Study On Superplastic Forming And Precision Control Of TC4 Alloy Deep Cups

Since the first application of superplastic forming in the production of large-scaled titanium alloy aerospace components in 1970, the using of titanium alloy in aerospace industry has widely extended. As one of the most typical technics of superplastic forming, superplastic bulging is now more and more used in the manufacturing of titanium alloy aerospace components, at the same time, the controlling of forming precision is bound to be a hotspot of research in the area.In this paper, the precision control of the superplastic bulging process of a TC4 sheet into a deep cup was studied through finite element analysis and bulging tests. In the study, factors that affect the forming precision were analyzed systematically and accordingly a series of optimizing methods were carried out.First, a direct-reverse superplastic bulging method was adopted by the analysis of manufacture procedure of the part and the direct bulging die and reverse bulging die were designed. Then a bulging test of Zn5Al sheet was performed, which to some extent showed the direct-reverse bulging method was feasible for controlling the thickness distribution of the part.FEA simulation of TC4 alloy superplastic bulging was carried out on MARC, followed by a TC4 alloy superplastic bulging experiment. Experimental results showed that finite element simulation data tallied comparatively well with the results of practical test. Then, the effect of the pre-forming shape and friction coefficient on the thickness distribution of the part was analyzed using FEA. It was shown that appropriate pre-form design and a higher friction coefficient of the pre-forming die could improve the situation of non-uniform thickness distribution of the bulged part. The FEA results also showed the friction in the direct bulging process was a main factor that increases the non-uniform thinning, so a lower friction coefficient of direct bulging die was helpful for the uniform thickness distribution of the part. At last, the heat expansion effect in TC4 superplastic forming was studied. A FEA simulation was carried out to analyze the contact and mechanical behavior between the die and workpiece during the cooling stage after high temperature bulging. The FEA results showed that during the cooling stage, the workpiece was in a nearly elastic deformation. Plastic deformation happened in only a few area of the workpiece, and the equivalent plastic strain of the area was lower than 10-4 which was negligible comparing to the elastic deformation. Under the assumption of pure elastic deformation, a formula was developed to compute the quantity of dimensional modification of the die, to compensate the dimensional precision loss from heat expansion effect. By comparing the theoretical computing value of the formula with the experimental value from bulging test, the absolute error was obtained to be less than 0.01mm in every 100mm, which was acceptable to the required precision. Based on the heat expansion compensation formula, a simple scheme of die modification was proposed, which was applicable in different cases of TC4 superplastic forming.