Effects of different cooling conditions on microstructure and precipitation behavior of fine-grained Al–Zn–Mg–Cu alloy friction stir welding joint

Abstract

The microstructure evolution and precipitation phase distribution of 7075 aluminum alloy friction stir welding (FSW) joint were investigated under liquid CO2 cooling conditions. The fine grained 7075 aluminum alloy was developed by four-pass equal channel angular pressing (ECAP) and FSW joints with CO2 cooling at room temperature. It was found that CO2 cooling could effectively reduce the peak temperature of the weld, leading to a refinement of the welded grain and an improvement of the welded joint. Various recrystallization mechanisms were found in different areas of the weld. The grain boundary and dislocation density of large angles gradually decreased from the weld stir-zone (SZ) to the heat-affected-zone (HAZ) regions, and the anisotropy increased. The type of precipitated phase and the relationship with the matrix interface were determined by detecting the spacing between the precipitated phase and the crystal plane of the matrix. CO2 forced cooling not only enhanced the matrix dislocation strengthening and grain boundary strengthening, but also effectively improved the size and type of precipitated phases and the colattice strengthening effect. In addition, liquid CO2 cooling can greatly reduce the PFZ width and enhance the Zener pinning effect.