Initial holding time dependent warm deformation and post-ageing precipitation in an AA7075-T4 aluminum alloy

Abstract

• Warm deformation at 180 ℃ significantly enhances the formability of AA7075-T4 alloy. • The through-process microstructure evolution during warm compression is revealed. • Initial soaking of 10−30 s benefits the balance of warm-deformability and post-ageing. • A TTP sketch considering the reversion of clusters and reprecipitation is proposed. Process parameters such as soaking time and forming temperature are critical for the isothermal warm forming performance and post-form mechanical properties. The isothermal warm forging process of AA7075-T4 aluminum alloy has been simulated through the warm compression at 180 ℃. The warm deformation schedule including fast heating and short-term initial holding is found to be advantageous for enhancing the formability of high-strength AA7075-T4 alloy due to the reduced flow strength and significantly increased ductility. As revealed by differential scanning calorimetry and transmission electron microscopy, the natural ageing clusters quickly dissolve, then the GP zones and even η'/η 2 phases form during initial holding at 180 ℃ for 4 min. The optimum balance of warm-deformability and subsequent age hardening potential at 120 ℃ is obtained with an initial holding for less than 30 s, by restricting the growth of coarsening precipitates before isothermal warm deformation. The warm compression induced dislocations are largely retained during post-ageing, accelerating the hardening kinetics and promoting the formation of a high density of η'/η 2 precipitates in the peak-aged sample. Through considering the reversion of natural ageing clusters and solutes reprecipitation during the initial holding and post-ageing, a temperature-time-property sketch is proposed to explain the relationship between the warm deformation temperature, initial holding time and the obtained hardness for the AA7075-T4 alloy. This study could guide the optimization of warm forming process and is fruitful for establishing an accurate microstructure-based model for the warm deformation of AA7075-T4 aluminum alloy.