Research On Theoretical Forming Limit Diagram Of AA5083 Thin-shell At Elevated Temprature

It is the research hotspot that how to evaluate the formability of high strength aluminium alloy at elevated temperature. Forming limit diagram(FLD) is an intuitional method to evaluate the forming property of sheet metal so that it has been widely concerned. By establishing the model for predicting the forming limit of aluminium sheet at elevated temperatures, the designer can predict the instability trend of material in the study on technology theory and numerical simulation and reasonably evaluate its forming property.In this paper, AA5083 thin-shell is object for study. The constitutive equation of AA5083 sheet, which is related to temperature, is established by hot uniaxial tensile test on sheet metal. Based on the M-K model which contains through-thickness normal stress, the theoretical FLD is obtained by using Newton-Raphson iteration method. By comparing the theoretical FLDs which are respectively obtained under plan stress state and three dimensional stress state, the results show that through-thickness normal stress can effectively improve the theoretical forming limit curve(FLC) especially in the first quadrant. The influence of parameters, such as strain hardening exponent n, strain rate sensibility exponent m, forming temperature T and initial thickness imperfection ratio f0, on the theoretical FLC is studied. And the effect of through-thickness normal stress on the sensitivity of theoretical FLC to forming parameters is also studied. To some extent, the sensitivity of theoretical FLC to strain path is discussed. By theoretical derivation, the limit strain can be converted to limit stress, so the forming limit stress diagram(FLSD) and the extended stress-based forming limit diagram(XSFLD) can be obtained. The influence of through-thickness normal stress and material property parameters on FLSD and XSFLD is respectively studied, and the sensitivities of FLSD and XSFLD to strain path are also respectively discussed, the results show that they are basically identical.The simulation analysis for the Hot Granule Medium Pressure Forming(HGMF) technology on AA5083 tube is implemented, the results show that the influence of through-thickness normal stress on the area in danger of fracture on tube in the stepped shaft bulging process predicted by theoretical FLD is obvious, and the simulation result is basically coincident with the result of technological test, namely the through-thickness normal stress can not be ignored for theoretical prediction in some forming technologies on thin-shell. By comparing the results respectively predicted by FLD and FLSD, it can be found that theoretical FLSD can further narrow the prediction interval of the area in danger of fracture.