Research On The Size Effect Of Micro Forming For Rolled Foil Of AZ31Magnesium Alloy

With the growing requirement for miniaturized product, miniaturizedtechnology has a constant development in recent years. Gas blow micro-bulging is amicro plastic forming technology with excellent characteristics such as low cost,without terrace die, particularly for quantity production. People’s concern isgradually aroused by the research on gas blow micro-bulging. Micro plasticforming has different characteristics compared with macro plastic forming, which iscalled size effect phenomenon. In this paper, a series of experiments are carried outto study the size effect phenomenon of AZ31magnesium alloy rolled foil in micro-tensile at room temperature and gas blow micro-bulging.The tensile tests of AZ31magnesium alloy rolled foil are performed at roomtemperature to study the size effect on foil thickness, grain size and strain rate toflow stress. The experimental results show that the tensile strengthes of tensilesamples are less than that of the macro-tensile samples in the same condition, whichis the size effect phenomenon. The tensile strength for rolled foil of AZ31magnesium alloy is not sensitive to the strain rate at room temperature.The performance of the size effect phenomenon in micro-tensile tests is thathigher strength and elongation could be acquired by larger thickness and smallergrain size. The size effect of the mechanical properties could be interpreted by themodel which is established on the basis of the surface layer and the size effect ofthe elongation could be interpreted by the relationship between the internal strainand the deformation of grain boundary.The gas blow micro-bulging experiment for AZ31magnesium alloy rolled foilis carried out to study the size effect caused by the foil thickness, the diameter ofdie cavity and initial grain size. It is found that the flow stress in micro-bulging islarger than that in the macro-bulging, the flow stress is larger when the die diameteris smaller and the foil thickness is thinner, the flow stress increased with the innergrain size decreased which is the same as the phenomenon in the macro-bulging;however the difference in the stress between different grain size is decreased whenthe strain is same. Above phenomenon is the size effect phenomenon which can be explained by the strain gradient hardening theory.The observation of the wall thickness distribution in specimens gas blowmicro-bulging is performed to find the size effect phenomenon of materials flow.The experimental results show that the wall thickness distribution of microhemisphere parts is more homogeneous than that of macro-parts when the height todiameter ratio is constant, which is the size effect phenomenon; the wall thicknessdistribution is more homogeneous when the diameter of die cavity and the foilthickness is smaller, the inner grain size is lager; the thinning rate of the apexposition at the bulging specimens decreased with the bugling process while thethinning rate of other positions rose; the wall thickness become gradually morehomogeneous. The size effect in the materials flow could be explained by t he straingradient hardening theory and microstructure evolution.