Friction Stir Welding Method, Friction Stir Welding Tool, and Friction Stir Welding Tool Set

Abstract

This friction stir welding method causes a friction stir welding tool to pass through a material to be welded, in the thickness direction of the material to be welded, and performs friction stir welding in a state in which a refrigerant is flowing through a through-hole provided in a rotating shaft of the friction stir welding tool.

Description

Technical Field

[0001] The present invention relates to a friction stir welding method and a tool for friction stir welding.

Background Art

[0002] As a typical solid-phase joining method for metal materials, friction stir welding (FSW) is known. In friction stir welding, two metal materials to be joined are joined as follows. That is, the two metal materials are opposed to each other at the portion to be joined, and a probe provided at the tip of the rotating tool is inserted into the portion to be joined. The rotating tool is rotated and moved along the interface to be joined. By causing the metal materials to flow due to frictional heat and the stirring force of the rotating tool, the two metal materials are joined. Friction stir welding has the characteristics that the maximum temperature reached during joining does not reach the melting point of the material to be joined, and the strength reduction at the joint is small compared to conventional fusion welding, and has been rapidly put into practical use in recent years.

[0003] However, while friction stir welding has various excellent characteristics, in addition to the need to press-fit a tool having a higher strength than the material to be joined, a large stress is applied to the tool. Therefore, depending on the material to be joined, the cost and life of the tool become major problems. Specifically, it is as follows. That is, when joining thin plates of relatively soft metals such as aluminum or magnesium, the load on the tool is also small, and there are no particular problems regarding tool life and joining conditions. However, when joining high melting point metals such as steel or titanium, the tool life becomes extremely short.

[0004] In contrast, Patent Document 1 (Japanese Patent Application Laid-Open No. 2003-532543) proposes a friction stir welding tool capable of friction stir welding metal matrix composites (MMCs), ferrous alloys, non-ferrous alloys, and superalloys. Specifically, the friction stir welding tool described in Patent Document 1 includes a shaft portion, a shoulder portion, and a pin, and a high wear-resistant material disposed on at least a part of the shoulder portion and the pin, and the shoulder portion is mechanically fixed to the shaft portion to prevent rotational movement of the shoulder portion with respect to the shaft portion. The high wear-resistant material has a first phase and a second phase, and the high wear-resistant material is manufactured under ultra-high temperature and ultra-high pressure. The friction stir welding tool can functionally friction stir weld MMCs, ferrous alloys, non-ferrous alloys, and superalloys. Further, Patent Document 1 discloses that polycrystalline boron nitride (PCBN) or polycrystalline diamond (PCD) is used as the high wear-resistant material.

[0005] In the friction stir welding tool described in Patent Document 1, materials that cannot be joined when using conventional friction stir welding methods and tools, namely, (i) ferrous alloys such as stainless steel, and (ii) materials including high melting point superalloys containing a small amount of iron or no iron, can be subjected to friction stir welding.

[0006] Also, in general friction stir welding, the probe of the rotating tool is inserted into the portion to be joined from only the surface side of the opposed metal materials. Therefore, there is also a drawback that a kissing bond, which is an unjoined portion between the metal materials, is likely to occur on the back side of the metal materials.

[0007] In order to improve this drawback, for example, as disclosed in Patent Document 2 (Japanese Patent Application Laid-Open No. 2003-181654), a method of performing friction stir welding from both the surface and back sides of a metal material using a rotating tool called a bobbin tool has been proposed.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

[0009] However, since the tool for friction stir welding disclosed in Patent Document 1 is manufactured under ultra-high temperature and ultra-high pressure conditions, the manufacturing cost increases significantly. As a result, it becomes a tool that is prohibitively expensive for general industrial applications. In addition, the manufacture of tools under ultra-high temperature and ultra-high pressure conditions is not suitable for mass production. Therefore, as the demand for tools increases, the tool price will further increase.

[0010] Also, when using the bobbin tool disclosed in Patent Document 2, since the thin probe part moves laterally in a situation where it penetrates the material to be joined, the probe part is likely to break. Therefore, the bobbin tool disclosed in Patent Document 2 cannot basically be applied to the joining of high melting point metals such as steel or titanium.

[0011] In view of the problems in the prior art as described above, an object of the present invention is to provide a friction stir welding method for inexpensively and efficiently obtaining a low-defect joint of high melting point metals such as steel and titanium, and a friction stir welding tool that can be suitably used for the friction stir welding. [Means for Solving the Problems]

[0012] As a result of intensive research on the friction stir welding method and the friction stir welding tool in order to achieve the above object, the present inventor found that it is extremely effective to provide a through hole in the rotation axis of the friction stir welding tool and circulate a refrigerant through the through hole to cool the friction stir welding tool, and thus arrived at the present invention.

[0013] That is, one aspect of the present invention is penetrating the friction stir welding tool in the thickness direction of the material to be joined, Performing friction stir welding in a state where a refrigerant is circulated through a through hole provided in the rotation axis of the friction stir welding tool. Provided is a friction stir welding method characterized by the above.

[0014] Also provided is a friction stir welding tool characterized by having a through hole in the rotation axis.

Advantages of the Invention

[0015] According to the present invention, it is possible to provide a friction stir welding method for inexpensively and efficiently obtaining a low-defect joint of high melting point metals such as steel and titanium, and a friction stir welding tool that can be suitably used for the friction stir welding.

Brief Description of the Drawings

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Embodiments for Carrying Out the Invention

[0017] Hereinafter, representative embodiments of the friction stir welding method and the friction stir welding tool of the present invention will be described in detail with reference to the drawings. However, the present invention is not limited to only these embodiments. In the following description, the same or corresponding parts may be denoted by the same reference numerals, and duplicate descriptions may be omitted. Further, since the drawings are for conceptually explaining the present invention, the dimensions of each component shown and the ratios of those dimensions may be different from the actual ones.

[0018] (1) Friction stir welding method (1-1) Friction stir welding using a bobbin tool Figure 1 shows a schematic diagram of friction stir welding when using a bobbin tool. In Figure 1, the joining direction is indicated by the black straight arrow, and the flow of the refrigerant within the tool is indicated by the thick arrow. The bobbin tool 2 used in the friction stir welding of the present invention has an upper shoulder portion 4 and a lower shoulder portion 6 connected by a probe portion 8 to form an integral unit. Further, the bobbin tool 2 has a through-hole 10 provided on the rotation axis, and the through-hole 10 is formed in the upper shoulder portion 4 - probe portion 8 - lower shoulder portion 6.

[0019] The bobbin tool 2 rotated at high speed is inserted from the end of the joint portion 14 of the workpiece 12, and the workpiece 12 is sandwiched between the upper shoulder portion 4 and the lower shoulder portion 6 to generate frictional heat and processing heat, and friction stir welding can be achieved by frictionally stirring the softened workpiece 12.

[0020] Here, during friction stir welding, by circulating the refrigerant through the through-hole 10, the temperature rise of the bobbin tool 2 is suppressed, and by making the bobbin tool 2 at a lower temperature than the workpiece 12, the bobbin tool 2 can be made to have a longer service life. In addition, friction stir welding can be performed using the bobbin tool 2 made of a material having a room temperature strength lower than that of the workpiece 12.

[0021] The type of refrigerant is not particularly limited as long as the effects of the present invention are not impaired, and various conventionally known refrigerants can be used. However, by using water, an increase in the process cost can be suppressed. Rust inhibitors or the like may be added to the water as necessary. As refrigerants other than water, for example, liquid CO2, liquid nitrogen, etc. can be used.

[0022] By circulating the refrigerant discharged from the through-hole 10 of the lower shoulder portion 6 and re-introducing it from the through-hole 10 of the upper shoulder portion 4, an increase in the process cost can be suppressed. When using liquid CO2 as the refrigerant, since the liquid CO2 vaporizes, such a circulation mechanism is unnecessary.

[0023] The flow velocity and flow rate of the refrigerant may be appropriately adjusted according to the material, shape, and size of the bobbin tool 2 and the material, shape, and size of the material to be joined 12 so that a desired friction stir portion is formed. Here, by providing a mechanism for circulating the refrigerant through the through hole 10, the flow velocity and flow rate of the refrigerant can be easily controlled. When water is used as the refrigerant, for example, when the through hole has a diameter of φ2 mm, the flow rate (related to the flow velocity) can be set to about 1.0 to 5.0 L / min.

[0024] Regarding the friction stir welding conditions such as the tool rotation speed and the tool movement speed, they may be appropriately adjusted according to the material, shape, and size of the bobbin tool 2 and the material, shape, and size of the material to be joined 12 so that a desired friction stir portion is formed.

[0025] (1-2) Reverse friction stir welding FIG. 2 shows a schematic diagram of reverse friction stir welding. In FIG. 2, the joining direction is indicated by a black straight arrow, and the flow of the refrigerant in the tool is indicated by a thick arrow. The probe portion 8 is inserted into the joint portion 14 of the material to be joined 12, and the shoulder portion 20 is brought into contact with the back surface of the material to be joined 12. A surface plate 22 is brought into contact with the surface of the material to be joined 12, the material to be joined 12 is sandwiched between the shoulder portion 20 and the surface plate 22, and the shoulder portion 20 and the probe portion 8 are rotated and moved along the joint interface to join the materials to be joined 12 together. The control method of the joining is not particularly limited, and various conventionally known control methods can be used. For example, tool position constant control, load constant control, and torque constant control can be used.

[0026] Here, the reverse friction stir welding tool 30 used in the friction stir welding method of the present invention has a through hole 10 provided in the rotation axis, and the through hole 10 is formed in the probe portion 8 to the shoulder portion 20.

[0027] The reversed friction stir welding tool 30 rotated at high speed is pulled up from the back surface of the workpiece 12, and the workpiece 12 is sandwiched between the shoulder part 20 and the surface plate 22 to generate frictional heat and processing heat. By frictionally stirring the softened workpiece 12, friction stir welding can be achieved. Note that the surface plate 22 is provided with an opening through which the probe part 8 can penetrate.

[0028] Here, during friction stir welding, by circulating a refrigerant through the through-hole 10, the temperature rise of the reversed friction stir welding tool 30 is suppressed. By making the reversed friction stir welding tool 30 have a lower temperature than the workpiece 12, the reversed friction stir welding tool 30 can be made to have a longer service life. In addition, friction stir welding can be performed using the reversed friction stir welding tool 30 made of a material whose room temperature strength is lower than that of the workpiece 12.

[0029] Here, by fixing the distance between the shoulder part 20 and the surface plate 22 to be constant, reversed friction stir welding by position control can be performed. Also, by performing friction stir welding while pulling up the shoulder part 20 and the probe part 8, reversed friction stir welding by load control can be performed. Note that the reversed friction stir welding by load control can easily follow changes in the plate thickness. Also, although butt welding is shown in FIG. 2, it may be lap welding. In addition, it can also be used as a processing method such as surface treatment.

[0030] Since friction stir welding is performed from the back surface of the workpiece 12, even when, for example, the stirring shaft (probe part 8) of the tool is not inclined (tool forward angle 0°), an unjoined part such as a kissing bond does not occur on the back surface of the workpiece 12. In addition, the stirring shaft (probe part 8) of the tool can be inclined, and a tool forward angle can be imparted during friction stir welding. Due to this forward angle, the material flow proceeds smoothly, so the joinable conditions are expanded and it becomes possible to prevent joining defects. Note that the tool forward angle is preferably more than 0° and 7° or less.

[0031] Also, in the example shown in FIG. 2, members that rotate are not brought into contact with both surfaces of the workpiece 12 around the joint like the bobbin tool, and the undulations of the surface plate 22 are transferred to the surface of the workpiece 12. Therefore, by bringing the surface plate 22 having a smooth surface into contact, it is possible to obtain a good joint surface without causing marks after friction stir welding on the surfaces of the workpieces 12 around the joint.

[0032] Furthermore, in the reverse friction stir welding, it is not necessary to perform operations from both the front and back surfaces of the workpiece 12 like the conventional friction stir welding method in which the probe of the rotating tool is inserted into the joint from only the front surface side of the workpiece 12 while bringing the backing plate into contact with the back surface side. Most of the operations can be performed from only the front surface side of the workpiece 12. Therefore, similar to the case of using the bobbin tool, the workpiece 12 can be easily joined by operations from only the outside for structures with a closed structure such as the chassis of equipment and vehicles.

[0033] The cooling method of the reverse friction stir welding tool 30 and the adjustment of various friction stir welding conditions are the same as those in the case of using the bobbin tool.

[0034] (1-3) Other friction stir weldings FIG. 3 schematically shows an example of a mode in which a friction stir welding tool 40 having a general shape is pressed into the workpiece 12 from the front surface side. In FIG. 3, the welding direction is indicated by a black straight arrow, and the flow of the refrigerant in the tool is indicated by a thick arrow. The friction stir welding tool 40 has the shape of a conventional general friction stir welding tool in which a probe portion 8 is provided at the center of the shoulder portion 20 except that a through hole 10 is formed.

[0035] Insert the probe portion 8 into the joint portion 14 of the workpiece 12, and bring the shoulder portion 20 into contact with the surface of the workpiece 12. Bring the back plate 24 into contact with the back surface of the workpiece 12, sandwich the workpiece 12 between the shoulder portion 20 and the back plate 24, and move along the joint interface while rotating the shoulder portion 20 and the probe portion 8 to join the workpieces 12 together. Note that the joining control method is not particularly limited, and various conventionally known control methods can be used. For example, tool position constant control, load constant control, and torque constant control can be used.

[0036] Here, in the friction stir welding tool 40 having a general shape used in the friction stir welding method of the present invention, there is a through hole 10 provided in the rotating shaft, and the through hole 10 is formed in the shoulder portion 20 to the probe portion 8.

[0037] Press the friction stir welding tool 40 having a general shape that is rotated at high speed against the surface of the workpiece 12, and sandwich the workpiece 12 between the shoulder portion 20 and the back plate 24 to generate frictional heat and processing heat, and achieve friction stir welding by frictionally stirring the softened workpiece 12. Note that the back plate 24 is provided with a recess or an opening through which cooling water can flow at the penetration position of the probe portion 8.

[0038] Here, during friction stir welding, by circulating a refrigerant through the through hole 10, the temperature rise of the friction stir welding tool 40 having a general shape is suppressed, and the friction stir welding tool 40 having a general shape is made to have a lower temperature than the workpiece 12, so that the friction stir welding tool 40 having a general shape can be made to have a longer life. In addition, friction stir welding can be performed using a friction stir welding tool 40 having a general shape made of a material having a room temperature strength lower than that of the workpiece 12.

[0039] Here, by fixing the distance between the shoulder part 20 and the back plate 24 to be constant, friction stir welding can be performed by position control. Also, by performing friction stir welding while pressing the shoulder part 20 and the probe part 8, friction stir welding by load control can be performed. Note that friction stir welding by load control can easily follow changes in the plate thickness. Also, although butt welding is shown in FIG. 3, it may be lap welding.

[0040] Since the probe part 8 performs friction stir welding in a state of penetrating the workpiece 12 in the plate thickness direction, for example, even when no inclination is provided on the stirring shaft (probe part 8) of the tool (tool advance angle 0°), an unwelded part such as a kissing bond does not occur on the back surface of the workpiece 12. In addition, the stirring shaft (probe part 8) of the tool can be inclined, and a tool advance angle can be imparted during friction stir welding. Due to this advance angle, material flow proceeds smoothly, the weldable conditions are expanded, and it becomes possible to prevent welding defects. Note that the tool advance angle is preferably more than 0° and 7° or less.

[0041] Also, since a rotating member does not contact both surfaces of the workpiece 12 around the joint like a bobbin tool, and the undulations of the back plate 24 are transferred to the back surface of the workpiece 12, by bringing the smooth-surfaced back plate 24 into contact, it is possible to obtain a good joint surface without causing marks after friction stir welding on the back surface of each of the workpieces 12 around the joint.

[0042] Regarding the cooling method of the friction stir welding tool 40 having a general shape, the adjustment of various friction stir welding conditions, etc., it is the same as when using a bobbin tool. In addition, it can also be used as a processing method such as surface treatment.

[0043] (2) Friction Stir Welding Tool (2-1) Bobbin Tool Figures 4 and 5 show schematic diagrams of the bobbin tool 2. In Figure 4, the through-hole 10 has a diameter of φ2 mm, and in Figure 5, the through-hole 10 has a diameter of φ3 mm. In Figures 4 and 5, the upper shoulder portion 4 and the lower shoulder portion 6 have a diameter of φ12 mm, and the probe portion 8 has a diameter of φ6 mm. The diameter of the through-hole 10 may be appropriately adjusted according to the material, diameter, etc. of the bobbin tool 2. However, when the absolute strength of the probe portion 8 is required, it is preferably made thinner, and when it is desired to increase the cooling effect, it is preferably made thicker.

[0044] The shape and size of the upper shoulder portion 4, the probe portion 8, and the lower shoulder portion 6 are not particularly limited as long as the effects of the present invention are not impaired, and can be the shapes and sizes of various conventionally known bobbin tools. Further, by forming a coating layer such as TiAlN, TiN, CrN, HfO2, etc. having good heat resistance and wear resistance on the surface of the tool, the bobbin tool 2 can be made to have a longer service life.

[0045] The material of the bobbin tool 2 is not particularly limited as long as the effects of the present invention are not impaired, but it is possible to friction stir weld steel materials with a steel-made bobbin tool 2. As the material of the tool, for example, carbon tool steel, low alloy tool steel, cold die steel, hot die steel, high speed tool steel, etc. can be used. In addition, for example, by making the bobbin tool 2 made of a copper alloy or the like having excellent thermal conductivity, the cooling effect by the refrigerant can be maximally exhibited.

[0046] (2-2) Tool for Reverse Friction Stir Welding Figures 6 and 7 show schematic diagrams of the tool 30 for reverse friction stir welding. In Figure 6, the through-hole 10 has a diameter of φ2 mm, and in Figure 7, the through-hole 10 has a diameter of φ3 mm. In Figures 6 and 7, the shoulder portion 20 has a diameter of φ12 mm, and the probe portion 8 has a diameter of φ6 mm. The diameter of the through-hole 10 may be appropriately adjusted according to the material, diameter, etc. of the tool 30 for reverse friction stir welding. However, when the absolute strength of the probe portion 8 is required, it is preferably made thinner, and when it is desired to increase the cooling effect, it is preferably made thicker.

[0047] The shapes and sizes of the shoulder portion 20 and the probe portion 8 are not particularly limited as long as the effects of the present invention are not impaired, and can be the shapes and sizes of various conventionally known tools for friction stir reverse welding. Further, by forming a coating layer having good heat resistance, wear resistance, etc. on the surface of the tool, the tool 30 for friction stir reverse welding can be made to have an even longer service life.

[0048] The material of the tool 30 for friction stir reverse welding is not particularly limited as long as the effects of the present invention are not impaired, but it is possible to friction stir weld steel materials with a steel-made tool 30 for friction stir reverse welding. As the material of the tool, for example, carbon tool steel, low alloy tool steel, cold die steel, hot die steel, high speed tool steel, etc. can be used. In addition, for example, by using a tool 30 for friction stir reverse welding made of a copper alloy having excellent thermal conductivity, the cooling effect by the refrigerant can be maximally exhibited.

[0049] As described above, the representative embodiments of the present invention have been described, but the present invention is not limited to these only, and various design changes are possible, and all of these design changes are included in the technical scope of the present invention.

Example

[0050] ≪Example 1≫ A cold rolled steel sheet (SPCC) of 100 mm × 50 mm × 2.0 mm was used as the test material, and butt friction stir welding was performed using the bobbin tool shown in FIG. 4. The bobbin tool is made of tool steel (SKD61), and has a shape with a shoulder diameter of 12 mm, a probe diameter of 6 mm, and a through hole diameter of 2 mm for the rotating shaft. Further, a TiAlN-based coating with a film thickness of about 5 μm is formed on the surface of the tool.

[0051] The tool rotation speed was 400 rpm, and the joining speed was changed between 10 and 150 mm / min during joining. Also, water was used as the refrigerant and circulated at 2.0 L / min from the upper shoulder portion to the lower shoulder portion of the bobbin tool. The discharged water was circulated so as to flow from the upper side of the bobbin tool.

[0052] The appearance photograph of the obtained friction stir welded joint is shown in Fig. 8. The figure shown on the upper side of Fig. 8 is the appearance photograph of the upper surface which is the tool gripping side in the friction stir welded joint. The figure shown on the lower side of Fig. 8 is the appearance photograph of the lower surface which is the drainage side in the friction stir welded joint. In Example 1, butt friction stir welding was started at a joining speed of 10 mm / min in the direction of the straight line arrow shown between the figure shown on the upper side and the figure shown on the lower side of Fig. 8. Then, the joining speed was accelerated to 150 mm / min from the position where the end point (tip) of the left straight line arrow in Fig. 8 is connected to the start point of another straight line arrow on the right side and where a vertical line is shown in the figure, and butt friction stir welding was performed. In Fig. 8, the rotation direction of the tool is indicated by a curved arrow. The side where the rotation direction of the tool coincides with the joining direction is called AS (Advancing side), and the side where the rotation direction of the tool faces the opposite to the joining direction is called RS (Retreating side). In the figure shown on the upper side of Fig. 8, the upper side is AS and the lower side is RS. In the figure shown on the lower side of Fig. 8, the upper side is RS and the lower side is AS.

[0053] At 400 rpm and 10 - 100 mm / min, periodic disturbances due to tool vibration were observed, but at 150 mm / min, the tool was stable and a joint with a homogeneous and good surface quality was formed. Similarly, a stable and homogeneous joint was formed even when changing directly from 10 mm / min to 150 mm / min at the beginning of joining. By selecting appropriate joining conditions (400 rpm, 150 mm / min), it became clear that friction stir welding of steel materials is possible with a steel bobbin tool.

[0054] The microstructure of the cross-section of the joint obtained at 400 rpm and 150 mm / min is shown in Fig. 9. In the figure shown in the upper part of Fig. 9, with the direction from the front to the back of the paper as the joining direction, the upper side is the upper surface, the lower side is the lower surface, the left side is the above-mentioned AS, the right side is the above-mentioned RS, and the cross-section of the joint is shown such that the stirring part is located in the center. The figure shown in the lower part of Fig. 9 shows the microstructure of the base material in the cross-section. The figure shown in the middle part of Fig. 9 shows, in order from left to right, the microstructure of the base material / stirring part (AS), the microstructure of the stirring part, and the microstructure of the stirring part / base material (RS) in the cross-section.

[0055] No tunnel defects or the like are observed in the cross-section, indicating that a good joint has been formed. The width of the stirring part is wider than the probe diameter and slightly narrower than the shoulder diameter, with the width on the upper surface side being slightly wider. The base metal structure consists of equiaxed ferrite grains about 10 μm in size, while the stirring part structure is composed of fine equiaxed ferrite, and in some parts, the formation of sub-grain boundaries within the grains is observed. Since SPCC has a low carbon content and a high A3 point, the stirring part during joining is at a two-phase region temperature, and it is considered that the stirring part is a mixture of phase transformation ferrite and recrystallized / recovered ferrite.

[0056] The appearance and cross-sectional shape of the bobbin tool used for friction stir joining before and after joining are shown in FIGS. 10 and 11, respectively. The upper, middle, and lower diagrams shown in FIG. 10 represent the appearance of the bobbin tool at the initial stage, after 1 pass, and after 2 passes, respectively. The diagram shown on the upper side of FIG. 11 shows the shape of the bobbin tool. The diagram shown on the lower side of FIG. 11 is a graph showing the shape of the bobbin tool at the initial stage, after 1 pass, and after 2 passes, with the horizontal axis representing the tool longitudinal position and the vertical axis representing the tool radius. In this graph, the initial shape of the bobbin tool is shown by a solid line, the shape after 1 pass is shown by a dashed line, and the shape after 2 passes is shown by a dotted-dashed line. In the graph, the position indicated by the arrow shows the part where the tool radius is 3 mm, that is, the φ6 mm part. No changes in appearance and cross-sectional shape are observed before and after joining, and it can be seen that there is no change in the tool shape for a total joint length of 20 cm.

[0057] <<Example 2>> Using the tool for reverse friction stir joining shown in FIG. 6, reverse friction stir joining was performed in the same manner as in Example 1, except that the tool rotation speed was 400 rpm, the tool travel speed was 10 mm / min, and the lead angle was 3°.

[0058] Fig. 12 shows an external photograph depicting the state of friction stir joining with reverse rotation. In Fig. 12, the direction from the front to the back of the paper is defined as the tool feed direction. The shoulder part is brought into contact with the lower surface (back surface) of the material to be joined, and the state of friction stir joining with reverse rotation conducted under a water-cooled condition is shown. It can be confirmed that the cooling water is discharged from the bottom surface of the shoulder part and that the temperature of the shoulder part has hardly risen (not glowing red) during the joining process.

[0059] Fig. 13 shows an external photograph of the obtained friction stir welded joint. The figure shown on the upper side of Fig. 13 is an external photograph of the upper surface, which is the fixed part side, of the friction stir welded joint. The figure shown on the lower side of Fig. 13 is an external photograph of the lower surface, which is the drainage side, of the friction stir welded joint. In Fig. 13, the butting line is indicated by a dashed line. In Fig. 13, the rotation direction of the tool is indicated by a curved arrow. In the figure shown on the upper side of Fig. 13, the upper side is AS and the lower side is RS. In the figure shown on the lower side of Fig. 13, the upper side is RS and the lower side is AS. Immediately after the start of joining, initial crack-like defects that can also be seen in normal friction stir joining with reverse rotation are confirmed, but from the middle stage onwards, the appearance of the joint part is good. From these results, it became clear that friction stir joining of steel materials is possible using a steel-made tool for friction stir joining with reverse rotation.

[0060] Fig. 14 shows an optical microscope photograph of the cross-section of the joint part of the obtained friction stir welded joint. In the figure shown in the upper row of Fig. 14, with the direction from the front to the back of the paper defined as the joining direction, the upper side is the upper surface, the lower side is the lower surface, the left side is the above-mentioned AS, the right side is the above-mentioned RS, and the cross-section of the joint part is shown such that the stirring part is located in the center. The figure shown on the left side in the middle row of Fig. 14 shows the microstructure of the base material in the cross-section, and the figure shown on the right side shows the microstructure of the heat-affected zone / stirring part (AS). The figure shown on the left side in the lower row of Fig. 14 shows the microstructure of the stirring part in the cross-section, and the figure shown on the right side shows the microstructure of the stirring part / heat-affected zone (RS). It can be seen that no joining defects are observed and a good joint part is formed. The base material is composed of equiaxed ferrite with a size of several tens of μm, and a microstructure in which fine equiaxed ferrite is distributed in the worked ferrite from the heat-affected zone to the stirring part is confirmed.

[0061] <<Comparative Example 1>> Friction stir welding was performed in the same manner as in Example 1, except that the bobbin tool was not provided with a through hole and water cooling was not applied. The tool rotation speed was 400 rpm and the tool travel speed was 10 mm / min.

[0062] Immediately after the start of joining, the temperature of the tool rose to 700 - 800°C, and it was confirmed that the tool became red-hot. It broke from the center of the probe part about 2 minutes after the start of joining. The external appearance photograph of the obtained joint is shown in Fig. 15, and the external appearance photographs of the bobbin tool before and after friction stir welding are shown in Fig. 16. The figure shown on the upper side of Fig. 15 is the external appearance photograph of the upper surface, which is the tool gripping side, of the obtained joint. The figure shown on the lower side of Fig. 15 is the external appearance photograph of the lower surface, which is the drainage side, of the obtained joint. In Fig. 15, the rotation direction of the tool is indicated by a curved arrow. In the figure shown on the upper side of Fig. 15, the upper side is AS and the lower side is RS. In the figure shown on the lower side of Fig. 15, the upper side is RS and the lower side is AS. Friction stir welding was performed at a joining speed of 10 mm / min in the direction of the straight arrow shown between the figure shown on the upper side and the figure shown on the lower side of Fig. 15. Also, the figures shown on the upper side and the lower side of Fig. 16 show the external appearance of the bobbin tool before and after joining, respectively. From the fracture mode of the tool, it is considered that the tool life is due to the strength reduction caused by high temperature rather than wear. Since no through hole was provided, the tool strength is higher than that of the bobbin tool used in Example 1, but it was shown that friction stir welding is impossible when water cooling is not used.

[0063] ≪Comparative Example 2≫ In the same manner as in Example 2, reverse friction stir welding was performed, and the cooling water was stopped about 6 minutes after the start of joining (corresponding to a joining length of 60 mm). While water cooling was being applied, as in the case of Example 2 shown in Fig. 11, the red-hot state of the tool could not be confirmed with the naked eye. Fig. 17 shows an external appearance photograph showing the state immediately after the water cooling was stopped, and it can be seen that the shoulder is red-hot.

[0064] Figure 18 shows the changes in the tool appearance over time during the reverse friction stir welding process. The figures shown in the upper part of Figure 18, from left to right in sequence, depict the tool appearance at the start of welding (-5′58″) at 4:12:91, just before the water cooling stops (-0′00″) at 10:10:71, and at the moment when the water cooling stops (0′00″) at 10:11:00. In the figure shown on the left side of the upper part of Figure 18, the tool is indicated by a dotted line, and the flow of cooling drainage is indicated by a downward arrow. The figures shown in the middle part of Figure 18, from left to right in sequence, depict the tool appearance after the water cooling stops (0′05″) at 10:16:01, after the water cooling stops (0′25″) at 10:36:00, and after the water cooling stops (0′30″) at 10:41:00. The figures shown in the lower part of Figure 18, from left to right in sequence, depict the tool appearance after the water cooling stops (0′50″) at 11:01:10, at the moment of tool breakage (0′55″) at 11:06:00, and after the tool breakage (1′25″) at 11:36:00.

[0065] Five seconds after the water cooling stopped, the red heat of the entire shoulder became apparent. After about 30 seconds (equivalent to a joint length of 5 mm), eccentricity of the shoulder and generation of sparks were observed. Subsequently, as the probe extended, the shoulder separated from the material and completely broke after 55 seconds (equivalent to a joint length of 10 mm). In the case of welding with water cooling, in the joining of two pieces of SPCC with a length of 100 mm and two passes, the tool did not break, indicating that the tool life was improved by at least 20 times or more due to water cooling.

[0066] The appearance photo of the obtained joint is shown in Fig. 19. The figure shown on the upper side of Fig. 19 is the appearance photo of the upper surface which is the fixed part side of the obtained joint. The figure shown on the lower side of Fig. 19 is the appearance photo of the lower surface which is the shoulder side of the obtained joint. In the figure shown on the upper side of Fig. 19, the upper side is AS and the lower side is RS. In the figure shown on the lower side of Fig. 19, the upper side is RS and the lower side is AS. In Fig. 19, the straight line arrows extending upward and downward respectively indicate the time when water cooling stops. Also, the tool is indicated by a white straight line arrow, the burr is indicated by a black diagonal straight line arrow, and the hole is indicated by a bracket on one side. In the water-cooled area, the joint surface is relatively uniform, but large holes are formed near the tool broken without water cooling. From the above results, it was shown that friction stir welding is impossible without using water cooling.

[0067] (Summary) As described above, one aspect of the present invention provides a friction stir welding method characterized by performing friction stir welding in a state where a refrigerant is circulated through a through hole provided in the rotation axis of a friction stir welding tool that penetrates in the thickness direction of a workpiece.

[0068] In the friction stir welding method of one aspect of the present invention, by performing friction stir welding in a state where a refrigerant is circulated through a through hole provided in the rotation axis of a friction stir welding tool, it is possible to suppress the temperature rise of the friction stir welding tool during friction stir welding. More specifically, by using the through hole, for example, it is easier to increase the flow velocity and flow rate of the refrigerant compared to the case where a refrigerant flow path is formed inside the friction stir welding tool, and the temperature rise of the friction stir welding tool can be extremely effectively suppressed.

[0069] In addition, in the friction stir welding method according to one aspect of the present invention, by performing friction stir welding with a refrigerant flowing through a through hole provided in the rotation axis of the friction stir welding tool, while ensuring the temperature rise of the outermost surface of the friction stir welding tool necessary for friction stirring the workpiece, the temperature of the entire friction stir welding tool can be kept low. As a result, not only can good friction stir welding characteristics and a long service life of the friction stir welding tool be achieved simultaneously, but also an inexpensive friction stir welding tool made of a material with a room temperature strength equal to or lower than that of the workpiece can be used.

[0070] In addition, the friction stir welding tool used in the friction stir welding method according to one aspect of the present invention is not particularly limited as long as it does not impair the effects of the present invention, except that a through hole is provided in the rotation axis, and the material, shape, and size of the friction stir welding tool can be various conventionally known ones according to the material, shape, and size of the workpiece, etc.

[0071] In addition, in the friction stir welding method according to one aspect of the present invention, the mode of penetrating the friction stir welding tool in the thickness direction of the workpiece is not particularly limited as long as it does not impair the effects of the present invention, but it is preferable to penetrate the probe portion of the friction stir welding tool in the thickness direction of the workpiece. For example, the probe portion of a friction stir welding tool having a conventionally common shape may be penetrated from the back surface of the workpiece (a recess or opening is provided at the probe penetration position of the back plate disposed on the back surface of the workpiece), and inverse friction stir welding or a bobbin tool may be used.

[0072] In addition, in the friction stir welding method according to one aspect of the present invention, it is preferable to use the friction stir welding tool as a bobbin tool. Since the probe portion of the bobbin tool penetrates the workpiece in a normal use mode, the flow of the refrigerant through the through hole can be easily and simply performed. In addition, since the shoulder portions contact the front and back surfaces of the workpiece, the formation of a kissing bond can be suppressed.

[0073] Also, in the friction stir welding method according to one aspect of the present invention, it is preferable to use reverse friction stir welding for the friction stir welding. Reverse friction stir welding is a technique in which a shoulder portion is brought into contact with the back surface of the workpiece to be joined, a probe portion provided on the shoulder portion penetrates the workpiece to the surface side, and friction stir welding is performed while pulling up the probe portion. Since the probe portion penetrates the workpiece, the flow of the refrigerant through the through-hole can be easily and simply performed. Further, since the shoulder portion is in contact with the back surface of the workpiece and the probe portion penetrates the workpiece, the formation of a kissing bond can be suppressed.

[0074] Also, in the friction stir welding method according to one aspect of the present invention, it is preferable to use water as the refrigerant. The type of the refrigerant is not particularly limited as long as the effects of the present invention are not impaired, and various conventionally known refrigerants can be used. However, by using water, an increase in the process cost can be suppressed. An anti-rust agent or the like may be added to the water as necessary. As refrigerants other than water, for example, liquid CO2 (including those in which a part of gaseous CO2 or solid CO2 is mixed) and liquid nitrogen can be used.

[0075] Also, in the friction stir welding method according to one aspect of the present invention, it is preferable that the friction stir welding tool is made of steel and the workpiece to be joined is made of steel. Conventionally, in order to friction stir weld steel materials, it has been necessary to use expensive tools such as PCBN and tungsten alloys. However, in the friction stir welding method of the present invention, since the friction stir welding tool is efficiently cooled, steel materials can be friction stir welded with an inexpensive steel tool. Further, by friction stir welding steel materials with a steel tool, the influence of contamination from the tool can be minimized. As the material of the tool, for example, carbon tool steel, low alloy tool steel, cold die steel, hot die steel, high speed tool steel, etc. can be used.

[0076] Further, in the friction stir welding method according to one aspect of the present invention, it is preferable to circulate the refrigerant discharged from the friction stir welding tool and cause it to flow into the friction stir welding tool. By reusing the refrigerant, an increase in the process cost can be suppressed. When liquid CO2 is used as the refrigerant, since the liquid CO2 vaporizes, the circulation mechanism is not required.

[0077] Moreover, one aspect of the present invention also provides a friction stir welding tool characterized by having a through hole in the rotating shaft. The friction stir welding tool of the present invention can be suitably used in the friction stir welding method of the present invention. The friction stir welding tool of the present invention is not particularly limited except that a through hole is provided in the rotating shaft as long as the effects of the present invention are not impaired, and can be made of the materials, shapes, and sizes of various conventionally known friction stir welding tools. Further, the hole formed in the friction stir welding tool only needs to be through, and does not need to be linear, nor does the diameter of the hole need to be constant.

[0078] The friction stir welding tool according to one aspect of the present invention is preferably a bobbin tool. The bobbin tool has a relatively simple shape in which two shoulder portions are connected by a probe portion to form an integral body, and a through hole in the rotating shaft can be easily formed. Further, the friction stir welding tool according to one aspect of the present invention is preferably a reverse friction stir welding tool. Since the probe portion of the reverse friction stir welding tool penetrates the workpiece to be joined, it is easy to form a through hole in the rotating shaft.

[0079] Furthermore, the friction stir welding tool according to one aspect of the present invention is preferably made of steel. In addition to being inexpensive, steel is easy to process in various ways, and a friction stir welding tool having an arbitrary shape and size can be easily manufactured. As the steel material for the friction stir welding tool, for example, carbon tool steel, low alloy tool steel, cold die steel, hot die steel, high speed tool steel, etc. can be used.

Explanation of reference numerals

[0080] 2 ··· Bobbin tool (tool for friction stir welding) 4 ··· Upper shoulder part 6 ··· Lower shoulder part 8 ··· Probe part 10 ··· Through hole 12 ··· Workpiece to be joined 14 ··· Joint part 20 ··· Shoulder part 22 ··· Front plate 24 ··· Back plate 30 ··· Tool for reverse friction stir welding 40 ··· Friction stir welding tool with a general shape

Claims

1. A friction stir welding tool is passed through the thickness direction of a workpiece, and friction stir welding is performed with a refrigerant flowing through a through hole provided on the rotation axis of the friction stir welding tool. A friction stir welding method characterized by the above.

2. The probe portion of the friction stir welding tool is passed through the thickness direction of the workpiece. The friction stir welding method according to claim 1, characterized by the above.

3. The friction stir welding tool is a bobbin tool. The friction stir welding method according to claim 1 or 2, characterized by the above.

4. Reverse friction stir welding is used for the friction stir welding. The friction stir welding method according to claim 1 or 2, characterized by the above.

5. Water is used as the refrigerant. The friction stir welding method according to claim 1 or 2, characterized by the above.

6. The friction stir welding tool is made of steel, and the workpiece is made of steel. The friction stir welding method according to claim 1 or 2, characterized by the above.

7. The refrigerant discharged from the friction stir welding tool is circulated and made to flow into the friction stir welding tool. The friction stir welding method according to claim 1 or 2, characterized by the above.

8. A friction stir welding tool having a through hole in the rotation axis, wherein the through hole is used to cool the friction stir welding tool by performing friction stir welding with a refrigerant flowing through the through hole. A friction stir welding tool characterized by the above.

9. It has a through hole in the rotation axis, and is a bobbin tool. A friction stir welding tool characterized by the above.

10. It has a through hole in the rotation axis, and is a reverse friction stir welding tool. A friction stir welding tool characterized by the above.

11. It is made of steel. The friction stir welding tool according to any one of claims 8 to 10, characterized by the above.

12. A friction stir welding tool set used for friction stir welding a workpiece, comprising the friction stir welding tool according to claim 8, and a backing plate disposed on the opposite side of the friction stir welding tool with respect to the workpiece, wherein the friction stir welding tool includes a shoulder portion and a probe portion provided at the center of the shoulder portion, and the through hole penetrates through the shoulder portion and the probe portion. The back plate has a recess or an opening that accommodates the tip of the probe part and allows the refrigerant flowing out from the through hole to flow through, in a state where the probe part is inserted into the joint part of the material to be joined and the shoulder part is brought into contact with the surface of the material to be joined, and the material to be joined is sandwiched between the shoulder part and the back plate. A friction stir welding tool set.

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