Properties of hybrid fibre reinforced shell for investment casting

In order to improve the properties of silicon sol shell for investment casting process, a varying content of hybrid fibres (aluminium silicate and polypropylene) was introduced into slurry for preparation of fibre-reinforced shell in the present work. The bending strength, self-load deformation at elevated temperature, and the permeability of fibre-reinforced shell specimens were investigated and the fracture surfaces of shell specimens were observed by SEM. The results show that the bending strength of green shell increases with content of fibres in it. The maximum bending strength of 4.96 MPa was obtained in the fired shell with 0.6 wt% hybrid fibres addition. The high temperature self-loaded deformation of specimens of shell reinforced with a hybrid fibre addition above 0.6 wt% is higher than that of the unreinforced. However, the shell with a hybrid fibre addition up to 0.4 wt% exhibits the lower self-loaded deformation at high temperature compared to the unreinforced. It is also found that the permeability of shell specimens can be improved by hybrid fibres addition. Based on the fracture surfaces observation using a scanning electron microscope (SEM), the failure mode of the green shell reinforced with hybrid fibres is identified as fibre rupturing from the substrate of shell specimens, and/ or debonding from adhesive film surrounding it in shell. Even though the specimens of shell being fired at 900 °C for 2 h, the same failure features also exist in the fracture surfaces of specimens. This indicates that the specimens of shell can still be reinforced with aluminium silica fibres (residue of hybrid fibres) for their interpenetrating fibres network structure although go through firing.