Macrosegregation Characteristics of Commercial Size Aluminum Alloy Ingot Cast by the Direct Chill Method

A metallurgical investigation of the macrosegregation occurring in commercial size direct chill cast Al-Zn-Mg-Cu alloy ingot is described. The extent of the chemical homogeneity from the surface to the center of the rectangular ingot produced by the bi-level transfer method is compared to ingot produced by the level pour method. The relative severity of macrosegregation for each major alloying element is discussed. Existing metallurgical theories of macrosegregation including interdendritic liquid flow and convection in the liquid pool are used to explain the observed variation in chemical composition. An analysis is carried out to qualitatively illustrate the importance of the volume fraction of the detached dendrites and the partition coefficient of each major alloying element contributing to the level of negative macrosegregation at the centerline of the ingot.