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 500 rpm, 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 monotonie 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 × 107 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

A Novel Approach to Enhance the Mechanical Strength and Electrical and Thermal Conductivity of Cu-GNP Nanocomposites

Copper/graphene nanoplatelet (GNP) nanocomposites were produced by a wet mixing method followed by a classical powder metallurgy technique. A qualitative evaluation of the structure of graphene after mixing indicated that wet mixing is an appropriate dispersion method. Thereafter, the effects of two post-processing techniques such as repressing-annealing and hot isostatic pressing (HIP) on density, interfacial bonding, hardness, and thermal and electrical conductivity of the nanocomposites were analyzed. Density evaluations showed that the relative density of specimens increased after the post-processing steps so that after HIPing almost full densification was achieved. The Vickers hardness of specimens increased considerably after the post-processing techniques. The thermal conductivity of pure copper was very low in the case of the as-sintered samples containing 2 to 3 pct porosity and increased considerably to a maximum value in the case of HIPed samples which contained only 0.1 to 0.2 pct porosity. Electrical conductivity measurements showed that by increasing the graphene content electrical conductivity decreased