Deformation behavior of saddle surface part during multi-point forming under normally full constraint condition

In order to solve the problem of wrinkling in traditional multi-point forming (MPF), a new multi-point forming process, namely multi-point forming with individually controlled force-displacement (MPF-ICFD), was proposed. The core idea of this process is to impose a full-area normal constraint on the sheet metal during the whole forming process. Therefore, the deformation sequence of the sheet metal is changed from the local constraint and the whole deformation in the traditional MPF to the whole constraint and the local deformation so as to completely eliminate the noncontact region and avoid wrinkling. In this paper, the saddle surface part was taken as the research object. The influence of normal restraint on critical wrinkle stress is studied by mechanical analysis, and the wrinkle restraint mechanism under normal restraint is revealed. The deformation behavior of saddle surface parts was studied by forming experiments, and the wrinkling law of saddle surface parts under different constraints is obtained. The deformation characteristics of saddle surface parts were analyzed by numerical simulation, and the critical stress distribution rules of the sheet metal were given. It is shown that the critical wrinkling stress of the sheet metal is significantly increased in MPF-ICFD compared with that in traditional MPF, and the increased value is proportional to the magnitude of normal constrained force. Besides, the wrinkling phenomenon of saddle surface parts is significantly suppressed, and the surface quality of workpiece is obviously improved. In addition, the stress state change from compressive stress to tensile stress in the core region of the sheet metal was prone to wrinkle.