ABSTRACT
Multiple alternating of dendritic and nondendritic fronts was attempted in continuous casting. It should be taken into account that a change of the crystallization front without an external impact on the solidifying melt is almost impossible. Different smeltings of the same alloy can produce different structures in different ingots, even in those of identical size, because of the differing constitution of impurities. Consequently, a regular variation of structure in one and the same ingot without any external impact requires local changes in the composition.The addition of a modifying alloy into the solidifying melt must be periodic because of the slow and non-uniform diffusion rate of inoculant to the crystallization front. The initial composition of the alloy in the pool recovers at the same slow but regular rate, thus recovering the initial dendritic structure.In casting with electromagnetic stirring inside or outside the mold, one can sharply reduce the grain size by varying the strength and direction of the electromagnetic field to change the motion of melt in the liquid pool. However, the return to the initial dendritic structure is hard to control because motion in the pool is not suppressed quickly enough.A more appropriate procedure for discrete changing of the crystallization front seems to be the ultrasonic treatment of melt, in view of its ability to rapidly introduce and remove a cavitation field in the liquid pool by switching the ultrasonic source. However, the source has to be removed from the melt to be examined. Below we present the results of an experimental casting of round 125 x 1000mm ingots of alloy 1161 (2324)—an Al-Cu-Mg-Zr system. The bottom and top parts of these ingots were cast without ultrasound, and the middle part was cast with ultrasound. The casting rate was 100 mm/min. The puddle depth was 85 mm.The casting technology included a short break to lift the ultrasonic source from the liquid pool before casting the top section of the ingot. Metallographic studies revealed a refined dendritic structure in parts cast without ultrasound. The mean grain size of 50-60 pm was 2-3 times larger than the dendritic
parameter (20-25 jim) corresponding to the crystallization rate of an identical ingot.The transition from the dendritic front to the nondendritic front after the application of ultrasound occupies a layer of 1 to 2 mm depending on the distance from the ingot surface.In the ultrasonically cast section, the nondendritic grain size was 20-25 |xm, corresponding to the theoretical dendritic parameter. The transition layer from nondendritic to dendritic structure, which appears after the removal of ultrasound, was 2 mm high at one half of the ingot radius.The dendritic front also changed into a nondendritic front in 112mm diameter ingots of alloy 1960 (7055) which had a coarser dendrite grain of 0.5-1 mm than alloy 1161 (2324). These ingots were cast with an ultrasound source installed in the mold, but not powered at the beginning. The bottom part of the ingot was cast without ultrasound. The structure of a longitudinal template shown in Figure 5.3 refers to the point at which ultrasound was switched on. The transition zone to the nondendritic front (grain size « 20 pm) was found to be within 6 mm.These studies are insufficient to determine to what extent the mechanism of transition from a nondendritic front to a dendritic front (and vice versa) is determined by the motion of particles on the crystallization front and to what extent by variations in the nucleation pattern.In castings of the alloy 1161 (2324), Dobatkin and Eskin (1995) observed that a short break, associated with the removal of the ultrasonic source from the melt, caused the formation of a columnar structure on the segment of the meniscus adjacent to the mold. This pattern is characteristic of crystallization caused by a further removal of heat from the open ingot surface. This structure corresponds to the conditions for the formation of cavities. It is unusual that a nondendritic structure is formed over the region of large grains. The nondendritic grain size is 20-25 p.m, is equal to or smaller than their average size, which precludes their appearance due to their departure from the crystallization front.The presence of a nondendritic structure in a layer formed after the removal of the ultrasonic source from the liquid pool and the resumption of casting seems to be associated with the
movement of the melt in the pool once the source has been removed and filling a space over the forming cavity with molten metal from the liquid pool.This pattern implies that the volume of a pool with a large concentration of ultrasonically activated crystallization nuclei is an important factor, and that in order to obtain nondendritic grains, it is important to create an excess of crystallization nuclei even before a noticeable growth of grains and an increase in melt viscosity occurs.
