The effects of porosity on mechanical properties of cast discontinuous reinforced metal-matrix composite

This paper presents the findings of porosity effects on the mechanical properties of cast discontinuous reinforced metal-matrix composites (DRMMCs). Aluminum-silicon alloy composite specimens are produced by the stir casting method with varied content of reinforcing silicon carbide (SiC) particles for metallographic study, porosity measurement, and tensile and fatigue testing. The factors of porosity formation are assessed from three different levels of stirring speeds, 100, 200, and 500rpm, varied SiC content, and modification of the stir casting method. The evaluated porosity data exhibits increased SiC particle content and stirring speed has increased the porosity formation in the cast DRMMC whereas the modification of the stir casting method significantly has decreased the porosity content. The least content of porosity evaluated is at 0.09% in the modified stir cast DRMMC, while the highest is at 12.45% in the conventionally stir cast DRMMC. In monotonic tensile testing, increasing porosity content has decreased the ductility, tensile strength, and yield strength of cast DRMMC. Though, based on fully reversed (R = -1) fatigue test, porosity formation has increased the cast DRMMC fatigue strength at 1 x 10(7) cycles. The fatigue strengths of cast DRMMCs at 5, 10, and 15% reinforcing SiC particle are 129.7, 141.5, and 157.3 MPa, respectively. These are due to either isolated porosity formation or porosity presence among particle clusters as observed in the metallographic studies.