Microstructure and mechanical properties of ultrafine grained 5052 Al alloy fabricated by multi-pass differential speed rolling

This study examined the microstructural evolution and mechanical properties of an ultrafine grained (UFG) Al alloy processed by multi-pass differential speed rolling (DSR). For DSR operations with a thickness reduction of 30% in each pass and a roll speed ratio of 1:4 for the lower and upper rolls, respectively, plastic strain was imparted to the samples that were rotated 180° along the longitudinal axis between the adjacent passes. As the number of operations increased, the deformed microstructure developed gradually from a band-like structure, which was observed after a single pass, to one consisting of nearly equiaxed ultrafine grains (≈0.7 μm) divided by high angle grain boundaries (GB) after four-pass DSR. A close microstructural observation using high resolution transmission electron microscopy showed that the GBs of the UFG Al sample were curved and wavy. Based on microstructural observations, the grain refinement mechanism occurring in the Al sample during multi-pass DSR was found to be governed by geometric dynamic recrystallization and continuous dynamic recrystallization. In addition, the deformation texture of the DSR-treated samples developed into the ideal shear texture consisting of a rotated Cube {001}<110> orientation and γ-fiber (<111>//ND) with increasing number of operations. The tensile test results showed that the strength of the deformed samples increased with decreasing grain size while its tensile ductility was sacrificed. Moreover, the contribution of the strengthening mechanisms, such as solid solution, precipitation, grain size and dislocation strengthening, were discussed.