captures the moment of molten droplet transition by integrating machine vision technology, obtains the current molten droplet transition state by image processing, develops intelligent control algorithms for different transition states to achieve real-time monitoring and adaptive control, and realizes closed-loop control of the molten droplet transition mode and distance by controlling the Z-direction height of the substrate, thereby solving the existing problems such as discontinuous deposition, avoiding the generation of splashes, effectively improving the stability of the additive process, ensuring the molding accuracy and quality of the components, and improving the additive efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a flow chart of the method for identifying and controlling the droplet transition mode in the electron beam fuse deposition additive manufacturing process proposed by the present invention.

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

[0022] Figure 3 The difference between the melt pool size in the X and Y directions. (a) Schematic diagram of the transition state in the Y direction and the parameters h2, W, and Wmax, (b) x-direction addition.

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

[0024] Figure 5 Schematic diagram of compensation parameters for long and short liquid bridges. Schematic diagram of liquid bridge transition h4.

[0025] Figure 6 Y-axis additive analysis diagram, (a) calculated expected adjustment amount and (b) actual single-channel morphology.

[0026] Figure 7 Schematic diagram of the desired adjustment calculated for X-axis additive manufacturing.

[0027] Figure 8 This is a comparison diagram of the additive sample and the open loop. DETAILED DESCRIPTION

[0028] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0029] Example

[0030] The present invention provides a method for identifying and controlling transition modes in electron beam fuse deposition, comprising the following steps:

[0031] (1) First, pixel analysis is performed on the ideal liquid bridge transition state image. Generally speaking, the diameter of the wire used corresponds to a pixel width size D in the image. The liquid bridge is connected to the molten pool. Generally, the width of the liquid bridge increases from the fuse 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 fuse 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 are used 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) An annular high-energy electron beam is emitted onto the workpiece or substrate to be deposited. The electron beam melts the continuously fed wire to form a molten droplet or liquid bridge. The molten metal transitions to the workpiece to be deposited, forming a molten pool on the surface of the workpiece to be deposited.

[0033] (3) When deposition begins, the substrate to be deposited moves along a set trajectory to perform electron beam fuse deposition.

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

[0035] (4-1) Obtain an image of the droplet transition area, enhance and denoise the image, and perform grayscale, binarization, corrosion dilation, and area filtering on the image in turn to obtain a droplet transition image of good quality.

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

[0037] (4-2-1) The situation judged as droplet transition in step (4-2) is highly extracted. In the wire coaxial electron beam fuse additive, 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 based on the image. The calculation equation F1 for the expected adjustment amount Δh is:

[0038]

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

[0040] (4-2-2) For step (4-2), it is judged as a non-droplet transition. First, extract the transition morphology contour and calculate the maximum width Wmax of the contour. The transition image taken in the X direction has no molten pool length morphology, but the Y direction image contains the overall molten pool length. Set the XY distinction threshold W. When Wmax is greater than W, it is judged as Y-direction additive and enters the Y-direction analysis process; when Wmax is not greater than W, it is judged as X-direction additive and enters the X-direction judgment process. W depends on the specific field of view, which is generally above 5D. It can be used as a judgment standard for the bottom of the liquid bridge while ensuring that it is greater than the maximum molten pool width in the X-direction image.

[0041] (4-2-2-1) Calculate the length of the contour line from top to bottom for the Y-axis additive in step (4-2-2) to find the dividing line. The transition zone is above the dividing line, and the molten pool is below it. Find the first line with a width greater than W from top to bottom, and define it as the dividing line between the liquid bridge and the molten pool. The distance from the fuse point to the dividing line is the current liquid bridge height h2. The expected adjustment amount Δh is calculated by equation F2:

[0042]

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

[0044] (4-2-2-2) Calculate the contour length of the X-axis additive in step (4-2-2) and determine whether it is an intermediate transition. The basis for judgment is: the liquid bridge transition contour widens and gradually increases from top to bottom, while the intermediate transition contour has a necking phenomenon between the liquid bridge and the molten pool. The overall contour width first increases, then decreases, and then increases again. Therefore, only the middle part needs to be analyzed to distinguish the difference. The judgment method is: the contour is analyzed from top to bottom, starting from the top, and the current line width is compared with the next line width in turn. For the width at any position, if the line width is always less than or equal to the next line, the contour is judged as a liquid bridge transition. Otherwise, as long as the current line width is greater than the next line width, 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 interval between the reduced width and the maximum width (③ between ② and ① in the figure). Ideally, the minimum value is the boundary between the liquid bridge and the molten pool. Then search upward from the minimum value to find the local maximum value in the interval between the top and the minimum value (④ between ① and ③ in the figure). Then search downward from the minimum value to find the width that matches the absolute error of the local maximum value within a certain range within the interval between the minimum and the maximum value (⑤ between ① and ③ in the figure). The error starts from 1 and is searched cyclically, and the maximum error is set to 5. Finally, the distance h3 between the local maximum value and the matching value (i.e., between ④ and ⑤) is calculated based on the image. This design ensures that the concave area in the contour can be found, and thus ensures that the parameters that need to be adjusted can be calculated. The expected adjustment amount Δh calculation equation F3:

[0046]

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

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

[0049]

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

[0051] (4-3) For the various judgment methods and control strategies in (4-2), after they are implemented once, secondary sampling is performed at t+0.5s, and the above control strategy is implemented again, that is, the frequency of closed-loop control is 0.5s / time. This cycle can control the metal transition state and droplet transition distance in the additive process.

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

[0053] First, we tested the Y-axis additive process. During the single-pass additive process, we manually adjusted the substrate movement to form two droplet transfers. The expected adjustment amount given by the algorithm is as follows: Figure 6 It can be seen that the algorithm can well reflect the movement changes of the substrate, and can give reasonable calculation values ​​for both normal transition and droplet transition. The continuous increase in the expected adjustment amount means that the substrate needs to be continuously moved up to maintain the ideal liquid bridge transition, and the same is true for reducing it.

[0054] Then, we tested the X-axis additive process and manually adjusted the substrate movement to form various transition states. The expected adjustment results given by the algorithm are as follows: Figure 7 The algorithm can also give reasonable expected adjustments for different additive processes.

[0055] Finally, a closed-loop control test was conducted, and the comparison results between the additive sample and the open-loop control were as follows: Figure 8 The closed-loop sample has better surface quality, no collapse, coverage or flow measurement, and the overall height and width are more uniform.

Claims

1. A method for identifying and controlling multi-directional metal transition modes in an electron beam fuse deposition additive manufacturing process, characterized in that: The specific steps are as follows: Step 1, firstly, pixel analysis is performed on the ideal liquid bridge transition state image; Determine the diameter of the wire used, and the pixel width corresponding to it in the image is D; Determine that the maximum width d of the ideal liquid bridge is between 1.05D and 1.1D; Determine that the height from the fuse point to the maximum width d of the liquid bridge is set as the ideal liquid bridge height h, which is calculated based on the profile of the liquid bridge; Take d and h as the subsequent control parameter targets; adjust the size according to different wire diameters and control requirements, and the expected adjustment amount calculated for each transition state is Δh; Step 2, emitting a circular high-energy electron beam onto the additive workpiece or substrate to be deposited, the electron beam melts the continuously fed wire to form a molten droplet or a liquid bridge, the molten metal transitions to the additive workpiece to be deposited, and a molten pool is formed on the surface of the additive workpiece to be deposited; Step 3, when the deposition starts, the substrate to be deposited moves along a set trajectory to perform electron beam fuse deposition and formation; Step 4, at the sampling time t, the image of the droplet transition area is collected in real time, the image of the droplet transition area is processed, the current droplet transition mode is identified and the droplet transition distance that needs to be adjusted is extracted, and the control of the transition distance is achieved by adjusting the Z-direction height of the additive substrate.

2. The method for identifying and controlling multi-directional metal transition modes in the electron beam fuse deposition additive manufacturing process according to claim 1, characterized in that: In step 4, the analysis control process specifically includes the following steps: Step 4-1, obtaining an image of the droplet transition area, enhancing and denoising the image, and sequentially performing grayscale, binarization, corrosion expansion, and area filtering on the image to obtain a droplet transition image of good quality; Step 4-2, performing connected domain calculation on the image processed in step 4-1; if the connected domain calculation result is 2, it means that the molten droplet and the molten pool exist independently, and it is determined to be a molten droplet transition; if the calculation result is 1, it means it is not a molten droplet transition, and the subsequent processing flow is performed; Step 4-3, for the various judgment methods and control strategies in step 4-2, after being implemented once, secondary sampling is performed at t+0.5s, and the above control strategy is implemented again, that is, the frequency of closed-loop control is 0.5s / time; such a cycle can control the metal transition state and droplet transition distance in the additive process.

3. The method for identifying and controlling multi-directional metal transition modes in the electron beam fuse deposition additive manufacturing process according to claim 2, characterized in that: The situation judged as droplet transition in step 4-2 is highly extracted. In the wire coaxial electron beam fuse additive, 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 based on the image. The calculation equation F1 for the expected adjustment amount Δh is: Among them, h1 is the difference between the molten droplet and the molten pool, h is the desired controlled liquid bridge height, Δh is the height of the substrate that needs to be adjusted in the Z direction; a is the control coefficient.

4. The method for identifying and controlling multi-directional metal transition modes in the electron beam fuse deposition additive manufacturing process according to claim 2, characterized in that: Step 4-2 determines the situation as non-droplet transition; first extract the transition morphology contour, calculate the maximum width Wmax of the contour, the transition image taken in the X direction has no molten pool length morphology but the Y direction image contains the overall molten pool length; set the XY distinction threshold W, when Wmax is greater than W, it is judged as Y-direction additive and enters the Y-direction analysis process; when Wmax is not greater than W, it is judged as X-direction additive and enters the X-direction judgment process.

5. The method for identifying and controlling multi-directional metal transition modes in the electron beam fuse deposition additive manufacturing process according to claim 4, characterized in that: For Y-axis additive, the length of the contour line is calculated from top to bottom to find the dividing line. The transition zone is above the dividing line, and the molten pool is below it. The first line with a width greater than W is found from top to bottom, and the line is defined as the dividing line between the liquid bridge and the molten pool. The distance from the fuse point to the dividing line is the current liquid bridge height h2. The expected adjustment amount Δh is calculated by equation F2: Among them, h2 is the calculated liquid bridge height, h is the desired controlled liquid bridge height, Δh is the height of the substrate that needs to be adjusted in the Z direction, and a is the control coefficient; if the calculated result of Δh is a positive number, it moves upward by Δh height, and if it is a negative number, it moves downward by |Δh| height.

6. The method for identifying and controlling multi-directional metal transition modes in the electron beam fuse deposition additive manufacturing process according to claim 4, characterized in that: For X-axis additive manufacturing, calculate the contour length and determine whether it is an intermediate transition; The basis for discrimination is: the liquid bridge transition profile widens and gradually increases from top to bottom, while the middle transition profile has a necking phenomenon between the liquid bridge and the molten pool, and the overall profile width first increases, then decreases, and then increases again. Therefore, only the middle part needs to be analyzed to distinguish the difference; The judgment method is: the contour is analyzed from top to bottom, starting from the top, and the current line width is compared with the next line width in turn; for the width at any position, if the line width is always less than or equal to the next line, the contour is judged as a liquid bridge transition; otherwise, as long as the current line width is greater than the next line width, it is judged as an intermediate transition.

7. The method for identifying and controlling multi-directional metal transition modes in the electron beam fuse deposition additive manufacturing process according to claim 6, characterized in that: When it is an intermediate transition, first find the minimum value in the interval between the width reduction and the maximum width; ideally, the minimum value is the boundary between the liquid bridge and the molten pool; then search upward from the minimum value to find the local maximum value in the interval between the top and the minimum value; Then search downward from the minimum value to find the width that matches the local maximum value within a certain range of absolute error between the minimum and maximum values. The error starts from 1 and the maximum error is set to 5; Finally, the distance h3 between the local maximum value and the matching value is calculated based on the image; The expected adjustment Δh is calculated by equation F3: Where h3 is the calculated height difference, Δh is the height of the substrate that needs to be adjusted in the Z direction, and a is the control coefficient.

8. The method for identifying and controlling multi-directional metal transition modes in the electron beam fuse deposition additive manufacturing process according to claim 6, characterized in that: When it is a liquid bridge transition, find the first row with a width dimension of d in the liquid bridge transition contour from top to bottom. The corresponding row height of this row is h4. If there is no equal d, the error can be appropriately increased; The calculation equation for the expected adjustment amount Δh is F4: Wherein, h4 is the liquid bridge height corresponding to the ideal liquid bridge width in the current transition state, h is the desired controlled liquid bridge height, a is the control coefficient, and Δh is the height of the substrate that needs to be adjusted in the Z direction; if Δh is a positive number, it means that the liquid bridge is too long and the substrate is lifted upward by a height of Δh; if Δh is a negative number, it means that the liquid bridge is too short and the substrate moves downward by a height of |Δh|.

9. The method for identifying and controlling multi-directional metal transition modes in the electron beam fuse deposition additive manufacturing process according to claim 8, characterized in that: In order to reduce control oscillation, it is set that when h4 is less than 0.95h, the liquid bridge is too short; when h4 is greater than 1.05h, the liquid bridge passes through the field, and the liquid bridge at the middle height is not processed.

10. The method for identifying and controlling multi-directional metal transition modes in the electron beam fuse deposition additive manufacturing process according to claim 3, 5, 7 or 8, characterized in that: The control coefficient a is set to 2 because the upper and lower parts of the molten metal meet together.

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