High-temperature annealing behavior of cold-rolled electrolytic tough-pitch copper.

Pure copper is very soft, however, hardening the pure copper with most strengthening mechanisms leads to a significant reduction in electrical conductivity. Grain refinement is a better strengthening mechanism to maintain high enough electrical conductivity. Plastic deformation at room temperature followed by post-annealing is one of the best methods to achieve fine-grained metals and alloys. In this research, the high-temperature annealing behavior of cold-rolled electrolytic tough-pitch (ETP) copper sheets was studied. 90 % asymmetric cold rolling followed by high-temperature post-annealing at 673 K for 1, 2, 5, 10, 30, 60, and 120 min were applied on the copper. The microstructure was significantly changed with increasing annealing time from 1 to 2 min owing to full recrystallization. With increasing the annealing duration, the grain size is increased. The formation of equiaxed grains with a smaller size (∼9 µm) compared to the full-annealed (initial) sample (∼68 µm) is observed after the longest time of post-annealing (120 min) due to the pinning effect of Cu2O particles. The post-annealed copper sheets processed by asymmetric rolling (in this work) exhibited a more homogeneous microstructure through the thickness compared to the symmetric rolling due to more uniform stored strain energy. The results showed that the first deformed grains that undergo recrystallization during post-annealing are Goss-oriented grains. With an increase in the post-annealing time, the S and Copper components were eliminated and a strong Cube and P texture orientations were formed. Interestingly, after 1 min of post-annealing, the yield and tensile strength enhanced to 410.2 MPa and 418.6 MPa owing to the annealing hardening phenomenon. The hardness and strength reached a constant value after the post-annealing for 10 min and above. With increasing the post-annealing duration, the central area of fracture surfaces (consisting of ductile dimples) became larger and the outer region (consisting of flat surfaces and shear dimples) became smaller, showing a shift towards perfect ductile fracture. With the increase of post-annealing time from 1 min to 120 min, the electrical conductivity was increased from 77.6 to 97.5 %IACS. The presence of the Cube texture increased the electron mobility compared to the P orientation, by reducing the mean distance that they can travel without scatter. From the obtained results, it can be concluded that the asymmetric cold rolling followed by high-temperature post-annealing is capable of strength improvement and maintaining electrical conductivity in copper.