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The stress and strain fields of AA7050 aluminum alloy billets during direct-chilling (DC) casting process from an aluminum production company were simulated by directly coupling thermal, fluid and stress fields for three different start-up casting speeds. The results show that the center of the billet is subjected to large tri-axial tensile stress whereas the periphery demonstrates obvious compressive stress. According to the relatively higher circumferential and radial stress and strain combined with lower axial stress and strain in the mushy zone, it can be predicted that hot tears tend to propagate along the axial direction while hot tears perpendicular to axial direction are not likely to occur. It is necessary to apply a lower casting speed at start-up phase in the casting process to decrease thermal stress and strain in the mushy zone in order to reduce cracking tendency.