On The Dependency Of Grain Refinement Efficiency On Processing Route During Equal Channel Angular Extrusion Of Pure Copper

Equal channel angular extrusion (ECAE) is an important technique with great potential to fabricate ultra-fine grained material in bulk form. The die angle (Φ), number of passes and processing routes are factors having significant effects on grain refinement. To date, the effects of die angle and number of passes are clear but the relationship between processing routes and grain refinement is not very clear. In this article, finite element method (FEM) was preformed to investigate the deformation behavior during ECAE with emphasis on the effects of outlet channel length and billet length on the distribution of strain and the characteristics of plastic deformation zone. The simulation results were used to optimize the ECAE die design. Then, in a greater range of processing route, the effects of processing routes on the microstructure and mechanical property in pure copper were investigated in detail, and the relationship between processing routes and grain refinement was discussed.The results of FEM simulations suggest that the outlet channel length and billet length have influences on steady state deformation of billet. A shorter outlet channel generally leads to a longer steady state region, but a higher tendency of upward bending of the deformed billet. The relative length of the steady state region generally increases with the billet length-to width ratio; however, it does not increase further when the billet length-to-width ration reaches a critical value due to severe heterogeneous billet deformation in the inlet channel. For the typical strain hardening material under realistic friction condition, a billet length-to-width ratio of about 6 ~ 8 and an outlet channel length-to-width ratio of 0.5 ~ 2 are recommended for the sake of both cost saving and production efficiency; accordingly, ECAE dies with a billet length-to-width ratio of 6.5 and an outlet channel length-to-width ratio of 0.5 were designed and applied to the ECAE experiments of the present pure copper.The experimental results suggest that with the increase of number of passes (from 1 to 8 passes), the microstructure evolves from coarse lamellar structure towards fine equiaxed structures, together with an increase of the proportion of high-angle grain boundaries. Correspondingly, the strength of copper increases gradually and the elongation decreases significantly after 1 pass and then increases slightly after the subsequent passes. Meanwhile, the processing route has important effect on the microstructure and mechanical properties of pure copper, and different processing routes induce different types of microstructure: For aΦ= 90o die, route R90 leads to a more rapid development of the lamellar structure into an equiaxed structure and consequently higher strength and elongation, compared to routes R180 and R0; for aΦ= 120o die, route R75 leads to a more rapid development of the lamellar structure into an equiaxed structure and consequently higher strength and elongation compared to routes R180 and R0. The optimum route for grain refinement varies with the die angle, i.e. R90 is most effective forΦ= 90o but R75 is most effective forΦ= 120o, and the latter result is different from existing results in the literature.The dependencies of grain refinement on the processing route revealed by the present experimental results can be partially explained in terms of the characteristics of shearing plane intersection, strain path change or slip activity. Nonetheless, none of the existing theories can fully explain these results. The relationship between processing route and grain refinement needs to be further investigated.