Ultrafine grained materials through mechanical processing: an assessment

Abstract

In this paper severe plastic deformation (SPD) and friction stir processing/ welding are examined. The structural changes due to SPD are reflected in improved mechanical properties. Advantages of SPD are pointed out. Within the SPD technique, a number of approaches are possible, e.g., equi-channel angular pressing/extrusion, high pressure torsion, accumulative roll bonding/fold - roll process, reciprocating extrusion - compression, cyclic close die forging, repetitive corrugation and straightening. Analyses available are elementary and often assume uniform stress and strain distribution. These processes are easily adapted to suit standard metal working equipment fitted with inexpensive devices and tools. However, scaling up the processes to handle large billets and achieve large tonnage production is difficult. In the near future, medium and small-scale industrial production only is likely. Friction stir process, a solid state technique for joining similar or dissimilar materials of equal or different thickness, has some key metallurgical, environmental and energy benefits. It is already being considered for applications in aerospace and automotive industries. Significant improvements in surface properties and superplastic flow have been established in friction stir processed materials. Velocity of tool movement and power input needed for fast rotation of the tool are the major variables. Since significant temperature rise is there during processing, in a proper analysis, the boundary conditions arising from thermal and mechanical constraints have to be satisfied simultaneously, which is an extremely difficult. A few key issues have to be addressed before large-scale production can be attempted. An integral approach that takes into account the total system of material, design, mechanics and component forming is likely to lead to industrially relevant solutions.