Evolution of titanium arc weldment macro and microstructures - Modeling and real time mapping of phases

Macro and microstructural features in gas tungsten are (GTA) welded titaniu m were modeled for the first time based on a combination of transport pheno mena and phase transformation theory. A transient, three-dimensional, turbu lent heat transfer and fluid flow model was developed to calculate the temp erature and velocity fields, thermal cycles, and the shape and size of the fusion zone. The kinetics of the alpha-->beta allotropic transformation dur ing continuous heating and the corresponding (alpha+beta)/beta phase bounda ry were calculated using a modified Johnson-Mehl-Avrami (JMA) equation and the calculated thermal cycles. The modeling results were compared with the real-time phase mapping data obtained using a unique spatially resolved X-r ay diffraction technique with synchrotron radiation. The real-time evolutio n of grain structure within the entire weld heat-affected zone (HAZ) was mo deled in three dimensions using a Monte Carlo technique. The following are the major findings. First, the rates of heat transfer and fluid flow in the titanium weld pool during gas tungsten are welding (GTAW ) are significantly enhanced by turbulence, and previous calculations of la minar fluid flow and heat transfer in are-melted pools need to be re-examin ed. The fusion zone geometry, and the alpha/(alpha+beta) and (alpha+beta)/b eta phase boundaries in the HAZ could be satisfactorily predicted. Second, comparison of real-lime alpha-->beta transformation kinetics with the rates computed assuming various alternative reaction mechanisms indicates the tr ansition was most likely controlled by the transport of Ti atoms across the alp interface. Third, comparison of the experimental data with the simulat ed results indicates the real-rime evolution of the grain structure around the weld pool could be simulated by the Monte Carlo technique. Finally, the in sight developed in this research could not have been achieved without c oncomitant modeling and experiments.