Abstract
Experimental work was carried out to develop a technique for making titanium castings free from porosity by static molds. Plates, 2cm thick, 6cm wide and 8 cm high were vertically cast without risers and used to assess porosity. The soundness was ascertained from a simple visual inspection of a longitudinal section through the casting. Castings had extensive gas pores regardless of types of melting stock, e. g. sponge or rolled bar when melted and poured under reduced pressure of argon. Porosity was also observed in castings made in the copper mold previously heated at 500 K in vacuum to exclude the effect of interaction of the metal with the mold and the effect of gases in the mold. The addition of various elements less than 2 mass per cent, bottom gating and control of melting rate were not effective for the prevention of porosity insofar as melting and casting were done under reduced conditions. Soundness was improved by increasing the pressure of argon during melting and casting processes and porosity free castings were obtained under the pressure above 250 kPa. Bridging of the charge in the crucible could be avoided when melting was conducted under pressurized conditions. From these results it was deduced that calcium gas generated by the reaction between lime crucible and titanium melt is one of the causes of porosity in the castings produced by this process and the boiling of calcium is suppressed by the pressure of argon.
