Using aluminum wheels, the fatigue properties of large squeeze cast (AC4CH) were investigated and compared with those of wrought alloy (A6061). The fatigue strength of squeeze cast was slightly lower than that of wrought alloy, reflecting the static strength difference. The crack propagation characteristics were analyzed using the in situ direct surface observation by means of a plastic-replica method. Though most cracks of both alloys originated from the transgranular slip bands, in the squeeze cast case, some cracks were initiated from eutectic Si particles. In particular, when the applied stresses were high, some cracks were found to originate from segregated eutectic Si and sometimes the crack propagated along with eutectic Si. These phenomena coincided with the shorter life of squeeze cast under high stresses. Moreover, in squeeze cast, eutectic Si was occasionally observed to delay the crack propagation. The notch sensitivities of both alloys were explained from the fatigue crack initiation and propagation behavior caused by the microstructural difference. In both alloys, the crack initiation mostly originated from slip bands and was therefore controlled by the stress that was determined by the stress concentration factor. That is, the notch sensitivities for the crack initiation of both alloys were almost the same. On the other hand, the notch sensitivities for crack propagation were different. That is, the squeeze cast was more insensitive than the wrought alloy, because of the blocking effect of Si on crack propagation.
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