Hybrid superplastic forming of AZ31 magnesium alloy
Date of Issue2017-11-06
School of Mechanical and Aerospace Engineering
Hybrid superplastic forming (SPF) has recently been developed as a sheet forming technique, combining a hot drawing (mechanical pre-forming) process and a gas forming process. The hot drawing stage greatly enhances the formability and the forming efficiency of the metal sheet. The blank sheet with a desired amount of material is drawn into the die to form an intermediate hollow shape by a punch. During the gas forming stage, the mechanical pre-formed part is further formed at a target strain rate by applying the gas pressure. It is considered as an efficient way to form components with complex shapes in automotive and aerospace industries. In this work, non-superplastic grade AZ31B magnesium sheets were successfully formed by hybrid SPF at 400 °C. The dome height of formed part was 60 mm under gas pressure cycle S1, approximately 60 % of the die diameter. In contrast, the part formed by conventional SPF cracked at the same height. The influence of punch shape on the formability of AZ31B was also investigated. The minimum thickness of 1.31 mm after hybrid SPF was found at location where the material first came into contact with the punch, and the corresponding thickness reduction was 59%. Compared with the conventional SPF process, the thickness distribution of part formed by hybrid SPF was significantly improved. Additionally, the microstructure evolution of AZ31B in uniaxial tensile tests and hybrid SPF were examined by electron backscatter diffraction. The static and dynamic grain growth at grip region and gauge region were also investigated at various strain levels. Many subgrains with low misorientation angle were observed in the coarse grains during SPF process. The average misorientation angle and fraction of high angle grain boundaries decreased with increasing strain. This explains well the generation of subgrain via dislocation motion during SPF. Therefore, the main deformation mechanisms during hybrid SPF was determined to be a combination of recrystallization, dislocation climb and grain boundary sliding (GBS). Based on the tensile test results, parameters of hyperbolic sine creep law model and two-term material constitutive model were determined at 400 ºC. The hybrid SPF behavior of non-superplastic grade AZ31B was predicted by ABAQUS using these two material forming models. The FEM results of thickness distribution, thinning characteristics and forming height were compared with the experimental results. For the hot drawing process, the simulation results of both two models agree well with the experimental results. However, neither the hyperbolic sine creep law material model nor the two-term material model could predict the dome height and thickness distribution well during hybrid SPF. There is an increase in deviation between the prediction and experimental results when the material undergoes larger strain deformation during SPF. As a result, the part formed by hybrid SPF was done faster with higher dome height and a more even thickness distribution than conventional SPF. The forming time for hybrid SPF was significantly shortened as the hot drawing step formed the pre-formed component rapidly before the final more time-consuming gas blow forming. Accordingly, AZ31B part formed by hybrid SPF is structurally stronger due to the presence of fewer and smaller cavities as compared to conventional SPF.