Time-dependent springback of various sheet metals: an experimental study

In the present study, draw-bending tests were conducted to investigate the effects of holding time at the bottom dead center (holding-time dependency) and elapsed time after releasing from dies (elapsed-time dependency) on springback in steel, Al alloy, Mg alloy, and CP-Ti sheets experimentally. The amount of springback decreased with increasing the holding time, irrespective of the material. The decreasing amounts from the holding time of 5 min to 600 min were 29%, 11%, 8.1%, and 5.7% respectively for the Mg alloy, CP-Ti, Al alloy, and steel sheets. On the contrary to previous studies, it was presumed from simple analyses that the holding-time dependency would presumably be explained in terms of not only stress relaxation but also unloading behavior following stress relaxation. The amount of springback gradually increased with the elapsed time regardless of the material. The amounts of increase from just after releasing from dies were approximately 5.9% for one month in the CP-Ti sheet, 4.1% for 1.5 months in the Al alloy sheet, 1.6% for 1.5 months in the Mg alloy sheet, and 1.1 % for 1.5 months in the steel sheet. This magnitude relationship was different from that of creep strains, indicating that the mechanism of the elapsed-time dependency could not be explained only from the creep behavior and there would be other factors that affect the elapsed-time dependency.