Deformation heating and its effect on grain size evolution during equal channel angular extrusion

Abstract

The size effect of equal-channel angular pressing (ECAP) is investigated systematically, by examining the similarity of geometrical dimensions, mechanical parameters and thermal behaviors. The strain and strain rate distribution, microstructure and mechanical property evolution upon scaling up were characterized experimentally, followed by finite element simulations and theoretical analyses of thermal behavior. Steady and consistent microstructure recovery and mechanical properties softening observed in the eight-pass ECAP processed ultrafine-grained (UFG) copper upon scaling-up, demonstrate a significant size effect. The billet temperature profile rises upon scaling-up in simulation that accounts for the observed size effect. Theoretical analyses show that neither the heat conduction within the billet, nor the heat exchange between the billet and its surroundings could maintain similarity upon scaling. In other words, the size effect in ECAP is inherently rooted in the laws of thermal physics. Plunger velocity scaling down was proposed as an effective strategy to control the temperature rise and size effect upon scaling up. The fundamental discoveries uncovered by present work will benefit the scaling of ECAP for commercialization. And the frame of similarity applied to the examination of the ECAP process is believed to be also applicable to the scaling of other thermal mechanical processing.