New heat treatment to improve the mechanical properties of low copper aluminum primary foundry alloy

Abstract

The purpose of this work is to design a new heat treatment in order to improve the mechanical properties of primary 356 + 0.5 wt%Cu aluminum alloy. This heat treatment combines a multi step solution treatment with a quench and a multi step overaging treatment. It takes advantage of the evolution of the solubility limit of the matrix as a function of temperature in order to allow the appearance of successive precipitation sequences during excessive aging in several stages and leads to an increase in the volume fraction of precipitates.After T64 air quench, T6 air quench, T7 air quench and T7 water quench treatments and overaging 100 h at 150 °C, a secondary precipitation of Al2Cu particles is seen using conventional TEM, mostly after the T7 treatments. This secondary precipitation leads to an increase of the static mechanical properties. This dual-step overaging is then transposed to a dual-step aging T7 heat treatment, with aging 2h at 200 °C and 2h at 170 °C, and further compared to a classic T7 treatment with aging during 4h at 200 °C. Both heat treatments are also modeled using a KWN model. The model captures the secondary supersaturation of the proeutectic α-phase that leads to the secondary precipitation during the dual-step aging treatment. This translates to an increase of the mechanical properties by 6% and 3% for the yield strength and the ultimate tensile strength respectively. The elongation decreases by 33%.