Microstructure and Mechanical Properties in Friction Stir Welded Thick Al-Zn-Mg-Cu Alloy Plate

In this work, 20-mm-thick aluminum-alloy plates were joined via friction stir welding. The temperature gradient was reduced by reducing the surface welding heat input to achieve uniformity of the mechanical properties across the thick plate joints. The welding temperature was measured using thermocouples. The microstructures were observed via electron backscatter diffraction and transmission electron microscopy. The tensile properties of the samples sliced along the thickness direction of the joint were evaluated. The results show that the highest welding peak temperature is 430 degrees C on the advancing side on the top surface of the joint. The grain size gradually decreased along the thickness direction, and grain refinement was due to the combination of continuous, discontinuous, and geometric dynamic recrystallization. The tensile properties of the sliced samples were found to be uniform, and the ultimate tensile strength reached 62% of that of the base metal. The main strengthening mechanism of the Al-Zn-Mg-Cu alloy joints consists of precipitation strengthening. In addition, the eta` -> eta phase transition and grain coarsening in the heat-affected zone were found to be responsible for the fracture of the joints.