Method for globally monitoring deformation of metal laser additive manufacturing component based on reverse DIC

Abstract

A method for globally monitoring deformation of a metal laser additive manufacturing component based on reverse DIC comprises the steps that after a target area image in the printing process is collected, noise reduction processing is carried out on the image, the sequence is changed, a cooled final image serves as an initial state, and the images from the last layer to the beginning of printing are arranged in an inverted sequence; inputting the images into calculation software according to a newly arranged sequence, setting a target monitoring area, a seed point and a two-dimensional proportion according to requirements, adjusting a coordinate system, and carrying out association calculation on the images; and finally, outputting a calculation result to obtain a horizontal X-direction displacement field and a vertical Y-direction displacement field of the target area. According to the method, the manufacturing process of the whole metal laser additive manufacturing component is recorded in a reverse deconstruction mode, subsequent processing and calculation are carried out, the image processing flow is completely opposite to the printing process, and the applicability of the DIC to the metal laser additive manufacturing printing process is improved.

Description

Technical Field

[0001] The invention relates to the technical field of metal laser additive manufacturing, and in particular to a method for globally monitoring the deformation of a metal laser additively manufactured component based on reverse DIC. Background Art

[0002] Laser metal additive manufacturing technology (also known as laser metal deposition) uses high-energy lasers to melt synchronously supplied metal powders and uses a special nozzle to quickly build target components on a substrate. It is generally used for welding repair and parts manufacturing. The process is controlled by the complex mutual application of layer-by-layer welding, and there are cyclic heating, melting, solidification and other links. These links will produce large residual stresses inside the printed components, causing plastic deformation, which will lead to a decrease in the accuracy of the finished parts or even cracking. In response to the deformation problem of components in metal laser additive manufacturing, some scholars have tried to monitor the printing process in real time through various monitoring methods. Among them, digital image correlation (DIC) technology, as a non-contact optical measurement experimental technology, has been applied to on-site monitoring of additive manufacturing in recent years due to its advantages such as good optical path adaptability, wide detection range and high degree of automation.

[0003] DIC quantifies the changes in target features in 2D or 3D by comparing characteristic speckles in the captured optical image. For some extrusion-type or high-temperature-free additive manufacturing processes, DIC can be directly monitored without other pre-processing. However, due to the characteristics of high temperature and strong light in laser metal additive manufacturing, some preparation and processing are required before DIC monitoring, such as the preparation of identification speckles or the preparation of solutions to resist interference from high temperature and strong light. At present, most of the research on DIC for online monitoring of metal additive manufacturing still uses the method of artificially making speckles, that is, spraying white primer on the area to be measured as an identification background, and then making random black spots in it as identification speckles. However, this method still has many limitations, such as the raw materials for speckle production are easily ablated by high temperature and cannot be identified, the printing process needs to be interrupted, and continuous monitoring cannot be achieved.

[0004] For metal laser additive manufacturing, a patent application entitled "Method for Real-time Measurement of Part Strain in Additive Manufacturing Process" (publication number CN107560560B) uses the natural texture of the specimen as an identification speckle for subsequent calculations. This method avoids some problems with artificially produced speckles, but the method only improves the monitoring method and field test settings. The monitoring process is still to print a certain number of layers first, set the target monitoring area, and then reprint, photograph and calculate the impact of the newly printed part on the set target monitoring area. Under this image processing process, the monitorable area and the monitorable time range are still limited, and the interaction process between layers within the target area and the deformation of subsequent deposited layers cannot be obtained. Summary of the invention

[0005] In order to overcome the shortcomings of the above-mentioned prior art, the purpose of the present invention is to provide a method for globally monitoring the deformation of metal laser additive manufacturing components based on reverse DIC, improve the monitoring and image processing process based on natural speckle recognition, record the entire manufacturing process of the metal laser additive manufacturing component in a "reverse deconstruction" manner and perform subsequent processing and calculation. The image processing process is completely opposite to the printing process, thereby expanding the applicability of DIC to the metal laser additive manufacturing printing process.

[0006] In order to achieve the above object, the present invention is implemented by the following technical solutions:

[0007] A method for globally monitoring the deformation of a metal laser additive manufacturing component based on reverse DIC includes image processing, performing noise reduction processing on the collected images, changing the order, taking the final image after cooling as the initial state, and arranging the images from the last layer to the beginning of printing in reverse order; inputting the images into a calculation acquisition software in the newly arranged order, setting a target monitoring area according to requirements, setting a seed point and a two-dimensional scale, adjusting a coordinate system, and setting nodes that need to output displacement changes with time, and performing correlation calculation on the images; and finally outputting the calculation results to obtain a horizontal X-direction displacement field and a vertical Y-direction displacement field of the target area.

[0008] The noise reduction process is to reduce the background contrast except the target component and the substrate, or directly reduce the intensity to 0 and display black.

[0009] The coordinate system adjustment operation is to rotate the positive direction of the horizontal X direction and the positive direction of the vertical Y direction by 180°.

[0010] A method for globally monitoring the deformation of metal laser additive manufacturing components based on reverse DIC also includes image acquisition, building a DIC online monitoring system near a processing area, connecting a DIC camera gimbal to a computer installed with image processing system software; installing a blue light source, setting a filter corresponding to the blue light wavelength on a camera lens, and calibrating the camera; starting to print a target component, continuously acquiring images starting from the first layer of printing, stopping when the target number of printing layers is reached, and acquiring another image after cooling.

[0011] The corresponding wavelength of the filter must be consistent with the wavelength of the corresponding illumination light source, and the corresponding wavelength should be as different as possible from the wavelength of the additive manufacturing heat source laser.

[0012] Compared with the prior art, the present invention has the following beneficial effects:

[0013] The present invention improves the monitoring process and image processing technology, and records the entire manufacturing process of metal laser additive manufacturing components in a "reverse deconstruction" manner, thereby realizing global monitoring of the entire printing process of the printed component, from the first layer printing to the end of printing. The monitoring range is included; at the same time, the monitoring area is no longer fixed, and the monitoring time is no longer restricted. Any area of ​​interest can be set according to demand to obtain the deformation cloud map and displacement data of specific points in the entire printing process. In general, this reverse DIC monitoring method solves the problems of fixed target area and limited monitoring time when DIC is used for additive manufacturing monitoring to a certain extent. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 Schematic diagram of DIC monitoring according to an embodiment of the present invention, wherein (a) is a DIC camera gimbal, (b) is a schematic diagram of the continuous image acquisition principle, and (c) is an original image captured by DIC during the printing process.

[0015] Figure 2 This is a flow chart of the reverse DIC method according to an embodiment of the present invention.

[0016] Figure 3 Schematic diagram of image background processing effect according to an embodiment of the present invention.

[0017] Figure 4 1 is a comparison diagram of the image processing flow of the reverse DIC method of an embodiment of the present invention and the general DIC method; wherein (a) is the general DIC image processing flow; (b) is the reverse DIC image processing flow.

[0018] Figure 5 The following are the displacement cloud diagrams of the components in the X horizontal direction and the Y vertical direction at different times of the components of the embodiment of the present invention; wherein (a) is the horizontal displacement cloud diagram in the X direction; and (b) is the vertical displacement cloud diagram in the Y direction. DETAILED DESCRIPTION

[0019] The present invention is described in detail below in conjunction with embodiments and drawings. The embodiment is a laser metal deposition GH4169 alloy thin-walled component printing process.

[0020] A method for global monitoring of deformation of metal laser additive manufacturing components based on reverse DIC, including image acquisition and image processing;

[0021] 1) Image acquisition includes the following steps:

[0022] 1.1) Reference Figure 1In (a) and (b), a DIC online monitoring system is built near the processing area. First, the DIC camera and the blue light source are assembled on the DIC camera gimbal. A filter corresponding to the wavelength of the light source is installed in front of the DIC camera. The wavelength corresponding to the filter must be consistent with the wavelength of the corresponding illumination light source, and the wavelength should be as different as possible from the wavelength of the additive manufacturing heat source laser. This is to ensure that the DIC online monitoring system can still work normally during the cladding head operation and obtain a complete image with identification features. If the wavelength of the light source is close to that of the laser or the wavelength of the filter is inconsistent with that of the light source, it is possible that the component image during the laser operation cannot be obtained due to overexposure, or the image cannot identify surface speckles due to problems such as light intensity. After installation, the DIC camera gimbal is connected to the computer.

[0023] 1.2) Make various preparations before printing and image shooting, place the substrate in the processing area and fix it, adjust the position of the DIC camera gimbal, and include the designated processing area in the DIC camera field of view. Different from the traditional DIC monitoring process, it is not calibrated after printing for a period of time, but it is necessary to determine the expected deposition position of the component before printing, and calibrate the DIC camera at the predetermined position using the calibration plate, and adjust the light source and focal length to make the captured image clear and bright;

[0024] 1.3) Use the laser cladding head to process the component at the specified position, and start the DIC camera to continuously shoot the printing process. When each layer is finished and cooled, the shooting is paused. 0.5s before the next layer starts printing, the DIC camera is turned on to continue the image acquisition of the next layer. Repeat the above operation until the component is printed; after cooling for 30 minutes, take the image of the complete deposited component, such as Figure 1 As shown in (c), the continuous image acquisition interval time and cooling time of this embodiment are only applicable to the same material and the same process. If other materials need to be acquired, they can be modified according to actual requirements and circumstances;

[0025] 2) If Figure 2 As shown, image processing includes the following steps:

[0026] 2.1) Arrange the collected images in reverse order and perform noise reduction processing. The image of the complete component after cooling is used as the first reference image, followed by the nth layer, n-1th layer, n-2th layer, and so on. After the sorting is completed, if Figure 3As shown, the corresponding image is subjected to noise reduction processing, and the background contrast except for the target component and the substrate is reduced, or the intensity is directly reduced to 0 and displayed in black. Because the minimum pixel size of the DIC camera used in this embodiment is only 3.45 μm, the background at the edge of the component may reduce the recognizability of the speckle, causing the edge of the target monitoring area to be distorted, that is, the edge position displacement data is not in line with the actual situation. When there is no interference factor in the actual background of the image, such as pure black, pure white, or the target control area has a clear contrast with the background, the noise reduction processing can be omitted;

[0027] 2.2) Images are continuously input and calculated, refer to Figure 4 , input the arranged images into the DIC calculation software, at this time, the cooling complete component is the reference image, select the region of interest in the reference image, and set the subset size and step size to 12 pixels and 6 pixels respectively; select the seed point in the reference image framed area, and then perform the two-dimensional scale setting, complete the image scale setting, and rotate the horizontal X direction and the vertical Y direction by 180° at the same time. This is because the printing end time is the initial state after the image is arranged in reverse order. If it is not modified, the displacement direction will be exactly opposite to the actual situation; the specific parameter settings mentioned in the image processing flow, such as subsets and step sizes, are only applicable to this embodiment, and researchers in this field can change and modify them according to the actual situation such as component size;

[0028] 2.3) Start the calculation, such as Figure 5 As shown, the horizontal X-direction and vertical Y-direction displacement cloud maps of the target component's area of ​​interest at different times are obtained.

[0029] In this embodiment, the DIC monitoring object is a single-pass multi-layer thin-walled component of GH4169 alloy laser metal deposition. The image of the component after deposition is completed and cooled to room temperature is used as a reference image in order to be able to compare with the images at all times during the printing process. Although this situation can organize and calculate the overall printing process, it cannot explain the displacement of a specific component part at a certain moment in the printing process. When there is a need in this regard, it is only necessary to set the image at a specific target moment as the original reference image in the calculation software, and process the image before this moment according to step 2.1), and repeat steps 2.2) and 2.3) to obtain the target displacement area at a specific printing moment.

[0030] The related equipment of the DIC online monitoring system of this embodiment is described as follows: the DIC camera model is Basler acA2440-75μm, the resolution is 2448px×2048px, the pixel size is 3.45μm x 3.45μm, the lens model is Fujinon HF50XA-5M, the focal length is 50mm, it can be equipped with a 31.5mm filter, and the blue light source wavelength is 455nm.

[0031] In this embodiment, the reverse DIC method is used to monitor the laser additive manufacturing printing process of GH4169 alloy thin-walled components to obtain the horizontal X-direction and vertical Y-direction displacement fields, such as Figure 5 As shown in the figure, the monitoring area is complete, and it is possible to analyze and calculate the displacement field of the formed thin-walled component at the beginning of printing (9th layer) and in the middle of printing (20th layer) as well as during the laser action. Figure 5 The displacement field distribution can be analyzed to show that the thin-walled component is in the shape of a cat's ear when it is continuously printed, that is, the left and right sides bulge upward and extend outward. Figure 1 (c), which matches the actual shape of the component. At the same time, the displacement of the area near the molten pool will change significantly during the laser action to produce a new cladding layer. This is because the temperature near the molten pool is high during the laser action, and the material expands due to heat and deforms.

[0032] In summary, this embodiment improves the traditional DIC monitoring and image processing process, making it possible to monitor the entire printing process of the component through DIC. At the same time, after changing the original reference image, the displacement of a specific area at a specific time can be calculated and analyzed, which to a certain extent solves the problems of fixed target area and limited monitoring time when DIC is used for additive manufacturing monitoring.

Claims

1. A method for global monitoring of deformation of metal laser additive manufacturing components based on reverse DIC, including image processing, characterized in that: The collected images are subjected to denoising and the order is changed. The final image after cooling is used as the initial state, and the images from the last layer to the beginning of printing are arranged in reverse order. The images are input into the calculation acquisition software in the newly arranged order. The target monitoring area is set according to the requirements, the seed point and the two-dimensional scale are set, the coordinate system is adjusted, and the nodes that need to output the displacement change with time are set at the same time, and the image is correlated. Finally, the calculation results are output to obtain the horizontal X-direction displacement field and the vertical Y-direction displacement field of the target area.

2. The method for global monitoring of deformation of metal laser additive manufacturing components based on reverse DIC according to claim 1, characterized in that: The noise reduction process is to reduce the background contrast except the target component and the substrate, or directly reduce the intensity to 0 and display black.

3. The method for global monitoring of deformation of metal laser additive manufacturing components based on reverse DIC according to claim 1, characterized in that: The coordinate system adjustment operation is to rotate the positive direction of the horizontal X direction and the positive direction of the vertical Y direction by 180°.

4. The method for global monitoring of deformation of metal laser additive manufacturing components based on reverse DIC according to claim 1, characterized in that: It also includes image acquisition, building a DIC online monitoring system near the processing area, connecting the DIC camera gimbal to a computer installed with image processing system software; installing a blue light source, setting a filter corresponding to the blue light wavelength on the camera lens, and calibrating the camera; starting to print the target component, continuously acquiring images from the first layer of printing until the target number of printing layers is reached, and then acquiring another image after cooling.

5. The method for global monitoring of deformation of metal laser additive manufacturing components based on reverse DIC according to claim 4, characterized in that: The corresponding wavelength of the filter must be consistent with the wavelength of the corresponding illumination light source, and the corresponding wavelength should be as different as possible from the wavelength of the additive manufacturing heat source laser.

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