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Induction Skull Melting (ISM) of Titanium Alloys

Titanium alloys are so reactive that they must be melted in a vacuum.  Because they would react with linings normally used in melting systems,
they are produced in a copper crucible that is water cooled to prevent it melting at the temperature of molten titanium – around 1700ºC.  If the
crucible failed to contain the titanium, and contact was made between the cooling water and the molten metal, a serious explosion would occur. 
Consequently, melting systems for titanium alloys are complex engineering solutions to a considerable technical challenge that must also
address serious safety issues.

The standard practice has been to strike an arc between a titanium alloy electrode and pieces of the same alloy (electrode ends, for example)
placed in the water-cooled copper crucible.  A molten pool is established and the electrode progressively melts.  When sufficient molten metal
is available, the electrode is withdrawn and the crucible tilted to pour the metal into a mould.  This Vacuum Arc Re-melting (VAR) technique has
several drawbacks:

  • The titanium electrodes are expensive due either to the high cost of titanium billets or forgings, or the high labour cost of creating an
    electrode from certified scrap or revert material.
  • The requirement for a pre-alloyed electrode makes it difficult and expensive to produce non-standard alloys.
  • The water-cooled crucible limits the degree of superheat achievable in the metal, which in turn affects fluidity, leading to difficulty in
    filling thin-wall castings.
  • The highest temperature exists where the arc strikes the metal, and high temperature gradients exist in the molten metal. This also
    affects the filling of moulds and sets up poor temperature gradients in the solidifying casting.
  • The molten alloy may not be homogeneous in composition and no stirring action exists to promote uniformity of composition or temperature.

Induction skull or ‘cold crucible’ melting overcomes these constraints.  This note explains why Cti selected this technology for the two melting
systems currently installed at a cost of £2m in its premises on the Advanced Manufacturing Park in South Yorkshire.

Induction skull melting (ISM) technology also uses a water-cooled copper crucible but in this case it is segmented in such a way that an induction
field can be established in the alloy placed inside the crucible, without the copper itself being heated by the induction field. 


Schematic of the means by which the electromagnetic field levitates the metal and the mechanism by which the skull forms at the base of the crucible.  The electromagnetic stirring provides uniform temperature and composition
Schematic of the cold crucible assembly of water cooled 'palisades' that are electrically insulated from each other to prevent the crucible heating in the electromagnetic field induced by the coil

As the titanium alloy melts, it solidifies against the walls of the segmented crucible, forming a thin skin or ‘skull’ on the surface.  Titanium has a
low thermal conductivity, so the skull insulates the molten metal from the cooling effect of the crucible.  Moreover, the effective power input is so
high that the molten metal is partially levitated, which further reduces heat exchange between the liquid metal and the skull.  This results in a
much higher superheat being achieved and the stirring effect created by the induction field achieves a uniform temperature distribution throughout
the melt. 

Semi-levitated molten titanium alloy in the cold crucible.  The individual water-cooled copper segments or 'palisades' can be clearly seen
The titanium alloy 'skull' remaining in the crucible after the metal has been poured.  As the metal flows over the crucible wall during pouring, it solidifies.  To minimise the weight of the skull, the crucible is tilted very quickly

The advantages of ISM are, therefore:

  • Higher superheat is achieved and more uniform metal temperature, both of which facilitate casting manufacture.
  • The melting stock can be ingot, plate, tubing, turnings, sponge, compacts, powder and revert (recycled material from the casting process) –
    basically, anything that can fit into the crucible.  Even fully-certified material (forged or rolled premium quality off-cuts) in these forms is far
    cheaper and more readily available than pre-formed electrodes as used in the VAR process.






    Different forms of certified
    alloy 'scrap' that is readily
    available at lower cost than
    the electrodes used in VAR
    melting of titanium alloys


  • The strength of titanium alloys is controlled by the oxygen content and it is easier to control the content by selection of the charge materials.

  • Control of charge materials allows alloys of any composition to be produced, with extended holding times and vigorous stirring enabling
    complete dissolution of refractory metal additions such as tantalum and tungsten.

  • The power input allows fast melting times to be achieved, approximately 20 minutes for 30kg and 35 minutes for 100kg melts.



M.C. Ashton
28th March, 2007