Residual stress characterization of dissimilar Ti/Al spot welds: A comparative study using synchrotron X-ray and neutron diffraction

Abstract

Residual stress (RS) plays a critical role in the structural integrity of dissimilar titanium/aluminum (Ti/Al) spot welds, yet its reliable determination and interpretation remain challenging due to strong microstructural heterogeneity and steep stress gradients. In this study, RS distributions in Ti/Al spot welds produced by refill friction stir spot welding (refill FSSW) and spot friction melt bonding (FMB) are investigated using Neutron Diffraction (ND) and Energy-Dispersive Synchrotron X-Ray Diffraction (EDXRD). This work provides an original study on the quantification of RS using diffraction-based methods in dissimilar Ti/Al spot welds and presents a direct comparison between ND and EDXRD to assess their respective capabilities for RS determination. The results show that RS evolution is strongly dependent on the joining process. In spot FMB, RS is dominated by thermally induced stresses in the Ti, where constrained thermal contraction leads to high stress magnitudes and steep through-thickness gradients. In contrast, refill FSSW leads to a more uniform RS distribution owing to homogeneous deformation in the Al and limited influence of the process on the Ti. The comparison between ND and EDXRD reveals differences in the measured RS distributions, particularly in regions with steep gradients and pronounced microstructural heterogeneity, where spatial averaging effects influence ND measurements. In refill FSSW welds, where the microstructure is more homogeneous, both techniques show comparable RS trends. These findings provide new insights into the relationship between welding process, microstructure, and RS evolution in dissimilar Ti/Al joints.