Two step–ageing of 7xxx series alloys with an intermediate warm-forming step

Abstract

Two-step ageing of 7xxx series alloys can reduce ageing time while obtaining similar mechanical properties as a longer isothermal heat treatment. In this paper, an intermediate deep-drawing step at 120–160 °C is introduced to produce elongated cup–shaped parts with near-T6 strength. A Cu-containing (Alclad 7075) and a non-standard low-Cu alloy (7021+) are used. Formability and the final mechanical properties after various second ageing steps are reported. Hardening precipitates are studied by means of transmission electron microscopy and differential scanning calorimetry. We found that the pre-aged 7021+ alloy can be deep-drawn at lower temperature (120 °C) than pre-aged Alclad 7075 (160 °C). Both alloys respond well to the two-step ageing regime with the parts achieving up to 99% and 98% of the T6 yield strength of Alclad 7075 and 7021+, respectively. Compared to T6, the distribution of hardening nanoscale Mg–Zn-precipitates is slightly coarser after two-step hardening regimes with intermediate warm-forming. Mechanical properties are closest to T6 for a final ageing step at 165 °C for 130 min and coarsening of hardening precipitates is less pronounced compared to a simulated paint-bake cycle (185 °C, 34 min).

Keywords

• Aluminium alloy
• Automotive
• Retrogression
• Paint-bake
• Deep-drawing
• Transmission electron microscopy