Macro and microstructural features in gas tungsten arc (GTA) welded titanium were modeled for the first time based on a combination of transport phenomena and phase transformation theory. A transient, three-dimensional, turbulent heat transfer and fluid flow model was developed to calculate the temperature and velocity fields, thermal cycles, and the shape and size of the fusion zone. The kinetics of the α→β allotropic transformation during continuous heating and the corresponding (α+β)/β phase boundary 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-ray diffraction technique with synchrotron radiation. The real-time evolution of grain structure within the entire weld heat-affected zone (HAZ) was modeled 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 arc welding (GTAW) are significantly enhanced by turbulence, and previous calculations of laminar fluid flow and heat transfer in arc-melted pools need to be re-examined. The fusion zone geometry, and the α/(α+β) and (α+β)/β phase boundaries in the HAZ could be satisfactorily predicted. Second, comparison of real-time α→β transformation kinetics with the rates computed assuming various alternative reaction mechanisms indicates the transition was most likely controlled by the transport of Ti atoms across the α/β interface. Third, comparison of the experimental data with the simulated results indicates the real-time evolution of the grain structure around the weld pool could be simulated by the Monte Carlo technique. Finally, the insight developed in this research could not have been achieved without concomitant modeling and experiments.
|Original language||English (US)|
|Journal||Welding Journal (Miami, Fla)|
|State||Published - Apr 2000|
All Science Journal Classification (ASJC) codes
- Mechanics of Materials
- Mechanical Engineering
- Metals and Alloys