The influence of Cryo-ECAP and aging on the microstructure and mechanical behavior of AA6061 Al alloy

The combination of Equal-Channel Angular Pressing (ECAP) with cryogenic processing temperatures can be an efficient method to obtain very refined microstructures. However, there is a lack of research on the combination of these processes and the effects they have on microstructure and mechanical properties, particularly in precipitation hardened aluminum alloys. The present study describes the effects of cryogenic ECAP-processing and subsequent heat treatments on the microstructure and mechanical properties of AA6061 Al alloy. Samples in the solutionized state were processed by ECAP at 173 K (Cryo) and at room temperature (RT) up to an equivalent strain of 4.2. After Cryo-ECAP, the microstructure was very refined, with an average grain size close to 0.5 mu m and strain concentrated within microscopic shear bands. EBSD analysis after Cryo-ECAP shows new fine equiaxed grains with a higher proportion of low-angle grain boundaries. The temperature of 173 K was effective in suppressing partially precipitation during ECAP, and made the process possible without extensive cracking. However, recovery mechanisms were not fully hindered, which undermined the strengthening effect of Cryo-ECAP. The yield stress was higher after RT-ECAP, while uniform elongation was improved by Cryo-ECAP plus T6 treatment, a result attributed to the higher fraction of low-angle grain boundaries observed in the microstructure. Finally, it was demonstrated that cryogenic processing is not in itself a guarantee of improvements in strength and ductility, in comparison with RT processing. Both recovery and precipitation processes dictated the feasibility of the process, the final microstructure and the mechanical response of the ECAP-processed AA6061 Al alloy.