Processing And Mechanism Of Pulse Current Auxiliary Superplastic Forming Of Light Alloy Sheets

As one of the effective method to manufacture complex and thin wall lightalloy structures, superplastic forming has been applied in many fields. However, thistechnology has the defect of high cost owing to high energy consumption and lowefficiency caused by the deformation condition of high forming temperature and lowstrain rate. Therefore, the pulse current heating method of superplastic foming hasbeen studied in this paper to achieve fast and high efficiency heating. Moreover, theelectro-superplastic effect of pulse current has been utilized to improve the fomingproperty of the light alloy sheet.Most present studies about the electro-superplastic focused on the theory, andthere has little literatures concerning the craft application of electro-superplstic.Therefore, the pulse current has been applied into the process of light alloy sheetsuperplastic forming in this paper, and the Joule heating effect andelectro-superplstic effect of pulse current have been associated and applied. A newprocessing method named pulse current auxiliary superplastic forming (PCASPF)has been proposed, and the processing device has been design, and the technologicalprocess and the processing parameter have been determined. From the results of theforming test, it is known that this method is effective for superplastic foming of lightalloy sheet such as AZ31magnesium alloy and Ti-6Al-4V titanium alloy. The timeof the whole processing reduced to50%, and the energy consumption of the wholeprocessing reduced to35%.Based on the electrothermics theory, the heating rate and heating temperatureof light alloy sheets with different performances during pulse current heating hasbeen studied systematically, which is necessary to determine the heating processingparameters. The temperature fields of the flat plate and semisphere plate duringpulse current heating have been analysed by the method of finite element simulation.The uniformity of the temperature field has been improvement by placing theTi-6Al-4V titanium alloy sheet between the specimen and the electrode. The heatingtest has been executed to check the theory analysis. Form the results of heating test,it is known that this heating method is high efficiency and energy conservation.To disclosure the deformation mechanism of light alloy superplstic forming bythe action of pulse current, the analysis methods of OM and TEM have been used toanalyse the microstructure evolution, involving grain growth dynamicrecrystallization (DRX) and grain boundary slipping (GBS), of AZ31magnesium alloy and Ti-6Al-4V titanium alloy with different original microstructure duringPCASPF, and the effect of pulse current on the dislocation morphology andmovement has also been studied. Form the research, it is known that the graingrowth speed has been reduced, the dynamic recrystallization has been accelerated,and the microstructure has been refined by the pulse current. Furthermore, the grainboundary slipping and the dislocation movement have also been accelerated by thepulse current, which are benefit to improve the superplastic forming ability of thelight alloy sheet.Based on the electrothermics and thermoelasticity theory, the temperature andstress fields by the action of pulse current near the cavities have been studied. Theeffect of pulse current on the cavity nucleate and cavity growth has been analysed,and the cavitied morphology of AZ31magnesium alloy during PCASPF has beenobserved by the method of SEM. Form the results, it is concluded that there exist thelocal high temperature and compression stress zone near the cavity tips, whichrestrained the cavity continues growth along the tip direction. Moreover, the cavitynucleate rate and small cavity growth have been accelerated by the pulse current,which made the cavities size and cavities distribution more uniform, and avoided thepremature fracture during superplastic forming.