Abstract
Reduction in vehicular weight by ~50% is a vital step in meeting automotive requirements such as (i) fuel-economy; (ii) life-cycle greenhouse gas emissions; and (iii) crash- and impact-worthiness and safety. Vehicle weight reduction will also play major part in future long-range battery-electric cars which, compared to their internal combustion counterparts, have much lower energy storage density and are substantially heavier. There has been intense development of the high-strength, precipitation-hardened 7000-series sheet aluminum alloys for use in the structural components of vehicles to meet these requirements. These high-strength heat-treatable alloys are typically supplied in peak-strength, T6, temper and possess poor room temperature formability and are prone to stress corrosion cracking (SCC). Die quenching, warm forming, and intermediate quench and form processing routes have been suggested to improve formability of these sheets; however, their SCC susceptibility in these conditions has not been ascertained. The overall objective of the present research is to determine the SCC resistance of warm-formed peak-aged and pre-aged AA7075 alloys, but the results presented here are for the T6 temper – the most susceptible temper – to provide a basis for evaluation of the other tempers. This work employed a 4-point bend test set-up following the ASTM G39 standard procedure and was compared to direct tension test in ASTM G49. Unlike samples under direct tensile in ASTM G49, the stress variation between 70 – 90% of yield strength applied on samples exposed to 3.5% NaCl content did not cause cracking after 40 days under bend test, despite testing the most susceptible temper to SCC, i.e. T6 temper. However, when the chloride amount was tripled on day 41, and monitored until day 120-150, through-thickness corrosion cracking was observed on these specimens and the severity increased with stress level.