Modeling and Experiments on Thermomechanics of DC Casting

Direct chill (DC) casting is the major technology for manufacturing wrought aluminum alloy ingots. In this semi-continuous manufacturing process, thermal strains can cause cracks during and after solidification, leading to rejection of the ingot. Numerical simulations have been applied to study factors influencing the thermal stress development. Currently, there are two major challenges in modeling the DC casting: 1. Lack of a time-dependent constitutive behavior to describe the alloy behavior under thermal and mechanical load; 2. Scarcity of thermomechanical testing data representative of materials in genuine as-cast states. In this work, a finite element model with coupled thermal-structural simulation of DC casting process was developed as a basis to study transient stress development inside the ingot. The traditional time-independent plasticity constitutive law was firstly used in the model to study the effect of wiper position and ingot geometry. In order to address the first challenge, a time-dependent stress relaxation-based constitutive equation was developed with revitalized mechanical testing methods and numerical implementation scheme. As for the second challenge, industrial AA7050 (Al – 6.2% Zn - 2.3% Cu – 2.3% Mg) was remelted and cast to reproduce specimens with DC cast ingot grain size and morphology by controlling casting conditions. Stress relaxation and flow stress tests were performed on the as-cast specimens. As a step to implement the developed stress relaxation constitutive law, simple test cases were first created to understand the different regimes in stress relaxation. After the validation using simple test cases, the stress relaxation approach was applied to the full DC casting analysis for AA7050, and the results were compared to those with plasticity approach. The two approaches agreed with each other mostly, but the stress relaxation approach was able to resolve the difference in residual stress under different cooling rates. Results showed the main distinction between the two methods is that stress keeps increasing at low temperatures (less than 100 °C) for stress relaxation approach, but is nearly a constant for plasticity approach. In addition, the current work correlated material properties to microstructure by performing mechanical testing on both as-cast and homogenized AA7050 specimens. Eutectic microconstituent was found to play an important role, and its thermal instability causes the flip of relative strength of two specimens at different temperatures. These mechanical tests provided the mechanical data for the constitutive law in the finite element model