Experimental and theoretical analysis of the thickness distribution in multistage two point incremental sheet forming

Incremental sheet forming (ISF) has received increasing attention with its advantages of economy and flexibility for small batch sheet metal parts. However, the excessive thinning may be resulted at large forming angles for complex parts, which cannot meet the design requirements and seriously impedes the industrial application of ISF. Although the multistage incremental sheet forming (MISF) has been proposed to overcome the dilemma of excessive thinning in ISF, the design of the optimal strategy which can control material flow and improve thickness distribution is a critical issue. In this paper, three promising multistage two point incremental sheet forming (MTPIF) path strategies, i.e., parallel line strategy, variable angle strategy, and stretch-bend-assisted strategy, are selected to investigate the influence of tool paths on thickness distributions, material flow, and geometric error. Firstly, experimental thickness distributions of truncated pyramids formed with three path strategies and single-stage TPIF strategy are compared. Meanwhile, the theoretical models are introduced to illustrate the difference of thickness distributions both at the wall and the corner. Compared with single-stage TPIF strategy, MTPIF path strategies studied in this paper are effective to improve the thickness distribution of truncated pyramids at both the component wall and the corner. Through the theoretical analysis, it is found that the thickness distribution at the wall can be controlled by adjusting intermediate forming angles. Meanwhile, the intermediate shape in circumferential direction has significant influence on the thickness distribution for complex parts. Then, the material flows of three path strategies at different forming stages are analyzed. It is found that the materials at the flange as well as transition arcs are deformed into wall to increase the thickness distribution. Finally, considering both thickness distributions and geometric errors, the variable angle strategy is found as the effective MTPIF strategy for the studied process.