Method and device for evaluating and measuring microscopic mechanical properties of TC4 alloy

Abstract

The invention provides a method and a device for evaluating and measuring microscopic mechanical properties of TC4 alloy, relates to the technical field of microscopic mechanical properties, and realizes accurate evaluation of material properties through combination of multi-scale characterization and numerical simulation. Firstly, a standard sample is prepared through selective laser melting, and residual stress field distribution is obtained through X-ray diffraction; dividing a stress field into sub-regions, and establishing a quantitative relationship between microscopic structure parameters and residual stress by combining SEM and EBSD analysis; the method comprises the following steps: acquiring local mechanical properties through a nanoindentation test, and constructing a stress-performance correlation model; and finally, establishing a finite element model, transmitting stress and tissue parameters by adopting trilinear interpolation and an inverse distance weighting method, verifying data reliability through a stress difference criterion, and calculating mechanical properties of each unit. According to the method, multi-scale correlation from microstructure characteristics to macroscopic mechanical properties is innovatively realized, so that the mechanical property distribution of the material is accurately evaluated.

Description

Technical Field

[0001] The present invention relates to the technical field of microscopic mechanical properties, and in particular to a method and device for evaluating and measuring the microscopic mechanical properties of a TC4 alloy. Background Art

[0002] Selective Laser Melting (SLM) is a new 3D printing technology that emerged in the 1990s. Unlike traditional manufacturing processes like cutting, drilling, milling, and welding, this technology creates three-dimensional solid parts by layering materials. This eliminates the geometric limitations of traditional processing methods and allows for the simple and precise creation of personalized, integrated, and functional complex three-dimensional structures. Furthermore, the resulting metal possesses high density and excellent mechanical properties, finding widespread application in industrial manufacturing, packaging, architecture, art, medicine, aviation, aerospace, and film and television.

[0003] Titanium alloys have the advantages of high specific strength, good corrosion resistance, and high heat resistance, and are mainly used in aerospace, automotive, medical and other fields. Ti-6Al-4V (TC4) alloy is a widely used titanium alloy with high impact toughness, low Young's modulus, corrosion resistance, and excellent high and low temperature performance. It is mainly used to manufacture spiral blades for automobile engines, aircraft landing gear, medical artificial joints, etc. With the increasing application of TC4 titanium alloy, traditional casting technology can no longer meet the needs of preparing precision and complex parts. TC4 titanium alloy prepared by SLM technology not only has the advantages of traditional casting TC4 titanium alloy, but also can produce metal devices with perfect metallurgical bonding and high density.

[0004] With the development and application of SLM technology, some problems have gradually emerged. The most important of these is that during the manufacturing process, the superimposed layers are affected by rapid thermal cycling, forming large temperature gradients and generating residual stress within the material. Residual stress can cause component deformation, cracks, and interlayer separation, thereby compromising the normal function and structural integrity of the component. Therefore, in-depth research on the residual stress distribution of SLM-manufactured materials and its impact on mechanical properties is of great significance.

[0005] The above information disclosed in this Background section is only for enhancement of understanding of the background of the present disclosure and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art. Summary of the Invention

[0006] The purpose of the present invention is to provide a method and device for evaluating the microscopic mechanical properties of TC4 alloy to solve the problems raised in the above background technology.

[0007] To achieve the above object, the present invention provides the following technical solutions: A method for evaluating and measuring the microscopic mechanical properties of TC4 alloy, comprising the following steps: Step 1: Prepare a standard sample of TC4 alloy. After the standard sample is formed, use X-ray diffraction to measure the residual stress distribution on its surface to obtain the residual stress field of the standard sample. Step 2: Divide the residual stress field into multiple sub-regions and calculate the average residual stress value of each sub-region as the characteristic stress value; obtain the SEM image of the standard specimen, extract the microstructural parameters of each sub-region, and form a data pair with the characteristic stress value of the corresponding sub-region to establish the relationship between the microstructural parameters of the standard specimen and the residual stress value; Step 3: Select several typical micro-regions in each sub-region, determine the indentation points of the typical micro-regions, and perform nanoindentation tests at the indentation points to obtain the mechanical properties of each indentation point; Step 4: Discretize the residual stress field into several three-dimensional grids, determine the residual stress value of each grid by interpolation, and match each indentation point to the nearest grid to establish the relationship between the mechanical properties of the standard specimen and the residual stress value; Step 5: Construct a finite element model of the standard specimen and obtain the interpolated residual stress value and interpolated microstructure parameters of each finite element unit through the interpolation method; analyze the differential microstructure parameters based on the relationship between the microstructure parameters and the residual stress values ​​of the standard specimen, calculate the estimated residual stress value of the finite element unit, and establish a stress difference criterion by combining the interpolated residual stress value and the estimated residual stress value to determine the final residual stress value and calculate the mechanical properties of each finite element unit.

[0008] Furthermore, the process parameters for preparing the standard sample of TC4 alloy include laser power, scanning speed, powder thickness and scanning spacing; the standard sample of TC4 alloy is prepared by laser selective melting technology, and after the standard sample is formed, the residual stress distribution on the surface of the standard sample is measured by X-ray diffraction method. Method is used to measure residual stress, determine the distance between measuring points, and obtain the residual stress field distribution through three-dimensional reconstruction; The specific criteria for determining whether a standard specimen is formed are as follows: process monitoring parameters and specimen macro-monitoring parameters are within the preset threshold range; process monitoring parameters include melt pool characteristics and interlayer temperature, and specimen macro-monitoring parameters include geometric dimensions, surface roughness, porosity, and defect length.

[0009] Furthermore, obtaining the microstructure parameters of each sub-region specifically includes: The residual stress field is spatially divided into N sub-regions, and the fluctuation range of each sub-region is determined to be lower than a preset residual stress value range threshold, and the mean of the residual stress values ​​of the sub-region is used as the characteristic stress value of the sub-region; Scanning electron microscopy was used to observe each sub-area to obtain SEM images of the standard specimens, as follows: Three representative points are selected in each sub-region. The representative points meet the following conditions: the difference between the X-ray diffraction residual stress value of the representative point and the residual stress value of the sub-region must be within 10%. The center of the representative point is measured by electron backscatter diffraction, and the radius is The strain field in the region represents the strain gradient at the point is not greater than A scanning electron microscope is used to obtain an SEM image at each representative point. The specific method is as follows: the position of the representative point on the standard specimen is used as the measurement point, and a microscopic analysis area group of the measurement point is determined with each measurement point as the center. The microscopic analysis area group is centered on the measurement point, and five analysis areas are determined at the measurement point, including a central area and four surrounding areas. The central area is the area centered on the representative point, and the representative point coincides with the X-ray measurement point. The surrounding areas are areas offset to the left and right along the X-axis, and areas offset to the up and down along the Y-axis, respectively. The method for determining the actual offset is as follows: the absolute value of the residual stress gradient in the X and Y directions at the current measurement point is calculated, and the ratio of the residual stress gradient to the material sensitivity coefficient is defined as the theoretical offset. The theoretical offset is then compared with a preset maximum safety offset limit, and the minimum of the two is ultimately taken as the actual offset. Determine the residual stress value of each analysis area by interpolation method, and ensure that the fluctuation range of the residual stress value of each analysis area is still lower than the preset residual stress value range threshold; otherwise, mark it as a cross-interval area and eliminate it; The SEM images of each analysis area were analyzed using image analysis technology and electron backscatter diffraction technology to measure the microstructural parameters of each analysis area. The microstructural parameters include Phase volume fraction, Phase volume fraction and grain size, in each sub-region, the mean of the microstructural parameters of all analysis areas except the cross-interval area is defined as the microstructural parameters of the sub-region.

[0010] Furthermore, the relationship between the microstructural parameters and the residual stress values ​​of the standard specimens is constructed specifically including: The microstructural parameters and characteristic stress values ​​of each sub-region are defined as data pairs of the corresponding sub-region. Based on the data pairs of all sub-regions, the relationship between the microstructural parameters and residual stress values ​​of the standard specimen is constructed, including The relationship between phase volume fraction and residual stress value, The relationship between phase volume fraction and residual stress value, the relationship between grain size and residual stress value; Substituting all the data pairs of microstructural parameters and residual stress values ​​of the standard specimen into the relationship, the formula is expressed as: ; in, express Phase volume fraction, Indicates the datum under zero stress state Phase volume fraction, Residual stress caused by The maximum phase change, represents the residual stress value, represents the critical stress of phase transition, represents the phase change rate control parameter, for Phase volume fraction, is the grain size, is the grain size under zero stress state, is the yield stress, is the hardening index.

[0011] Furthermore, obtaining the mechanical properties of each indentation point specifically includes: In each sub-region, M typical micro-regions are selected, and the typical micro-regions satisfy the stress uniformity rule and the tissue consistency rule. The stress uniformity rule is that the standard deviation of the residual stress value in the typical micro-region is within the preset stress fluctuation threshold, and the tissue consistency rule is that the standard deviation of the residual stress value in the typical micro-region is within the preset stress fluctuation threshold. Phase volume fraction and its sub-region, all typical micro-regions The average value of the phase volume fraction is within a preset deviation range, and the deviation between the grain size of a typical micro-region and the average value of the grain sizes of all typical micro-regions in its sub-region is within a preset deviation range; Mark characteristic points in the SEM image of a typical micro-area, where the characteristic points are locations within the typical micro-area that have microstructural characteristics and coincide with the coordinates of the X-ray measurement points. Record the coordinates of the characteristic points, map the characteristic points in the SEM image to the standard specimen using the coordinate conversion matrix of the nanoindenter, arrange the indentation points in a grid array centered on the characteristic points mapped to the standard specimen, perform nanoindentation testing at the indentation points, and obtain the mechanical properties of each indentation point, including hardness and elastic modulus values.

[0012] Furthermore, determining the relationship between the residual stress value and the hardness of the standard specimen specifically includes: Presetting fiducial markers on a standard specimen to verify that all test equipment, including an X-ray diffractometer, SEM, and nanoindenter, uses the same coordinate system; Define the geometric center of the standard specimen as the origin. The X, Y, and Z axes of the coordinate axis correspond to the length, width, and height of the standard specimen, respectively. If there is inconsistency in the coordinate system of the equipment, the rigid body transformation is used to align the coordinate system. If all test equipment uses the same coordinate system, no processing is required. The residual stress field is discretized into a three-dimensional grid with a grid size consistent with the nanoindentation spacing. The residual stress value of each grid is determined by interpolation. The coordinates and mechanical properties of each indentation point are recorded by the nanoindenter. The indentation point is matched to the nearest grid by the nearest neighbor search method. Each grid generates a set of matching data. Each set of data includes the residual stress value, hardness and elastic modulus of the grid. Based on the residual stress values ​​and hardness of all grids, the relationship between the residual stress value and hardness of the standard specimen is determined, which is expressed as follows: ; in, represents the hardness of the j-th grid, represents the zero stress hardness, represents the residual stress value of the jth grid, represents the linear term coefficient, The coefficient of the quadratic term, represents the exponential decay amplitude, represents characteristic stress.

[0013] Furthermore, determining the relationship between the residual stress value and the elastic modulus of the standard specimen specifically includes: According to the residual stress values ​​and elastic modulus of all grids, the relationship between the residual stress value and elastic modulus of the standard specimen is determined and expressed as follows: ; in, represents the elastic modulus of the j-th grid, represents the zero stress modulus, It represents the sensitivity coefficient of elastic modulus to residual stress.

[0014] Furthermore, determining the mechanical properties of each finite element unit specifically includes: Determine the dimensions of the standard specimen and input them into the finite element software to determine the element type, element size, and number of elements to complete the element division. Assign a local material coordinate system to each element based on the grain orientation analyzed by electron backscatter diffraction. The discretized three-dimensional grid residual stress value is transferred to the center point of the finite element unit using the trilinear interpolation method to obtain the interpolated residual stress value of each finite element unit; Interpolation of microstructural parameters for each finite element, including: interpolation using inverse distance weighted method Phase volume fraction, using Interpolation of phase volume fraction conservation relations Phase volume fraction and grain volume interpolated using Kriging, the The phase volume fraction conservation relationship is Phase volume fraction and The sum of the phase volume fractions is 1; According to the relationship between the microstructure parameters and residual stress values ​​of the standard specimen, the estimated residual stress value of each finite element is obtained by substituting the microstructure parameters obtained by interpolation of each finite element; Compare the interpolated residual stress value and the estimated residual stress value of each finite element unit. When the absolute difference between the interpolated residual stress value and the estimated residual stress value is not greater than 0.05 times the yield stress, take the average of the interpolated residual stress value and the estimated residual stress value as the final residual stress value. When the absolute difference between the interpolated residual stress value and the estimated residual stress value is greater than 0.05 times the yield stress, re-collect the X-ray diffraction data of the physical position of the center of the finite element unit and use the measured value as the final residual stress value. According to the relationship between mechanical properties and residual stress values, the final residual stress value is input to obtain the hardness and elastic modulus of each finite element; Set the boundary conditions and solution settings of the finite element model. The boundary conditions are to import the residual stress field, apply the discretized residual stress field to the corresponding unit in the form of the initial stress field, and satisfy the equilibrium equation ,in, Represents the residual stress value of the corresponding element, F represents the body force; the solver is set to use implicit dynamic analysis, and the time step is set.

[0015] The present invention further provides a device for evaluating and measuring the microscopic mechanical properties of a TC4 alloy. The device is used to implement the above-mentioned method for evaluating and measuring the microscopic mechanical properties of a TC4 alloy, comprising: The residual stress field calibration module is used to prepare the standard sample of TC4 alloy. After the standard sample is formed, the residual stress distribution on its surface is measured using X-ray diffraction to obtain the residual stress field of the standard sample; The microstructure and stress correlation module is used to divide the residual stress field into multiple sub-regions and calculate the average residual stress value of each sub-region as the characteristic stress value; obtain the SEM image of the standard sample, extract the microstructure parameters of each sub-region, and form a data pair with the characteristic stress value of the corresponding sub-region to establish the relationship between the microstructure parameters of the standard sample and the residual stress value; The nanoindentation mechanical testing module is used to screen out several typical micro-regions in each sub-region, determine the indentation points of the typical micro-regions, and perform nanoindentation tests at the indentation points to obtain the mechanical properties of each indentation point; The mechanical properties and stress mapping module is used to discretize the residual stress field into several three-dimensional grids, determine the residual stress value of each grid by interpolation, and match each indentation point to the nearest grid to establish the relationship between the mechanical properties and residual stress values ​​of the standard specimen; The multi-scale finite element analysis module is used to construct a finite element model of a standard specimen, obtain the interpolated residual stress value and interpolated microstructure parameters of each finite element unit through interpolation; analyze the differential microstructure parameters based on the relationship between the microstructure parameters and the residual stress value of the standard specimen, calculate the estimated residual stress value of the finite element unit, and establish a stress difference criterion by combining the interpolated residual stress value and the estimated residual stress value to determine the final residual stress value and calculate the mechanical properties of each finite element unit. In the above technical solution, the technical effects and advantages provided by the present invention are: This approach combines multi-scale experiments with numerical modeling to accurately evaluate the microscopic mechanical properties of TC4 alloy after selective laser melting. To address the difficulty of traditional methods in correlating residual stress, microstructure, and mechanical properties, an innovative quantitative mapping relationship between the discretized mesh of the residual stress field and microstructural parameters was established, and a stress-property coupling model was constructed using nanoindentation test data. This approach not only solves the spatial matching problem of multi-source heterogeneous data (X-ray stress measurement, SEM microstructure analysis, and nanoindentation mechanical data), but also significantly improves the physical realism of the mechanical property parameterization of finite element units by introducing phase transformation kinetic equations and grain size evolution models. In particular, for the gradient stress-microstructure characteristics of TC4 alloy manufactured by laser additive manufacturing, the local modulus and hardness distribution can be accurately predicted. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 Schematic diagram of the overall method flow of the present invention; Figure 2 Schematic diagram comparing the interpolated residual stress value and the estimated residual stress value of the present invention; Figure 3 This is a schematic diagram comparing the final residual stress value and the average value of the present invention; Figure 4 Schematic diagram of the structure of the device of the present invention. DETAILED DESCRIPTION

[0017] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to specific embodiments.

[0018] It should be noted that, unless otherwise defined, the technical or scientific terms used in the present invention should have the usual meanings understood by people with ordinary skills in the field to which the present invention belongs. The "first", "second" and similar words used in the present invention do not indicate any order, quantity or importance, but are only used to distinguish different components. "Include" or "comprise" and similar words mean that the elements or objects appearing before the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects. "Connect" or "connected" and similar words are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "Up", "down", "left", "right" and the like are only used to indicate relative position relationships. When the absolute position of the object being described changes, the relative position relationship may also change accordingly.

[0019] Example: See also Figure 1-Figure 3 , the present invention provides a technical solution: A method for evaluating and measuring the microscopic mechanical properties of TC4 alloy, comprising the following steps: Step 1: Prepare a standard sample of TC4 alloy. After the standard sample is formed, use X-ray diffraction to measure the residual stress distribution on its surface to obtain the residual stress field of the standard sample. In this example, a standard sample of TC4 alloy was prepared using the laser selective melting technique. The key process parameters during the preparation process include laser power, scanning speed, powder thickness, and scanning spacing. Laser power affects the depth of the molten pool and the forming quality, and the preferred range is [100W, 400W]. The scanning speed determines the energy input rate, and the preferred range is [500mm / s, 1500mm / s]. The powder thickness affects the interlayer bonding and density, and the preferred range is [20 ,50 ], the scanning spacing affects the overlap rate and residual stress distribution, and the preferred range is [50 ,120 ]. In order to ensure the forming quality of standard specimens, it is necessary to monitor the process monitoring parameters and specimen macro-monitoring parameters in real time to determine whether they meet the preset threshold range: For molten pool characteristics (such as molten pool width and depth), high-speed camera or infrared thermal imaging monitoring is used to ensure the stability of the molten pool to avoid unfusion or overburning. The interlayer temperature is controlled within the range of [100℃, 300℃] to reduce the accumulation of thermal stress. Among the specimen macro-monitoring parameters, the geometric dimensions are monitored by optical measurement or three-coordinate measuring machine, and the error does not exceed ±0.1mm. The surface roughness is measured by a profilometer, and the surface roughness is required to be no more than 10 The porosity is evaluated by X-ray tomography or metallographic analysis and is required to be no more than 0.5%. The length of defects (such as cracks and pores) is evaluated by microscopic observation or CT scanning and is required to be no more than 50 If the above parameters exceed the threshold, the sample is judged to be unqualified and needs to be re-prepared after adjusting the process parameters. Otherwise, the sample is judged to be qualified and will be formed as a standard sample.

[0020] After the standard sample is formed, the residual stress distribution on the surface of the standard sample is measured using the X-ray diffraction method. The specific steps are as follows: First, Method, by measuring different Residual stress is calculated by measuring the diffraction peak shift at angles (e.g., 0°, 15°, 30°, and 45°). The measurement point spacing is then set based on the standard specimen size and stress gradient, preferably between 0.5 mm and 2 mm, to ensure data representativeness. Finally, a three-dimensional residual stress field is constructed using multi-point measurement data combined with an interpolation algorithm, such as Kriging.

[0021] Specifically, the steps for determining the distance between measurement points based on the standard specimen size and stress gradient are as follows: The distance between measurement points should be evenly distributed on the sample surface while avoiding data redundancy. The following empirical formula is recommended: ; in, is the distance between measurement points, is the proportional coefficient, which usually ranges from [0.01, 0.05], that is, the distance between the measuring points is 1% to 5% of the standard specimen size. L is the characteristic size of the standard specimen, which can be length, width or diameter, and the unit is mm.

[0022] Since the residual stress distribution may have local drastic changes (such as welds and boundary areas), the measurement point spacing needs to be further optimized according to the stress gradient: After the preliminary measurement, the stress change rate between adjacent measurement points is calculated using the following formula: ; in, represents the stress gradient between the i-th measurement point and the i+1-th measurement point, represents the residual stress value of the i-th measurement point, Represents the residual stress value of the i+1th measurement point, where i is the measurement point index; Adaptive adjustment rules, if , then keep the initial spacing; if , then reduce the spacing to ;like , then further narrowed to , and encrypt measurements in this area.

[0023] The stress gradient is usually higher at the edge, hole or weld area of ​​the standard specimen. It is recommended to use a default spacing of 0.1mm to 0.5mm. In flat or low stress areas, it can be appropriately relaxed to 2mm to 3mm to improve measurement efficiency. The low stress area refers to area.

[0024] Step 2: Divide the residual stress field into multiple sub-regions and calculate the average residual stress value of each sub-region as the characteristic stress value; obtain the SEM image of the standard specimen, extract the microstructural parameters of each sub-region, and form a data pair with the characteristic stress value of the corresponding sub-region to establish the relationship between the microstructural parameters of the standard specimen and the residual stress value; In this embodiment, obtaining the microstructure parameters of each sub-region specifically includes: The residual stress field is spatially divided into N subregions. The fluctuation range of each subregion is ensured to be below a preset residual stress threshold (-20 MPa, 20 MPa) to ensure uniform stress distribution. The size of the subregions can be dynamically adjusted based on the stress gradient. For example, in low-gradient regions (stress gradients no greater than 10 MPa / mm), the subregion size can be set to 1 mm × 1 mm, while in high-gradient regions (stress gradients greater than 50 MPa / mm), the subregion size can be set to 0.2 mm × 0.2 mm. The arithmetic mean of the residual stress values ​​at all measurement points within each subregion is taken as the characteristic stress value for that subregion.

[0025] Scanning electron microscopy was used to observe each sub-region with an accelerating voltage of 15 kV, a working distance of 10 mm, and backscattered electron mode imaging with a resolution of 1 , thereby obtaining the SEM image of the standard sample, as follows: In each sub-region, three representative points are selected. The representative points meet the following conditions, including that the difference between the X-ray diffraction residual stress value of the representative point and the residual stress value of the sub-region must be within 10%. The center of the representative point is measured by electron backscatter diffraction (EBSD) with a radius of The strain field in the region, the strain gradient satisfies no greater than , to ensure uniform local deformation.

[0026] The purpose of the difference between the X-ray diffraction residual stress value and the residual stress value of the sub-region must be within the range of 10% to ensure that the residual stress of the representative point can truly reflect the overall stress state of the sub-region. If the stress value of the representative point differs too much from the average value of the sub-region (such as >10%), it may be a local abnormal point (such as micro defects, impurities, etc.) and cannot represent the general characteristics of the sub-region. The basis for setting the 10% threshold is that the residual stress distribution of TC4 alloy usually shows a continuous gradient change. Experimental statistics show that the normal fluctuation range is generally no more than ±15%, so a stricter 10% is selected to ensure data consistency. The typical grain size of TC4 alloy is Phase grains are usually 1 to 10 , is equiaxed, and the phase grains are usually 5 to 20 , distributed in strips or nets. The radius is The rationality of the region is reflected in the fact that it contains at least 25 to 100 grains (based on the average grain size of 5 Calculation), meet the statistical representativeness requirements (such as ASTM E112 standard recommends a minimum of 50 grains). ), which may contain only 1 to 2 grains, resulting in the strain field measurement being greatly affected by the orientation of a single grain and lacking statistical significance.

[0027] The purpose of the local strain gradient constraint is to exclude the inhomogeneous regions caused by local plastic deformation or lattice distortion. ) may correspond to local deformations such as grain boundary sliding and dislocation accumulation. The microstructure (such as grain orientation and phase distribution) of such areas may be significantly different from those in low gradient areas, affecting statistical representativeness. The threshold is set based on the EBSD calibration test, which shows that when the strain gradient exceeds this value, the TC4 alloy or The phase distribution will show obvious local segregation, such as Phase is enriched at grain boundaries.

[0028] A scanning electron microscope is used to obtain SEM images at each representative point. The specific method is as follows: the position of the representative point on the standard specimen is used as the measurement point, and a microscopic analysis area group of the measurement point is determined with each measurement point as the center. The microscopic analysis area group is centered on the measurement point, and five analysis areas are determined at the measurement point, including a central area and four surrounding areas. The central area is the area centered on the representative point, and the representative point coincides with the X-ray measurement point. The surrounding areas are areas offset to the left and right along the X-axis, and areas offset to the up and down along the Y-axis, respectively. The method for determining the actual offset is as follows: the absolute value of the residual stress gradient in the X and Y directions at the current measurement point is calculated, and the ratio of the residual stress gradient to the material sensitivity coefficient is defined as the theoretical offset. The theoretical offset is then compared with a preset maximum safe offset limit, and the minimum of the two is ultimately taken as the actual offset.

[0029] The actual offset is expressed as: ; in, Indicates the offset in the X-axis direction. Indicates the residual stress value in the X-axis direction, The material sensitivity coefficient is determined by TC4 alloy calibration test, which indicates the maximum stress change allowed per micron distance. If the value exceeds this value, it is considered as a stress mutation area and more sampling is required. is the maximum allowable offset, set , the unit is To ensure that the analysis area does not exceed the single field of view of the SEM and avoid coordinate mapping distortion, Indicates the offset in the Y-axis direction. Indicates the residual stress value in the Y-axis direction.

[0030] The stress gradient directly reflects the severity of the change in residual stress in the X or Y direction. The larger the gradient, the more uneven the stress distribution, and the offset needs to be reduced to avoid cross-region sampling. Make sure that the stress fluctuations in the analysis area do not exceed the sub-region threshold. The calculation method is similar to the stress gradient calculation method in step 1, except that the stress gradient in the X direction or in the Y direction is analyzed separately here. The material sensitivity coefficient characterizes the sensitivity of TC4 alloy to residual stress changes. In the above formula, the stress gradient is normalized and converted into the actual allowable offset distance. The larger the material sensitivity coefficient, the smaller the allowable offset. In addition to obtaining the material sensitivity coefficient through experimental calibration, it can also be determined by referring to the typical range. For example, the material sensitivity coefficient of TC4 alloy is usually 0.3 to 1 The maximum allowable offset is to prevent the offset from being too large, which may cause the SEM field of view to overflow or the coordinate mapping to be distorted. It can be set according to the empirical value. For example, the SEM single field of view range is usually 100. to 200 , so the maximum allowable offset is 100 .

[0031] The purpose of coinciding the representative point with the X-ray measurement point is to achieve accurate spatial matching of macro stress and microstructure. The residual stress measurement area of ​​X-ray diffraction is usually 1mm to 2mm in diameter, while the observation area of ​​SEM / EBSD is at the micron level. If the representative point does not coincide with the X-ray measurement point, it may cause misalignment of macro-micro data and fail to establish an accurate stress-structure relationship. Coincidence verification method: First, make a microhardness indentation mark on the surface of the sample as a positioning reference for the X-ray measurement point. During SEM observation, the indentation position is identified by the backscattered electron image to ensure that the deviation between the center of the representative point and the center of the indentation is no more than 5 .

[0032] The residual stress value of each analysis area is determined by interpolation method, and the fluctuation range of the residual stress value of each analysis area is ensured to be still lower than the preset residual stress value range threshold. Otherwise, it is marked as a cross-interval area and eliminated.

[0033] The SEM images of each analysis area were analyzed using image analysis technology and electron backscatter diffraction technology to measure the microstructural parameters of each analysis area. The microstructural parameters include Phase volume fraction, Phase volume fraction and grain size, in each sub-region, the mean of the microstructural parameters of all analysis areas except the cross-interval area is defined as the microstructural parameters of the sub-region. Phase volume fraction and The phase volume fraction is measured using the grayscale threshold segmentation method, such as Use dark contrast, Phase was determined using bright contrast, and grain size was obtained by calculating the equivalent circle diameter based on the electron backscatter diffraction grain boundary map.

[0034] In this embodiment, the relationship between the microstructure parameters and the residual stress values ​​of the standard specimen is constructed as follows: The microstructural parameters and characteristic stress values ​​of each sub-region are defined as the data pairs of the corresponding sub-regions. The data pairs are represented as Data Pair ( Phase volume fraction, Phase volume fraction, grain size, characteristic stress value). Based on the data pairs of all sub-regions, the relationship between the microstructural parameters and residual stress values ​​of the standard specimen is constructed, including The relationship between phase volume fraction and residual stress value, The relationship between phase volume fraction and residual stress value, the relationship between grain size and residual stress value; Substituting all the data pairs of microstructural parameters and residual stress values ​​of the standard specimen into the relationship, the formula is expressed as: ; in, express Phase volume fraction, Indicates the datum under zero stress state The phase volume fraction can be calibrated by SEM / EBSD measurement of stress-free annealed samples, and then optimized by fitting. It can also refer to the typical value of TC4, such as 90%. Residual stress caused by Maximum phase change, fitting at different stress levels The change in phase fraction, represents the residual stress value, represents the critical stress of phase transition, that is to Phase transition critical stress, corresponding to the stress threshold at which phase transition begins, is determined by inflection point analysis. Represents the phase change rate control parameter, controls the slope of the tanh function, and reflects the sensitivity of the phase change to stress changes for Phase volume fraction, is the grain size, is the grain size under zero stress state, and the initial value can be obtained by EBSD statistics of annealed sample. It is the yield stress, which is related to the yield behavior of the material and needs to be calibrated in combination with the tensile test data. You can also refer to the typical value of TC4, such as 800MPa. is the hardening index, which describes the rate at which grain size changes with stress. The above parameters are obtained by substituting all data pairs and fitting them using the least squares method.

[0035] The relationship between phase volume fraction and residual stress value is described by hyperbolic tangent function The change of phase volume fraction with residual stress value is reasonable because arrive Phase transformation is a typical stress-driven process. Tensile stress promotes phase (high temperature phase), while compressive stress is beneficial to Phase retention, this transition shows an S-shaped trend near the critical stress, which perfectly matches the saturation characteristics of the tanh function. Phase fraction from the baseline value at zero stress Gradually decreases, the change range is Control, and Determines the speed of phase change. near hour, It begins to decrease significantly and eventually stabilizes in the high stress region, which is consistent with the stress-induced phase transformation behavior observed in the experiment.

[0036] The phase volume fraction is directly given by The phase volume fraction is derived, and its rationality comes from the and The volume conservation property of the phase. The stress is stable phase in the phase, and its fraction increases monotonically with the increase of tensile stress, and the change area is This linear complementary relationship avoids the physical contradictions that may result from independent modeling, such as , and simplifies the parameter calibration process. hour, The phase volume fraction increases rapidly and reaches saturation in the high stress region, i.e. , which is consistent with the phase composition evolution law measured by X-ray diffraction.

[0037] The grain size is related to the residual stress value through a power law function, and its core mechanism is that the residual stress value affects the dynamic recrystallization behavior through the change of dislocation density. and n together determine the sensitivity of grain refinement to stress: low stress (i.e. ), the grain size approaches the zero stress value As stress increases, dislocation multiplication leads to accelerated grain boundary migration, according to This nonlinear relationship can simultaneously describe the slow grain coarsening of TC4 alloy in the low stress region and the rapid grain refinement in the high stress region, which is consistent with the statistical distribution of grains observed by EBSD.

[0038] Step 3: Select several typical micro-regions in each sub-region, determine the indentation points of the typical micro-regions, and perform nanoindentation tests at the indentation points to obtain the mechanical properties of each indentation point; In this embodiment, obtaining the mechanical properties of each indentation point specifically includes: In each sub-region, M typical micro-regions are selected, and M is required to be no less than 3 to ensure statistical representativeness, and the typical micro-regions meet the stress uniformity rule and the organizational consistency rule. The stress uniformity rule is that the standard deviation of the residual stress value in the typical micro-region is within the preset stress fluctuation threshold range, that is, the standard deviation of the residual stress value of all measuring points in the typical micro-region must not exceed 15MPa. The yield strength of TC4 alloy is about 800-900MPa, and the fluctuation threshold of 15MPa is equivalent to a yield strength of less than 2%, which is far below the critical value of the macroscopic plastic deformation of the material, thereby ensuring that the micro-region is still in the elastic deformation-dominated range. In addition, there are errors in X-ray diffraction measurements. The stress measurement accuracy of commercial XRD equipment is usually ±10MPa to 20MPa. The 15MPa threshold can effectively distinguish between true stress gradients and instrument noise. Furthermore, in TC4 alloy The phase fraction fluctuates by about 1% to 2% when the stress changes by ±20MPa. The 15MPa threshold can capture the significant tissue response while avoiding data redundancy caused by over-subdivision of regions. Phase volume fraction and its sub-region, all typical micro-regions The average value of the phase volume fraction is within the preset deviation range, that is, the typical micro-region The phase volume fraction differs from the sub-region average by no more than 5%. , then the typical micro-region It should be in the range of 80% to 90%. TC4 alloy is in equilibrium The phase fraction is about 90% ( Phase fraction is 10%), and the 5% deviation (i.e. 85%-95%) covers the natural fluctuation range of phase fraction caused by common processes (such as annealing and solution treatment). Exceeding this range may indicate abnormal phase transformation (such as martensite formation). In addition, there are errors in EBSD phase analysis. When the statistical confidence level is 95%, the phase fraction detection error of EBSD is about ±3%. The 5% threshold can distinguish between real organizational differences and measurement noise. The deviation between the grain size of a typical micro-region and the average grain size of all typical micro-regions in the sub-region where it is located is within the preset deviation range, that is, the difference between the grain size of a typical micro-region and the average value of the sub-region is no more than 10%. For example, if the average grain size of a sub-region is 5 , the typical micro-region grain size needs to be 4.5 to 5.5 The grain size deviation is set based on the Hall-Petch relationship. Grain size measurement error is also taken into account. The statistical error of EBSD for equivalent diameter is approximately ±5% (requiring at least 50 grains). A threshold of 10% ensures data reliability.

[0039] Mark the characteristic points in the SEM image of the typical micro-region. The characteristic points are the positions in the typical micro-region that have microstructural characteristics and coincide with the coordinates of the X-ray measurement points. The intersection of phase interface and grain boundary should coincide with the coordinates of X-ray measurement points, and the deviation should not exceed 2 To ensure that stress and performance data are spatially aligned. Mark the feature points in the SEM image and record their pixel coordinates, such as , the characteristic points in the SEM image are mapped to the standard specimen through the coordinate transformation matrix of the nanoindenter and converted into the actual coordinates of the standard specimen, such as , using a standard grid template (such as 1 The grating is calibrated to ensure that the conversion error is no more than 1 The indentation points are arranged in a 5×5 grid array with a spacing of 20 , covering 50 ×50 The spacing is set in this way to avoid the indentation to see the charging of the plastic zone. The indentation radius of TC4 is about 3 times the indentation depth. Based on a depth of 100nm, the spacing is not less than 6 , so take 20 Ensure safety. Nanoindentation test is performed at the indentation point to obtain the mechanical properties of each indentation point, including hardness and elastic modulus values. In the nanoindentation test, the equipment uses a Berkovich indenter, the loading curve is a maximum load of 10mN, and the loading, holding and unloading times are 10s, 5s, and 10s, respectively. In the continuous stiffness measurement mode, the hardness and elastic modulus change curves with depth are obtained. The hardness and elastic modulus values ​​at 10% of the indentation depth are taken, such as when the depth is 100nm, to avoid the influence of surface roughness. Eliminate abnormal indentations (such as slip line interference) and retain at least 9 valid data points to calculate the average value as the final mechanical properties.

[0040] Step 4: Discretize the residual stress field into several three-dimensional grids, determine the residual stress value of each grid by interpolation, and match each indentation point to the nearest grid to establish the relationship between the mechanical properties of the standard specimen and the residual stress value; In this embodiment, determining the relationship between the residual stress value and the hardness of the standard specimen specifically includes: Pre-set reference marks on the standard specimen to verify that all test equipment uses the same coordinate system. The test equipment includes X-ray diffractometer, SEM and nanoindenter. To set the reference marks, process at least 3 microhardness indentations or laser marking points on the surface of the standard specimen to form non-collinear reference marks (triangular distribution is recommended). The coordinate accuracy is not greater than 5 The function of the fiducial marker is to serve as the common coordinate system reference of the X-ray diffractometer, SEM and nanoindenter to ensure the spatial alignment of the data.

[0041] Define the geometric center of the standard specimen as the origin, and the X, Y, and Z axes of the coordinate axis correspond to the length, width, and height of the standard specimen respectively. If there is inconsistency in the device coordinate system, the rigid body transformation is used to align the coordinate system. If all test devices use the same coordinate system, no processing is required. The rigid body transformation calculation requires measuring the coordinates of the reference points in each device, forming a point set, solving the rotation matrix and translation vector, and minimizing the objective function: ,in, It represents the three-dimensional coordinates of the qth reference mark point in the X-ray diffractometer coordinate system. The position of the mark point is recorded by the sample stage encoder or laser tracker of the XRD equipment. represents the three-dimensional coordinates of the qth fiducial marker in the scanning electron microscope coordinate system, R is the rotation matrix, t is the translation vector, and q is the fiducial marker index of different devices. The singular value decomposition algorithm is used to achieve the optimal transformation. The verification standard is that the residual error of the fiducial coordinate after transformation is no more than 10 The essence of minimizing the objective function is to achieve the optimal rigid body alignment between multiple coordinate systems through the least squares method.

[0042] The residual stress field is discretized into a three-dimensional grid with a grid size consistent with the nanoindentation spacing, such as 20 ×20 × layer thickness, ensure that each grid contains 1 indentation point, and crop the edge grid of the standard specimen to retain only the complete grid data. The residual stress value of each grid is determined by the Kriging interpolation method. Specifically, based on the residual stress value of the X-ray measurement point, a variation function model (such as Gaussian type) is constructed, and then the grid unit stress value is solved. The root mean square error of the difference result and the measured point is controlled to be no more than 15MPa. Then, according to the coordinates and mechanical properties of each indentation point recorded by the nanoindenter, the indentation point is matched to the nearest grid through the nearest neighbor search method. Each grid generates a set of matching data, and each set of data includes the residual stress value, hardness and elastic modulus of the grid. The nearest neighbor search algorithm constructs a kd tree spatial index at the center point of the grid, and the indentation point is matched to the nearest grid through the Euclidean distance; the matching distance threshold is not greater than 50% of the grid diagonal length. For example, 20 Cubic grid, tolerance no greater than 17 Each grid outputs a set of data, including the grid interpolation residual stress value, the mean value of the hardness of the indentation points in the grid, and the arithmetic mean value of the elastic modulus of the indentation points in the grid.

[0043] According to the residual stress values ​​and hardness of all grids, the relationship between the residual stress value and hardness of the standard specimen is determined and expressed as follows: ; in, represents the hardness of the j-th grid, Indicates zero stress hardness, which can refer to the typical value of TC4, which is about 340HV. represents the residual stress value of the jth grid, represents the linear term coefficient, The coefficient of the quadratic term, represents the exponential decay amplitude, Denotes the characteristic stress. Substituting the residual stress values ​​and hardness of all meshes, the data are fitted by the least squares method to determine the correlation coefficient and exponential decay amplitude.

[0044] The relationship between hardness and residual stress is expressed in the form of a polynomial plus exponential decay, which is reasonable due to the micromechanical response mechanism of TC4 alloy. Residual stress directly regulates hardness by affecting dislocation motion and interfacial resistance: tensile stress promotes dislocation slip, resulting in a decrease in hardness, i.e., the linear term is negative, while under high stress dislocation entanglement and Phase precipitation may induce nonlinear hardening, i.e., the quadratic term The exponential term describes the stress saturation effect, that is, when the residual stress value exceeds the characteristic value After that, dislocation proliferation tends to balance and hardness changes slow down. As residual stress transitions from compressive stress to tensile stress, hardness first increases due to the enhanced dislocation hindering effect, and then decreases due to phase transformation softening and increased dislocation mobility. The overall trend is first increasing and then decreasing, which is consistent with the stress and hardness experimental curve of titanium alloy.

[0045] In this embodiment, determining the relationship between the residual stress value and the elastic modulus of the standard sample specifically includes: According to the residual stress values ​​and elastic modulus of all grids, the relationship between the residual stress value and elastic modulus of the standard specimen is determined and expressed as follows: ; in, represents the elastic modulus of the j-th grid, Indicates zero stress modulus, which can be referred to the typical value of TC4, which is about 114GPa. This coefficient represents the sensitivity of the elastic modulus to residual stress. It is usually negative, reflecting the decrease in modulus with tensile stress. Substituting the residual stress and elastic modulus values ​​for all meshes, the least squares method is used to fit the data to determine the sensitivity coefficient.

[0046] The linear relationship between the elastic modulus and the residual stress value reflects the regulation of the sound velocity by the lattice strain. The residual stress changes the atomic spacing, thereby affecting the atomic bond stiffness: tensile stress weakens the bond force and reduces the elastic modulus, while compressive stress strengthens the bond. However, in TC4 The presence of the phase partially offsets this effect, resulting in a nearly linear change in the modulus. Negative values ​​of (e.g., -0.03) are consistent with the acoustoelastic theory of titanium alloys, which states that tensile stress reduces the elastic wave velocity by 1% to 3% GPa. The elastic modulus decreases linearly with increasing residual stress. This model is simplified to a linear model because the engineering application stress of TC4 is typically less than 10 GPa, making the linearity assumption negligible.

[0047] When interpolating data, outliers must be removed. For example, for hardness data, values ​​exceeding ±3 times the absolute deviation of the implanted value are removed, and for elastic modulus, values ​​exceeding ±2 times the standard deviation of the elastic modulus data set are removed. Invalid grid marking conditions include: the number of matching indentation points is less than 3, and the deviation between the interpolated stress and the measured value is greater than 20 MPa.

[0048] Step 5: Construct a finite element model of the standard specimen and obtain the interpolated residual stress value and interpolated microstructure parameters of each finite element unit through the interpolation method; analyze the differential microstructure parameters based on the relationship between the microstructure parameters and the residual stress values ​​of the standard specimen, calculate the estimated residual stress value of the finite element unit, and establish a stress difference criterion by combining the interpolated residual stress value and the estimated residual stress value to determine the final residual stress value and calculate the mechanical properties of each finite element unit.

[0049] In this embodiment, determining the mechanical properties of each finite element unit specifically includes: Based on the size of the standard specimen, such as 50mm×10mm×5mm, a collective model is established in finite element software, such as ABAQUS or ANSYS. The coordinates of the reference mark points need to be retained to match the experimental data coordinate system. The element type uses an eight-node hexahedral linear element, which takes into account both computational efficiency and stress gradient resolution. The residual stress mesh size is consistent with that in step 4, with a side length of 20 , to ensure data transmission accuracy. High gradient areas (such as welds) can be locally encrypted to 10 Based on the EBSD grain orientation data, a local coordinate system is defined for each unit, such as aligning the unit X axis with the 100 crystal orientation to take into account the crystal anisotropy. Constrain the standard specimen to all degrees of freedom at one end and apply a small displacement (such as 0.1) at the other end. ) Verify that the model stiffness matrix has no singularities.

[0050] The discretized 3D mesh residual stress values ​​are transferred to the finite element center using trilinear interpolation to obtain the interpolated residual stress value for each finite element. The input data for the trilinear interpolation method are the discrete 3D mesh residual stress values ​​generated in step 4, in the form of mesh center coordinates and residual stress values. A weighted average is performed on the element center and then on the eight mesh nodes surrounding it, with the weights determined by the distance from the element center to the mesh nodes.

[0051] Interpolation of microstructural parameters for each finite element, including: interpolation using inverse distance weighted method Phase volume fraction, the power is set to 2. Interpolation of phase volume fraction conservation relations Phase volume fraction and grain volume interpolated using Kriging, the The phase volume fraction conservation relationship is Phase volume fraction and The sum of the phase volume fractions is 1; According to the relationship between the microstructure parameters and residual stress values ​​of the standard specimen, the estimated residual stress value of each finite element is obtained by substituting the microstructure parameters obtained by interpolation of each finite element; Compare the interpolated residual stress value and the estimated residual stress value of each finite element unit. When the absolute difference between the interpolated residual stress value and the estimated residual stress value is not greater than 0.05 times the yield stress, take the average of the interpolated residual stress value and the estimated residual stress value as the final residual stress value. When the absolute difference between the interpolated residual stress value and the estimated residual stress value is greater than 0.05 times the yield stress, re-collect the X-ray diffraction data of the physical position of the center of the finite element unit and use the measured value as the final residual stress value.

[0052] The determination of the final residual stress value combines the efficiency of numerical simulation with the reliability of experimental verification. While the interpolated residual stress field can quickly cover the entire standard specimen area, due to the limitations of X-ray diffraction measurements (such as sampling spacing and surface effects), the interpolated results may deviate in localized high-gradient regions. Estimating the relationship between microstructural parameters and residual stress values ​​provides another layer of evidence for the stress state from the perspective of the material's microscopic response mechanism. When the difference between the two calculation results is small, it indicates that the stress distribution in the region is within an acceptable range due to the combined influence of microstructural uniformity and measurement error. In this case, taking the average value can smooth random errors while preserving the physical meaning of both methods.

[0053] However, when the difference between the interpolated value and the estimated result is large, it means that there are complex factors in the area that are not fully described by the model, such as local phase transformation anomalies, grain boundary stress concentration, or cross-gradient distribution of measurement points. In such cases, relying solely on numerical averaging will obscure the real physical phenomena, so re-experimental measurement is used to obtain real data. In this way, computational optimization strategies are used to improve efficiency in areas with small errors, and regression experimental verification is used to ensure accuracy in areas with large errors. This hybrid method is particularly suitable for multiphase materials such as TC4 alloy, because its residual stress distribution is affected by both macroscopic process parameters and closely coupled with microstructural evolution.

[0054] In addition, the threshold setting of 0.05 times the yield stress has a clear physical meaning. For TC4 alloy, the stress difference of this magnitude is sufficient to induce observable microstructural responses, such as Phase fraction changes exceeding 5% but not reaching a level that significantly affects macroscopic mechanical properties—approximately 5% of the yield stress—are used to effectively distinguish between acceptable statistical fluctuations and significant deviations that must be corrected. This avoids both the efficiency loss caused by over-reliance on experiments and the systematic errors introduced by blind interpolation.

[0055] In this embodiment, relevant data of 30 sample finite element units were collected, and the interpolated residual stress value and the estimated residual stress value of each finite element unit were analyzed respectively. The absolute difference between the two was calculated, the absolute error was judged, and the final residual stress value of the finite element unit was given. The specific relevant multi-source data are shown in Table 1 below: Table 1: Relevant multi-source data for determining the final residual stress values ​​of the finite element

[0056] Reference Figure 2 and Figure 3 As can be seen from the data in the table above, the absolute error between the interpolated residual stress value and the estimated residual stress value for some finite element units exceeds the preset threshold, further verifying that the method used in this application to determine the final residual stress value is accurate. This achieves a dynamic closed-loop optimization of theoretical prediction and experimental verification during the residual stress field reconstruction process.

[0057] like Figure 2 As shown in the figure, the two broken lines above show the relationship between the interpolated residual stress value and the estimated residual stress value. From the absolute difference broken line below, it can be seen that for samples with absolute differences exceeding the threshold, the gap between the corresponding interpolated residual stress value and the estimated residual stress value is larger. Figure 3 In the above example, for samples whose absolute difference exceeds the threshold, the number of samples whose corresponding final residual stress values ​​coincide with the average value is smaller, while for samples whose absolute difference does not exceed the threshold, the corresponding final residual stress values ​​coincide with the average value. The threshold here is the preset yield stress of 0.05 times, the average value refers to the arithmetic mean of the interpolated residual stress value and the estimated residual stress value, and the absolute difference also refers to the absolute difference between the interpolated residual stress value and the estimated residual stress value.

[0058] According to the relationship between mechanical properties and residual stress values, the final residual stress value is input to obtain the hardness and elastic modulus of each finite element.

[0059] Set the boundary conditions and solution settings of the finite element model. The boundary conditions are to import the residual stress field, apply the discretized residual stress field to the corresponding unit in the form of the initial stress field, and satisfy the equilibrium equation ,in, represents the residual stress value of the corresponding element, and F represents the body force, which is usually zero. The Newton-Raphson iteration is used, with a convergence tolerance of no more than 0.1%. The solver is set to use implicit dynamic analysis, with an initial step size of 1e-5s and a maximum step size of 1e-3s.

[0060] See also Figure 4 The present invention further provides a device for evaluating and measuring the microscopic mechanical properties of a TC4 alloy. The device for evaluating and measuring the microscopic mechanical properties of a TC4 alloy is used to implement the above-mentioned method for evaluating and measuring the microscopic mechanical properties of a TC4 alloy, comprising: The residual stress field calibration module is used to prepare the standard sample of TC4 alloy. After the standard sample is formed, the residual stress distribution on its surface is measured using X-ray diffraction to obtain the residual stress field of the standard sample; The microstructure and stress correlation module is used to divide the residual stress field into multiple sub-regions and calculate the average residual stress value of each sub-region as the characteristic stress value; obtain the SEM image of the standard sample, extract the microstructure parameters of each sub-region, and form a data pair with the characteristic stress value of the corresponding sub-region to establish the relationship between the microstructure parameters of the standard sample and the residual stress value; The nanoindentation mechanical testing module is used to screen out several typical micro-regions in each sub-region, determine the indentation points of the typical micro-regions, and perform nanoindentation tests at the indentation points to obtain the mechanical properties of each indentation point; The mechanical properties and stress mapping module is used to discretize the residual stress field into several three-dimensional grids, determine the residual stress value of each grid by interpolation, and match each indentation point to the nearest grid to establish the relationship between the mechanical properties and residual stress values ​​of the standard specimen; The multi-scale finite element analysis module is used to construct a finite element model of a standard specimen and obtain interpolated residual stress values ​​and interpolated microstructural parameters for each finite element through interpolation. Based on the relationship between the microstructural parameters and residual stress values ​​of the standard specimen, the differential microstructural parameters are analyzed to calculate the estimated residual stress values ​​of the finite element. A stress difference criterion is established by combining the interpolated and estimated residual stress values ​​to determine the final residual stress value and calculate the mechanical properties of each finite element. The above formulas are all dimensionless numerical calculations. The formulas are derived from the most recent real-world results obtained through software simulation using a large amount of data. The preset parameters in the formulas are set by technicians in this field based on actual conditions.

[0061] The above embodiments can be implemented in whole or in part by software, hardware, firmware, or any other combination thereof. When implemented using software, the above embodiments can be implemented in whole or in part in the form of a computer program product. Those skilled in the art will appreciate that the units and algorithm steps of each example described in conjunction with the embodiments disclosed herein can be implemented by electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are performed by hardware or software depends on the specific application and design constraints of the technical solution.

[0062] The units described as separate components may or may not be physically separate, and the components shown as units may or may not be physical units, and may be located in one place or distributed across multiple network units. Some or all of these units may be selected to achieve the purpose of this embodiment as needed.

[0063] The above is only a specific implementation method of the present application, but the scope of protection of the present application is not limited thereto. Any technician familiar with this technical field can easily think of changes or replacements within the technical scope disclosed in this application, which should be covered by the scope of protection of the present application.

Claims

1. A method for evaluating and measuring the microscopic mechanical properties of TC4 alloy, characterized in that: The specific steps include: Step 1: Prepare a standard sample of TC4 alloy. After the standard sample is formed, use X-ray diffraction to measure the residual stress distribution on its surface to obtain the residual stress field of the standard sample. Step 2: Divide the residual stress field into multiple sub-regions and calculate the average residual stress value of each sub-region as the characteristic stress value; obtain the SEM image of the standard specimen, extract the microstructural parameters of each sub-region, and form a data pair with the characteristic stress value of the corresponding sub-region to establish the relationship between the microstructural parameters of the standard specimen and the residual stress value; Step 3: Select several typical micro-regions in each sub-region, determine the indentation points of the typical micro-regions, and perform nanoindentation tests at the indentation points to obtain the mechanical properties of each indentation point; Step 4: Discretize the residual stress field into several three-dimensional grids, determine the residual stress value of each grid by interpolation, and match each indentation point to the nearest grid to establish the relationship between the mechanical properties of the standard specimen and the residual stress value; Step 5: Construct a finite element model of the standard specimen and obtain the interpolated residual stress value and interpolated microstructure parameters of each finite element unit through the interpolation method; analyze the differential microstructure parameters based on the relationship between the microstructure parameters and the residual stress values ​​of the standard specimen, calculate the estimated residual stress value of the finite element unit, and establish a stress difference criterion by combining the interpolated residual stress value and the estimated residual stress value to determine the final residual stress value and calculate the mechanical properties of each finite element unit.

2. The method for evaluating and measuring the microscopic mechanical properties of TC4 alloy according to claim 1, characterized in that: The process parameters for preparing the standard sample of TC4 alloy include laser power, scanning speed, powder thickness and scanning spacing; the standard sample of TC4 alloy is prepared by laser selective melting technology. After the standard sample is formed, the residual stress distribution on the surface of the standard sample is measured by X-ray diffraction method. Method is used to measure residual stress, determine the distance between measuring points, and obtain the residual stress field distribution through three-dimensional reconstruction; The specific criteria for determining whether a standard specimen is formed are as follows: process monitoring parameters and specimen macro-monitoring parameters are within the preset threshold range; process monitoring parameters include melt pool characteristics and interlayer temperature, and specimen macro-monitoring parameters include geometric dimensions, surface roughness, porosity, and defect length.

3. The method for evaluating the microscopic mechanical properties of TC4 alloy according to claim 2, wherein: Obtaining the microstructure parameters of each sub-region specifically includes: The residual stress field is spatially divided into N sub-regions, and the fluctuation range of each sub-region is determined to be lower than a preset residual stress value range threshold, and the mean of the residual stress values ​​of the sub-region is used as the characteristic stress value of the sub-region; Scanning electron microscopy was used to observe each sub-area to obtain SEM images of the standard specimens, as follows: Three representative points are selected in each sub-region. The representative points meet the following conditions: the difference between the X-ray diffraction residual stress value of the representative point and the residual stress value of the sub-region must be within 10%. The center of the representative point is measured by electron backscatter diffraction, and the radius is The strain field in the region represents the strain gradient at the point is not greater than A scanning electron microscope is used to obtain an SEM image at each representative point. The specific method is as follows: the position of the representative point on the standard specimen is used as the measurement point, and a microscopic analysis area group of the measurement point is determined with each measurement point as the center. The microscopic analysis area group is centered on the measurement point, and five analysis areas are determined at the measurement point, including a central area and four surrounding areas. The central area is the area centered on the representative point, and the representative point coincides with the X-ray measurement point. The surrounding areas are areas offset to the left and right along the X-axis, and areas offset to the up and down along the Y-axis, respectively. The method for determining the actual offset is as follows: the absolute value of the residual stress gradient in the X and Y directions at the current measurement point is calculated, and the ratio of the residual stress gradient to the material sensitivity coefficient is defined as the theoretical offset. The theoretical offset is then compared with a preset maximum safety offset limit, and the minimum of the two is ultimately taken as the actual offset. Determine the residual stress value of each analysis area by interpolation method, and ensure that the fluctuation range of the residual stress value of each analysis area is still lower than the preset residual stress value range threshold; otherwise, mark it as a cross-interval area and eliminate it; The SEM images of each analysis area were analyzed using image analysis technology and electron backscatter diffraction technology to measure the microstructural parameters of each analysis area. The microstructural parameters include Phase volume fraction, Phase volume fraction and grain size, in each sub-region, the mean of the microstructural parameters of all analysis areas except the cross-interval area is defined as the microstructural parameters of the sub-region.

4. The method for evaluating the microscopic mechanical properties of TC4 alloy according to claim 3, wherein: The relationship between the microstructural parameters and residual stress values ​​of the standard specimens is specifically as follows: The microstructural parameters and characteristic stress values ​​of each sub-region are defined as data pairs of the corresponding sub-region. Based on the data pairs of all sub-regions, the relationship between the microstructural parameters and residual stress values ​​of the standard specimen is constructed, including The relationship between phase volume fraction and residual stress value, The relationship between phase volume fraction and residual stress value, the relationship between grain size and residual stress value; Substituting all the data pairs of microstructural parameters and residual stress values ​​of the standard specimen into the relationship, the formula is expressed as: ； in, express Phase volume fraction, Indicates the datum under zero stress state Phase volume fraction, Residual stress caused by The maximum phase change, represents the residual stress value, represents the critical stress of phase transition, represents the phase change rate control parameter, for Phase volume fraction, is the grain size, is the grain size under zero stress state, is the yield stress, is the hardening index.

5. The method for evaluating and measuring the microscopic mechanical properties of TC4 alloy according to claim 1, wherein: Obtaining the mechanical properties of each indentation point specifically includes: In each sub-region, M typical micro-regions are selected, and the typical micro-regions satisfy the stress uniformity rule and the tissue consistency rule. The stress uniformity rule is that the standard deviation of the residual stress value in the typical micro-region is within the preset stress fluctuation threshold, and the tissue consistency rule is that the standard deviation of the residual stress value in the typical micro-region is within the preset stress fluctuation threshold. Phase volume fraction and its sub-region, all typical micro-regions The average value of the phase volume fraction is within a preset deviation range, and the deviation between the grain size of a typical micro-region and the average value of the grain sizes of all typical micro-regions in its sub-region is within a preset deviation range; Mark characteristic points in the SEM image of a typical micro-area, where the characteristic points are locations within the typical micro-area that have microstructural characteristics and coincide with the coordinates of the X-ray measurement points. Record the coordinates of the characteristic points, map the characteristic points in the SEM image to the standard specimen using the coordinate conversion matrix of the nanoindenter, arrange the indentation points in a grid array centered on the characteristic points mapped to the standard specimen, perform nanoindentation testing at the indentation points, and obtain the mechanical properties of each indentation point, including hardness and elastic modulus values.

6. The method for evaluating and measuring the microscopic mechanical properties of TC4 alloy according to claim 5, characterized in that: Determining the relationship between the residual stress value and hardness of the standard specimen specifically includes: Presetting fiducial markers on a standard specimen to verify that all test equipment, including an X-ray diffractometer, SEM, and nanoindenter, uses the same coordinate system; Define the geometric center of the standard specimen as the origin. The X, Y, and Z axes of the coordinate axis correspond to the length, width, and height of the standard specimen, respectively. If there is inconsistency in the coordinate system of the equipment, the rigid body transformation is used to align the coordinate system. If all test equipment uses the same coordinate system, no processing is required. The residual stress field is discretized into a three-dimensional grid with a grid size consistent with the nanoindentation spacing. The residual stress value of each grid is determined by interpolation. The coordinates and mechanical properties of each indentation point are recorded by the nanoindenter. The indentation point is matched to the nearest grid by the nearest neighbor search method. Each grid generates a set of matching data. Each set of data includes the residual stress value, hardness and elastic modulus of the grid. Based on the residual stress values ​​and hardness of all grids, the relationship between the residual stress value and hardness of the standard specimen is determined, which is expressed as follows: ； in, represents the hardness of the j-th grid, represents the zero stress hardness, represents the residual stress value of the jth grid, represents the linear term coefficient, The coefficient of the quadratic term, represents the exponential decay amplitude, represents characteristic stress.

7. The method for evaluating and measuring the microscopic mechanical properties of TC4 alloy according to claim 5, characterized in that: Determining the relationship between the residual stress value and elastic modulus of the standard specimen specifically includes: According to the residual stress values ​​and elastic modulus of all grids, the relationship between the residual stress value and elastic modulus of the standard specimen is determined and expressed as follows: ； in, represents the elastic modulus of the j-th grid, represents the zero stress modulus, It represents the sensitivity coefficient of elastic modulus to residual stress.

8. The method for evaluating and measuring the microscopic mechanical properties of TC4 alloy according to claim 1, wherein: Determining the mechanical properties of each finite element specifically includes: Determine the dimensions of the standard specimen and input them into the finite element software to determine the element type, element size, and number of elements to complete the element division. Assign a local material coordinate system to each element based on the grain orientation analyzed by electron backscatter diffraction. The discretized three-dimensional grid residual stress value is transferred to the center point of the finite element unit using the trilinear interpolation method to obtain the interpolated residual stress value of each finite element unit; Interpolation of microstructural parameters for each finite element, including: interpolation using inverse distance weighted method Phase volume fraction, using Interpolation of phase volume fraction conservation relations Phase volume fraction and grain volume interpolated using Kriging, the The phase volume fraction conservation relationship is Phase volume fraction and The sum of the phase volume fractions is 1; According to the relationship between the microstructure parameters and residual stress values ​​of the standard specimen, the estimated residual stress value of each finite element is obtained by substituting the microstructure parameters obtained by interpolation of each finite element; Compare the interpolated residual stress value and the estimated residual stress value of each finite element unit. When the absolute difference between the interpolated residual stress value and the estimated residual stress value is not greater than 0.05 times the yield stress, take the average of the interpolated residual stress value and the estimated residual stress value as the final residual stress value. When the absolute difference between the interpolated residual stress value and the estimated residual stress value is greater than 0.05 times the yield stress, re-collect the X-ray diffraction data of the physical position of the center of the finite element unit and use the measured value as the final residual stress value. According to the relationship between mechanical properties and residual stress values, the final residual stress value is input to obtain the hardness and elastic modulus of each finite element; Set the boundary conditions and solution settings of the finite element model. The boundary conditions are to import the residual stress field, apply the discretized residual stress field to the corresponding unit in the form of the initial stress field, and satisfy the equilibrium equation ,in, Represents the residual stress value of the corresponding element, F represents the body force; the solver is set to use implicit dynamic analysis, and the time step is set.

9. A device for evaluating and measuring the microscopic mechanical properties of TC4 alloy, characterized in that: The device for evaluating and measuring the microscopic mechanical properties of a TC4 alloy is used to implement the method for evaluating and measuring the microscopic mechanical properties of a TC4 alloy according to any one of claims 1 to 8, comprising: The residual stress field calibration module is used to prepare the standard sample of TC4 alloy. After the standard sample is formed, the residual stress distribution on its surface is measured using X-ray diffraction to obtain the residual stress field of the standard sample; The microstructure and stress correlation module is used to divide the residual stress field into multiple sub-regions and calculate the average residual stress value of each sub-region as the characteristic stress value; obtain the SEM image of the standard sample, extract the microstructure parameters of each sub-region, and form a data pair with the characteristic stress value of the corresponding sub-region to establish the relationship between the microstructure parameters of the standard sample and the residual stress value; The nanoindentation mechanical testing module is used to screen out several typical micro-regions in each sub-region, determine the indentation points of the typical micro-regions, and perform nanoindentation tests at the indentation points to obtain the mechanical properties of each indentation point; The mechanical properties and stress mapping module is used to discretize the residual stress field into several three-dimensional grids, determine the residual stress value of each grid by interpolation, and match each indentation point to the nearest grid to establish the relationship between the mechanical properties and residual stress values ​​of the standard specimen; The multi-scale finite element analysis module is used to construct a finite element model of the standard specimen and obtain the interpolated residual stress value and interpolated microstructure parameters of each finite element unit through the interpolation method; based on the relationship between the microstructure parameters and the residual stress values ​​of the standard specimen, the differential microstructure parameters are analyzed, and the estimated residual stress value of the finite element unit is calculated. The stress difference criterion is established by combining the interpolated residual stress value and the estimated residual stress value, the final residual stress value is determined, and the mechanical properties of each finite element unit are calculated.

Images