Three dimensional Cellular Automaton – Finite Element (CAFE) modeling for the grain structures development in Gas Tungsten / Metal Arc Welding processes

Abstract : Grain structure formation during fusion welding processes has a significant impact on the mechanical strength of the joint. Defects such as hot cracking are also linked to the crystallographic texture formed during the solidification step. Direct simulation of three-dimensional (3D) grain structure at industrial scale for welding processes is rarely modeled. In this work, a 3D coupled Cellular Automaton (CA) – Finite Element (FE) model is proposed to predict the grain structure formation during multiple passes Gas Tungsten Arc Welding (GTAW) and Gas Metal Arc Welding (GMAW). At the macroscopic scale, the FE model solves the mass, energy and momentum conservation equations for the whole system based on an adaptive mesh. For GMAW with metal addition, the FE model is enriched and established in a level set (LS) approach in order to model the evolution of the metal/air interface due to the weld bead development. The FE domain then contains the workpiece and the surrounding air where the weld bead forms. FE computations are coupled with the CA approach used to model the grain structure evolution. A fixed mesh, referred to as CA mesh, is superimposed to the adaptive FE mesh. FE fields are interpolated between the adaptive FE mesh and the fixed CA mesh. A new dynamic allocation/deallocation strategy of a CA grid of cells is then used based on the dynamic activation/deactivation of the elements of the CA mesh. The CA grid is made of a regular lattice of cubic cells superimposed onto the welded domain. At the micro scale, this grid is used in order to simulate the melting and solidification steps at the boundaries between the mushy domain and the liquid pool during the welding process. The temperature evolutions of the cells are computed by interpolation from the CA mesh. Coupling with tabulated transformation paths and phase enthalpy is also implemented, which permits to track the phase amount and latent heat release during the process. In order to master the resolution time and memory cost of the simulations, a management of the FE/CA mesh dimensions and CA cell size is considered for both FE and CA models. The 3D CAFE model is applied to simulate the formation of solidification structures during multiple passes GTAW and GMAW processes on a duplex stainless steel UR 2202. In GTAW, the evolution of the grain structures with respect to the welding process parameters is considered. The normal orientation of the grains with the increase of the heat source velocity is shown. In GMAW, the model is shown to compute the remelting and growth of successively deposited layers. Overall, the predicted structures qualitatively reveal the expected evolutions presented in the literature.
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Shijia Chen. Three dimensional Cellular Automaton – Finite Element (CAFE) modeling for the grain structures development in Gas Tungsten / Metal Arc Welding processes. Materials. Ecole Nationale Supérieure des Mines de Paris, 2014. English. ⟨NNT : 2014ENMP0010⟩. ⟨pastel-01038028⟩

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