Multipoint forming process of aluminum sheet considering its forming limit diagram: Experimental and numerical investigations

Considering the significance of sheet metal forming in various industries, metal sheets, and lightweight alloys find extensive applications. It is essential to predict the formability limit of sheets. Whereas the previous studies used the Nakazima test, this study aims to determine the forming limit diagram (FLD) numerically and experimentally using multipoint forming, considering the strain path's effect on formability. Also, it is compared with the conventional FLD obtained via the Nakazima test. The results show that multipoint forming improves sheet metal forming. The experimental FLD for an AL 2024-O sheet with a 12 mm pin size was found to have higher formability than that obtained from the Nakazima test. The effect of the polyurethane layer on the forming limit diagram was investigated. The experimental results showed that the polyurethane layer prevents local strain and increases the limits on the forming limit diagram. The numerical forming limit diagram was predicted by the second derivative method. Finally, the experimental results were compared with the numerical simulation results, showing that the results had an agreement above 90%. This method can be used as a reference for assessing the effectiveness of the multipoint forming technique.