Joint evaluation method, joint evaluation device and welding system

Abstract

To provide a method capable of evaluating a joint state between metal plates in a non-destructive manner.SOLUTION: A joint evaluation method of a metal joined body (M) in which plate sets (m) formed by overlapping at least a first metal plate (1) and a second metal plate (2) are welded between the opposite inner surfaces of the plate sets in a spot state includes: a measurement step of receiving a reflection wave of an ultrasonic wave transmitted to a joint evaluation area from the surface side of the first metal plate, and acquiring amplitude information (interface reflection amplitude image) of the reflection wave in the vicinity of a joint interface, and positional information (surface shape image) in the vicinity of the surface of the first metal plate; and an analysis step of determining a size (IMC diameter) of a weld part and a size (HAZ diameter) of a heat affected part generated on the outer peripheral side of the weld part, on the basis of the amplitude information and the positional information. The metal joined body is formed by resistance spot welding an Al alloy plate and a steel plate.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for evaluating the bonding condition of a metal bonded body in which metal plates are spot-welded. [Background technology]

[0002] Metal plates that make up car bodies, aircraft bodies, housings, structures, etc. are often resistance spot welded. Resistance spot welding is a type of resistance welding that uses Joule heating and is achieved by passing a large current for a short period of time from an electrode pressed against the outer surfaces of a stack of metal plates. This forms a molten pool between the opposing inner surfaces of the stacked metal plates (between the surfaces to be joined), which cools and solidifies to form a weld (nugget), joining the metal plates together.

[0003] The plates to be joined may consist of the same type of metal (for example, steel plates) or different types of metal (for example, steel and aluminum alloy plates). The number of metal plates to be joined is not limited to two, but can also be three or more. Recently, in addition to thin cold-rolled steel plates, thick high-tensile steel plates and hot-stamped steel plates are also used.

[0004] In order to respond to the diversification of resistance spot welding and to ensure stable joint quality, a method for non-destructively inspecting or evaluating the joint state between metal sheets is required. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 3-233352 [Patent Document 2] Patent Publication No. 2007-278809 Summary of the Invention [Problem to be solved by the invention]

[0006] In Patent Document 1, the nugget diameter is determined based on the time interval and height of the reflected waves obtained by scanning the surface of the spot weld with an ultrasonic probe. In Patent Document 2, the quality of the spot weld is determined based on the intensity distribution of the reflected waves obtained by using a robot to optimally position an ultrasonic sensor relative to the workpiece.

[0007] All of the patent documents are concerned with resistance spot welding of thin steel plates, and merely judge the quality of the welded state based on the intensity of the reflected wave obtained from the vicinity of the spot weld (nugget).

[0008] The present invention has been made in view of the above circumstances, and aims to provide a new bonding evaluation method and the like that can perform evaluation more accurately than conventional methods. [Means for solving the problem]

[0009] As a result of intensive research aimed at solving this problem, the inventors came up with the idea of ​​accurately evaluating the bonding state of a metal bonded body using amplitude information and position information obtained from reflected ultrasonic waves, and have realized this idea. By further developing this idea, the present invention, which will be described below, has been completed.

[0010] <Joint evaluation method> The present invention is a method for evaluating a joint of a metal joint formed by spot-welding a plate set consisting of at least a first metal plate and a second metal plate, between opposing inner surfaces of the plate set, and includes a measurement step of receiving reflected waves of ultrasonic waves transmitted from the surface side of the first metal plate to a joint evaluation area, and acquiring amplitude information of the reflected waves near the joint interface and position information near the surface of the first metal plate, and an analysis step of determining the size of the weld and the size of the heat-affected zone formed on the outer periphery of the weld based on the amplitude information and the position information.

[0011] In the bonding evaluation method of the present invention (simply referred to as the "evaluation method"), the sizes of the weld and the heat-affected zone are estimated with high accuracy based on amplitude information near the bonded interface of a metal bonded body and position information near the surface. By using these sizes, the bond quality of the metal bonded body can be accurately evaluated.

[0012] <Bonding evaluation device> The present invention can be understood as not only a bonding evaluation method but also a bonding evaluation device. For example, the present invention may be a bonding evaluation device for a metal joined body formed by spot-welding a sheet pair including at least a first metal sheet and a second metal sheet, the sheet pair being formed by overlapping the first metal sheet and the second metal sheet, the bonding evaluation device including: a measuring means for receiving a reflected wave of an ultrasonic wave transmitted from the surface side of the first metal sheet to a bonding evaluation region, and acquiring amplitude information of the reflected wave near the bonding interface and position information of the surface of the first metal sheet; and an analyzing means for calculating the size of the weld and the size of a heat-affected zone formed on the outer periphery of the weld based on the amplitude information and the position information.

[0013] Welding System The present invention can be understood as a welding system that not only evaluates the bonding state of a metal bonded body, which is a welded product, but also reflects (feeds back) the evaluation on welding conditions. For example, the present invention may be a welding system that includes a resistance spot welding apparatus and the above-mentioned bonding evaluation apparatus, and that can change welding conditions based on the evaluation of the bonding state of the metal bonded body.

[0014] According to the welding system of the present invention, desired joining quality, working environment, etc. can be stably ensured for a variety of metal joined bodies.

[0015] "others" (1) The elements of a method, such as "process" or "step," and the elements of a product, such as "means" or "part," are interchangeable. Steps or means executable by a computer are also elements of a program (including recording media, data structures, etc.) that can be loaded into a computer.

[0016] (2) The term "size" as used in this specification is sufficient as long as it indicates the size of a joint (heat-affected zone, weld zone, etc.), and may be an actual dimension (maximum length, diameter, etc.), a coordinate that reflects or indexes the actual dimension, a normalized (non-dimensional) numerical value, etc.

[0017] For ease of explanation, regardless of shape (whether circular or not), the size of the heat affected zone (HAZ) is referred to as the "HAZ diameter" and the size of the weld is referred to as the "nugget diameter." Furthermore, when the nugget is made of an intermetallic compound (IMC), the size is referred to as the "IMC diameter."

[0018] The size of each part obtained by measuring the actual fracture surface is called the "measured diameter," the base size estimated using reflected ultrasonic waves is called the "base diameter," and the evaluated size corrected from that base diameter is called the "evaluated diameter."

[0019] (3) Unless otherwise specified, "x to y" in this specification includes a lower limit value x and an upper limit value y. Any numerical value included in the various numerical values ​​or numerical ranges described in this specification may be used as a new lower limit or upper limit value to create a new range such as "a to b." Unless otherwise specified, "x to y MHz" in this specification means x MHz to y MHz. The same applies to other unit systems (mm, μm, etc.). [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a schematic diagram illustrating an overview of a bonding evaluation device. [Figure 2] 10 is a flowchart showing a processing procedure for bonding evaluation. [Figure 3] 1 shows an interface reflection amplitude image and a surface shape image generated from three-dimensional data of reflected waves (first embodiment). [Figure 4] The process of determining the base diameter based on the amplitude profile obtained from the interface reflection amplitude image is shown. [Figure 5] This shows how the feature quantity (opening angle) of the surface shape is identified based on the Z position profile obtained from the surface shape image. [Figure 6] FIG. 10 is a scatter diagram showing the relationship between the error in the base diameter relative to the actually measured diameter and the opening angle. [Figure 7A] 1 is a graph showing the relationship between the measured diameter and the base diameter or evaluation diameter for the heat affected zone (HAZ). [Figure 7B] 1 is a graph showing the relationship between the measured diameter and the base diameter or evaluation diameter for a weld (IMC). [Figure 8] FIG. 1 is a scatter plot showing the relationship between the joint strength of the joint and the measured IMC diameter. [Figure 9] 10 shows how the base diameter is determined based on an amplitude profile obtained from an interface reflection amplitude image of a welded portion (nugget) (second embodiment). [Figure 10] 10 is a graph showing the relationship between the base diameter and the actually measured diameter. [Figure 11] FIG. 10 is a scatter diagram showing the relationship between the error in the base diameter relative to the actually measured diameter and the opening angle. [Figure 12] 10 is a graph showing the relationship between the evaluated diameter corrected for the base diameter and the actually measured diameter. DETAILED DESCRIPTION OF THE INVENTION

[0021] One or more components selected from this specification may be added to the above-described components of the present invention. The contents described in this specification may also be applied to devices, systems, etc. as appropriate. Which embodiment is best depends on the target, required performance, etc.

[0022] 《Metal joint》 (1) Metal plate / plate assembly A metal joined body is formed by spot welding at least two metal plates between opposing joined surfaces. In the case of a plate assembly in which three or more metal plates are overlapped, only some of the metal plates may be spot welded to each other. The object of joint evaluation may be the entire joint or only some of the joints. The metal plates constituting the plate assembly may be made of the same material or different materials. For example, the plate assembly may be a pair of plates made of the same material or a pair of plates made of different materials. Metal plates made of the same material include, for example, steel plates or aluminum-based plates. Metal plates made of different materials include, for example, a steel plate and an aluminum-based plate. Metal plates made of the same material may have different shapes (such as thickness), metal structures, etc. The thickness of the metal plates is, for example, about 0.1 to 10 mm, or about 0.5 to 5 mm.

[0023] The metal plate referred to in this specification is not necessarily a plate (sheet) plate as a whole, as long as at least the area to be spot-welded is plate-shaped. For example, at least one of the metal plates may be a part (such as a flange) of a three-dimensional shaped member (such as a housing or case).

[0024] (2) Joint evaluation area The bonding evaluation area typically includes a weld and a heat-affected zone, in that order from the center to the outer periphery (in the radial direction or the radial direction from the center). A sheet separation zone may also be present. In this specification, the weld and the heat-affected zone are collectively referred to as the "bond" as appropriate. The heat-affected zone may include a solid-state bond (corona bond).

[0025] A weld is a solidified or reacted area (e.g., an intermetallic compound layer) formed after metal sheets melt near the joining interface. A heat-affected zone is an unmelted or unreacted area heated during welding. The specific structure of the heat-affected zone varies depending on the material of the metal sheets being welded, the surface treatment (e.g., plating), the welding conditions, etc. For example, the heat-affected zone can include areas where the metal structure has changed from the base material (base metal) before welding, solid-state welded areas (corona bonds), and areas where the plating layer has melted and solidified. In other words, the heat-affected zone is simply the intimate contact between the metal sheets on the outer edge of the weld. In addition to the strictly defined area academically, it can also be considered a pressure-welded area or an electrode-pressured area. A sheet separation area is an area where a gap exists between the joined surfaces of metal sheets. Note that minute, discontinuous voids can also form in the heat-affected zone.

[0026] Ultrasound (1) Frequency Ultrasonic waves are generally high-frequency sound waves that exceed the audible range (approximately 20 Hz to 20 kHz). The frequency may be selected taking into consideration the material, thickness, characteristics, etc. of the bonding evaluation area, and is, for example, 0.1 to 30 MHz or 1 to 10 MHz.

[0027] (2) Reflected wave Ultrasonic waves can generate strong reflected waves (i.e., reflected waves with large amplitude) at interfaces where the acoustic impedance (∝ density of medium × speed of sound in the medium) changes significantly (interfaces where the difference in acoustic impedance between media is large).

[0028] Near the interface of a weld made of the same type of plate material, the change in acoustic impedance is usually small, and the amplitude of the reflected wave also tends to be small. On the other hand, near the interface of a joint made of different types of plate material, in addition to changes in the material properties of the plate material, a material different from the plate material (e.g., an intermetallic compound) may also be formed. Near the joint interface of such a weld, the amplitude of the reflected wave may show a characteristic profile (amplitude distribution).

[0029] The heat-affected zone may contain small gaps where the joined surfaces are not firmly joined together, so the amplitude of the reflected wave generally tends to increase from the heat-affected zone to the sheet separation zone.

[0030] When a probe (probe) that transmits (oscillates) ultrasound also receives reflected waves, the amplitude of the received reflected waves is affected by the shape of the reflecting surface (reflection direction). For example, if the reflecting surface is not perpendicular to the direction of ultrasound propagation, part of the reflected waves will be scattered, and the amplitude of the reflected waves received by the probe may decrease.

[0031] In this way, the characteristics of the weld evaluation area can be reflected in the amplitude of the reflected wave. Therefore, by focusing on the amplitude information (amplitude distribution) of the reflected wave, the basic size of the weld and heat-affected zone can be determined. In addition, by focusing on the time information from when the ultrasonic wave is transmitted until the reflected wave is received, the characteristics (position information) near the surface of the metal plate can also be determined.

[0032] <<Bonding evaluation>> (1) Measurement Ultrasonic waves are emitted from the surface side of the joined metal plates, and the reflected waves are received and measured (and analyzed). This provides information on the amplitude of the reflected waves near the joint interface and position information near the metal plate surfaces.

[0033] Ultrasound waves are transmitted and received by an ultrasound probe. The probe may be a single probe or an array probe. A single probe may be moved (scanned) in a centralized manner, but an array probe with transducers arranged in a matrix can be used to efficiently obtain three-dimensional data from reflected waves.

[0034] The amplitude information and position information are obtained by measuring the reflected waves along at least one measurement line that crosses the weld interface (welded surface) or the metal plate surface within the weld evaluation area. The more measurement lines there are per weld evaluation area, the more accurate the evaluation (inspection) of the weld can be. It is preferable that multiple measurement lines pass through the approximate center of the weld and are set around the center of the weld.

[0035] If three-dimensional data of reflected waves is obtained using an array probe or the like, amplitude information and position information can also be generated as two-dimensional data on the evaluation surface (XY plane). Two-dimensional data can be obtained by applying a gate to the three-dimensional data. By using a gate, it is possible to cut out only the reflected waves within a certain time range, and extract only information on the reflected waves generated in a specific region (Z coordinate region) in the direction of ultrasonic transmission (Z direction) (amplitude information near the joint interface, position information near the surface of the metal plate).

[0036] (2) Analysis By analyzing the amplitude information (e.g., amplitude distribution) of the reflected waves generated near the joint interface, the base size of the weld (nugget diameter before correction) and the base size of the heat-affected zone (HAZ diameter before correction) can be determined.

[0037] For example, the base size of the heat-affected zone can be determined based on the position of the approximate minimum or inflection point of the amplitude distribution near the outer periphery of the heat-affected zone, and the base size of the weld can be determined based on the position of the approximate maximum of the amplitude distribution near the outer periphery of the weld.

[0038] The base size calculated from the amplitude information may have an error (deviation) from the actual measured size. This is thought to be because the reflected wave (particularly the amplitude) is affected by the surface morphology where the ultrasonic waves are irradiated. It is recommended to use the base size calculated from the amplitude information corrected according to the feature value that reflects the surface morphology to evaluate the bonding condition of the metal bonded body.

[0039] It is preferable to select a feature quantity that has a strong correlation with the base size error. Examples of such feature quantities include the opening angle generated in resistance spot-welded metal plates and the shape (size (diameter), depth, etc.) of the electrode mark formed on the surface of the metal plate. If a database or correction formula showing the relationship between such feature quantities and errors is prepared in advance, the correction size can be efficiently determined from the base size of each part. Note that suitable feature quantities and correction contents can be appropriately selected depending on the configuration of the plate assembly (material, number, thickness, etc. of the metal plates), the shape of the metal joint, the welding method, etc.

[0040] (3) Evaluation The bonding condition of the metal bonded body may be determined based on the evaluation size (HAZ diameter, nugget diameter) corrected for the base size. For example, first, the HAZ diameter is compared with a threshold value (first threshold value). If the HAZ diameter is larger than the threshold value, the nugget diameter is compared with another threshold value (second threshold value). If the nugget diameter is larger than the threshold value, the metal bonded body is determined to have a good bond. Conversely, if the HAZ diameter is smaller than the threshold value, the nugget diameter may not be evaluated, and the metal bonded body may be determined to have a poor bond. When the size is equal to the threshold value, the metal bonded body may be determined to be good or bad depending on the threshold value.

[0041] When the base size cannot be identified from the amplitude information, it may be determined that evaluation is impossible or that there is a poor connection.

[0042] If the joining condition is good, the joining strength between the metal sheets corresponding to the evaluated size of the weld may be displayed. If correlation data (database) between the measured size of the weld and the joining strength between the metal sheets is prepared in advance, the joining strength between the metal sheets can be estimated from the evaluated size (nugget diameter).

[0043] "welding" Any welding method can be used as long as a weld (nugget) and its surrounding heat-affected zone (including corona bond) are formed when metal sheets are spot-welded together. Examples of welding methods include resistance spot welding, arc welding, laser welding, and electron beam welding.

[0044] Typical welding conditions (energization patterns) for resistance spot welding mainly include the electrode pressure, the amount of current (current value), and the time of current application. The pressure and current value may be constant or may vary (upslope or downslope). The energization pattern may include a non-energized section. These welding conditions may be changed (corrected, revised, etc.) based on the evaluation diameter (e.g., HAZ diameter, nugget diameter, etc.), the quality of the joint, the joint strength, etc. The welding conditions may be changed as needed during the manufacturing process of the metal joined body, or may be changed periodically or irregularly. [Example]

[0045] The present invention will be specifically described with reference to an example in which the joining condition of a metal joined body in which metal plates are stacked and resistance spot welded together is evaluated.

[0046] [First Example] An outline of a metal bonded body M (referred to as "bonded body M") and a bonding evaluation device D (referred to as "evaluation device D") according to this example is shown in Fig. 1. Unless otherwise specified, the directions indicated by arrows in Fig. 1 are up / down, left / right, or X-axis, Y-axis, and Z-axis.

[0047] 《Metal joint》 The joined body M is formed by resistance spot welding (simply referred to as "spot welding") a plate assembly m (joined materials) in which an Al alloy plate 1 (first metal plate), a steel plate 2 (second metal plate), and a steel plate 3 are stacked in this order. The spot welding was performed by clamping the upper surface of the Al alloy plate 1 and the lower surface of the steel plate 3 between a pair of electrodes and passing current between the electrodes. Specifically, the process is as follows.

[0048] The aluminum alloy plate 1 was a wrought material equivalent to JIS A6022 / thickness: 1.2 mm), the steel plate 2 was an unplated cold-rolled steel plate (440 MPa grade / thickness: 0.8 mm), and the steel plate 3 was an unplated cold-rolled steel plate (590 MPa grade / thickness: 1.2 mm). Each plate was cut into strips (30 mm x 100 mm) and used for spot welding as is without surface polishing or other treatment.

[0049] A pair of electrodes (not shown) was used, each equipped with a commercially available chromium copper DR-type (JIS C9304) tip (manufactured by OBARA Corporation). The electrodes had a tip diameter (nominal diameter): φ16 mm, a tip surface curvature radius: 40 mm, and a tip diameter: 12 mm. The tips were cooled by cooling water circulating inside.

[0050] Spot welding was performed using a servo pressure spot welder (WTC AS-SV-MFDC91X2). The electrode pressure (F) applied to the plate assembly m was set to 5.8 kN (constant). The DC current value and its duration were controlled.

[0051] First, a first current was applied at a constant current value of 6 or 7 kA for a current application time of 150 ms. This resulted in spot welding between steel sheet 2 and steel sheet 3. After this current application, a 100 ms non-energized state was followed by a second current application at a current value of 13 to 15 kA for a current application time of 100 to 300 ms. This resulted in spot welding between Al alloy sheet 1 and steel sheet 2. The current value during the second current application was monotonically increased during the current application time, and the current value and current application time were adjusted for each sample (test piece) to change the amount of heat input (and therefore the size of the joint). In this way, multiple metal joints with different joint states (joint w) between Al alloy sheet 1 and steel sheet 2 were produced.

[0052] The above-described spot welding was carried out with reference to the contents of Japanese Patent Application Laid-Open Nos. 2023-63070 and 2023-66438. The contents of these publications are incorporated herein by reference.

[0053] The steel plates 2 and 3 are spot-welded by a nugget 23 (weld) made of molten and solidified iron alloy. The Al alloy plate 1 and the steel plate 2 are joined at a weld w formed between the welded surfaces 11 and 21. The weld w has a weld w1 and a heat-affected zone w2 on its outer periphery. A sheet separation zone s with an opening angle (θ) is formed on the outer periphery of the heat-affected zone w2. The opening angle is the angle of the gap that widens in a wedge-like shape from near the outer periphery of the weld w in the radial direction. In this embodiment, the low-rigidity Al alloy plate 1 is pressed by the electrode and deforms upward relative to the steel plate 2, which can create a gap with an opening angle (θ).

[0054] As described in the above publication, the weld zone w1 is made of an intermetallic compound (IMC) formed by a melting reaction, and its thickness is approximately 2 to 4 μm. The heat-affected zone w2 contributes little to the joint strength, and often has a small gap due to the difference in thermal deformation that occurs between the dissimilar materials, the Al alloy plate 1 and the steel plate 2, during spot welding. The thickness of the IMC layer in the weld zone w1 is preferably 2 to 10 μm or 3 to 5 μm.

[0055] The sizes of the weld w1 and heat-affected zone w2 are determined by the diameter or width (the length in the left-right direction in Figure 1) of the cross section along the joint interface. Regardless of whether they are circular or not, the size of the weld w1 is referred to as the IMC diameter (d1), and the size of the heat-affected zone w2 is referred to as the HAZ diameter (d2).

[0056] Furthermore, the size of each part (whether circular or not) obtained by measuring the fracture surface is referred to as the measured diameter, and the size of each part estimated using ultrasonic waves is referred to as the estimated diameter. Of the estimated diameters, the size of each part specified based on the amplitude distribution of the joint interface, as described below, is referred to as the base size or base diameter (base IMC diameter, base HAZ diameter), and the base size corrected in consideration of the shape change (feature value of the surface shape / e.g., opening angle) of the surface of the joint M (the upper surface of the Al alloy plate 1) is referred to as the corrected diameter or simply the evaluated diameter (IMC diameter, HAZ diameter).

[0057] <Bonding evaluation device> The evaluation device D includes an ultrasonic wave transmission / reception sensor 61, an ultrasonic wave transmission / reception source 62, a reflected wave waveform processing unit 63, a reflected wave information analysis unit 64, and a bonding state evaluation unit 65.

[0058] A matrix-shaped ultrasonic array probe (8.5M0808G2015-C0303P10T0 manufactured by Toshiba Inspection Solutions) was used as the transmitting and receiving sensor 61. A device (TMUT064D600 manufactured by Toshiba Inspection Solutions) that serves as both an ultrasonic oscillator and a reflected wave receiver was used as the transmitting and receiving source 62. The ultrasonic oscillation frequency was, for example, 8.5 MHz. The transmitting and receiving sensor 61 was attached to the surface of the Al alloy plate 1 via gel or the like.

[0059] The transmitting / receiving source 62 converts the waveform of the reflected wave into a digital signal and outputs it to the processing unit 63. Based on the three-dimensional data of the reflected wave thus acquired (including the amplitude for each coordinate position, etc.), the processing unit 63 generates an interface reflection amplitude image (amplitude information) and a surface topography image (position information) (see FIG. 3). The interface reflection amplitude image shows the amplitude distribution of the reflected wave near the bonded interface. The surface topography image shows the distribution of Z coordinates on the surface of the bonded body M (the upper surface of the Al alloy plate 1). The transmitting / receiving sensor 61, the transmitting / receiving source 62, and the processing unit 63 realize the measuring means or measuring step referred to in the present invention.

[0060] The analysis unit 64 (analysis means) identifies the base diameter (base HAZ diameter, base IMC diameter) of the welded portion w from the interface reflection amplitude image (see FIG. 4). Also, it identifies the opening angle (feature value) of the Al alloy plate 1 from the surface shape image (see FIG. 5). Furthermore, it identifies the evaluation diameter (IMC diameter, HAZ diameter) corrected according to the opening angle (see FIGS. 6, 7A, and 7B).

[0061] The evaluation unit 65 (evaluation means) judges the bonding state (good or bad) of the bonded portion w based on the evaluation diameters (HAZ diameter and IMC diameter) specified by the analysis unit 64. The processing unit 63, analysis unit 64, and evaluation unit 65 are realized by executing a program (algorithm) on a computer (personal computer).

[0062] <Joint evaluation method> The processing flow using the evaluation device D is shown in FIG. (1) Measurement Measurement is performed using a transmitting / receiving sensor 61 and a transmitting / receiving source 62 to generate three-dimensional data of the reflected waves. A processing unit 63 applies a gate to the three-dimensional data to generate an interface reflection amplitude image and a surface shape image as shown in Fig. 3 (step S1). The gate extracts only the reflected waves received within a specified time, making it possible to observe only the reflected waves generated at Z coordinate positions (areas) within a specified range.

[0063] The interface reflection amplitude image is generated by setting a gate so that the Z coordinate position for observing the reflected wave is near the bond interface, and displaying the amplitude (distribution) of the obtained reflected wave on the XY plane. In the interface reflection amplitude image shown in Figure 3, the magnitude of the amplitude is indicated by color (shade).

[0064] The surface topography image was generated by setting a gate so that the Z coordinate position for observing the reflected wave was near the surface of the Al alloy plate 1, and displaying the Z coordinate (distribution) of the obtained reflected wave on the XY plane. The Z coordinate was determined by measuring the time from when the ultrasonic wave was transmitted until the extracted reflected wave was received. In either case, the reflected wave showing the maximum peak within the gate was extracted and processed. In addition, when generating the interface reflection amplitude image, the gate range was adjusted according to the surface position, taking into account the surface topography of the test piece (for example, depressions from electrode marks on the Al alloy plate 1).

[0065] (2) Analysis The analysis unit 64 analyzes the interface reflection amplitude image to determine the base diameter of the bonded portion w (step S211), and analyzes the surface shape image to determine the feature amount (opening angle) of the surface shape (step S212).

[0066] The base diameter was determined as follows. As shown in Figure 4, the amplitude profile (amplitude distribution) was cut off along a straight line (measurement line) passing through the center of the interface reflection amplitude image. In the case of a joint M consisting of a three-plate assembly m in which an Al alloy plate 1, a steel plate 2, and a steel plate 3 are laminated in this order, as in this example, if the joint w is formed normally, when the amplitude profile is observed from the outer edge of the joint w toward the center, a minimum point (lower peak) of the amplitude appears on the outer edge, followed by a maximum point (upper peak) of the amplitude on the inside. The position of the outer lower peak corresponds to the outer edge of the heat-affected zone w2, and the position of the inner upper peak corresponds to the weld w1 (IMC).

[0067] The angle of the measurement line is changed and the operation of extracting peak positions is repeated in the same way to scan the entire interface reflection amplitude image. The locus of each peak position is approximately concentric, as shown in Figure 4. The outer approximate circle corresponds to the outer periphery of the heat-affected zone w2, and its diameter is taken as the base HAZ diameter. The inner approximate circle corresponds to the outer periphery of the weld w1, and its diameter is taken as the base IMC diameter. In this way, the base diameter of the joint w is determined (step S211). Note that if there is a joint failure, the peak positions as described above will not appear on the amplitude profile.

[0068] The feature quantity (opening angle) of the surface shape was determined as follows. As shown in Figure 5, the Z position profile was cut along a straight line (measurement line) that passes through the center of the surface shape image. From this Z position profile, the inclination angle (angle with respect to the XY plane) of the smooth parts (diagonal straight parts) on both sides of the electrode mark could be calculated. The same operation of calculating the inclination angle was repeated by changing the angle of the measurement line. The arithmetic average value of the inclination angles thus obtained was taken as the opening angle (θ) (step S212).

[0069] The measured diameters (measured HAZ diameter, measured IMC diameter) were obtained from the fracture surface of the joint w of the test specimen. Figure 6 shows the relationship between the error (Δφ(%) = 100 × (φe - φa) / φa) of the base diameter (φe) described above relative to the measured diameter (φa) and the opening angle described above. As is clear from Figure 6, there was a positive correlation between the two. By regression analysis (least squares method), the relationship between the error (y%) and the opening angle (x°) was expressed by the approximate formula (correction formula) shown in Figure 6.

[0070] The error (y) corresponding to the opening angle is calculated from the correction formula, and the base diameter is corrected. That is, the base diameter is multiplied by the correction coefficient (c = 100 / (100 + y)) to calculate the corrected diameter. This corrected diameter is set as the evaluation diameter (IMC diameter, HAZ diameter) of the joint w (step S22).

[0071] The effectiveness of the above-mentioned correction was confirmed as follows. In addition to the test specimens used to create the scatter diagram in Figure 6, several new test specimens were produced with different welding conditions (joint diameter). The base diameter and evaluation diameter of these test specimens were also determined according to the procedure described above. The actual diameter of the fracture surface of each test specimen was also determined.

[0072] As shown in Figure 7A, the base diameter and evaluation diameter were compared with the measured diameter for the heat-affected zone w2. Also, as shown in Figure 7B, the base diameter and evaluation diameter were compared with the measured diameter for the weld w1. As is clear from Figures 7A and 7B (collectively referred to as "Figure 7"), the base diameter before correction was larger than the measured diameter, but the evaluated diameter after correction was nearly identical to the measured diameter. Therefore, it was confirmed that the size of the weld w1 and the heat-affected zone w2 can be predicted with high accuracy by using the corrected evaluated diameter.

[0073] (3) Evaluation The evaluation unit 65 uses the evaluation diameters (HAZ diameter, IMC diameter) to determine the quality of the bonded portion w as follows: First, the HAZ diameter is compared with threshold value I (step S3). If the HAZ diameter is less than threshold value I, it is determined that the bond is poor (step S5). On the other hand, if the HAZ diameter is greater than or equal to threshold value I, the IMC diameter is compared with threshold value II (step S4). If the IMC diameter is less than threshold value II, it is determined that the bond is poor, and if the IMC diameter is greater than or equal to threshold value II, it is determined that the bond is good (step S5).

[0074] (4)Join strength Tensile shear tests were conducted using various test pieces in which Al alloy sheet 1 and unplated steel sheet 2 were spot-welded to determine the joint strength. The IMC diameter was also measured from the fracture surface of each test piece. The relationship between the two is shown in Figure 8.

[0075] As is clear from Figure 8, the joint strength of the welded body was approximately proportional to the IMC diameter of the weld. Using such a database not only allowed us to judge the quality of the welded joint, but also enabled us to estimate the joint strength from the evaluated diameter.

[0076] Welding System The IMC diameter (quality of the bonded state) and bond strength evaluated non-destructively by the evaluation device may be fed back to the welding conditions. Such processing may be performed while the welding device is in operation or during periodic maintenance.

[0077] [Second Example] The joining state of a metal joined body (simply referred to as a "jointed body") in which two overlapping steel plates were spot-welded was evaluated in the same manner as in Example 1. The details are as follows.

[0078] 《Zygote》 The two steel sheets were the same unplated cold-rolled steel sheets (440 MPa grade / sheet thickness: 0.75 mm). Spot welding was performed under three conditions of current values ​​of 7.4 kA, 8.4 kA, and 9.4 kA to change the joint diameter, with a current flow time of 150 ms and an electrode pressure of 1.7 kN. Other conditions (such as the shape of the test piece and the welding machine) were basically the same as in Example 1.

[0079] "evaluation" As in Example 1, the evaluation device D was used to determine the base diameter of the welded portion (nugget) of the bonded body based on the interface reflection amplitude image and the evaluated diameter obtained by correcting the base diameter based on the opening angle determined from the surface shape image. After the measurement and analysis, the fracture surface (cross section) of each test piece was measured to determine the actual diameter.

[0080] When spot welding two steel plates of the same type, it is known that the reflectivity of ultrasonic waves at the joint interface is approximately 100% in the non-bonded area and approximately 0% in the bonded area. Therefore, the amplitude profile (amplitude distribution) of the interface reflection amplitude image changes suddenly at the boundary between the two. Therefore, in this example, ultrasonic waves were irradiated onto the evaluation area of ​​the bonded structure, and the bonded state was explored from the outer periphery toward the center. The base point where the reflectivity suddenly changes was determined to be the outer periphery of the heat-affected zone (HAZ diameter). The position where the amplitude of the reflected wave becomes equal to or less than a predetermined threshold and the slope becomes maximum was determined to be the outer periphery of the weld (nugget diameter). This is schematically shown in Figure 9.

[0081] The relationship between the base diameter and the measured diameter obtained for the nuggets of multiple test specimens is shown in Figure 10. In the test specimen group in which almost no gaps were observed around the periphery of the nugget (referred to as "Sample 0"), the base diameter and the measured diameter were almost the same. On the other hand, in the test specimen group in which gaps were observed around the periphery of the nugget (referred to as "Sample 1"), the base diameter was larger than the measured diameter.

[0082] For each test piece, the relationship between the opening angle determined based on the surface profile image and the error of the base diameter relative to the actually measured diameter is shown in Fig. 11. As is clear from Fig. 11, in the case of this example as well, there was a positive correlation between the opening angle (x°) and the error (y%). This relationship was analyzed by regression analysis (the least squares method) to obtain the approximate formula (correction formula) shown in Fig. 11.

[0083] Using this correction formula, the relationship between the evaluated diameter obtained by multiplying the base diameter by the correction coefficient (c = 100 / (100 + y)) and the actually measured diameter is shown in Figure 12. As is clear from Figure 12, even when the metal plates are deformed by pressure welding of the electrodes, etc., and a considerable opening angle occurs around the outer periphery of the joint interface, it was confirmed that according to the present invention, the joint condition (size of the welded portion) can be appropriately evaluated regardless of the configuration of the joint (whether the plates are the same or different, the number of plates combined, etc.). By reflecting this evaluation in the welding conditions, desired welding can be performed stably.

[0084] In addition, taking into consideration the influence on the error in the actual diameter and the welding conditions, other features related to the surface shape of the spot-welded plate material (such as the diameter and depth of the electrode marks) may be used in place of the opening angle, or multiple features (including the opening angle) may be used in combination. [Explanation of symbols]

[0085] 1 Al alloy plate 2, 3 Steel plate m board set w Joint w1 Welded joint (IMC) w2 Heat affected zone (HAZ) D Bonding evaluation equipment M Metal joint

Claims

1. A method for evaluating a joint of a metal joined body in which a plate assembly in which at least a first metal plate and a second metal plate are stacked is spot-welded between opposing inner surfaces of the plate assembly, a measurement step of receiving a reflected wave of an ultrasonic wave transmitted from the surface side of the first metal plate to a bonding evaluation region, and acquiring amplitude information of the reflected wave near the bonding interface and position information near the surface of the first metal plate; an analysis step of determining the size of the weld and the size of the heat-affected zone formed on the outer periphery of the weld based on the amplitude information and the position information; A bonding evaluation method comprising:

2. The analyzing step determines that the metal bonded body has a poor bond when the size of the heat-affected zone is smaller than a threshold value, The bonding evaluation method according to claim 1 , wherein when the size of the heat-affected zone is larger than the threshold value, the bonding condition of the metal bonded body is evaluated based on the size of the welded zone.

3. 3. The joining evaluation method according to claim 1, wherein the size of the weld and / or the size of the heat-affected zone is an evaluation size obtained by correcting a base size calculated from the amplitude information based on a feature calculated from the position information.

4. 4. The bonding evaluation method according to claim 3, wherein the base size of the heat-affected zone is determined based on an approximate minimum point or approximate inflection point of the amplitude distribution appearing near the outer periphery of the heat-affected zone.

5. 4. The joining evaluation method according to claim 3, wherein the base size of the weld is determined based on a local maximum point of the amplitude distribution that appears near the outer periphery of the weld.

6. The joining evaluation method according to claim 3 , wherein the feature is an opening angle generated between the first metal plate and the second metal plate that are resistance spot welded, or the shape of an electrode mark formed on the surface of the first metal plate.

7. The bonding evaluation method according to claim 1 , wherein the amplitude information and the position information are generated from three-dimensional data of the reflected waves.

8. The bonding evaluation method according to claim 1 , wherein the first metal plate and the second metal plate are made of different materials.

9. the first metal plate is an aluminum alloy plate, the second metal plate is a steel plate, The bonding evaluation method according to claim 8 , wherein the welded portion contains an intermetallic compound.

10. The bonding evaluation method according to claim 1 , wherein the first metal plate and the second metal plate are made of the same material.

11. 11. The bonding evaluation method according to claim 1, 8 or 10, wherein the plate set comprises a three-plate set in which a third metal plate is further superimposed on the second metal plate.

12. A joining evaluation device for a metal joined body in which a sheet set in which at least a first metal plate and a second metal plate are stacked is spot-welded between opposing inner surfaces of the sheet set, a measuring means for receiving a reflected wave of an ultrasonic wave transmitted from the surface side of the first metal plate to a bonding evaluation region, and acquiring amplitude information of the reflected wave in the vicinity of the bonding interface and position information in the vicinity of the surface of the first metal plate; an analysis means for determining the size of the weld and the size of the heat-affected zone formed on the outer periphery of the weld based on the amplitude information and the position information; A bonding evaluation device comprising:

13. a resistance spot welding device; The bonding evaluation device according to claim 12, A welding system in which welding conditions can be changed based on an evaluation of the bonding state of the metal bonded body.

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