Theoretical analysis, numerical simulation, and experimental verification of critical thickness of upper sheet in aluminum alloy double-layer sheets hydroforming

A novel hydroforming process called as double-layer sheets hydroforming process (DLSHP) was proposed in my previous research to decrease the maximum fluid pressure in the hydroforming of wrinkle-free semi-ellipsoid curved sheet parts with a small thickness-to-diameter ratio. In DLSHP, the two sheets involved are not bonded together, the upper sheet is introduced to increase the critical wrinkling stress of the lower sheet, and then the stress state of unsupported area of lower sheet will be changed during the hydroforming process. Through proper selection of upper sheet and application of reasonable forming pressure, semi-ellipsoid curved sheet parts with lower thickness-diameter ratio (less than 2‰) can be obtained and the shortcomings occurred in traditional hydroforming process can be overcome. However, the realization of the technical advantages of DLSHP depends on the proper selection of upper sheet, which means the determination of the critical thickness of upper sheet. Although the FE simulations and experiments can well solve this problem, the low efficiency and high cost is not commercially acceptable. Thus, analytical models were developed through geometrical and stress analysis to determine the critical thickness of upper sheet to overcome this question in this paper. The critical thickness of the upper sheet was deduced theoretically, through geometrical and stress analysis in the traditional deep drawing process, and was verified and determined through several experimental as well as numerical studies considering the effects of the axial length ratio and the thickness of the formed sheet part. Moreover, the effects of the blank holding force and the initial size of the sheet on the critical thickness of the upper sheet were investigated through serial experiments.