Mechanical and microstructural characterisation of A356 castings realised with full and empty cores

Abstract

The use of full cores, for producing automotive components by means of the permanent mould casting technique, allows good castings with a low level of defects to be obtained. Nevertheless, an extra phase of the production cycle is necessary to have an optimal emptying of the internal cavities of the castings, with an increase in costs. This scenario motivated the present research, in particular to study the effect of replacing full cores with empty cores. The component analysed is one of the three parts of a motorcycle frame, realised in aluminum alloy A356 by permanent mould casting and T6 heat-treated. Several castings have been produced with both full and empty cores; tensile strength tests have been performed on samples drawn from four different zones, in order to compare the mechanical and microstructural properties of the castings realised with the two different kinds of core. A slight decrease in elongation at fracture has been verified in the samples drawn from the castings realised with empty cores, but yield strength and ultimate tensile strength are well comparable. A finer microstructure always corresponds to higher mechanical properties; an inverse correlation between secondary dendrite arm spacing and ultimate tensile strength has been found. The effect of secondary phases, porosities and morphology and distribution of eutectic silicon particles has been considered. The Japanese Taikai methodology has been improved in order to compare the current production cycle with the one optimised by the use of empty cores. Cost, weight and timecycle reductions in the production of the component, due to the elimination of the extra phase using empty cores, have been evaluated