Effect of copper to magnesium ratio on precipitation induced anisotropy during ageing of recrystallized Al-Zn-Mg-Cu alloy

Abstract

High strength Al–Zn–Mg–Cu alloys (7xxx series aluminium alloys) have been widely used in military and aerospace industries due to high strength, easy formability and low density. Other important properties that must be considered for these applications are strength, ductility, modulus, corrosion and damage tolerance (e.g. fracture toughness and fatigue resistance). Most of these properties can be controlled through appropriate alloying, processing or a combination of these. Age-hardenable 7xxx series aluminium alloys for high-performance structural applications are typically processed in the form of plates, extrusions or forgings. For thick plate products, a typical processing schedule involves casting, homogenising, hot rolling, solution treating, quenching and age hardening.

In this work, the effects of variation of Mg and Cu contents in some predefined ratios on the microstructures and mechanical properties of a 7xxx alloy both in longitudinal and transverse directions were studied. Initial cast and homogenised microstructure revealed dendritic structure which was lost completely during hot rolling. After hot rolling, solution treatment and ageing responses were observed at different temperature and time combustions. Interestingly, it was found that under same ageing condition, hardness and strength of the alloys were predominantly controlled by magnesium content, attributed to η-phase. Albeit similar behaviour was shown by copper content, the response was not as effective as that obtained with higher amount of magnesium. From microstructural analysis in optical microscopy and scanning electron microscopy, it can be asserted that higher magnesium to copper ratio yielded higher amount of second phase particles, which was validated by thermodynamic modelling of microstructural phases. In both longitudinal & transverse directions, better mechanical properties (ultimate tensile strength and hardness) were found for the alloy having Cu/Mg ratio of 1.32, owing to S-phase. Along longitudinal direction, fracture surfaces were heavily dimpled, and intergranular features were found that yielded better ductility with high UTS value for alloy of 1.32 Cu/Mg ratio. Along transverse direction, microstructure investigation revealed transgranular and cleave features, attributed to stable S-phases.