ISSUE 1/2012

/// 04
Anyalebechi, P. N.
Effects of mold material and casting speed on shell morphology and attendant subsurface microstructure of aluminum alloy 3003

Casting defects such as laps, liquation, and exudation and their associated subsurface segregation are often attributed to the events at the mold-melt interface at the early stages of solidification. They reportedly can be alleviated by proper control of casting process conditions such as casting speed, melt superheat, mold material, and mold surface topography. In this study, an immersion tester was used to investigate the effects of mold material and casting speed on shell morphology and attendant subsurface microstructure formed in the early stages of solidification of aluminum alloy 3003. Instrumented chill (mold) blocks of pure copper, aluminum alloy 6061, and graphite were used to make shell castings at different casting speeds. The surface topography of each chill block was as-machined. For a given mold material, increase in casting speed (between 1 mm/s and 50 mm/s) and melt superheat (between 10 °C and 50 °C) exacerbated uneven shell growth on the casting-melt interface but ameliorated lap formation at the mold-casting interface. For the range of casting speed and melt superheat investigated, shells solidified on the copper and aluminum alloy molds exhibited uneven growth front and laps were formed on the mold-casting interface. In contrast, the shells solidified on the graphite chill were comparatively more uniform and almost free of laps. However, the attendant microstructure of the shells solidified on the three different mold materials contained unacceptable levels of subsurface segregation, suggesting that the subsurface segregation on as-cast aluminum alloy ingots and slabs are caused by two different remelting-induced mechanisms. There was an apparent correlation between the macro-morphological features (i. e., the depressions and humps) of the uneven shell growth and the subsurface microstructure. Microstructurally, the depressed thinner regions of the shell showed signs of remelting with attendant liquation and subsurface segregation whereas the humps were microstructurally uniform and tended to be free from subsurface segregation.




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