Influence of electric pulse treatment on structure and hardness of cryorolled aluminum

Abstract

The influence of the energy of electric pulse treatment (EPT) in the range of integral current densities (K j ) from 0.06 ×10 5 to 0.29 ×10 5 A 2 s / mm 4 on the structure and hardness of coarse-grained high-purity Al, isothermally rolled up to a total strain of 90 % at a liquid nitrogen temperature, was studied. It was found that EPT with an energy up to К j = 0.104 ×10 5 A 2 s / mm 4 practically did not affect the microhardness obtained in the cryorolled aluminum (45 -50 HV). An increase in the EPT energy to К j = 0.121×10 5 A 2 s / mm 4 led to a rapid drop of the hardness to 30 HV followed by its gradual stabilization near 25 HV at higher К j values. It was established that an enhanced microhardness of rolled Al resulted from formation of a well developed cellular structure with a crystallite size of about 2 μm, containing less than 10 % of ultrafine grains of about 4 μm in size. The minor hardness changes after EPT with К j up to 0.104 ×10 5 A 2 s / mm 4 were related to occurrence of recovery and continuous recrystallization, resulted only in improving the deformation structure without noticeable changes in its type and the crystallite sizes. Therewith the softening caused by a partial decrease in the scalar dislocation density and the microdeformation of the crystal lattice was compensated by increasing the high angle boundaries fraction. At EPT with К j = 0.121×10 5 A 2 s / mm 4 , the deformation structure was severely replaced by the fine-grain recrystallized one with the grain size of 19 μm, and resulted in the loss of the strenthening effect, caused by rolling. With further increase in the EPT energy, an extensive grain growth was observed, leading to the formation of a non-uniform structure grains and to appropriate material softening, owing to the grain coarsening. It was concluded that the restoration processes that took place during EPT were similar in nature to those that occur during furnace annealing of heavily deformed materials. Therewith, the short time of the thermal exposure on the deformed metal during EPT was compensated by high applied energies.