Continuous Differential Speed Rolling for Grain Refinement of Metals: Processing, Microstructure, and Properties

In recent years, considerable research has been devoted to the development of ultrafine-grained (UFG) structural metals fabricated by various severe plastic deformation (SPD) techniques. Among these techniques, differential speed rolling (DSR) has been considered as the continuous forming method in which bulk sheet materials pass through two identical rolls with different speeds of the rolls, effectively imposing intense plastic strain based on shear deformation. This review describes the principles of the DSR process, including the strain imposed by DSR, processing routes, and macro-shear characteristics associated with the processing routes; it also outlines the processing factors that affect grain refinement, such as total thickness reduction, thickness reduction per pass, rolling speed, roll speed ratio, number of operations, and rolling temperature. The essential features of the resultant microstructural variables, including the grain size and morphology, grain boundaries, deformation textures, and dislocation density developed in materials with HCP, FCC, and BCC crystals deformed by the DSR process are also summarized. In addition, the structural properties of metals subjected to DSR are reviewed, including strength, hardness, and ductility at room and elevated temperatures.