Die design of large spherical curvature hot forming process for improving the dimensional accuracy

Abstract

Large spherical curvature hot forming (LSCHF) process using square grid die was developed to manufacture moss-type LNG tanks. This process commonly realizes less spring-back of the thick plate after hot forming and more productivity than the conventional cold forming. Nevertheless, it is very difficult to shape the plate into a perfectly spherical curvature shape because of the thermal deformation and elastic recovery during cooling after hot forming. The final spherical curvature shape of the thick plate depends on many process variables such as the dimensions of the initial die surface, the size of the grid of the die, and the initial plate and design of the cooling device. In particular, the final spherical curvature shape of the aluminum thick plate is determined by the spherical curvature of the gridtyped die. Thus, the object of this study is to propose a method to design the spherical-curvature of the square grid die. To design the spherical-curvature of the square grid die, a Displacement adjustment (DA) method and a numerical approach by Finite element analysis (FEA) were applied. The die surface design achieved by the DA method and FEA were suggested to adjust the errors in its spherical curvature after final cooling. Finally, an experiment was conducted on a small-scale model (310 × 310 mm) to test the process to verify the design method for the die surface. The FEA results agreed very well with the experiment results within the maximum average errors 0.008% (0.122 mm). Further, the DA method yielded relative errors of 0.011% (0.163 mm) to objective spherical curvature.