Evaluation of the mechanical anisotropy and the deformation mechanism in a multi-pass friction stir processed Al-Zn-Mg-Cu alloy

Multi-pass friction stir processing (MP-FSP) can produce extensive bulk nanostructured areas in various metallic materials, such as the Al 7075 alloy (Al-Zn-Mg-Cu), providing fine and highly misoriented grains. This allows obtaining superplastic deformations by the activation of grain boundary sliding (GBS) mechanism at certain strain rate and temperature window, crucial for performing a further superplastic forming (SPF). The superplastic performance of the MP-FSP Al 7075 alloy in two uniaxial testing directions was evaluated and compared to that of a single pass FSP (SP-FSP), reporting a mechanical anisotropy in the MP-FSP attributed to the heterogeneous crystallographic orientation distribution along the transverse testing direction. The operative deformation mechanism was determined by uniaxial tensile test and small punch test (SPT), the former a test with biaxial with radial symmetry. GBS was corroborated as the main operative deformation mechanism in the SP-FSP and MP-FSP, reporting high strain rate superplasticity in the temperature range 350–400 °C at 10 s, and maximum elongation values between 290–540% at 400 °C depending on the testing direction. The Al 7075 alloy processed by MP-FSP showed a great potential for a further SPF as GBS remains as the global deformation mechanism under biaxial testing conditions.Financial support from MINECO, Projects MAT2012-38962 and MAT2015-68919-R (MINECO/FEDER) is gratefully acknowledged. A. Orozco-Caballero also thanks CSIC for a JAE-Pre fellowship, co-funded by the European Social Fund FSE 2007-2013. M. Alvarez-Leal thanks MINECO for a FPI fellowship, number BES-2013-063963