An experimental method for investigating phase transformations in the heat affected zone of welds using synchrotron radiation

Although welding is an established technology used in many industrial settings, it is least understand terms of the phases that actually exist, the variation of their spatial disposition with time, and the rate of transformation from one phase to another at various thermal coordinates in the vicinity of the weld. With the availability of high flux and, more recently, high brightness synchrotron x-radiation sources, a number of diffraction and spectroscopic methods have been developed for structural characterization with improved spatial and temporal resolutions to enable in-situ measurements of phases under extreme temperature, pressure and other processing conditions not readily accessible with conventional sources. This paper describes the application of spatially resolved x-ray diffraction (SRXRD) for in-situ investigations of phase transformations in the heat affected zone (HAZ) of fusion welds. Results are presented for gas tungsten (GTA) welds in commercially pure titanium that show the existence of the high temperature bcc {beta}-phase in a 3.3 {plus_minus} 0.3 mm wide HA band adjacent to the liquid weld pool. Phase concentration profiles derived from the SRXRD data further show the co-existence of both the low temperature hcp ({alpha}-phase and the {beta}-phase in the partially, transformed region of the HA. These results represent the first direct observations of solid state phase transformations and mapping of phase boundaries in fusion welds. SRXRD experiments of this type are needed as experimental input for modeling of kinetics of phase transformations and microstructural evolution under the highly non-isothermal conditions produced during welding