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Molten Nitinol is highly reactive and must be processed in vacuum

Having close to fifty percent Ti, molten Nitinol is highly reactive and must be processed in a vacuum. Both vacuum induction melting (VIM) and consumable vacuum arc melting (VAR) processes are commonly used for production. Other melting processes, such as non-consumable arc, electron beam, and plasma melting, are also used in experimental scales. Graphite or calcium (CaO) crucible is preferred in the VIM process. Other crucible material, such as alumina or magnesia, contaminates the molten Nitinol with oxygen. Molten Nitinol does pick up carbon contaminants from graphite crucibles. By keeping the melting temperature below 1450°C when using a graphite crucible, the carbon content of a VIM Nitinol ingot can be controlled between 200 and 500 ppm. The transformation temperature can be controlled within +/- 5°C in a VIM ingot. To achieve more precise control, an in-situ composition control process can be implemented where samples are taken from the molten metal, and the transformation temperatures are quickly analyzed for instant composition adjustments. One also needs to bear in mind that because Nitinol transformation temperatures are so sensitive to small variations in chemistry, the analytical methods are not sufficiently accurate to predict the transformation temperature. The transformation temperature correlates much better with the charge chemistry used more reliably for ingot formulation. In the VAR process, consumable electrodes of Nitinol are melted and solidified in a water-cooled copper mold. Because there is no contamination from the crucible, cleaner materials with less than 200 ppm carbon content are produced. However, the molten pool of a VAR process is limited to only a small zone. Lacking complete mixing within the entire ingot leads to less homogeneous chemistry and the distribution in transformation temperature compared to the VIM ingot. Multiple re-melts are required in the VAR process to achieve acceptable homogeneity. A double melting process using VIM primary melting followed by VAR re-melt produces good results. Ingots of 1,000 Kg and 14 inches (35.5 cm) diameter are routinely produced using the VIM/VAR double melt process. If you are looking for high quality, high purity, and cost-effective Nitinol, or if you require the latest price of Nitinol, please feel free to email contact mis-asia.

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