Effect of electric-current pulses on grain-structure evolution in cryogenically rolled copper

The effect of electric-current pulses on the evolution of microstructure and texture in cryogenically rolled copper was determined. The pulsed material was found to be completely recrystallized, and the recrystallization mechanism was deduced to be similar to that operating during conventional static annealing. The microstructural changes were explained simply in terms of Joule heating. A significant portion of the recrystallization process was concluded to have occurred after pulsing; i.e., during cooling to ambient temperature. The grain structure and microhardness were shown to vary noticeably in the heat-affected zone (HAZ); these observations mirrored variations of temper colors. Accordingly, the revealed microstructure heterogeneity was attributed to the inhomogeneous temperature distribution developed during pulsing. In the central part of the HAZ, the mean grain size increased with current density and this effect was associated with the temperature rise per se. This grain size was slightly smaller than that in statically recrystallized specimens

EFFECT OF ELECTRIC PULSING ON THE STRUCTURE, TEXTURE AND HARDNESS OF CRYOROLLED FINE-GRAIN COPPER

A synergy effect of cryorolling and high-dense electric pulsing on the structure, texture and hardness of fine-grain Cu is analyzed. More than twice Cu strengthening under rolling with 90% reduction at –196°С was caused by strong rolling texture and work-hardened nanostructure with ~300 nm crystallites and ~30% fraction of high-angle boundaries. Further single pulsing with current intensity Kj = 3.5×104 A2s/mm4 resulted in a static recovery and slight Cu softening due to the formation of a more equilibrium structure with lower dislocation density and lattice microstrain. Increasing Kj to 3.8×104 A2s/mm4 led to a sharp drop in the Cu hardness owing to continuous recrystallization and texture randomization. At Kj near 5.0×104 A2s/mm4 homogeneous ultrafine-grain structure with 1 μm defect free equiaxed grains and about 30% of twin boundaries is formed. Normal grain growth to 3 µm and gradual decrease of the Cu hardness were taking place at higher pulsing energies, up to 8.1×104 A2s/mm4