Modeling weldment macro- and microstructure from fundamentals of transport phenomena and phase transformation theory.

摘要

The weldment macro and microstructures were modeled based on the fundamentals of transport phenomena and phase transformation theory. A three dimensional (3D) turbulent heat transfer and fluid flow model was developed to calculate temperature and velocity fields, the weld geometry, and the thermal cycles in the weldment. The phase transformations during welding were calculated by coupling available phase transformation models with the calculated thermal cycles. A 3D Monte Carlo (MC) based grain growth simulation model was developed and coupled with the calculated thermal cycles to simulate the grain growth in the entire heat affected zone (HAZ).; To check the capabilities of the developed models, several cases were studied, which included (1) weld metal macro and microstructures of HSLA-100 steel using gas-metal-arc (GMA) welding, (2) weld metal microstructure of C-Mn steel produced by gas-tungsten-arc (GTA) welding, and (3) macro and microstructures in the GTA welded commercially pure titanium weldment. The calculated results were compared with the experimental data.; Through the present study, it was found that the computed values of turbulent viscosity and thermal conductivity were much higher than the corresponding molecular values, which indicated that the transport of heat and momentum in the weld pool was significantly aided by turbulence. The weld geometry from both GMA welding and GTA welding can be satisfactorily predicted from the present model. Specially, the “Finger penetration”, a unique weld geometric feature in GMA welding, was well predicted. The predicted cooling rates agreed well with the corresponding experimental data. The phase volume fractions in the steel welds can be well predicted by coupling the calculated cooling rates with the continuous-cooling-transformation (CCT) diagrams from an available phase transformation model. The predicted spatial phase distribution in the commercially pure titanium weldments was found in good agreement with the real-time experimental results. The salient features of the grain growth in the entire HAZ has been effectively simulated by coupling the 3D MC model with the 3D thermal model. The capability of the 3D MC model to quantitatively predict the spatial distribution of grain size in the HAZ of commercially pure titanium has been tested for different welding conditions. The agreement between the calculated and experimental results in this thesis indicates significant promise for modeling weldment macro and microstructure from fundamentals of transport phenomena and phase transformation theory.