Method for measuring mechanical property of resistance spot welding heat affected zone

Abstract

The invention discloses a method for measuring the mechanical property of a resistance spot welding heat affected zone, and relates to the technical field of welding detection.The method comprises the steps that a welding nugget and a sample structure with the heat affected zone partially exposed are prepared, the sample structure comprises a welding spot, and the welding spot comprises the welding nugget and the heat affected zone surrounding the welding nugget; according to the hardness information corresponding to the position points on the sample structure, a hardness distribution diagram of the sample structure with the weld nugget as the center is established, position information of a heat affected zone is determined according to the hardness distribution diagram, speckle spraying is conducted on the position corresponding to the sample structure according to the position information of the heat affected zone, and a uniaxial stretching acting force is applied to the sample structure; a load time curve of the sample structure and a target strain time curve of the heat affected zone are obtained, a stress-strain curve of the heat affected zone is obtained according to the load time curve and the target strain time curve, and the stress-strain curve is used for determining material mechanical property parameters of the heat affected zone. And the precision and reliability of the mechanical property parameters of the welding spot heat affected zone material are effectively improved.

Description

Technical Field

[0001] The invention relates to the technical field of welding detection, in particular to a method for measuring mechanical properties of a heat-affected zone of resistance spot welding. Background Art

[0002] Resistance spot welding is widely used in car body manufacturing because of its advantages of high strength, good reliability, stable performance and easy automation. It is mainly used for the interconnection between car body sheet metal parts. The mechanical properties of the welds themselves have an important influence on the performance of the car body, especially in high-speed vehicle collision conditions. The failure and fracture strength of the welds have a huge impact on the deformation mode, collision acceleration and structural intrusion of the vehicle itself. According to welding theory, the process of weld formation is essentially a metallurgical process. A circle of heat-affected zone will be formed around the weld core. It is the weakest area in the weld position and the fracture occurs in this area. Therefore, the problem of weld cracking is converted into fracture of the heat-affected zone. The key to the problem is to obtain accurate mechanical property parameters (stress-strain curve, elongation at fracture, etc.) of the heat-affected zone.

[0003] In the related art, the method for obtaining the mechanical property parameters of the material in the heat affected zone of the weld is mainly through the indirect measurement and calibration method, for example, by establishing a refined mesh model of the weld, establishing a refined mesh model of the heat affected zone of the weld, calibrating the material of the heat affected zone of the weld, establishing a refined mesh model of the heat affected zone of the weld in the whole vehicle simulation model, whole vehicle simulation, whole vehicle testing, and correction of the material of the heat affected zone of the weld. However, the indirect measurement and calibration method is a reverse method, which uses the performance of the entire weld to indirectly infer the mechanical property parameters of the heat affected zone. It is essentially a "make do" method with very low reliability. In addition, because there are many parameters that need to be calibrated, multiple calibrations and multiple corrections of parameters are required, which is time-consuming and labor-intensive, and the accuracy of the measured mechanical property parameters of the heat affected zone is not high. Summary of the invention

[0004] The present invention aims to solve the problem that the reliability and accuracy of obtaining the mechanical property parameters of the heat affected zone of the weld by indirect measurement and calibration methods in the prior art need to be improved. Therefore, a method for measuring the mechanical properties of the heat affected zone of resistance spot welding is proposed.

[0005] In order to achieve the above-mentioned purpose, the present application provides a method for measuring the mechanical properties of the heat affected zone of resistance spot welding, comprising: Prepare a sample structure with a weld nugget and a heat-affected zone partially exposed, the sample structure comprising a weld spot, the weld spot comprising the weld nugget and the heat-affected zone surrounding the weld nugget; Establishing a hardness distribution map of the sample structure centered on the weld core according to hardness information corresponding to a plurality of position points on the sample structure; Determining the position information of the heat affected zone according to the hardness distribution map; According to the position information of the heat-affected zone, performing speckle spraying on the position corresponding to the sample structure; Applying a uniaxial tensile force to the sample structure to obtain a load-time curve of the sample structure and a target strain-time curve of the heat-affected zone; According to the load time curve and the target strain time curve, a stress strain curve of the heat affected zone is obtained, and the stress strain curve is used to determine the material mechanical property parameters of the heat affected zone.

[0006] In some embodiments, the step of preparing a sample structure with the weld nugget and the heat affected zone partially exposed specifically includes: at least partially overlapping two pieces of parent metal and resistance spot welding the overlapping area to form a weld; Cutting is performed around the weld point to form a test specimen structure in which the weld core and the heat-affected zone are partially exposed.

[0007] In some embodiments, the step of establishing a hardness distribution map of the sample structure centered on the weld nugget specifically includes: Measuring hardness information parameters of each position point, wherein a plurality of the position points are arranged around the weld core, and the spacing between adjacent position points is the same; Determine the center point of the weld nugget and hardness information corresponding to the center point; The hardness distribution map is established according to the center point, the plurality of position points and the corresponding hardness information.

[0008] In some embodiments, the step of determining the position information of the heat affected zone according to the hardness distribution map specifically includes: According to the curve change of the hardness distribution diagram, determining the hardness abrupt change area corresponding to the continuous change of the hardness value; The position information of the heat affected zone distribution is determined according to the spacing width corresponding to the hardness sudden change area.

[0009] In some embodiments, performing speckle spraying on a position corresponding to the sample structure according to the position information of the heat affected zone includes: According to the position information of the heat affected zone, speckle spraying is performed on the surface of the sample structure corresponding to the weld nucleus as the center to obtain a speckle area, and the speckle area is used to characterize the distribution of the heat affected zone on the sample structure.

[0010] In some embodiments, applying a uniaxial tensile force to the sample structure to obtain a load-time curve of the sample structure and a target strain-time curve of the heat-affected zone specifically includes: marking strain measurement sites at intervals in the speckle area, applying a unidirectional force to one end of the marked sample structure for axial stretching, measuring a set of tensile load parameters that vary with time, and obtaining a load-time curve; and The strain parameter set corresponding to each strain measurement site that changes with time is collected to obtain a target strain-time curve.

[0011] In some embodiments, marking strain measurement sites at intervals in the speckle region comprises: According to the speckle area formed by the speckle spraying, a plurality of line segments are marked on the speckle area along the axial direction of the sample structure as the strain measurement sites, and the plurality of line segments are arranged in parallel at intervals.

[0012] In some embodiments, collecting a set of strain parameters corresponding to each strain measurement site that changes with time to obtain a target strain-time curve includes: The strain parameter set corresponding to each strain measurement site that changes with time is collected, and a plurality of initial strain time curves are obtained from the strain parameter set. According to the multiple initial strain-time curves, the average strain value of each same time node is determined, and the target strain-time curve is obtained based on the average strain value.

[0013] In some embodiments, applying a uniaxial tensile force to the sample structure to obtain a load-time curve of the sample structure and a target strain-time curve of the heat-affected zone further includes: An electronic tensile universal testing machine is used to perform a uniaxial tensile test on the sample structure, and a VIC-2D full-field strain measurement test device is used to measure the strain at the strain measurement site.

[0014] In some embodiments, determining the material mechanical property parameters of the heat affected zone according to the stress-strain curve includes: The material mechanical property parameters of the heat affected zone including elastic modulus, hardening index and elongation at break are determined according to the stress-strain curve.

[0015] The technical solution provided in the present application above, compared with the prior art, includes at least the following beneficial effects or advantages: By preparing a specific test structure, hardness testing is performed on the surrounding area of ​​the middle weld of the test structure, the hardness distribution around the weld core is measured, and a hardness distribution map is drawn. Then, the width corresponding to the mutation area of ​​the hardness value in the hardness distribution map is used to accurately determine the position information of the heat affected zone corresponding to the test structure, and speckle spraying is performed on the heat affected zone position corresponding to the sample structure, so as to directly measure the strain changes of multiple sites in the heat affected zone. By applying a unidirectional tensile force to the sample structure, the load-time curve of the sample structure and the target strain-time curve of the heat affected zone can be directly obtained; according to the load-time curve and the target strain-time curve, the stress-strain curve of the heat affected zone is obtained, and the stress-strain curve is used to determine the material mechanical property parameters of the heat affected zone. Compared with the indirect measurement and calibration method, the measurement method proposed in the present application can directly measure the material mechanical property parameters of the heat affected zone, and the method is simple and efficient, and the measured data is reliable.

[0016] Additional aspects and advantages of the present invention will be given in part in the following description and in part will be obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0018] Figure 1 is a schematic diagram of the deformation structure of a vehicle longitudinal beam under different considerations of welding points according to an embodiment of the present application; Figure 2 is a schematic diagram of a process of forming a solder joint according to an embodiment of the present application; Figure 3 It is a diagram showing different areas of a solder joint and corresponding hardness distribution according to an embodiment of the present application; Figure 4 is a simplified structural schematic diagram of a welding point in a CAE model according to an embodiment of the present application; Figure 5 It is a flow chart of a method for measuring mechanical properties of heat-affected zone of resistance spot welding according to an embodiment of the present application; Figure 6 is a flow chart of sample structure preparation according to an embodiment of the present application; Figure 7 is a schematic structural diagram of a sample structure according to an embodiment of the present application; Figure 8 It is a hardness distribution diagram and a corresponding heat affected zone distribution diagram shown according to an embodiment of the present application; Fig. 9 It is a site distribution diagram of strain measurement using VIC-2D according to an embodiment of the present application. DETAILED DESCRIPTION

[0019] The embodiments of the present invention are described in detail below. The embodiments described with reference to the accompanying drawings are exemplary. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0020] It should be noted that, unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used in the specification of the present invention herein are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more of the related listed items.

[0021] The terms "first", "second", "third", etc. are only used to distinguish descriptions and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. For example, the first end plate and the second end plate are only used to distinguish different end plates, and their order is not limited. The first end plate can also be named as the second end plate, and the second end plate can also be named as the first end plate without departing from the scope of the various described embodiments. And the terms "first", "second", "third", etc. do not limit the indicated features to be different.

[0022] In the description of the embodiments of the present application, unless otherwise clearly specified and limited, the terms "connected", "connection", etc. should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, and it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in this application can be understood according to the specific circumstances. The meaning of "plurality" is at least two, that is, two or more.

[0023] It should be noted that, in the present application, words such as "in one embodiment", "exemplarily", "for example", etc. are used to indicate examples, illustrations or descriptions. Any embodiment or design described in the present application as "in one embodiment", "exemplarily", "for example" should not be interpreted as being more preferred or more advantageous than other embodiments or designs. Specifically, the use of words such as "in one embodiment", "exemplarily", "for example", etc. is intended to present related concepts in a concrete way, meaning that specific features, structures or characteristics described in conjunction with the embodiment may be included in at least one embodiment of the present application. The appearance of the above words in various locations in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0024] See also Figures 1 to 4 , Figure 1 A schematic diagram of the deformation structure of the vehicle longitudinal beam provided by this embodiment under different considerations of the welding points is shown; Figure 2 A schematic diagram showing a process of forming a solder joint provided by this embodiment; Figure 3 shows different areas of the solder joint provided in this embodiment and the corresponding hardness distribution diagram; Figure 4 A simplified structural schematic diagram of a welding point in the CAE model provided in this embodiment is shown.

[0025] like Figure 1 As shown in the figure, resistance spot welding is widely used in car body manufacturing because its welds have the advantages of high strength, good reliability, stable performance and easy automation. It is mainly used for the interconnection between car body sheet metal parts. The mechanical properties of the welds themselves have an important influence on the performance of the car body. Especially in the high-speed collision conditions of vehicles, the failure and fracture strength of the welds have a huge impact on the deformation mode, collision acceleration and structural intrusion of the vehicle itself. There is a significant difference in the deformation of the longitudinal beam of the vehicle body after the collision when the weld failure is not considered and when the weld failure is considered. Therefore, in order to study the fracture of the welds under high-speed collision conditions, it is necessary to analyze and test the weld area to obtain the material mechanical performance parameter information.

[0026] like Figure 2 and Figure 3As shown in the figure, the principle of resistance spot welding is to pressurize and energize the welding circuit so that the two stacked parent materials between the two electrode heads form a weld. The weld corresponds to the weld core in the middle and the surrounding heat-affected zone. According to welding theory, the process of weld formation is essentially a metallurgical process. A circle of heat-affected zone will be formed around the weld core. It is the weakest area in the weld position, and the fracture occurs in this area. Therefore, the problem of weld cracking is converted into fracture in the heat-affected zone. The key to the problem is to be able to obtain accurate mechanical properties parameters (stress-strain curve, elongation at break, etc.) of the heat-affected zone.

[0027] like Figure 4 As shown in Figure 1, currently, in order to accurately simulate the fracture of welds under high-speed collision conditions, the failure of welds is mainly handled in the CAE collision simulation model. Figure 2 as shown) and the hardness distribution around the weld (i.e. Figure 3 As shown in Figure 2), the solder joints in the simulation model are simplified into Figure 4 The structure shown in the figure, the inventors found that the indirect measurement calibration method is a reverse method, which uses the performance of the entire solder joint to indirectly infer the mechanical performance parameters of the heat affected zone. It is essentially a "make do" method with very low reliability. In addition, because there are many parameters that need to be calibrated, multiple calibrations and multiple parameter corrections are required, which is time-consuming and labor-intensive, and the effect is often not very good.

[0028] Based on this, the inventor proposed a method for measuring the mechanical properties of the heat affected zone of resistance spot welding. This method can directly measure the mechanical performance parameters of the heat affected zone through a special sample structure. This method makes the measurement process more convenient and efficient, and at the same time, the measured performance parameters are highly reliable.

[0029] See also Figures 5 to 9 , Figure 5 The flowchart of the method for measuring the mechanical properties of the heat affected zone of resistance spot welding provided in this embodiment is shown. Figure 6 The flow chart of sample structure preparation provided in this embodiment is shown. Figure 7 The schematic diagram of the structure of the sample provided in this embodiment is shown. Figure 8 The hardness distribution diagram and the corresponding heat affected zone distribution diagram provided in this embodiment are shown. Fig. 9 The figure shows the site distribution diagram of strain measurement using VIC-2D provided in this embodiment.

[0030] like Figures 5 to 9As shown, this embodiment provides a method for measuring the mechanical properties of the heat affected zone of resistance spot welding. It should be noted that the method can be used in the vehicle field, and can also be used in other fields involving resistance spot welding, such as electrical equipment, home appliance industry, aerospace, electronics industry, and metal product processing, etc., without limitation here. Specifically, the method includes: Step S100: preparing a sample structure with a weld core and a heat-affected zone partially exposed, wherein the sample structure comprises a weld spot, and the weld spot comprises the weld core and the heat-affected zone surrounding the weld core; In this step, for the preparation of the sample structure, first, according to the research needs, a suitable welding material can be selected as the base material, two base materials are at least partially overlapped and the overlapping area is resistance spot welded to form a weld, and then the area around the weld is cut to form a sample structure with the weld core and the heat affected zone partially exposed. For the preparation before welding, such as surface treatment of the base material, removal of oxides and oil stains, design of a suitable fixture, ensuring the accurate position and angle of the sample during welding, and welding parameter settings, such as current, voltage and welding speed, etc., can be selected according to actual needs and are not limited here.

[0031] In one example, if Figure 6 and Figure 7 As shown, in the process of preparing the sample structure, two pieces of base material are overlapped, and then a weld including a weld core and a heat-affected zone is formed by resistance welding, and then cutting is performed around the weld core to obtain the sample structure. Preferably, clamping ends for clamping the stretching device are reserved on both sides of the weld core, and the clamping ends on both sides are arranged in a mirror-symmetrical manner. A narrowed observation area in the middle is formed by cutting between the two clamping ends, and the weld core and the heat-affected zone of the weld core are exposed on the side end surface of the observation area.

[0032] It should be noted that the figure only shows one of the sample structures. For other structures of the sample structure, such as the clamping end can be rectangular or polygonal, the specific selection can be made according to the requirements of the tensile testing machine equipment. The material and thickness of the base material can be selected and set according to the properties of the material used in the equipment to be tested. However, in this sample structure, the position of the weld point needs to be cut to reveal the weld core area and the heat affected zone to facilitate the subsequent strain measurement.

[0033] At the same time, for the accuracy of measurement, the end faces on both sides of the weld need to be set in parallel by cutting, and in order to facilitate the measurement of stress and strain, preferably, the end faces on both sides of the weld need to be cut flat. If necessary, the cut sample structure can be ground and cleaned to remove burrs and residues generated during the cutting process.

[0034] Step S200: establishing a hardness distribution map of the sample structure centered on the weld core according to hardness information corresponding to a plurality of position points on the sample structure; In this step, the hardness information parameters of each position point can be measured, wherein multiple position points are set around the weld core, and the spacing between adjacent position points can be the same or different. The center point of the weld core and the hardness information corresponding to the center point can be determined, and a hardness distribution map can be established based on the center point, multiple position points and the corresponding hardness information.

[0035] In one example, a suitable hardness testing method can be selected according to the material and requirements, such as Vickers hardness (HV), Brinell hardness (HB) or Rockwell hardness (HR). The hardness testing equipment can be selected according to actual needs. In the test process, the center point of the weld nugget can be determined first. The center point of the weld nugget can be distinguished by the color depth of the weld. The center point of the weld nugget is used as the origin of the coordinate axis to establish a coordinate system for hardness distribution, in which the horizontal axis is the distance from the center point of the weld nugget, and the vertical axis is the Vickers hardness (HV). Starting from the center point of the weld nugget, measurements are made on both sides at equal or unequal intervals. In this way, the hardness value of the corresponding position is recorded, and then the hardness value is recorded in the coordinate axis. The hardness values ​​of each point are connected into a curve in turn, thereby establishing a hardness distribution diagram.

[0036] Step S300: determining the position information of the heat affected zone according to the hardness distribution map; In this step, if Figure 8 As shown in the figure, according to the curve change of the hardness distribution diagram, the hardness steep change area corresponding to the continuous change of hardness value is determined; according to the spacing width corresponding to the hardness steep change area, the position information of the heat affected zone distribution is determined. From the previous analysis, it can be known that the circle of heat affected zone formed around the weld core is the weakest area in the weld position. Therefore, the hardness measured in the heat affected zone will change. By analyzing the established hardness distribution diagram, the position of the heat affected zone can be accurately found through the corresponding projection method.

[0037] It should be noted that since the weld point is a three-dimensional structure, that is, the weld core is surrounded by the heat-affected zone. Therefore, after determining that the spacing width corresponding to the area where the hardness suddenly changes is the width of the heat-affected zone, the corresponding position of the sample structure can be found to accurately mark the heat-affected zone. Of course, in this example, the angle can also be adjusted by measuring the hardness at different locations on the horizontal line. Similarly, the center point of the weld core is used as the origin of the coordinate axis to measure the direction that is at an angle to the horizontal direction.

[0038] Step S400: performing speckle spraying on a position corresponding to the sample structure according to the position information of the heat affected zone; In this step, according to the position information of the heat affected zone, the corresponding area of ​​the sample structure surface is subjected to speckle spraying to obtain a speckle area, and the speckle area is used to characterize the distribution of the heat affected zone on the sample structure. Specifically, according to the position information of the heat affected zone of the sample structure, the corresponding position of the sample structure is subjected to speckle spraying. For the speckle spraying process, a spray gun, a sprayer or a plasma spraying device can be used, and a suitable spraying material (such as aluminum powder, alloy powder) can be selected. The specific spraying equipment and the spraying process can be selected according to actual needs, and no selection is made here. It should be noted that the position of the heat affected zone can be further made clear by speckle spraying, which is convenient for subsequent strain measurement.

[0039] It should be noted that after the speckle spraying is completed, the strain measurement sites are marked at intervals in the speckle area, wherein the specific form of the marking is as follows: Fig. 9 As shown, according to the speckle area formed by speckle spraying, the speckle area horizontally extends from one end close to the weld core to one end outside the heat affected zone as a strain measurement site. At the same time, in order to improve the measurement accuracy, multiple strain measurement sites can be set at intervals in the vertical direction. Preferably, the spacing between the multiple strain measurement sites can be set to be the same. Step S500: applying a uniaxial tensile force to the sample structure to obtain a load-time curve of the sample structure and a target strain-time curve of the heat-affected zone; In this step, a unidirectional force is applied to one end of the marked sample structure for axial stretching, and the tensile load parameter set that changes with time is measured to obtain a load-time curve; and the strain parameter set that changes with time corresponding to each strain measurement site is collected to obtain a target strain-time curve, wherein the strain parameter set that changes with time corresponding to each strain measurement site is collected, and a plurality of initial strain-time curves are obtained from the strain parameter set, and the average strain value of each same time node is determined based on the plurality of initial strain-time curves, and the target strain-time curve is obtained based on the average strain value.

[0040] Optionally, the sample structure is mounted on an electronic tensile universal testing machine to perform a unidirectional tensile test, measure the corresponding tensile load, and obtain a load-time curve. Specifically, the sample fixture is firmly clamped between the upper and lower fixtures of the testing machine to ensure alignment and adjust to a suitable position. The test parameters, including loading rate, maximum load, and deformation, are entered into the testing machine control software. Tensile test, compression test, or bending test, etc. are selected as needed. After starting the test, the testing machine will gradually apply tensile force while recording stress and strain data in real time. The test process is observed to ensure that the equipment and sample are in normal condition, and then the load-time curve is output through the electronic tensile universal testing machine.

[0041] Optionally, the initial strain time curve is acquired using a VIC-2D full-field strain measurement test device. During the measurement process, the camera of the VIC-2D device is calibrated to ensure measurement accuracy, usually by using a calibration plate of known size. The camera's mounting position and angle are adjusted as required by the test to ensure that the entire test area is captured. The load is applied gradually according to the test design, which may be static or dynamic loading. The VIC-2D system is started, and image data is monitored and recorded in real time while the load is applied. The acquired images are analyzed using the VIC-2D software to calculate the full-field strain distribution.

[0042] It can be understood that in order to improve the measurement accuracy of strain, the strain curve can be measured at multiple positions in the heat-affected zone, and then the average curve of the strain curves measured at multiple positions can be used as the strain-time curve of the uniaxial tensile test in the heat-affected zone. For example, the strain values ​​corresponding to multiple strain curves at the same time node can be averaged to obtain the average strain value, so as to obtain the target strain-time curve.

[0043] Step S600: obtaining a stress-strain curve of the heat-affected zone according to the load-time curve and the target strain-time curve, wherein the stress-strain curve is used to determine material mechanical property parameters of the heat-affected zone.

[0044] In this step, when studying the mechanical properties of the material in the heat affected zone, we first need to start from the load-time curve. The load-time curve usually records the relationship between the load and time that the material is subjected to during the stretching process, reflecting the mechanical response of the material under specific conditions. In order to better understand the material behavior, it is necessary to combine the target strain-time curve, which describes the strain change of the material in the corresponding time.

[0045] Specifically, the load-time curve and the target strain-time curve can be comprehensively analyzed. With known load and target strain, the stress value can be derived according to the basic mechanical theory of the material. The stress is usually calculated by dividing the load by the cross-sectional area of ​​the material. According to the characteristics of the heat-affected zone, factors such as temperature, initial state of the material, and loading rate may need to be considered to obtain the corresponding relationship between stress and strain, and the stress-strain curve of the heat-affected zone can be drawn.

[0046] It can be understood that the stress-strain curve not only reflects the various stages that the material goes through during the stretching process, but also reveals the elastic and plastic properties of the material. Typically, the initial linear part of the curve represents the elastic behavior of the material, while the subsequent part shows the yielding and hardening process of the material.

[0047] Finally, a series of material mechanical properties parameters can be extracted using the obtained stress-strain curve of the heat-affected zone. These parameters include elastic modulus, yield strength, tensile strength, and elongation at break. Through these performance parameters, engineers and material scientists can better understand the performance of materials in practical applications, thus providing an important basis for material selection and engineering design.

[0048] For example, in a stress-strain curve, the elastic modulus (E) is usually defined as the ratio of stress to strain. In the linear region of the curve, by selecting a range of data points, the elastic modulus can be calculated using the following formula: E = In the formula, σ is stress and ϵ is strain. By selecting the points of the initial linear part of stress and strain for calculation, a stable elastic modulus value can be obtained.

[0049] For example, the elongation at break (ϵf) refers to the strain of a material when it is stretched to break, which can usually be calculated using the following formula: ; Where Lf is the length after fracture and L0 is the initial length. The length data before and after fracture can be obtained through the fracture point in the stress-strain curve.

[0050] Other parameters of the mechanical properties of the heat-affected zone can also be obtained by similar methods. After obtaining the above performance parameters, they can be sorted together to form a mechanical properties table of the material. These parameters are of great significance for evaluating the material properties of the heat-affected zone and predicting its performance in practical applications. In addition, these parameters can also be compared with standard values ​​to determine whether the material meets the design requirements. Through a systematic analysis process, a deeper understanding of the material's behavioral characteristics under different environmental conditions can be achieved.

[0051] In the above method steps, a specific test structure is prepared, and hardness testing is performed on the peripheral area of ​​the middle weld of the test structure, the hardness distribution around the weld core is measured, and a hardness distribution diagram is drawn. According to the hardness distribution diagram, the width corresponding to the mutation area of ​​the hardness value in the hardness distribution diagram is used to accurately determine the position information of the heat affected zone corresponding to the test structure, and then speckle spraying is performed on the heat affected zone position corresponding to the sample structure to directly measure the strain changes of multiple sites in the heat affected zone, so that by applying a unidirectional tensile force to the sample structure, the load time curve of the sample structure and the target strain time curve of the heat affected zone can be directly obtained; according to the load time curve and the target strain time curve, the stress strain curve of the heat affected zone is obtained, and the stress strain curve is used to determine the material mechanical property parameters of the heat affected zone. Compared with the indirect measurement and calibration method, the measurement method proposed in the present application can directly measure the material mechanical property parameters of the heat affected zone, and the method is simple and efficient, and the measured data is reliable.

[0052] In the description of the present invention, it is to be understood that the terms “center”, “longitudinal”, “lateral”, “length”, “width”, “thickness”, “up”, “down”, “front”, “back”, “left”, “right”, “vertical”, “horizontal”, “top”, “bottom”, “inside”, “outside”, “clockwise”, “counterclockwise”, “axial”, “radial”, “circumferential”, etc., indicating orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation to the invention.

[0053] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "illustrative embodiments", "examples", "specific examples", or "some examples" means that the specific features, structures, materials, or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example.

[0054] Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Mentioning "embodiment" in this article means that the specific features, structures or characteristics described in conjunction with the embodiment may be included in at least one embodiment of the present embodiment application. The appearance of this phrase in various positions in the specification is not necessarily the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It can be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0055] Although the embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the claims and their equivalents.

[0056] Those skilled in the art will readily appreciate other embodiments of the present application after considering the specification and practicing the disclosure disclosed herein. The present application is intended to cover any modification, use or adaptation of the present application, which follows the general principles of the present application and includes common knowledge or customary techniques in the art that are not disclosed in the present application. The specification and examples are intended to be exemplary only, and the true scope and spirit of the present application are indicated by the following claims.

Claims

1. A method for measuring the mechanical properties of the heat affected zone of resistance spot welding, characterized in that: include: Prepare a sample structure with a weld nugget and a heat-affected zone partially exposed, the sample structure comprising a weld spot, the weld spot comprising the weld nugget and the heat-affected zone surrounding the weld nugget; Establishing a hardness distribution map of the sample structure centered on the weld core according to hardness information corresponding to a plurality of position points on the sample structure; Determining the position information of the heat affected zone according to the hardness distribution map; According to the position information of the heat-affected zone, performing speckle spraying on the position corresponding to the sample structure; Applying a uniaxial tensile force to the sample structure to obtain a load-time curve of the sample structure and a target strain-time curve of the heat-affected zone; According to the load time curve and the target strain time curve, a stress strain curve of the heat affected zone is obtained, and the stress strain curve is used to determine the material mechanical property parameters of the heat affected zone.

2. The method for measuring mechanical properties of heat affected zone of resistance spot welding according to claim 1, characterized in that: The step of preparing a sample structure with the weld nugget and the heat affected zone partially exposed specifically includes: at least partially overlapping two pieces of parent metal and resistance spot welding the overlapping area to form a weld; Cutting is performed around the weld point to form a test specimen structure in which the weld core and the heat-affected zone are partially exposed.

3. The method for measuring mechanical properties of heat affected zone of resistance spot welding according to claim 1, characterized in that: The step of establishing a hardness distribution diagram of the sample structure centered on the weld nugget specifically includes: Measuring hardness information parameters of each position point, wherein a plurality of the position points are arranged around the weld core, and the spacing between adjacent position points is the same; Determine the center point of the weld nugget and hardness information corresponding to the center point; The hardness distribution map is established according to the center point, the plurality of position points and the corresponding hardness information.

4. The method for measuring mechanical properties of heat affected zone of resistance spot welding according to claim 1, characterized in that: The step of determining the position information of the heat affected zone according to the hardness distribution map specifically includes: According to the curve change of the hardness distribution diagram, determining the hardness abrupt change area corresponding to the continuous change of the hardness value; The position information of the heat affected zone distribution is determined according to the spacing width corresponding to the hardness sudden change area.

5. The method for measuring mechanical properties of heat affected zone of resistance spot welding according to claim 1, characterized in that: The step of performing speckle spraying on a position corresponding to the sample structure according to the position information of the heat affected zone comprises: According to the position information of the heat affected zone, speckle spraying is performed on the corresponding area on the surface of the sample structure to obtain a speckle area, and the speckle area is used to characterize the distribution of the heat affected zone on the sample structure.

6. A method for measuring mechanical properties of heat affected zone of resistance spot welding according to claim 5, characterized in that: The step of applying a uniaxial tensile force to the sample structure to obtain a load-time curve of the sample structure and a target strain-time curve of the heat-affected zone specifically includes: marking strain measurement sites at intervals in the speckle area, applying a unidirectional force to one end of the marked sample structure for axial stretching, measuring a set of tensile load parameters that vary with time, and obtaining a load-time curve; and The strain parameter set corresponding to each strain measurement site that changes with time is collected to obtain a target strain-time curve.

7. A method for measuring mechanical properties of heat affected zone of resistance spot welding according to claim 6, characterized in that: The step of marking strain measurement sites at intervals in the speckle area comprises: According to the speckle area formed by the speckle spraying, a plurality of line segments are marked on the speckle area along the axial direction of the sample structure as the strain measurement sites, and the plurality of line segments are arranged in parallel at intervals.

8. A method for measuring mechanical properties of heat-affected zone of resistance spot welding according to claim 6, characterized in that: The collecting of a strain parameter set corresponding to each strain measurement site that changes with time to obtain a target strain time curve includes: Collecting a strain parameter set corresponding to each strain measurement site that changes with time, and obtaining a plurality of initial strain time curves from the strain parameter set; According to the multiple initial strain-time curves, the average strain value of each same time node is determined, and the target strain-time curve is obtained based on the average strain value.

9. The method for measuring mechanical properties of heat affected zone of resistance spot welding according to claim 1, characterized in that: The step of applying a uniaxial tensile force to the sample structure to obtain a load-time curve of the sample structure and a target strain-time curve of the heat-affected zone further comprises: An electronic tensile universal testing machine is used to perform a uniaxial tensile test on the sample structure, and a VIC-2D full-field strain measurement test device is used to measure the strain at the strain measurement site.

10. The method for measuring mechanical properties of heat affected zone of resistance spot welding according to claim 1, characterized in that: Determining the material mechanical property parameters of the heat affected zone according to the stress-strain curve includes: The material mechanical property parameters of the heat affected zone including elastic modulus, hardening index and elongation at break are determined according to the stress-strain curve.

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