The effect of interface defect on mechanical properties and its formation mechanism in friction stir lap welded joints of aluminum alloys

As a typical characteristic of the joints obtained by friction stir lap welding (FSLW), the interface defect has a significant influence on the mechanical properties. The evolution of interface defect and its formation mechanism were investigated, based on which some innovative insights into the material flow were proposed. The heights of interface defect affected the tensile-shear properties of the lap joints. The deformation process of the original lap interface during FSLW was divided into three stages. The upward movement of the original lap interface before contacted to the pin, the enlargement of stir zone (SZ) and the migration of original lap interface and the formation of the interface defects. The upward flow of the material was arisen in the leading edge of the pin tool resulting from the combination effects of inhomogeneous heat distribution and extrusion from the proceeding pin. The downward flow of the material was occurred in the trailing edge of the pin tool arising from the combination effects of shoulder pressure and force from the rotational threads on the pin surface. The formation mechanism of interface defect was revealed clearly by means of schematic illustration, contributing to the further development of the FSLW. (C) 2016 Elsevier B.V. All rights reserved.