Severe friction stir processing of an Al-Zn-Mg-Cu alloy: Misorientation and its influence on superplasticity

An overaged Al 7075 alloy was severe friction stir processed using two different backing anvils using different heat extraction rates. A range of processing parameters. (rotation speed) and V (traverse speed) involving low heat index (HI) for obtaining severe processing conditions were employed. As a result, we obtained microstructures with a wide variety of very fine grain sizes and misorientations that gave rise to very different values of stresses and elongations. We found a relation between mean misorientation ((delta) over bar) and grain size (L) that allows discerning two processing regimes: severe ((delta) over bar = <()over bar>(L)) and standard ((delta) over bar. 41 degrees). Additionally, we found analytical relations between the misorientation, the grain size and the FSP parameters that predict misorientation and grain size processing maps for each backing anvil. The further experimental data analysis of high temperature tensile tests provided the relationship sigma(y) = sigma(y)(omega, V) in the grain boundary sliding (GBS) region considering the influence of the misorientation in the superplastic behavior. Finally we propose a modified GBS constitutive equation including the influence of the misorientation. This equation describes the superplastic behavior in the severe FSP region (low HI), while it tends to the classical GBS constitutive equation in the standard FSP region (high HI). (C) 2017 Elsevier Ltd. All rights reserved.