A very reactive metal in the molten state, titanium must be melted and poured in a vacuum to yield a quality casting. Most titanium investment cast parts are made with the vacuum arc remelting (VAR) technique, which uses a vacuum arc furnace to melt a portion of a titanium electrode into a water- cooled copper crucible.
The consumable electrodes are generally forged billets, wrought revert material, or selected foundry returns, the extra processing of which tends to increase the cost of the raw material. Because the VAR method tends to yield a melt of inhomogeneous temperature, it often must be poured into centrifugal or preheated static molds.
Titanium’s uses have been somewhat limited because of the relatively high cost associated with the metal, but continually improving manufacturing techniques resulting in near net shapes and greater precision have helped improve titanium’s affordability, thereby broadening its markets.
Important to know:
A titanium charge is placed in the crucible with granular calcium fluoride added to act as an electrical insulator between the titanium and the crucible to prevent arcing and crucible damage. The chamber is then evacuated and power is supplied to the induction coil. Here is where the segmented crucible becomes important, because if it weren’t nearly all the field generated by the induction coil would dissipate before it could melt the charge.
When power is applied, the titanium charge heats up, but the calcium fluoride melts first, effectively coating the crucible. As the titanium melts, the water cooled crucible freezes the outer shell of the melt, forming a titanium "skull" similar to that created in arc melting.
Named for the "skull" of material that lines the crucible after each pour, ISM is purported to offer several advantages over other reactive metal melting and casting techniques. The charge material, for example, does not have to be an expensive electrode. Rather, the charge stock can be in any form as long as it fits into the crucible. The charge is typically composed of revert and titanium scarp.
Also, titanium dioxide can easily be added to the melt to obtain desired oxygen content in the castings, and alloying in general is more easily accomplished. For example, high vapor pressure alloying elements such as manganese can be added late in the melt.
Additionally, the molten pool can be held for a long time to allow additions with high melting points, such as tungsten, to fully dissolve. These advantages has been used to produce many different reactive alloys. Many alloys have been based on Ti-Al-type formulations, but Zr, Nb, Cr, V, Ni and Al alloys has also been produced, to name a few.
The well-known refractories used in metallurgy (presented with the leading chemical component):
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