Influence Of Electric Current On Mechanical Properties Of Metal Alloys And Mechanism

The flow of electric current through metal during forming has been observed to reduce flow stress and increase formability significantly. According to this feature, electrically assisted forming technology has developed and received considerable attention for the characteristic of energy efficiency. The influence of electric current on material properties contains two aspects:on one hand, joule heating generated by electric current affects the thermal-mechanical behavior; on the other hand, without considering the effect of temperature, electric current helps to reduce flow stress and increase elongation, deemed as 'electroplasticity'. However, the existence of electroplasticity is debated in material research, some researchers believe that only temperature affects the mechanical behavior of metal.This dissertation provides an in-depth study of the mechanics associated with electrically-assisted deformation in order to uncover the dominant mechanisms behind the observed behavior. A new experimental mechanical testing system is built up which decouples thermal-mechanical from electroplastic behavior and characterizes the effects of electrical and mechanical loading. The contents of the paper are stated as follows:(1) The influence of continuous electric current on the thermal and mechanical behavior of metal alloy has been studied during tension tests. A larger decrease in material flow stress is observed in electrically assisted tension compared to oven heated thermal tension tests at similar temperature. The flow stress decreases with the increase of current density. The relationship between grain size and mechanical behavior of brass alloy is established. Decreasing grain size is observed to increase joule heating and increase stress reductions. The experimental results of aluminum alloy shows that, The strain rate sensitivity changes from negative to positive and keeps increasing with the increase of current density. Furthermore, the strain rate sensitivity obtained from the electrically assisted test is greater than that from thermally assisted test. The tendency toward serrated flow is characterized by the onset of PLC instabilities, electric current helps suppress the serrated flow at the similar temperature level of oven-heating.(2) The influence of pulse current on the thermal and mechanical behavior of metal alloy has been studied. The peak temperature generated by pulse current induces the stress recovery of brass alloy during electrically assisted tension tests. The recovered stress is near to the initial yield stress when the temperature is over than 600?. Considering the influences of pulse current with the same temperature level, as the current density increases, the instant drop of stress increases as well as the elongation, although the maximum flow stress remains almost unchange.(3) A generalized model is developed to relate electric current density to thermally activated behavior and provides insight into an observed'current density threshold'in certain materials. The relationhip of thermal softening parameters and normalized electric current is established with the consideration of Joule heating effect. A new material parameter,'current density sensitivity,'is introduced to provide a metric for the relative influence of current density on thermally softening behavior. Instant stress drops caused by electric pulse are predicted through modeling the effects of thermal expansion and strain hardening.The calculated results accord with the experimental values.(4) Changes in material microstructure, including grain and grain boundary, dynamic strain aging, integranular cavitation growth, are observed with respect to the deformation mechanisms present during electrically-assisted deformation. Intergranular cavitation and local grain boundary melting during high current density experiments are observed. This phenomenon may be caused by the local electric current density increasing along grain boundaries and strain-induced grain boundary premelting. Grain recrystallization and grain growth are observed on brass after electrically assisted tension. Grain refinement and dendritic growth occurs near the fracture area. Additionally, electric current helps to suppress the serrated flow on aluminum alloy, which may be attributed to the current induced dissolution of small precipitates.(5) Finally, Electrically-assisted micro-rolling(EA?R) system equipped with surface texturing function is utilized to characterize channel forming behavior of micro-rolling. This method combines the effect of electric current with micro-rolling technology, helps to produce various mophologies on machined surfaces rapidly, the experimental and numerical characterizations of both EA?R and non-electrically-assisted micro-rolling(NEA?R) processes are conducted. By comparing the pressure data and channel geometry, deeper textured features of aluminum alloy under electric current are observed as compare to no current rolling cases. The adhensive strengths of samples on the surfaces with textures are significantly enhanced through the evolution by sinlg-lap-joint shear strength test.