Effect Of Electric Current On Micro-macro Mechanism Of Light Alloy Superplastic Deformation

At present, the resistance heating forming technology has received widespreadattention for the characteristic of energy-efficient and has been used in manymaterial process fields. Due to the need of high temperature and low forming speed,superplastic forming technology is restricted in civilian products. In this paperresistance heating has been used in superplastic bulging to improve formingefficiency and reduce energy consumption. In addition, the influence of current ondeformation mechanism was analyzed. The electro plastic effect, electrosuperplastic effect, crack arrest effect and microstructures improvement effect wasadopted to improve the material formability, reduce technological requirements andimprove the quality of the parts.There are differences in electro-thermal performance and formability betweendifferent alloys. In this paper, TA15 titanium alloy, 5083 aluminium alloy, AZ31 magnesium alloy and Si Cp/Al composite were chosen to be research material andfirst their performance of resistance heating was tested. Then their technologicalparameters were determined and the device of resistance heating bulging wasdesigned and produced. The refractory mortar was used to make mould to meet theneed of insulation. By rational technology, different light alloy samples wereformed. The results show that, with the introduction of current, forming time wasshortened and energy consumption was reduced significantly. Because of highspeed of heating and cooling, the problem of surface oxidation was restrained tosome extent.The deformation was carried out with the action of current, so the deformationcharacteristic would be impacted by the current. By comparing conventionalbulging with resistance heating bulging of three kinds of light alloys, it was foundthat the current can increase the material plasticity and decreased the flow stress. Inresistance heating forming process, the deformation would generate the change ofsheet thickness and that could lead to higher temperature in thinning area, so thewall thickness distribution of formed sample would be more uneven. In order toreveal the functional mechanism of current on material deformation, themicroscopic structure of three kinds of light alloys was observed and analyzed. Itwas found that the current could improve the motion of dislocation. By reducing theactivation energy and generating electron wind force, the speed of dislocationsslipping and climbing is accelerated and the dislocation tangle was also decrease, somaterial plastic property was increased. For TA15 titanium alloy, there was phasetransition occurred during superplastic forming process and the microstructure ofdeformed specimens showed some directional features. After resistance heatingdeformation, the resulting structure of 5083 aluminium alloy is fine; Due to thepassivation of electric current on the tip of cavity, the V-cavities in the substratechanged into O-cavities, so the probability of cavity extension is greatly reduced.For AZ31 magnesium alloy, the current could improve nucleation in the crystallinegrain boundary that induced twinning.Comparing conventional hot forming with resistance heating forming ofSi Cp/Al composite material, the height/diameter ratio increased from 0.2 to 0.38.Under the action of current, the material plasticity has been greatly improved. Byanalyzing the deformation mechanism of composite, it was found that the speed ofdiffusion creep and slipping were accelerated due to higher diffusion coefficient andaction of electron wind force. The electrons in the metal tended to centre at grainboundary and the phenomenon of detour ?ow would appear around the tip of Si Cparticles. All of these would lead to local boundary zone has higher temperaturethat even makes the aluminium matrix melt somewhere. So there might be a thinliquid layer on the surface of Si C particle. The deformation mechanism changed to?owing of non-Newtonian ?uid. As lubricant, this thin liquid layer could preventinterfacial crack, improve material deformability and reduce deformation resistance.The electric current could lead to polarity effect during AZ31 magnesium alloyheating and deformation process. The direction of current can influence itstemperature distribution, flow stress and macroscopical deformation characteristicof formed sample. The analysis showed that temperature polarity was caused byPeltier effect and the deformation polarity was influenced by electron wind force. Inthis paper, the value of electron wind force was calculated and added into finiteelement simulation of resistance heating forming and the effect of electron windforce was verified.