Abstract
Aluminium alloy (7075 series) contains scandium (Sc) promoting fine grains due to their potent inoculation effects formation of homogeneous primary particles, Al3Sc (L12) during solidification and subsequent age- hardening phenomenon. A supersaturated aluminium matrix can ensure the driving force for the coherent precipitates, and high density (∼0.45 wt% Sc) nanoparticles Al3Sc and further enhanced by raising the ageing temperature. FSP is a solid-state surface modification technique via frictional heat and tool stirring during processing. The effects of double-pass friction stir processing (DP-FSP) on the microstructure, and nano-sized particle (MgZn2, Al3Sc) distribution, as well as the microstructural evolution after T6 heat treatment, were investigated by OM, FESEM, TEM, SEM, Vicker’s hardness measurement, and tensile properties. Zn vaporization is a primary defect in the stirring zone which hinders the high strength even though having fine grain microstructures. SEM fractography revealed coarse precipitates, defects, and propagation of fracture plane of the alloy. The present investigation aims to study the enhancement of strength and ductility for nano-sized precipitates in the matrix by a combination of T4 + DP-FSP and subsequent post ageing treatment at 140 °C for 2 h (T6 treatment).
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