Friction stir bonding method

[0002] The present invention relates to a friction stir welding method.

[0003] A friction stir welding method is a technique in which a round shaft (called a rotary tool) is rotated and inserted into the joint between members to be welded, and the rotating rotary tool is moved along the junction line of the joint, thereby heating, mobilizing and plasticising the material in the welding region, so as to effect a solid-phase welding of the members. The rotary tool comprises a large-diameter portion and the small-diameter portion at the leading end thereof. The welding is performed by inserting the small-diameter portion into the joint between the members to be bonded, such that the end surface of the large-diameter portion comes into contact with the aforementioned members. A screw thread is formed an the small-diameter portion. The end surface of the large-diameter portion on the screw thread side has a concave shape. Moreover, friction stir welding may be performed by positioning members to be bonded between two large-diameter portions of the rotary tool. This technique is disclosed in Japanese Patent No. 2,712,838 (U.S. Pat. No. 5,460,317), and Japanese Patent National Publication of PCT Application No. 9-508073 (EP 0752926B1).

[0004] A case of friction stir welding the abutting region formed between two extruded members 10, 20 made of aluminum alloy will be considered with reference to FIG. 4. A trapezoidal concave portion 12 is provided at the end surface of the face plate 11 of the member 10, and a trapezoidal convex portion 22 is provided at the end surface of the face plate 21 of the member 20. The convex portion 22 is inserted into the concave portion 12 when the two members 10, 20 are abutted. The concave portion 12 and the convex portion 22 have a trapezoidal shape to simplify insertion. Also, the end portions of the members 10, 20 are provided with raised portions 13, 23 projecting upwardly. In the case where the members 10, 20 are used to form a car body of a railway car, the length of the members 10, 20 is approximately 20 m. Therefore, the members 10, 20 at the abutting region may each tend to constitute a substantially different plane. In order to prevent such situation, the end surfaces of the abutting region are provided with the concave portion 12 and the convex portion 22 and are fitted against each other.

[0005] In such a state, friction stir welding is performed by rotating and inserting a small-diameter portion 51 of a rotary tool 50 from above into the welding joint until the end surface of a large-diameter portion 53 (end surface on the small-diameter portion 51 side) is inserted slightly into the raised portions 13, 23. The rotating rotary tool 50 is then moved along the welding joint in the abutting region, as is illustrated in FIG. 9. Reference number 100 indicates a bed for fixing the members 10, 20.

[0006] The rotary tool 50 used for welding is shown in FIG. 10. The rotary tool 50 has a small-diameter portion 51 attached to the leading end of the large-diameter portion 53. The small-diameter portion 51 has a tapered shape with a larger diameter portion located towards the leading end. This is to stir the material in the abutting region of the concave portion 12 and the convex portion 22 sufficiently. The tapered surface of the small-diameter portion 51 is provided with a male screw thread. The end surface 53hof the large-diameter portion 53 is contoured so as to be concave towards the large-diameter portion 53.

[0007] The small-diameter portion 51 in the vicinity of the large-diameter portion 53 (the small-diameter portion 51 on the base end side) is not provided with a screw thread. This is because it is difficult to provide the screw thread on the small-diameter portion 51 close to the base end side due to the fact that the small-diameter portion 51 is tapered.

[0008] As seen in FIG. 9, during welding, the region of the small-diameter portion 51 without the screw thread is placed at the position of the upper slopes (called sides) 12b, 22bof the trapezoidal concave portion 12 and the trapezoidal convex portion 22. Therefore, the stirring in this region tends to be insufficient. When stirring is insufficient, this is considered to have an undesirable influence on the strength of the weld. In the case where the material to be welded is an aluminum alloy, the surfaces of the members 10, 20 tend to be oxidized. When stirring is insufficient, it is considered that such metal oxide has a bad influence on the strength of the weld.

[0009] An object of the present invention is to provide a friction stir welding method that is capable of providing good welding at the abutting region with the concave portion and convex portion fitted together.

[0010] The present invention is characterized by the face that friction stir welding is performed by positioning a screw thread on the rotary tool in the fitted region of the concave portion and the convex portion.

[0011] FIG. 1 is a longitudinal cross-sectional view of members being welded according to one embodiment of the present invention;

[0012] FIG. 2 is a side view of a rotary tool used in the welding illustrated in FIG. 1;

[0013] FIG. 3 is a disassembled longitudinal cross-sectional view of the rotary tool shown in FIG. 2;

[0014] FIG. 4 is a longitudinal cross-sectional view of a pair of extruded members to be welded as illustrated in FIG. 1;

[0015] FIG. 5 is a perspective view of a car body of a railway car;

[0016] FIG. 6 is a longitudinal cross-sectional view showing welding according to another embodiment of the present invention.

[0017] FIG. 7 is a disassembled longitudinal cross-sectional view of the rotary tool shown in FIG. 6;

[0018] FIG. 8 is a longitudinal cross-sectional view of the assembled rotary tool shown in FIG. 6;

[0019] FIG. 9 is a longitudinal cross-sectional view showing welding according to a conventional technique; and

[0020] FIG. 10 is a longitudinal cross-sectional view of a conventional rotary tool.

[0021] A preferred embodiment of the present invention will now be explained with reference to FIG. 1 through FIG. 5. Referring first to FIG. 5, a car body 500 of a railway car is comprised of a side structure 501 constituting the side surface, a roof structure 502 constituting the roof, an under frame 503 constituting the floor, and an end structure 504 constituting the end portion of the car in the longitudinal direction. The side structure 501, the roof structure 502, and the underframe 503 are formed by joining and welding plural extruded members 10, 20, respectively. The longitudinal direction (extruded direction) of the extruded members 10, 20 is positioned along the longitudinal direction of the car body 500. The extruded members 10, 20 are made of an aluminum alloy. There exists a plurality of ribs at the inner side of the car body, that is, at the side of the face plates 11, 21 from which the raised portion 13, 23 protrude, that are not shown in the figures. The structure of the abutting region of the members 10, 20 is as mentioned earlier.

[0022] The parts of a rotary tool 50 are comprised of a round shaft 50b, including a small-diameter portion 51, and a member 53bwhich forms part of the large-diameter portion 53. To the end portion of the small-diameter portion of the round shaft 50b, from the leading end side, there exists a screw thread portion 51b, including a screw thread on a tapered small-diameter portion, and an axis portion 51con which the member 53bis fixed for forming a lower end of the large-diameter portion. The leading end of the screw thread portion 51bhas a larger diameter. A pin hole 51fis provided on the axis portion 51cfor fixing the member 53bthereon via a corresponding pin hold 53fto form the large-diameter portion.

[0023] The member 53bcorresponds to the lower end of the large-diameter portion 53, and has a circular outer shape. The member 53bhas an axial hole 53cinto which the axis portion 51cis fitted. The difference between the diameter of the axis portion 51cand the diameter of the hole 53cis small. The member 53bincludes the pin hole 53f. The member 53bis mounted on the axis portion 51cand is fixed thereto with a knock pin 58.

[0024] The end surface of the member 53bfacing the screw portion 51bis provided with a concave surface 53g. The depth of the concave surface 53gis deeper towards the screw portion 51band is shallower towards the outer peripheral portion. The concave surface 53gholds the metal to be bonded and restricts the outflow of the metal from the end surface of the large-diameter portion 53. This operation is similar to that of the concave surface 53hshown in FIG. 10.

[0025] The length L1 of the axis portion 51cis shorter than the length L3 of the member 53b, as seen in FIG. 3. The length L2 of the screw portion 51bis longer than the length of the conventional screw portion 51 by the amount of the difference (L3−L1). The length of the screw thread is elongated relative to the increase of the length L2 of the screw thread portion 51b. The end portion 53dat the inner diameter side of the concave surface 53gof the member 53bis positioned at the end surface 51dof the axis portion 51c. Therefore, when the member 53bis mounted on the axis portion 51c, the screw is positioned inside the bounds of the concave surface 53g. Or, the screw thread is positioned in the vicinity of the end portion in the axial direction of the outer peripheral portion of the member 53b.

[0026] The diameter D of the large-diameter portion 53 is smaller than the sum W of the width W1 and the width W2 of the two raised portions 13, 23, respectively. The metal which forms the raised portions 13, 23 becomes the source for filling the gap at the abutting region between the two members 10, 20. Also, during friction stir welding, the lowermost end of the large-diameter portion 53 is inserted to a level below the surface of the raised portions 13, 23 and above the upper surface of the face plates 11, 21 excluding the raised portions 13, 23.

[0027] As is well-known in the art, the axis of rotation of the rotary tool 50 is inclined relative to the direction of movement. The axis of rotation is inclined so that the axial center of the small-diameter portion 51 is positioned in the direction of movement forward of the axial center of the large-diameter portion 53. The lowermost end at the rear end of the lower end of the large diameter portion 53 is positioned below the surface of the raised portions 13, 23.

[0028] The friction stir welding is performed by placing the axial center of the rotary tool 50 at approximately the center of the concave portion 12 in the depth direction. Therefore, the width WI of the convex portion 13 is larger than the width W2 of the convex portion 23. A driving device for rotating and moving the tool is coupled to the upper end of the round shaft 50b.

[0029] With such a structure, the screw thread of screw portion 51bis formed to the vicinity of the end surface of the large-diameter portion (end surface on the side of the screw portion 51b, the base end side of the screw portion 51b). Therefore, the upper slopes (called sides) 12b, 22bof the abutting trapezoidal portions are stirred by the screw thread in the axial direction. Therefore, the problem concerning reduction in the strength of the weld arising from the formation of metal oxide is inhibited.

[0030] In the above-mentioned embodiment, the screw portion 51bis tapered. However, even in the case of a non-tapered (called a linear) screw portion 51b, a similar problem occurs because it is difficult to form the screw to the vicinity of the end surface of the large-diameter portion 53. Therefore, at least a part of the large-diameter portion should be formed as an independent member, even in the case of provision of a linear screw portion, as illustrated in FIG. 7.

[0031] In the above-mentioned embodiment, the large-diameter portion 53bis connected to the axis portion 51cwith the pin 58. However, these parts may be fixed by shrinking. Moreover, the large-diameter portion 53bmay be fixed by forming a screw thread on the axis portion 51c, and screwing it thereon. In this case, the screw should be a reverse screw formed opposite to the direction of rotation.

[0032] Furthermore, in the case of performing welding using a conventional rotary tool 50 having no screw thread near the end surface of the large-diameter portion 53, the upper slopes 12b, 22bof the trapezoidal concave portion 12 and convex portion 22 are adjusted to the position of the screw thread of the small-diameter portion 51.

[0033] Also, the upper slopes 12b, 22bof the trapezoidal concave portion 12 and convex portion 22 are provided inside the raised portions 13, 23, and welding is performed using a conventional rotary tool 50 having no screw thread near the end surface of the large-diameter portion 53. Subsequently, in order to eliminate the incompletely stirred portion therefrom, the raised portions 13, 23 are cut out. By doing so, an insufficient stirred portion does not exist in the remaining portion, so that there exists no problem concerning strength.

[0034] Another embodiment, shown in FIG. 6 through FIG. 9, will now be explained. The embodiment involves the use of a friction stir welding tool having large-diameter portions 63, 64 at either end of the screw thread portion 61bof the small-diameter portion 61 for performing friction stir welding while interposing the abutting region of the members between the two large-diameter portions 63, 64.

[0035] A rotary tool 60 is comprised of a round shaft 60bincluding a small-diameter portion 61, and members 63b, 64bforming large-diameter portions 63, 64. The members 63b, 64bare each-fixed to the axis portions 61c, 61d, respectively, of the round shaft 60bby means of a knock pin. The small-diameter portion 61 is provided with, from the leading end side, the axis portion 61dfor holding the member 64b, a screw thread portion 61b, and the axis portion 61cfor holding the member 63b. The screw portion 61bis linear. The diameter of the axis portion 61cis identical to that of the screw thread portion 61b. The diameter of the axis portion 61dis smaller than that of the screw thread portion 61b. The length of ordinary screw thread of the screw thread portion 61bis longer than the length L. The position of the screw thread portion 61bdepends on the shape of the concave surfaces 63h, 64h. There are provided pin holes 61f, 61gin the axis portions 61c, 61d, respectively.

[0036] The members 63b, 64binclude axial holes 63c, 64chaving a diameter fitting the axis portions 61c, 61d. The difference between the diameter of the axis portions 61c, 61dand that of the axial holes 63c, 64cis small. There are provided pin holes 63d, 64din the members 63b, 64b. The members 63b, 64bare provided with concave surfaces 63h, 64hhaving a similar objective as the aforementioned concave surfaces 53h, 53gat the end surface facing the screw thread portion 61b. The end portions 63j, 64jat the inner diameter side of the concave surfaces 63h, 64hare in a concaved position relative to axial ends 63k, 64kat the outer peripheral side of the members 63b, 64b. By doing so, the end portion of the screw thread on the side of the large-diameter portion 63 is positioned inside the bounds of the concave surface 63hof the large-diameter portion 63, as is illustrated in the assembly view of FIG. 8. The end portion of the screw thread on the side of the large-diameter portion 64 is positioned inside the bounds of the concave surface 64hof the large-diameter portion 64.

[0037] During welding, the lower end of the large-diameter portion 63 is inserted into the raised portions 13, 23, as in the case of the aforementioned embodiment. The rotary tool 60 is inclined, as in the case of the aforementioned embodiment. The front end of the large-diameter portion 64 of the leading end on the screw thread portion 61bside is inserted into the lower portion of the face plates 11, 21. When performing friction stir welding, the metal is slightly raised against the lower surface of the face plates 11, 21 at the rearward side of the large-diameter portion 64. When using the lower surface of the face plates 11, 21 as the outer surface of the car body, the raised metal mentioned above is cut out to obtain a flat surface.

[0038] By doing so, the upper slopes 12b, 22bof the trapezoidal portion can be sufficiently stirred. Also, the lower side of the face plates 11, 21 can be sufficiently stirred. Therefore, the whole range of the abutting region of the face plates 11, 21 can be sufficiently stirred.

[0039] In the above-mentioned embodiment, the end portions 63j, 64jof the concave surfaces 63h, 64hon the inner diameter side are formed inwardly relative to the end portions 63k, 64kof the large portions 63, 64. However, they may be formed at the identical position. Furthermore, the concave surfaces 63h, 64hmay be formed nearer the end portion of the screw thread. Also, a screw thread may be formed on the axis portion 61ccontinuing from the screw thread portion 61b, in order to fix the large-diameter portion 63 thereon. This could also be applied to a rotary tool having no large-diameter portion at the leading end.

[0040] The technical scope of the present invention is not limited to the terms used in the claims or in the summary of the present invention, but is extended to a range in which a person skilled in the art could easily find equivalents based on the present disclosure.

[0041] The present invention enables good welding, in a case of friction stir welding a fitted region formed by a trapezoidal concave portion and convex portion.