Investigation of forming parameters influence on pillow defect in a new vacuum-assisted incremental sheet forming process

The geometrical accuracy of incrementally formed products is the key focus in the incremental sheet forming (ISF) process. Pillow defect hinders the material’s formability, which limits geometrical accuracy. Therefore, considering the pillow defect as the major objective, the present study is aimed to evaluate the benefits of adding vacuum to the conventional ISF process for limiting the pillow effect. This work also studies the impact of forming parameters like tool diameter and vertical step increments on pillowing. Using the digital image correlation (DIC) technique, the material characteristics of AA5052 Al alloy were determined from a room-temperature tensile test. The vacuum-assisted ISF (VISF) experiments were then conducted, and the shape error was computed using the 3D scan data to check the part’s geometrical accuracy. The test results revealed that the forming tool diameter is the most important parameter, followed by the vertical step increment, affecting pillowing; with a small forming tool diameter, vacuum presence greatly reduced material pillowing. Besides, the vacuum uniformly deforms materials and controls shear deformation by balancing local element strain accumulation; however, when the tool diameter increased, the produced parts had no shear deformation. The numerical results showed that pillow formation largely relied on stress values from the forming tool motion and transverse directions and that the pillow height was lowered when the stress values were decreased in magnitude; however, there was no change in direction with a vacuum. In conclusion, the meaningful outcomes from the VISF process confirm the benefits of using vacuum to control pillowing.