Time resolved X-ray diffraction observations of phase transformations in transient arc welds

In situ X-ray diffraction methods have been developed at Lawrence Livermore National Laboratory for direct observation of microstructural evolution under quasi-steady state and transient welding conditions. Using intense highly collimated synchrotron radiation, the crystal structures in the weld heat affected and fusion zones are probed in real time to monitor solidification and solid state phase transformations during welding. Here the authors review recent work on the development and use of the time resolved X-ray diffraction (TRXRD) technique during transient welding and illustrate its unique capabilities to: directly observe the solidification mode; discover, in real time, the definitive sequence of phase transformations that lead to the final microstructure; and provide quantitative kinetic data of phase transformations through synthesis of TRXRD data with the temperature history obtained through heat transfer modelling. The TRXRD technique has been used to investigate welding induced phase transformations in titanium alloys, low alloy steels, and stainless steel alloys. The results show some of the first real time observations of the weld solidification mode and the evolution of equilibrium and non-equilibrium phases during rapid heating and cooling. When combined with numerical modelling, quantitative phase transformation kinetic data are obtained, allowing for its use in a wide variety of isothermal and non-isothermal processing. The potential for future applications of this and similar techniques is also addressed.