Critical conditions in the ironing process of thin wall metal cups. (3rd report Mechanisms of fracture of metal cups)

Abstract

The mechanisms of the fracture of metal cups during ironing process have been examined, using aluminum cups under various lubricating conditions on the die and punch surfaces. When the welding occurs, the cup ruptures even though the average axial stress in the cup wall at the die exit is far smaller than the tensile fracture stress (σcr) of the ironed wall with a smooth surface. This is mainly because of the ununiform distribution of the axial stress in the cup wall due to the partial welding. The surface damage of the ironed wall caused by welding has a little effect on the tensile strength of the ironed cup wall. In any case, the initial cracking arises at a point of the cup wall, where the axial stress reaches partially the tensile fracture stress σcr. The critical condition for the fracture during ironing process has been clarified.