Resistance spot joint method and resistance spot joint device

Abstract

To provide a resistance spot joint method and a resistance spot joint device which can improve joint quality.SOLUTION: A resistance spot joint method includes: a projection formation step of pressurizing a first workpiece W10 and a second workpiece W20 with a first load by a pressurization shaft 11 and a pressurization member 12, and thereby forming a projection W11 on at least one of the first workpiece and the second workpiece; a load removing step of removing the load of pressurization force by the pressurization shaft and the pressurization member so that a second load F2 acts on the first workpiece and the second workpiece, and thereby reducing a contact area between the first workpiece and the second workpiece; an energization step of energizing the first workpiece and the second workpiece, in a state in which the second load acts on the first workpiece and the second workpiece, and electrodes 21 and 22 are brought into contact with each of the first workpiece and the second workpiece; and a joint step of pressurizing the first workpiece and the second workpiece with a third load, and thereby joining a contact part interface between the workpieces W10 and W20.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present disclosure relates to a resistance spot bonding method and apparatus. [Background technology]

[0002] International Publication No. 2021 / 182444 discloses a solid-phase spot joining device including a pressing unit and a pair of welding electrodes. The pressing unit can press two metal plates in a direction perpendicular to the metal plates. The pair of welding electrodes heat each metal plate by passing current through the two metal plates while sandwiching the two metal plates from both sides in the thickness direction. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2021 / 182444 Summary of the Invention [Problem to be solved by the invention]

[0004] In the solid state spot joining apparatus described in WO 2021 / 182444, it is desirable to improve the joining quality.

[0005] An object of the present disclosure is to provide a resistance spot bonding method and a resistance spot bonding apparatus that can improve bonding quality. [Means for solving the problem]

[0006] A resistance spot joining method according to one aspect of the present disclosure is a method for joining a first workpiece made of metal and a second workpiece made of metal, the method including a protrusion forming step of forming a protrusion on at least one of the first workpiece and the second workpiece by applying a first load to the first workpiece and the second workpiece from both sides in a thickness direction of the first workpiece and the second workpiece using a pressure shaft arranged on one side in the thickness direction and a pressure member arranged on the other side, and removing the pressure from the pressure shaft and the pressure member so that a second load smaller than the first load acts on the first workpiece and the second workpiece from the pressure shaft and the pressure member. the second load acting on the first workpiece and the second workpiece from the pressure shaft and the pressure member and with electrodes in contact with each of the first workpiece and the second workpiece; and a joining process for joining the contact interface of the first workpiece and the second workpiece to each other by applying a third load larger than the second load to the first workpiece and the second workpiece from both sides in the thickness direction using the pressure shaft and the pressure member.

[0007] A resistance spot joining device according to one aspect of the present disclosure includes a pair of pressure shafts which apply pressure to a first workpiece and a second workpiece from both sides in a thickness direction thereof, a pair of electrodes respectively arranged around the pair of pressure shafts, a protrusion forming means which applies a first load to the first workpiece and the second workpiece from the pair of pressure shafts so that protrusions which contact each other are formed on each of the first workpiece and the second workpiece, and a pressure applying means which applies a second load smaller than the first load to the first workpiece and the second workpiece from the pair of pressure shafts, thereby bonding the protrusions to each other. the second load is applied to the first workpiece and the second workpiece from the pair of pressure shafts, and current is passed through the first workpiece and the second workpiece while the pair of electrodes is in contact with an area surrounding a portion of the first workpiece that is pressurized by one of the pair of pressure shafts and an area surrounding a portion of the second workpiece that is pressurized by the other of the pair of pressure shafts; and joining means for applying a third load greater than the second load to each of the protrusions from the pair of pressure shafts when a softening condition indicating that the protrusions have been softened is established. Effect of the Invention

[0008] According to the present disclosure, it is possible to provide a resistance spot bonding method and a resistance spot bonding apparatus that can improve bonding quality. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic diagram of a resistance spot bonding apparatus according to an embodiment of the present disclosure; [Diagram 2] FIG. 13 is a diagram illustrating a current application step after a load removing step. [Diagram 3] FIG. 3 is an enlarged view of the area indicated by solid line III in FIG. [Figure 4] 4 is a flowchart showing the control contents of a control unit. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present disclosure will now be described with reference to the accompanying drawings, in which the same or corresponding components are designated by the same reference numerals.

[0011] 1 is a schematic diagram of a resistance spot joining device according to an embodiment of the present disclosure. The resistance spot joining device 1 is a device that forms softened regions in multiple workpieces W10, W20 by passing an electric current through multiple workpieces W10, W20 stacked on top of each other, and joins the multiple workpieces W10, W20 together in a solid phase by plastically deforming the softened regions.

[0012] The plurality of workpieces W10, W20 include a first workpiece W10 and a second workpiece W20. Each of the workpieces W10, W20 is made of a metal such as iron or aluminum. Each of the workpieces W10, W20 is formed, for example, in a flat plate shape.

[0013] 1, the resistance spot bonding device 1 includes a pressure shaft 11, a pressure member 12, a pair of electrodes 21 and 22, a control unit 30, and a sensor 40. In this embodiment, a pressure shaft paired with the pressure shaft 11 is used as the pressure member 12. For this reason, hereinafter, the pressure shaft 11 and the pressure member 12 will be described as "a pair of pressure shafts 11 and 12."

[0014] The pair of pressure applying shafts 11, 12 can apply pressure to the first workpiece W10 and the second workpiece W20 from both sides in the thickness direction. The pair of pressure applying shafts 11, 12 are driven by a drive source (such as a servo press machine) (not shown). The pair of pressure applying shafts 11, 12 includes a first pressure applying shaft 11 and a second pressure applying shaft 12.

[0015] The first pressure shaft 11 has a shape that extends long in one direction (the vertical direction in FIG. 1). The first pressure shaft 11 can press the first workpiece W10 so that the first workpiece W1 is plastically deformed. Specifically, the first pressure shaft 11 can press the first workpiece W10 so that a protrusion W11 is formed on the first workpiece W10. The first pressure shaft 11 is made of, for example, tungsten carbide. In this embodiment, the first pressure shaft 11 is formed in a cylindrical shape. The first pressure shaft 11 has a pressing surface 11a that presses the first workpiece W10. The pressing surface 11a is formed by an end surface of the first pressure shaft 11. The pressing surface 11a is formed in a circular shape.

[0016] The sensor 40 is provided, for example, on the first pressure applying shaft 11. In this embodiment, a load cell is used as the sensor 40. Note that the location where the sensor 40 is installed is not limited to the first pressure applying shaft 11.

[0017] The second pressure applying shaft 12 has the same configuration as the first pressure applying shaft 11. The second pressure applying shaft 12 is disposed in such a manner that the central axis of the second pressure applying shaft 12 is located on an extension line of the central axis of the first pressure applying shaft 11, and the pressing surface 12a of the second pressure applying shaft 12 faces the pressing surface 11a of the first pressure applying shaft 11.

[0018] The pair of electrodes 21, 22 can pass electricity through the first workpiece W10 and the second workpiece W20 while in contact with the first workpiece W10 and the second workpiece W20. A voltage is applied to the pair of electrodes 21, 22 from a power supply unit (not shown). The pair of electrodes 21, 22 has a first electrode 21 and a second electrode 22.

[0019] The first electrode 21 can contact a portion of the first workpiece W10 surrounding a portion of the first workpiece W10 that is pressed by the first pressure shaft 11. In this embodiment, the first electrode 21 is formed in a cylindrical shape surrounding the first pressure shaft 11. A gap is provided between the inner peripheral surface of the first electrode 21 and the outer peripheral surface of the first pressure shaft 11. The first electrode 21 is made of, for example, copper. The first electrode 21 has a contact surface 21a that contacts the first workpiece W10. The contact surface 21a is formed in an annular shape. The shape of the contact surface 21a is not limited to an annular shape.

[0020] The second electrode 22 has the same configuration as the first electrode 21. The second electrode 22 can come into contact with a portion of the second workpiece W20 surrounding a portion pressurized by the second pressurizing shaft 12. The second electrode 22 is disposed in such a manner that the central axis of the second electrode 22 is located on an extension of the central axis of the first electrode 21, and the contact surface 22a of the second electrode 22 faces the contact surface 21a of the first electrode 21.

[0021] The control unit 30 controls the pair of pressure applying shafts 11, 12 and the pair of electrodes 21, 22. Specifically, the control unit 30 controls the load acting on the first workpiece W10 and the second workpiece W20 from the pair of pressure applying shafts 11, 12, the load acting on the first workpiece W10 and the second workpiece W20 from the pair of electrodes 21, 22, the voltage applied to the pair of electrodes 21, 22, and the pressing amount of the pair of pressure applying shafts 11, 12 and the pair of electrodes 21, 22. The control unit 30 has a load control unit 31, an electric current control unit 32, an acquisition unit 33, and a determination unit 34.

[0022] The load control unit 31 performs an operation to apply a first load F1 (see FIG. 1) from the pair of pressure shafts 11, 12 to the first work W10 and the second work W20 so that protrusions W11, W21 that come into contact with each other are formed on the first work W10 and the second work W20, respectively. In other words, the load control unit 31 constitutes a protrusion forming means. At this time, the load control unit 31 also performs an operation to apply an adjustment load smaller than the first load F1 (for example, a load of 1 kN or less) from the pair of electrodes 21, 22 to the first work W10 and the second work W20.

[0023] Thereafter, the load control unit 31 performs an operation to reduce the contact area between the protrusions W11, W21 by removing the pressure from the pair of pressure shafts 11, 12 so that a second load F2 (see FIG. 2) smaller than the first load F1 acts on the first workpiece W10 and the second workpiece W20 from the pair of pressure shafts 11, 12. In other words, the load control unit 31 constitutes a load removing means. The second load F2 may be zero. In other words, "removing load" means removing at least a part of the load acting on each of the workpieces W10, W20. At this time, the load control unit 31 may continue to apply the adjusted load from the pair of electrodes 21, 22 to the first workpiece W10 and the second workpiece W20.

[0024] Next, the current control unit 32 performs an operation to apply current to the first workpiece W10 and the second workpiece W20. Specifically, the load control unit 31 applies the second load F2 from the pair of pressure shafts 11, 12 to the first workpiece W10 and the second workpiece W20, and causes the contact surface 21a of the first electrode 21 to contact a portion of the first workpiece W10 surrounding a portion of the first workpiece W10 that is pressurized by the first pressure shaft 11, and causes the contact surface 22a of the second electrode 22 to contact a portion of the second workpiece W20 surrounding a portion of the second workpiece W20 that is pressurized by the second pressure shaft 12. In other words, the current control unit 32 constitutes a current supplying means. In addition, in FIG. 2 and FIG. 3, the flow of current during this operation is indicated by a dashed line.

[0025] Next, when the softening condition indicating that each of the protrusions W11, W21 has been softened is established, the load control unit 31 performs an operation of applying a third load larger than the second load F2 to each of the protrusions W11, W21 from the pair of pressure shafts 11, 12. In other words, the load control unit 31 constitutes a joining means.

[0026] The determination unit 34 determines whether or not the softening condition is satisfied, and sends the determination result to the load control unit 31. For example, the determination unit 34 receives the detection value of the sensor 40 from the acquisition unit 33, which acquires the detection value of the sensor 40, and determines that the softening condition is satisfied when the detection value becomes equal to or less than a threshold value.

[0027] Next, specific control contents of the control unit 30 will be described with reference to FIG.

[0028] First, the control unit 30 moves the pair of pressurizing shafts 11, 12 toward the first workpiece W10 and the second workpiece W20 (step S11).

[0029] Thereafter, the control unit 30 judges whether or not the detection value of the sensor 40 is equal to or greater than a reference value (step S12). The reference value is set to a value when the pair of pressure shafts 11, 12 contact the first workpiece W10 and the second workpiece W20.

[0030] If the detection value of the sensor 40 is less than the reference value, the control unit 30 returns to step S11, and if the detection value of the sensor 40 is equal to or greater than the reference value, the control unit 30 records the positions of the pair of pressure shafts 11, 12 at that time (step S13).

[0031] Next, the control unit 30 sets the end positions of the pair of pressure shafts 11, 12 (step S14). The end positions are set to positions that are a predetermined distance away from the positions where the pair of pressure shafts 11, 12 contact the first workpiece W10 and the second workpiece W20, in other words, positions where the protrusions W11, W21 are formed on the first workpiece W10 and the second workpiece W20.

[0032] Then, the control unit 30 starts pressing the first workpiece W10 and the second workpiece W20 by the pair of pressing shafts 11, 12 (step S15).

[0033] After that, the control unit 30 judges whether the pair of pressure applying shafts 11, 12 have reached the end position (step S16). When the pair of pressure applying shafts 11, 12 have reached the end position, a first load F1 is applied from the pair of pressure applying shafts 11, 12 to the first workpiece W10 and the second workpiece W20.

[0034] As a result, if the pair of pressure applying shafts 11, 12 have not reached the end position, the control unit 30 returns to step S15, and if the pair of pressure applying shafts 11, 12 have reached the end position (if a first load F1 is applied to the first workpiece W10 and the second workpiece W20 from the pair of pressure applying shafts 11, 12), the control unit 30 releases the pressure applying force from the pair of pressure applying shafts 11, 12 so that a second load F2 is applied to the first workpiece W10 and the second workpiece W20 (step S17).

[0035] As a result, the amount of elastic deformation of each workpiece W10, W20 is reduced, and the contact area between the protrusions W11, W21 is reduced because the contact areas between the protrusions W11, W21 are only at the tips (centers) of the protrusions W11, W21, as shown in Fig. 3. In Fig. 3, the outlines of the protrusions W11, W21 after unloading are shown by solid lines, and the outlines of the protrusions W11, W21 before unloading are shown by two-dot chain lines.

[0036] Next, the control unit 30 applies a second load F2 to the first workpiece W10 and the second workpiece W20 from the pair of pressure application shafts 11, 12, and also brings the contact surface 21a of the first electrode 21 into contact with the area surrounding the area of ​​the first workpiece W10 that is pressurized by the first pressure application shaft 11, and brings the contact surface 22a of the second electrode 22 into contact with the area surrounding the area of ​​the second workpiece W20 that is pressurized by the second pressure application shaft 12, and passes an electric current through the first workpiece W10 and the second workpiece W20 (step S18).

[0037] As a result, the central portions of the protrusions W11, W21 effectively generate heat, as indicated by hatched areas in Fig. 3. In step S18, the control unit 30 may detect the degree of unloading based on the detection value of the sensor 40.

[0038] Next, the control unit 30 determines whether or not the protrusions W11, W21 of the first workpiece W10 and the second workpiece W20 have been softened based on the detection value of the sensor 40 (step S19). Note that the control unit 30 determines whether or not the protrusions W11, W21 have been softened based on whether or not the detection value of the sensor 40 is equal to or less than the threshold value.

[0039] When the detection value of the sensor 40 is greater than the threshold value (when the protrusions W11, W21 are not softened), the control unit 30 returns to step S18 or continues with step S19. When the detection value of the sensor 40 is equal to or less than the threshold value (when the protrusions W11, W21 are softened), the control unit 30 presses the protrusions W11, W21 with the pair of pressure shafts 11, 12 so that a third load greater than the second load F2 acts on the protrusions W11, W21 from the pair of pressure shafts 11, 12 (step S20).

[0040] As a result, the softened regions of the protrusions W11, W21 (regions including the interface of the contact portion between the first workpiece W10 and the second workpiece W20) are plastically deformed, and are joined while remaining in a solid phase state.

[0041] Finally, the control unit 30 judges whether the pair of pressure shafts 11, 12 have reached the end position (step S21). As a result, if the pair of pressure shafts 11, 12 have not reached the end position, the control unit 30 returns to step S20, and if the pair of pressure shafts 11, 12 have reached the end position, the control unit 30 ends the control.

[0042] The resistance spot bonding apparatus 1 described above is capable of carrying out the following resistance spot bonding method. This resistance spot bonding method includes a projection forming step, a load removing step, a current passing step, and a bonding step.

[0043] The protrusion forming step is a step of forming the protrusions W11, W21 that come into contact with each other on the first work W10 and the second work W20 by applying a first load F1 to both sides of the first work W10 and the second work W20 in the thickness direction by a pair of pressure shafts 11, 12. This step corresponds to the above-mentioned steps S15 and S16.

[0044] The unloading process is a process for reducing the contact area between the protrusions W11 and W21 by unloading the pressure from the pair of pressure shafts 11 and 12 so that a second load F2 smaller than the first load F1 acts on the first workpiece W10 and the second workpiece W20 from the pair of pressure shafts 11 and 12 after the protrusion forming process. This process corresponds to step S17.

[0045] The current application step is a step of applying current to the first workpiece W10 and the second workpiece W20 in a state in which, after the unloading step, a second load F2 is acting on the first workpiece W10 and the second workpiece W20 from the pair of pressure application shafts 11, 12, and the contact surface 21a of the first electrode 21 is in contact with a portion of the first workpiece W10 surrounding a portion of the first workpiece W10 that is pressurized by the first pressure application shaft 11, and the contact surface 22a of the second electrode 22 is in contact with a portion of the second workpiece W20 surrounding a portion of the second workpiece W20 that is pressurized by the second pressure application shaft 12. This step corresponds to the above steps S18 and S19.

[0046] The joining step is a step of joining the protrusions W11, W21 to each other by applying pressure to the protrusions W11, W21 formed on the first workpiece W10 and the protrusions W21 formed on the second workpiece W20 from both sides in the thickness direction with a third load larger than the second load F2 by a pair of pressure shafts 11, 12 after the current passing step. This step corresponds to the above-mentioned steps S20 and S21.

[0047] As described above, in the resistance spot joining method of this embodiment, the unloading process is performed after the projections are formed, so that the contact area between the projections W11, W21 after the unloading process is smaller than the contact area between the projections W11, W21 before the unloading process. As a result, only the central parts of the projections W11, W21 become contact parts, that is, current-carrying areas, and the contact resistance of the contact parts also increases, so that the central parts effectively generate heat in the current-carrying process. Furthermore, if a plating layer is formed on the surfaces of the first workpiece W10 and the second workpiece W20, the plating layer is effectively removed in the current-carrying process. Then, in the joining process, the projections W11, W21 are joined while undergoing plastic deformation from their central parts to their peripheral parts. This improves the joining quality.

[0048] In the above embodiment, an example has been shown in which the first workpiece W10 and the second workpiece W20 are joined to each other, but the above resistance spot joining method can also be applied to the case of joining three or more workpieces to each other.

[0049] In the above embodiment, the detection value of the load cell (sensor 40) is used in both steps S13 and S18, but in step S18, the softening of each workpiece W10, W20 may be detected by a sensor other than the load cell. As such a sensor, a camera capable of photographing the contact portion between the protrusions W11, W12 may be used. In this case, the determination unit 34 determines whether or not the softening condition is satisfied based on the brightness of the contact portion acquired by the acquisition unit 33. Alternatively, a contact or non-contact thermometer may be used as such a sensor, and the determination unit 34 may determine whether or not the softening condition is satisfied based on the temperature of the contact portion acquired by the acquisition unit 33. Alternatively, the determination unit 34 may determine whether or not the softening condition is satisfied based on the result of the heat amount calculation of each workpiece W10, W20 acquired in advance and a timer of the power supply.

[0050] It will be appreciated by those skilled in the art that the exemplary embodiments described above are examples of the following aspects.

[0051] [Aspect 1] A method for joining a first workpiece made of a metal and a second workpiece made of a metal, comprising: a protrusion forming process of forming a protrusion on at least one of the first workpiece and the second workpiece by applying a first load to both sides in a thickness direction of the first workpiece and the second workpiece using a pressure shaft arranged on one side in the thickness direction and a pressure member arranged on the other side; an unloading process of unloading a pressure applied by the pressure shaft and the pressure member so that a second load smaller than the first load acts on the first workpiece and the second workpiece from the pressure shaft and the pressure member, thereby reducing a contact area between the first workpiece and the second workpiece; a current application process for applying a current to the first workpiece and the second workpiece while the second load is acting on the first workpiece and the second workpiece from the pressure shaft and the pressure member and electrodes are in contact with each of the first workpiece and the second workpiece; a joining process for joining the contact interface of the first workpiece and the second workpiece to each other by applying a third load greater than the second load using the pressure shaft and the pressure member to the first workpiece and the second workpiece from both sides in the thickness direction.

[0052] In this joining method, an unloading process is performed after the formation of the protrusion, so that the contact area between the workpieces after the unloading process is smaller than the contact area between the workpieces before the unloading process. As a result, only the center part of the protrusion is the contact part, i.e., the current-carrying area, so that the center part effectively generates heat in the current-carrying process. Then, in the joining process, the protrusion is joined while undergoing plastic deformation from the center part to the peripheral part. This improves the joining quality.

[0053] [Aspect 2] In the protrusion forming step, a pressure shaft that is paired with the pressure shaft is used as the pressure member to form the protrusions on each of the first workpiece and the second workpiece, 2. The resistance spot joining method according to claim 1, wherein in the joining step, the protrusion formed on the first workpiece and the protrusion formed on the second workpiece are joined to each other.

[0054] [Aspect 3] The resistance spot joining method according to aspect 1 or 2, wherein in the joining step, after the contact portions of the first workpiece and the second workpiece are softened, the contact interface of the first workpiece and the second workpiece are joined to each other.

[0055] In this embodiment, the joining quality is further improved.

[0056] [Aspect 4] A pair of pressure shafts that apply pressure to the first workpiece and the second workpiece from both sides in a thickness direction thereof; A pair of electrodes disposed around the pair of pressure shafts, respectively; a protrusion forming means for applying a first load to the first workpiece and the second workpiece from the pair of pressure shafts so that protrusions that come into contact with each other are formed on the first workpiece and the second workpiece; an unloading means for unloading a pressure force from the pair of pressure shafts so that a second load smaller than the first load acts on the first workpiece and the second workpiece from the pair of pressure shafts, thereby reducing a contact area between the protrusions; an electric current supplying means for applying the second load to the first workpiece and the second workpiece from the pair of pressure shafts and supplying electric current to the first workpiece and the second workpiece while the pair of electrodes are in contact with a portion of the first workpiece surrounding a portion of the first workpiece that is pressurized by one of the pair of pressure shafts and a portion of the second workpiece surrounding a portion of the second workpiece that is pressurized by the other of the pair of pressure shafts; and joining means for applying a third load greater than the second load to each of the protrusions from the pair of pressure shafts when a softening condition indicating that the protrusions have been softened is satisfied.

[0057] [Aspect 5] The resistance spot joining device according to aspect 4, further comprising a sensor capable of detecting a load acting on the first workpiece and the second workpiece from the pair of pressure shafts, and a determination means for determining that the softening condition is satisfied when the detection value of the sensor becomes equal to or lower than a threshold value.

[0058] It should be noted that the embodiments disclosed herein are illustrative in all respects and should not be considered as limiting. The scope of the present invention is indicated by the claims, not by the description of the embodiments above, and further includes all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0059] 1 Resistance spot joining device, 11 First pressure shaft, 11a Pressure surface, 12 Pressure member (second pressure shaft), 12a Pressure surface, 21 First electrode, 21a Contact surface, 22 Second electrode, 22a Contact surface, 30 Control unit, 31 Load control unit, 32 Current flow control unit, 33 Acquisition unit, 34 Determination unit, 40 Sensor, F1 First load, F2 Second load, W10 First workpiece, W11 Protrusion, W20 Second workpiece, W21 Protrusion.

Claims

1. A method for joining a first workpiece made of a metal and a second workpiece made of a metal, comprising: a protrusion forming process of forming a protrusion on at least one of the first workpiece and the second workpiece by applying a first load to both sides in a thickness direction of the first workpiece and the second workpiece using a pressure shaft arranged on one side in the thickness direction and a pressure member arranged on the other side; an unloading process of unloading a pressure applied by the pressure shaft and the pressure member so that a second load smaller than the first load acts on the first workpiece and the second workpiece from the pressure shaft and the pressure member, thereby reducing a contact area between the first workpiece and the second workpiece; a current application process for applying a current to the first workpiece and the second workpiece while the second load is acting on the first workpiece and the second workpiece from the pressure shaft and the pressure member and electrodes are in contact with the first workpiece and the second workpiece, respectively; a joining process for joining the contact interface of the first workpiece and the second workpiece to each other by applying a third load greater than the second load to the first workpiece and the second workpiece from both sides in the thickness direction using the pressure shaft and the pressure member.

2. In the protrusion forming step, a pressure shaft that is paired with the pressure shaft is used as the pressure member to form the protrusions on each of the first workpiece and the second workpiece, The resistance spot joining method according to claim 1 , wherein in the joining step, the protrusion formed on the first workpiece and the protrusion formed on the second workpiece are joined to each other.

3. 3 . The resistance spot joining method according to claim 1 , wherein in the joining step, an interface of the contact portions of the first workpiece and the second workpiece is joined to each other after the contact portions of the first workpiece and the second workpiece are softened.

4. A pair of pressure shafts that pressurize the first workpiece and the second workpiece from both sides in a thickness direction thereof; A pair of electrodes disposed around the pair of pressure shafts, respectively; a protrusion forming means for applying a first load to the first workpiece and the second workpiece from the pair of pressure shafts so that protrusions that come into contact with each other are formed on the first workpiece and the second workpiece; an unloading means for unloading a pressure applied by the pair of pressure shafts so that a second load smaller than the first load acts on the first workpiece and the second workpiece from the pair of pressure shafts, thereby reducing a contact area between the protrusions; an electric current supplying means for applying the second load to the first workpiece and the second workpiece from the pair of pressure shafts and supplying electric current to the first workpiece and the second workpiece while the pair of electrodes are in contact with a portion of the first workpiece surrounding a portion of the first workpiece that is pressurized by one of the pair of pressure shafts and a portion of the second workpiece surrounding a portion of the second workpiece that is pressurized by the other of the pair of pressure shafts; and joining means for applying a third load, which is greater than the second load, to each of the protrusions from the pair of pressure shafts when a softening condition indicating that the protrusions have been softened is satisfied.

5. 5. The resistance spot joining device according to claim 4, further comprising: a sensor capable of detecting a load acting on the first workpiece and the second workpiece from the pair of pressure shafts; and a determination means for determining that the softening condition is satisfied when the detection value of the sensor becomes equal to or lower than a threshold value.

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