Resistance spot welding method

Abstract

To improve a quality of a dent, in continuously performing resistance spot welding to different metals.SOLUTION: A resistance spot welding method, which welds a material to be welded made of a plurality of metal members superposed with each other, by energizing the member while sandwiching the member between a pair of electrodes, comprises a pre-electrifying step of applying electric currents between the pair of electrodes to preheat the material to be welded, and a main electrifying step of applying electric currents between the pair of electrodes to form a nugget on the material to be welded to weld the material, after the pre-electrifying step. An electrification time in the main electrifying step is shorter than an electrifying time in the pre-electrifying step, and a current value of welding currents in the main electrifying step is larger than a current value of welding currents in the pre-electrifying step.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present disclosure relates to a resistance spot welding method. [Background technology]

[0002] Patent Document 1 discloses a resistance spot welding method for welding multiple steel plates by pulsation current. In this method, the occurrence of surface and center flash is suppressed by variably controlling the current flow time, current rest time, and welding current of the current pulses that make up the pulsation current. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2015 / 093568 Summary of the Invention [Problem to be solved by the invention]

[0004] Generally, when workpieces are welded by resistance spot welding, a portion of the workpiece adheres to the electrode tip surface. When welding an aluminum workpiece after welding an iron workpiece, the melting point of iron is higher than that of aluminum, so the iron adhering to the tip surface does not adhere to the dent. However, when welding an iron workpiece after welding an aluminum workpiece, the temperature of the tip surface exceeds the melting point of aluminum, and the aluminum adhering to the tip surface adheres to the dent, potentially causing rust or corrosion. Therefore, a technology that can improve the quality of the dent when resistance spot welding dissimilar metals consecutively is needed. [Means for solving the problem]

[0005] The present disclosure can be realized in the following forms.

[0006] According to one aspect of the present disclosure, there is provided a resistance spot welding method for joining workpieces, which are overlapping metal members, by sandwiching the workpieces between a pair of electrodes and passing a current therethrough, the method including: a pre-current step for preheating the workpieces by passing a current between the pair of electrodes; and a main current step, performed after the pre-current step, for forming a nugget in the workpieces and joining them by passing a current between the pair of electrodes, wherein the main current step has a shorter current passing time than the pre-current step and a greater current value of the welding current in the main current step than the pre-current step. This resistance spot welding method can suppress heat generation on the electrode surface during the main current application step, thereby suppressing melting of the workpieces attached to the electrode that were joined prior to the workpieces to be joined in the main current application step and are made of a different metal than the workpieces to be joined in the main current application step. This prevents the workpieces attached to the electrode from adhering to the dent. As a result, the quality of the dent can be improved when resistance spot welding different metals consecutively. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a diagram showing a schematic configuration of a resistance spot welding device. [Figure 2] 3 is a flowchart of a welding process performed by a resistance spot welding device. [Figure 3] FIG. 4 is a diagram illustrating a change in welding current over time in a welding process. DETAILED DESCRIPTION OF THE INVENTION

[0008] A. First embodiment: FIG. 1 is a diagram showing a schematic configuration of a resistance spot welding apparatus 100. The resistance spot welding apparatus 100 melts and joins a workpiece W, which is a stack of multiple metal members. In this embodiment, the workpiece W is composed of a first metal plate W1 and a second metal plate W2 that are stacked on top of each other. Note that the workpiece W may be composed of three or more metal plates.

[0009] Resistance spot welding apparatus 100 includes a welding gun 10 and a control device 30. Welding gun 10 includes a gun body 11, a pair of electrodes, a lower electrode 12 and an upper electrode 13, and an electrode lifting device 14. Gun body 11 has an upper arm 11T and a lower arm 11B. The lower electrode 12 is attached to lower arm 11B. The upper electrode 13 is attached to upper arm 11T via electrode lifting device 14. The tip of upper electrode 13 and the tip of lower electrode 12 are positioned opposite each other.

[0010] The electrode lifting device 14 is an electric device that holds and raises and lowers the upper electrode 13. The electrode lifting device 14 is attached to the tip of the upper arm 11T. The electrode lifting device 14 includes a servo motor 15 and a lifting member 16 that is connected to the drive shaft of the servo motor 15 via a gear. The electrode lifting device 14 raises and lowers the lifting member 16 by operating the servo motor 15 in accordance with a command signal from the control device 30. In other words, the electrode lifting device 14 moves the upper electrode in a direction along the opposing direction of the upper electrode 13 and the lower electrode 12. When the lifting member 16 is lowered, the workpiece W is sandwiched between the upper electrode 13 and the lower electrode 12.

[0011] When welding workpieces W, the resistance spot welding device 100 applies a welding current to the workpieces W via the upper electrode 13 and the lower electrode 12 while clamping and pressurizing the workpieces W between them. In this manner, the workpieces W melt due to Joule heat generated by the current flow. The workpieces W then cool and solidify, joining the first metal plate W1 and the second metal plate W2. The workpieces W expand as they melt due to the current flow, and contract as they cool after the current flow is completed. A nugget is formed at the joint interface between the welded metal members.

[0012] Cooling water channels (not shown) are formed inside the upper electrode 13 and the lower electrode 12. Flowing cooling water through the cooling water channels prevents the electrodes from becoming too hot and thus prevents deformation or wear of the electrodes. FIG. 1 shows an enlarged cross-sectional view of an electrode tip 20 attached to the tip of the upper electrode 13. The electrode tip 20 is provided with a first hole 21 and a second hole 22 that form part of the cooling water channel. The first hole 21 is a hole that opens toward the top of the electrode tip 20. The second hole 22 is a hole that opens toward the bottom of the first hole 21. Cooling water flows through the first hole 21 and the second hole 22 when the resistance spot welding apparatus 100 welds the workpiece W. The electrode tip 20 in this embodiment is more easily cooled by cooling water than an electrode tip that has the first hole 21 but not the second hole 22. This makes it easier to lower the surface temperature of the upper electrode 13. An electrode tip having the same structure as the electrode tip 20 may also be attached to the lower electrode 12 .

[0013] The control device 30 is configured by a computer including a CPU and a storage device. The control device 30 controls the operation of the resistance spot welding device 100 by having the CPU execute a program stored in advance in the storage device. Specifically, the control device 30 comprehensively controls the welding current flowing through the workpiece W, the current application time, the electrode pressure, the current application timing, the pressure application timing, and the like. Note that some or all of the functions of the control device 30 may be realized by hardware circuits.

[0014] FIG. 2 is a flowchart of the welding process performed by the resistance spot welding apparatus 100. The welding process is performed when the resistance spot welding apparatus 100 joins workpieces W made of aluminum and then successively joins workpieces W made of iron. The resistance spot welding method is realized by the resistance spot welding apparatus 100 performing the welding process. FIG. 3 is a diagram illustrating the change in welding current over time during the welding process. In FIG. 3, the horizontal axis represents time, and the vertical axis represents the welding current. The welding process will be described below with reference to FIGS. 2 and 3.

[0015] In step S10 of FIG. 2, a pre-energization process is performed. In the pre-energization process, the resistance spot welding apparatus 100 preheats the workpiece W by passing current between the electrodes. The pre-energization process is performed to cause a high-temperature reaction between the opposing surfaces of the first metal sheet W1 and the second metal sheet W2, thereby removing or reducing the oxide film on the metal sheet surfaces and facilitating melting of the first metal sheet W1 and the second metal sheet W2 in the main energization process. As shown in FIG. 3, in the pre-energization process, the control device 30 applies a welding current with a current value I1 and a current application time T1. Hereinafter, the current value of the welding current will be simply referred to as the current value. In this embodiment, the current value I1 in the pre-energization process is constant. The current value I1 is preferably, for example, 3 kA or more and 5 kA or less, and the current application time T1 is preferably, for example, 100 ms or more and 200 ms or less.

[0016] In step S20 of FIG. 2, the main current application process is performed. In this current application process, the resistance spot welding apparatus 100 applies current between the electrodes to form and join a nugget on the workpiece W. As shown in FIG. 3, the control device 30 applies current at a current value I2 and a current application time T2 in this current application process. In this embodiment, the current value I2 in this current application process is constant. Here, the current value I2 is greater than the current value I1, and the current application time T2 is shorter than the current application time T1. The current value I2 and the current application time T2 are set so that the temperature of the electrode surface in this current application process does not exceed the melting point of aluminum. The current value I2 is preferably, for example, 10 kA or more and 20 kA or less, and the current application time T2 is preferably, for example, 30 ms or more and 50 ms or less.

[0017] According to the first embodiment described above, when joining a workpiece W made of aluminum and then joining another workpiece W made of iron, the resistance spot welding apparatus 100 applies current at a higher value and for a shorter time than in the pre-current application process in the main current application process executed after the pre-current application process. By applying current in this manner, heat generation on the surface of the electrode during the main current application process can be suppressed, thereby suppressing melting of aluminum adhering to the electrode. This suppresses aluminum from adhering to the dent in the workpiece W made of iron. Therefore, the quality of the dent can be improved when resistance spot welding different metals consecutively.

[0018] B. Other Embodiments: (B-1) In the above embodiment, the welding process is performed when the resistance spot welding apparatus 100 joins workpieces W made of aluminum and then successively joins workpieces W made of iron. In contrast, the welding process may be performed when the resistance spot welding apparatus 100 joins first workpieces W and then successively joins second workpieces W made of a metal with a higher melting point than the first workpieces W. In this case, the current value and current application time in the main current application step are set so that the temperature of the electrode surface in the main current application step does not exceed the melting point of the first workpieces W.

[0019] (B-2) In the above embodiment, the current value I1 in the pre-current passing step and the current value I2 in the main current passing step are constant. In contrast, the current value in the pre-current passing step and the current value in the main current passing step do not have to be constant, as long as the average current value in the pre-current passing step is smaller than the average current value in the main current passing step.

[0020] The present disclosure is not limited to the above-described embodiments and can be realized in various configurations without departing from the spirit thereof. For example, the technical features in the embodiments corresponding to the technical features in each aspect described in the Summary of the Invention section can be appropriately replaced or combined to solve some or all of the above-described problems or achieve some or all of the above-described effects. Furthermore, if a technical feature is not described as essential in this specification, it can be appropriately deleted. [Explanation of symbols]

[0021] 10...welding gun, 11...gun body, 11B...lower arm, 11T...upper arm, 12...lower electrode, 13...upper electrode, 14...electrode lifting device, 15...servo motor, 16...lifting member, 20...electrode tip, 21...first hole, 22...second hole, 30...control device, 100...resistance spot welding device, W...workpiece, W1...first metal plate, W2...second metal plate

Claims

[Claim 1] A resistance spot welding method for joining workpieces, which are made up of a plurality of overlapping metal members, by sandwiching the workpieces between a pair of electrodes and passing current through the electrodes, comprising: a pre-current application process of preheating the workpiece by applying current between the pair of electrodes; a main current application process that is performed after the pre-current application process, and that forms a nugget in the workpiece by applying current between the pair of electrodes and joining the workpiece, The energization time of the main energization process is shorter than the energization time of the pre-energization process, The current value of the welding current in the main current application step is greater than the current value of the welding current in the pre-current application step. Resistance spot welding method.

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