Joint evaluation method, joint evaluation apparatus, and welding system

Abstract

This invention provides a non-destructive, robust, and efficient method for evaluating the bonding conditions between metal plates. [Solution] The present invention is a method for evaluating the bonding of a metal joint (M) obtained by spot welding a plate assembly (m) made of at least a first metal plate (1) and a second metal plate (2) stacked on top of each other by applying current to an electrode pressed against the plate assembly. The present invention comprises a measurement step of receiving reflected ultrasonic waves transmitted from the surface side of the first metal plate on which an electrode mark (e) is formed to a bonding evaluation area and acquiring morphological information of the electrode mark and amplitude information near the bonding interface; a identification step of identifying the outer edge of the electrode mark based on the morphological information; and an evaluation step of evaluating the bonding status within the outer edge of the electrode mark based on the amplitude information. By evaluating the bonding status within the electrode mark, robustness and efficiency can be achieved simultaneously.

Description

Technical Field

[0001] The present invention relates to a method for evaluating the joining state of a metal joined body formed by spot welding metal plates and the like.

Background Art

[0002] Metal plates constituting a vehicle body, an aircraft body, a housing, a structure, etc. are often (resistance) spot welded. Spot welding is a kind of resistance welding using Joule heating, and is performed by passing a large current through electrodes pressure-welded to the outer surface of a stack of metal plates for a short time. As a result, a welded portion (a molten and solidified portion, an intermetallic compound layer, etc.) is formed between the opposing inner surfaces (the surfaces to be joined) of the stacked metal plates, and the metal plates are joined together.

[0003] The stack of plates to be joined may be composed of the same type of metal plates (for example, steel plates or aluminum alloy plates) or different types of metal plates (for example, a steel plate and an aluminum alloy plate). The metal plates to be joined are not limited to two, and there may be three or more. In addition to thin cold-rolled steel plates, thick high-tensile steel plates, hot-stamped steel plates, etc. are also used for the metal plates.

[0004] In order to ensure the joining quality stably while coping with such diversification of resistance spot welding, a method for non-destructively inspecting (evaluating) the joining state between metal plates is required. Descriptions related to this are, for example, in the following patent documents.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] Patent Document 1 describes a method for identifying the welded area (nugget diameter / IMC diameter) and the heat-affected zone (HAZ diameter) based solely on the amplitude distribution of the reflected waves (interface reflection amplitude image) obtained from near the joint interface by irradiating a spot-welded metal joint with ultrasound, and determining the quality of the weld within the heat-affected zone. According to the method of Patent Document 1, the joint condition of the weld can be evaluated non-destructively and efficiently.

[0007] However, the heat-affected zone (HAZ diameter) determined based on the amplitude distribution can be affected by the morphology (deformation) of the metal plate surface irradiated with ultrasound, so the bonding evaluation method in Patent Document 1 was not necessarily robust.

[0008] This invention is based on such new discoveries and aims to provide a new method for stably evaluating the bonding condition between spot-welded metal plates. [Means for solving the problem]

[0009] The inventors diligently researched how to solve this problem and conceived and implemented a method to evaluate the bonding condition within the outer edge of the electrode marks formed on the surface side of a metal plate irradiated with ultrasound. By further developing this result, the inventors completed the present invention described below.

[0010] 《Method for evaluating joints》 (1) The present invention relates to a method for evaluating the joining of a metal joint obtained by spot welding a plate assembly, which is made by applying an electric current to an electrode pressed against a plate assembly, which is made by stacking at least a first metal plate and a second metal plate, and comprises a measurement step of receiving a reflected wave of ultrasonic waves transmitted from the surface side of the first metal plate on which an electrode mark has been made to a joining evaluation area, and acquiring morphological information of the electrode mark and amplitude information near the joining interface; a identification step of identifying the outer edge of the electrode mark based on the morphological information; and an evaluation step of evaluating the joining condition within the outer edge of the electrode mark based on the amplitude information.

[0011] (2) According to the joining evaluation method of the present invention (simply referred to as the "evaluation method"), the joining condition between metal plates (for example, the size of the weld) can be evaluated stably (robustly) and efficiently. The reason for this is as follows.

[0012] Spot welding is performed by passing an electric current through an electrode pressed against a metal plate. As a result, electrode marks are created on the surface of the metal plate according to the shape of the electrode tip and the characteristics of the metal plate (material, strength, rigidity, thickness, etc.). These electrode marks (outer edges) roughly correspond to the area (extension) of current flow from the electrode to the metal plate, and the welded portion of the metal plate is formed inside and below these marks.

[0013] Therefore, by searching for the welded area based on amplitude information within the outer edge of the electrode trace identified based on morphological information, the bonding condition between metal plates can be accurately evaluated.

[0014] Furthermore, since it is not necessary to search for the weld in a wide area (for example, the heat-affected zone outside the electrode mark) that is not always clearly identifiable using only amplitude information (amplitude distribution), the joining condition can be evaluated efficiently without misjudgment.

[0015] Even if the welded area extends slightly beyond the outer edge of the electrode mark, the joint strength will increase in proportion to its size (area, outer diameter, etc.), so this does not pose a problem for evaluating the joint condition (determining the quality of the joint).

[0016] Furthermore, as long as the bonding condition is evaluated within the outer edge of the electrode mark, the evaluation starting point (search starting point) of the bonding condition does not need to perfectly coincide with the outer edge of the electrode mark. For example, the weld may be searched from slightly inside the outer edge of the electrode mark. Also, the outer edge of the electrode mark may be determined by taking into account information other than morphological information (e.g., the outer diameter (nominal diameter) of the electrode, tip shape, etc.). For example, the upper limit range of the outer edge of the electrode mark (maximum value of the electrode mark diameter) may be limited to within the (maximum) outer diameter of the electrode.

[0017] 《Bonding Evaluation Device》 The present invention can be understood not only as a joining evaluation method but also as a joining evaluation apparatus. For example, the present invention is a joining evaluation apparatus for a metal joint obtained by spot welding a plate assembly, which is formed by applying current to an electrode pressed against a plate assembly, which is made of at least one first metal plate and one second metal plate stacked on top of each other, and the apparatus comprises: a measurement means that receives reflected waves of ultrasonic waves transmitted from the surface side of the first metal plate, on which an electrode mark has been made, to a joining evaluation area, and acquires morphological information of the electrode mark and amplitude information near the joining interface; a identification means that identifies the outer edge of the electrode mark based on the morphological information; and an evaluation means that evaluates the joining condition within the outer edge of the electrode mark based on the amplitude information.

[0018] Welding System The present invention is not limited to evaluating the joining condition of welded objects (metal joints), but can also be understood as a welding system that reflects (feeds back) that evaluation into the welding conditions. For example, the present invention may be a welding system comprising a resistance spot welding apparatus and the above-described joining evaluation apparatus, wherein the welding conditions can be changed based on the evaluation of the joining condition of the metal joint.

[0019] According to the welding system of the present invention, desired joining quality and working environment can be stably ensured for various metal joints.

[0020] "others" (1) The components relating to a method, "~process" or "~step," and the components relating to an object, "~means" or "~part," are interchangeable. Steps or means that can be executed by a computer may also be components of a program (including recording media, data structures, etc.) loaded into the computer.

[0021] (2) For the sake of convenience of explanation, regardless of shape (whether circular or not), the size of the electrode mark (outer edge) is also called the "electrode mark diameter," and the size of the weld (outer edge) is also called the "nugget diameter." The electrode mark diameter and nugget diameter can be determined, for example, as the diameter of a circle that approximates the outer edge of the electrode mark or nugget (for example, by the least squares method). In this specification, the weld (nugget) includes not only molten solidified material of the same type of material but also reactants of dissimilar materials (for example, Fe-Al intermetallic compounds (IMC)).

[0022] The size of electrode marks or welds may be the actual dimensions (maximum length, diameter, etc.), or an index value or normalized value (dimensionless numerical value) that reflects them. The size measured on the actual object (such as the fracture surface of the weld) is also referred to as the "measured diameter", and the size estimated (evaluated) using ultrasonic waves is also referred to as the "evaluated diameter".

[0023] (3) Unless otherwise specified, "x to y" as used in this specification includes the lower limit value x and the upper limit value y. Any numerical value included in the various numerical values or numerical ranges described in this specification can be used as a new lower limit value or upper limit value to newly establish a range such as "a to b". Unless otherwise specified, "x to y MHz" as used in this specification means x MHz to y MHz. The same applies to other unit systems (mm, μm, etc.).

Brief Description of the Drawings

[0024] [Figure 1] It is a schematic diagram showing the outline of the joining evaluation device. [Figure 2] It is a flowchart showing the processing procedure of joining evaluation. [Figure 3] It shows the state of generating a surface shape image and an interface reflection amplitude image from three-dimensional data of reflected waves. [Figure 4] It shows the state of specifying the outer edge of the electrode mark based on the profile obtained from the surface shape image. [Figure 5] It shows the state of specifying the outer edge of the weld (nugget) based on the profile obtained from the interface reflection amplitude image. [Figure 6] It schematically shows the state of ultrasonic waves irradiated near the outer edge of the electrode mark being scattered. [Figure 7] It is a graph showing the relationship between the measured diameter and the evaluated diameter of the weld between an aluminum alloy plate and a steel plate. [Figure 8] It is a graph showing the relationship between the measured diameter and the evaluated diameter of the weld between steel plates. [Figure 9] It schematically shows the outer edge position of the weld specified based on the interface reflection amplitude image between steel plates.

Modes for Carrying Out the Invention

[0025] One or more components, arbitrarily selected from this specification, may be added to the components of the present invention described above. The contents described herein may also apply to apparatus, systems, etc., in addition to methods. Which embodiment is best depends on the subject, required performance, etc.

[0026] 《Metal plate / plate assembly》 A metal joint is formed by spot welding at least two metal plates between their opposing surfaces. In the case of a plate assembly consisting of three or more overlapping metal plates, it is sufficient to spot weld only some of the metal plates together. The joint evaluation may cover the entire joint or only a portion of it. The metal plates constituting the plate assembly may be of the same material or different materials; for example, the assembly may consist of two plates or three or more plates. Metal plates of the same material may be, for example, steel plates or aluminum plates. Metal plates of different materials may be, for example, steel plates and aluminum plates. Metal plates of the same material may differ in shape (thickness, etc.) and metal structure. The thickness of the metal plates may be, for example, 0.1 to 10 mm or even 0.5 to 5 mm.

[0027] In this specification, a metal plate only needs to be plate-shaped in the area to be spot-welded, and does not necessarily have to be plate-shaped (sheet-shaped) in its entirety. For example, spot welding may be performed on a part of a component (such as a housing or case) (such as a flange).

[0028] Ultrasound (1) Frequency Ultrasound is generally a high-frequency sound wave that exceeds the audible range (approximately 20 Hz to 20 kHz). Its frequency should be selected considering the material, thickness, and characteristics of the bonding evaluation range, for example, 0.1 to 30 MHz or 1 to 10 MHz.

[0029] (2)Reflected wave Ultrasound can generate strong reflected waves (i.e., reflected waves with large amplitude) at interfaces where the acoustic impedance (∝ density of the medium × speed of sound in the medium) changes significantly (interfaces with a large difference in acoustic impedance between media). Based on the time it takes to receive these reflected waves, the intensity (amplitude) of the reflected waves, and their profiles, it becomes possible to understand the characteristics of the bonding evaluation area (for example, the surface of electrode marks, interfaces where the material changes, gaps between surfaces to be bonded (around the bonding interface), etc.).

[0030] 《Joint Evaluation》 (1) Measurement Ultrasonic waves are emitted from the surface side of the joined metal plates, and the reflected waves are received, measured (and even analyzed). This provides morphological information of the electrode marks formed on the surface side of the metal plates, as well as amplitude information of the reflected waves near the bonding interface.

[0031] Ultrasound is transmitted and received by an ultrasound probe. The probe can be a single probe or an array probe. A single probe can be moved (scanned) in a single path, but using an array probe in which transducers are arranged in a matrix allows for efficient acquisition of three-dimensional data from reflected waves.

[0032] Morphological and amplitude information is obtained by measuring reflected waves along at least one measurement line that crosses the metal plate surface or joint interface (joined surface) within the joint evaluation area. The more measurement lines there are in a single joint evaluation area, the higher the evaluation (inspection) accuracy can be. The measurement lines should ideally pass through approximately the center of the weld and be set up in multiple locations around the center of the weld.

[0033] By obtaining three-dimensional data of reflected waves using an array probe or similar device, morphological and amplitude information can also be generated as two-dimensional data on the evaluation surface (XY plane). The two-dimensional data is obtained by gatedting the three-dimensional data. Gating allows for the extraction of only the reflected waves within a certain time range, and information on reflected waves generated in a specific region (Z coordinate) in the direction of ultrasonic wave transmission (Z direction) (morphological information near the metal plate surface, amplitude information near the bonding interface) can be extracted.

[0034] (2) Identification Morphological information of the surface can be obtained from the reflected waves of ultrasonic waves emitted from the surface of the metal plate to which the electrodes were pressed. For example, the height (Z coordinate) in the direction of ultrasonic wave emission (Z direction) can be determined from the morphological information displayed on the XY plane, where the Z coordinate corresponding to the maximum reflection peak is shown. Using such morphological information, it becomes possible to determine the outer edge of the electrode mark. For example, the outer edge (contour) of the electrode mark is defined by connecting the boundary points (groups) between the depression and the flat area surrounding the depression, or the vertices (groups) of the raised areas that form around the depression. An approximate shape (such as a circle) determined from each point may also be used as the outer edge of the electrode mark. The same applies to welded areas.

[0035] (3) Evaluation By transmitting ultrasonic waves into the joint evaluation area, amplitude information (amplitude distribution) near the joint interface can be obtained from the reflected waves. From this amplitude information, the joint status between opposing surfaces of metal plates can be understood. Normally, ultrasonic waves easily penetrate the welded area, so the amplitude (intensity) of the reflected wave is small, while the amplitude (intensity) of the reflected wave is large in the non-jointed area. Therefore, for example, by using amplitude information that indicates the presence or absence or magnitude of the maximum reflection peak near the joint interface, it is possible to understand the outer edge and size of the weld, which correlate with the joint status. Furthermore, within the outer edge of the electrode mark, the surface shape is usually smooth along the tip surface of the electrode, and the amplitude of the reflected wave near the joint interface within that range is also stable. Therefore, within the electrode mark, the evaluation (judgment) of the weld (outer edge) can be performed robustly.

[0036] Spot welding The electrodes used for spot welding are not limited in size or type. The shape of the electrode tip that forms the electrode mark is also not limited. The electrode tip can be convex or concave, as long as the outer edge of the electrode mark can be identified and the weld can be evaluated. For example, it is preferable to use a convex electrode with one of the basic tip shapes specified in JIS C9304 (1999) (flat type (F type), radius type (R type), dome type (D type), dome radius type (DR type), frustoconical type (CF type), frustoconical radius type (CR type), etc.).

[0037] The electrodes typically consist of a pair that sandwich the plate assembly between the front (upper) and back (lower) sides. Only one electrode (one side) may be used, as long as current can be conducted. As long as electrode marks are formed on the surface side of the first metal plate, the bonding condition according to the present invention can be evaluated.

[0038] Welding conditions such as the pressure applied to the electrodes or its pattern, the amount of current (current × time) and the current flow pattern from the electrodes to the plate assembly, etc., can be appropriately adjusted according to the metal plate and the desired joint strength. [Examples]

[0039] The present invention will be specifically explained with an example of evaluating the bonding condition of a metal joint formed by spot welding a plate assembly made of stacked metal plates.

[0040] [First Embodiment] Figure 1 shows an overview of the metal joint M ("joint M") and the joint evaluation device D ("evaluation device D") according to this embodiment. Unless otherwise specified, the directions indicated by arrows in Figure 1 are up / down, left / right, or X-axis, Y-axis, and Z-axis.

[0041] 《Metal joint》 The joined body M is formed by spot welding a plate assembly m (materials to be joined) which consists of an Al alloy plate 1 (first metal plate), a steel plate 2 (second metal plate), and a steel plate 3 stacked in order. Spot welding was performed by clamping the upper surface of the Al alloy plate 1 and the lower surface of the steel plate 3 with a pair of electrodes and passing an electric current between the electrodes. Specifically, it was done as follows:

[0042] For Al alloy plate 1, a wrought alloy equivalent to JIS A6022 (plate thickness: 1.2 mm) was used; for steel plate 2, an unplated cold-rolled steel plate (440 MPa class / plate thickness: 0.8 mm) was used; and for steel plate 3, an unplated cold-rolled steel plate (590 MPa class / plate thickness: 1.2 mm) was used. Each plate material was cut into strips (30 mm x 100 mm) and subjected to spot welding without surface polishing or other treatments.

[0043] A pair of electrodes (not shown in the diagram) used commercially available DR-type (JIS C9304) tips made of chromium copper (manufactured by OBARA Corporation). The electrodes had a tip diameter (nominal diameter) of φ16 mm, a tip surface radius of curvature of 40 mm, and a tip diameter of 12 mm. The tips were cooled by cooling water circulating inside.

[0044] Spot welding was performed using a servo-pressure type spot welding machine (WTC AS-SV-MFDC91X2). The pressure (F) applied to the plate assembly m by the electrodes was set to 5.8kN (constant). Current application was controlled as follows by controlling the DC current value and the application time.

[0045] First, a first energization was performed with a constant current of 6-7kA and an energization time of 150ms. This spot-welded steel plate 2 and steel plate 3. After the first energization was completed, there was a 100ms period without energization, followed by a second energization with a current of 13-15kA and an energization time of 100-300ms. This spot-welded Al alloy plate 1 and steel plate 2. During the second energization, the current was monotonically increased during the energization time, and the amount of heat input (and therefore the size of the joint) was varied by adjusting the current and energization time for each test piece. In this way, multiple metal joints with different joint conditions (joint w) between Al alloy plate 1 and steel plate 2 were fabricated.

[0046] Regarding the implementation of the spot welding described above, reference was made to the contents of Japanese Patent Publication No. 2023-63070 and Japanese Patent Publication No. 2023-66438. The contents of those publications are incorporated into this specification as appropriate.

[0047] Steel plate 2 and steel plate 3 are spot-welded together by nuggets 23 (welds) made of molten and solidified iron alloy. Al alloy plate 1 and steel plate 2 are joined at a joint w formed between the joining surfaces 11 and 21. The joint w has a welded portion w1 and a heat-affected zone w2 on its outer periphery.

[0048] In this embodiment, electrode marks e, consisting of roughly cylindrical or hemispherical depressions, were formed on the surface side of the low-rigidity Al alloy plate 1 that was pressurized by the electrodes. The outline (outer edge) of the electrode marks e was roughly circular. Furthermore, the Al alloy plate 1 deformed upward relative to the steel plate 2, and a slight gap (sheet separation s) was observed on the outer periphery of the heat-affected zone w2.

[0049] As described in the above publication, the welded joint w1 consists of an intermetallic compound (IMC) produced by the melting reaction, and its thickness (IMC layer thickness) is preferably, for example, 2 to 10 μm or 3 to 5 μm.

[0050] The size of the weld w1 (nugget diameter: d1) is determined by the diameter or width of the cross-section along the joint interface (length in the left-right direction in Figure 1). The outline (outer edge) of the weld w1 formed below (inside) the approximately circular electrode mark e is not strictly circular, but can be treated as an approximate circle.

[0051] Since the weld w1 is mainly composed of IMC, the nugget diameter is also appropriately referred to as the IMC diameter (d1). Furthermore, the nugget diameter measured from the fracture surface (average diameter of several points) is called the measured diameter, and the nugget diameter estimated using ultrasound (approximate circular diameter) is called the evaluated diameter.

[0052] 《Bonding Evaluation Device》 The evaluation device D comprises an ultrasonic transmitting and receiving sensor 61, an ultrasonic transmitting and receiving source 62, a reflected wave waveform processing unit 63, a reflected wave information analysis unit 64, and a bonding status determination unit 65.

[0053] A matrix-shaped ultrasonic array probe (Toshiba Inspection Solutions Co., Ltd. 8.5M0808G2015-C0303P10T0) was used as the transmitting / receiving sensor 61. A device that serves as both an ultrasonic oscillator and a reflected wave receiver (Toshiba Inspection Solutions Co., Ltd. TMUT064D600) was used as the transmitting / receiving source 62. The ultrasonic oscillation frequency was set to, for example, 8.5 MHz. The transmitting / receiving sensor 61 was attached to the surface of the Al alloy plate 1 via gel or the like.

[0054] 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 acquired in this way (including amplitude for each coordinate position, etc.), the processing unit 63 generates a surface shape image (morphological information) and an interface reflection amplitude image (amplitude information) (see Figure 3).

[0055] The surface topography image shows the distribution of Z coordinates on the upper surface of the Al alloy plate 1 (the surface of electrode mark e). The interface reflection amplitude image shows the amplitude distribution of the reflected wave near the bonding interface. The transmitting / receiving sensor 61, the transmitting / receiving source 62, and the processing unit 63 realize the measurement means or measurement step as defined in the present invention.

[0056] The analysis unit 64 (identification means, evaluation means) identifies the size of the electrode mark e (electrode mark diameter: d2) formed on the surface of the Al alloy plate 1 from the surface shape image (identification step / see Figure 4). The analysis unit 64 also identifies the size of the welded part w1 (nugget diameter (IMC diameter): d1) from the interface reflection amplitude image (evaluation step / see Figure 5).

[0057] The determination unit 65 (determination means) determines the quality (bonding status) of the joint w based on the identified nugget diameter (evaluation diameter). The processing unit 63, analysis unit 64, and determination unit 65 are implemented by executing a program (algorithm) on a computer (personal computer).

[0058] 《Method for evaluating joints》 An example of the processing flow using evaluation device D is shown in Figure 2. The details are as follows.

[0059] (1) Measurement Measurements are performed using the transmitting / receiving sensor 61 and the transmitting / receiving source 62 to generate three-dimensional data of the reflected wave. The processing unit 63 applies a gate to this three-dimensional data to generate a surface shape image and an interface reflection amplitude image as shown in Figure 3 (step S1). The gate extracts only the reflected waves received within a predetermined time corresponding to the Z coordinate.

[0060] The surface topography image is generated by setting a gate in the Z direction so that the Z-coordinate position where the reflected wave is observed is near the surface of the Al alloy plate 1, and then displaying the obtained Z-coordinate (distribution) of the reflected wave on the XY plane. The interface reflection amplitude image is generated by setting a gate in the Z direction so that the Z-coordinate position where the reflected wave is observed is near the bonding interface, and then displaying the obtained amplitude (distribution) of the reflected wave on the XY plane.

[0061] In each image, the maximum reflection peak within the gate at each XY position was recorded and generated as an XY distribution image. The Z coordinate corresponding to the maximum reflection peak was determined by measuring the time from ultrasonic emission to the reception of the extracted reflected wave. The interface reflection amplitude image was generated considering the surface shape of the test piece (electrode marks on the Al alloy plate 1). In other words, the gate range of the interface reflection image was adjusted (moved) to match the surface (height) of the electrode marks e. Thus, as shown in Figure 3, a surface shape image showing the height of the electrode mark surface in color (shade) and an interface reflection amplitude image showing the magnitude of the amplitude of the reflected wave obtained from near the bonding interface in color (shade) were obtained.

[0062] (2) Analysis In the analysis unit 64, the surface shape image is analyzed to determine the outer edge of the electrode mark e and its size (electrode mark diameter: d2) (step S2). Within that outer edge (electrode mark diameter), the interface reflection amplitude image is analyzed to determine the size of the welded part w1 (nugget diameter: d1) (step S3).

[0063] The electrode mark diameter was determined as follows: As shown in Figure 4, a profile of the Z position (height) is cut out along a straight line (measurement line) passing through the center of the surface shape image. On this profile, for example, the boundary point between the recess and the flat part, or the vertices of the raised parts that form around the recess, are searched for. The same operation is repeated for at least one rotation, changing the angle of the measurement line. The outer edge of the electrode mark e is determined from each boundary point or vertex obtained in this way, and the electrode mark diameter (d2) is determined from the approximate circle of that outer edge. Note that the outer edge of the electrode mark e (electrode mark diameter) may change depending on the plate assembly, welding conditions, electrode shape, etc., but it is sufficient if it is determined within the maximum diameter (nominal diameter) of the electrode.

[0064] The nugget diameter was determined as follows: As shown in Figure 5, the amplitude profile (amplitude distribution) is cut along a straight line (measurement line) passing through the center of the interface reflection amplitude image. On this profile, for example, points where the amplitude of the reflected wave changes (decreases) abruptly are searched for. This search is performed starting from the outer edge of the electrode mark e identified from the surface shape image or its approximate circle (a circle determined from the electrode mark diameter), and moving inward (towards the center). The operation is repeated in the same manner for at least one rotation, changing the angle of the measurement line. From the points of abrupt change in amplitude obtained in this way, the outer edge of the welded part w1 is identified, and the nugget diameter (d1) is determined from the approximate circle of that outer edge.

[0065] Incidentally, the reason why abrupt changes in the amplitude of the reflected wave appear on the interface reflection amplitude image within the outer edge of the electrode mark can be thought to be as follows: Outside the welded part w1 (non-welded area including the pressure-welded area), reflection close to 100% can occur due to the interposition of air (layer). On the other hand, strong reflection is unlikely to occur inside the welded part w1. For example, in the welded part w1 of dissimilar metal plates (aluminum alloy plate and steel plate) as in this embodiment, the change in the density (acoustic impedance) of the medium is small, so the reflection of ultrasonic waves is at most about 50%. Also, in the welded part of the same type of metal plates (steel plates) as described later, the density (acoustic impedance) of the medium hardly changes, so ultrasonic waves are unlikely to be reflected and are almost transmitted. For this reason, by using the abrupt change in amplitude that appears on the interface reflection amplitude image as a reference, it is possible to determine the boundary (outer edge) of the welded part w1 and evaluate the nugget diameter.

[0066] Typically, the welded area w1 is formed within the electrode mark e from the current path during spot welding (nugget diameter: d1 ≤ electrode mark diameter: d2). Therefore, as described above, it is sufficient to evaluate the size of the welded area w1 within the outer edge of the electrode mark e.

[0067] Furthermore, as shown in Figure 6, ultrasonic waves are easily scattered by raised areas and other structures around the electrode marks, and a position where the amplitude of the reflected wave decreases sharply may appear outside the electrode marks. As shown in Figure 5, by not searching the area outside such electrode marks e, the welded joint w1 can be evaluated accurately and efficiently.

[0068] Furthermore, the evaluation of the welded joint w1 may be based on the point of abrupt change in amplitude on the interface reflection amplitude image, or it may be performed by comparing its amplitude with a threshold.

[0069] (3) Judgment The determination unit 65 determines the joining status of the welded part w1 based on the nugget diameter (evaluation diameter) (step S4). For example, if the nugget diameter (d1) within the electrode trace diameter (d2) is greater than or equal to a threshold (α) (α≦d1≦d2), the joining is determined to be good, and if the nugget diameter is less than the threshold (d1<α), the joining is determined to be poor.

[0070] In addition to determining the quality of the joint, the strength of the joint M may be determined from a pre-prepared database or empirical formula showing the relationship between nugget diameter and joint strength. Furthermore, the nugget diameter (evaluation diameter) evaluated nondestructively may be fed back into the welding conditions. Such processing may be performed while the welding equipment is running, or during its periodic maintenance.

[0071] Experimental Examples (1) Metal joint of aluminum alloy plate and steel plate (set of 3) Numerous test specimens (joints M) were fabricated by spot welding together plate assemblies m consisting of the Al alloy plate 1, steel plate 2, and steel plate 3 described above. For each test specimen, the welded joint w1 between the Al alloy plate 1 and steel plate 2 was irradiated with ultrasound for non-destructive evaluation (evaluation diameter). After this evaluation, the fractured welded joint w1 (joint fracture surface) was measured (measured diameter).

[0072] In this process, along with the evaluation diameter within the electrode trace identified using the surface shape image (Sample 1), the evaluation diameter within the heat-affected zone (HAZ diameter) identified solely from the interface reflection amplitude image, in accordance with the description in Japanese Patent Publication No. 7556118 (Sample C1), was also determined for comparison. The relationship between each evaluation diameter and the measured diameter is shown in Figure 7. The contents of Japanese Patent Publication No. 7556118 are also incorporated into this specification as appropriate.

[0073] As is clear from Figure 7, the evaluated diameter within the electrode trace showed little error (variation) compared to the measured diameter, confirming excellent robustness.

[0074] (2) Metal joints of two steel plates Similarly, the evaluated diameter and measured diameter were determined for test specimens (joints) formed by spot welding together two steel plates. The test specimens were made by spot welding together plates made of the same cold-rolled steel plate (440 MPa class / plate thickness: 0.75 mm). To vary the joint diameter, welding was performed under three conditions: current values ​​of 7.4 kA, 8.4 kA, and 9.4 kA, with an energizing time of 150 ms and an electrode pressure of 1.7 kN on the plate assembly. In this case, two test specimens were prepared: one in which the steel plates were simply stacked and spot-welded (no spacer) and another in which a spacer was interposed between the steel plates and spot-welded (with spacer). An annular (perforated) plate material (thickness 1 mm) surrounding the joint area was used as the spacer. The spacer formed deeper electrode marks on the surface of the steel plate where the electrodes were pressed. Otherwise, spot welding was performed under basically the same conditions as for the three-plate assembly described above.

[0075] Figure 8 also shows the relationship between the measured diameter and the evaluated diameter within the electrode mark (sample 2) or the evaluated diameter within the heat-affected zone (HAZ diameter) (sample C2). Here, the evaluated diameter (sample 2) was identified as follows: The reflectivity of ultrasound near the interface between two steel plates is approximately 100% at the non-jointed area and approximately 0% at the jointed area, and the amplitude of the reflected wave decreases sharply at the boundary from the non-jointed area to the jointed area. Therefore, as shown in Figure 9, when searching for a profile based on the interface reflection amplitude image from the outside to the inside, the point where the amplitude is below a certain value and its slope is maximum was defined as the outer edge position of the weld (evaluation position of the nugget diameter).

[0076] As is clear from Figure 8, even when spot welding two steel plates together, the evaluated diameter obtained within the electrode mark showed little error (variation) compared to the measured diameter, confirming excellent robustness. [Explanation of symbols]

[0077] 1 Al alloy plate 2, 3 Steel plate m board set e Electrode trace w joint w1 weld D Bonding evaluation device M metal bonding

Claims

1. A method for evaluating the joint of a metal joint obtained by spot welding a plate assembly, which is formed by applying current to an electrode pressed against a plate assembly, which is made of at least a first metal plate and a second metal plate stacked on top of each other, A measurement step involves receiving the reflected wave of ultrasonic waves transmitted from the surface side of the first metal plate on which electrode marks have been made to the bonding evaluation area, and acquiring morphological information of the electrode marks and amplitude information near the bonding interface. A selection step to identify the outer edge of the electrode trace based on the morphological information, An evaluation step of evaluating the bonding status within the outer edge of the electrode mark based on the amplitude information, A joint evaluation method comprising the following:

2. The bonding evaluation method according to claim 1, wherein the outer edge of the electrode mark is demarcated by the boundary point between the recess and the flat portion surrounding the recess, or by the apex of the raised portion formed around the recess.

3. The joining condition is indicated by the nugget diameter, which is the size of the welded portion within the plate assembly, according to claim 1.

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

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

6. The first metal plate is an aluminum alloy plate, The bonding evaluation method according to claim 5, wherein the second metal plate is a steel plate.

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

8. The joining evaluation method according to claim 7, wherein the first metal plate and the second metal plate are steel plates or aluminum alloy plates.

9. The bonding evaluation method according to claim 1, 5, or 7, wherein the aforementioned plate assembly further comprises a three-piece assembly in which a third metal plate is stacked on top of the second metal plate.

10. A device for evaluating the joining of a metal joint, wherein an electrode pressed against a plate assembly consisting of at least a first metal plate and a second metal plate is energized, and the plate assembly is spot-welded, A measurement means that receives the reflected wave of ultrasonic waves transmitted from the surface side of the first metal plate on which electrode marks have been made to the bonding evaluation area, and acquires morphological information of the electrode marks and amplitude information near the bonding interface. A means for identifying the outer edge of the electrode trace based on the morphological information, An evaluation means for evaluating the bonding condition within the outer edge of the electrode mark based on the amplitude information, A bonding evaluation device equipped with the following features.

11. Resistance spot welding equipment, The device comprises a bonding evaluation apparatus as described in claim 10, A welding system in which welding conditions can be changed based on an evaluation of the bonding status of the aforementioned metal joint.

Images