Rotary tool for double-sided friction stir welding

Abstract

The present invention provides a rotating tool for double-sided friction stir welding that can simultaneously suppress the occurrence of defects during welding, increase the welding speed, and improve the durability of the rotating tool. The tip of the rotating tool is made of a material with a dynamic friction coefficient of less than 0.30 and a Vickers hardness of 2500 HV or more.

Description

[Technical field]

[0001] The present invention relates to a rotary tool for double sided friction stir welding. [Background technology]

[0002] As a technique related to friction welding, for example, Patent Document 1 describes: "producing relative motion between a pair of members while biasing the members together; sensing a rate of relative motion at least toward the end of a welding cycle; and biasing the members together under a force controlled in response to the sensed speed of relative motion between the members. has been disclosed.

[0003] However, the friction welding of Patent Document 1 requires the rotation of the members to be joined, such as metal plates, and therefore places limitations on the shapes and dimensions of the members to be joined, such as metal plates.

[0004] In addition, as a technique related to friction stir welding, for example, Patent Document 2 describes: "A method of friction welding comprising providing a continuous or substantially continuous surface of a workpiece with a probe of a material harder than the material of the workpiece, bringing the probe and the workpiece together by frictional heat generated as the probe enters the workpiece to create a flexible layer of the workpiece material around the probe, stopping the relative circular motion and providing a flexible material by hardening around the probe." has been disclosed. In this disclosure, the area where the joined materials are butted or overlapped and not yet joined is referred to as the "unjoined area," and the area that has been joined and integrated is referred to as the "joined area."

[0005] As described above, friction welding as in Patent Document 1 is a method of rotating the members such as metal plates to be welded together and welding them by the frictional heat between the members. On the other hand, friction stir welding as in Patent Document 2 is a method of welding by moving a rotating tool while rotating the metal plates to be welded together while fixing the metal plates to be welded together. For this reason, friction stir welding has the advantage that it can continuously perform solid-phase welding even if the members to be welded are members that are substantially infinitely long in the welding direction. In addition, friction stir welding is a solid-phase welding that utilizes the material flow of metal caused by the frictional heat between the rotating tool and the members to be welded, so it can be welded without melting the unwelded parts. In addition, friction stir welding has many advantages, such as less deformation after welding because the heating temperature is low, less defects in the welded parts because the members to be welded are not melted, and no filler metal is required. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 183979 / 1983 [Patent Document 2] Special Publication No. 07-505090 [Patent Document 3] Patent No. 3261433 [Patent Document 4] Patent No. 4838385 [Patent Document 5] Patent No. 4838388 [Patent Document 6] Patent No. 6825630 [Patent Document 7] Patent No. 6737347 [Patent Document 8] Patent No. 4827359 [Patent Document 9] JP 2007-237258 A [Patent Document 10] Patent No. 5971616 [Patent Document 11] Patent No. 6491363 [Patent Document 12] Patent No. 5156948 [Patent Document 13] International Publication No. 2018 / 030309 [Patent Document 14] Patent No. 5185103 [Patent Document 15] JP 2015-127063 A [Patent Document 16] JP 2003-181655 A [Patent Document 17] JP 2003-290936 A [Patent Document 18] JP 2004-195480 A [Patent Document 19] JP 2011-115846 A Summary of the Invention [Problem to be solved by the invention]

[0007] The above-mentioned friction stir welding has been widely used in the fields of aircraft, ships, railway cars, automobiles, etc. as a method for joining low-melting-point metal materials such as aluminum alloys and magnesium alloys. The reason for this is as follows. When joining the above-mentioned low-melting-point metal materials, it is difficult to obtain satisfactory characteristics of the joint by the conventional arc welding method. In addition, the application of friction stir welding improves productivity and allows the production of high-quality joints.

[0008] Furthermore, in recent years, the application of friction stir welding to structural steel, which is mainly used as a material for structures such as buildings, ships, heavy machinery, pipelines, and automobiles, has also been considered. This makes it possible to avoid solidification cracking and hydrogen cracking, which are issues with conventional fusion welding. Changes in the steel's structure are also suppressed. Therefore, improved joint performance can be expected. In addition, by stirring the joint interface with a rotating tool, clean surfaces can be created and the clean surfaces can come into contact with each other. Therefore, there is also the expectation that there is no need for a preparatory process like diffusion welding. In this way, the application of friction stir welding to structural steel is expected to have many advantages.

[0009] However, friction stir welding of structural steel has problems in terms of workability, such as suppressing the occurrence of defects during welding and increasing the welding speed, and therefore has not become more widespread compared to friction stir welding of low melting point metal materials. Here, examples of defects during welding include defective shapes and defective welding on the joint surface or inside the joint.

[0010] In order to solve the above problems, the application of double-sided friction stir welding to structural steel as disclosed in, for example, Patent Documents 3 to 7 has been considered. In double-sided friction stir welding, as shown in Figs. 1 and 2, a pair of opposing rotating tools press one side and the other side of the unjoined portion of the workpieces to be welded. Therefore, a homogeneous and sufficient material flow is obtained in the thickness direction of the workpieces to be welded. This is expected to suppress the occurrence of defects during welding while increasing the welding speed.

[0011] In addition, in order to popularize friction stir welding of structural steel, it is also necessary to improve the durability of the rotating tool. In other words, if the durability of the rotating tool is insufficient, it is predicted that the rotating tool will need to be repaired due to breakage and wear, and there will be a high probability of poor welding. Therefore, even if the rotating tool has the above-mentioned advantages, if the durability of the rotating tool is insufficient, it will be judged that it is difficult to actually apply friction stir welding to structural steel.

[0012] Here, Patent Documents 8 to 13 disclose rotary tools made of various materials. Patent Documents 14 to 19 disclose rotary tools that do not have a probe (pin) that is subject to a large load during joining and is prone to breakage and wear (hereinafter, also referred to as a probe-less rotary tool).

[0013] However, even if the rotating tools disclosed in Patent Documents 8 to 19 are used in double-sided friction stir welding of structural steel, it is not possible to simultaneously achieve the following points. - Suppresses the occurrence of defects during joining, especially the following defects: When the rotation speed of the rotating tool is increased to increase the joining speed, excessive frictional heat is generated between the rotating tool and the workpieces, which causes the workpieces to burn onto the tip of the rotating tool and gouge out the surface of the joint. -Increase the joining speed. -Improve durability of rotary tools. Therefore, there is a demand to simultaneously achieve the above points.

[0014] The present invention has been developed in consideration of the above-mentioned current situation, and aims to provide a rotating tool for double-sided friction stir welding that makes it possible to simultaneously suppress the occurrence of defects during welding, increase the welding speed, and improve the durability of the rotating tool. [Means for solving the problem]

[0015] The inventors have conducted extensive research in order to achieve the above object. As a result, the inventors found that the above object can be achieved by forming the tip of the rotary tool from a material having a dynamic friction coefficient of less than 0.30 and a Vickers hardness of 2500 HV or more. The present invention was completed based on the above findings and through further investigation.

[0016] That is, the gist and configuration of the present invention are as follows. 1. A rotary tool for double-sided friction stir welding, comprising: The rotating tool for double-sided friction stir welding has a tip made of a material having a dynamic friction coefficient of less than 0.30 and a Vickers hardness of 2500 HV or more.

[0017] 2. A rotating tool for double-sided friction stir welding as described in 1 above, wherein the rotating tool is a probe-less rotating tool and the tip surface of the rotating tool is a flat surface, a convex curved surface, or a concave curved surface.

[0018] 3. A rotating tool for double-sided friction stir welding as described in 2 above, wherein the tip surface of the rotating tool has a spiral-shaped step portion facing in the opposite direction to the rotation direction.

[0019] 4. The rotating tool for double-sided friction stir welding according to any one of 1 to 3 above, which satisfies the relationship of the following formula (1). 4×t < D < 20×t (1) Where: t: thickness of the material to be joined (mm) D: diameter of the tip of the rotating tool (mm) It is. Effect of the Invention

[0020] According to the present invention, there is provided a rotating tool for double-sided friction stir welding that can simultaneously suppress the occurrence of defects during welding, increase the welding speed, and improve the durability of the rotating tool. Furthermore, the rotating tool for double-sided friction stir welding of the present invention can be more advantageously applied to various workpieces such as structural steel, and is extremely advantageous in industry. [Brief description of the drawings]

[0021] [Figure 1] FIG. 1 is a side perspective view showing an example of butt welding by double-sided friction stir welding. [Diagram 2] FIG. 1 is a side perspective view showing an example of lap welding by double-sided friction stir welding. [Figure 3A] FIG. 2 is a top view showing an example of butt welding by double-sided friction stir welding. [Figure 3B] 3B is a view taken along the line AA' in FIG. 3A. [Figure 4A] FIG. 13 is a schematic diagram showing an example of the shape of a rotation tool with a probe. [Figure 4B] FIG. 13 is a schematic diagram showing an example of the shape of a rotation tool with a probe. [Figure 5A] FIG. 13 is a schematic diagram showing an example of the shape of a flat-tip rotating tool without a probe. [Figure 5B] FIG. 13 is a schematic diagram showing an example of the shape of a flat-tip rotating tool without a probe. [Figure 6A] FIG. 13 is a schematic diagram showing an example of the shape of a convex-tip rotating tool without a probe. [Figure 6B] FIG. 13 is a schematic diagram showing an example of the shape of a convex-tip rotating tool without a probe. [Figure 7A] FIG. 13 is a schematic diagram showing an example of the shape of a probe-free concave-tip rotating tool. [Figure 7B] FIG. 13 is a schematic diagram showing an example of the shape of a probe-free concave-tip rotating tool. [Figure 8A] FIG. 13 is a schematic diagram showing an example of a convex-tip rotating tool provided with a stepped portion. [Figure 8B] FIG. 13 is a schematic diagram showing an example of a convex-tip rotating tool provided with a groove-shaped step portion. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] The present invention will be described based on the following embodiments. [1] Rotation tool A rotary tool according to one embodiment of the present invention has a tip portion made of a material having a dynamic friction coefficient of less than 0.30 and a Vickers hardness of 2500 HV or more.

[0023] Material for the tip of the rotating tool: Coefficient of dynamic friction: less than 0.30 and Vickers hardness: material of 2500HV or more The inventors conducted extensive research to simultaneously suppress the occurrence of defects during joining, increase the joining speed, and improve the durability of the rotating tool in double-sided friction stir joining, particularly double-sided friction stir joining of structural steel, and obtained the following findings. In double-sided friction stir welding of structural steel, if the rotation speed of the rotating tool is increased to increase the welding speed, excessive frictional heat is generated between the rotating tool and the workpiece. This causes the workpiece to burn onto the tip of the rotating tool, gouging out the surface of the weld and causing defects. The above phenomenon occurs when the workpieces are suddenly exposed to high temperatures due to excessive frictional heat, causing them to soften and adhere to the tip of the rotating tool. To prevent the above phenomenon, it is effective to suppress the sudden generation of frictional heat. To do so, it is effective to use a material with a dynamic friction coefficient of less than 0.30 at the tip of the rotating tool. In addition, when the rotation speed of the rotating tool is increased to increase the joining speed, a large load is applied to the rotating tool. This makes the rotating tool prone to damage and wear. To prevent this, it is effective to use a high-hardness material, specifically a material with a Vickers hardness of 2500HV or more, at the tip of the rotating tool. For these reasons, the tip of the rotating tool is made of a material with a dynamic friction coefficient of less than 0.30 and a Vickers hardness of 2500 HV or more.

[0024] The kinetic friction coefficient (hereinafter, simply referred to as the kinetic friction coefficient) of the material of the tip of the rotary tool is preferably 0.25 or less. There is no particular lower limit to the kinetic friction coefficient. The kinetic friction coefficient is preferably, for example, 0.12 or more.

[0025] Here, the dynamic friction coefficient is measured by the ball-on-disk method in accordance with JIS (Japanese Industrial Standards) R 1613 (2010). The ball (spherical test piece) is made of the material to be measured (the same material as the material constituting the tip of the rotating tool), and the disk (disk-shaped test piece) is made of SKD11 as specified in JIS G 4404 (2015). The temperature at which the dynamic friction coefficient is measured may be room temperature (e.g., 25°C).

[0026] The Vickers hardness (hereinafter, simply referred to as Vickers hardness) of the material of the tip of the rotary tool is preferably 2700 HV or more, more preferably 2900 HV or more. There is no particular upper limit to the Vickers hardness. For example, the Vickers hardness is preferably 5000 HV or less.

[0027] Here, the Vickers hardness is measured by a Vickers hardness test method in accordance with JIS R 1610 (2003). The test force is 19.61 N. The temperature when measuring the Vickers hardness may be room temperature (for example, 25° C.).

[0028] Here, a material with a dynamic friction coefficient of less than 0.30 and a Vickers hardness of 2500HV or more can be formed, for example, from polycrystalline cubic boron nitride (hereinafter also referred to as PCBN). PCBN can be manufactured by mixing cubic boron nitride (CBN) crystal grains with a binder under a high pressure and high temperature environment. In addition, the dynamic friction coefficient and Vickers hardness of PCBN can be variously adjusted by the size and content of the CBN crystal grains and the type of binder.

[0029] The tip of the rotating tool is the area that comes into contact with the workpieces and their flowing part (softened part) during joining. For example, in the case of a rotating tool with a probe as shown in Figs. 4A and B, the tip of the rotating tool has a probe and a shoulder. In the case of a rotating tool without a probe as shown in Figs. 5A, 5B, 6A, 6B, 7A and 7B, the tip of the rotating tool has a shoulder. It is preferable that the tip of the rotating tool includes an area of ​​0 to 5 mm from the shoulder toward the base part on the opposite side of the rotating tool in the direction of the rotation axis of the rotating tool.

[0030] The shoulder has a flat shape formed by a substantially flat surface or a gently curved surface. The shoulder has a function of contacting the workpieces while rotating during welding and generating frictional heat. The shoulder also has a function of pressing the area softened by heat to prevent the material from scattering and promote plastic flow in the direction of rotation.

[0031] The probe is discontinuous with the shoulder and protrudes almost perpendicularly toward the workpieces. The probe penetrates the softened portion of the workpieces toward the center of the thickness during welding, improving the stirring ability in the vicinity of the center of the thickness. The probe is usually located at the center of the tip.

[0032] The portion of the rotary tool other than the tip may be made of the same material as the tip, or may be made of a different material from the tip. For example, a material harder than the material to be joined is preferable. Specifically, tungsten carbide (WC) or a nickel-based alloy can be exemplified.

[0033] The shape of the rotating tool is not particularly limited, and examples thereof include the above-mentioned rotating tool with a probe (FIGS. 4A and B) and a rotating tool without a probe. However, since the probe is located in a region close to the center in the thickness direction of the materials to be joined during joining, a larger stress is applied to the probe than to the shoulder, and it is prone to breakage and wear. Therefore, from the viewpoint of tool durability, a rotating tool without a probe is preferable. As a rotating tool without a probe, for example, a flat-tip rotating tool in which the tip surface of the rotating tool is a flat surface as shown in FIGS. 5A and B, a convex-tip rotating tool in which the tip surface is a convex curved surface as shown in FIGS. 6A and B, and a concave-tip rotating tool in which the tip surface is a concave curved surface as shown in FIGS. 7A and B are preferable. In addition, the shape of the tip surface in a plane perpendicular to the tool rotation axis (projected area when the tip surface of the rotating tool is projected in a direction parallel to the rotation axis) of both the rotating tool with a probe and the rotating tool without a probe is circular.

[0034] In order to further promote the material flow, it is preferable that the tip surface of the rotary tool has a spiral (spiral) step, particularly a spiral (spiral) step in the opposite direction to the rotation. The spiral step is defined by a radial curve (swirl) that starts from the center of the tip surface of the rotary tool or the periphery of a central circle (a circle of any diameter centered on the center of the tip surface of the rotary tool) of the tip surface of the rotary tool as shown in Figures 5B, 6B, and 7B and extends to the outer periphery of the tip surface of the rotary tool. Note that the number of swirls is four in all of Figures 5B, 6B, and 7B.

[0035] That is, in friction stir welding, frictional heat is generated when the workpieces are pressed and stirred by the rotating tool, softening and flowing the metal material to be joined. By providing a spiral-shaped step portion on the tip surface of the rotating tool in the opposite direction to the rotation direction of the rotating tool, frictional heat is generated from the outside to the inside of the rotating tool. This makes it possible to prevent the metal material from flowing out to the outside of the part pressed by the rotating tool. As a result, it is possible to promote plastic flow in the pressed part. It is also possible to prevent the thickness of the joint from decreasing in proportion to the thickness of the base material. Furthermore, it is possible to form a joint surface with fewer burrs.

[0036] The number of vortices defining the step portion may be one or more. However, if the number of vortices defining the step portion exceeds six, the effect of promoting material flow is reduced. In addition, the complicated shape may make the step portion more susceptible to breakage. Therefore, it is preferable that the number of vortices defining the step portion is six or less.

[0037] In one example, the step portion changes the height position in a step-like manner for each region between the vortices. For example, when the tip surface of the rotary tool is a convex curved surface, the step portion is configured by gradually lowering from the periphery of the center toward the outer periphery of the tip surface of the rotary tool (the end of the tip surface of the rotary tool) as shown in FIG. 8A. Note that, when the tip surface of the rotary tool is a concave curved surface, the step portion is configured by gradually raising from the periphery of the center toward the outer periphery of the tip surface of the rotary tool (the end of the tip surface of the rotary tool). Hereinafter, such a form of the step portion is also referred to as a step-like form. Also, in a cross section including the rotation axis and parallel to the rotation axis, each step portion may be, for example, approximately horizontal.

[0038] In another example, the step portion is formed by providing a recess (groove portion) or a protrusion at the position of the swirl, as shown in Fig. 8B. Hereinafter, such a shape of the step portion is also referred to as a recess-shaped or protrusion-shaped. Examples of the cross-sectional shape of the recess (groove portion) include a U-shape (including a square shape and a semicircular shape), a V-shape, and a V-shape, but any shape that can obtain the above-mentioned effect is acceptable.

[0039] It is also preferable that the relationship of the following formula (1) is satisfied. 4×t < D < 20×t (1) During the ceremony, t: thickness of the material to be joined (mm) D: diameter of the tip of the rotating tool (mm) It is. In butt joining, when the thicknesses of the materials to be joined are different, t is the average thickness of the materials to be joined. In the case of lap joining, t is the thickness of the overlapping part of the materials to be joined (total thickness of the materials to be joined). Also, D is the diameter of the tip of the rotating tool on a plane perpendicular to the rotation axis (the diameter of the projected area when the tip of the rotating tool is projected in a direction parallel to the rotation axis).

[0040] That is, the diameter of the tip of the rotary tool (hereinafter also referred to as tip diameter) is appropriately controlled according to the thickness of the workpiece, particularly to satisfy the above formula (1). This allows the temperature rise caused by the frictional heat generated between the rotary tool and the workpiece and the shear stress caused by the frictional force to be effectively applied to the workpiece. Here, if D is less than 4×t (mm), homogeneous material flow in the thickness direction of the workpiece may not be effectively obtained. On the other hand, if D exceeds 20×t (mm), the region where plastic flow occurs unnecessarily widens, and an excessive amount of heat is input to the joint. This may cause an excessive load to be applied to the joining device. D is preferably 5×t (mm) or more, more preferably 6×t (mm) or more. D is preferably 15×t (mm) or less, more preferably 10×t (mm) or less.

[0041] The base portion of the rotary tool on the opposite side to the tip portion is only required to be attachable to a conventionally known friction stir welding apparatus, and the shape of the base portion is not particularly limited.

[0042] The shape of the rotary tool other than the above is not particularly limited and may follow a conventional method. For example, the rotary tool shown in FIG. 4A has a tip diameter of 12 mm, a probe diameter (hereinafter also referred to as pin diameter): 4 mm, a probe length (hereinafter also referred to as pin length): 0.5 mm, and a concave depth: 0.3 mm. In the rotary tool shown in FIG. 4B, the tip diameter is 20 mm, the pin diameter is 6.7 mm, the pin length is 0.7 mm, and the concave depth is 0.3 mm.

[0043] Furthermore, in double-sided friction stir welding, a pair of rotating tools having the same shape is usually used.

[0044] [2] Friction stir welding Next, friction stir welding using a rotary tool according to one embodiment of the present invention will be described. In friction stir welding, it is sufficient to use a rotary tool according to one embodiment of the present invention, and the joint type and welding conditions may be in accordance with conventional methods.

[0045] First, preferred examples of the joint type include a butt joint as shown in FIG. 1 and a lap joint as shown in FIG. In butt joining, the end faces of the workpieces are placed facing each other, and a rotating tool is pressed against the butt section including the end faces (butt surfaces) of the workpieces while rotating. In this state, the rotating tool is moved in the joining direction to join the workpieces. In lap joining, at least a portion of the ends of the workpieces are overlapped, and a rotating tool is pressed against the overlapping portion while rotating. In this state, the rotating tool is moved in the joining direction to join the workpieces. In Figures 1 and 2, reference numeral 1-1 denotes a rotating tool (front surface side rotating tool), reference numeral 1-2 denotes a tip portion, reference numeral 2-1 denotes a rotating tool (rear surface side rotating tool), reference numeral 2-2 denotes a tip portion, reference numeral 3 denotes a material to be joined, and reference numeral 4 denotes a joining portion.

[0046] Butt joining and lap joining differ only in the shape of the unjoined part; otherwise, the configuration of the equipment is basically the same. Therefore, here, we will explain the case where butt joining is performed by double-sided friction stir welding, as shown in Figure 1.

[0047] In double-sided friction stir welding, for example, a friction stir welding device is used that includes a pair of opposing rotating tools, a driving device for the rotating tools, a gripping device, and a control device that controls the operation of the rotating tools. Note that the control device controls, for example, the tilt angle α of the rotating tools, the distance G between the tips of the rotating tools, the welding speed, and the rotation speed of the rotating tools.

[0048] Then, as shown in FIG. 1, the rotating tools of the friction stir welding apparatus are placed on both sides of the workpieces. The rotating tool placed on the front side (vertical upper side) of the workpieces may be called the front side rotating tool, and the rotating tool placed on the back side (vertical lower side) of the workpieces may be called the back side rotating tool. The workpieces are placed parallel to the welding center line shown in the figure, and are each held by a holding device (not shown). Then, the rotating tool is rotated and pressed against both sides of the unjoined portion (area to be joined) located on the welding center line, that is, the butt joint between the ends of the workpieces. Next, in this state, the rotating tool is moved in the joining direction. As a result, the workpieces are softened by frictional heat between the rotating tool and the workpieces. Then, the softened portion is stirred with the rotating tool to cause plastic flow and join the workpieces. A joint (butt joint) is formed in the part where the joining is completed.

[0049] Here, it is preferable that the rotation direction of the front side rotating tool and the back side rotating tool are opposite when viewed from the front side (or back side) of the workpieces. This allows the rotation torques applied to the workpieces from the front side rotating tool and the back side rotating tool to be cancelled out. As a result, it is possible to simplify the structure of the jig that restrains the workpieces, compared to one-sided friction stir welding, in which the unjoined parts are pressed from one side to be joined.

[0050] If the rotation direction of the front side rotating tool and the rotation direction of the back side rotating tool are the same as seen from the front side (or back side) of the workpieces, the relative speed of one rotating tool with respect to the other rotating tool approaches zero. As a result, the plastic flow of the workpieces approaches a homogeneous state, and the plastic deformation also becomes small. Therefore, heat generation due to the plastic deformation of the material is not obtained, and it becomes difficult to achieve a good joining state. Therefore, from the viewpoint of obtaining a sufficient temperature rise and shear stress homogeneously in the thickness direction of the workpieces to achieve a good joining state, it is preferable to rotate the front side rotating tool and the back side rotating tool in the opposite directions as seen from the front side (or back side) of the workpieces.

[0051] Furthermore, when a rotary tool with a probe is used, it is preferable that the inclination angle α of the rotary tool satisfies the relationship of the following formula (2). 0°< α ≦ 3° (2) Here, α is the inclination angle of the rotation axis of the rotary tool (hereinafter also referred to as the rotation axis of the tool) from the thickness direction of the workpieces (direction perpendicular to the surface of the workpieces) in a plane including the joining direction and the thickness direction (direction perpendicular to the surface of the workpieces), as shown in FIG. 3B. The direction (angle) in which the tip of the rotary tool leads the joining direction is defined as +. FIG. 3A is a top view showing an example of butt joining by double-sided friction stir welding, that is, a view of the state in which double-sided friction stir welding is performed, as seen from above in the vertical direction where the front-side rotary tool is placed. FIG. 3B is a view seen from the A-A' arrow of FIG. 3A. In FIGS. 3A and B, reference numeral 1-1 denotes the rotary tool (front-side rotary tool), reference numeral 1-2 denotes the tip, reference numeral 1-3 denotes the probe, reference numeral 2-1 denotes the rotary tool (rear-side rotary tool), reference numeral 2-2 denotes the tip, reference numeral 2-3 denotes the probe, reference numeral 3 denotes the workpieces, and reference numeral 4 denotes the joining portion.

[0052] That is, the rotating tool is usually made of a material harder than the materials to be joined. However, when a force in the bending direction is applied to the probe, stress is concentrated locally, which may lead to destruction. In this regard, if the rotation axis of the tool is tilted from the thickness direction of the materials to be joined and the tip of the probe is advanced in the joining direction, the load on the rotating tool is received by the rotating tool as a component force that compresses in the rotation axis direction, and the force in the bending direction can be reduced. This makes it possible to improve the durability of the rotating tool.

[0053] Here, when α exceeds 0°, the above-mentioned effect can be obtained. However, when the tilt angle α of the rotary tool exceeds 3°, the front and back surfaces of the joint become concave, which may adversely affect the joint strength. Therefore, it is preferable to set α in the range of 0°<α≦3° for both the front side rotary tool and the back side rotary tool.

[0054] Furthermore, when a rotation tool without a probe is used, it is preferable that α satisfies the relationship of the following formula (3). 0°≦α≦3° (3)

[0055] That is, when using a rotary tool without a probe, there is no need to consider reducing the load of the force in the bending direction on the probe. Therefore, even if the rotation axis of the tool is parallel to the thickness direction of the materials to be joined, that is, even if joining is performed with α = 0°, there is no particular effect on the durability of the rotary tool. However, as mentioned above, if the inclination angle α of the rotary tool exceeds 3°, the front and back surfaces of the joint become concave, which may adversely affect the joint strength. Therefore, it is preferable to set α in the range of 0°≦α≦3° for both the front side rotary tool and the back side rotary tool.

[0056] It is also preferable that the distance G between the tips of the rotary tools satisfies the relationship of the following formula (4). 0.25×t-0.2×D×sinα≦ G ≦0.8×t-0.2×D×sinα (4) Here, G corresponds to the distance (shortest distance) in the thickness direction of the workpieces between the shoulder of the front side rotating tool and the shoulder of the back side rotating tool as shown in Fig. 3B. The definitions of t, D and α are as described above.

[0057] In double-sided friction stir welding, it is advantageous to appropriately control the distance between the tips of the rotating tools from the viewpoint of homogenizing the sufficient temperature rise and the shear stress applied during welding in the thickness direction of the workpieces. Therefore, it is preferable for G to satisfy the relationship of the above formula (4). When α=0°, that is, when the rotation axis of the tool is not tilted from the thickness direction of the workpieces, it is preferable to control G in the range of 0.25×t to 0.8×t. On the other hand, when the rotation axis of the tool is tilted from the thickness direction of the workpieces, it is preferable to satisfy the relationship of the above formula (4) according to α and D.

[0058] Here, by satisfying the relationship of the above formula (4) for G, the tips of the opposing front side rotating tool and back side rotating tool are pressed against the workpieces with a sufficient load, and heat generation and material flow are sufficiently promoted. This promotes uniform material flow in the thickness direction of the workpieces, and a good joint can be obtained. On the other hand, if G exceeds 0.8×t-0.2×D×sinα, the tips of the front side rotating tool and back side rotating tool are not pressed against the workpieces with a sufficient load, and the above effect may not be obtained. Also, if G is less than 0.25×t-0.2×D×sinα, the front and back surfaces of the joint may become concave, which may adversely affect the joint strength.

[0059] The other conditions may be the same as usual. For example, the rotation speed of the rotary tool is preferably 100 to 5000 r / min. By setting the rotation speed of the rotary tool within this range, it is advantageous to suppress the deterioration of mechanical properties due to the input of excessive heat while maintaining a good surface shape of the joint. The rotation speed of the rotary tool is more preferably 500 r / min or more. The rotation speed of the rotary tool is more preferably 3000 r / min or less.

[0060] From the viewpoint of improving the efficiency of construction, the joining speed is preferably 1000 mm / min or more, and more preferably 2000 mm / min or more.

[0061] [3] Material to be joined The material to be joined is not particularly limited. Examples of the material to be joined include high melting point alloys such as steel plates. Specific steel types include general structural steel and carbon steel, such as rolled steel for welded structures of JIS G 3106 (2020) and carbon steel for mechanical structures of JIS G 4051 (2016). High-strength structural steel with a tensile strength of 800 MPa or more can also be advantageously applied as the material to be joined. Even in this case, a tensile strength of 85% or more, or even 90% or more, and even more preferably 95% or more of the tensile strength of the steel material to be joined can be obtained at the joint. EXAMPLES

[0062] The operation and effects of the present invention will be described below using examples, but the present invention is not limited to the following examples.

[0063] Steel plates having the thickness, chemical composition (the balance being Fe and unavoidable impurities), and tensile strength shown in Table 1 were butted together as the materials to be welded, and double-sided friction stir welding was performed using a pair of rotating tools shown in Table 2 under the following conditions (the same conditions for both the front-side rotating tool and the back-side rotating tool) to obtain a welded joint. α:0° G: 0.80 mm Rotating tool speed: 1500r / min Joining speed: 3000mm / min Joint length: 0.5m In addition, for butt joints, a so-called I-groove was made on the end faces of the two steel plates to be joined, with no groove angle, and the two steel plates were butt-joined with their surfaces in a state similar to that of milling.

[0064] The front side rotating tool arranged on the upper side in the vertical direction was rotated clockwise when viewed from above in the vertical direction, and the back side rotating tool arranged on the lower side in the vertical direction was rotated counterclockwise when viewed from above in the vertical direction. In other words, when the tip of each rotating tool is viewed from the front, both rotate counterclockwise. In addition, the front side rotating tool and the back side rotating tool used were rotating tools with the same cross-sectional dimensions and shape. Conditions not specified were set according to the usual method.

[0065] Next, the obtained welded joint was inspected for (I) surface defects and (II) internal defects in the following manner.

[0066] (I) Surface defects The presence or absence of surface defects was confirmed by visually observing the presence or absence of depressions (hereinafter also referred to as depressions) caused by gouging out the surface of the joint. If the above-mentioned depressions were confirmed, their depth D d The displacement (mm) of the welded joint was measured using a laser displacement meter. The presence or absence of surface defects and their significance were judged according to the following criteria. The evaluation results are also shown in Table 2. The above observations were carried out in the steady state, that is, in the region where welding was carried out when the welding speed reached 3000 mm / min. <Judgment criteria> No defects (pass, excellent): No dents observed. Minor Defects (Pass): A dent is found, but the D of the dent is d / t is 0.1 or less in all cases. Significantly defective (failed): D d A concave portion with a / t of more than 0.1 (including a groove-like unbonded portion that penetrates from the front surface to the back surface of the bonded portion) is confirmed. (Note that if a non-bonded state was confirmed, where the joint penetrated from the front to the back, it was deemed that the joint was not established, and the following evaluation was omitted.)

[0067] (II) Internal defects The welded joint was cut in the thickness (vertical) direction so that the surface including the weld perpendicular direction and the thickness direction was the observation surface, and three test pieces were taken. The cut positions (observation surfaces) in the weld direction were 20 mm from the end of the steady part on the weld start side, 20 mm from the end of the steady part on the weld end side, and the middle position of the steady part, and the test pieces were taken so that the cut surfaces at the cut positions were the observation surfaces. Next, the observation surfaces of the obtained test pieces were observed with an optical microscope (magnification: 10 times). The presence or absence of internal defects and their significance were judged according to the following criteria. <Judgment criteria> No defects (pass, excellent): No tunnel-shaped unbonded portions were observed in any of the three test pieces. Minor defects present (pass): A tunnel-shaped unbonded state was observed in one of the three test pieces. Significant defects (fail): Tunnel-like unbonded states were observed in two or more of the three test pieces.

[0068] In addition, No. 1 test pieces as specified in JIS Z 3121 (2013) were taken from the joints, and tensile tests were conducted in accordance with JIS Z 3121 (2013) to measure the tensile strength. The measurement results are shown in Table 2.

[0069] (III) Durability of Rotating Tools Furthermore, the durability of the rotary tool was evaluated as follows. In other words, if the rotating tool breaks or wears, there is a high probability of a poor weld due to internal defects. Therefore, under the same conditions as above, welds were repeatedly performed with a weld length of 0.5 m, and the presence or absence of internal defects was determined for the welded joints obtained using the determination method shown in "(II) Presence or absence of internal defects" above. The durability of the rotary tool was evaluated according to the following criteria based on the maximum number of weldings that maintains the number of joints that are determined to be free of internal defects at 90% or more of the total (hereinafter, also referred to as the maximum number of weldings that maintains 90%). The evaluation results are shown in Table 2. Good (passed, especially excellent): 90% maintained, maximum number of joints is 200 or more Unacceptable (failed): 90% maintained maximum number of joints is less than 200

[0070] Here, the maximum number of weldings with 90% maintenance is the maximum value of N that satisfies the following formula (a) when the presence or absence of internal defects of the welded joints obtained in the joining order is checked and the number of welded joints checked for the presence or absence of internal defects is N. [Number of welded joints that were determined to have no internal defects among the welded joints that were checked for the presence of internal defects] ÷ [Number of welded joints that were checked for the presence of internal defects] × 100 ≧ 90 Formula (a)

[0071] For example, if the welded joints obtained from the first to fourth welds are judged to have no internal defects, and the welded joint obtained from the fifth weld is judged to have an internal defect, For N=4, [Total number of welded joints without internal defects among welded joints confirmed to have internal defects] ÷ [Number of welded joints confirmed to have internal defects N] × 100 =4÷4×100=100≧90 And then, For N=5, [Total number of welded joints without internal defects among welded joints confirmed to have internal defects] ÷ [Number of welded joints confirmed to have internal defects N] × 100 =4÷5×100=80<90 It becomes. That is, in this case, formula (a) is satisfied up to N=4, and formula (a) is not satisfied until N=5, so the maximum number of joinings while maintaining 90% is 4.

[0072] In addition, if the welded joints obtained from the 1st to 10th and 12th to 19th welds are judged to have no internal defects, and the welded joints obtained from the 11th, 20th, and 21st welds are judged to have internal defects, For N=11, [Total number of welded joints without internal defects among welded joints confirmed to have internal defects] ÷ [Number of welded joints confirmed to have internal defects N] × 100 =10÷11×100≒90.9≧90 And then, For N=20, [Total number of welded joints without internal defects among welded joints confirmed to have internal defects] ÷ [Number of welded joints confirmed to have internal defects N] × 100 =18÷20×100=90≧90 And then, For N=21, [Total number of welded joints without internal defects among welded joints confirmed to have internal defects] ÷ [Number of welded joints confirmed to have internal defects N] × 100 =18÷21×100=85.7<90 It becomes. That is, in this case, formula (a) is satisfied up to N=20, and formula (a) is not satisfied until N=21. Therefore, the maximum number of joinings while maintaining 90% is 20.

[0073] [Table 1]

[0074] [Table 2]

[0075] As shown in Table 2, in all of the inventive examples, even when the welding speed was increased to 3000 mm / min and the tool rotation speed was increased to 1500 r / min, the occurrence of defects during welding was suppressed. The durability of the rotating tool was also excellent. Furthermore, in all of the inventive examples, the tensile strength of the weld was 95% or more of the tensile strength of the welded materials, and sufficient joint strength was obtained. On the other hand, the durability of the rotary tool was insufficient in all of the comparative examples. Also, in comparative examples 6 and 7, significant surface defects were observed.

[0076] In addition, when the rotating tool of the above-mentioned invention was used to perform double-sided friction stir welding of various workpieces under various welding conditions with a butt joint or lap joint as the joint type, it was possible to simultaneously suppress the occurrence of defects during welding, increase the welding speed, and improve the durability of the rotating tool, as described above. Also, sufficient joint strength was obtained at the welded portion. [Explanation of symbols]

[0077] 1-1: Rotation tool (front side rotation tool) 1-2:Tip 1-3: Probe 2-1: Rotation tool (reverse side rotation tool) 2-2:Tip 2-3: Probe 3: Material to be joined 4: Joint

Claims

1. A rotary tool for double-sided friction stir welding, comprising: A rotating tool for double-sided friction stir welding, the tip of which is made of a material having a dynamic friction coefficient of less than 0.30 and a Vickers hardness of 2500 HV or more.

2. The rotary tool for double-sided friction stir welding according to claim 1 , wherein the rotary tool is a probe-less rotary tool, and a tip surface of the rotary tool is a flat surface, a convex curved surface, or a concave curved surface.

3. The rotary tool for double-sided friction stir welding according to claim 2 , wherein a tip surface of the rotary tool has a spiral-shaped step portion facing in an opposite direction to the rotation.

4. The rotating tool for double-sided friction stir welding according to any one of claims 1 to 3, which satisfies the following formula (1): 4×t ≦ D ≦ 20×t ・・・(1) Where: t: thickness of the material to be joined (mm) D: diameter of the tip of the rotating tool (mm) It is.

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