Enhancing properties of Al-Zn-Mg-Cu alloy through microalloying and heat treatment

This paper aims to understand the influence of microalloying constituents (Ti, Zr, Sr and Sc), and three heat treatment conditions i.e., T6, retrogression and reaging (RRA) and high temperature aging (HTA) on mechanical and thermal characteristics of modified Al-Zn-Mg-Cu-X alloy using microstructural analyses consist of precipitation characteristics, phase fraction and phase morphology. Addition of 0.25 wt % Sc significantly refines the grain structure, reducing the grain size from 62 mu m to 34 mu m and increasing Al3Zr, Al-3(Zr, Ti), and Al-3(Sc, Zr) precipitate fraction. After T6 aging, the alloy exhibits primary precipitates of fine eta and T phases, accompanied by secondary intermetallics as well as S phases. RRA treatment led to a slightly larger fraction of coarse Zn and Cu-rich phases, while HTA results in larger eta, T, and S phases due to high-temperature aging. The addition of Sc reduces the cooling rate by raising the transition temperature to 532 degrees C with an enthalpy of 22.1 J/g. T6 treatment shows dissolution of most eta phases within the matrix, although RRA and HTA exhibit higher enthalpy due to a greater fraction of eta phases. The tensile strength of Al-Zn-Mg-Cu-X alloy follows the following sequence: T6 > RRA > HTA > as-cast condition, regardless of micro-alloy additions. Under T6 conditions, Sc added alloy achieves 30 % higher strength, i.e., 575 MPa, than base alloy due to fine dispersoids. RRA treatment enhances elongation without compromising mechanical properties, resulting in a shift from transgranular brittle fracture to intergranular-dominated mix-mode fracture, with the presence of secondary cracking.