Theoretical investigation on the springback behavior of AA7B04 sheet in hydraulic bulge process

Abstract

The derivation of theoretical formula about the amount of springback in the hydraulic bulge process using aviation aluminum alloy 7B04 was studied. Firstly, the traditional formula of plate bending theory was amended when the thickness normal stress was introduced into it. Based on it, the theoretical formula of amount of springback in hydraulic bulge process with thickness normal stress was deduced. Secondly, by using the numerical simulation and the experiment, the theoretical formula was proved to be correct. Thirdly, several classic analytical models for radius of curvature at the bulging vertex and bulging vertex thickness were verified by use of the hydraulic bulge experiments. When the cavity pressure was 30 MPa, the measured springback value of radius of curvature at the bulging vertex was 1.2 mm, while the result calculated by the theoretical formula was 0.8 mm and the theory error was 33.3 %. When the cavity pressure was 50 MPa, the measured springback value of radius of curvature at the bulging vertex was 3.13 mm, while the result calculated by the theoretical formula was 1.98 mm and the theory error was 36.7 %. According to the actual measurement, the predicted values of Hill’s model concerning radius of curvature at the bulging vertex agreed well with experimental values and the predicted values of the Jovane’s model concerning bulging vertex thickness agreed well with experimental values. The theoretical formula of amount of springback with the thickness normal stress which is obtained by the derivation is proved to be correct and reasonable by the plate hydraulic bulge experiment. In addition, the calculation of the springback value in sheet forming process which is in three-dimensional stress state can be guided by the theoretical formula.