Elementary model of severe plastic deformation by KoBo process

Abstract

Self-consistent model of generation, interaction, and annihilation of point defects in the gradient of oscillating stresses is presented. This model describes the recently suggested method of severe plastic deformation by combination of pressure and oscillating rotations of the die along the billet axis (KoBo process). Model provides the existence of distinct zone of reduced viscosity with sharply increased concentration of point defects. This zone provides the high extrusion velocity. Presented model confirms that the Severe Plastic Deformation (SPD) in KoBo may be treated as non-equilibrium phase transition of abrupt drop of viscosity in rather well defined spatial zone. In this very zone, an intensive lateral rotational movement proceeds together with generation of point defects which in self-organized manner make rotation possible by the decrease of viscosity. The special properties of material under KoBo version of SPD can be described without using the concepts of nonequilibrium grain boundaries, ballistic jumps and amorphization. The model can be extended to include different SPD processes.

Review of principles and methods of severe plastic deformation for producing ultrafine-grained tubes

“…These include, ECAP followed by rolling, 50 ECAP followed by extrusion, 51 integrated extrusion-ECAP, 52 ECAP-conform and drawing, 53 hydrostatic extrusion (HE), 54 asymmetric rolling, 55,56 torsion extrusion, 57 cryogenic rolling, 58 severe torsion straining, 59 HE combined with torsion, 60 twist extrusion and subsequent rolling, 61 a method without SPD, 62 integrating forward extrusion and torsion deformation, 63 calibre rolling 64 and SPD using the KoBo process. 65,66…”

Microstructure of titanium deformed by warm extrusion with forward- backward rotating die

“…Gusak et al [17] claim that the material behaves like a liquid changing the flow direction without changes the local texture. They have proposed a model describing this process.…”

An Overview on the Continuous Severe Plastic Deformation Methods

“…The material billet tolerates reversible plastic twisting just before entering to the extrusion deformation zone during the KoBo extrusion process. 139) 4. Discussion…”