Abstract:
Ti-64, which accounts for more than 50% of the worldwide titanium
tonnage, has found commercial importance in industries requiring
components with high specific strength and resistance to corrosion.
Investment casting is the preferred production method due to the
difficult machinability of the alloy. This study was aimed at investigating
the mechanism and the extent of alpha case formation on Ti-
64 components cast using the investment casting method with YFSZ
(yttria fully-stabilized zirconia) shell moulds after vacuum
induction melting. The extent of the reaction between the mould hot
face and the molten metal has been studied by varying parameters
such as soaking temperature and mould hot face composition, and
examining their effects on the reaction with the mould. An increase
in the soaking temperature had an effect on the alpha case, both in
appearance and hardness, but had no effect on contamination levels
by carbon, oxygen, and nitrogen. The depth of alpha case increased
with soaking temperature, increasing from 35 m to 161 m with an
increase in temperature from 1200°C to 1400°C. The micro-hardness
profiles provided insight into the effect of the alpha case on the
mechanical properties of the Ti-64 alloy by displaying hardness
values of 1000 HV0.1 and above, but could not be solely utilized to
determine the alpha case penetration depth due to microstructural
differences in the unaffected Ti-64, in particular the martensitic
microstructure that formed with a fast cooling rate from a higher
temperature. Levels of expected contaminants such as Zr, Y, O, and
C were low. The addition of the colloidal zirconia binder affected the
interfacial reactions. YFSZ proved to be a thermodynamically stable
refractory material, with the alpha case possibly forming as a result
of segregation.
