G. M. Kramer, Metalwerks PMD, Inc., Aliquippa, PA
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 a second time in a VAR. The second method utilizes a Vacuum Induction Melting unit to alloy the nickel and titanium, utilizing 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. The ISM produces small ingots which are assembled into an electrode for VAR melting. For each of the melting methods, 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, are critical to the quality of the final product. The melting method has the potential to strongly influence the final chemistry, as well as the type and number of inclusions present in the final product. The chemistry and micro-cleanliness of product manufactured by each method will be compared to determine the melting method that will produce NiTi with the lowest residual elements, such as carbon, as well as the lowest size and number of inclusions.
Summary: NiTi alloys can be melted using three distinct processing paths: VAR/VAR, VIM/VAR, and ISM/VAR. Each melt path will affect the final chemistry and micro-cleanliness of the product. Through melting of standard NiTi in the same facility using each method, the method that produces NiTi with the lowest residual elements, the lowest number of inclusions, and the lowest distribution of inclusion size will be determined.
