Influence of temper on the performance of a high-strength Al-Zn-Mg alloy sheet in the warm forming processing chain

Al-Zn-Mg aluminium alloys have greater strength than conventional aluminium alloys used in the automotive industry, but little has been reported on their formability and paint bake response. In this paper the strength and formability of age-hardenable AW-7921 alloy sheet across a range of tempers from the under-aged to peak-aged condition is investigated by warm tensile tests at temperatures between room temperature and 230 °C, and cross-die deep-drawing tests at 230 °C. Differential scanning calorimetry (DSC) was used to study the precipitation states of the AW-7921 sheet in the as-received and warm cross-die deep-drawn conditions. Formability was found to improve at 230 °C and was sensitive to the initial temper of the sheet. Light aging for 10 min at 100 °C resulted in only GP Zones present in the sheet, while both GP Zones and η' precipitates formed after a subsequent moderate aging at 120 °C for 50 min. Dynamic recovery and the partial dissolution of precipitates were found to contribute to the increase in formability at elevated temperatures. Across the tempers investigated, the T4 temper has shown the best combination of warm formability and high post-paint baking strength.