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The fatigue properties of an A356(AlSi7Mg) alloy have been investigated. Both materials free from and containing casting defects have been compared. In the defect free material persistent slip bands (PSBs) are formed in the inter-dendritic triple points. By extrusion, however, the inter-dendritic distribution of primary silicon particles is replaced by an almost uniform distribution of smaller Si particles. Persistent slip bands are formed near the primary Si particles and an increase in the density of PSBs are observed resulting in a more homogeneous distribution of the strain. A significant increase in fatigue life is measured. Material containing pores shows approximately 50% reduction in fatigue life compared to the defect free material, and the fatigue strength at 107 cycles is reduced by 20%. Cracks are initiated from the casting defects and most of the fatigue life is spent in the fatigue crack growth regime. A model has been used to predict the fatigue life.