Effect of Sc on solidification segregation and subsequent friction stir processing of aluminium alloy

Abstract

Al-Zn-Mg alloys (7xxx series) are used in aerospace engineering due to their high strength-to-weight ratio and good mechanical properties. Al-Zn-Mg alloy is exhibited strong age-hardening properties due to the Sc (0.39 wt%) inoculation effects, which implies decomposition of the supersaturated solid solution and the formation of a fine distribution of coherent GP-zones precipitates, ή precipitates, η(stable) and Al3Sc(L12) precipitates. In addition, the ageing sequence consists of mainly three steps such as solutionizing at 465 °C for 1 h then immediately water quenching (T4), subsequently, room temperature ageing, and again proceed artificial ageing at 140 °C for 6 h (T6). The microstructure at peak strength is shown to depend on the mainly dissolution of GP zones, nucleation of ή and Al3Sc precipitates or formation of GP zones, nucleation of dislocations and dislocation with Al3Sc particles interaction. Thus, at higher temperature around 140 °C the ή transition phase is being formed and thermal stability of Al3Sc phase at > 300 °C. It has also identified those ageing treatments both operation eliminated micro-segregations and improved mechanical properties of this alloy. The influence of various process parameters on the microstructure and mechanical properties of the as-cast alloy and FSP of aluminium alloy are studied by OM, EPMA, FESEM, DSC, TEM, Vicker’s hardness measurements and tensile properties. The combined effect of Sc addition, FSP then later on post ageing treatment at 140 °C for 2 h significantly enhanced better tensile properties (278.8 MPa, ductility 6.7%) when compared to the T6 ageing treatment with Sc addition (340 MPa, ductility 4.8%). The aim of this paper is to investigate in details the ageing behaviour at T6 condition, and examine microstructural effect after optimized FSP parameters at 1000 rpm, traverse speed of 70 mm/min, specified tool design and induced heat input (1.07 kJ/mm) on Al-Zn-Mg-Sc alloy to enhances mechanical properties after post ageing treatment at 140 °C for 2 h.