Second phase structure effect to the failure of an Al–Si casting

Abstract

An Al–Si casting (blower pump) failed during pressure test. The material of the casting was aluminum–silicon alloy (BS-LM 9), in solution treated and aged condition. During pressure test the casting failed around 130 bar, while it was designed to sustain a pressure of 160 bar. The failed body was examined with the help of stereo microscope, optical microscope and scanning electron microscope. Fracture started from a sharp edge of a race present at the interior of the body. A granular dull gray fracture surface was observed during unaided visual examination. Fractographic studies showed that the mode of the fracture was transgranular and the material failed in brittle mode. Optical microscopic examination showed a dendritic structure having a continuous network of the secondary phase. A sample taken from the fracture region unveiled the crack propagation along the secondary phase network. It was recommend to use modification process during melting practice to avoid the stress raisers.