Analysis of wall curling in incremental forming of a sheet metal: role of residual stresses, stretching force and process conditions

The present work, first detailed study in its nature, introduces wall curl as a source of geometric inaccuracy in the novel Incremental Sheet Forming (ISF) process, and aims at investigating role of the residual stresses, stretching force and process conditions on formation of the curl. A series of metal components are formed by varying the conditions. The curl height, residual stresses (surface, mean and equivalent) and stretching force are determined and their correlations are analyzed. The analysis reveals that curling is strongly influenced by the residual stresses in a way that the curl height increases linearly as their magnitude increases. Similarly, an increase in degree of non-uniformity in the through-thickness stress distribution promotes curling. The increase in stretching force, however, diminishes curling reasoning to reduction in the residual stresses. As regard the conditions, feed rate does not show any effect while mild annealing, greater tool diameter, smaller step size, and lower forming angle and rotation are found conducive to minimize curling. The geometric errors in wall and bottom of the produced parts (i.e., curling and pillowing) are found to show an interesting inverse relationship. The reported results are very useful to deepen the process mechanics and will also prove helpful in simultaneously controlling the two types of errors.