Thermal–microstructural analysis of the mechanism of liquation cracks in wire-arc additive manufacturing of Al–Zn–Mg–Cu alloy

High-strength Al–Zn–Mg–Cu aluminum alloys are widely used in large-scale aerospace parts. However, high crack sensitivity limits their development during additive manufacturing. The mechanism of crack formation in Al–Zn–Mg–Cu alloy parts manufactured by wire-arc additive manufacturing was studied through a combination of thermal stress simulations and microstructure analyses. Increasing number of deposition layers gradually expanded the partial melting zone, leading to heat accumulation. In addition, the equivalent stress at both sides was considerably higher than that at the center of the parts, and the equivalent stress at the bottom was the highest, which gradually decreased with increasing distance from the bottom. However, in the initial stage of the deposition, the area of the partial melting zone in the bottom was small; the crack sensitivity was low (no cracks were found). With further deposition, liquation cracks occurred 40–60 mm from the bottom and were initiated at sufficient stress and temperature. The increase in the deposition height increased the stress and crack propagation to form a macro crack.