Patent ID: 7360677

Claim:
A spot joining method of metal members, in which different kinds of metal members are joined by at least partly lapping the metal members, comprising the steps of: lapping a first metal member and a second metal member, a melting point of the second metal member being higher than that of the first metal member, a metal film being formed on a joining face of the second metal member; placing a rotating tool under a rotation on an outer surface of the first metal member by making the rotating tool approach in a rotational axis direction, the rotating tool comprising a cylindrical body portion and a cylindrical pin portion that projects from a tip of the body portion, the body portion having a concave portion at a tip surface thereof, the concave portion comprising a slant bottom face such that a depth becomes deeper gradually inwardly, the pin portion being located at a center of the concave portion to project from the bottom face of the concave portion beyond the tip of the body portion, an axis of the pin portion being located on a rotational axis of the body portion; pressing the rotating tool against the first metal member with a pressing force in such a manner that the rotating tool goes into the first metal member without reaching the second metal member, whereby only the first metal member can be softened and made in a plastic flow state by a frictional heat generated by the rotation of the rotating tool and spot faces of the first and second metal members are joined thereby in a solid state, wherein said step of pressing the rotating tool against the first metal member comprises at least an initial step of pressing the rotating tool with a relatively small pressing force enough to generate a frictional heat which can soften the outer surface of the first metal member to provide proper centering of the rotating tool by the pin portion and stabilize a rotational center of the rotating tool, and a subsequent step of pressing the rotating tool with a relatively large pressing force that is greater than said relatively small pressing force and enough to make the first metal member in the plastic flow state and push away outwardly said metal film softened by the frictional heat generated by the rotation of the rotating tool so that respective new uncovered surfaces of said first and second metal members can directly contact each other, whereby the spot faces of the first and second metal members can be joined in the solid state substantially without said metal film and with a direct contact of the new uncovered surfaces of the first and second metal members.