Understanding of macro-segregation in direct-chill cast 2024 alloy with intensive melt shearing based on the evolution of microstructure and temperature field

Abstract

The influence of intensive melt shearing (IMS) on macro-segregation has been systematically investigated. Experimental results show that the negative centerline segregation is effectively suppressed when the IMS is applied. To reveal the mechanism of IMS on macro-segregation, a quantitative study on the evolution of the slurry region, mushy region, solidification interface, morphology, and concentration of microstructure was carried out. It was confirmed that the IMS parameters determine the dominant mechanism affecting centerline segregation by controlling its action region. When IMS is applied above the liquidus, the fine dendritic structure not only has higher solute content, but also effectively limits the penetration depth of the liquid melt. Moreover, the velocity of the shrinkage flow in the horizontal direction decreases due to the flattening of the solidification interface. At this condition, the above factors play a more dominant role in macro-segregation than the increase of the mushy region thickness. When the action region of IMS changes to the slurry region, the solidification interface profile transforms from a “V” shape to a “VVV” shape. In this case, the influence of the magnitude and direction of the shrinkage flow on macro-segregation is balanced by the near-spherical structure (solute-poor), the increase of permeability, and the thickness of the mushy region. A physical model has been proposed to explain the IMS parameters on macro-segregation.

Keywords

• Macro-segregation
• Temperature field
• Intensive melt shearing
• Direct-chill casting
• Aluminum alloy