Abstract
Superplastic blow forming usually poses difficulty in controlling thickness variation on formed articles because of its deformation style of free bulging. Also, when a large deformation is formed, microcavities are formed in the micro-structure of aluminum alloy, which degrade mechanical properties of the alloy. In order to overcome these demerits, a new process using two stages of forming has been proposed. In this process, a blank sheet is bulged locally by incremental forming using a bar tool in the preliminary stage, then it is superplastically formed in the finishing stage. Some forming experiments were conducted to determine the feasibility of this new forming process using fine-grain-quality 7475 aluminum alloy as blank sheets. Firstly, blank sheets were preformed into quadrilateral pans with a given depth by incremental forming. Then, their bottoms were formed into a quadrilateral cone by superplastic forming. Lastly, each article was formed successfully with a thickness distribution as predicted from the controlled height of the preformed articles. It was also found that the new forming process performs effectively under certain range conditions.
