Time-resolved X-ray diffraction studies of solidification microstructure evolution in welding

High-brilliance and high-energy polychromatic X-ray diffraction (XRD) has been used for time-resolved studies of rapid solidification microstructure evolution in situ during simulated spot welding in austenitic steels. Weld pools of 5.0 mm diameter and 0.6-1.5 mm depth were formed at the steel plate surfaces by radiation heating from halogen lamps. Solidification was initiated by powering off the lamps, and completed within 1.5 s while measuring diffraction from the solidifying grains of the pool at a 1 kHz frame rate. The data contain time-resolved information on individual grain growth and overall solid fraction evolution, and furthermore reveal prominent individual and collective motion of grains during early stages of solidification, presumably caused by convective currents in the pool. Ultrafast polychromatic high-energy XRD is novel in studies of microstructure evolution during welding. The experimental technique could quite readily be used for similar studies in real welding.