A novel application of cryogenics in dieless sheet metal piercing

In tube punching, if the internal die is necessary to properly pierce the tube avoiding its collapse, it also represents a bottleneck to a rapid change of the punching set. In this research an innovative dieless tube punching approach has been conceived and studied. The use of a cryogenic fluid to force the material ductile-brittle transition is a way to limit the sheet deformation during the piercing process. The analysis of the innovative cryogenic punching was carried out both adopting numerical and experimental methodologies. A finite element FE model of the cryogenic punching was developed and updated in two stages. First, experimental tensile tests, performed at cryogenic temperatures, were used to characterize some material properties. Secondly, some piercing tests in cryogenic conditions were performed at different velocities and temperatures to fine update the model. A validation session was carried out to assess the model and the process feasibility. It was found that the FE model reproduced the experimental results within a maximum estimation error of 10% on both the punching force and tube deflection. Results showed that both the increment of the punching velocity and especially the decrement of the punching temperature could be the only viable solution for making the tube dieless punching industrially feasible.