Effects of Al₂O₃ and SiC micro-scale powder addition on mechanical, thermal and physical properties of reinforced shell for investment casting

This study investigates the effect of adding 15 wt% of Al₂O₃ and SiC micro-scale powders to the backup coat slurry of shell for investment casting on the mechanical, thermal and, physical properties of shell molds. These reinforced shells were implemented in the 4.5 to 8.5-layer shells where the main objective is to replace the original formulation of shells with the fewer number of layers. The performance indicators that will be used as a comparison benchmark with the original shells include measurements of modulus of rupture (MOR), porosity, gas permeability, and heat transfer coefficient (HTC). The measurement results show that the modulus of rupture (MOR) of the Al₂O₃ and SiC-reinforced shells at 8.5 layers construction reached 8.41 MPa and 8.10 Mpa or 26.28 % and 21.62 % higher than the original shell, respectively. The Al₂O₃-reinforced shells were found to have denser surface and lower porosities with a range of 12–15 % compared with the SiC-reinforced shells (13–17 %), which was almost similar to the original formulation). Al₂O₃-reinforced shells were superior in term of MOR, density, and porosity measurement results, but the gas permeability measurement results were much lower than the SiC-reinforced and original shells. This condition was due to the presence of calcium aluminosilicate particles produced during the sintering process and inhibiting gas flow in the permeability measurement. The addition of Al₂O₃ and SiC powders successfully increased the heat transfer coefficient (HTC) measurements of the shell, with no significant difference values between the two types of powder. Therefore, the evaluation of replacing the original shells with the reinforced shells was assessed through a MOR vs. permeability graph which revealed that SiC-reinforced shells with 5.5 and 6.5 layers were qualified to replace the original shells with 7.5 and 8.5 layers, respectively. These findings are very beneficial to reduce cost and time of production.