Abstract
Melt-conditioned, direct-chill (MC-DC) casting is an emerging technology to manipulate the solidification process by melt conditioning via intensive shearing in the sump during DC casting to tailor the solidification microstructure and defect formation. When using MC-DC casting technology in an industrial scale DC cast billet of an A4032 aluminum alloy, significant grain refinement and uniform microstructure can be achieved in the primary α-Al phase with fine secondary dendritic arm spacing (SDAS). Improved macrosegregation is quantitatively characterized and correlated with the suppression of channel segregation. The mechanisms for the prevention of channel segregation are attributed to the increased local cooling rate in the liquid–solid phase region in the sump and the formation of fine equiaxed dendritic grains under intensive melt shearing during MC-DC casting. A critical cooling rate has been identified to be around 0.5 to 1 K/s (°C/s) for the channel segregation to happen in the investigated alloy based on quantitative metallographic results of SDAS. Reduction and refinement of microporosity is attributed to the improved permeability in the liquid–solid phase region estimated by the Kozeny–Carman relationship. The potential improvement in the mechanical properties achievable in MC-DC cast billets is indicated by the finer and more uniform forging streamline in the forgings of MC-DC cast billet.
Highlights
IN direct chill (DC) casting of Al alloys, a grain-refined as-cast microstructure is generally desirable
In conventional direct chill cast billets of A4032 alloy, even with grain refiner additions (Al-Ti-B-Sr), the as-cast structure displays coarse and nonuniform structures consisting of coarse dendritic primary a-Al phase and blocks/flakes of eutectics
MC-DC casting technology has been successfully implemented in the production of industrial-scale DC cast billets to manipulate microstructure and solidification defects by incorporating an intensive melt shearing mechanism in the sump during the DC casting process
Summary
IN direct chill (DC) casting of Al alloys, a grain-refined as-cast microstructure is generally desirable. McCartney has defined grain refinement as deliberate suppression of columnar grain growth in castings and formation of a fine equiaxed solidification structure throughout the material.[1] Effective grain refinement brings many direct and indirect benefits, including the possibility of faster production of DC castings. During DC casting, grain refinement directly affects the formation of porosity, the tendency of hot tearing, the pattern of macro-segregation, and the scale of micro-segregation.
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