The effect of silicon content on long crack fatigue behaviour of aluminium–silicon piston alloys at elevated temperature

Abstract

The microstructure of aluminium piston alloys comprises primary and eutectic silicon together with numerous intermetallics. Previous research has shown that primary silicon strongly influences both fatigue crack initiation and subsequent propagation behaviour, however, the detailed effects of varying silicon volume fraction and morphology have not been fully addressed. Therefore, the fatigue properties of a number of candidate piston alloys with varying volume fractions of silicon have been studied. Long crack fatigue tests have been performed at room and elevated temperature typical of the gudgeon pin boss (200 °C) using a test frequency of 15 Hz (a typical engine frequency at engine idle condition). Microstructural characterisation using image analysis approaches combined with optical profilometry has been used to assess the fracture surfaces of test samples. The role of primary Si in enhancing crack growth rates at high Δ K levels, whilst affording improvements in crack growth rates at lower Δ K levels due to local crack deflections and shielding, has been confirmed. In the absence of primary Si (lower Si content alloys) the low Δ K level crack growth behaviour is dominated by matrix properties (intra-dendritic crack growth pre-dominates) whilst the high Δ K level crack growth behaviour is inter-dendritic and occurs along the weak path of the eutectic Si and/or intermetallic network.