PREDICTION OF SHRINKAGE POROSITY IN TI-46AL-8NB TILT-CASTING USING THE NIYAMA CRITERION FUNCTION

Practical measurements, in conjunction with computer simulation modelling, were carried out to study the effects of cooling rate and temperature gradient on shrinkage porosity in a Ti-46Al-8Nb investment casting, by investigating the influence of casting geometry and preheat mould temperature on shrinkage porosity. A tilt-fill solidification model (ProCast) Ti-46Al-8Nb investment casting was developed and used to compute Niyama criterion function in different test bars with different taper angles, at 500C (932F) and 1000C (1832F) mould temperatures. Analyzing the model results and matching them with the experimentally measured area percentage of porosity revealed that enhancing directional solidification by increasing the taper coefficient significantly reduced the amount of porosity, due to the influence of improved temperature gradient on the formation of the shrinkage porosity. The Niyama criterion function was validated to predict shrinkage-related porosity in Ti-46Al-8Nb castings. Positions in a casting with Niyama values above 4 (degrees C.min)(0.5)cm(-1) could be considered completely sound, with the critical value of the Niyama function being 2 (degrees C.min)(0.5)cm,(-1) below which the local area would be susceptible to macroshrinkage porosity.