Abstract: A method of friction stir welding and a non-consumable retractable shoulderless variable penetration friction stir welding tool. The tool includes a substantially cylindrical body portion, a head portion, and a tip section, each integral to the tool. The body portion has a longitudinal axis about which it is rotable, a diameter, a sidewall substantially parallel to the longitudinal axis, a proximal end, and a distal end. The head portion is located at the distal end of the body portion. The head portion has a base with a diameter substantially equal to the diameter of the body portion thereby forming a smooth transition between the body portion and the head portion, thus the tool has no shoulder. The head portion includes a face that converges to the tip section. The tool is retractable, reduces overheating, improves weld quality by reducing internal voids and lack of fusion, and facilitates variable penetration welds.