Influence of Natural Aging After T6 Heat Treatment on Mechanical Properties of Age-Hardenable Al Alloys

It is well known that residual stress is detrimental to mechanical properties of metal materials. This paper investigated the relief of residual stress during long-time natural aging as well as its influence on tensile properties of partial age-hardenable Al alloys. It was found that uniform elongation and ultimate tensile stress were increased after a long-time natural aging of T6 heat-treated 2024, A356, 6063 and 6061 alloys. For example, uniform elongation and ultimate tensile stress of 2024 alloy were respectively increased from 10.2% and 496.7 MPa to 11.1% and 507.5 MPa. According to fracture behavior observation for 2024 and A356 alloys, cracks or/and voids could generate from micron-sized secondary particles or particle/α-Al interface in matrix, which led to fracture of tensile samples eventually. And reduced residual stress during natural aging delayed formation of the cracks or voids based on stress distribution analysis, which led to an increase in uniform elongation and tensile stress of Al alloys.