Effect of thermal cycles on the microstructure and properties of the Al–Zn–Mg–Cu alloy during wire-arc additive manufacturing

Abstract

Al–Zn–Mg–Cu alloys are important for aerospace applications, and wire-arc additive manufacturing (WAAM) provides a way to produce large-scale metal structures. However, research on the effects of WAAM thermal cycles on the microstructure and mechanical properties of Al–Zn–Mg–Cu alloys is still lacking. In this study, quantitative thermal cycle data was obtained to investigate evolution of the microstructure, and then the mechanical behavior of the as-deposited part was investigated. Results indicated that the part mainly contained microcracks (predominantly liquation cracks), pores, and the interdendritic crystalline phase, which decreased the tensile properties. However, the grain morphology was dominated by orientations with direction of approximately 65° with respect to the deposition direction, which led to better tensile properties in the longitudinal direction than in the transverse direction. In addition, the microhardness reached the maximum (∼130 HV) after four to seven times thermal cycles, which was mainly attributed to the Guinier–Preston (GP) zone. The continuous high temperature resulted in the hardness minimum (∼104 HV), which was a significant factor in forming the η′ and coarse η phases in the part. Combined with the precipitation behavior, hardness of the whole part can be divided into two zone: unstable and stable zone.