Enhanced properties and homogeneity of Al-Zn-Mg-Cu alloy thick plate by non-isothermal aging

Abstract

The desirable properties and homogeneity of Al-Zn-Mg-Cu alloy thick plate are urgent to acquire in the industrial field. An efficient method for determining the optimal non-isothermal aging (NIA) of Al-Zn-Mg-Cu alloy plates was proposed by employing an in-situ resistance testing method, and the microstructure, properties, and inhomogeneity of plates with 150 mm and 120 mm in thickness during NIA process were investigated. The in-situ resistance analysis suggested that the NIA process was divided into four stages, which were dominated by nucleation and atomic diffusion. The 190 °C (Peak temperature) and 30 °C/h (Temperature change rate) were the optimum parameters of the 150 mm plate, and the obtained ultimate tensile strength, elongation, and electrical conductivity were 569 MPa, 11%, and 40% IACS, respectively. The η' phases were the primary precipitates for strengthening after NIA. The excellent comprehensive properties of the non-isothermal aged alloy benefited from the secondary precipitation of fine precipitates in the cooling stage and the discontinuous distribution of grain boundary precipitates. The continuous temperature phase transition curves were conducted to formulate the optimal regime of 120 mm plate, which was optimized as 195 °C and 34 °C/h. The NIA treatment regulated the relative phase transformation variable of different positions of the thick plate to 42–46%, which verified the optimization of aging homogeneity along the thickness direction. Under the precondition of reducing the aging time by more than 76%, both plates after NIA maintained satisfactory strength and ductility and were characterized by an impressive corrosion resistance far beyond the two-step aging state.