Abstract:The microstructural evolution during the laser welding and subsequent post-weld heat treatment processes of laser welded TB9 joints was investigated. Results show that, during the laser welding process, the average size of β grains in the fusion zone increases with the increase in laser power. During the aging treatment, the size of the α phase increases with the increase in temperature. Concurrently, the quantity of the α phase decreases. The size of the α phase also increases with the prolongation of holding time. Meanwhile, the morphology of α phase transforms from a needle-like structure to an elliptical one. After the heat treatments, the precipitation-free zones (PFZs) are observed in the fusion zone, heat-affected zone (HAZ), and base metal of the welded joint. The formation of PFZs is due to the inhomogeneous precipitation and growth of α phase. PFZs exist between the dendrite arms in the fusion zone, near the grain boundaries in HAZ, and near the low angle grain boundaries (LAGBs) and grain boundaries of the base metal. In the fusion zone, the formation of PFZs is due to the enrichment of element Cr between dendrite arms. In HAZ and base metal, the formation of PFZs is attributed to vacancy depletion around grain boundaries as well as LAGBs.