The melting point of nickel and titanium is high compared to copper. But, in melting the Ni-Ti shape memory alloys, copper hearth is used in the Arc melting furnace, Why? which grade of copper ?
Dear Bala,
this is very simple, the copper hearth is water cooled and thermal conductivity of copper is very high. This is why remelting copper hearth with arc is harder in comparison to other metals even refractory metals as tungsten.
For arc melting the copper grade usually would be atleast OFHC or even higher. This ensures high thermal conductivity of the base to enable melting of Ni, Ti and other high melting metals.
Thanks Rohit.
If we melt those metals with arc in the copper crucible without water cooling in the open atmosphere, does it affects the casting or any problems?
It is not possible to do that in open atmosphere:
1. Titanium is highly reactive with oxygene
2. Copper will be heated over wetting angle and reaction between copper and melt will occur
Moreover, for arc melting of NiTi shape memory alloys you need to get as high purity as possible, high vacuum before venting chamber with argon, high purity of argon and Ti gettering of oxygen before melting. Martensite-austenite transformation temperatures are very sensitive to small differences in chemical composition and you have to keep that in mind.
As you know the copper hearth is water cooled and thermal conductivity of copper is very high. This is why remelting copper hearth with arc is harder in comparison to other metals even refractory metals as tungsten..
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