Patent ID: 7137545

Claim:
A friction stir welding method of joining an abutment portion by moving a rotating probe relatively along said abutment portion while pressing said probe against one surface of said abutment portion at which an end of a first cylindrical member and an end of a second cylindrical member are butted together, said friction stir welding method comprising the steps of: externally fitting said first and second cylindrical members on a backing jig having an outer circumferential surface of a completely circular shape and a different thermal expansion relative to said first and second cylindrical members while said first and second cylindrical members are relatively expanded and then relatively contracted as compared with said backing jig so that the entire circumference of another surface of said abutment portion is in close contact with the entire outer circumferential surface of said backing jig, wherein the circumferences of said ends of said first and second cylindrical members have an identical length; performing friction stir welding on said one surface of said abutment portion while retaining said other surface of said abutment portion with said backing jig; and separating said backing jig from said other surface of said abutment portion after said friction stir welding by relatively thermally expanding said first and second cylindrical members as compared with said backing jig.