Prediction of Hot Tear Formation in Vertical DC Casting of Aluminum Billets Using a Granular Approach

Abstract

A coupled hydro-mechanical granular model aimed at predicting hot tear formation and stress-strain behavior in metallic alloys during solidification is applied to the semi-continuous direct chill casting of aluminum alloy round billets. This granular model consists of four separate 3D modules: (i) a solidification module that is used for generating the solid-liquid geometry at a given solid fraction; (ii) a fluid flow module that is used to calculate the solidification shrinkage and deformation-induced pressure drop within the intergranular liquid; (iii) a semi-solid deformation module that is based on a combined finite element / discrete element method and simulates the rheological behavior of the granular structure; and (iv) a failure module that simulates crack initiation and propagation. To investigate hot tearing, the granular model has been applied to a representative volume within the direct chill cast billet that is located at the bottom of the liquid sump, and reveals that semi-solid deformations imposed on the mushy zone open the liquid channels due to localization of the deformation at grains boundaries. At a low casting speed, only individual pores are able to form in the widest channels since liquid feeding remains efficient. However, as the casting speed increases, the flow of liquid required to compensate for solidification shrinkage also increases and as a result the pores propagate and coalesce to form a centerline crack.