Friction stir welding of dissimilar AA7050 and AA5083 aluminium alloys: Microstructural and mechanical characterization

Abstract

In this study, dissimilar aluminium alloys, AA7050-T7651 and AA5083-H111, with 5 mm thickness were friction stir butt welded and the microstructural and mechanical properties of the joints are investigated. The experiments were carried out at tool rotational speeds of 800, 1000 and 1200 rpm while keeping the tool traverse speed, axial force and tool tilt angle, respectively, at constant values of 30 mm/min, 3.5 kN and 1.5°. A tool axis offset of 0.25 mm towards the 7xxx alloy side that fixed by a detailed preliminary study was used in all the trials. All the fabricated joints were externally defect-free and the fractured tensile specimens showed that the joints have fractured in the thermo-mechanically affected zone (TMAZ) at the AA5083-H111 alloy side. The highest ultimate tensile strength of 294 MPa (96% joint efficiency) was observed for the joint fabricated at tool rotation speed of 1000 rpm. The location of fracture together with the microstructure observation at the joint zone showed that the coarsening of grains and loss of cold work are the causes of fracture of the tensile specimen at the TMAZ of the 5083-alloy side. In addition, significant grain coarsening and dissolution of precipitates are observed at the heat-affected zone and TMAZ at the 7050-alloy side at tool rotational speeds of 1000 rpm and 1200 rpm. The study testified that the tool rotational speed has a significant influence on the microstructure and tensile properties of the dissimilar joints.