Strategy for severe friction stir processing to obtain acute grain refinement of an Al-Zn-Mg-Cu alloy in three initial precipitation states

Abstract

An Al-Zn-Mg-Cu, Al 7075, alloy was subjected to friction stir processing (FSP) using several processing conditions, two different backing anvils and three initial precipitation states in order to reach the maximum feasible processing severity to produce ultrafine grain sizes. Microstructures formed by fine, equiaxed and highly misoriented grains were obtained. Grain sizes were situated in the range of 200-1000 nm, making FSP competitive with other severe plastic deformation techniques. No influence of the initial precipitation state in the processed grain size was perceived. In fact, the processing conditions and the cooling rate determine the observed grain size. It was found that the selection of the appropriate processing conditions delivered an ultrafine grain size, thus allowing suitable microstructural control.