Inverse segregation in directionally solidified Al-Cu-Ti alloys with equiaxed grains
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By using a device which could achieve the 1-Dimensional shrinkage instead of 3D shrinkage of the casting during the solidification and cooling, the cooling curves and the relationship between the shrinkage / temperature and time were investigated. Then, the thermal shrinkage behaviors of pure Mg and Al ingots were obtained. As a result, the characterization of the thermal shrinkage behavior of Mg and Al was systematically investigated. The results indicate that the contraction rate of pure Mg ingot increases with the increase of growth rate of dendrites in quantity during solidification. The maximum contraction rate νl, max will be reached when the formationof primary dendrite ends, and the value of νl, max is 1.12×10-2 mm·s-1. However, the shrinkage will not be present until the solid state cooling starting for the pure Al ingot. The contraction of pure Mg ingot in solid state can be divided into two stages: high temperature solid shrinkage stage and low temperature solid shrinkage stage. And the rate of shrinkage firstly increases then decreases with the temperature dropping at the stage of high temperature solid shrinkage. The rate of solid shrinkage of pure Al ingot is present at the beginning of solid-state cooling. And its value is 9.69×10-3 mm·s-1, which is more than 3 times of that of pure Mg ingot. The shrinkage behaviors of both pure Mg and Al ingots tend to be uniform shrinkage when the temperature is below 400°C.
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A series of experiments were conducted by Al-2·0Si%Si alloy to investigate the effect of mould D/H ratio and shape on solidification structure under ultrasonic vibration (UV). Finally, the mechanism of grain refining was discussed. The results show that equiaxed grain occupancy is maximized when D/H is equal to 0·7 for ingots with mould volume of 80 and 120 cm3, however, the equiaxed grain occupancy has the maximum when D/H is equal to 2·0 for the ingot with mould volume of 200 cm3. Otherwise, the equiaxed grain occupancy has downtrend with increasing of mould volume. The columnar structure is easily formed in ultrasonic ingots with complex mould shape and more corners.
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A mathematical model of equiaxed grains movement and macro-segregation has been built to analyze the fluid flow and species distribution during the metal solidification. The mushy zone was divided into dendrites region and free equiaxed grains region by the dendritic coherency point. Macro-segregation of a steel ingot in a rectangular mold with a riser was simulated and the calculation result was compared with that of an experiment. It shows that the species distribution obtained by the grain movement model is more consistent with experiment comparing to that by the solid skeleton model in mush. The equiaxed grains move with the fluid and accumulate at the bottom center of the ingot during solidification. The cone-shape negative segregation forms after solidification. The positive segregation in the upper center and the negative segregation in the exterior region of the ingot are found at the same time.
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In the present work macrosegregation during solidification of a 2.45 ton steel ingot is simulated with a pure equiaxed model, which was tested previously via modeling of a benchmark experiment. While the columnar structure is not taken into account, a packed layer formed over inner walls of the mold at an early stage of solidification reproduces to some extent phenomena generally related to zones of columnar dendrites. Furthermore, it is demonstrated that interaction of free-floating equiaxed grains with ascending convective flow in the bulk liquid results in flow instabilities. This defines the irregular form of the negative segregation zone, the formation of which at the ingot bottom corresponds to experimental observation. Vertical channels reported in experimental measurements are reproduced in simulations. It is confirmed that intensification of ingot cooling may decrease segregation in the ingot.
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Abstract The control of the carbon macrosegregation level in steel ingots is important for the structural integrity of the final component. During solidification, the fragmentation of the columnar dendrites is an important source of equiaxed grains, and has a large influence on the macrosegregation and the grain structure. The goal of this study is to show that a numerical model that takes into account fragmentation can describe the formation of the structures and the macrosegregation during solidification of a large steel ingot. The present article describes the multiphase numerical model used in the simulations. The simulation results are compared to experimental data from a 9.8 t ingot cast in A5/6 steel by ArcelorMittal Industeel. The model can then be used to explain the formation of the observed structures. For example, we show that the structural transitions, first from columnar to equiaxed globular and then to equiaxed dendritic at the bottom of the ingot are a consequence of the concurrent transport and growth of the dendrite fragments from the columnar zone. Furthermore, we show that most of the structures are formed very early on during solidification, whereas macrosegregation develops much more gradually.
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Morphology
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