Friction stir spot welding apparatus and friction stir spot welding method

A method of operating a friction stir spot welding apparatus that partially stirs a workpiece including a first workpiece and a second workpiece to weld the first workpiece and the second workpiece to each other, the first workpiece including fiber reinforced plastic,

• the friction stir spot welding apparatus comprising:
• a pin that is cylindrical or columnar, rotates about an axis, and advances or retreats in a direction along the axis;
• a shoulder that is cylindrical, rotates about the axis, and advances or retreats in the direction along the axis, the pin being within the shoulder;
• a rotary driver to rotate the pin and the shoulder about the axis;
• and
• a tool driver to advance and retreat the pin and the shoulder along the axis,
• the method comprising:
• (A) operating the rotary driver and the tool driver such that the pin and the shoulder are brought into contact with the workpiece while rotating about the axis;
• (B) operating, after the step (A), the rotary driver and the tool driver such that the pin moves in a direction away from the workpiece while the pin and the shoulder rotate about the axis, or such that the shoulder is further pressed in the workpiece while the pin and the shoulder rotate about the axis;
• and
• (C) operating, after the step (B), the rotary driver and the tool driver such that the pin advances toward the workpiece while the pin and the shoulder rotate about the axis, or such that the shoulder moves in the direction away from the workpiece while the pin and the shoulder rotate about the axis,
• wherein the tool driver operates such that rotational frequencies of the pin and the shoulder in the step (C) are lower than rotational frequencies of the pin and the shoulder in the step (B),
• the method further comprising step (D) operating, after the step (C), the rotary driver and the tool driver such that an end surface of the pin and an end surface of the shoulder are located on a front surface of the welded workpiece while the pin and the shoulder rotate about the axis,
• wherein the tool driver operates such that pressing force applied to the workpiece from the pin and the shoulder in the step (D) is larger than the pressing force applied to the workpiece from the pin and the shoulder in the step (C).