5th International Conference on Advances in Solidification Processes (ICASP-5) 5th International Symposium on Cutting Edge of Computer Simulation of Solidification, Casting and Refining (CSSCR-5)

Comprehensive thermomechanical casting simulations offer the opportunity to increase the dimensional accuracy of cast parts. These simulations require accurate material properties, which are delicate to measure experimentally in the mushy zone. These properties are often specified only as function of the temperature, neglecting the impact of the non-equilibrium character of the evolving microstructure. Therefore, in order to derive a more precise description of effective properties of the mushy state, a mutiscale approach is presented here. It couples quantitative multi-phase field simulations of the microstructure evolution with a homogenization technique. Effective thermo-elastic properties of the predicted directional and equiaxial 3D microstructures are derived and their impact on the thermomechanical casting simulation of an axisymmetric A356 bowl outlined. Moreover, in this paper we want to develop for each semi-solid regime, the coherent and non-coherent one, a specific constitutive law and to ensure continuity of the stress state at traction coherency and at Tsol. Both constitutive laws depend on microstructural features like fraction solid and grain size, which are extracted from the coupled multi-phase field simulation. The nonlinear behaviour of the coherent semi-solid phase on the macro-scale is described by an original single surface viscoplastic flow potential that includes micro-structural parameters and takes the internal cohesion of the solid skeleton into account. The variation of the cohesion with the fraction solid is described by a simple law and compared to Ludwig’s expression [1]. The proposed viscoplastic potential takes the effect of isotropic hardening, pressure dependence of yielding and the strength difference in tension and compression of the coherent semi-solid state into account. For the non-coherent mushy state, presenting some shear resistance, a simplified viscoplastic potential is adopted, neglecting pressure effects on the solid dendrites and describes large plastic flow under shear loading. The stress continuity condition at mechanical coherence allows us the introduction of only one additional material parameter. The developed constitutive laws are implemented in the FE program Abaqus via a dedicated CREEP user routine. Simulations of uniaxial, isothermal tension, compression or pure shear experiments permits us to identify the model parameters of an A356 aluminium alloy. [1] O. Ludwig, J.-M. Drezet, Ch. Martin and M. Suéry: „Rheological behaviour of Al-Cu alloys during solidification. Constitutive modeling, experimental identification and numerical study”, Metallur. and Mat. Trans. A, vol. 36A, pp. 1525-1535, 2005.