Physical modeling of the effect of refilling the melt into an ingot knock-off head on solidification and structure formation

The paper presents the results of a laboratory study of the effect of refilling the ingot knock-off head with melt in a certain time interval after pouring the ingot body on solidification and structure formation of the model ingot. The research was carried out by the method of physical (cold) modeling for which a laboratory installation (casting form-mold) was developed and manufactured. It allows visually studying the processes occurring during solidification and structure formation on a 19.6-ton model ingot. We used sodium sulfuric acid (crystalline hyposulfite) as a modeling solution. Correspondence of the processes occurring on the model and in real conditions of industrial ingots casting was evaluated using similarity criteria obtained on the basis of dimension theory with analysis of physico-chemical processes occurring during casting and crystallization of the ingot. Casting of the melt into the casting form-mold was downhill. In order to assess changes in the temperature field during casting and crystallization of the ingot in the entire solidification time, we performed thermometry of the mold model surface. Analysis of the conducted studies results showed that refilling the melt before 40 min leads to stimulation of early settling of crystals (“rain of crystals”), which contributes to an increase in the crystallization directivity in vertical direction. It was established that in a conventional ingot up to 40 min solidification proceeds by a sequential mechanism, and after that the crystals begin to settle (“rain of crystals”) and the solidification of the ingot passes through a volume-sequential mechanism. Refilling the ingot knock-off head with melt 40 min after pouring the ingot body contributed to the continuation of the sequential mechanism of ingot solidification, which led to the formation of a monolithic defect-free structure in the ingot body and the least development of shrinkage shell in the knock-off head. The results obtained make it possible to develop a technology for differentiated ingots casting when filling their knock-off heads with melt in a certain time interval after pouring the ingot body, which will affect the process of metal structure formation and reduce defective zones.