Multiple direct extrusion: A new technique in grain refinement

A novel high-straining bulk deformation technique based on repeating conventional direct extrusion is presented. This technique, named multiple direct extrusion (MDE), uses a square container with a rectangular die aperture that can achieve a minimum 50% reduction/pass in the cross section of the billet. After extrusion, the new billet is cut perpendicular to the longitudinal axis. The resulting halves are then joined to obtain a square shape again so that the direct extrusion process can be repeated. Two processing routes are possible before reintroducing the billet into the container: no rotation and 90° rotation around the longitudinal axis. During each cycle, the billets change their geometrical shape and as a result, the cross section area gets smaller. A mechanism of grain fragmentation during MDE based on the analysis of velocity discontinuities along slip lines in the deformation zone is suggested. Four cycles of MDE were applied to commercial copper and the potential for grain refinement, and the improvement in mechanical properties were evaluated.