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The effect of microstructural parameters on channel-type segregations formed during direct chill casting of Al–Mg billets is investigated through numerical simulation by solving the momentum, mass, heat, and species balance equations coupled with phase diagram. The numerical results indicate that the strip-patterned channel-type segregations are formed under conditions when the critical solid fraction is high. The dendrite arm spacing is found to slightly affect the macrosegregation formation. Also, the diameter of floating particles in the slurry zone has a significant effect on the macrosegregation, and the larger the particle diameter is, the greater the extent of macrosegregation. These phenomena are explained in terms of flow conditions. The solutal buoyancy flow in the slurry zone is enhanced with increase of the critical solid volume fraction and floating particle diameter that result in the remarkable formation of channel-type segregations. [ABSTRACT FROM AUTHOR]

Channel-type segregation is one of the defects in aluminum casting technology, particularly in the direct chill (DC) process, having a significant influence on the cast quality. Nevertheless, the formation mechanism of three-dimensional (3D) channel-type segregations remains poorly understood. In order to clarify the formation mechanism of defects of this type, we conducted a 3D numerical simulation of the DC casting process of an Al-Mg alloy billet considering the melt flow, heat and mass transfer, solidification, and the motion of the solidified ingot, coupled with the alloy phase diagram. The simulation results showed that the channel-type segregations have strip-patterns and formed easily at high casting speeds, being localized at a distance of a half-radius from the billet centerline. We also compared two-dimensional axisymmetric and 3D simulations, and the results indicated that a two-dimensional axisymmetric simulation is incapable of properly predicting the behavior of channel-type segregations. [ABSTRACT FROM AUTHOR]

Although channel-type macro-segregations are a well-known defect formed in Direct Chill (DC) casting of aluminum alloys, their formation mechanism remains unclear up to now. To investigate the possible mechanisms, we carried out numerical simulation considering thermo-solutal flow and solidification coupled with phase diagram. The main emphasis is placed on the effect of shrinkage flow and slurry flow on the macro-segregation characteristics. The shrinkage flow causes negative segregations to form along the center axis of billet especially at high casting speeds, while channel-type segregations are not formed under these conditions. On the other hand, the slurry flow in mushy zone results in formation of the channel-type segregations in large billets at low casting speeds. In addition, formation of three types of channel-type segregations is predicted in the following locations (1) flow separation zone, (2) flow stagnant zone, (3) mushy zone where the upward solutal buoyant flow is generated. The formation condition of channel-type segregation is considered using solutal Rayleigh number for binary alloys. [ABSTRACT FROM AUTHOR]