Retractable shoulderless variable penetration friction stir welding tool

A non-consumable retractable shoulderless variable penetration friction stir welding tool for use in joining a first workpiece, having a first thickness, and a second workpiece, having a second thickness, by friction stir welding, comprising:

• a substantially cylindrical body portion, a head portion, and a tip section, each integral to the tool, the body portion having a longitudinal axis about which it is rotatable, a diameter, a sidewall substantially parallel to the longitudinal axis, a proximal end, and a distal end;
• and
• the head portion located at the distal end of the body portion having a base with a diameter substantially equal to the diameter of the body portion forming a smooth transition between the body portion and the head portion and a face that converges to the tip section having a diameter and a center wherein the center is located substantially on the longitudinal axis, a height from the distal-most portion of the tip section to the base along the longitudinal axis.