CASTING DEFECTS AND PROPERTIES OF CAST A356-ALUMINUM ALLOY REINFORCED WITH SIC PARTICLES

This paper is concerned with casting defects and properties of test bars cast from an A356 aluminium alloy reinforced with SiC particles. The composite materials were produced by the molten aluminium mechanical stirring technique. Particular attention was given to the characteristics of the casting defects which appeared at the tensile fracture surfaces of the cast test bars by means of extensive analytical electron microscopy. These observations showed that the casting defects on the fracture surface were agglomerates of SiC particles, aluminium and silicon oxides, mixtures of SiC particles with aluminium and silicon oxides, and brittle chemical compounds containing aluminium and silicon elements. These defects either were present in the as-received ingots or were introduced in the casting process during which oxide films were easily incorporated into the liquid metal. The numbers and areas of the defects on the fracture surface have an obvious effect on the mechanical properties of the cast test bars. Appropriate methods of improving the quality of the composite ingots as well as of optimizing the casting technique must be used to minimize these defects.