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Influence of Silicon Morphology and Mechanical Properties of Piston Alloys

In this paper, the influence of silicon particle morphology on mechanical properties of two piston alloys has been reported. Alloys having nominal composition Al-12%Si-1%Ni-0.8%Cu-0.6%Mg (LM13) and Al-17%Si-1%Ni-0.8%Cu-0.6%Mg (LM28) were prepared by melting and casting. The morphology of silicon crystals was changed using additives during the melt treatment and subsequently by heat treatment. Mechanical properties such as hardness, ductility, and tensile strength of experimental alloys were tested. Test results showed that the melt treatment and heat treatment of both the alloys increased the tensile strength, hardness, and ductility. However, the influence of melt treatment on mechanical properties was not as significant as that of heat treatment. Tensile strength and ductility of LM13 was found to be higher than the LM28 alloy under similar conditions. Hardness of LM28 was higher than the LM13 alloy. Optical and SEM studies showed a change in eutectic silicon morphology on melt treatment of LM13. However, change in morphology of primary silicon particles was not significant as for LM28. Heat treatment of both alloys caused spheroidization and better distribution of eutectic silicon crystals. Sharp-edged primary silicon particles were rounded off after the heat treatment of LM28. Scanning electron microscopy (SEM) of tensile fractured surfaces was carried out to study the influence of microstructure on fracture mode. Heat treatment of both alloys promotes dimple fracture. However, as-cast and melt-treated alloys show predominantly cleavage fracture.