Grain Floatation During Equiaxed Solidification of an Al-Cu Alloy in a Side-Cooled Cavity: Part I—Experimental Studies

Abstract

Experimental studies were performed to investigate the role and influence of grain movement on macrosegregation and microstructure evolution during equiaxed solidification. Casting experiments were performed with a grain-refined Al-Cu alloy in a rectangular sand mold. For the aluminum alloy studied, the equiaxed grains are lighter than the bulk melt and thus float up. Experiments were designed to investigate floatation phenomena of equiaxed grains in the presence of thermosolutal convection. Cooling curves were recorded at key locations in both the casting and the chill. Quantitative image analysis and spatial chemical analysis were performed on the solidified casting to observe the chemical and microstructural inhomogeneity created by the melt convection and solid floatation. Several notable features that can be attributed to grain movement were observed in temperature histories, macrosegregation patterns, and microstructures. In our experiments, the floatation of grains influences the thermal conditions and the overall flow direction in the casting cavity. In some cases, the induced flow resulting from the grain movement caused a flow reversal. This in turn influences the solidification direction, microstructure evolution, and the overall macrosegregation behavior.

Effect of Super-gravity Field on Grain Refinement and Tensile Properties of Cu–Sn Alloys

“…Wu's simulation study did not take account of the movement of equiaxed grains, which can markedly influence the solidification structure of castings. 22,23) Also, the solidification structure refinement of Al-Cu alloys in super-gravity field has been investigated systematically in our recent study, 24) which demonstrated that the "crystal rain" caused by super-gravity contributed to the grain refinement.…”

Grain Floatation During Equiaxed Solidification of an Al-Cu Alloy in a Side-Cooled Cavity: Part II—Numerical Studies

“…IN a companion paper, Kumar et al [1] reported a controlled set of experiments to study the influence of the floatation of solid crystals on macrosegregation and microstructure formation during equiaxed solidification.…”

“…[3,5,26] Yang et al [3] and Chang and Stefanescu [26] proposed a simplified treatment by using a singledomain framework for movement of equiaxed grains in which the solid grains and the liquid move with the same velocity, and an effective viscosity is used. Their model assumes zero relative velocity between the solid and the liquid phases, which is not the case for larger floating equiaxed grains as has been shown in Kumar et al [1] A splitting scheme for coupling macroscopic transport and grain growth during solidification within a framework of a multicomponent, two-phase solidification model was presented by Combeau and coworkers. [27][28][29] A key feature in the proposed solution scheme is that the contribution to the solid mass and solute balance caused by transport and grain growth are solved in two stages by applying an operator splitting algorithm.…”

“…This model is applied to study the equiaxed solidification phenomena in Al-Cu alloys in a rectangular cavity. In part I of this work, [1] the authors reported qualitative observations and quantitative metallurgical data gathered from controlled experiments on equiaxed solidification of Al-Cu alloys. The current modeling results, in terms of temperature evolution, segregation patterns, and eutectic fraction distribution, have been compared with data from the experiments reported in part I.…”

“…The current modeling results, in terms of temperature evolution, segregation patterns, and eutectic fraction distribution, have been compared with data from the experiments reported in part I. [1] To highlight the influence of the motion of solid phase, the results of the current model are compared with those of conventional solidification models, which assumes immobile solid phase. A sensitivity study of the key fitting parameters, to include coherency point and characteristic size of the moving solid phase on the macrosegregation results, is also presented.…”