Microstructure and precipitation behaviours of laser clad 7075 aluminium alloy

Abstract

Laser cladding is a promising technique for repairing metal components. However, the extreme thermal conditions introduced by laser heat can lead to complex precipitation behavior in the cladding layer and heat-affected zone (HAZ). In this paper, the laser cladding technique was applied to repair a widely used aircraft structural material - 7075 aluminium alloy. Specimens of the cladding layer, HAZ, and their interface were site-specifically prepared by using focused ion beam (FIB) and micro-manipulator. The microstructure and precipitation behaviours of these regions were characterized individually using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that the cladding layer forms various species of precipitates, such as coarse MgZn2 particles, Zn-rich particles, and CuMgZnAl compounds. Coarse inclusions and voids are formed at the interface between the cladding layer and HAZ. In the HAZ, solute redistribution and re-precipitation take place due to the laser heating cycle. The majority of Mg and Zn atoms segregate at grain boundaries in the HAZ, resulting in low Mg and Zn contents in the grain interior. The number density of nano-particles was significantly reduced in the HAZ, compared to the unaffected 7075 aluminium alloy substrate, which explains the strength reduction in the HAZ.