Joining evaluation method, joining evaluation device, and welding system

Abstract

To provide a method enabling simple and non-destructive evaluation of the joining state between metal plates.SOLUTION: The present invention is a joining evaluation method for a metal joint body (M), in which a plate assembly (m), comprising a plurality of stacked metal plates (1, 2, 3), is spot-welded between opposing surfaces of the metal plates. The joining evaluation method includes: a measurement step (S1) of receiving reflected waves of ultrasound emitted toward a joining evaluation region to determine an amplitude distribution of the reflected waves; an analysis step (S2) of identifying, based on the amplitude distribution, a size of a welded portion (IMC diameter) and a size of a heat-affected zone (HAZ diameter) formed on the outer peripheral side of the welded portion; and an evaluation step (S3-S5) of evaluating a joining state of the metal joint body based on the size of the welded portion when the size of the heat-affected zone is larger than a threshold value. When the size of the heat-affected zone is smaller than the threshold value, it is determined as indicating a joining defect.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a method for evaluating the joining condition of a metal joint in which metal sheets are spot-welded. [Background technology]

[0002] Vehicle bodies, aircraft bodies, housings, structures, etc. are often manufactured by joining multiple components (e.g., plate materials) in spots. A typical example of such a joining method is resistance spot welding. Resistance spot welding is a type of resistance welding that utilizes Joule heating, and is performed by passing a large current for a short period of time from an electrode that is pressed against the outer surface of a sheet assembly of overlapping metal plates. This forms a molten pool between the opposing surfaces to be joined of the overlapping metal plates, and the metal plates are joined together by a weld (nugget) that cools and solidifies.

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

[0004] In order to respond to such diversification and to stably ensure the joining quality, a method for non-destructively inspecting or evaluating the joining condition between metal sheets is required. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent Publication No. 2013-122414 [Patent Document 2] Patent Publication No. 2016-83670 Summary of the Invention [Problem to be solved by the invention]

[0006] In Patent Document 1, the weld condition is evaluated based on an infrared image obtained by irradiating a weld with a laser beam, while in Patent Document 2, the weld condition is evaluated by focusing on the amount of depression formed in the welded part by friction stir spot welding.

[0007] In addition, ultrasonic inspection methods have been known for some time that distinguish between joints and non-joints between resistance spot-welded homogeneous steel sheets based on the presence or absence of reflected ultrasonic waves and specify the nugget diameter from the results. However, such methods cannot accurately evaluate the joining conditions of diversified metal joints.

[0008] In addition, ultrasonic inspection devices capable of three-dimensional imaging of resistance spot welded joints are also commercially available. However, even with such dedicated devices, it is difficult to easily and nondestructively evaluate the bonding condition of diversified metal joints.

[0009] The present invention has been made in consideration of the above circumstances, and has an object to provide a new method, etc., capable of evaluating the bonding state between metal plates. [Means for solving the problem]

[0010] As a result of intensive research into solving this problem, the inventors have discovered that it is possible to evaluate at least the size of the heat-affected zone by utilizing the amplitude distribution of the reflected ultrasonic waves. By developing this result, the present invention, which will be described below, has been completed.

[0011] <Joint evaluation method> The present invention is a bonding evaluation method for a metal joined body in which a plate group consisting of multiple overlapping metal plates is spot-welded between opposing surfaces of the metal plates, and the bonding evaluation method includes a measurement step of receiving reflected waves of ultrasonic waves transmitted to a bonding evaluation area and determining an amplitude distribution of the reflected waves, an analysis step of identifying a size of a weld and a size of a heat-affected zone formed on the outer periphery of the weld based on the amplitude distribution, and an evaluation step of evaluating the bonding state of the metal joined body based on the size of the weld when the size of the heat-affected zone is larger than a threshold value.

[0012] According to the bonding evaluation method of the present invention (simply referred to as the "evaluation method"), the bonding state of a metal bonded body can be evaluated in a non-destructive and simple manner. The reason for this is believed to be as follows.

[0013] Using the amplitude distribution of the reflected wave, it is first possible to determine the size of the heat-affected zone. If the heat-affected zone is too small, the weld inside it will naturally be too small, and the joint condition will be deemed poor, so there is no need to specify the size of the weld. Therefore, it is sufficient to consider the joint condition only when the heat-affected zone is the desired size (or larger).

[0014] Next, if the heat-affected zone is not too small, it is possible to accurately identify the size of the weld from the characteristics appearing in the amplitude distribution of the reflected wave. Based on the size of the weld thus identified, the joining condition of the metal joint can be accurately judged.

[0015] In this way, the size of the heat-affected zone determined based on the amplitude distribution of the reflected wave makes it possible to easily and efficiently evaluate the size of the weld and the joining state of the metal joint body in a non-destructive manner. Furthermore, according to the present invention, unlike the conventional method in which the size of the weld was directly evaluated based on the presence or absence of the reflected wave of the ultrasonic wave, the size of the weld can be prevented from being underestimated or overestimated.

[0016] <Joint 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 system for a metal joined body in which a sheet set in which a plurality of metal plates are stacked is spot-welded between opposing surfaces of the metal plates, the bonding evaluation device including: a measuring means for receiving a reflected wave of an ultrasonic wave transmitted to a bonding evaluation region and determining an amplitude distribution of the reflected wave; an analyzing means for specifying a size of a weld and a size of a heat-affected zone formed on an outer periphery of the weld based on the amplitude distribution; and an evaluating means for evaluating a bonding state of the metal joined body based on the size of the weld when the size of the heat-affected zone is larger than a threshold value.

[0017] Welding System The present invention is understood as a welding system that not only evaluates the joining state of a metal joint, which is a welded product, but also reflects (feeds back) the evaluation on welding conditions. For example, the present invention is The present invention may be a welding system comprising a welding device capable of spot welding a plate set consisting of multiple overlapping metal plates between opposing surfaces of the metal plates, and the above-mentioned joining evaluation device, wherein the welding device is capable of changing welding conditions based on the size of the heat-affected zone and / or the size of the welded zone.

[0018] According to the welding system of the present invention, it is possible to appropriately change (update) the welding conditions based on the accurately estimated size of the welded portion, and it is possible to stably ensure the joining quality. In addition, since it is possible to avoid erroneous revision of the welding conditions (current value, current application time, etc.) based on an underestimated or overestimated size of the welded portion as in the conventional method, according to the present invention, it is also possible to reduce energy loss and spatter generation during welding.

[0019] "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. In addition, the steps or means executed by a computer and the programs (including recording media, data structures, etc.) loaded into a computer are interchangeable.

[0020] (2) In this specification, the term "size" refers to a joint (heat-affected zone, welded zone, etc.) formed between the surfaces to be joined, and is sufficient as long as it is an index of the size of the area along the joint interface. "Size" may be an actual dimension (maximum length, diameter, etc.), or it may be a coordinate that reflects or indexes the actual dimension, a standardized (dimensionless) numerical value, a symbol indicating a classification or division, etc.

[0021] For ease of explanation, regardless of shape (whether circular or not), the size of the heat affected zone (HAZ) is called the "HAZ diameter" and the size of the weld is called the "nugget diameter." Furthermore, when it is made of intermetallic compounds (IMC), the size of the nugget is called the "IMC diameter."

[0022] The HAZ diameter and IMC diameter determined from the amplitude distribution of the reflected wave are called the "evaluated HAZ diameter" and the "evaluated IMC diameter", respectively, and collectively called the "evaluated diameter". The HAZ diameter and IMC diameter determined by measuring the actual fracture surface are called the "actually measured HAZ diameter" and the "actually measured IMC diameter", respectively, and collectively called the "actually measured diameter".

[0023] (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 description of the drawings]

[0024] [Figure 1] FIG. 1 is a schematic diagram showing an overview of a bonding evaluation device. [Diagram 2] 13 is a flowchart showing a processing procedure for bond evaluation. [Diagram 3] 1 shows an ultrasonic scan image of a joint and an amplitude distribution diagram of the reflected wave. [Figure 4] 1 is an explanatory diagram showing the shape of a joint and characteristics of an amplitude distribution. FIG. [Diagram 5] 13 is a scanned image of a joint with the measurement line of the amplitude distribution and the outer circumferential edge of the joint determined from the amplitude distribution added. [Figure 6] 1 is a graph showing the relationship between the actually measured diameter and the evaluated diameter in Verification Example 1. [Figure 7] 13 is a graph showing the relationship between the actually measured diameter and the evaluated diameter in Verification Example 2. [Figure 8] FIG. 1 is a scatter plot showing the relationship between the joint strength of the joint and the measured IMC diameter. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] One or more components selected from the present specification may be added to the above-mentioned components of the present invention. The contents described in this specification may be applicable to devices, systems, etc. as well as methods. Which embodiment is best depends on the target, required performance, etc.

[0026] 《Metal joint》 (1) Metal plate / plate assembly A metal joint is formed by spot welding at least two metal plates between the 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 joint evaluation may be performed on the entire joint or only on some of the joints.

[0027] The metal plates constituting the plate assembly may be of the same or different materials. Metal plates of the same material may differ in shape (thickness, etc.), metal structure, etc. When the deformation characteristics differ between the opposing joined surfaces, the amplitude distribution of the reflected wave is more likely to show features indicating the outer periphery (edge) of the heat-affected zone. Examples of deformation characteristics include the rigidity of the metal plates and the amount of thermal expansion at the joining interface. Differences in deformation characteristics between joined surfaces are not limited to when metal plates with different deformation characteristics are included in the plate assembly, but can also occur depending on the configuration of the plate assembly (the number of metal plates, the position of the joint, etc.).

[0028] The metal plate in this specification is sufficient as long as at least the area to be spot-welded is plate-shaped, and does not necessarily have to be entirely plate-shaped (sheet-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).

[0029] (2) Joint evaluation area The joining evaluation area usually includes, in order from the center to the outer periphery (in the radial direction or the radial direction from the center), a welded part, a heat-affected part, and a sheet separation part. In this specification, the welded part and the heat-affected part are collectively referred to as the "jointed part" as appropriate.

[0030] A welded zone is a solidified or reacted zone (such as an intermetallic compound layer) that is formed after metal sheets melt near the joint interface. A heat-affected zone is an unmelted or unreacted zone that is heated during welding. The specific configuration of the heat-affected zone varies depending on the material of the metal sheets to be welded, the surface treatment (such as plating), the welding conditions, etc. For example, the heat-affected zone may include a part where the metal structure has changed from the base material (base material) before welding, a solid-phase bonded part (corona bond), a part where the plating layer has melted and solidified, etc. A sheet separation zone is a part where the surfaces of the metal sheets to be joined have separated to produce a gap. The gap is usually larger than the fine, discontinuous voids that form in the heat-affected zone.

[0031] Ultrasound (1) Frequency Ultrasonic waves are high-frequency sound waves that exceed the audible range (approximately 20 Hz to 20 kHz). The ultrasonic waves may pass through the metal plate in the bonding evaluation area and generate reflected waves that reflect the changes in the form and material of the medium. The thickness of the metal plate is, for example, approximately 0.1 to 10 mm, or 0.5 to 5 mm. The frequency of the ultrasonic waves may be, for example, 0.1 to 30 MHz or 1 to 10 MHz, taking into account the wavelength.

[0032] (2) Reflected wave Ultrasonic waves 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).

[0033] Welds are usually partitioned by the interface between solid phases, so the difference in acoustic impedance between the media is small and the amplitude of the reflected wave is stable and small. Conversely, at sheet separations, an interface is formed between the metal plate (solid phase) and the void (gas phase), and the acoustic impedance between the media changes suddenly, so the amplitude of the reflected wave is large.

[0034] The heat-affected zone is the transition area. In the heat-affected zone, the welded surfaces are not firmly welded together, and may contain small gaps. For this reason, the number of gaps increases from the outer periphery of the weld (the inner periphery of the heat-affected zone) to the inner periphery of the sheet separation (the outer periphery of the heat-affected zone), and the amplitude of the reflected wave generally tends to increase.

[0035] When a probe that transmits (oscillates) ultrasonic waves also receives reflected waves, the strength (amplitude) of the received reflected waves is affected by the direction of reflection. In other words, when the propagation direction of the transmitted ultrasonic waves is not perpendicular to the extension direction of the interface that reflects the ultrasonic waves, part of the reflected waves is scattered and not received by the probe, and the apparent amplitude of the reflected waves received by the probe decreases.

[0036] This situation is likely to occur, for example, in areas where the distance between the joined surfaces changes suddenly (for example, near the boundary between the heat-affected zone and the sheet separation zone). This tendency is particularly strong when the weld or heat-affected zone is of the desired size and the joining condition is good. For this reason, approximate minimum points or approximate inflection points are likely to appear in the amplitude distribution, which tends to increase from near the outer periphery of the weld to near the outer periphery of the heat-affected zone. The location where such points appear may indicate (near) the outer periphery of the heat-affected zone.

[0037] Furthermore, when an approximate minimum point appears in the increasing amplitude distribution, an approximate maximum point appears correspondingly inside (closer to the center). The position where this approximate maximum point appears may indicate (the vicinity of) the outer periphery of the weld.

[0038] <Joint evaluation> (1) Measurement The amplitude distribution is obtained by receiving the reflected waves of ultrasonic waves transmitted to the bonding evaluation area. Ultrasonic waves are transmitted and received by an ultrasonic probe. The probe may be a single probe or an array probe. A single probe may be moved (scanned) centrally, but a wide area can be measured efficiently by using an array probe in which transducers are arranged in a matrix.

[0039] The amplitude distribution is formed at least along one measurement line that crosses the bonding interface (bonded surfaces) within the bonding evaluation area. The more measurement lines along which the amplitude distribution is formed per bonding evaluation area, the more accurate the evaluation (inspection) of the bond can be.

[0040] The measurement line may be set to pass through approximately the center of the weld. By acquiring the amplitude distribution along multiple measurement lines rotated around the center of the weld, it is possible to determine the size (e.g., diameter) of the weld or heat-affected zone that reflects its overall contour.

[0041] (2) Analysis By analyzing the amplitude distribution, the size of the weld (nugget diameter) and the size of the heat-affected zone (HAZ diameter) can be determined. The amplitude distribution can vary depending on the state of the metal joint (material, shape, welding conditions, etc. of the metal plates). It is advisable to determine the nugget diameter and HAZ diameter based on the characteristics of the amplitude distribution ascertained for each type of metal joint.

[0042] For example, the size of the heat-affected zone may be determined based on the location of the approximate minimum or inflection point of the amplitude distribution near the outer periphery of the heat-affected zone, and the size of the weld is determined based on the location of the approximate maximum of the amplitude distribution near the outer periphery of the weld.

[0043] The range for analyzing the amplitude distribution may be the entire amplitude distribution or a part of it. If a range where the characteristics of the amplitude distribution appear is predicted from the welding conditions, etc., it is sufficient to analyze within that specific range (for example, near the outer periphery). This allows for efficient and highly reliable evaluation.

[0044] The size of the heat-affected zone or weld may be determined for each amplitude distribution obtained along the measurement line, or may be determined on an average or integrated basis from the size or contour (outer peripheral edge) obtained from multiple amplitude distributions.

[0045] (3) Evaluation The bonding condition of the metal bonded body is evaluated based on the size of the heat-affected zone (HAZ diameter) and the size of the weld (nugget diameter). 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 is not 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 set to either good or bad depending on the threshold value.

[0046] When the size of the heat-affected zone cannot be determined from the amplitude distribution, it may be determined that the heat-affected zone is not evaluable or that the joint is defective. The same applies to the size of the welded zone.

[0047] If the joining condition is good, the joining strength between the metal sheets corresponding to the size of the weld may be indicated. If correlation data (database) between the 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 obtained from the size of the weld by comparing with the data.

[0048] "welding" Any welding method can be used as long as a weld (nugget) and its 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.

[0049] In a typical spot welding process, both sides of a plate assembly are pressed together with electrodes, the parts to be joined (between the surfaces to be joined) are melted by current heating, and then cooled (particularly quenched) through the electrodes. The welding conditions (current pattern) mainly include the electrode pressure, current amount (current value), and current flow time. The pressure and current value may be constant or may vary (upslope, downslope). The current pattern may also include non-current-flow sections.

[0050] The 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 or periodically during the manufacturing process of the metal joined body. EXAMPLES

[0051] The present invention will be specifically described with reference to the example of evaluating the joining condition of a metal joined body obtained by resistance spot welding a sheet assembly in which two steel sheets and an Al alloy sheet are stacked in order.

[0052] FIG. 1 shows an overview of a metal bonded body M (hereinafter referred to as "bonded body M") according to this embodiment and a bond evaluation device D (hereinafter referred to as "evaluation device D").

[0053] 《Metal joint》 The joint M is formed by resistance spot welding (simply called "spot welding") a plate assembly m (joined materials) in which an Al alloy plate 1, a steel plate 2, and a steel plate 3 are laminated in this order. The spot welding was performed by passing electricity through a pair of electrodes that clamp the plate assembly m from both outer surfaces. Specifically, the process is as follows.

[0054] An aluminum alloy plate 1 (JIS A6022 equivalent wrought material / plate thickness: 1.2 mm) was used, an uncoated cold-rolled steel plate (440 MPa class / plate thickness: 0.8 mm) was used for steel plate 2, and an uncoated cold-rolled steel plate (590 MPa class / plate thickness: 1.2 mm) was used for steel plate 3. Each plate material was cut into strips (30 mm x 100 mm) and used for spot welding as it is without surface polishing or the like.

[0055] A pair of electrodes (not shown) was made of chromium copper DR type (JIS C9304) commercially available tips (manufactured by OBARA Co., Ltd.). 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.

[0056] Spot welding was performed using a servo pressure spot welding machine (AS-SV-MFDC91X2 manufactured by WTC). The electrode pressure (F) applied to the plate assembly m was set at 4 kN (constant). Electric current was applied by controlling the DC current value and its duration.

[0057] First, a first current was applied with a current value of 8 kA (constant) and a current application time of 50 ms. As a result, the steel plate 2 and the steel plate 3 were spot-welded first. After the end of the current application, a non-current-applied state of 100 ms was followed by a second current application with a current value of 9 to 13 kA and a current application time of 100 to 300 ms. As a result, the Al alloy plate 1 and the steel plate 2 were spot-welded. The current value during the second current application was set to increase monotonically during the current application time, and the amount of heat input was changed by adjusting the current value and current application time for each sample (test piece). In this way, a plurality of metal joints with different joint states (joint w) of the Al alloy plate 1 and the steel plate 2 were produced.

[0058] Regarding the implementation of the above-mentioned spot welding, the contents of JP 2023-63070 A and JP 2023-66438 A were referred to. The contents of those publications are incorporated into this specification as appropriate.

[0059] 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 joint w formed between the joining surface 11 and the joining surface 21. The joint w has a weld w1 and a heat-affected zone w2 on its outer periphery. A sheet separation zone s is formed on the outer periphery of the heat-affected zone w2.

[0060] As described in the above publication, the weld w1 is made of an intermetallic compound (IMC) generated by a melting reaction, and has a thickness of about 2 to 4 μm. In this embodiment, the sizes of the weld w1 and the heat-affected zone w2 are defined by the cross-sectional width along the joint interface (the length in the left-right (Y) direction in FIG. 1), and the width of the weld w1 is appropriately referred to as the IMC diameter (d1), and the width of the heat-affected zone w2 is appropriately referred to as the HAZ diameter (d2).

[0061] <Joint evaluation> (1) Equipment The evaluation device D includes an ultrasonic wave transmission / reception sensor 61, an ultrasonic wave transmission / reception source 62, a reflected wave processing unit 63, an amplitude distribution analysis unit 64, and a bonding state evaluation unit 65.

[0062] A matrix-shaped ultrasonic array probe (8.5M0808G2015-C0303P10T0 manufactured by Toshiba Inspection Solutions) was used for the transmitting / receiving sensor 61. The transmitting / receiving source 62 is a device (TMUT064D600 manufactured by Toshiba Inspection Solutions) that functions as both an ultrasonic oscillator and a reflected wave receiver, and converts the intensity (amplitude) of the reflected wave into a digital signal and outputs it to the processing unit 63. The processing unit 63 generates an amplitude distribution based on the digital signal of the reflected wave obtained from the transmitting / receiving source 62 and its receiving position. In this embodiment, the ultrasonic oscillation frequency is set to 8.5 MHz. The measuring means or measuring step referred to in the present invention is realized by the transmitting / receiving sensor 61, the transmitting / receiving source 62, and the processing unit 63.

[0063] The analysis unit 64 (analysis means / step) identifies the HAZ diameter and IMC diameter of the joint w from the features appearing in the amplitude distribution. The evaluation unit 65 (evaluation means / step) judges the quality of the joint w from the HAZ diameter and / or IMC diameter obtained from the analysis unit 64. The processing unit 63, analysis unit 64, and evaluation unit 65 are realized by a program (algorithm) executed on a computer (personal computer).

[0064] (2) Processing The process flow using the evaluation device D is shown in Fig. 2. First, a measurement is performed using a transmitting / receiving sensor 61 and a transmitting / receiving source 62, and an amplitude distribution is generated in a processing unit 63 (step S1). Next, an analysis unit 64 analyzes the amplitude distribution obtained by measurement to obtain the HAZ diameter and IMC diameter of the joint w (step S2).

[0065] The evaluation unit 65 first compares the HAZ diameter with threshold value I (step S3). If the HAZ diameter is less than threshold value I, it is determined that the joint 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 joint is poor, and if the IMC diameter is greater than or equal to threshold value II, it is determined that the joint is good (step S5).

[0066] (3) Amplitude distribution Measurement data obtained from a transmitting / receiving sensor 61 attached to the surface of the Al alloy plate 1 via gel or the like was processed by a processing unit 63 to generate an amplitude distribution. The amplitude distribution along a measurement line (X=0) passing through approximately the center (X=Y=0) of the joint w, and an overall scanned image of the joint w are also shown in Figure 3. The left side of Figure 3 shows the amplitude distribution of a sample with a good bond, and the right side of Figure 3 shows the amplitude distribution of a sample with a poor bond. Different characteristics appear in each amplitude distribution.

[0067] When the joint was good (left side of Fig. 3), a maximum amplitude point appeared near the outer edge of the weld w1, and a minimum amplitude point appeared near the outer edge of the heat-affected zone w2. When the joint was poor (right side of Fig. 3), no minimum or maximum amplitude points appeared near those outer edges. However, an inflection point appeared in the curve showing the amplitude distribution near the outer edge of the heat-affected zone w2.

[0068] (4) Size specification The above-mentioned characteristics of the amplitude distribution were compared with the fracture surface or cross section of the actually observed joint w. In general, the positions where the minimum points or inflection points of the amplitude distribution appeared corresponded to the outer peripheral edge of the heat-affected zone w2. In addition, the positions where the maximum points of the amplitude distribution appeared corresponded to the outer peripheral edge of the weld w1 (IMC).

[0069] Based on these results, the HAZ diameter (d2) is determined from the position (Y coordinate) of the minimum point or inflection point that appears in the amplitude distribution, as shown in Figure 4. For example, the distance between the minimum points or the distance between the inflection points that appear on both sides can be taken as the HAZ diameter. If the amplitude distribution is symmetrical with respect to the center, the HAZ diameter can be twice the distance from the center to one of the minimum points or inflection points.

[0070] When the HAZ diameter is equal to or greater than a predetermined value (threshold I), the IMC diameter (d1) is also determined from the position (Y coordinate) of the maximum point appearing in the amplitude distribution. For example, the distance between the maximum points appearing on both sides can be determined as the IMC diameter. If the amplitude distribution is symmetrical, the IMC diameter can be twice the distance from the center to one of the maximum points.

[0071] The maximum point, minimum point, or inflection point may be determined by differentiating the amplitude distribution curve. The amplitude distribution curve may be a curve obtained by extrapolating discrete coordinate points, or may be a moving average line obtained from three to five coordinate points.

[0072] Of course, it is also possible to sequentially calculate the change in amplitude (slope) for each point and extract the extreme point (minimum if negative to positive, maximum if positive to negative) from the position where the sign of the slope is reversed, or to extract the inflection point (the boundary between increase and decrease) from the position where the increase and decrease of the slope is reversed.

[0073] Incidentally, the reason why the minimum points and inflection points in the amplitude distribution of the reflected wave appear near the outer periphery of the heat-affected zone is thought to be as follows. As shown by the arrows in Figure 4, even if the incident direction of the ultrasonic wave is perpendicular to the outer surface of the metal plate, the reflected direction of the ultrasonic wave is not perpendicular to that outer surface. The reflected wave can be scattered in various directions due to factors such as the inclination of the boundary surface between the joined surfaces of the metal plates and the gap. For this reason, the amplitude (intensity) of the reflected wave and its rate of increase can decrease near the outer periphery of the heat-affected zone. This tendency is reflected in the amplitude distribution and appears as the above-mentioned poles (minimum points, maximum points) and inflection points.

[0074] The reflected wave is measured from the outer surface of the Al alloy plate 1, and there is usually an electrode mark near the center of the surface. This is also reflected in the amplitude distribution of the reflected wave, and a maximum point may appear near the center of the amplitude distribution. Such a maximum point is not related to the size of the joint. Therefore, the analysis of the amplitude distribution may be performed by excluding the center and focusing on the characteristics that appear in the area expected from the electrode specifications, welding conditions, etc. (near the outer periphery of the welded part and heat-affected part).

[0075] (5) Measurement line As described above, the reflected wave can be measured along a linear measurement line passing through the approximate center of the joint. As shown in Fig. 5, the measurement line may be rotated around the center of the joint, and the measurement of the reflected wave may be repeated for each measurement line. By connecting the outer peripheral end positions (dots in Fig. 5) of the heat-affected zone or welded zone identified along the measurement line for each angle, an approximate circle showing the entire outline (outer peripheral edge) of the heat-affected zone or welded zone can be drawn. The diameter of each approximate circle may be the size of the heat-affected zone (HAZ diameter) or the size of the welded zone (IMC diameter).

[0076] By repeating measurements along multiple measurement lines (called "rotation lines") around the center of the joint, the size of each part can be determined stably and with high accuracy, ensuring high robustness.

[0077] Evaluation example: (1) Verification example 1 Using four samples (test pieces A to D) with different bonding conditions between the Al alloy plate 1 and the steel plate 2, the HAZ diameter and IMC diameter (evaluation diameter) of each test piece identified by the above-mentioned method, as well as the HAZ diameter and IMC diameter (actual diameter) measured on the fracture surface of each test piece after the tensile shear test, were determined.

[0078] Each test piece was produced by changing the amount of heat input (current value x current flow time) when spot welding the Al alloy plate 1 and the steel plate 2 (second current flow process). Other welding conditions and the plate combinations (metal plates) used were as described above. From test piece A to test piece D, the amount of heat input during the second current flow decreased in order.

[0079] The evaluation diameter was the diameter of the approximate circle obtained by measuring along the rotation line described above. The actual measurement diameter was the value measured from the size of the heat affected zone (HAZ) and the welded joint (IMC) that appeared on the fracture surface, based on the images taken with an optical microscope.

[0080] The relationship between the measured diameter and the evaluated diameter for the HAZ diameter and the IMC diameter is shown in Fig. 6. In this example, the threshold I for the evaluated HAZ diameter was set to 8 mm.

[0081] As can be seen from Fig. 6, the evaluated HAZ diameter was almost the same as the measured HAZ diameter. When the evaluated HAZ diameter is equal to or greater than threshold I, the evaluated IMC diameter is also almost the same as the measured IMC diameter, so it was found that the evaluated IMC diameter can be used as an index for judging the bonding condition.

[0082] On the other hand, in the case of test piece D, where the evaluated HAZ diameter is smaller than the threshold I (when the joint is poor), the evaluated IMC diameter does not match the measured HAZ diameter, and is significantly smaller than the measured HAZ diameter. This is because, as shown in Fig. 3, in the case of a joint failure, the maximum point of the amplitude distribution is unlikely to appear at the outer periphery of the IMC, and the maximum point that appears near the center of the amplitude distribution is likely to be erroneously detected as the outer periphery of the IMC. In this way, when the evaluated HAZ diameter is smaller than the threshold, unlike the cases of the other test pieces, it becomes difficult to use the evaluated IMC diameter as an index for judging the joint condition.

[0083] (2) Verification example 2 Three samples (test pieces E to G) using different steel plates were also produced in the same manner as in Verification Example 1. A galvannealed hot-dip galvanized steel plate (270 MPa class / plate thickness: 0.75 mm) was used as steel plate 2, and an uncoated cold-rolled steel plate (440 MPa class / plate thickness: 1.4 mm) was used as steel plate 3. The galvanized layer had a thickness of approximately 8 μm and a melting point of approximately 420°C.

[0084] The evaluated diameters and actually measured diameters of test pieces E to G were compared in the same manner as in Verification Example 1. The results are shown in Fig. 7. In this verification example, the amount of heat input during the second current application was also decreased in the order from test piece E to test piece G.

[0085] As can be seen from Fig. 7, even in the joint where plated steel sheets were spot welded, the trends between the evaluated diameter and the measured diameter were similar to those in Verification Example 1. Therefore, it was also confirmed that regardless of whether the steel sheets were plated or not, if the evaluated HAZ diameter was larger than the threshold value, the evaluated IMC diameter could be used as an index for judging the joining condition of the joint.

[0086] (3)Join strength Tensile shear tests were carried out using various test pieces obtained by spot welding the above-mentioned Al alloy sheet 1 and non-plated steel sheet 2. The relationship between the joint strength thus obtained and the IMC diameter actually measured from the fracture surface of each test piece is shown in Figure 8.

[0087] As is clear from FIG. 8, there is a positive correlation between the size of the weld (IMC diameter) and the joint strength of the joint. It was also confirmed that if a database of this correlation exists, it would be possible to estimate the joint strength from the size of the weld.

[0088] Welding System The evaluation diameter, the quality of the joint state, or the joint strength obtained as described above can also be fed back to the welding conditions. For example, when the evaluation HAZ diameter is larger than a threshold value, the joint state of the joint is judged from the evaluation IMC diameter. If the evaluation IMC diameter is too large or too small, the welding conditions set in the control unit of the welding device are changed. Such processing may be performed while the welding process is proceeding, or may be performed during periodic maintenance. By feeding back the evaluation results of the joint to the welding conditions, a high-quality welded product (metal joint) can be stably manufactured. [Explanation of symbols]

[0089] 1 Al alloy plate 2, 3 Steel plate w1 weld w2 Heat affected zone w Joint s Sheet separation section D Bonding evaluation equipment M Metal joint m board set 61 Transmitting and receiving sensor (ultrasonic array probe)

Claims

1. A method for evaluating a joint of a metal joint body in which a plurality of metal plates are stacked and spot-welded between opposing surfaces of the metal plates, comprising: a measuring step of receiving a reflected wave of the ultrasonic wave transmitted to the bonding evaluation area and determining an amplitude distribution of the reflected wave; An analysis step of identifying a size of a weld and a size of a heat-affected zone formed on an outer periphery of the weld based on the amplitude distribution; an evaluation step of evaluating a joining condition of the metal joined body based on a size of the weld when the size of the heat affected zone is larger than a threshold value; A bonding evaluation method comprising:

2. The bonding evaluation method according to claim 1 , wherein the evaluation step judges the metal bonded body to be defective when the size of the heat-affected zone is smaller than the threshold value.

3. The bonding evaluation method according to claim 1 , wherein the size of the heat-affected zone is specified based on an approximate minimum point or approximate inflection point of the amplitude distribution that appears near an outer periphery of the heat-affected zone.

4. The joining evaluation method according to claim 1 , wherein the size of the weld is specified based on a substantial maximum point of the amplitude distribution that appears near an outer periphery of the weld.

5. The joint evaluation method according to claim 1 , wherein the plate assembly includes metal plates having different deformation characteristics.

6. The joining evaluation method according to claim 1 , wherein the welded portion is a nugget formed by resistance spot welding.

7. The joint evaluation method according to claim 1 , wherein the welded portion contains an intermetallic compound formed between the welded surfaces of the steel plate and the aluminum alloy plate.

8. The joint evaluation method according to claim 1 , wherein the size of the heat-affected zone and / or the size of the welded portion is identified based on a plurality of amplitude distributions related to the joint evaluation area.

9. A joining evaluation system for a metal joined body in which a plurality of metal plates are stacked and spot-welded between opposing surfaces of the metal plates, a measuring means for receiving a reflected wave of an ultrasonic wave transmitted to the bonding evaluation area and determining an amplitude distribution of the reflected wave; an analysis means for identifying a size of a weld and a size of a heat-affected zone formed on an outer periphery of the weld based on the amplitude distribution; an evaluation means for evaluating a joining state of the metal joined body based on a size of the weld when the size of the heat affected zone is larger than a threshold value; A bonding evaluation device comprising:

10. The joint evaluation device according to claim 9 , wherein the evaluation means outputs the joint strength between the metal plates corresponding to the size of the welded portion based on a database.

11. A welding device capable of spot welding a sheet set in which a plurality of metal sheets are stacked on each other between opposing surfaces of the metal sheets; The bonding evaluation device according to claim 9 or 10, The welding system may include a welding device that can change welding conditions based on the size of the heat-affected zone and / or the size of the weld.

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