Synchrotron EDXRD strain-temperature measurement during laser welding

Localised heat input, as it occurs in welding with moving heat sources, induces residual stresses and distortion in materials. The quantitative determination of residual stress evolution is difficult. Despite existing models, residual stress build-up with temperature progression is not fully understood. High-flux density X-rays from a synchrotron source allow the measurement of local strains in materials and improve the resolution of stress gradients as it permits small measurement volumes (Gibmeier et al., 2014). A laser beam welding process was used to perform linear bead-on-plate welds on bar steel samples. The X-ray diffraction system recorded the transient strain evolution. Multiple repetitions at different locations in the specimen were combined to develop a map of the strains present within the specimen. The temperature was measured locally at the surface of the sample. As the strain was determined within a measurement volume inside the sample, the temperature history over time had to be obtained as well. A numerical model was employed to determine the temperature inside the measurement volume. This model was calibrated using the transient surface temperatures and metallographic cross-sections. The result was a representation of the local strain superimposed on the temperature distribution. Analysis of this data correlation showed that a strain maximum occurs as a function of time and distance from the heat source, which is likely to coincide with the austenite-ferrite phase transformation temperature.