Influence of double passes weld on interface migration and mechanical property of friction stir lap joint

Friction stir lap joints of 3 mm thick plates of LY12(CZ)aluminum alloy were conducted using the cylindrical tool with clockwise and counter-clockwise screw thread.The interface migration and mechanical property of friction stir lap joint of double passes weld were investigated.The results show that,the interfaces at both sides of the welds pull-up using the tool with counter-clockwise screw thread when the tool rotating clockwisely,while the interfaces pull-down using the tool with clockwise screw thread.With single pass weld,the effective sheet thickness(EST)is reduced because the interface migratory always exists on the loaded side.The interface appearance at one side of the weld pull-up and the other side pull-down was obtained if the tool with clockwise and counter-clockwise screw thread were used alternately by double passes weld.The strength of double passes welds could reach more than twice as the single pass weld because of avoiding the influence of the interface migration on the loaded side