Bands of defects are commonly found in high pressure die cast (HPDC) aluminium alloys. In the work presented here, similar defects have been observed in laboratory gravity die castings. Castings were poured with an initial solid fraction in the range fs= 0-0.3 to simulate the externally solidified crystals (ESCs) injected in cold chamber HPDC. The effect of the fraction of ESCs on the segregation bands at die temperatures in the range 130 – 450°C was investigated. In addition, the behaviour of the ESCs during filling has been studied. A mechanism describing the formation of the segregation bands has been proposed by considering flow past a solidifying wall layer accounting for mushy zone dynamics and rheology.
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