Improvement of the Forming Limit of Sheet Metals by Removal of Surface Roughening with Plastic Strain

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In sheet metal forming, press shops are forced to use thin sheets to reduce the weight of products. However, the press forming of thin sheets is very difficult because the formability of a sheet metal generally decreases with decreasing the thickness. Such thickness dependence of the formability of sheet metal is closely related to the surface roughening behavior with plastic deformation of the sheet.In this investigation, the surface roughening of sheet metals with plastic strain is removed at a given strain during the forming process, and its effect on the improvement of the forming limit is discussed.As a preliminary examination to estimate how the onset of a localized necking is delayed by the removal of surface roughness, a rigid-plastic finite element simulation is first made. Experiments using aluminium and killed steel sheets are also carried out, and the increase in forming limit strain is examined under a few strain paths.It is shown that the improvement of the forming limit is considerable in the stretch forming region where the major and minor strains in the plane of a sheet are positive. For balanced biaxial stretching, for example, the forming limit strain can be increased up to about 1.3 to 1.4 times as large as that for ordinary in-plane stretching. The effect of surface roughness removal becomes large as the thickness of sheet decreases.