Rotary tool for friction stir welding and friction stir welding method

Abstract

To provide a highly durable rotary tool for friction stir welding which makes damage of a rotary tool less likely to occur even when friction stir welding is conducted on highly rigid joined materials, such as steel plates.SOLUTION: A rotary tool 1 is used to join two joined materials by friction stir welding and includes a tip 10 having a convex curved surface 11. The curved surface 11 has: a first circular smooth surface 12 disposed at a center part; an annular undulation shape surface 13 disposed at an outer periphery of the first smooth surface 12; and an annular second smooth surface 14 disposed at an outer periphery of the undulation shape surface 13.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a friction stir welding rotating tool used for friction stir welding for joining two steel plates, and a friction stir welding method. [Background technology]

[0002] The friction stir welding method is a method of joining steel plates by inserting a rotating tool (e.g., a rotating tool) into the unjoined part of overlapping or butted materials to be joined (e.g., steel plates), and moving this rotating tool while rotating, thereby generating frictional heat in the steel plates, softening them, and then stirring the softened part with the rotating tool to induce plastic flow.

[0003] The friction stir welding method is a solid-state welding method that utilizes the plastic flow of metal caused by frictional heat between the rotating tool and the materials to be welded, so it can join without melting the unjoined parts. Since the materials to be welded are not melted, there are many advantages to this method, such as fewer defects in the welded parts, less deformation after joining because the heating temperature is low, and no filler metal is required for joining.

[0004] In this specification, the overlapping or butting portions where the materials to be joined, such as steel plates, are simply overlapped or butted together but not yet joined, are referred to as "unjoined portions," and the portions that have been joined and integrated are referred to as "joined portions."

[0005] There is a strong demand for weight reduction in transportation machinery such as automobiles and ships in order to improve fuel efficiency. In response to this trend, spot welding and laser welding are widely used for components used in transportation machinery. However, since these are fusion joining methods, when ultra-high tensile steel or high carbon materials are welded, the welded part becomes a full martensite structure. As a result, hardening becomes significant, resulting in a brittle welded part.

[0006] As a method of friction stir welding, there are techniques described in, for example, Patent Documents 1 and 2 below.

[0007] Patent Document 1 describes the application of the friction stir welding method to a tailored blank member of an aluminum material. In the technique described in Patent Document 1, the axis of rotation of the rotating tool (rotor of the joining tool) is inclined toward the joining member on the lower side and inserted into the mating portion of the joining members. That is, the rotating tool joins from the surface side of the joining members by moving in the joining direction along the mating portion in a state where it is directed in the direction perpendicular to the joining direction.

[0008] Patent Document 2 describes the application of the friction stir welding method to a tailored blank member of a workpiece made of a material such as aluminum or magnesium. In the technique described in Patent Document 2, a bobbin tool having a curved surface-shaped shoulder is used as the rotating tool. This bobbin tool is configured such that the upper base, upper shoulder, lower base, lower shoulder, and the probe between the shoulders can rotate integrally.

[0009] As described above, in the friction stir welding method, the joining interface is stirred by a rotating tool to join a workpiece such as a steel plate. Since the rotating tool receives a large load during joining, there is a risk of poor joint formation due to this. Therefore, it is required that the rotating tool suppress the occurrence of poor joint formation during joining.

[0010] Examples of techniques for suppressing the occurrence of poor joint formation include the techniques of Patent Documents 3 and 4 below.

[0011] Patent Document 3 describes a technique for making the tool depth constant in order to suppress defects caused by fluctuations in the pressing depth of the tool against the workpiece that occur when the friction stir welding tool is pressed and inserted while rotating and moved to the joining portion of the workpiece. In the technique described in Patent Document 3, in a tool having a shoulder and a pin, the outer peripheral side of the end face of the shoulder that contacts the workpiece when the pin is pressed and inserted into the workpiece is formed as an inclined surface, and the tool pressing depth is controlled by this inclined surface.

[0012] Patent Document 4 describes a technique for joining using a tool formed in a frustum of a cone shape having spiral grooves on the shoulder surface. In the technique described in Cited Document 4, the above tool can suppress the sinking of the shoulder portion of the tool during friction stir processing, reduce the generation of burrs and the reduction of wall thickness in the processed portion, and also make the processing marks less conspicuous.

Prior Art Documents

Patent Documents

[0013]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0014] However, the technique described in Patent Document 1 targets aluminum or its alloys as joining members, and does not consider at all materials with high rigidity such as steel materials. Further, in the technique described in Patent Document 1, when joining steel plates with different thicknesses, the rotation axis of the rotating tool is inclined toward the joining member side on the lower position side (the direction perpendicular to the joining direction). Therefore, when attempting to join a material with high rigidity, the rotating tool cannot withstand the large load generated by inclining the rotating tool and breaks, making joining difficult. Also, in order to withstand the above-described large load, the device for installing the rotating tool needs to have sufficient rigidity.

[0015] In the technology described in Patent Document 2, a bobbin tool having curved upper and lower shoulders is used for joining. However, because the bobbin tool is used, the rotation directions of the upper and lower bobbin tools are the same, and as a result, the plastic flow moves in the same way on the upper and lower sides. Therefore, when joining thin plates in which the distance between the upper and lower bobbin tools is short or when joining at high speed, there is a problem that voids are generated by plastic flow when the bobbin tool passes on the retreating side where the joining direction and the rotation direction of the bobbin tool are opposite, and linear defects are likely to occur in the joint.

[0016] The technology described in Patent Document 3 requires a mechanism for controlling the pressing depth of the tool in the device. In addition, the technology described in Patent Document 3 uses an aluminum alloy as the workpiece, but Patent Document 3 does not specifically mention the strength characteristics of the tool. Therefore, when using another metal, particularly a high melting point material or a high hardness material, as the workpiece, there is a problem that the strength of the tool is insufficient and joining is not possible.

[0017] In the technology described in Patent Document 4, in order to prevent burrs from being generated due to the softened material biting into the material, the softened material is bitten into a spiral groove during processing, and is appropriately held and secured on the shoulder surface. For this reason, Patent Document 4 describes that the deeper the groove is, the closer it is to the center of the tool from the outer periphery. However, if the amount of metal wrapped around the tool increases, the tool may break and joining may become difficult when high-rigidity materials such as steel are used.

[0018] The present invention has been made in consideration of the above-mentioned problems, and has an object to provide a highly durable rotating tool for friction stir welding that is less likely to break even when friction stir welding highly rigid workpieces such as steel plates. [Means for solving the problem]

[0019] The inventors of the present invention have intensively studied to solve the above problems. Here, with reference to FIGS. 3 and 4, a conventional rotary tool for friction stir welding (hereinafter, also simply referred to as "rotary tool") will be described. FIG. 3 is a diagram for explaining an example of a conventional rotary tool, where (a) is a side view and (b) is a plan view. The rotary tool 3 shown in FIG. 3 is a rotary tool whose tip 30 is composed only of a probe 31 and shoulders 32 and 33. Further, FIG. 4 is a diagram for explaining another example of a conventional rotary tool, where (a) is a side view and (b) is a plan view. The rotary tool 4 shown in FIG. 4 is a rotary tool whose tip 40 has a convex curved surface 41, and the curved surface 41 is formed only of a smooth surface.

[0020] When friction stir welding a workpiece using the conventional rotary tool 3 shown in FIG. 3, since the tip 30 has a probe 31, a large load is applied to the probe 31 during welding, and the rotary tool 3 is likely to be damaged. Also, when performing friction stir welding using the conventional rotary tool 4 shown in FIG. 4, the workpiece cannot be sufficiently stirred, and poor bonding is likely to occur.

[0021] Therefore, the inventors of the present invention have intensively studied means for suppressing damage to the rotary tool and sufficiently stirring the workpiece, and obtained the following conclusions.

[0022] (1) When performing friction stir welding, by making the surface of the tip of the rotary tool a convex smooth surface, the load applied to the rotary tool is reduced, and it becomes less likely to be damaged. (2) By making the surface of the peripheral part, where the stress load is smaller compared to the central part (the most tip part) of the tip, into a undulating shape, the plastic fluidity can be enhanced.

[0023] Based on the above conclusions, the inventors of the present invention have completed a rotary tool for friction stir welding that has a small load on the rotary tool and can sufficiently stir the workpiece.

[0024] That is, the present invention for solving the above problems is as follows. [1] A rotary tool used for friction stir welding of two workpieces to be joined, comprising a tip portion having a convex curved surface, wherein the curved surface has a circular first smooth surface disposed at the center, an annular undulating surface disposed on the outer periphery of the first smooth surface, and an annular second smooth surface disposed on the outer periphery of the undulating surface, and a rotary tool for friction stir welding characterized by this.

[0025] [2] The rotary tool for friction stir welding according to [1], wherein the undulating surface has a spiral shape.

[0026] [3] The rotary tool for friction stir welding according to [2], wherein the undulating surface is disposed in a range of 0.20r to 0.90r from the central axis of the rotary tool, where r is the radius of the rotary tool.

[0027] [4] The rotary tool for friction stir welding according to any one of [1] to [3], wherein the rotary tool is made of a material harder than the workpiece to be joined.

[0028] [5] In a two-sided friction stir welding method in which a pair of rotary tools disposed on one side and the other side of the butted portion or the overlapped portion of two workpieces to be joined are rotated in opposite directions and pressed against the butted portion or the overlapped portion of the workpieces to be moved in the joining direction, while softening the unjoined portion of the workpieces to be joined by the frictional heat between the rotary tool and the unjoined portion of the workpieces to be joined, and generating plastic flow by stirring the softened portion with the rotary tool to join the workpieces to be joined, A friction stir welding method characterized by using the rotary tool for friction stir welding according to any one of [1] to [4] as the rotary tool.

Effect of the Invention

[0029] According to the present invention, it is possible to provide a highly durable rotary tool for friction stir welding in which breakage of the rotary tool hardly occurs even when friction stir welding a high-rigidity workpiece such as a steel plate.

Brief Description of the Drawings

[0030]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Embodiments for Carrying Out the Invention

[0031] (Rotary tool for friction stir welding) Hereinafter, with reference to each figure, an embodiment of a rotary tool for friction stir welding according to the present invention will be described. Note that the present invention is not limited to this embodiment. FIG. 1 is a figure for explaining an example of a rotary tool for friction stir welding according to the present invention, (a) is a side view, and (b) is a plan view. The rotary tool 1 shown in FIG. 1 is a rotary tool used for friction stir welding of two workpieces to be joined, and includes a tip portion 10 having a convex curved surface 11. Here, the curved surface 11 is characterized by having a circular smooth surface (first smooth surface) 12 disposed at the center, an annular undulating surface 13 disposed on the outer periphery of the first smooth surface 12, and an annular smooth surface (second smooth surface) 14 disposed on the outer periphery of the undulating surface 13. As shown in FIG. 1(b), the tip portion 10 of the rotary tool 1 is circular in plan view.

[0032] The first smooth surface 12 is the surface of the most distal part (i.e., the part near the rotation axis of the rotary tool 1) at the distal end portion 10 of the rotary tool 1. By having the surface of this part as a smooth surface, when joining a highly rigid workpiece such as a steel plate, the load on the rotary tool 1 can be reduced, and breakage of the rotary tool 1 can be suppressed. In this specification, the "smooth surface" means a surface where, when the rotary tool 1 is arranged with its rotation axis along the vertical direction, the height position changes continuously and monotonically as it goes radially outward from the rotation axis of the rotary tool 1. Specifically, the first smooth surface 12 is a surface without grooves, steps, or the like.

[0033] The undulating surface 13 is the surface of the portion arranged on the outer periphery of the first smooth surface 12 (i.e., radially outward from the rotation axis of the rotary tool 1). By having the distal end portion 10 of the rotary tool 1 have the undulating surface 13, sufficient plastic flow of the workpiece can occur, and joining defects can be suppressed. In the rotary tool 1 shown in FIG. 1, the undulating surface 13 is formed by providing a groove G in the convex smooth curved surface 11 of the distal end portion 10. In this specification, the "undulating surface" means a surface that has a portion where the height position changes discontinuously as it goes radially outward from the rotation axis of the rotary tool 1 when the rotary tool 1 is arranged with its rotation axis along the vertical direction. Specifically, the undulating surface 13 is a surface having grooves, steps, or the like.

[0034] The shape of the groove G is not particularly limited and can be any shape such as a rectangular cross-section as shown in FIG. 1, semi-circular, semi-elliptical, V-shaped, polygonal, or the like. Among these, a rectangular shape is preferable because it can cause the plastic flow of the workpiece to occur most favorably.

[0035] Further, the width Gw of the groove G is preferably 0.1 mm or more and 5 mm or less. Thereby, plastic flow of the material to be joined can be further caused, and joining failure can be more suppressed. Further, the depth Gd of the groove G is preferably 0.1 mm or more and 3 mm or less. Thereby, plastic flow of the material to be joined can be further caused, and joining failure can be more suppressed. The width Gw of the groove G preferably satisfies the above range at any position in the extending direction of the groove G. Further, the depth Gd of the groove G preferably satisfies the above range at any position of the central portion, outer peripheral portion, and inner peripheral portion of the groove G.

[0036] The undulating surface 13 preferably has a spiral shape (volute shape). Thereby, plastic flow of the material to be joined can be further caused. Then, the material of the material to be joined such as metal softened by frictional heat can be selectively flowed in the central direction or the normal direction of the rotation axis of the rotary tool 1, and the stirring of the material can be controlled better.

[0037] The volute constituting the spiral shape is preferably provided in a direction opposite to the rotation direction of the rotary tool 1 (that is, so as to face the central portion (first smooth surface 12) side along the rotation direction). Further, the number of volutes is preferably one or more. The number of volutes depends on the diameter of the tip portion 10, and it is preferable that the larger the diameter of the tip portion 10 is, the more the number is, and the smaller the diameter of the tip portion 10 is, the fewer the number is. Specifically, when the diameter of the tip portion 10 is less than 6 mm, the number of volutes is preferably 2 or less, and when the diameter of the tip portion 10 is 6 mm or more, the number of volutes is preferably 3 to 6. In the rotary tool 1 shown in FIG. 1, the number of volutes is 4.

[0038] When the radius of the rotary tool 1 is r, the undulating shaped surface 13 is preferably arranged in the range of 0.20r to 0.90r from the central axis of the rotary tool 1. More preferably, when the radius of the rotary tool 1 is r, the undulating shaped surface 13 is arranged only within the range of 0.20r to 0.90r from the central axis of the rotary tool 1. Even more preferably, it is arranged only within the range of 0.40r to 0.70r. Thereby, the load on the rotary tool 1 can be reduced, and sufficient compositional fluidity of the material to be joined can be ensured.

[0039] The second smooth surface 14 is a surface arranged on the outer periphery of the undulating shaped surface 13. By the tip portion 10 of the rotary tool 1 having the second smooth surface 14, similar to the first smooth surface 12, the load on the rotary tool 1 can be reduced when joining a high-rigidity material to be joined such as a steel plate, and breakage of the rotary tool 1 can be suppressed. Also, by arranging the second smooth surface 14 on the outer periphery of the undulating shaped surface 13, the plastic fluidity of the material to be joined by the undulating shaped surface 13 can be adjusted.

[0040] The rotary tool 1 is preferably made of a material harder than the material to be joined. Thereby, frictional heat can be generated better in the material to be joined such as a steel plate and softened, and the softened portion can be stirred with the rotary tool to cause better plastic flow and better joining. The above-mentioned hardness of the material to be joined such as a steel plate and the rotary tool 1 can be measured using, for example, the high-temperature Vickers hardness test method (JIS Z 2252).

[0041] Note that the material to be joined is not particularly limited, but steel plates such as ultra-high tensile (high-tensile steel plates) can be preferably joined. Also, the plate thickness of the material to be joined is preferably 20 mm or less. When joining two materials to be joined with different plate thicknesses, the plate thickness ratio (plate thickness of the thicker steel plate / plate thickness of the thinner steel plate) is preferably 1.6 or less. By setting the plate thickness and plate thickness ratio of the material to be joined within the above ranges, the material to be joined can be joined better.

[0042] When using a steel plate as the workpiece to be joined, common structural steels or carbon steels can be preferably used as the target steel grades, such as hot-rolled steel for welded structures in JIS (Japanese Industrial Standards) G 3106 and carbon steel for mechanical structures in JIS G 4051. In addition, high-strength structural steels with a tensile strength of 800 MPa or more can also be preferably used. Even when using such a steel plate, at the joint, a strength of 85% or more, more preferably 90% or more, and even more preferably 95% or more of the tensile strength of the steel plate (base material) can be obtained.

[0043] The rotary tool 1 according to the present invention can be preferably used in a two-sided friction stir welding method for butt-joining two workpieces to be joined.

[0044] Thus, the tip 10 of the rotary tool 1 according to the present invention has a convex curved surface 11. As a result, the load on the rotary tool 1 when joining the workpieces to be joined is reduced, and breakage of the rotary tool 1 can be suppressed. Further, since the tip 10 has a convex curved surface 11, not only two workpieces to be joined with the same thickness but also two workpieces to be joined with different thicknesses can be joined well. Furthermore, in the rotary tool 1, since the convex curved surface 11 has an annular undulating surface 13 on the outer periphery of the first smooth surface 12, sufficient plastic flow of the workpiece to be joined can occur, and defective joining can be suppressed.

[0045] FIG. 2 is a diagram for explaining another example of the rotary tool for friction stir welding according to the present invention, where (a) is a side view and (b) is a plan view. The rotary tool 2 shown in FIG. 2 includes a tip 20 having a convex curved surface 21. Here, the curved surface 21 has a circular smooth surface (first smooth surface) 22 disposed at the center, an annular undulating surface 23 disposed on the outer periphery of the first smooth surface 22, and an annular smooth surface (second smooth surface) 24 disposed on the outer periphery of the undulating surface 23. As shown in FIG. 2(b), the tip 20 of the rotary tool 2 is circular in plan view.

[0046] In the rotary tool 2 shown in FIG. 2, the undulating surface 23 is formed by providing a step S. Similar to the rotary tool 1 shown in FIG. 1, the rotary tool 2 having such an undulating surface 23 can also cause plastic flow of the workpiece to be joined and suppress joining defects. Further, since the tip 20 of the rotary tool 2 has a convex curved surface 21, the load on the rotary tool 2 during joining can be reduced, the breakage of the rotary tool 2 can be suppressed, and two workpieces to be joined with different thicknesses can be joined well.

[0047] The width Sw of the step S is preferably 0.1 mm or more and 5 mm or less. Thereby, plastic flow of the workpiece to be joined can be further caused, and joining defects can be more suppressed. Further, the depth Sd of the step S is preferably 0.1 mm or more and 3 mm or less. Thereby, plastic flow of the workpiece to be joined can be further caused, and joining defects can be more suppressed. The width Sw of the step S preferably satisfies the above range at any position in the extending direction of the step S. Further, the depth Sd of the step S preferably satisfies the above range at any position of the central portion, the outer peripheral portion, and the inner peripheral portion of the step S.

[0048] (Friction Stir Welding Method) The friction stir welding method according to the present invention is a friction stir welding method in which a pair of rotary tools arranged on one side and the other side of the butted portion or the overlapped portion of two workpieces to be joined are rotated in opposite directions and pressed against the butted portion or the overlapped portion of the workpieces to be joined and moved in the joining direction, while softening the unjoined portion of the workpieces to be joined by the frictional heat between the rotary tool and the unjoined portion of the workpieces to be joined, and causing plastic flow by stirring the softened portion with the rotary tool to join the workpieces to be joined together. Here, as the rotary tool, the rotary tool for friction stir welding according to the present invention described above is used.

[0049] As described above, the rotating tool according to the present invention has a convex curved surface at its tip. Therefore, by joining the workpieces to be joined using the rotating tool according to the present invention, the load on the rotating tool can be reduced, and breakage of the rotating tool can be suppressed. In addition, two workpieces to be joined with different thicknesses can be joined well. Furthermore, the rotating tool according to the present invention has an annular undulating surface on the outer periphery of the first smooth surface of the convex curved surface. Therefore, by joining the workpieces to be joined using the rotating tool according to the present invention, sufficient plastic flow of the workpieces to be joined can be caused, joining defects can be suppressed, and a joint without defects can be produced.

[0050] By the friction stir welding method according to the present invention, joining can be performed on both the overlapping portion where two workpieces to be joined are overlapped and the butting portion where two workpieces to be joined are butted against each other. In particular, good joining can be performed on the butting portion.

[0051] FIG. 7 is a diagram for explaining an example of the friction stir welding method according to the present invention, and is a diagram relating to the case of joining the butting portion of two steel plates. As shown in FIG. 7, the end faces (butting faces) of two steel plates are butted against each other and arranged, and are gripped by a gripping device. Next, a pair of rotating tools (for example, the rotating tool 1 shown in FIG. 1) arranged on one side and the other side of the unjoined portion of the steel plate are rotated in opposite directions to each other, and the rotating tools are inserted into the unjoined portion of the steel plate located on the joining center line. Then, the rotating tools are moved in the joining direction along the portion to be joined while rotating while pressing the steel plate at the unjoined portion. Thereby, while softening both steel plates by the frictional heat between the rotating tools and the steel plates, plastic flow is generated by stirring the softened portions with the rotating tools, and the steel plates can be joined. The portion where joining is completed at the butted portion is the joined portion.

[0052] Regarding the joining conditions, when the rotary tool according to the present invention is used in the two-sided friction stir welding method, the rotational speed of the rotary tool is preferably 100 to 5000 r / min, more preferably 300 to 3000 r / min. By setting the rotational speed within this range, while maintaining a good surface shape, there is an effect of suppressing a decrease in mechanical properties due to excessive heat input.

[0053] Also, the joining speed is preferably 300 mm / min or more, more preferably 500 mm / min or more. By setting the joining speed within this range, there is an effect of suppressing a decrease in mechanical properties due to excessive heat input.

[0054] Furthermore, the inclination angle of the rotary tool (the angle formed between the rotation axis of the rotary tool and the vertical direction when the rotation axis of the rotary tool is inclined rearward in the joining direction from the perpendicular line in the vertical direction) is preferably 3 degrees or less.

[0055] Thus, by performing friction stir welding of the workpieces using the rotary tool according to the present invention, it is possible to reduce the load on the rotary tool and produce a joint without defects.

Examples

[0056] Hereinafter, the actions and effects of the present invention will be described using examples. Note that the present invention is not limited to the following examples.

[0057] (Inventive Examples 1 to 8) Two workpieces were joined by the friction stir welding method according to the present invention. At that time, as the workpieces, two 1180 MPa grade cold rolled steel sheets with different thicknesses (thickness 1.2 mm, 1.6 mm) were used. The average Vickers hardness of 5 measurement points measured for the steel sheet base material is 401.

[0058] The end faces of two steel plates with different thicknesses were flattened by milling, and the end faces were butted as shown in Fig. 7. Next, the rotary tool 1 shown in Fig. 1 was arranged on each of one side and the other side of the unjoined portion of the steel plates. At that time, the rotary tool 1 on the upper surface side was arranged without inclining in the direction perpendicular to the joining direction (that is, with the rotation axis of the rotary tool 1 perpendicular to the surface of the steel plate), and a pair of rotary tools were arranged on one side (upper surface side) and the other side (lower surface side) of the unjoined portion.

[0059] The dimensions of the rotary tool 1 are as follows: the diameter D1 of the rotary tool 1 is 25 mm, the diameter D2 of the first smooth surface 12 is 12 mm, the diameter D3 of the undulating surface 13 is 16 mm, and the radius of curvature of the curved surface 11 is 20 mm. Also, the depth Gd of the groove G is 0.5 mm, and the width Gw of the groove G is 0.5 mm. Note that the width Gw and the depth Gd of the groove G are the values at the central groove G and the central position, and there is a difference of ±0.1 mm as an unavoidable difference in the shape of the rotary tool 1. The same applies to the following examples.

[0060] The rotary tool 1 on the upper surface side arranged as described above was rotated counterclockwise, and the rotary tool 1 on the lower surface side was rotated clockwise, and the pair of rotary tools 1 were rotated in opposite directions to each other. In this state, the rotary tool 1 was pressed against the steel plate from both the upper surface side and the lower surface side of the steel plate, and the pair of rotary tools 1 were moved in the joining direction, and the joining of the steel plate with a joining length of 1 m was carried out 50 times to produce a joined joint with a joining length of 50 m. The joining conditions are shown in Table 1.

[0061]

Table 1

[0062] (Inventive Examples 9 - 18) In the same manner as in Invention Examples 1 to 8, two workpieces were joined by the friction stir welding method according to the present invention. However, as the rotating tool, the rotating tool 2 shown in FIG. 2 was used. The dimensions of the rotating tool 2 are as follows: the diameter D1 of the rotating tool 2 is 25 mm, the diameter D2 of the first smooth surface 22 is 12 mm, the diameter D3 of the undulating surface 23 is 16 mm, and the radius of curvature of the curved surface 21 is 20 mm. Also, the depth Sd of the step S is 0.1 mm, and the width Sw of the step S is 0.5 mm. Note that the depth Sd and the width Sw of the above step S are the values at the central step S and the central position, and there is a difference of ±0.03 mm as an inevitable difference in the shape of the rotating tool 2. The joining conditions are shown in Table 1.

[0063] (Conventional Examples 1 to 18) In the same manner as in Invention Examples 1 to 8, two workpieces were joined by the friction stir welding method according to the present invention. However, as the rotating tools in Conventional Examples 1 to 9, the rotating tool 3 shown in FIG. 3 was used. Also, as the rotating tools in Conventional Examples 10 to 18, the rotating tool 4 shown in FIG. 4 was used. The dimensions of the rotating tool 3 are as follows: the diameter D1 of the rotating tool 3 is 25 mm, the diameter D4 of the probe 31 is 5 mm, the diameter D5 of the shoulder 32 is 10 mm, the probe length L is 0.5 mm, and the concave depth (that is, the difference in height between the lowermost end of the probe 31 in FIG. 3(a) and the height position of the connection portion between the shoulder 32 and the shoulder 33) is 0.3 mm. Also, the dimensions of the rotating tool 4 are as follows: the diameter D1 of the rotating tool 4 is 25 mm, and the radius of curvature of the curved surface 41 is 20 mm. The joining conditions are shown in Table 1.

[0064] (Comparative Examples 1 to 18) Similar to Invention Examples 1 to 8, two workpieces were joined by the friction stir welding method according to the present invention. However, for Comparative Examples 1 to 9, the rotary tool shown in FIG. 5 was used as the rotary tool. For Comparative Examples 10 to 18, the rotary tool shown in FIG. 6 was used as the rotary tool. The dimensions of the rotary tool 5 are as follows: the diameter D1 of the rotary tool 5 is 25 mm, the diameter D2 of the first smooth surface 52 is 12 mm, the diameter D3 of the undulating surface 53 is 25 mm, the radius of curvature of the curved surface 51 is 20 mm, the depth Gd of the groove G is 0.5 mm, and the width Gw of the groove G is 0.5 mm. The dimensions of the rotary tool 6 are as follows: the diameter D1 of the rotary tool 6 is 25 mm, the diameter D2 of the first smooth surface 62 is 6 mm, the diameter D3 of the undulating surface 63 is 25 mm, the radius of curvature of the curved surface 61 is 20 mm, the depth Gd of the groove G is 0.5 mm, and the width Gw of the groove G is 0.5 mm. The joining conditions are shown in Table 1.

[0065] Note that for the above rotary tools 1 to 6, tungsten carbide (WC) with a Vickers hardness of 1090 was used as the material.

[0066] Using the joined joints obtained for Invention Examples 1 to 18, Conventional Examples 1 to 18, and Comparative Examples 1 to 18, the following evaluations were performed.

[0067] Using 10 rotary tools, the above 50 m joining was performed 10 times each, and the following criteria were used for evaluation from the viewpoints of damage to the rotary tool and the appearance of the joint bead. The obtained results are shown in Table 2. <Criteria> ·◎: No damage to the rotary tool, and no burrs or non-uniform bead widths are observed in the joint material at all. ·○: No damage to the rotary tool, and burrs or non-uniform bead widths are observed in the joint material one or more times. ·△: No damage to the rotary tool, and linear surface defects are observed in the joint material one or more times. ·×: There is damage to the rotary tool.

[0068]

Table 2

[0069] As is clear from Table 2, in Invention Examples 1 to 18, it was possible to perform a joint with a joint length of 50 m without breakage of the rotary tool, and burrs or non-uniform bead widths were never confirmed. On the other hand, in Conventional Examples 1 to 18 and Comparative Examples 1 to 18, breakage of the rotary tool was confirmed, or although breakage of the rotary tool was not confirmed, linear surface defects were confirmed one or more times.

Industrial Applicability

[0070] According to the present invention, even when friction stir welding a high-rigidity workpiece such as a steel plate, it is possible to provide a highly durable rotary tool for friction stir welding in which breakage of the rotary tool is unlikely to occur.

Explanation of Signs

[0071] 1, 2, 3, 4, 5, 6 Rotary tool 10, 20, 30, 40, 50, 60 Tip 11, 21, 41, 51, 61 Convex curved surface 12, 22, 52, 62 First smooth surface 13, 23, 53, 63 Undulating surface 14, 24 Second smooth surface 31 Probe 32, 33 Shoulder G Groove Gd Depth of groove Gw Width of groove S Step Sd Depth of groove Sw Width of groove

Claims

1. A rotary tool used for friction stir welding of two workpieces to be joined, comprising a tip portion having a convex curved surface, wherein the curved surface has a circular first smooth surface disposed at the center, an annular undulating surface disposed on the outer periphery of the first smooth surface, and an annular second smooth surface disposed on the outer periphery of the undulating surface, and the undulating surface is disposed only within a range of 0.40r to 0.70r from the central axis of the rotary tool, where r is the radius of the rotary tool. A rotary tool for friction stir welding, characterized in that.

2. The rotary tool for friction stir welding according to claim 1, wherein the undulating surface has a spiral shape.

3. The rotary tool for friction stir welding according to claim 1 or 2, wherein the rotary tool is made of a material harder than the workpiece to be joined.

4. In a two-sided friction stir welding method in which a pair of rotary tools disposed on one side and the other side of the butting portion or the overlapping portion of two workpieces to be joined are rotated in opposite directions and pressed against the butting portion or the overlapping portion of the workpieces to be joined and moved in the joining direction, while softening the unjoined portion of the workpiece to be joined by the frictional heat between the rotary tool and the unjoined portion of the workpiece to be joined, and causing plastic flow by stirring the softened portion with the rotary tool to join the workpieces to be joined together, A friction stir welding method, characterized in that the rotary tool according to any one of claims 1 to 3 is used as the rotary tool.

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