Patent ID: 7051916

Claim:
A friction stir welding method for welding a first member and a second member, the first member comprising a plate, the plate being provided with a protrusion protruding substantially parallel to said plate from an end portion of said plate and arranged at a displaced position from one surface of said plate, said friction stir welding method comprising the steps of: superimposing an end portion of said second member on said protrusion, and abutting said end portion of said second member against the end portion of said plate, thereby providing abutted portions; and friction stir welding the abutted portions using a rotary tool having a small-diameter portion mounted on an end of a large-diameter portion, by inserting said small-diameter portion into a joint between said abutted portions; wherein: the rotary tool is positioned so that when observed along a direction in which said tool moves along said joint during the friction stir welding, a distance from a tip portion of said inserted small-diameter portion to a surface of said first member opposite from said large-diameter portion is greater on the right side of the axial center of said rotary tool than on the left side thereof, said distance from the tip portion of said inserted small-diameter portion to the surface of the first member opposite said large-diameter portion on the right side being sufficiently large to avoid flow-out of material of said first member at the right side; said protrusion extends to said right side of the welding line as seen in the direction of movement of said tool when performing said friction stir welding; and said rotary tool is rotated in a clockwise direction as observed from the large-diameter portion side while being moved along said joint during the friction stir welding.