| In recent years, with the development of modern manufacturing, materials and processes are required for the continuous improvement. Quick plastic forming(QPF) technology of magnesium alloy have become a hot spot in manufacturing. In this paper, a series experiments were carried out to draw the forming limits diagram and to study cavity evolution for AZ31B magnesium alloy sheet under QPF condition.Basing on the Mises yield criterion, the mechanical model for QPF was established. The QPF bulging process was analyzed by Marc finite element simulation software and the bulging gas pressure corresponding to targeted strain rate were predicted.The forming limit of AZ31B magnesium alloy sheet with 1.2mm in thickness was researched under different conditions by changing the shape of the female die. As a result, the forming limit diagrams of AZ31B magnesium alloy sheet which were divided into fraction area, danger area and safety area were drew at the bulging temperature of 350℃, 375℃and 400℃at the strain rate 2×10-3s-1 and 2×10-2s-2, respectively. It was found that both increase of the forming temperature and decrease of strain rate could improve the forming limit and the bulging height of the workpiece. The forming limit value decrease only about 0.2-0.3 as the strain rate increase 10 times and the feasibility of QPF under high strain rate was demonstrated.It was found there were two types of cavity in the alloy sheet: one was diffusion type and the other was stress type. With the pressure loading, the sheet deformed rapidly in the early stage of bulging. The cavities formatted due to grain boundary sliding. Along with the bulging process, the cavities grew up gradually. When the cavities grew to a certain size, they would combine and the vast scale combination of cavities led to t the breakdown of the sheet. As a result, cavity was the main reason of the breakdown of the sheet.Different places with varied strains of domes were analyzed and it was found as the strain got larger, the rates of cavity got larger. The two got a relationship of exponential distribution. The top of the bulging piece was the place of biggest strain, where the rate of cavity was the largest. When the bulging temperature was 350℃, the largest rates of cavity were 21% and 16% when without backing pressure and with backing pressure. In addition, when the strain was equal, backing pressure could significantly lower the rate of cavity. And also, raising bulging temperature and reducing strain rate could also lower the rate of cavity. However, the cavity could only be reduced to some extent, and there was no way to thoroughly eliminate the cavity completely.When the bulging temperature was 350℃and the bulging time was 300s, the bulging height of the dome was 42.3mm and the biggest thinning rate of sheet was 73% without backing pressure while the bulging height 43.1mm and the biggest thinning rate 78% with a back pressure of 1MPa. It was found that back pressure could increase the bulging height and the biggest thinning rate of sheet, and improve wall thickness distribution. As a result, the sheet could be deformed better. |
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