Enhanced microstructure and mechanical properties in narrow gap ring-core adjustable laser welding of titanium alloys: Insights from molten pool morphology

Ring-core adjustable laser welding is gaining increasing acceptance due to its ability to bridge gaps, modulate microstructure, and enhance the mechanical properties of welds. However, the origins of molten pool morphology in narrow gap structure and mechanisms affecting through-thickness heterogeneity in microstructure and mechanical properties is not well understood. To address this need, here we report a combined experimental and computational study to investigate the effects of groove gap and ring-core ratio on molten pool morphology in ring-core adjustable laser welding with a narrow gap structure (NGR-CALW). The molten pool flow behavior and temperature gradient distribution under varying groove gaps and ring-core ratios were thoroughly analyzed through numerical flow field simulations, elucidating the evolution mechanisms of weld morphology and microstructural characteristics. The results demonstrate that an increased groove gap enables a transition in the weld cross-sectional shape from narrow and deep to wide and shallow. Moreover, an increase in the ring-core ratio significantly widens the effective sidewall melting area, resulting in a more rounded and fuller upper weld profile. The morphological diversity of molten pool stems from differential heat dissipation dynamics and cooling gradients, consequently inducing spatially heterogeneous microstructures between concave-shaped, flat-shaped and convex-shaped molten pools. Finally, NGR-CALW with optimizing groove gap and ring-core ratio matching was found to favor rapid solidification and the development of finer solidification microstructure in joining 22 mm thick TC4 titanium alloys. This study may provide a perspective in understanding the phenomena involved in the NGR-CALW of titanium alloys with a narrow gap structure and lay the foundation for optimization of the narrow gap laser welding process.