Inspection method for spot welds

Abstract

This enables simple and accurate inspection of the quality of spot welds. [Solution] An inspection method for inspecting the quality of a nugget M, which is a spot weld formed at the contact portion C1 between metal plates 21, 22 in a plate assembly 20 made by stacking multiple metal plates 21, 22. The nugget M is formed by applying pressure to the plate assembly 20 with a pair of electrodes 4, 5 positioned opposite each other on either side of the plate assembly 20 and then passing current between the pair of electrodes 4, 5. After that, the current between the pair of electrodes 4, 5 is temporarily stopped, and then the quality of the nugget M is inspected based on the electrical resistance of the nugget M measured by re-applying current between the pair of electrodes 4, 5.

Description

Technical Field

[0001] The present invention relates to a method for inspecting a spot welded joint.

Background Art

[0002] Spot welding, which is a type of resistance welding, joins metal plates by sandwiching a stack of multiple metal plates between a pair of positive and negative electrodes under pressure and passing an electric current between the electrodes to form a spot welded joint, also called a nugget, at the contact portion between the metal plates. Since this spot welding can quickly and firmly join metal plates, it is widely used, for example, in manufacturing automobile bodies and outer panel sheets. However, if the spot welded joint does not meet the required quality, it will have an adverse effect on the strength and durability of the vehicle body, etc., and ultimately on the driving performance of the automobile. Therefore, during or after the formation of the spot welded joint, the quality of the spot welded joint (whether the spot welded joint has the desired joint strength) is inspected.

[0003] As a method for inspecting a spot welded joint, a chisel test is known in which a chisel inserted between two metal plates welded to each other is used to check whether the spot welded joint peels off (breaks). The chisel test, which is a type of destructive test, has the advantage of being able to accurately (with high precision) inspect the quality of the spot welded joint. However, it has disadvantages such as requiring time and labor for inspection and being unable to use the stack of plates being inspected as a product. In particular, when inspecting the quality of a spot welded joint formed in a stack of plates made of high-strength and high-tensile steel, such as ultra-high-tensile steel (steel with a tensile strength of 980 MPa or more) whose use as a vehicle body material is increasing in order to reduce the weight of automobiles (hereinafter, these are collectively referred to as "high-strength steel plates"), the possibility of defects such as cracks occurring in the spot welded joint during inspection increases, so special care is required for inspection and the time and labor for inspection increase.

[0004] Therefore, in particular, for inspecting spot welds formed on plate assemblies including the high-strength steel plates mentioned above, there is a strong tendency to use so-called non-destructive testing methods that do not destroy the spot welds during inspection. Examples of non-destructive testing methods include ultrasonic testing methods (for example, Patent Document 1 below) and quality determination methods based on the amount of deflection that occurs in the welding gun during spot welding (in-process inspection methods). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2012-93307 [Overview of the project] [Problems that the invention aims to solve]

[0006] To perform an ultrasonic inspection as disclosed in Patent Document 1, it is essential to prepare a dedicated inspection device. Furthermore, although not mentioned in Patent Document 1, in order to perform an ultrasonic inspection accurately, it is necessary to apply a gel to the measurement site (the area to be irradiated with ultrasound) before the inspection and wipe off the gel applied before the inspection after the inspection. Therefore, the inspection is time-consuming to perform and it is difficult to increase the frequency (inspection efficiency) of inspections. Consequently, ultrasonic inspections, like destructive tests such as chisel inspections, can only be performed on a small number of samples as a sampling inspection.

[0007] On the other hand, in-process inspection methods, which inspect the quality of spot welds based on the amount of deflection of the welding gun during spot welding (while the main current is applied), can significantly increase the inspection frequency (inspection efficiency) compared to ultrasonic inspection. However, because the theoretical framework is not yet complete, there is a possibility that the quality of spot welds cannot be inspected with high accuracy.

[0008] Therefore, the present invention aims to provide an inspection method that can easily and accurately inspect the quality of spot welds formed on plate assemblies including high-strength steel plates. [Means for solving the problem]

[0009] The inventors have discovered a correlation between the electrical resistance of spot welds formed at the contact points between metal plates in a plate assembly made by stacking multiple metal plates, and the size of the spot welds (nugget diameter). Therefore, they hypothesized that by forming spot welds in the plate assembly by passing current between a pair of electrodes, and then re-applying current between the electrodes to determine the electrical resistance of the spot welds, the size of the spot welds could be estimated, and the quality of the spot welds could be accurately inspected. This invention was conceived based on these findings.

[0010] In other words, the present invention was devised to achieve the above objective, A method for inspecting spot welds, which involves inspecting the quality of spot welds formed at the contact points between metal plates in a plate assembly made by stacking multiple metal plates, The method is characterized by forming a spot weld by applying pressure to the plate assembly with a pair of electrodes positioned opposite each other on either side of the plate assembly while applying current between the pair of electrodes, then temporarily stopping the current between the pair of electrodes, and then reapplying current between the pair of electrodes to measure the electrical resistance of the spot weld and inspecting the quality of the spot weld.

[0011] In the spot weld inspection method according to the present invention, the quality of the spot weld is inspected based on the electrical resistance of the spot weld measured (measured when the current is reapplied) by sequentially applying current to a pair of electrodes that are pressurized with a plate assembly in between to form the spot weld, temporarily suspending the current, and then reapplying the current. In this way, the quality of the spot weld can be inspected using the same spot welding equipment that was used to form the spot weld. Therefore, it is possible to easily inspect the quality of the spot weld without the need for a dedicated inspection device or the various procedures required when using a dedicated inspection device or method. Accordingly, with the inspection method according to the present invention, it is possible to increase the inspection frequency and even perform 100% inspection.

[0012] Furthermore, with the inspection method according to the present invention, the quality of spot welds can be inspected with high accuracy, based on the findings discovered through the inventors' diligent research. In particular, in the present invention, after forming a spot weld by applying current between a pair of electrodes, the current to the pair of electrodes is temporarily stopped, so that the constituent metal of the spot weld (molten metal generated at the contact point between the metal plates) can be completely hardened before measuring the electrical resistance of the spot weld. Moreover, in the present invention, current is applied to the pair of electrodes while the plate assembly is pressed by the pair of electrodes, the current is temporarily stopped, and then re-applied (re-applied to measure the electrical resistance of the spot weld), so a decrease in the accuracy of the inspection of the quality of spot welds due to misalignment of electrodes can be avoided. Combined with the above effects, the quality of spot welds can be inspected with high accuracy according to the present invention. [Effects of the Invention]

[0013] Based on the above, the inspection method for spot welds according to the present invention allows for simple and accurate inspection of the quality of spot welds formed on plate assemblies including high-strength steel plates. [Brief explanation of the drawing]

[0014] [Figure 1] This is a schematic diagram of a spot welding machine. [Figure 2] These are schematic cross-sectional views showing the spot welding process of two overlapping metal plates. Figure (a) is a schematic cross-sectional view at the start of welding, Figure (b) is a schematic cross-sectional view after a predetermined time has elapsed since the start of welding, and Figure (c) is a schematic cross-sectional view after a further predetermined time has elapsed from the stage shown in Figure (b). [Figure 3] This figure shows embodiments (execution time and current value) of the three steps included in the inspection method according to the present invention. [Figure 4] This figure shows the correlation between the nugget diameter formed in the plate assembly and the electrical resistance of the nugget. [Modes for carrying out the invention]

[0015] Hereinafter, embodiments of the present invention will be described based on the drawings.

[0016] FIG. 1 is a partial schematic view of a spot welding apparatus, and is a partial schematic view of a spot welding apparatus 1 used when implementing an inspection method for a spot welding portion according to the present invention. The spot welding apparatus 1 shown in the figure is used, for example, when forming a spot welding portion on a plate assembly 20 formed by overlapping a plurality (two sheets) of metal plates 21 and 22 shown in FIG. 2. For example, it is used in a state fixed to a frame not shown or attached to the tip of a robot arm that can move three-dimensionally, and includes a welding gun 2 and a measuring device 8. Hereinafter, for convenience of explanation, terms such as "upper side" and "lower side" are used, but these simply refer to the "upper side of the paper surface" and "lower side of the paper surface" in FIGS. 1 and 2, and do not intend to limit the posture of the welding gun 2 during welding.

[0017] Here, the plate assembly 20 is, for example, used for an automobile body, and at least one of the two metal plates 21 and 22 is a plate material made of a high-strength and high-tensile steel having a tensile strength of 980 MPa or more, such as ultra-high tensile steel, that is, a high-strength steel plate. For such a high-strength steel plate, even when the plate thickness of the metal plate (metal plate made of high-strength steel plate) is set to 2 mm or less, preferably 1.5 mm or less, aiming at weight reduction of the automobile, it is possible to realize the plate assembly 20 having the required mechanical strength.

[0018] The welding gun 2 includes a pair of electrodes 4 and 5, an electrode holding member 3, a pressurizing mechanism 6, and a transformer 7.

[0019] The pair of electrodes 4 and 5 are held at one end and the other end of an electrode holding member 3 formed in a substantially C shape from a conductive metal material in a coaxially arranged state (arranged vertically side by side in the illustrated example) such that the tip portions of both face each other. The pair of electrodes 4 and 5 have an internal structure through which a coolant can flow, and are constantly cooled by the coolant flowing through the inside.

[0020] The pressing mechanism 6 applies a pressing force in the plate thickness direction to the plate assembly 20 disposed between the pair of electrodes 4 and 5 by moving up and down one of the pair of electrodes 4 and 5 (here, the upper electrode 4). The electrode 4 is held at one end of the electrode holding member 3 via the pressing mechanism 6. As the pressing mechanism 6, one that can precisely adjust the amount of up and down movement of the electrode 4, that is, the pressing force to be applied to the welding object 20, for example, a linear actuator using a servo motor as a drive source is preferably used. During the execution of spot welding, the pressing mechanism 6 (the servo motor constituting it) operates according to a predetermined pressing pattern determined in advance according to the plate thickness, material, etc. of the metal plate constituting the plate assembly 20.

[0021] The transformer 7 is electrically connected to a power source (welding power source) not shown and generates a high welding current between the pair of electrodes 4 and 5. The pair of electrodes 4 and 5 are electrically connected to the positive terminal (or negative terminal) and the negative terminal (or positive terminal) of the transformer 8 via a conductive member also called a shunt formed of copper or a copper alloy and the electrode holding member 3, respectively. During the execution of spot welding, the transformer 7 energizes between the pair of electrodes 4 and 5 according to a predetermined energization pattern determined in advance according to the plate thickness, material, etc. of the metal plate constituting the plate assembly 20.

[0022] The measuring device 8 measures the voltage applied between the electrodes 4 and 5 and the current flowing between the electrodes 4 and 5 when energized between the pair of electrodes 4 and 5.

[0023] The inspection method for the spot welded part according to the present invention uses the spot welding device 1 having the above configuration, and after executing the main energization process of forming a spot welded part (nugget) by joining two metal plates 21 and 22 at their contact part, the cooling process and the inspection energization process are continuously executed. Hereinafter, first, based on FIGS. 2(a) to 2(c), the embodiment of the main energization process of forming a nugget as the spot welded part will be described.

[0024] First, as shown in Figure 2(a), the plate assembly 20, consisting of two overlapping metal plates 21 and 22, is placed horizontally (flat) between the electrodes 4 and 5. Then, when the pressure mechanism 6 (consisting of the servo motor) of the welding gun 2 is driven, the plate assembly 20 is clamped from both the top and bottom sides in the thickness direction by the pair of electrodes 4 and 5. After the plate assembly 20 is clamped, current is passed between the pair of electrodes 4 and 5 that are clamping the plate assembly 20.

[0025] When current is passed between electrodes 4 and 5, heat is generated at the contact area C1 between the metal plates 21 and 22, and a nugget M (formed by the melting and mixing of the constituent metals of the metal plates 21 and 22, and then hardening) is formed at the contact area C1 as a spot weld. The nugget M is first formed within the contact area C1 between the metal plates 21 and 22, specifically within the contact area between electrodes 4 and 5 and the plate assembly 20 (the contact area C2 between electrode 4 and metal plate 21, and the contact area C3 between electrode 5 and metal plate 22) [see Figure 2(b)], and then grows by expanding radially outward from electrodes 4 and 5. As shown in Figure 2(c), when the nugget M grows to a predetermined size, the current is stopped from flowing between the pair of electrodes 4 and 5. This forms a nugget M formed by joining the two metal plates 21 and 22. Figure 2(c) illustrates the case where the nugget M grows until its diameter (nugget diameter) d2 becomes larger than the diameters of the contact portions C2 and C3.

[0026] As shown in Figure 3, when the current is stopped between the pair of electrodes 4 and 5, a cooling process is performed to maintain this stopped state for a predetermined time. During this cooling process, the clamping state of the plate assembly 20 by the pair of electrodes 4 and 5 (electrode 4 in contact with metal plate 21 and electrode 5 in contact with metal plate 22) is maintained. As described above, since cooling liquid is constantly flowing inside electrodes 4 and 5, the metal plates 21 and 22 in contact with electrodes 4 and 5 are cooled, and the constituent metals of the nugget M harden completely.

[0027] Following the cooling process described above, in which the current is temporarily stopped from flowing between the pair of electrodes 4 and 5 that hold the plate assembly 20, an inspection current-flow process is performed to re-enable current between the pair of electrodes 4 and 5. As shown in Figure 3, the duration of this inspection current-flow process (the time for re-energing current between the pair of electrodes 4 and 5) is shorter than the duration of the main current-flow process described above (the time for current flowing between the pair of electrodes 4 and 5), and longer than the duration of the cooling process (the time for which current is stopped between the electrodes 4 and 5). The current value flowing between the electrodes 4 and 5 in this inspection current-flow process (the magnitude of the welding current) is smaller than the current value flowing between the electrodes 4 and 5 in the main current-flow process. For example, if the current value flowing between the electrodes 4 and 5 in the main current-flow process is 7 to 8 kA, the current value flowing between the electrodes 4 and 5 in the inspection current-flow process should be approximately 6 to 6.5 kA. The total duration of the main current-flow process, the cooling process, and the inspection current-flow process is less than 1 second. In other words, the nugget M, which is a spot weld, can be formed with a total energizing time of less than 1 second.

[0028] During the test energizing process, in which current is re-energed between the pair of electrodes 4 and 5, the measuring device 8 measures the voltage between electrodes 4 and 5 and the current flowing between them. Since electrodes 4 and 5 are sandwiching the plate assembly 20 (the upper electrode 4 is in contact with the metal plate 21, and the lower electrode 5 is in contact with the metal plate 22), the current flowing between electrodes 4 and 5 flows through the contact points between the metal plates 21 and 22 that constitute the plate assembly 20, i.e., the nugget M which is a spot weld formed on the plate assembly 20. Therefore, by substituting the voltage (voltage value) between electrodes 4 and 5 and the current (current value) flowing between electrodes 4 and 5 measured by the measuring device 8 into Ohm's law, the electrical resistance of the nugget M can be determined.

[0029] The inspection method for spot welds according to the present invention inspects the quality of a spot weld based on the electrical resistance of the nugget M measured (determined) as described above. This is because the inventors have found that there is a correlation between the diameter (nugget diameter) [mm] of the spot weld and the electrical resistance [Ω] of the spot weld. Figure 4 shows the nugget diameter on the horizontal axis and the electrical resistance of the spot weld on the vertical axis. The approximation curve shown in the figure shows that there is a correlation between the nugget diameter and the electrical resistance of the spot weld.

[0030] Therefore, as described above, after performing the main energizing process to form a nugget M as a spot weld on the plate assembly 20 by applying current between the pair of electrodes 4 and 5, an inspection energizing process is performed to re-enable current between the pair of electrodes 4 and 5 in order to determine the electrical resistance of the nugget M. This allows for the estimation of the nugget diameter and accurate inspection of the quality of the spot weld (whether or not the spot weld has the desired joint strength).

[0031] In particular, in the above method, after forming the nugget M by passing current between the pair of electrodes 4 and 5 (the main current-passing process is performed), the current to the pair of electrodes 4 and 5 is temporarily suspended (a cooling process is provided in which the current is temporarily suspended), so that the constituent metal of the nugget M can be completely hardened before measuring the electrical resistance of the nugget M. Furthermore, since the current is passed between the pair of electrodes 4 and 5, temporarily suspended, and then re-passed while the plate assembly 20 is held (pressurized) by the pair of electrodes 4 and 5, it is possible to avoid the occurrence of measurement errors (measurement errors) in electrical resistance caused by misalignment of the electrodes 4 and 5, and consequently, a decrease in inspection accuracy. Combined with the above effects, the inspection method according to the present invention can accurately inspect the quality of the nugget M formed on the plate assembly 20 including high-strength steel plates.

[0032] Furthermore, in the inspection method according to the present invention, as described above, current is applied to the pair of electrodes 4 and 5 that are pressed with the plate assembly 20 sandwiched between them to form the nugget M, the current is temporarily stopped, and then the current is reapplied in sequence, and the quality of the nugget M is inspected based on the electrical resistance of the nugget M measured and calculated when the current is reapplied. In this way, the quality of the nugget M can be inspected using the spot welding apparatus 1 that was used to form the nugget M. Therefore, it is possible to easily inspect the quality of the nugget M without the need for a dedicated inspection device, and furthermore, the various troubles required when using a dedicated inspection device or method. Accordingly, with the inspection method according to the present invention, it is possible to increase the inspection frequency, and even to perform 100% inspection.

[0033] Although one embodiment of the present invention has been described above, the present invention is not limited to the embodiments described above, and various modifications can be made without departing from the spirit of the invention.

[0034] For example, in the embodiments described above, the plate assembly 20 was made by overlapping two metal plates 21 and 22, but the inspection method according to the present invention can also be applied when spot welding a plate assembly 20 made by overlapping three or more metal plates. [Explanation of symbols]

[0035] 1 Spot welding machine 3 Electrode holding member 4,5 electrodes 6. Pressurization mechanism 7 Transformers 8. Measuring device 20 board assembly 21,22 metal plate d2 nugget diameter C1 Contact area between metal plates Contact area between electrodes C2 and C3 and the metal plate M Nugget (spot weld)

Claims

[Claim 1] A method for inspecting spot welds, which involves inspecting the quality of spot welds formed at the contact points between metal plates in a plate assembly made of multiple overlapping metal plates, A method for inspecting a spot weld, characterized by forming the spot weld by applying pressure to the plate assembly with a pair of electrodes positioned opposite each other on either side of the plate assembly and then passing current between the pair of electrodes, temporarily suspending the current between the pair of electrodes, and then re-applying current between the pair of electrodes to inspect the quality of the spot weld based on the measured electrical resistance of the spot weld.

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