THERMAL STRESS ANALYSIS IN DIRECT CHILL CASTING OF MAGNESIUM-ALLOYS
Direct chill casting is a semi-continuous casting technique used to produce rolling ingots and extrusion billets. Cracks that develop in cast ingots or billets during and after solidification limit the productivity and challenges the continuous casting. In this work, mathematical models are presented to estimate the evolution of temperature and stress profiles in the ingot during direct chill casting of AZ31 alloy. Thermal, metallurgical and mechanical fields are modeled by solving the energy and momentum equations using Finite Element Method. The latent heat released during solidification is incorporated in the energy equation by modifying the specific heat. A 2-D computational domain that grows vertically up based on casting speed is considered as computational domain to simulate the direct chill casting process. A temperature dependent heat transfer coefficient is used to calculate heat flux boundary conditions in primary, secondary and bottom block cooling zones. An isothermal staggered approach is used, in which temperature profiles are solved first and then used as an input to the mechanical model. The simulated temperature profiles are validated by comparing the temperature measurements taken at real time magnesium casting plant trails that are reported in Hao et.al.. To find the stresses in the solid region, a Garafalo law is used to describe the material behavior. It is found that axial and circumferential stresses are tensile at the core of the billet. The axial stress at the surface is tensile and the circumferential stress is compressive.