Continuous grain refinement using severe torsion straining process

This study presents a new rapid continuous process for grain refinement in metallic materials through severe plastic deformation (SPD). The new process, designated in this study the severe torsion straining process (STSP), is applicable to a wide range of alloys based on aluminum, magnesium and copper including carbon steel. This process consists of producing a local heated zone in a rod and cooling both sides of the heated zone by spray water while rotating one end with the other. Thus, torsion strain is introduced in the local heated zone. The STSP can be continuous because the straining is achieved while the rod is shifted along the longitudinal axis of die rod. Furthermore, the process requires no die, suggesting a potential for commercialization of grain refinement through SPD. In this study, STSP was applied to an Al-Mg alloy and a Mg-Al-Zn alloy. It is shown that STSP is effective for both alloys so that the grain size is reduced to ∼1.5 μm for the Al alloy and ∼0.9 μm for the Mg alloy. Tensile testing showed that the strength is increased with a minimal decrease in uniform elongation. There is a critical ratio between rotation speed and moving speed, which defines the feasibility of STSP operation without breaking the rod. The grain size tends to be lowered as the ratio is close to the critical value.