A stress detection method for laser metal deposition layer

Abstract

This application provides a stress detection method for laser metal deposition layers, belonging to the field of stress detection technology. This method uses laser marking to produce a speckle pattern on the side of a substrate, avoiding the impact of laser-generated high temperatures on the speckle quality and improving the accuracy and reliability of strain measurement in high-temperature environments. Subsequently, by using digital image correlation technology during the laser metal deposition process, the strain and stress state of the laser metal deposition coating during the forming process can be detected online, thereby guiding the preparation of high-performance, crack-free laser metal deposition layers and providing theoretical support and process optimization opportunities for the development of crack-free, high-performance deposition layers.

Description

Technical Field

[0001] The invention belongs to the technical field of stress detection and relates to a laser metal deposition layer stress detection method. Background Art

[0002] Laser metal deposition is an alloy coating deposition layer formed by a high-energy laser beam. It improves the wear resistance and corrosion resistance of the metal through the excellent properties of the alloy. It has been widely used in aviation, aerospace, national defense, chemical industry, machinery, electricity, electronics and other industrial fields. It can achieve surface strengthening of high-end equipment, which is beneficial to improve its service life and performance.

[0003] However, the inherent rapid solidification characteristics of laser metal deposition often introduce defects such as cracks, element segregation, and residual stress, thereby reducing coating performance. In addition, due to the different solidification sequences of laser metal deposition layers, large temperature gradients exist between the deposited material and the substrate, as well as within the deposited material, resulting in a very complex thermal stress state within the deposited layer. When the thermal stress exceeds the yield strength of the material, the deposited layer will crack, greatly affecting the service life of the deposited layer and causing the deposited layer to fail.

[0004] Traditional methods for stress analysis of laser metal deposition layers mainly include finite element analysis and destructive stress testing. All of these methods rely on offline simulation or destructive testing, which are time-consuming and costly. This seriously hinders the in-depth understanding of the crack initiation mechanism during laser metal deposition, as well as the possibility of active process control based on real-time stress feedback, such as adjusting heat input and scanning strategies, to suppress cracks and optimize coating quality. Summary of the Invention

[0005] The purpose of the present invention is to provide a laser metal deposition layer stress detection method to solve the problem that existing stress detection methods all rely on offline simulation or destructive detection, have poor timeliness and high cost.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] The present application provides a method for detecting stress in a laser metal deposition layer. This method uses a laser marking method to produce a speckle pattern on the side of a substrate, thereby avoiding the influence of the high temperature generated by the laser on the speckle quality. Then, by using digital image correlation technology (DIC) to detect the speckle image during the laser metal deposition process, and importing the speckle image into a VIC full-field strain measurement system for strain analysis, the strain and stress state of the laser metal deposition coating during the forming process can be detected online, thereby guiding the preparation of high-performance crack-free laser metal deposition layers.

[0008] Specifically, the present application provides a method for detecting stress in a laser metal deposition layer, the method comprising:

[0009] S01: After sandpaper polishing the surface of the alloy substrate, Glare software was used to generate a digital speckle image, and a laser marking machine was used to etch the speckle pattern on the side of the alloy substrate.

[0010] The alloy substrate surface was polished with sandpaper to remove surface contaminants. A digital speckle pattern image was generated using Glare software. The software parameters were set as follows: speckle diameter: 5-50 pixels; speckle density: 30%-70%; packing: 30%-80%; and offset: 30%-80%. Based on the digital speckle pattern image, a 30W laser marker was used to etch speckles onto the side of the alloy substrate. This prevented the speckle pattern from being damaged by deposition temperatures exceeding 1000°C, thereby preventing burnout or deformation, and improving the accuracy and reliability of strain measurements in high-temperature environments.

[0011] S02: Laser deposition and a camera system are started synchronously to collect images of speckle changes on the surface of the alloy substrate during the deposition process, wherein the camera system has a shooting angle perpendicular to the side surface of the alloy substrate.

[0012] The shooting angle of the shooting system is perpendicular to the side of the alloy substrate. When the laser deposition starts, the shooting system is triggered to start synchronously. The surface speckle change image of the entire process before, during and after deposition is continuously captured in real time to obtain the speckle change image of the alloy substrate surface, realizing in-situ observation of the dynamic strain field of the laser metal deposition process.

[0013] S03: Analyze the speckle change image using VIC-2D software to obtain the matrix Von Mises equivalent strain value.

[0014] S04: Calculating a stress value according to the equivalent strain value.

[0015] Due to the mismatch in thermal expansion coefficients between the alloy substrate and the deposited layer, the temperature drops by ∆T. Therefore, in-plane stress will be generated between the alloy substrate and the laser deposited layer. The calculation formula for this in-plane stress is:

[0016] (1)

[0017] in, are the positive strains of the i-th layer of material in the y direction.

[0018] The equilibrium condition for in-plane stress is: (2)

[0019] From formulas (1) and (2), we can get the following relationship:

[0020] (3)

[0021] Since both sides of the alloy substrate are clamped and fixed by the clamp, the strain of the alloy substrate and the laser deposited layer in the horizontal direction can be assumed to be 0, that is, (4)

[0022] Therefore, the deformation coordination condition of the alloy matrix and the laser deposited layer is: (5)

[0023] From formulas (3) and (5), we can get (6)

[0024] Based on formula (6), the calculation formula of stress value can be obtained as follows: (7)

[0025] Where, i is the number of layers, For the i The stress value in the layer y direction, is the elastic modulus of the i-th layer, is the Poisson’s ratio of the i-th layer, is the thermal expansion coefficient of the i-th layer, is the height of the i-th layer, is the temperature difference.

[0026] It can be seen from formula (7) that the stress caused by the temperature drop in the i-th layer material is not only proportional to the temperature drop, but also related to the thermal expansion coefficient and elastic properties of the layer and other layers of materials.

[0027] The present invention has the following beneficial effects:

[0028] (1) This application uses a laser marking machine to etch the digital speckle image on the side of the alloy substrate instead of the traditional spraying method, which avoids the influence of the high temperature generated by the laser on the quality of the speckle, thereby avoiding the speckle burning or deformation, and improving the accuracy and reliability of strain measurement in high temperature environment.

[0029] (2) This application uses a high-speed camera system to capture the surface speckle change image during the deposition process in real time, thereby realizing in-situ observation of the dynamic strain field during the laser metal deposition process; at the same time, through the high-temperature adaptive speckle design, in-situ strain monitoring of the deposition process and the coupling of strain-stress physical models, the dynamic quantification of thermal strain evolution during the laser metal deposition process is realized.

[0030] (3) This application correlates the DIC strain data of the substrate with the strain of the deposited layer, and calculates the residual stress value of the coating after deposition using the DIC strain data, providing theoretical support and process optimization window for the development of crack-free high-performance deposited layers.

[0031] (4) This application uses an experimental method to obtain the temperature difference between the substrate and the deposited layer, the coating strain and stress, etc. during the laser metal deposition process. Compared with finite element simulation calculations, this method has the characteristics of simple operation and high calculation accuracy, and is more suitable for online stress detection of laser metal deposition.

[0032] (5) Compared with traditional detection technologies, the detection method in this application is simple to operate and has high detection accuracy. It can regulate stress during the deposition process and monitor stress changes online in real time. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 A schematic structural diagram of a laser metal deposition layer stress detection system provided in an embodiment of the present application;

[0034] Figure 2 A schematic structural diagram of a laser metal deposition layer provided in an embodiment of the present application;

[0035] Figure 3 The digital speckle image provided by the embodiment of the present application;

[0036] Figure 4 The speckle image etched on the side of the alloy substrate by the laser marking machine provided in the embodiment of the present application;

[0037] Figure 5 This is a graph showing the change in the Von Mises equivalent strain value of the matrix provided in the embodiments of the present application. DETAILED DESCRIPTION

[0038] The technical solution of the present invention is further explained and illustrated by means of specific embodiments below.

[0039] Example 1

[0040] The embodiment of the present application provides a laser metal deposition layer stress detection system, which includes: an alloy substrate disposed between two fixtures, a camera system located on one side of the alloy substrate, and a laser scanner located at the fixture, as shown in the attached Figure 1 The laser metal deposition process in the embodiment of the present application is a process in which metal powder is used as raw material, a laser beam is used as an energy source, and the metal powder is melted layer by layer and rapidly solidified and deposited layer by layer on the surface of the metal substrate according to a predetermined processing path.

[0041] Specifically, the alloy substrate is clamped between two fixtures, and a gray point camera system is installed on one side of the fixture to capture DIC images. A laser scanner is located above one fixture, and the laser scanner's laser scanning direction is scanned from one fixture to the other.

[0042] In order to obtain DIC data, it is necessary to perform speckle processing on the side of the alloy substrate, and the speckle image is obtained through Glare software. Taking into account the high-temperature forming characteristics of the laser metal deposition process, in order to reduce the impact of high temperature on the speckle image, a laser marking machine is used in the embodiment of the present application to etch the speckle image on the side of the alloy substrate. During the laser metal deposition process, the camera system is turned on to collect the substrate speckle change image. The image results are imported into the VIC full-field strain measurement system software for strain analysis to obtain the strain value of the substrate material. Since the above-mentioned DIC data collects the strain of the side of the substrate, in order to obtain the strain and stress of the laser metal deposition layer, a substrate-deposition layer stress-strain model is established, as shown in the schematic diagram. Figure 2 shown.

[0043] Example 2

[0044] The present invention provides a method for detecting stress in a laser-deposited metal layer. Taking the preparation of a high-entropy alloy FeCoCrNiAl laser-deposited layer on the surface of an FeNi alloy substrate as an example, the method includes:

[0045] S01: Use sandpaper to polish the surface of the alloy substrate to remove surface contaminants. Use Glare software to generate digital speckle images, as shown in the attached figure. Figure 3 The parameters of Glare software are as follows: speckle diameter: 5-50 pixels; speckle density: 30%-70%; close packing: 30%-80%; offset: 30%-80%. According to the digital speckle image, a 30W laser marking machine is used to etch speckles on the side of the alloy substrate to obtain the attached Figure 4 .

[0046] S02: Adjust the shooting angle of the camera system to be perpendicular to the side of the alloy substrate for easy observation Figure 2 The imaging system is triggered to start synchronously when laser deposition is started, continuously capturing real-time images of the surface speckle changes from before to during to after deposition. This image of the surface speckle changes of the alloy substrate is obtained, enabling in-situ observation of the dynamic strain field during the laser metal deposition process.

[0047] S03: Use VIC-2D software to analyze the speckle change image and obtain the matrix Von Mises equivalent strain value, as shown in the attached Figure 5 shown.

[0048] S04: Calculate the stress value based on the equivalent strain value.

[0049] Specifically, due to the mismatch in thermal expansion coefficients between the alloy substrate and the deposited layer, the temperature decreases by ∆T. Therefore, in-plane stress is generated between the alloy substrate and the laser deposited layer. The calculation formula for this in-plane stress is:

[0050] (1)

[0051] in, are the positive strains of the i-th layer of material in the y direction.

[0052] The equilibrium condition for in-plane stress is: (2)

[0053] From formulas (1) and (2), we can get the following relationship:

[0054] (3)

[0055] Since both sides of the alloy substrate are clamped by the clamp, the strain of the alloy substrate and the laser deposited layer in the horizontal direction can be assumed to be 0, that is, the strain in the x direction is (4)

[0056] Therefore, the deformation coordination condition of the alloy matrix and the laser deposited layer is: (5)

[0057] From formulas (3) and (5), we can get (6)

[0058] Based on formula (6), the calculation formula of stress value can be obtained as follows: (7)

[0059] Where, i is the number of layers, For the i The stress value in the layer y direction, is the elastic modulus of the i-th layer, is the Poisson’s ratio of the i-th layer, is the thermal expansion coefficient of the i-th layer, is the height of the i-th layer, is the temperature difference.

[0060] The thermal expansion coefficient of the alloy matrix was measured by thermal expansion instrument and was 1.2×10 -5 / k, the elastic modulus of the alloy matrix tested by nanoindentation is 80.2GPa, and the Poisson's ratio is 0.25.

[0061] The height of the alloy substrate is 10 mm, the elastic modulus is 80.2 GPa, and the thermal expansion coefficient is 1.2×10 -5 / k, Poisson's ratio 0.25 and Figure 5 Substituting the Von Mises equivalent strain value of the substrate shown in the figure into formula (6), the temperature difference between the deposited layer and the substrate can be obtained. =316K. The temperature difference and the thermal expansion coefficient of the laser deposited layer is 1.65×10 -5Substituting / K into formula (6), the strain value of the deposited layer is approximately 0.003. Then, substituting the strain value of the deposited layer and the elastic modulus of 145.4GPa into formula (7), the stress value of the deposited layer is approximately 0.7GPa.

[0062] The above description is merely a preferred embodiment of the present application and is not intended to limit the present application. Various modifications and variations are possible for those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present application shall be included within the scope of protection of the present application.

Claims

1. A method for detecting stress in a laser metal deposition layer, characterized in that: include: After sandpaper polishing the surface of the alloy substrate, a digital speckle image was generated using Glare software, and a laser marking machine was used to etch the speckle pattern on the side of the alloy substrate; The laser deposition and the camera system are started synchronously to collect images of speckle changes on the surface of the alloy substrate during the deposition process, wherein the camera system has a shooting angle perpendicular to the side of the alloy substrate; The speckle change image is analyzed using VIC-2D software to obtain the matrix Von Mises equivalent strain value; Calculating a stress value according to the equivalent strain value; Calculating the stress value according to the equivalent strain value includes: In-plane stress will be generated between the alloy substrate and the laser deposited layer. The calculation formula of the in-plane stress is: （1） in, are the positive strain of the i-th layer material in the y direction; The equilibrium condition for in-plane stress is: (2) From formulas (1) and (2), we can get the following relationship: （3） Since both sides of the alloy substrate are clamped and fixed by the clamp, the strain of the alloy substrate and the laser deposited layer in the horizontal direction is assumed to be 0, that is, (4) The deformation coordination conditions of the alloy substrate and the laser deposited layer are: (5) From formulas (3) and (5), we can get (6) The calculation formula of the stress value based on formula (6) is: (7) Where, i is the number of layers, For the i The stress value in the layer y direction, is the elastic modulus of the i-th layer, is the Poisson’s ratio of the i-th layer, is the thermal expansion coefficient of the i-th layer, is the height of the i-th layer, is the temperature difference.

2. The laser metal deposition layer stress detection method according to claim 1, characterized in that: The parameters of the Glare software were set as follows: speckle diameter: 5-50 pixels; speckle density: 30%-70%; close packing: 30%-80%; and offset: 30%-80%.

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