The Kinetics of Phase Transformation in Welds (2002)

The fundamentals of welding-induced phase transformations in metals and alloys are being investigated using a combination of advanced synchrotron based experimental methods and modem computational science tools. In-situ experimental methods have been developed using a spatially resolved x-ray probe to enable direct observations of phase transformations under the real non- isothermal conditions experienced during welding. These experimental techniques represent a major step forward in the understanding of phase transformations that occur during welding, and are now being used to aid in the development of models to predict microstructural evolution under the severe temperature gradients, high peak temperatures and rapid thermal fluctuations characteristic of welds. Titanium alloys, stainless steels and plain carbon steels are currently under investigation, and the phase transformation data being obtained here cannot be predicted or measured using conventional metallurgical approaches. Two principal synchrotron-based techniques have been developed and refined for in-situ investigations of phase transformation dynamics in the heat-affected zone (HAZ) and fusion zone (FZ) of welds: Spatially Resolved X-Ray Diffraction (SRXRD) and Time Resolved X-Ray Diffraction (TRXRD). Both techniques provide real-time observations of phases that exist during welding, and both have been developed at the Stanford Synchrotron Radiation Laboratory (SSRL) using a high flux wiggler beam line. The SRXRD technique enables direct observations of the phases existing in the HAZ of quasi-stationary moving arc welds, and is used to map the HAZ phases by sequentially jogging the weld with respect to the x-ray beam while taking x-ray diffraction (XRD) patterns at each new location. These spatially resolved XRD patterns are collected in linear traverses perpendicular to the direction of weld travel. The XRD data contained in multiple traverses is later compiled to produce an areal map of the phases that existed in the HAZ during welding. The TRXRD technique uses an x-ray beam positioned at one location, where XRD patterns are collected as a stationary ''spot'' weld rapidly heats and cools the base metal. Data can be collected from either the HAZ to observe solid-state phase transformations or from the FZ to observe solidification and subsequent solid-state phase transformations. Higher heating and cooling rates are produced in the spot welds than in the moving welds, requiring time resolution in the milli-second regime for TRXRD to record the rapid microstructural changes that occur. Numerical weld modeling is being performed to: (1) extract the important phase transformation kinetic information from the experimental data, (2) investigate mechanisms of phase transformations under non-isothermal welding conditions, and (3) develop models for predicting weld microstructural evolution. The core of our numerical weld modeling program is a 3-dimensional coupled heat and fluid flow code developed by the Penn State University. Additional modeling is being performed at Oak Ridge National Laboratory where Thermocalc is being used to understand phase equilibria in complex alloy systems, and where a diffusion-based model considering paraequilibrium is being developed to predict phase transformations in steels.