Effect of Isothermal Heat Treatment on Hardness of X7475 Aluminium Alloys

Abstract

The heterogeneous nature of microstructure in most as-cast alloys has created the unique properties for an ally before heat treatments. The scanty nature of scholarlyarticleson the effect of heat treatments on the hardness of new alloys produced from recycled aluminium alloys prompts this investigation. In this work, we upgraded from 3 series to 7 series alloys to form a new material. Previous attempts were also silent on the Alcao Green Letter specifications of the 7 series alloys, hence producing a 7475 alloy that may not be within the Alcao specifications. This paper presents theresult of preliminary studies on new experimental X7475 Al-alloy that was produced per Alcoa Green Letter specifications, with variations of Zn (4.0 -5.0 wt.%), Mg (1.00 -1.50 wt.%) and Mn (0.025-0.075 wt.%), thensubjected to microhardness probeunder a load of 490.3mN (0.05Hv) and 10sec duration afterisothermal and isochronous “O”, T4 and T6 heat treatments. Heating and cooling rates were controlled during heat treatment processes to investigate the effects on the formation of the MgZn2and other precipitates on the mechanical properties of the alloy. Peak obtainable hardness value of 140.45Hv and 134.32Hv are attained within the T6 and T4 conditionsin an alloy of 4% and 5% Zn respectively.The increase in wt.%of Zn favours mechanical properties while reverseis the case for Manganese. This result is an indication of the load bearing capacity of our alloy. It has also demonstrated the success in upgradingthe 3 series to 7 series alloysfrom secondary materials. However, the result could be improved onafter composition, heat treatmentsand production route optimization.