A Comparison of Chemistry and Inclusion Distribution and Morphology Versus Melting Method of NiTi Alloys

NiTi alloys are produced by three melting methods. The first method requires compaction of nickel and titanium raw material into sections that can be joined together for melting in a Vacuum Arc Remelt unit (VAR). This ingot is melted two or more times in a VAR. The second method utilizes a Vacuum Induction Melting (VIM) unit to alloy the nickel and titanium, with the use of a graphite crucible. The resulting ingot is prepared and remelted in a VAR. The third method begins with primary melting in a vacuum Induction Skull Melter (ISM). The ISM produces ingots that are assembled into an electrode for VAR melting. For each of the melting methods, the final product depends on the quality and handling of the raw materials, the control of the process at each unit, and the preparation of the intermediate ingots for further processing. The melting method influences the final chemistry as well as the type and number of inclusions present in the final product. This study compares the chemistry and microcleanliness of product manufactured by each method to determine the appropriate melting technique that produces NiTi with the lowest residual elements, such as carbon, as well as the lowest size, and number of inclusions.