| Unlike the traditional solid/fluid interface concept, in our model the material is treated as a continuum of multiple phases with changing volume fractions of solid and fluid. The energy equation is coupled to the equilibrium equations through various mechanical work terms. The fluid is treated as thermoviscous and incompressible. The solid is thermoviscoplastic described by Bodner-Partom/Walker type of constitutive equations. Traction free boundary condition is applied on the boundary of solid while surface tension boundary condition is applied on the molten surface. On the fusion line, a moving Gaussian heat source simulates the welding. The weldment is subjected to convection and radiation boundary conditions. The microstructure due to phase transformation is handled by tracking the transient percentage of transformation from one constituent to another. It is determined by a modified Avrami equation and CCT diagram. Implementation of the mathematical model was by ALEFEM (Arbitrary Lagrangian and Eulerian Finite Element Method).;A plane perpendicular to the welding line was chosen for analysis for the solid model and the solid-fluid model. The temperature, stress distributions, molten pool zone, plastic zone, plastic work, phase distributions, molten flow motion and grain growth orientation were displayed at various stages of welding. The influence of the change of direction of the circulatory molten flow on the residual stress was investigated. It was found that the positive surface tension temperature coefficient will result in a downward circulation flow along the seam, deeper penetration and lower residual tensile stress. Under the usual conditions of air cooling there would not be any martensitic transformation for weldable steel. Two computations were made for a fast cooling rate, one with phase transformation and the other without. Results show that the residual stress in the former was still higher than the case of slow cooling, even if the martensitic transformation induced some compressive stress. The residual stress has a tensile maximum near the solidified boundary, and can be reduced considerably when preheating together with postwelding annealing process was used. |
