Current profiles during VAR of Ti-6Al-4V
LMPC 2005 - Proceedings of the 2005 International Symposium on Liquid Metal Processing and Casting
Williamson, R.L.; Shelmidine, G.J.; Maroone, J.P.
Voltage and temperature distributions along the crucible were measured during VAR of 0.81 m diameter Ti-6Al-4V electrode into 0.91 m diameter ingot. These data were used to determine the current distribution along the crucible. Measurements were made for two furnace conditions, one with a bare crucible and the other with a painted crucible. The VAR furnace used for these measurements is of the non-coaxial type, i.e. current is fed directly into the bottom of the crucible through a stool (base plate) contact and exits the furnace through the electrode stinger. The data show that approximately 63% of the current is conducted directly between the ingot and electrode with the remaining conducted between the electrode and crucible wall. This partitioning does not appear to be sensitive to crucible coating. The crucible voltage data were successfully simulated using uniform current distributions for the current conduction zones, a value of 0.63 for the partitioning, and widths of 0.30 and 0.15 m for the ingot/crucible wall and plasma conduction zones, respectively. Successful simulation of the voltage data becomes increasingly difficult (or impossible) as one uses current partitioning values increasingly different from 0.63, indicating that the experimental value is consistent with theory. Current conducted between the ingot and crucible wall through the ingot/wall contact zone may vary during the process without affecting overall current partitioning. The same is true for current conducted through the ingot/stool and stool/crucible contact zones. There is some evidence that the ingot/stool current decreases with increasing ingot length for the case of the bare crucible. Equivalent circuit analysis shows that, under normal conditions, current partitioning is only sensitive to the ratio of the plasma resistance across the annulus to the plasma resistance across the electrode gap, thereby demonstrating the relationship between current partitioning and gap.