Correlation between structure and mechanical properties of cast SiCp AI MMCs

摘要

Ingots of 20% and 30% silicon carbide particle-reinforced (SiCp) aluminum-based alloy composites from two different suppliers, were remelted and cast into tensile bars by three foundries using the permanent mold and the sand mold processes. All the samples were T6 heat-treated. The mechanical properties of the composites were measured by the three independent test laboratories (X, Y and Z). The analysis showed that alloy producer, foundry, test lab, type of mold and SiC particle loading have measurable influence on the mechanical properties of the composites. However, no significant differences were observed between the mechanical properties of machined and unmachined tensile bars. The mechanical properties measured by one of the testing laboratories (Y) are higher than those measured by either of the two other testing laboratories (X and Z). The percent elongations reported by one of the laboratories (Z) are much higher (2-3 times) than those reported by the two other (X and Y). The range of tensile property data generated in this study are comparable to the property data published by other researchers. The macrostructure and microstructure of the fracture surface of the tensile test bars were also investigated using stereo optical microscope and SEM-EDX. The main factors, which appear to be associated with lower tensile properties in certain bars of the same compositions, were casting defects including shrinkage, gas porosity, oxide inclusions and cracks. The modulus of Al-30% SiC tensile bars was more than the modulus of Al-20% SiC bars. However the tensile strength and elongations of Al-30% SiC were lower than the tensile strength and elongations of Al-20% SiC bars. The results obtained further underline the need for standardization in foundry practice and testing of Al-SiC composites.