Size effects on flow stress behavior during electrically-assisted micro-tension in a magnesium alloy AZ31

As one of the promising micro-manufacturing technologies, micro-forming has economical and ecological advantages in terms of mass and near-net-shape production. However, size effects increasingly affect material performances with scaling down geometry and process parameters and consequently hinder applications of micro-forming. Electrically-assisted (EA) micro-forming may have the potential to minimize the size effects. In order to investigate the size effects in the EA micro-forming, uniaxial tension tests were conducted on miniaturized AZ31 tensile samples with varying grain sizes and geometry sizes at a constant DC current density. It was found that the normalized flow stress reduction, i.e., decreases of flow stresses/flow stresses at room temperature (RT), increased with the decrease of the grain size and with the increase of the geometry size at the constant current density of 91.1 A/mm². Additionally, tests for the isolation of bulk Joule heating such as oven-heated tests and air-cooled EA tests were also conducted on the smallest samples with the same grain size regime examined in the EA tests to comparatively analyze the effect of grain size on the flow stress behavior during EA micro-tension. It was found that the amount of the Hall-Petch slope decreases in order of the RT, the oven-heated and the EA micro-tension tests. Both the Hall-Petch effects in the EA tests and the air-cooled EA tests are insignificant, showing nearly levelled Hall-Petch curves across the grain size regime examined. Based on these observations, a composite material model taking account of localized Joule heating at grain boundaries was developed to qualitatively interpret the Hall-Petch slope differences in the above four conditions. It was found that the mechanism behind the effects due to electric current may not be exclusive to bulk thermal softening.