Abstract
The longitudinal surface cracks occurring on the DC-cast 7039 aluminium alloy were investigated. The cracks were characterized by a grooved appearance on the exterior surface and a branching and porous morphology in the interior area. Microscopic analysis showed that the crack opening was of an abnormal chilled structure, while the interior region was of slowly solidified structures. EPMA and DTA indicated that Mg-Zn intermetallic compounds were formed in the crack interior boundaries. The results suggested that the cracks originated from the uneven chill conditions of the mould. The liquid melt in front of the hot spot was enclosed by the rapidly growing solidified shell and the bridged liquid melt then suffered from a slow cooling rate, forming shrinkage cracks and pores. Formation of Mg-Zn intermetallic compound arising from the slow solidification may enhance the crack occurrence. The cracks were completely eliminated by the improvement in the mould cooling efficiency.
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