Stress field and cracking tendency of AA7050 alloy during direct-chill casting

AA7050 alloy has a very high hot tearing and cold cracking tendency during direct-chill casting, which can affect its product quality and productivity, even cause industrial accidents. The trial and error method is often used in a foundry factory to find the optimum process parameters. However, this kind of approach is very costly and inefficient. Numerical modeling can reproduce the evolution of multi-physical fields in direct-chill casting, and it has been proved to be an indispensable tool in improving the casting process of aluminum alloys. In this paper, a direct-chill casting process of AA7050 alloy was simulated through direct coupling of thermal, fluid flow and stress fields. Simulation results showed that in the mushy zone the largest stress and strain components could be observed in the width direction, especially at the start-up phase. As a result, hot tearing perpendicular to the width direction was most likely to occur at this stage. The formation of cold cracking was closely related to stress concentration in the ingot, and the highest cold cracking tendency can be seen when the ingot was cooled to 200℃. According to the position of the cold crack in the ingot and the critical crack size needed, a speculation could be made that the cold crack was possibly caused by the further propagation of hot tearing at lower temperatures.