Melting is a crucial step in the determination of the Nitinol devices. Key aspects to be controlled are C and O contamination, and composition homogeneity. The effects of furnace scale, heating schedule, raw materials, and crucible type on these parameters have been investigated using Vacuum Induction Melting, and characterizing ingots by means of DSC, ICP and AA analyses, and LECO analyzers.
The results of the studies we carried out showed the influence of furnace ( i.e. crucible) size on C content not directly related to the surface/melt volume ratio, while the melting time, once a critical value is overtaken, has little influence on the ingot purity and homogeneity.
In order to show the influence of ingot purity on wire performances, two wires have been drawn and trained with the same procedure, starting from two different ingots, with a different C content, showing higher strokes and transition temperature for the purer wire