Mechanisms of Grain Refinement in Direct-Chill Casting 2024 Aluminum Alloy With Intensive Melt Shearing

The mechanisms involved in the grain refinement of a 2024 aluminum alloy prepared by the direct-chill (DC) casting process with the introduction of intensive melt shearing were investigated through experiments and theoretical analyses. It was proven for the first time by theoretical calculations that the shear stress between the stator and rotor is capable of breaking the dendrite arms. In the experiment, 2024 aluminum alloy ingots (300 mm) were prepared with high-shear unit at different shear positions and rotation speeds. Furthermore, the temperature of the melt near the tip (the working part in the melt) of the high-shear unit and the variation in the melt temperature during solidification were measured. The experimental results reveal that during the casting process with intensive melt shearing, the melt is cooled to the liquidus temperature at a higher cooling rate than that in conventional DC casting. Moreover, when shearing is processed at a lower position with a higher rotation speed, the grain refining effect is significantly improved. It is suggested that when the melt around the tip is in the solid–liquid two-phase state in the DC casting, dendritic fragmentation is the dominant mechanism of the grain refinement.