Abstract: End portions of face plates 111 and 112 of a frame member 110 are connected by a rib 116A. To protrusion chips 117b1 and 117b2 end portions of face plates 101 and 102 of another frame member 100 are overlapped. The corners between the rib 116A and the rear faces of the face plates 111 and 112 are in the form of a convex circular arc 131 which protrudes into a hollow portion 119 of the frame member 110. Both end portions of the circular arc 131b1 are connected to the rib 116A and the face plates 111 and 112 by two smooth recessed circular arcs 132b1 and 133b1. Accordingly, since a flow-out of metal does not occur from this portion, the cross-section can be small and a light weight structure can be obtained. A structural body formed of hollow frame members which are joined by friction stir welding provides a light weight structure.

Abstract: Rotary tools 340A and 340B are disposed to weld along two joining lines. For this purpose spaced welding lines the two rotary tools are inserted and into respective welding joints moved at the same time along their respective welding lines. At a position P1 of a portion of a window 210, the rotary tool 340A is retreated from the welding joint so that the tool 340 A continues to move friction stir welding on that welding joint is stopped although the welding. At a position P3, the rotary tools 340B are gradually withdrawn from their respective welding joints while they continue to move along the welding line. At a position P4, the moves of the rotary tools 340A and 340B is stopped, the rotary tool 340B is made to retreat and the joining is made to stop. Next, the rotary tools 340A and 340B are inserted and the move are started again. An insertion amount of the rotary tool 340B is larger than an insertion amount during the stop of the move of the rotary tool.

Abstract: The rotary tools are disposed to weld along two spaced welding lines. The two rotary tools are inserted into respective welding joints and moved at the same time along the respective welding lines. At a position PI of a portion of a window one rotary tool is withdrawn from the welding joint so that the friction stir welding on that welding joint is stopped although the welding tool continues to move. At a position P3, the rotary tools are gradually withdrawn from their respective welding joints while they continue to move along the welding line. At a position P4, the movement of the rotary tools along the respective welding lines is stopped, the rotary tools are completely withdrawn and the welding is stopped. Next, the rotary tools are inserted into the respective welding joints once again and movement thereof is started again. An insertion amount of the rotary tools at this time is larger than the insertion amount thereof prior to the stopping of the movement of these rotary tools.

Abstract: End portions of face plates 111 and 112 of a frame member 110 are connected by a rib 116A. To protrusion chips 117b1 and 117b2 end portions of face plates 101 and 102 of another frame member 100 are overlapped. The corners between the rib 116A and the rear face of the face plates 111 and 112 are in the form of the corners circular arc 131 which protrudes into a hollow portion 119 of the frame member 110. Both end portions of the circular arc 131b1 are connected to the rib 116A and the face plates 111 and 112 by two smooth recessed circular arcs 132b1 and 133b1. Accordingly, since a flow-out of metal does not occur from this portion, the cross-section can be small and a light weight structure can be obtained. A structural body formed of hollow frame members which are joined by friction stir welding provides a light weight structure.

Abstract: A structural metal sheet employed as an outer sheet for a railway vehicle, for example, is generally pretensioned in order to prevent buckling. Hitherto, the pretensioning is effected by various methods after or during the assembly of a structure. According to the invention, the metal sheet itself is pretensioned by a plastic working for increasing the flexural rigidity thereof. Thus, there is no fear of buckling, and the workmanship is improved as compared with the prior art.