Modelling of the thermomechanical effects during the start-up Phase for DC casting of aluminum sheet ingots

摘要

Since the commercial viability of the Direct Chill (DC) semi-continuous casting process was established in the 1930's, it has been used most extensively by the nonferrous metal industry. In the aluminum industry, DC casting is used to produce a wide range of semi-finished products, such as billets for extrusions or forgings and ingots for rolled products. The most critical phase of the DC casting process from the standpoint of ingot quality is the start-up. During start-up, the ingot is subject to varying rates of heat transfer, which can give rise to cracking or hot tearing in crack-sensitive alloys. In addition, the base of the ingot undergoes a significant deformation (referred to as "butt curl"), which can give rise to sticking and breakouts. To try and avoid cracing and excessive butt curl the casting parameters are gradually adjusted over approximately the first meter in casting length until steady state operation is achieved where they are kept constant until the end of the cast. Historically, the trend in the industry has been to optimize the various parameters involved by a process of trial-error. Recently, there has been a move to develop thermal stress models of the casting process to help elucidate the mechanisms that cause cracking in the ingot to occur. This paper outlines the development of a 3D uncoupled thermal-stress model, using the commercial finite element code, ABAQUS, to predict the temperature and stress state in the ingot during the problematic start-up phase. Predictions from the model are compared to temperatures obtained from various locations in an ingot as well as to the total displacement of the base of the ingot measured in an industrial caster.