Ceramic shell mould

The ceramic shells were fabricated with ceramic slurry containing zirconia as filler material and colloidal silica binder to cast Zr-alloy 705. The investment shell mould characteristics in terms of hot bending strength, thermal expansion and metal-mould reaction were measured. On account of phase transformation (confirmed by Raman spectra) of zirconia at high temperatures the hot bending strength zirconia shells decreases with increasing temperature. A hard ZrO2 case layer was noticed beneath the surface of the casting.

The ceramic shells were fabricated with ceramic slurry containing yttria as filler material and colloidal silica binder. The slurry characteristics in terms of sedimentation and kinematic viscosity were measured. The green bending strength was determined to find the bonding strength of the ceramic shells. The yttria shells exhibit high strength for filler to binder ratio of 0.78 cc/ml.

The ceramic shells were fabricated with ceramic slurry containing magnesia as filler material and colloidal silica binder. The shell characteristics in terms of hot bending strength, thermal expansion and hardness were measured. The non–linear nature of thermal expansion in the ceramic shells is on account of phase transmission of stuccoing sand. The MgO was found at the interface of magnesia investment shell mould and AZ91E alloy.

The characteristics dip-casting slurries judge the performance of investment casting process.  The paper presents a hypothesis on the bonding mechanism in the ceramic shells developed from coal flyash. Experiments have been carried out to study the effects of filler loading, aging and air-drying on the strength of shells. The results indicate substantial reduction in the sedimentation under operating conditions and enhancement in the shells strength.

This paper discusses the development of a new technique for measuring the strength of ceramic shells. In this technique, compressed air was passed into the ceramic shells and the bursting pressure was measured using a diaphragm pressure gage. The hoop stresses were calculated. The results of the new technique realistically match the actual failure rate of shells in the precision casting process.