Elucidating the microstructure evolution and performance response of wire-arc directed energy deposited Al–Zn–Mg–Cu alloy at as-deposited and post-processing heat treatment states

Abstract

Wire-arc directed energy deposition technology was employed to manufacture ultra-high-strength Al–Zn–Mg–Cu alloy taking advantage of variable polarity cold metal transfer (CMT) process. The grain characteristics and precipitation behavior under as-deposited (AD) and post-processing heat treatment (PHT) states were comparatively analyzed. The relationship between the microstructure evolution and mechanical performances was fully disclosed. The obtained results show that the grain structure of AD sample exhibited a nearly fully-equiaxed feature (average size: ∼45 μm), which was caused by the low heat input and variable polarity of CMT advance mode. The microhardness distribution of AD sample consisted of slight fluctuation and stable areas along the building direction due to the different thermal history and precipitation process. After the PHT process, the average grain size and morphology almost remained unchanged. However, compared with the size and type of precipitation phases under the AD state, high-density nanoscale (∼5 nm) metastable η′ phases were precipitated in the grain interior of the PHT sample, which contributed to the crucial precipitation strengthening effect. The homogeneous microhardness distribution was achieved with a high level (∼180 HV0.2). Meanwhile, tensile properties and elongation significantly increased to 524.68 ± 7.31 MPa and 7.10 ± 2.09 % after the PHT process, respectively.