Mechanical properties evolution during post-welding-heat treatments of double-lap Friction Stir Welded joints

Research highlights: → Mechanical properties of double lap joints after post-welding-heat treatments. → Comparable mechanical properties after heat treatment at 200 and 300 oC. → Post-welding heat treatments at 400 and 450 oC induce Abnormal Grain Growth. → Tensile properties increased after high temperature heat treatments. -- Abstract: The present study focuses on double-lap Friction Stir Welded (FSW) joints in 2024T3 and 7075T6 aluminium alloys subjected to several post-welding-heat treatments at warm (typical aging) and high temperature (solution range) followed by room temperature deformation (tensile tests). The effect of post-welding-heat treatments on the microstructure and mechanical properties of double lap FSW joints were investigated. Polarized Optical Microscopy (POM) and Scanning Electron Microscopy (SEM) analysis reveal a progressive change in grain size and morphology in high temperature post-welding-heat treated joints, leading to Abnormal Grain Growth in the stir zone. Stress-strain curves are rather flat for 200o and 300 oC post-welded heat treated joints while, for the other set of samples, stress increases with strain to reach maximum stress of 140-160 MPa. Micro-hardness profiles measured on transversal sections of post-welded heat treated joints reveal conditions (temperature and time) of hardness homogeneity at top, bottom and central nugget zone and/or along the whole measured profile. When homogeneity is reached, fracture occurs in the nugget. A relationship between hardness and tensile properties has been applied in the nugget.