Friction stir spot welding method and welded assembly utilizing same

A welded assembly, comprising:

• a first member formed of a thermoplastic resin molding mixed with a fiber material;
• a second member formed of a molding containing at least a thermoplastic resin;
• an overlapping part in which the first member and the second member overlap each other while at least partially being in contact with each other, and
• a stirred weld in which the first member and the second member are welded by friction stir spot welding by a tool, the stirred weld being provided in the overlapping part, wherein
• the tool is a double-acting tool for friction stir spot welding including a pin and a shoulder including a hollow part into which the pin is inserted,
• the tool is disposed against the overlapping part so that a rotation axis of the tool is along an overlapping direction of the first member and the second member while rotating at least the pin around the rotation axis,
• the pin is plunged into the overlapping part from a first member side to perform friction stir so that extension fibers extended from the fiber material of the first member in the overlapping part remain around a plunging region of the pin while retracting the shoulder from the overlapping part in a direction of the rotation axis to release the thermoplastic resin molding overflowed by the plunging of the pin into a surrounding region of the pin by the retraction of the shoulder,
• the shoulder is brought close to the overlapping part in the direction of the rotation axis to wrap the extension fibers into a wrapped part when the resin molding material overflowed is backfilled into the plunging region from the surrounding region while retracting the pin from the overlapping part, and
• the stirred weld includes the wrapped part in which extension fibers extend from the fiber material in the overlapping part extend from a periphery of the stirred weld and are wrapped into a resin material constituting the stirred weld.