Microstructure, tensile and fatigue properties of the Al–10%Si–2%Cu alloy with different Fe and Mn content cast under controlled conditions

Abstract

The effect of solidification rate and chemical composition (different Fe and Mn content) on microstructure, tensile and fatigue properties of the Al–10%Si–2%Cu casting alloy was investigated. A special apparatus was used to produce cast specimens under controlled solidification conditions, in order to obtain two values of the secondary dendrite arm spacing (SDAS), equal to about 10 and 50 μm. Microstructural characterization of the alloy was carried out by optical and scanning electron microscopy, to evaluate the effect of cooling rate and chemical composition on solidification defects and others microstructural features, such as SDAS as well as morphology and composition of the Fe-rich intermetallic compounds. In the samples with high-Fe content these compounds were mainly β-(Al 5FeSi), with their typical needle-like morphology, while also the α-(Al 15(Fe,Mn) 3Si 2) and π-(Al 8FeMg 3Si 6) phases, with a Chinese-script morphology, were observed in the samples containing Mn. High cooling rate induced a reduction of the SDAS, as well as of the volume fraction and size of the solidification defects, with a consequent important increase of the tensile and fatigue properties. Microstructural factors, such as shape and composition of the Fe-rich intermetallic compounds, only influenced the tensile and fatigue response of the specimens with the lower SDAS and smaller solidification defects. The chemical composition had a negligible effect on tensile strength (UTS, YS), while it influenced the elongation to failure, that was significantly lower in the alloy with the larger volume fraction of Fe-rich intermetallics. The presence of high volume fraction of these intermetallics also increased the fatigue resistance at high applied stress, while it decreased the cycles to failure at low applied stress.