Effect of Surface Tension Anisotropy and Welding Parameters on Initial Instability Dynamics During Solidification: A Pha
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WELDING is an important joining method that is widely used in many fields of the manufacturing industry. The mechanical properties of weld joints depend on the microstructures which result from the weld solidification, which is a history-sensitive process.[1] Investigating the solidification dynamics can provide a theoretical basis for optimizing the welding process. In the past few decades, the development of computational materials science has advanced the understanding of solidification behavior during welding.[2,3] However, it is fundamentally challenging to integrate the whole welding process into one computational model in part due to the complex transient conditions in a molten pool.[4] As a compromise, most researchers begin microstructural simulations from the beginning of cellular crystal growth, regarding the simulated domain boundary as one simple solid layer and artificially seeding the nucleation.[5] These studies have typically ignored the establishment of the solute boundary layer during the initial stage of nucleation and growth, which affects the final microstructures significantly.[6] Moreover, the planar instability in the early stage could determine the
FENGYI YU and YANHONG WEI are with the College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China. Contact e-mial: [email protected] Manuscript submitted November 27, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
solidification structures in the late stages.[7] Therefore, it is important to investigate the initial instability dynamics of the solidification process in a molten pool. Theoretical modeling and numerical studies have been carried out to predict the initial instability in the past few decades. The theoretical analysis was developed from the constitutional undercooling[8] theory, the Mullins and Sekerka (MS)[9] theory, and the Warren and Langer (WL)[10] theory. The WL theory provides a method to predict the incubation time and the perturbation frequency of the initial instability. The simulation results from the WL model agreed well with the experimental observations of real-time synchrotron X-ray radiography,[11] but the detailed solid/liquid (S/L) interface morphologies could not be directly obtained. Wang et al.[12] developed an analytic model, based on the WL theory, which combines the Fourier synthesis method and the time-dependent linear stability analysis to predict the interface structures. The model was verified in the steady-state condition of directional solidification.[13] The steady-state model was extended to nonsteady-state by Dong el al.[14] The new model has been used in the transient conditions found in a gas tungsten arc welding (GTAW) process.[15] While there has been significant research into making the initial instability predictions, the effect of surface tension anisotropy and welding parameters on the initial instability dynamics have not been considered. The surface tension anisotropy and welding parameters have enormous impacts on the S/L interface morphology ev
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