making a low distortion frame construction

A method of making a low distortion frame construction, the construction having joint regions formed by first and second overlapping members defining a lapped interface, the joint regions being comprised of material that can be converted to a solid state deformable plastic condition by friction heat, comprising:

(a) providing a stir friction welding tool having
(i) a rotatable thermally conductive body presenting a shoulder to engage the joint region for storing friction-generated heat, and
(ii) a friction generating pin rotatable with said body about a pin axis and selectively extendable from the shoulder to progressively penetrate the overlapping members as the joint region in contact with the pin becomes plastic;

(b) placing the tool shoulder against the joint region with the pin axis transverse to the lapped interface and spinning the body while progressively extending the pin at a rotational speed effective to generate friction heat that converts the adjacent material of the joint to a plastic condition allowing the spinning of the probe to stir such plastically converted material; and

(c) controlling the depth of penetration of the spinning probe while translating the tool across the joint region to perfect a friction stirred welded zone that extends through the interface and between the overlapping members to provide a welded joint that is exceptionally strong in shea