Friction stir spot welding method and friction stir spot welding device

Abstract

A friction stir spot welding method comprises: press fitting a shoulder (12) to a prescribed first press-fitting position of an overlapping part (30) while retracting a pin (11) from the overlapping part (30), thereby joining a first member (31) and a second member (32) so that a coating film of the first member (31) is unevenly distributed at a specific position; retracting the shoulder (12) from the first press-fitting position while bringing the pin (11) close to the overlapping part (30); and press-fitting the shoulder (12) to a prescribed second press-fitting position of the overlapping part (30) while retracting the pin (11) from the overlapping part (30), thereby dispersing the coating film unevenly distributed at the specific position.

Description

Friction stir spot welding method and friction stir spot welding device

[0001] The present disclosure relates to a friction stir spot welding method and a friction stir spot welding apparatus for joining a plurality of members.

[0002] A joined body formed by joining a plurality of members may be used as a component of a structure such as an aircraft, a railway vehicle, or an automobile. One known method for such joining is friction stir spot joining.

[0003] For example, as disclosed in Patent Document 1, a double-action friction stir spot welding device is known that is provided with a pin member and a shoulder member that can rotate independently of each other about a predetermined axis and can move back and forth in the axial direction, and a clamp member that surrounds the outer periphery of the shoulder member.

[0004] Japanese Patent Application Laid-Open No. 2006-320924

[0005] In the above-described friction stir spot joining method, there are cases where a coating film is formed on at least one joining surface of a pair of workpieces to be joined. When a pair of workpieces having a coating film formed on at least one of them is friction stir spot joined by the technique described in Patent Document 1, there is a risk that the coating film will remain unevenly in the friction stirred region, resulting in a decrease in the joining strength.

[0006] The object of the present disclosure is to provide a friction stir spot welding method and a friction stir spot welding apparatus that can join multiple workpieces, at least one of which has a coating film formed on it, while preventing a decrease in joint strength.

[0007] A friction stir spot joining method according to one aspect of the present disclosure is a method for joining a first member and a second member, at least one of which has a coating film formed thereon, which are made of different materials, by softening them with frictional heat. The friction stir spot welding method includes: preparing a friction stir spot welding apparatus having a pin and a cylindrical shoulder including a hollow portion into which the pin is inserted; forming an overlapping portion where the first member and the second member overlap, and arranging the pin and the shoulder opposite the overlapping portion; retracting the pin from the overlapping portion while pressing the shoulder into a predetermined first press-fit position of the overlapping portion, thereby forming a raised anchor of the second member inside the shoulder, and distributing the coating film at a specific position, thereby joining the first member and the second member; retracting the shoulder from the first press-fit position while moving the pin toward the overlapping portion; retracting the shoulder from the first press-fit position while moving the pin toward the overlapping portion;

[0008] Furthermore, a friction stir spot joining method according to another aspect of the present disclosure is a friction stir spot joining method in which a first member and a second member, at least one of which has a coating film formed thereon, are made of different materials and are softened by frictional heat to join them. The joining method includes preparing a friction stir spot welding apparatus having a pin and a cylindrical shoulder including a hollow portion into which the pin is inserted; forming an overlapping portion where the first member and the second member overlap and arranging the pin and the shoulder facing the overlapping portion; retracting the pin from the overlapping portion while pressing the shoulder into a predetermined first press-fit position of the overlapping portion, thereby forming an anchor where the second member protrudes inside the shoulder, and joining the first member and the second member; retracting the shoulder from the first press-fit position while moving the pin toward the overlapping portion; retracting the pin from the overlapping portion while pressing the shoulder into a predetermined second press-fit position of the overlapping portion; retracting the shoulder from the second press-fit position while moving the pin toward the overlapping portion; and setting the first press-fit position to a position deeper than the surface of the second member.

[0009] According to another aspect of the present disclosure, there is provided a friction stir spot welding apparatus capable of joining a first member and a second member, at least one of which has a coating formed thereon, by softening the first member and the second member by frictional heat. The friction stir spot welding apparatus includes a joining tool having a pin and a cylindrical shoulder including a hollow portion into which the pin is inserted, a rotation mechanism capable of rotating the joining tool, a movement mechanism capable of independently moving the pin and the shoulder in a direction in which the pin extends, and a control unit that controls the rotation mechanism and the movement mechanism to join the first member and the second member while the pin and the shoulder are positioned opposite the overlapping portion where the first member and the second member overlap. The control unit controls the rotation mechanism and the movement mechanism to retract the pin from the overlapping portion while pressing the shoulder into a predetermined first press-fit position of the overlapping portion, thereby forming a raised anchor of the second member inside the shoulder, distributing the coating film at a specific position and joining the first member and the second member, retract the shoulder from the first press-fit position while moving the pin closer to the overlapping portion, and then retract the pin from the overlapping portion while pressing the shoulder into a predetermined second press-fit position of the overlapping portion, thereby dispersing the coating film that has been distributed at the specific position, and retract the shoulder from the second press-fit position while moving the pin closer to the overlapping portion.

[0010] FIG. 1 is a schematic diagram showing the configuration of a friction stir spot welding apparatus according to an embodiment of the present disclosure. FIG. 2 is a diagram showing a portion of a process using a friction stir spot welding method according to an embodiment of the present disclosure. FIG. 3 is a diagram showing a portion of a process using a friction stir spot welding method according to an embodiment of the present disclosure. FIG. 4 is a diagram showing a process chart of a friction stir spot welding method according to an embodiment of the present disclosure. FIG. 5 is a schematic cross-sectional view for explaining a welded portion of a welded body. FIG. 6 is a schematic cross-sectional view showing the distribution of a coating film when no anchor is formed in the welded portion or when the anchor is small. FIG. 7 is a schematic cross-sectional view showing the distribution of a coating film when an anchor is formed in the welded portion. FIG. 8 is a diagram showing a process using a friction stir spot welding method according to a modified embodiment of the present disclosure. FIG. 9 is a graph showing the tensile shear strength in an example and a comparative example of the present disclosure. FIG. 10 is a schematic perspective view showing how the tensile shear strength shown in FIG. 9 is measured. FIG. 11 is a cross-sectional photograph of a welded portion of a comparative example related to the present disclosure. FIG. 12 is an enlarged cross-sectional photograph of a portion of FIG. 11. FIG. 13 is a cross-sectional photograph of a welded portion of an example of the present disclosure. FIG. 14 is an enlarged cross-sectional photograph of a portion of FIG. 13 . FIG. 15 is a cross-sectional photograph of a joint in a comparative example related to the present disclosure. FIG. 16 is an enlarged cross-sectional photograph of the bottom surface of an aluminum column in a joint in a comparative example related to the present disclosure. FIG. 17 is an enlarged cross-sectional photograph of the bottom surface of the aluminum column in FIG. 15 . FIG. 18 is a cross-sectional photograph of a joint in an example of the present disclosure. FIG. 19 is an enlarged photograph of the bottom surface of an aluminum column in a joint in an example of the present disclosure. FIG. 20 is an enlarged cross-sectional photograph of the interface between the first member and the second member in FIG. 18 . FIG. 21 is a schematic diagram for explaining the position of the joint interface caused by the shoulder. FIG. 22 is a schematic diagram for explaining the position of the joint interface caused by the shoulder. FIG. 23 is a schematic diagram for explaining the position of the joint interface caused by the shoulder. FIG. 24 is a graph showing the transition of the press-fit amount of the shoulder during friction stir spot welding. FIG. 25 is a graph showing the cross tensile strength in an example of the present disclosure and a comparative example. Fig. 26 is a cross-sectional photograph of a welded portion of a reference example of the present disclosure. Fig. 27 is an enlarged cross-sectional photograph of a portion of Fig. 26. Fig. 28 is a graph showing the transition of the press-in amount of the shoulder during friction stir spot welding. Fig. 29 is a cross-sectional photograph of a welded portion of an example of the present disclosure.Fig. 30 is an enlarged cross-sectional photograph of a part of Fig. 29. Fig. 31 is an enlarged cross-sectional photograph of a part of Fig. 29.

[0011] The present disclosure will be described in detail below with reference to the drawings. The friction stir spot welding method according to the present disclosure can be applied to the manufacture of various joined bodies formed by spot-joining two or more overlapping structural materials, such as metal plates, frames, exterior materials, or columnar materials. The joined bodies thus manufactured are components of structures such as aircraft, railcars, and automobiles.

[0012] [Configuration of Friction Stir Spot Welding Apparatus] FIG. 1 is a schematic diagram showing the configuration of a friction stir spot welding apparatus M according to one embodiment of the present disclosure.

[0013] The friction stir spot welding apparatus M joins an upper plate 31 and a lower plate 32 by softening them with frictional heat. The friction stir spot welding apparatus M includes a tool 1 for friction stir spot welding, a tool drive unit 2 that drives the tool 1 to rotate and move up and down, and a controller C that controls the operation of the tool drive unit 2. Note that although the directions "up" and "down" are indicated in Fig. 1 for the convenience of explanation, this is not intended to limit the actual direction in which the tool 1 is used.

[0014] The tool 1 is supported by a tool fixing portion. The tool fixing portion can be, for example, the tip of an articulated robot. A backup 15 is arranged facing the lower end surface of the tool 1. At least two metal members to be joined are arranged between the tool 1 and the backup 15. FIG. 1 shows an example in which an overlapping portion 30, in which a portion of an upper plate 31 made of a flat plate and a portion of a lower plate 32 also made of a flat plate overlap in the vertical direction, is arranged between the tool 1 and the backup 15. Note that the tool 1 corresponds to the welding tool of the present disclosure.

[0015] The tool 1 includes a pin 11 having a central axis extending along a predetermined axial direction, a shoulder 12, a clamp 13, and a spring 14. The pin 11 is formed in a cylindrical shape and is disposed so that its central axis extends in the vertical direction. The pin 11 is rotatable about the central axis as a rotation axis R, and is also movable up and down, i.e., forward and backward, along the rotation axis R. When the tool 1 is in use, the rotation axis R is aligned with a spot joining position W at the overlapping portion 30. The tool 1 is a double-acting tool in which the pin 11 and the shoulder 12 move independently.

[0016] The shoulder 12 is a cylindrical member having a hollow portion into which the pin 11 is inserted. The axis of the shoulder 12 is coaxial with the axis of the pin 11, which is the rotation axis R. The shoulder 12 rotates around the rotation axis R and moves up and down along the rotation axis R, that is, moves forward and backward.

[0017] The shoulder 12 and the pin 11 inserted in the hollow portion rotate together around the rotation axis R, while moving relatively in the direction of the rotation axis R. That is, the pin 11 and the shoulder 12 not only move up and down simultaneously along the rotation axis R, but also can move independently, with one moving down and the other moving up.

[0018] The clamp 13 is a cylindrical member having a hollow portion into which the shoulder 12 is inserted. The axis of the clamp 13 is also coaxial with the rotation axis R. The clamp 13 does not rotate around its axis, but moves up and down along the rotation axis R, that is, moves forward and backward. The clamp 13 serves to surround the outer periphery of the pin 11 or the shoulder 12 when they perform friction stirring. The enclosure by the clamp 13 prevents the friction stir material from scattering, and enables the friction stir spot welded portion to be finished smoothly.

[0019] The spring 14 is attached to the upper end side of the clamp 13 and biases the clamp 13 downward, that is, toward the overlapping portion 30. The clamp 13 is attached to the tool fixing portion via the spring 14. As shown in FIG. 1 , the backup 15 has a support surface 15A that supports the underside of the overlapping portion 30, which is the object to be joined. The backup 15 is a backing member that supports the overlapping portion 30 when the pin 11 or shoulder 12 is press-fitted into the overlapping portion 30. The clamp 13, biased by the spring 14, presses the overlapping portion 30 against the backup 15.

[0020] The tool driving unit 2 includes a rotation driving unit 21 and an elevation driving unit 22. The rotation driving unit 21 includes a motor, a drive gear, etc., and drives the pin 11 and the shoulder 12 to rotate about the rotation axis R. The rotation driving unit 21 corresponds to the rotation mechanism of the present disclosure, which rotates the tool 1. The elevation driving unit 22 is a mechanism for moving the pin 11, the shoulder 12, and the clamp 13 forward and backward, i.e., raising and lowering, along the rotation axis R. The elevation driving unit 22 drives the pin 11 so as to press the pin 11 into the overlapping portion 30 and retract it from the overlapping portion 30. The elevation driving unit 22 also moves the shoulder 12 forward and backward along the rotation axis R, thereby pressing the shoulder 12 into and retracting the overlapping portion 30. The elevation driving unit 22 corresponds to the movement mechanism of the present disclosure, which can independently move the pin 11 and the shoulder 12 in the axial direction along which the pin 11 extends. The lifting drive unit 22 also moves the clamp 13 toward the overlapping portion 30, pressing the overlapping portion 30 against the backup 15. At this time, the biasing force of the spring 14 acts. In other words, in this embodiment, the lifting drive unit 22 is made up of three shafts: a first lifting shaft that drives the pin 11, shoulder 12, and clamp 13 together, a second lifting shaft that drives the pin 11 independently, and a central rotation shaft.

[0021] The controller C is comprised of a microcomputer or the like, and executes a predetermined control program to control the operation of each part of the tool driving unit 2. Specifically, the controller C controls the rotation driving unit 21 to cause the pin 11 and the shoulder 12 to perform a required rotational movement. The controller C also controls the elevation driving unit 22 to cause the pin 11, the shoulder 12, and the clamp 13 to perform a required forward / backward movement movement. The controller C corresponds to the control unit in this disclosure, and sets the rotation, movement amount, movement position, pressure force, etc. of each member described below.

[0022] The upper plate 31 is made of metal and has an upper plate surface 31A and an upper plate back surface 31B as shown in FIG. 1 . The upper plate surface 31A is the surface of the upper plate 31 and corresponds to the upper surface of the upper plate 31 in FIG. 1 . Similarly, the upper plate back surface 31B is the back surface of the upper plate 31 and corresponds to the lower surface of the upper plate 31 in FIG. 1 . The upper plate 31 corresponds to the first member in the present disclosure. The upper plate surface 31A corresponds to the first surface in the present disclosure, and the upper plate back surface 31B corresponds to the first back surface in the present disclosure. Also, as an example, the upper plate 31 is made of an aluminum alloy (aluminum). Note that the upper plate 31 may be a wrought material or a cast metal. Furthermore, the upper plate 31 is subjected to electrocoating. That is, a coating film is formed on the upper plate surface 31A and the upper plate back surface 31B of the upper plate 31. Note that in other embodiments, the upper plate 31 may be subjected to a chemical conversion treatment or the like. Also, aluminum in the present disclosure includes an aluminum alloy.

[0023] The lower plate 32 is made of metal and has a lower plate front surface 32A and a lower plate back surface 32B, as shown in FIG. 1 . The lower plate front surface 32A is the front surface of the lower plate 32 and corresponds to the upper surface of the lower plate 32 in FIG. 1 . Similarly, the lower plate back surface 32B is the back surface of the lower plate 32 and corresponds to the lower surface of the lower plate 32 in FIG. 1 . The lower plate 32 corresponds to the second member in the present disclosure. The lower plate front surface 32A corresponds to the second front surface in the present disclosure, and the lower plate back surface 32B corresponds to the second back surface in the present disclosure. As an example, the lower plate 32 is made of high-tensile steel. That is, the upper plate 31 and the lower plate 32 are made of different metals. The lower plate 32 may also be made of aluminum alloy, mild steel, or hot-stamp steel. The lower plate 32 may also be subjected to a non-plating treatment, a zinc-plating treatment, an Al-Si plating treatment, a chemical conversion treatment, an electrodeposition coating, or the like. In addition, an adhesive or a sealing material may be interposed between the upper plate 31 and the lower plate 32.

[0024] In this embodiment, examples of combinations in which the upper plate 31 and the lower plate 32 are made of different materials include, in this order, the following: (Example 1) Al and Fe, (Example 2) Resin and Fe, (Example 3) Al and Mg, (Example 4) Resin and Mg, and (Example 5) Resin and Al. In such combinations, the hardness of the lower plate 32 is higher than the hardness of the upper plate 31, and therefore the anchor 4B described below can be stably formed.

[0025] [Method of Using the Tool] Next, a method of using the tool 1 exemplified in this embodiment, i.e., a friction stir spot welding method, will be described. Methods of using the friction stir spot welding apparatus M can be roughly divided into a pin-first process in which the pin 11 of the tool 1 is first pressed into the overlapping portion 30 of the joining members, and a shoulder-first process in which the shoulder 12 is first pressed into the overlapping portion 30 of the joining members. Of these, the shoulder-first process is adopted in this embodiment.

[0026] 2 and 3 are diagrams showing a part of a process using the friction stir spot welding method by the shoulder-first process, where the process in Fig. 3 is carried out after the process in Fig. 2.

[0027] 4 is a process chart of the friction stir spot welding method according to this embodiment. Figures 2 to 4 show the process of friction stir spot welding of an overlapping portion 30 of an upper plate 31 and a lower plate 32.

[0028] 4, an overlapping portion 30 is formed by an upper plate 31 and a lower plate 32. In the overlapping portion 30, at least a portion of a lower plate surface 32A abuts against a lower plate back surface 31B, and the upper plate 31 and the lower plate 32 are arranged to overlap each other.

[0029] Next, as shown in process P11 of FIG. 2 and step S2 of FIG. 4, the tool 1 is placed at a predetermined position and rotated. Specifically, the tool 1 is placed facing the overlapping portion 30 so that the rotation axis R of the tool 1 is parallel to the overlapping direction of the upper plate 31 and the lower plate 32, i.e., the vertical direction. Note that this facing arrangement may also be such that the rotation axis R of the tool 1 is inclined relative to the vertical direction. In this embodiment, with the rotation axis R aligned with a predetermined spot joining position W as shown in FIG. 1, the lower end surface of the tool 1 abuts against the upper plate surface 31A of the upper plate 31. At this time, the clamp 13 presses the overlapping portion 30 against the backup 15 with the biasing force of the spring 14 of FIG. 1. With the lower end surface of the tool 1 abutting against the upper plate surface 31A of the upper plate 31, the controller C controls the rotation drive unit 21 of FIG. 1 to rotate the pin 11 and the shoulder 12 at a predetermined number of rotations around the rotation axis R.

[0030] This rotation causes friction to preheat the area of ​​the overlapping portion 30 where the pin 11 and the shoulder 12 are in contact with each other. That is, process P11 in FIG. 2 and step S2 in FIG.

[0031] Next, as shown in process P12 of FIG. 2 and step S3 of FIG. 4, a press-fitting process is performed in which the pin 11 retracts from the overlapping portion 30 and the shoulder 12 is press-fitted into the overlapping portion 30. In this process, the controller C controls the lifting drive unit 22 to lower the shoulder 12 as shown in process P12 of FIG. 2 while maintaining the rotation of the tool 1, and also controls the lifting drive unit 22 to raise, or retract, the pin 11. Note that the clamp 13 remains stationary. The shoulder 12 is press-fitted into the overlapping portion 30 from the upper plate surface 31A to a first press-fitting position deeper than the lower plate surface 32A. Note that the lower plate surface 32A in this case corresponds to the surface of the second member in this disclosure and is the upper metal surface of the lower plate 32. Even if the lower plate 32 has a coating, the surface of the second member refers to the surface of the metal portion.

[0032] Furthermore, the press-fitting operation described above agitates the material in the press-fit region of the shoulder 12, as indicated by arrow b1 in FIG. 2 , and the overflowing material OF that overflows from the overlapping portion 30 due to the press-fitting is released into the hollow space in the shoulder 12 created by the retraction of the pin 11. As a result, a base portion of the anchor 4B shown in FIG. 5 , which will be described in detail later, is formed. Note that in the press-fitting process, a pressurizing force along the axial direction is applied to the entire pin 11, shoulder 12, and clamp 13, i.e., a pressurizing force. Meanwhile, a pressurizing force is applied to the clamp 13, pressing the clamp 13 toward the backup 15. As a result, the pressurizing force with which the tool 1 is pressed toward the welded body 3 corresponds to the force obtained by subtracting the clamping force from the press-fitting force.

[0033] Next, as shown in process P13 in FIG. 2 and step S4 in FIG. 4 , while maintaining rotation of the tool 1, the controller C controls the lift drive unit 22 to retract the shoulder 12 from the first press-fit position and also controls the lift drive unit 22 to lower the pin 11 so that it enters the overlapping portion 30. This step represents a backfilling step of the overflowed material OF. In this step, the controller C controls the lift drive unit 22 to raise the shoulder 12, while also controlling the lift drive unit 22 to lower the pin 11. By the lowering of the pin 11, the overflowed material OF that has escaped into the hollow space is backfilled into the press-fit region of the shoulder 12, as shown by arrow b2 in FIG. 2 .

[0034] Next, as shown in process P14 in Fig. 2 and step S5 in Fig. 4, a leveling process is performed to smooth the overlapping portion 30. In this process, the controller C controls the lifting drive unit 22 to rotate the lower end surfaces of the pin 11 and shoulder 12 while positioning them slightly lower than the upper plate surface 31A of the upper plate 31, thereby smoothing the spot-joined portion. Thereafter, as shown in process P15 in Fig. 2 and step S6 in Fig. 4, the tool 1 is separated from the overlapping portion 30.

[0035] Next, as shown in process P21 in Fig. 3, the lower end surface of the tool 1 is again brought into contact with the upper plate surface 31A of the upper plate 31. Then, the controller C controls the rotation drive unit 21 in Fig. 1 to rotate the pin 11 and the shoulder 12 around the rotation axis R at a predetermined number of rotations. Due to this rotation, the region in the overlapping portion 30 where the pin 11 and the shoulder 12 are in contact is preheated by friction, as shown in step S7 in Fig. 4. That is, process P21 in Fig. 3 represents a preheating step for the overlapping portion 30, similar to process P11 in Fig. 2.

[0036] Next, as shown in process P22 of Fig. 3 and step S8 of Fig. 4, the pin 11 is retracted from the overlapping portion 30, and the press-fitting step is performed again to press-fit the shoulder 12 into the overlapping portion 30. This step is generally similar to process P12 of Fig. 2. The shoulder 12 is press-fitted into the overlapping portion 30 from the upper plate surface 31A to a predetermined second press-fit position.

[0037] Next, as shown in process P23 in Fig. 3 and step S9 in Fig. 4, while maintaining the rotation of the tool 1, the controller C controls the lifting drive unit 22 to retract the shoulder 12 from the second press-fit position, and also controls the lifting drive unit 22 to lower the pin 11 so that it enters the overlapping portion 30. This step is generally similar to process P13 in Fig. 2.

[0038] Next, as shown in process P24 of FIG. 3 and step S10 of FIG. 4, a leveling process is performed again to smooth the overlapping portion 30. In this process, the controller C controls the lifting drive unit 22 to rotate the lower end surfaces of the pin 11 and the shoulder 12 while positioning them slightly lower than the upper plate surface 31A of the upper plate 31, thereby smoothing the spot welded portion. As a result, an indent 4T (see FIG. 5), which will be described in detail later, is formed. Note that the indent 4T does not necessarily have to be formed. Thereafter, as shown in process P25 of FIG. 3 and step S11 of FIG. 4, the tool 1 is separated from the overlapping portion 30. Through the above process, the upper plate 31 and the lower plate 32 are joined together to form the stir welded portion 4 (see FIG. 1). The stir welded portion 4 corresponds to the welded portion of the present disclosure.

[0039] [Cross-sectional structure of the welded body] Figure 5 is a schematic cross-sectional view illustrating a stir-friction welded portion 4 of a welded body 3 formed by the above-described friction stir spot welding method. The stir-friction welded portion 4 has a weld bottom surface 4A, an anchor 4B, and an indent 4T. Note that Figure 5 shows thicknesses T1 and T2 of the upper plate 31 and lower plate 32.

[0040] The weld bottom surface 4A is the weld interface between the upper plate 31 and the lower plate 32, which is formed at a position deeper than the lower plate surface 32A relative to the upper plate surface 31A when pressed against the shoulder 12. When viewed along the rotation axis R, the weld bottom surface 4A has a ring shape corresponding to the shoulder lower surface 12S of the shoulder 12. Note that this ring shape is not limited to two perfect circles, and may be an irregular shape depending on the conditions of the friction stir spot welding. In other words, the weld bottom surface 4A has a substantially ring shape in a plan view.

[0041] The anchor 4B is a raised portion of the lower plate 32, located radially inward of the joint bottom surface 4A, raised to a position higher than the lower plate surface 32A relative to the joint bottom surface 4A. The anchor 4B is formed when the overflow material OF overflows from the overlapping portion 30 into the hollow space of the shoulder 12 during the press-fitting process, and the upper end of the overflow material OF is crushed by the pin 11 during the backfilling process. As shown in Figure 5, the outer edge of the tip of the anchor 4B tends to become claw-like. Note that the height of the raised anchor 4B increases as the amount by which the shoulder 12 is press-fitted into the lower plate 32 increases.

[0042] The indent 4T is disposed opposite the joint bottom surface 4A and the anchor 4B in the overlapping direction of the upper plate 31 and the lower plate 32, i.e., in the vertical direction, and corresponds to a recessed portion in the upper plate surface 31A of the upper plate 31. As described above, the indent 4T can be actively formed in the leveling process by rotating the lower end surface of the pin 11 and the lower end surface of the shoulder 12 while positioning them lower than the upper plate surface 31A of the upper plate 31. However, in the leveling process, the lower end surface of the pin 11 and the lower end surface of the shoulder 12 may be positioned flush with the upper plate surface 31A of the upper plate 31. Also, as described above, the indent 4T does not have to be formed.

[0043] [Regarding the Distribution of Coating Film in the Joint] As described above, in this embodiment, processes P11 to P15 in FIG. 2 are performed on one joint, followed by processes P21 to P25 in FIG. 3 . Conventionally, only processes similar to processes P11 to P15 in FIG. 2 have been performed on one joint. However, the present inventors have discovered a new problem with this type of friction stir spot welding method. Specifically, when a coating film is formed on at least one of the upper plate 31 and the lower plate 32, the conventional friction stir spot welding method described above has a problem in that the coating film remaining inside the joint significantly reduces the joint strength. The present inventors have then discovered a new friction stir spot welding method that can favorably distribute the coating film inside the joint and prevent a reduction in joint strength.

[0044] Specifically, in the friction stir spot welding method according to this embodiment, two welding processes are performed on one weld. In the first welding process Q1, the upper plate 31 and the lower plate 32 are welded so that the coating film is unevenly distributed at a specific position within the friction stir weld 4. The series of processes shown in FIG. 2 corresponds to this first welding process Q1. On the other hand, in the second welding process Q2, the coating film unevenly distributed at the specific position is dispersed. The series of processes shown in FIG. 3 corresponds to this second welding process Q2. The discloser further found that in this method of dispersing the coating film, the formation of anchor 4B shown in FIG. 5 is particularly effective.

[0045] FIG. 6 is a schematic cross-sectional view showing the coating film distribution when no anchors or small anchors are formed in the stir weld 4. FIG. 7 is a schematic cross-sectional view showing the coating film distribution when anchors are formed in the stir weld 4. FIGS. 6 and 7 correspond to processes P12, P13, and P14 in FIG. 2, respectively. The pin 11 and shoulder 12 are not shown. The following description will be given assuming that the upper plate 31 is made of aluminum and the lower plate 32 is made of steel, and that a coating film is formed on the surface of the upper plate 31, i.e., the upper plate front surface 31A and the upper plate back surface 31B shown in FIGS. 1 and 5. As an example, the coating film is formed by cathodic electrodeposition coating, which is formed for corrosion prevention, for example.

[0046] As shown in FIG. 6, when the press-fit amount of the shoulder 12 is set small in the first joining process Q1, the anchor 4B in FIG. 5, that is, the aluminum protrusion, is unlikely to be formed.

[0047] In this case, the pressure acting near the interface between the upper plate 31 and the lower plate 32 is relatively small, so the coating film tends to aggregate at the interface and in the center. In this state, when the pin 11 descends and the shoulder 12 rises in process P13, the overflowing material OF spreads the coating film, and as a result, the coating film returns to its original position, i.e., near the interface. As a result, the coating film is distributed in the same area even at the end of process P14.

[0048] On the other hand, in the first joining process Q1, if the shoulder 12 is pressed into the lower plate 32 by a larger amount than in FIG. 6, the aluminum bulges and an anchor 4B is formed.

[0049] In this way, by setting the first press-fit position of the shoulder 12 in the first joining process Q1 at a position deeper than the lower plate surface 32A of the lower plate 32, the anchor 4B can be stably formed. In this case, the pressing force of the shoulder 12 acts strongly near the interface between the upper plate 31 and the lower plate 32, so the coating film tends to aggregate inside the anchor 4B. In this state, when the pin 11 descends and the shoulder 12 rises in process P13, the overflowing material OF flows below the shoulder 12. At this time, because the coating film is held by the anchor 4B, the coating film spread by the overflowing material OF is distributed so as to spread from the upper end of the anchor to the shoulder path, i.e., the area passed by the shoulder 12, as shown in process P14 in Figure 7.

[0050] On the other hand, in process P12, the shoulder 12 is pressed into the lower plate 32 to a position deeper than the lower plate surface 32A thereof, so that the interface between the upper plate 31 and the lower plate 32 at the shoulder portion on the outside of the anchor is in a state of good adhesion without being affected by the coating film.

[0051] 7, when the second joining process Q2 is performed in a state where the coating is distributed and unevenly located at a position higher than the interface between the upper plate 31 and the lower plate 32, the coating can be stirred and efficiently dispersed as the shoulder 12 rotates when the shoulder 12 is pressed into the second press-fit position. As a result, the coating is prevented from concentrating in a part of the stir-jointed portion 4, and the joined body 3 can be prevented from breaking from this coating part.

[0052] [Modified Embodiment] FIG. 8 is a diagram showing a portion of a process using a friction stir spot welding method according to a modified embodiment of the present disclosure. In the above embodiment, as shown in FIGS. 2 and 3 , the clamp 13 of the tool 1 is separated from the upper plate 31 between the first welding process Q1 and the second welding process Q2. However, the present disclosure is not limited to this. In this modified embodiment, a series of processes are performed without the clamp 13 of the tool 1 separating from the upper plate 31. Specifically, processes P31 to P34 in FIG. 8 correspond to the first welding process Q1. In process P31, the preheating step is performed. In processes P32 and P33, the pin 11 is raised while the shoulder 12 is pressed into the first press-fit position. Thereafter, in process P34, the pin 11 is lowered and the shoulder 12 is raised. Thereafter, the second welding process Q2 is performed continuously. Specifically, in process P35, the pin 11 is raised while the shoulder 12 is pressed into the second press-fit position. Thereafter, in process P36, the pin 11 is lowered while the shoulder 12 is raised, and the backfilling step is performed. Thereafter, in process P37, the leveling step is performed, and then the tool 1 is separated from the upper plate 31.

[0053] Even in this joining method, the coating film can be unevenly distributed at a specific location in the first joining process Q1 and then dispersed in the second joining process Q2. This prevents the coating film from concentrating in a specific portion of the stir welded portion 4, thereby preventing the joined body 3 from fracture originating from the coating film. Furthermore, in this modified embodiment, the tool 1 does not move away from the upper plate 31 between the first joining process Q1 and the second joining process Q2. This prevents the rotation axis R of the tool 1 from shifting between the two joining processes, preventing the shoulder 12 from contacting the anchor 4B, as described below. Therefore, the strength of the joined body 3 can be stably maintained after joining. Even in the joining method according to the above embodiment, the same strength can be maintained by accurately reproducing the position of the tool 1 between the first joining process Q1 and the second joining process Q2.

[0054] As described above, according to each embodiment of the present disclosure, the anchor 4B is formed inside the shoulder 12 in the first joining process Q1 to unevenly distribute the coating at a specific position, and then the coating is dispersed in the second joining process Q2. This allows the members having the coating to be stably joined and maintain their strength. Therefore, after joining the upper plate 31 and the lower plate 32, it is not necessary to form a corrosion-preventing coating on the joined body 3, and multiple members can be joined using materials on which a coating has already been formed. Furthermore, as described in detail below, the strength of the stir-fry welded portion 4 can be improved by approximately 1.3 times compared to when members having a coating are joined using only a conventional single joining process.

[0055] In particular, by setting the first press-fitting position at a position deeper than the surface of the lower plate 32, the anchor 4B can be stably formed and the coating film can be raised above the bonding interface between the upper plate 31 and the lower plate 32. As a result, the coating film can be stably dispersed in the second bonding process Q2. In this case, by setting the first press-fitting position at a position deeper than the surface of the lower plate 32 by 0.09 mm or more, the anchor 4B can be formed more stably.

[0056] In this case, when the height of anchor 4B relative to the interface between upper plate 31 and lower plate 32 before joining is defined as HA and the position of the surface of upper plate 31 before joining is defined as tu, anchor 4B is preferably formed so as to satisfy the relationship HA / tu≧2.7(%). Furthermore, it is more preferable to form anchor 4B so as to satisfy the relationship HA / tu≧5.3(%).

[0057] Furthermore, it is desirable to form the anchors 4B so that the height of the anchors 4B is 0.16 mm or more, based on the interface between the upper plate 31 and the lower plate 32 before joining.

[0058] Then, by setting the second press-fitting position at a position deeper than the top of the anchor 4B, which is formed by the lower plate 32 being raised inside the shoulder 12 by pressing the shoulder 12 into the first press-fitting position, it becomes possible to disperse the raised coating film more stably. Here, the top of the anchor 4B means the point at the maximum vertical distance from the surface of the lower plate 32, in other words, the highest point of the anchor 4B.

[0059] The second press-fit position may be set deeper or shallower than the first press-fit position. However, by setting the second press-fit position shallower than the first press-fit position, the shoulder 12 is less likely to come into contact with the material of the lower plate 32 in the second joining process Q2, making it easier to maintain the joined state of the upper plate 31 and the lower plate 32 joined in the first joining process Q1.

[0060] Furthermore, when selecting the upper plate 31 and the lower plate 32, the lower plate 32 can be made to have a higher hardness than the upper plate 31. This makes it easier for a portion of the lower plate 32 to bulge during the first joining process Q1, stably forming the anchors 4B. As a result, the anchors 4B can stably hold the coating film in a specific position, enhancing the dispersion effect during the second joining process Q2.

[0061] Furthermore, as in the above-described modified embodiment, by continuing to bias the upper plate 31 by the clamp 13 at least from the time when the shoulder 12 is pressed into the first press-fitting position until the time when it is pressed into the second press-fitting position, it is possible to prevent the point-joining position W from shifting between the two joining processes, and it becomes easier to prevent deformation of the formed anchor 4B.

[0062] Next, the effects of the above-described embodiments will be described using examples, but the present disclosure is not limited to the following examples.

[0063] <Regarding Bonding Strength> Fig. 9 is a graph showing the tensile shear strength (TSS) in examples and comparative examples of the present disclosure. Fig. 10 is a schematic perspective view showing how the tensile shear strength shown in Fig. 9 is measured. Comparative Example 1 in Fig. 9 is a case where a coating film is not formed on either the upper plate 31 or the lower plate 32, and corresponds to a case where they are joined by a conventional single joining process. Comparative Example 2 is a case where a coating film is formed only on the upper plate 31, and they are joined by a conventional single joining process. Example 1 is a case where a coating film is formed only on the upper plate 31, and they are joined by two joining processes as shown in Figs. 2 and 3 .

[0064] 10, TSS indicates the load at which fracture occurs in the stir weld 4 when the upper plate 31 and the lower plate 32 of the welded body 3 after the stir weld 4 is formed are pulled in opposite directions parallel to the upper plate surface 31A. In other words, the larger the TSS value, the higher the tensile shear strength.

[0065] As shown in Figure 9, Comparative Example 1, in which no coating was formed, had a TSS of 5.6 (kN), while Comparative Example 2, in which a coating was formed on the upper plate 31, had a TSS of 3.8 (kN), resulting in a decrease in joint strength. This phenomenon is presumed to be due to the coating being unevenly distributed in the stir welded portion 4, making fractures more likely to occur as the starting point. Figure 11 is a cross-sectional photograph of the stir welded portion 4 of Comparative Example 2. Figure 12 is an enlarged cross-sectional photograph of region N in Figure 11. As shown in Figure 12, in Comparative Example 2, the coating remains unevenly in a layer around the anchor 4B, resulting in the above-mentioned decrease in strength.

[0066] On the other hand, in Example 1, although a coating film is formed, it is possible to ensure a TSS of 4.6 (kN) as shown in Figure 9, making it possible to improve the joining strength compared to Comparative Example 2. Figure 13 is a cross-sectional photograph of the stir welded portion 4 of Example 1. Figure 14 is an enlarged cross-sectional photograph of region N in Figure 13. As shown in Figure 14, in Example 2, no coating film concentrated around anchor 4B is observed, and it can be seen that the coating film is suitably dispersed by shoulder 12 in the second joining process.

[0067] <Relationship between anchor and coating> Next, experiments were conducted as a comparative example and an example to confirm how the coating distribution changes depending on whether or not the anchor 4B is present during the joining process. In these experiments, a technique called stop action was used to interrupt the joining process after the shoulder 12 was pressed into the first press-fit position, and each cross section of the stir welded portion 4 was cut and evaluated. In these experiments, the outer diameter of the shoulder 12 was set to 9 mm, and the outer diameter of the pin 11 was set to 6 mm.

[0068] In the comparative example, the first press-fit position of the shoulder 12 was set shallower than the back surface 31B of the upper plate 31, and the experiment was conducted under conditions intentionally preventing the formation of anchors 4B. Figure 15 is a cross-sectional photograph of the stir-fuse joint 4 of the comparative example. Figure 16 is an enlarged photograph of the bottom surface of the aluminum pillar in the stir-fuse joint 4 of the comparative example. Figure 17 is an enlarged cross-sectional photograph of the bottom N of the aluminum pillar in Figure 15. The aluminum pillar refers to the cylindrical region located below the pin 11 and inside the shoulder 12 when the process was interrupted. As shown in Figures 15 and 16, in the comparative example, a large portion of the coating remained on the steel interface side of the aluminum pillar, as described above with reference to Figure 6. Furthermore, because the material was pressed toward the center during the press-fit of the shoulder 12, the interface side portion of the aluminum pillar was recessed inward, as shown in Figure 17, resulting in the coating remaining inside the recessed portion.

[0069] On the other hand, in the example, the first press-fit position of the shoulder 12 was set deeper than the upper plate surface 31A of the lower plate 32, and an experiment was conducted under conditions in which an anchor 4B was formed. Fig. 18 is a cross-sectional photograph of a joint in an example of the present disclosure. Fig. 19 is an enlarged photograph of the bottom surface of an aluminum column at a joint in an example of the present disclosure. Fig. 20 is an enlarged cross-sectional photograph of the interface N between the upper plate 31 and the lower plate 32 in Fig. 18.

[0070] 18 and 19, in the example, the coating film was mainly distributed inside the anchor 4B, as explained above using FIG. 7. In particular, the presence of the anchor 4B prevented the coating film from spreading too far outside the anchor 4B. Furthermore, the presence of the anchor 4B prevented a large amount of coating film from being present near the interface between the upper plate 31 and the lower plate 32 outside the anchor 4B. As shown at the bonding interface CA in FIG. 20, it was confirmed that the bonding between the aluminum of the upper plate 31 and the steel of the lower plate 32 could be favorably improved even under conditions that included the coating film.

[0071] <Regarding the First Press-Fit Position> Next, we will describe the results of an experiment examining the coating film dispersion effect when the first press-fit position of the shoulder 12 during the first joining process Q1 is varied. FIG. 21 is a schematic diagram illustrating the joint interface position due to the shoulder 12. In FIG. 21 , the position of the upper plate surface 31A of the upper plate 31 is defined as the upper plate position tu, the height of the anchor 4B, i.e., the position of the tip of the anchor 4B, is defined as the anchor height HA, and the first press-fit position of the shoulder 12 during the first joining process Q1 is defined as the first-pass joint interface position HI. Note that the above values ​​are expressed as zero, i.e., the position of the base material interface between the upper plate rear surface 31B of the upper plate 31 and the lower plate surface 32A of the lower plate 32. The upper plate 31 side is represented by a positive sign and the lower plate 32 side is represented by a negative sign. In this experiment, the thickness T1 of the upper plate 31 in FIG. 5 , i.e., the upper plate position tu in FIG. 21 is 3.0 mm, and the thickness T2 of the lower plate 32 in FIG. 5 is 1.6 mm. The thickness of the coating on the upper plate 31 is 20 μm. The outer diameter of the shoulder 12 is 8 mm, and the outer diameter of the pin 11 is 5 mm. The upper plate position tu corresponds to the surface of the upper plate 31, i.e., the upper metal surface of the upper plate 31.

[0072] Table 1 shows the experimental conditions and the dispersion effect of the coating film based on cross-sectional observation of the stir-joint 4.

[0073]

[0074] Example 2 was carried out under the conditions of the first pass joint interface position HI -0.09 mm and anchor height HA +0.08 mm. As a result, the ratio of anchor height HA to upper plate position tu was 2.7%, and although there was a small amount of paint remaining on the outside of anchor 4B, this did not pose a problem in terms of strength in actual use, so the paint film dispersion effect was rated as fair.

[0075] On the other hand, Example 3 was performed under the conditions of the first pass joint interface position HI -0.16 mm and anchor height HA +0.26 mm. As a result, the ratio of anchor height HA to upper plate position tu was 8.7%, and no paint film residue was found near the joint interface on the outside of anchor 4B, so it was evaluated as good.

[0076] Similarly, Example 4 was performed under the conditions of the first pass joint interface position HI -0.20 mm and anchor height HA +0.42 mm. As a result, the ratio of anchor height HA to upper plate position tu was 14.0%, and no paint film residue was found near the joint interface on the outside of anchor 4B, so it was evaluated as good.

[0077] In Example 5, the first pass was performed at a joint interface position HI of -0.30 mm and an anchor height HA of +0.72 mm. As a result, the ratio of the anchor height HA to the upper plate position tu was 24.0%, and no paint film residue was found near the joint interface on the outside of anchor 4B, so the result was evaluated as good.

[0078] As described above, it is desirable that the first press-fitting position of the shoulder 12 for the first time be set at a position deeper than the lower plate surface 32A of the lower plate 32. In particular, by setting the first press-fitting position at a position 0.09 mm or more deeper than the lower plate surface 32A of the lower plate 32, a more stable effect can be obtained.

[0079] Furthermore, it is desirable that the first press-fit position be set at a position deeper than a position that is 0.09 mm deeper from the lower plate surface 32A of the lower plate 32. It is even more desirable that the first press-fit position be set at a position that is 0.16 mm or more deeper than the lower plate surface 32A of the lower plate 32.

[0080] Furthermore, the anchor height HA is preferably set to 0.08 mm or more, more preferably set to greater than 0.08 mm, and even more preferably set to 0.26 mm or more.

[0081] Furthermore, as shown in Table 1, it is desirable that the ratio of the anchor height HA to the upper plate position tu be set to 2.7% or more, more desirably to be set to greater than 2.7%, and even more desirably to be set to 8.7% or more.

[0082] The present inventors have also conducted the same experiment as above under conditions where the thickness of the coating film was 8 μm and 30 μm, and have confirmed that similar results were obtained.

[0083] <Regarding the Second Press-Fit Position> Next, we will explain the results of examining the coating film dispersion effect when the second press-fit position of the shoulder 12 in the second joining process Q2 is changed. Fig. 22 is a schematic diagram for explaining the joint interface position due to the shoulder 12. In Fig. 22, compared to Fig. 21, the second press-fit position of the shoulder 12 in the second joining process Q2 is defined as the second-pass joint interface position HI'. Note that the magnitude of the anchor height HA is an actual measurement value, and HI' is calculated based on the drive command signal to the controller C.

[0084] Table 2 shows the experimental conditions and the results of the dispersion effect of the coating film based on cross-sectional observation of the stir-joint 4. Table 2 compares Examples 3, 4, and 5 in Table 1 based on the second press-fit position of the shoulder 12 in the second joining process Q2. The method for evaluating the dispersion effect of the coating film was the same as in Table 1.

[0085]

[0086] As shown in Table 2, in Example 3, the second press-fit position of the shoulder 12 in the second joining process Q2 is set to a position 0.25 mm deeper than the tip of the anchor 4B.

[0087] Similarly, in Example 4, the second press-fit position of the shoulder 12 in the second joining process Q2 is set to a position 0.40 mm deeper than the tip of the anchor 4B. Furthermore, in Example 5, the second press-fit position of the shoulder 12 in the second joining process Q2 is set to a position 0.41 mm deeper than the tip of the anchor 4B. In either example, the coating film dispersion effect can be favorably achieved. That is, as schematically shown in FIG. 22 , it is desirable to set the second press-fit position above the interface between the upper plate 31 and the lower plate 32 and deeper than the tip of the anchor 4B in order to favorably disperse the coating film that has lifted up around the outer periphery of the tip of the anchor 4B.

[0088] <Relationship Between the First Press-Fit Position and the Second Press-Fit Position> FIG. 23 is a schematic diagram illustrating the position of the weld interface due to the shoulder 12. FIG. 24 is a graph showing the transition of the shoulder press-fit amount during friction stir spot welding. In the examples shown in Tables 1 and 2, the experiment was conducted under conditions in which the second press-fit position was set shallower than the first press-fit position. In this experiment, as shown in FIG. 23, the experiment was conducted under conditions in which the second press-fit position was set deeper than the first press-fit position. Table 3 shows the experimental conditions and the results of the dispersion effect of the coating film based on cross-sectional observation of the stir-fuse joint 4. In Example 6 based on this experiment, the first-pass weld interface position HI, corresponding to the first press-fit position, was set to −0.12 mm, and the second-pass weld interface position HI′, corresponding to the second press-fit position, was set to −0.21 mm. Note that in this experiment, as in the modified embodiment described above, the clamp 13 was maintained in contact with the upper plate 31 during welding.

[0089]

[0090] 24 , after the welding time reaches 0 sec, the tool 1 approaches the upper plate 31, and then a preheating process is performed from 0.5 sec to 1 sec. Thereafter, the shoulder 12 is pressed into the upper plate 31 and reaches the interface between the upper plate 31 and the lower plate 32 at around 1.3 sec. Furthermore, after the shoulder 12 reaches the first press-fit position at around 1.8 sec, the shoulder 12 temporarily retreats to a shoulder press-fit depth of around 1.8 mm so as to move away from the lower plate 32. Thereafter, the shoulder 12 is pressed again so as to approach the interface between the upper plate 31 and the lower plate 32, and at around 3.8 sec, it is pressed into a second press-fit position that is deeper than the first press-fit position. Thereafter, the shoulder 12 separates from the interface between the upper plate 31 and the lower plate 32 , and the smoothing process is performed when the shoulder press-fit amount is about 0.6 mm. After the indent 4T is formed, the tool 1 separates from the upper plate 31 .

[0091] Furthermore, as shown in Table 3, it was confirmed that even when the second press-fitting position was set at a position deeper than the first press-fitting position, the coating film unevenly distributed around the anchor 4B was dispersed appropriately.

[0092] The results of the experiments described above are summarized in Table 4.

[0093]

[0094] As shown in Table 4, Comparative Example 2, which did not perform the second joining process Q2, had difficulty achieving a coating film dispersion effect. However, Examples 2 to 6, which performed the second joining process Q2, demonstrated a coating film dispersion effect. As a result, it was possible to effectively prevent fracture of the joined body 3 due to localized coating film concentration. Forming anchors 4B in the first joining process Q1 and unevenly distributing the coating film above the interface between the upper plate 31 and the lower plate 32 enabled stable dispersion of the coating film in the second joining process Q2. Furthermore, anchors 4B are likely to be formed when the upper plate 31 and the lower plate 32 are made of dissimilar materials due to the difference in hardness. Therefore, the anchors 4B can be effectively used to achieve a coating film dispersion effect.

[0095] <Regarding the Continuous Two-Step Welding Process> Figure 25 is a graph showing CTS (Cross Tension Strength) in examples and comparative examples of the present disclosure. In evaluating this cross tensile strength, the upper plate 31 and the lower plate 32 are overlapped in a cross shape in a plan view, and the stir weld 4 is formed at the intersection. The CTS shown on the vertical axis of Figure 25 corresponds to the load at which fracture occurs when trying to peel the upper plate 31 and the lower plate 32 apart.

[0096] As in the embodiment described above, if the clamp 13 of the tool 1 moves away from the upper plate 31 between the first joining process Q1 and the second joining process Q2, there is a possibility that the spot joining position W, in other words, the position of the rotation axis R of the tool 1 relative to the joined body 3, may shift between the joining processes. Therefore, the present inventor intentionally caused the positional shift of the spot joining position W to occur, and evaluated the change in the CTS of the joined body 3.

[0097] 25 shows the CTS measurement results for each of Comparative Example 1, which does not have a coating and does not perform the second joining process, Comparative Example 2, which has a coating and does not perform the second joining process, Example 1, which has a coating and performs the first joining process Q1 and the second joining process Q2 separately, and Example 7, which, as in the above-mentioned modified embodiment, performs the first joining process Q1 and the second joining process Q2 consecutively without separating the clamp 13 from the upper plate 31. Data indicated by white circles corresponds to Example 1, and data indicated by black circles corresponds to a reference example in which the spot joining position W of the second joining process is intentionally shifted under the conditions of Example 1.

[0098] 25, Comparative Example 2, which has a coating film, has a lower CTS than Comparative Example 1, which does not have a coating film. On the other hand, in Examples 1 and 7, which perform the first bonding process Q1 and the second bonding process Q2, it was confirmed that the CTS was equal to or higher than that of Comparative Example 1, even under the condition that a coating film was present. On the other hand, in the Reference Example, in which the spot bonding position W was intentionally shifted, it was confirmed that the CTS was lower than that of Example 1.

[0099] FIG. 26 is a cross-sectional photograph of the stir-fuse joint 4 of a reference example of the present disclosure after fracture. FIG. 27 is an enlarged cross-sectional photograph of region N in FIG. 26 . When the spot welding position W was shifted as shown in FIGS. 26 and 27 , the shoulder 12 contacted the anchor 4B during the second welding process Q2, causing the anchor 4B to deform. In this case, the anchor 4B presumably prevented the upper plate 31 and the lower plate 32 from being strongly joined, resulting in a decrease in CTS, as shown in FIG. 25 . In other words, by sequentially performing the first welding process Q1 and the second welding process Q2, as in Example 7 of FIG. 25 , it is possible to suppress the misalignment of the spot welding position W and stably distribute the coating film.

[0100] Next, an experiment on the joining process based on Example 7 will be described in more detail. In this experiment, the shoulder 12 had an outer diameter of 8 mm, and the pin 11 had an outer diameter of 5 mm. The upper plate 31 was made of an Al-Si based casting with a thickness of 3.0 mm, and the electro-deposition coating film had a thickness of 20 μm. The lower plate 32 was a high-tensile steel plate with a thickness of 1.6 mm, such as a 780 MPa-class GA-plated steel plate.

[0101] FIG. 28 is a graph showing the transition of the shoulder 12 press-in amount during friction stir spot welding in this experiment. Table 5 also shows the press-in amount of the shoulder 12 corresponding to the end of each welding time in FIG. 28 . The press-in amount is expressed as zero for the surface of the upper plate 31 and plus for the lower plate 32. "1" corresponds to the preheating process, "2" corresponds to pressing the shoulder 12 into the upper plate 31, "3" corresponds to pressing the shoulder 12 into the lower plate 32, "4" corresponds to the backfilling process, "5" corresponds to re-pressing the shoulder 12 to disperse the coating, and "6" corresponds to the second backfilling and leveling process. In Table 5, the re-press-in amount for "5" is set to 3.14 mm. However, because the upper plate 31 and the lower plate 32 were already firmly joined, the press-in amount of the shoulder 12 actually slowed down and ended at approximately 2.6 mm, as shown in FIG. 28 .

[0102]

[0103] FIG. 29 is a cross-sectional photograph of the stir welded portion 4 after completion of the welding process in Example 7 of the present disclosure.

[0104] Figure 30 is an enlarged cross-sectional photograph of region N1 in Figure 29 , and Figure 31 is an enlarged cross-sectional photograph of region N2 in Figure 29 . Note that Figure 31 shows a virtual representation of a portion of the shoulder 12. As shown in Figures 29 and 30 , in this experiment, no contact marks of the shoulder 12 with the anchor 4B were observed, and it was confirmed that the coating was sufficiently dispersed near the joint interface between the upper plate 31 and the lower plate 32. As shown in Figure 25 , multiple experiments were performed on the joint 3 joined under the conditions of Example 7. In each experiment, fracture occurred along the aluminum-steel interface, as in the case without a coating, and no remaining coating was observed on the fracture surface. It was also confirmed that the coating remaining outside the anchor 4B, in other words, beyond the press-fit region of the shoulder 12, as shown in Figure 30 , did not affect the strength. It was also confirmed that there was no significant variation in the behavior of the tool 1 during joining and the joining time when comparing each experiment.

[0105] The aforementioned phenomenon of a reduction in the pressing speed of the shoulder 12 in the second joining process Q2 is presumably due to the anchor 4B formed in the first joining process Q1 making it difficult for the aluminum directly below the shoulder 12 to flow inside the shoulder 12, as shown in Figure 31.

[0106] Furthermore, when the first joining process Q1 and the second joining process Q2 are performed consecutively, as shown in Figure 28, control may be performed so that the shoulder 12 is pressed again inside the shoulder 12 without completely detaching the shoulder 12 from the upper plate 31.

[0107] The present inventors have confirmed that the effects of the present disclosure are particularly evident in the following ranges, including the above examples: Plate thickness of the upper plate 31 and the lower plate 32: 0.5 mm or more and 5 mm or less, more preferably 1 mm or more and 3 mm or less Tool rotation speed: 500 rpm or more and 4000 rpm or less, more preferably 1500 rpm or more and 2500 rpm or less

[0108] Furthermore, in each of the above embodiments and examples, the coating film is described as being formed on the upper plate 31, but the coating film may also be formed on at least one of the upper plate 31 and the lower plate 32.

[0109] [Summary of the Disclosure] The specific embodiments described above include disclosures having the following configurations.

[0110] A friction stir spot joining method according to a first aspect of the present disclosure is a friction stir spot joining method in which a first member and a second member, at least one of which has a coating film formed thereon, are made of different materials and are softened by frictional heat to join them. The joining method includes preparing a friction stir spot joining apparatus having a pin and a cylindrical shoulder including a hollow portion into which the pin is inserted; forming an overlapping portion where the first member and the second member overlap, and arranging the pin and the shoulder opposite the overlapping portion; retracting the pin from the overlapping portion while pressing the shoulder into a predetermined first press-fit position of the overlapping portion, thereby forming a raised anchor of the second member inside the shoulder and distributing the coating film at a specific position, thereby joining the first member and the second member; retracting the shoulder from the first press-fit position while moving the pin toward the overlapping portion; retracting the shoulder from the first press-fit position while moving the pin toward the overlapping portion;

[0111] According to this method, by distributing the coating film unevenly at specific locations in the first joining process and then dispersing the coating film in the second joining process, it is possible to stably join components having a coating film and maintain their strength. Therefore, after joining the first and second components, the need to form a corrosion-preventing coating film on the joined body is reduced, and multiple components can be joined using materials on which a coating film has already been formed. Furthermore, compared to joining components having a coating film using only a single joining process, it is possible to improve the strength of the joint by approximately 1.3 times. Thus, this method provides a friction stir spot joining method that can join multiple workpieces having a coating film formed on at least one side while preventing a decrease in joint strength.

[0112] A friction stir spot welding method according to a second aspect of the present disclosure is the friction stir spot welding method according to the first aspect described above, further comprising setting the first press-fit position at a position deeper than the surface of the second member.

[0113] According to this method, by setting the first press-fitting position at a position deeper than the surface of the second member, it is possible to stably form an anchor and raise the coating film above the interface between the first and second members, which makes it possible to stably disperse the coating film in the second joining process.

[0114] A friction stir spot welding method according to a third aspect of the present disclosure is the friction stir spot welding method according to the first or second aspect, further comprising forming the anchor so as to satisfy the relationship HA / tu≧2.7(%), where HA is the height of the anchor and tu is the position on the surface of the first member, based on the interface between the first member and the second member before joining.

[0115] According to this method, the height of the anchor is stably secured, and the coating film can be dispersed stably.

[0116] A friction stir spot welding method according to a fourth aspect of the present disclosure is the friction stir spot welding method according to the third aspect, further comprising forming the anchor so as to satisfy the relationship HA / tu≧5.3(%).

[0117] According to this method, the height of the anchors can be more stably ensured, and the coating film can be dispersed more stably.

[0118] Furthermore, a friction stir spot welding method according to a fifth aspect of the present disclosure is the friction stir spot welding method according to the first to fourth aspects, further comprising forming the anchor so that the height of the anchor is 0.16 mm or more based on the interface between the first member and the second member before joining.

[0119] According to this method, the height of the anchor is stably secured, and the coating film can be dispersed stably.

[0120] A friction stir spot welding method according to a sixth aspect of the present disclosure is the friction stir spot welding method according to the second aspect described above, further comprising setting the first press-fit position at a position that is 0.09 mm or more deeper than the surface of the second member.

[0121] According to this method, anchors can be formed more stably, and the coating film can be stably raised above the interface between the first member and the second member.

[0122] A friction stir spot welding method according to a seventh aspect of the present disclosure is the friction stir spot welding method according to any one of the first to sixth aspects, further comprising setting the second press-fit position at a position deeper than the top of the anchor.

[0123] According to this method, by setting the second press-in position at a position deeper than the top of the anchor, it is possible to stably disperse the lifted coating film in the second joining process.

[0124] A friction stir spot welding method according to an eighth aspect of the present disclosure is the friction stir spot welding method according to the seventh aspect, further comprising setting the second press-fit position to a position shallower than the first press-fit position.

[0125] According to this method, by setting the second press-fit position shallower than the first press-fit position, the shoulder is less likely to come into contact with the material of the second member, making it easier to maintain the joined state of the first and second members joined in the first joining process.

[0126] A friction stir spot welding method according to a ninth aspect of the present disclosure is the friction stir spot welding method according to any one of the first to eighth aspects, further comprising setting the hardness of the second member higher than the hardness of the first member.

[0127] According to this method, a portion of the second member is likely to bulge during the first joining process, and an anchor can be stably formed.

[0128] A friction stir spot welding method according to a tenth aspect of the present disclosure is the friction stir spot welding method according to the ninth aspect, further comprising: the first member is aluminum; and the second member is steel.

[0129] According to this method, a portion of the second member is likely to bulge during the first joining process, and an anchor can be stably formed.

[0130] A friction stir spot welding method according to an eleventh aspect of the present disclosure is the friction stir spot welding method according to the first to ninth aspects above, wherein the friction stir spot welding apparatus further includes a cylindrical clamp including a hollow portion into which the shoulder is inserted, and the clamp continues to bias the first member at least from the time the shoulder is pressed into the first press-fit position until the shoulder is pressed into the second press-fit position.

[0131] According to this method, deviation of the point bonding position between the two bonding processes is prevented, and deformation of the formed anchor is easily prevented.

[0132] A friction stir spot joining method according to a twelfth aspect of the present disclosure is a friction stir spot joining method in which a first member and a second member, at least one of which has a coating film formed thereon, are made of different materials and are softened by frictional heat to join them. The joining method includes preparing a friction stir spot welding apparatus having a pin and a cylindrical shoulder including a hollow portion into which the pin is inserted; forming an overlapping portion where the first member and the second member overlap and arranging the pin and the shoulder facing the overlapping portion; retracting the pin from the overlapping portion while pressing the shoulder into a predetermined first press-fit position of the overlapping portion, thereby forming an anchor where the second member protrudes inside the shoulder, and joining the first member and the second member; retracting the shoulder from the first press-fit position while moving the pin toward the overlapping portion; retracting the pin from the overlapping portion while pressing the shoulder into a predetermined second press-fit position of the overlapping portion; retracting the shoulder from the second press-fit position while moving the pin toward the overlapping portion; and setting the first press-fit position to a position deeper than the surface of the second member.

[0133] This method allows stable joining of components with a coating and maintains their strength. Therefore, the need to form a corrosion-preventing coating on the joined body after joining the first and second components is reduced, and multiple components can be joined using materials with a pre-formed coating. Furthermore, the strength of the joint can be improved by approximately 1.3 times compared to joining components with a coating using only a single joining process. In particular, by setting the first press-fit position deeper than the surface of the second component, anchors can be stably formed, causing the coating to rise above the interface between the first and second components. As a result, the coating can be stably dispersed during the second joining process. Thus, this method provides a friction stir spot joining method capable of joining multiple workpieces, at least one of which has a coating formed, while preventing a decrease in joint strength.

[0134] A friction stir spot welding apparatus according to a thirteenth aspect of the present disclosure is a friction stir spot welding apparatus capable of joining a first member and a second member, at least one of which has a coating formed thereon, by softening them with frictional heat. The friction stir spot welding apparatus includes a joining tool having a pin and a cylindrical shoulder including a hollow portion into which the pin is inserted, a rotation mechanism capable of rotating the joining tool, a movement mechanism capable of independently moving the pin and the shoulder in a direction in which the pin extends, and a control unit that controls the rotation mechanism and the movement mechanism to join the first member and the second member while the pin and the shoulder are positioned opposite each other at an overlapping portion where the first member and the second member overlap. The control unit controls the rotation mechanism and the movement mechanism to retract the pin from the overlapping portion while pressing the shoulder into a predetermined first press-fit position of the overlapping portion, thereby forming a raised anchor of the second member inside the shoulder, distributing the coating film at a specific position and joining the first member and the second member, retract the shoulder from the first press-fit position while moving the pin closer to the overlapping portion, and then retract the pin from the overlapping portion while pressing the shoulder into a predetermined second press-fit position of the overlapping portion, thereby dispersing the coating film that has been distributed at the specific position, and retract the shoulder from the second press-fit position while moving the pin closer to the overlapping portion.

[0135] According to this configuration, by distributing the coating film unevenly at a specific position during the first joining process and then dispersing the coating film during the second joining process, it is possible to stably join components having a coating film and maintain their strength. Therefore, after joining the first and second components, the need to form a corrosion-preventing coating film on the joined body is reduced, and multiple components can be joined using materials on which a coating film has already been formed. Furthermore, compared to joining components having a coating film using only a single joining process, it is possible to improve the strength of the joint by approximately 1.3 times. According to this configuration, it is possible to provide a friction stir spot joining device capable of joining multiple workpieces having a coating film formed on at least one side while preventing a decrease in joint strength.

[0136] A friction stir spot welding apparatus according to a fourteenth aspect of the present disclosure is the friction stir spot welding apparatus according to the thirteenth aspect described above, wherein the control unit controls the moving mechanism to set the first press-fit position to a position deeper than the surface of the second member.

[0137] According to this configuration, by setting the first press-fitting position at a position deeper than the surface of the second member, it is possible to stably form an anchor and raise the coating film above the interface between the first member and the second member, which makes it possible to stably disperse the coating film in the second joining process.

[0138] A friction stir spot welding apparatus according to a fifteenth aspect of the present disclosure is the friction stir spot welding apparatus according to the thirteenth or fourteenth aspect above, wherein the control unit controls the moving mechanism to set the second press-fit position to a position deeper than the top of the anchor.

[0139] According to this configuration, by setting the second press-fitting position at a position deeper than the top of the anchor, it becomes possible to stably disperse the lifted coating film in the second joining process.

[0140] A friction stir spot welding apparatus according to a sixteenth aspect of the present disclosure is the friction stir spot welding apparatus according to the fifteenth aspect described above, wherein the control unit controls the moving mechanism to set the second press-fit position to a position shallower than the first press-fit position.

[0141] According to this configuration, by setting the second press-fit position shallower than the first press-fit position, the shoulder is less likely to come into contact with the material of the second member, making it easier to maintain the joined state of the first and second members joined in the first joining process.

[0142] A friction stir spot welding apparatus according to a seventeenth aspect of the present disclosure is the friction stir spot welding apparatus according to any one of the thirteenth to sixteenth aspects above, wherein the welding tool further has a cylindrical clamp including a hollow portion into which the shoulder is inserted, the moving mechanism is capable of moving the clamp in the direction in which the pin extends, and the control unit controls the moving mechanism to continue to bias the first member by the clamp at least from the time the shoulder is pressed into the first press-fit position to the time the shoulder is pressed into the second press-fit position.

[0143] According to this configuration, deviation of the point joining position between the two joining processes is prevented, and deformation of the formed anchor can be easily prevented.

Claims

1. A friction stir spot welding method for joining a first member and a second member, at least one of which has a coating film formed thereon, which are dissimilar materials, by softening them with frictional heat, comprising the steps of: preparing a friction stir spot welding device having a pin and a cylindrical shoulder including a hollow portion into which the pin is inserted; forming an overlapping portion where the first member and the second member overlap, and arranging the pin and the shoulder opposite the overlapping portion; retracting the pin from the overlapping portion and pressing the shoulder into a predetermined first press-in position of the overlapping portion, thereby forming an anchor where the second member protrudes inside the shoulder, and distributing the coating film at a specific position, thereby joining the first member and the second member; retracting the shoulder from the first press-in position while moving the pin close to the overlapping portion; a friction stir spot joining method comprising: pressing the shoulder into the overlapping portion to a predetermined second press-in position while retracting the pin from the overlapping portion, thereby dispersing the coating film that is unevenly distributed at the specific position; and retracting the shoulder from the second press-in position while moving the pin closer to the overlapping portion.

2. The friction stir spot welding method according to claim 1, further comprising setting the first press-fit position at a position deeper than the surface of the second member.

3. The friction stir spot joining method according to claim 1, further comprising forming the anchor so as to satisfy the relationship HA / tu ≧ 2.7 (%), where HA is the height of the anchor and tu is the position on the surface of the first member based on the interface between the first member and the second member before joining.

4. The friction stir spot welding method according to claim 3, further comprising forming the anchor so as to satisfy the relationship HA / tu≧5.3(%).

5. The friction stir spot welding method according to claim 1, further comprising forming the anchor so that the height of the anchor is 0.16 mm or more based on the interface between the first member and the second member before joining.

6. A friction stir spot welding method as described in claim 2, further comprising setting the first press-fit position at a position 0.09 mm or more deeper than the surface of the second member.

7. A friction stir spot welding method according to any one of claims 1 to 6, further comprising setting the second press-in position at a position deeper than the top of the anchor.

8. The friction stir spot welding method according to claim 7, further comprising setting the second press-fit position at a position shallower than the first press-fit position.

9. The friction stir spot welding method according to claim 1, further comprising setting the hardness of the second member higher than the hardness of the first member.

10. The friction stir spot welding method of claim 9, further comprising: said first member being aluminum and said second member being steel.

11. The friction stir spot welding method according to claim 1, wherein the friction stir spot welding apparatus further comprises a cylindrical clamp including a hollow portion into which the shoulder is inserted, and the first member is continuously biased by the clamp at least during the period from when the shoulder is pressed into the first press-fitting position to when the shoulder is pressed into the second press-fitting position.

12. A friction stir spot welding method for joining a first member and a second member, at least one of which has a coating formed thereon, which are dissimilar materials, by softening them with frictional heat, comprising the steps of: preparing a friction stir spot welding device having a pin and a cylindrical shoulder including a hollow portion into which the pin is inserted; forming an overlapping portion where the first member and the second member overlap, and arranging the pin and the shoulder opposite the overlapping portion; retracting the pin from the overlapping portion and pressing the shoulder into a predetermined first press-fit position of the overlapping portion, thereby forming an anchor in which the second member protrudes inside the shoulder, and joining the first member and the second member; retracting the shoulder from the first press-fit position while moving the pin closer to the overlapping portion; pressing the shoulder into the predetermined second press-fit position of the overlapping portion while retracting the pin from the overlapping portion; a friction stir spot joining method comprising: retracting the shoulder from the second press-fit position while moving the pin closer to the overlapping portion; and setting the first press-fit position at a position deeper than a surface of the second member.

13. A friction stir spot welding device capable of joining a first member and a second member, at least one of which has a coating formed thereon, which are dissimilar materials, by softening them with frictional heat, the device comprising: a joining tool having a pin and a cylindrical shoulder including a hollow portion into which the pin is inserted; a rotation mechanism capable of rotating the joining tool; a movement mechanism capable of independently moving the pin and the shoulder in a direction in which the pin extends; and a control unit that controls the rotation mechanism and the movement mechanism to join the first member and the second member while the pin and the shoulder are disposed opposite to the overlapping portion where the first member and the second member overlap, the control unit controlling the rotation mechanism and the movement mechanism to: a friction stir spot joining device which, while retracting the pin from the overlapping portion, presses the shoulder into a predetermined first press-fit position of the overlapping portion to form an anchor in which the second member is raised inside the shoulder, thereby distributing the coating film at a specific position and joining the first member and the second member; a friction stir spot joining device which, while retracting the pin from the overlapping portion, retracts the shoulder from the first press-fit position, and then, while retracting the pin from the overlapping portion, presses the shoulder into a predetermined second press-fit position of the overlapping portion to disperse the coating film that is distributing at the specific position; and a friction stir spot joining device which, while retracting the pin from the overlapping portion, retracts the shoulder from the second press-fit position.

14. The friction stir spot welding apparatus according to claim 13, wherein the control unit controls the movement mechanism to set the first press-fit position to a position deeper than the surface of the second member.

15. The friction stir spot welding apparatus according to claim 13 or 14, wherein the control unit controls the moving mechanism to set the second press-in position to a position deeper than the top of the anchor.

16. The friction stir spot welding apparatus according to claim 15, wherein the control unit controls the movement mechanism to set the second press-in position to a position shallower than the first press-in position.

17. The friction stir spot welding apparatus according to claim 13, wherein the joining tool further has a cylindrical clamp including a hollow portion into which the shoulder is inserted, the moving mechanism is capable of moving the clamp in the direction in which the pin extends, and the control unit controls the moving mechanism to continue to bias the first member by the clamp at least during the period from when the shoulder is pressed into the first press-fitting position to when the shoulder is pressed into the second press-fitting position.

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