Change in microstructures and mechanical properties during deep wire drawing of copper

Evolution of microstructure and change in mechanical properties of a pure copper wire deformed to equivalent strain of 6.9 by deep wire-drawing (WD) process were investigated and compared with those in a copper sheet severely deformed by accumulative roll bonding (ARB) process. The deep-drawn copper had an ultrafine and elongated microstructure with mean high-angle grain boundary (HAGB) spacing of 380 nm. The fraction of HAGBs in the ultrafine microstructure was 62% and the elongated ultrafine grains involved dislocation substructures inside. The microstructural parameters in the WD copper at large strains were similar to those in the copper sheet ARB processed to large strains, though there were minor differences in the microstructure evolution between two processes due to small one-pass strain in the WD process. The tensile strength of the copper wire drawn to an equivalent strain of 6.9 reached 460 MPa, which was two times higher than that of the initial material. The strength of the deep-drawn wire was almost the same as that of the ARB processed copper deformed to the similar amount of strain. Uniform elongations of the WD and ARB specimens were both 1-3%, but the yield strength of the WD specimens decreased and the uniform elongation slightly increased with increasing equivalent strain applied in wire drawing. The electric conductivities of the WD specimens slightly decreased with increasing equivalent strain, and reached the value of 96.5%IACS at ɛWD = 6.9. The amount of change in electric conductivity during drawing process was approximately 3%IACS. Therefore, it could be concluded that deep wire drawing does not lower the electric conductivity of the copper wire so much. It was confirmed in the present investigations that the deep WD process acted as a kind of severe plastic deformation process to produce ultrafine microstructures in copper.