Investigations on warm forming of AW-7020-T6 alloy sheet

• Feasibility of warm forming of AW-7020-T6 alloy is studied.
• Forming at warm temperatures improved the formability of AW-7020-T6 sheet. Coarsening of hardening precipitates and dynamic recovery controls the formability of AW-7020-T6.
• Warm forming and paint baking both reduce the strength of the sheet from its initial T6 strength due to coarsening of the strengthening precipitates.
• Warm forming of AW-7020-T6 at 200 °C ensures minimum strength of 250 MPa in the formed parts after paint baking.

7000 series aluminium alloys have greater strength than conventional aluminium alloys used in the automotive industry, but little has been reported on their formability. In this paper the strength and formability of age-hardenable AW-7020 alloy sheet in the T6 temper condition was investigated at temperatures between 150 and 250 °C by warm tensile, Swift-cupping and cross-die deep-drawing tests. Differential scanning calorimetry (DSC) investigations were carried out to study the precipitation state of AW-7020 sheet in as-received, warm cross-die deep-drawn and post-paint-baked conditions. Formability was found to improve at temperatures above 150 °C and was sensitive to temperature and strain rate. There was also an onset of dynamic recovery from 150 °C. DSC results showed the presence of η′ precipitates in T6 temper and that these coarsen during the warm cross-die deep-drawing and paint baking processes with ∼30% drop in ultimate tensile and yield strengths. Dynamic recovery and coarsening of η′ precipitates were found to contribute to the increase in formability at elevated temperatures.