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

• An Al 7075 alloy was FSPed over two cooling anvils and four processing conditions.
• The initial precipitation state has no influence on the FSPed grain size.
• The higher the processing severity, the finer the grain size, reaching ~ 200 nm.
• The higher the heat extraction, the less coarsening after the pass of the tool.
• The control of the processing conditions makes FSP to be an effective SPD technique.

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.