Macrosegregation During Direct-Chill Casting Of Al-4.5%Cu Alloy

Macrosegregation is a common and serious detrimental defect during DC Casting of aluminum alloys. It is also irreversible defect and will always remain in the follow-up products, which results in performance differences and the impact on final product quality. Therefore, It is of significance for aluminum alloys to improve the quality and productivity by controlling the macrosegregation during DC casting.In this paper, Al-4.5%Cu alloys are studied, a coupled mathematic model of the interaction of multi-physical fields, which applies to both the conventional DC casting (DC) and low frequency electromagnetic casting (LFEC) processes, has been built firstly in this paper. This model is good at describing electromagnetic field, fluid flow, heat transfer, solidification and stress-strain fields in casting process. It uses the commercial finite element package ANSYS to calculate the electromagnetic field, and the commercial finite volume package FLUENT to calculate the magnetic driven fluid flow, heat transfer, solidification, solute transfer and macrosegregation. By using this model to simulate the DC casting and LFEC processes of O200mm billets of Al-4.5%Cu alloys, the obtained electromagnetic flux density, cooling temperature curve and distributions of Cu element fall good agreements with the measured results.Through observations the microstructures of the DC and LFEC billets, it is found that the microstructure consists of aluminum matrix and eutectic structure with α-Al and0phase (A12Cu).after applying low frequency electromagnetic field, the solidification structure of the billets were not change, but the eutectic structure was refined and uniform distribution.In this paper, the effects of model parameters, casting process parameters and electromagnetic field on the macrosegregation in billets are studied in detail from solidification shrinkage, float crystal and forced convection by the numerical simulation. The results show as follows:the macrosegregation in the billets is aggravated with the packing fraction and permeability increase; the macrosegregation in the billets is mitigated with the casting velocity reducing, cooling intension increasing and pouring temperature lowering; the central negative segregation is mitigated and the sub-surface positive segregation is aggravated by applying electromagnetic field; the central negative segregation is invariable and the sub-surface positive segregation is aggravated when the electromagnetic field frequency increases; both the central negative segregation and the sub-surface positive segregation are mitigated when the electromagnetic field intension increases.This study shows that reducing casting speed, increasing the cooling intensity, lowering pouring temperature and applying low-frequency high-strength electromagnetic fields can mitigate ingot macrosegregation in the billets, which provide a theoretical basis and technical support for the industrial production.