Fatigue properties of sand cast, stircast and extruded Al-7Si-0.3 Mg alloy with trace additions of Be and Mn

Studies the fatigue behaviour of Al-7Si-0.3 Mg sand cast alloys at varying iron levels and with beryllium and manganese trace additions and when the alloy is stircast and extruded. From the stress (S)-number of cycles to failure (N)-(S-N) curves it is observed that the presence of higher amount of iron (0.76%) in sand cast alloys leads to a shorter fatigue life. Beryllium and manganese additions to a higher iron containing alloy (0.76%) show better fatigue properties than low (0.29%) and high (0.76%) iron containing sand cast alloys, thus countering the detrimental effect of iron. The better fatigue life of beryllium and manganese added high iron alloy is due to the presence of (Be,Mn)-Fe phases only inside α-Al dendrites. The fatigue life of stircast and extruded low iron (0.44%) alloy is superior to sand cast low (0.29%) iron alloys. Observation of fractured surfaces reveals that porosity/inclusions is the high stress concentrating point where the crack originates (stage-I) and then propagates (stage-II) depending on the presence of the second phases. In the case of low iron alloys (sand and extruded stircast) a crack propagates along eutectic silicon, while in the high iron alloys a crack propagates through the brittle β-phase. In beryllium and manganese added alloys, even though a crack nucleates on the (Be,Mn)-Fe phase, it will be arrested as it approaches α-Al dendrites and hence, the crack has to propagate along silicon particles. The fractured surface of the stircast and extruded alloys has revealed fine fatigue fracture with striation as compared to sand cast alloys. This aspect of crack nucleation and propagation is explained schematically.