Modelling of an innovative cryogenic assisted dieless sheet metal piercing process

In tube piercing, if the internal die is necessary to properly pierce the tube avoiding its crushing, it also represents a bottleneck to a rapid change of the piercing/punching set. In this research, an innovative dieless tube piercing 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 dieless piercing process. The analysis of the innovative cryogenic piercing process was carried out both adopting numerical and experimental methodologies. A finite element FE model of the cryogenic piercing 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 carried out at different velocities and temperatures to fine update the model. A validation session was executed 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 required piercing force and on the deviation from the nominal dimension of the tube cross-section. Although the piercing tests were conducted at quite a low temperature (−80 °C), an extended analysis of the wall fractures confirmed that a proper ductile-to-brittle transition did not occur. Further increasing the punching velocity and especially decreasing the piercing temperature could be the only viable solutions to promote the transition and further reduce tube deformation.