钛合金是制造飞行器承力框梁结构的关键材料，超声辅助窄间隙激光焊接钛合金能够有效改善其组织并提高其综合性能。本工作结合温度场和流场的数值模拟结果，对比分析了超声对焊接接头不同层间区域微观组织的调控机理，并进一步探究了超声振动对不同层间力学性能的影响机制。结果表明，超声促进了温度的均匀化分布，同时提高了熔池内的流体流动速率。超声的引入会在晶界的剪切方向产生空化应力场和大量空化气泡，使得焊缝整体晶粒在尺寸细化的同时变得更加均匀，并且显著增加了焊缝区针状α'马氏体的数量，晶粒细化效果由焊缝区到热影响区显著降低。超声的施加使得各填充层显微硬度和冲击性能均有一定程度提升，且超声对焊接接头综合力学性能的提升效果具有一定的深度递减趋势。

TC4 titanium alloy is a key material for fabricating load-bearing frame beams in aircraft structures. Narrow gap laser welding offers considerable technical advantages and feasibility for joining thick TC4 titanium alloy plates. However, challenges remain because of substantial variations in the mechanical properties because different regions of the filler layer experience different thermal cycles and heat accumulations. These differences can lead to weak or defective layers, thereby compromising the overall mechanical performance of the weld. Ultrasonic vibration-assisted welding is an effective solution for improving the quality of narrow gap laser welding in thick-walled structures. This study combines the numerical simulations of the temperature and flow fields to investigate the mechanisms of microstructural regulation in different regions of the welded joint, focusing on the effect of ultrasonic vibration on mechanical properties across these areas. The results show that ultrasonic vibration produces a more uniform temperature distribution and increases the fluid flow velocity in the molten pool. Cavitation stress fields and the formation of numerous cavitation bubbles at grain boundaries in the shear direction facilitate grain refinement and promote a more homogeneous grain structure in the weld seam. In addition, the number of needle-like αʹ-martensite structures in the weld zone considerably increases. However, the grain refinement effect gradually weakens from the weld zone to the heat-affected zone. The application of ultrasonic energy improves the microhardness and impact toughness of the filler material layers. Notably, the improvement in the overall mechanical properties of the welded joint gradually declines as the depth from the weld seam increases.