Friction stir spot welding apparatus and joint structure

A friction stir spot welding apparatus that joins a to-be-joined object by softening the object with frictional heat, comprising:

• a pin member formed in a solid cylindrical shape;
• a shoulder member formed in a hollow cylindrical shape, the pin member being configured to be inserted in the shoulder member, a tip-end part of the shoulder member being formed in a tapered shape;
• a rotary actuator configured to rotate the pin member and the shoulder member on an axis along an axial center of the pin member;
• and
• a linear actuator configured to linearly move each of the pin member and the shoulder member along the axis, wherein
• the to-be-joined object includes a first member and a second member, the first member being disposed so as to oppose to the pin member and the shoulder member and being made of material with a melting point lower than the second member,
• the friction stir spot welding apparatus is further provided with a controller, the controller being programed to:
• (A) operate the rotary actuator and the linear actuator so that the pin member and the shoulder member press a joining part of the to-be-joined object while the pin member and the shoulder member rotate,
• (B) operate the linear actuator and the rotary actuator so that the tip-end part of the rotating shoulder member reaches a first position within the second member, and retreat the rotating pin member from the joining part of the to-be-joined object,
• (C), after (B), hold the tip-end part of the rotating shoulder member in a state where the tip-end part of the shoulder member reaches the first position for a first period of time, and
• (D), after (C), operate the rotary actuator and the linear actuator so that the rotating shoulder member is drawn out from the joining part of the to-be-joined object and the rotating pin member is advanced toward the joining part of the to-be-joined object.