Titanium and titanium alloys possess advantages such as high specific strength, heat resistance, and corrosion resistance. They have irreplaceable advantages in optimizing mechanical structures, enhancing the reliability of components, and achieving lightweightness. Titanium alloys have been widely used in the fields of aerospace, shipping, and weaponry equipment. Additionally, welding medium- and thick-plate titanium alloy structures has become a crucial process in manufacturing large-scale equipment with the expansion of defense equipment. Compared with other welding techniques, laser-arc hybrid welding (LAHW) has a high welding efficiency and excellent bridging ability. Therefore, the combination of LAHW and narrow gaps enables higher adaptability, efficiency, and quality in connecting thick-walled structures, presenting good application prospects for the welding of medium- and thick-plate titanium alloys. However, the coupling effect between the laser and arc and the spatial restraint of the narrow gap result in poor stability of the welding process, difficulty in controlling weld formation, and gas pore defects during the narrow-gap laser-arc hybrid welding of titanium alloys. An oscillating laser not only effectively enhances the stability of the welding process but also refines the weld grain and increases the joint strength. Thus, an oscillating laser has been introduced to improve welding quality. However, research till date has focused on stainless steel and aluminum alloys, and few scholars have investigated the influence of oscillating lasers on the microstructure and mechanical properties of titanium alloy welding joints. Therefore, in this study, narrow-gap TC4 welding joints with different beam modes are fabricated using LAHW technology. The microstructural characteristics of the joints are analyzed using electron backscatter diffraction (EBSD), revealing the relationship between the microstructure and joint mechanical properties. This study provides a theoretical basis for the rapid application of oscillating lasers in the laser welding of medium and thick titanium alloy plates.

TC4 titanium alloy plates with dimensions of 80 mm×80 mm×16 mm and a filler wire with a diameter of 1.2 mm are used. The laser-MIG hybrid welding (MIG, melt inert-gas) adopts the laser guided mode, and the shielding gas is high-purity argon (volume fraction 99.99%) at a gas flow rate of 50 L/min. After welding, the cross-sectional morphology of the welds is observed using a metallographic microscope. The microscopic morphology of the weld fracture is observed using a scanning electron microscope in secondary electron mode. For an in-depth understanding of the effect of an oscillating laser on the weld microstructure, the grain size and orientation are analyzed using an EBSD instrument with the Aztec Crystal 2.1 analysis software. The hardness distribution in each zone of the joint is measured using a digital microhardness tester, with a load of 200 g applied by a hardness tester indenter for a holding time of 15 s. The tensile and impact mechanical properties of the welded joints are determined using a universal testing machine.

Large-sized columnar and fine equiaxed grains are formed in the welded joints, regardless of whether the oscillating laser has been used. After the addition of the oscillating laser, the number of equiaxed grains is increased, and the size of the columnar grains is significantly decreased. EBSD characterization of the joint weld reveals that the orientation of some grains in the weld is shifted after the application of the oscillating laser. Simultaneously, the average grain size of the weld with the oscillating laser is approximately 3.73 μm, approximately 13.6% smaller than that without the addition of oscillation. Laser oscillation has a minor effect on the grain boundary distribution, and many high-angle grain boundaries are still distributed in the weld. Compared with the non-oscillating laser, the peak fraction at a grain boundary orientation difference of 64° is increased by approximately 16.3%. The mechanical properties of the joint improve with the application of the oscillating laser. The average hardness of the weld zone is increased by approximately 5.9%, and the average impact energy is increased by approximately 23%; this is mainly related to the fine-grain strengthening effect caused by laser oscillation.

Compared to conventional hybrid welding joints, the average hardness of the weld zone is increased by approximately 5.9%, and the average impact energy is increased by approximately 23% after applying a circular oscillating laser with an oscillation frequency of 300 Hz and amplitude of 1 mm. Simultaneously, the toughness of the joint is improved, and the dimple distribution in the impact fracture is uniform, dense, and free of obvious defects. In addition, the heterogeneity of the joint microstructure is improved, and the growth direction of the columnar grains is changed to 52°?64°. Simultaneously, the grain orientation of the weld seam is changed, and some hard-oriented grains are transformed into softer-oriented grains, which are more prone to slip deformation. The material properties improve because the increased proportion of large-angle grain boundaries enhances the ability to inhibit crack propagation. Fine-grain strengthening is considered to be the main reason for the improvement in the mechanical properties of the joints. The disturbance of the liquid metal in the mushy zone is enhanced by the stirring effect of the circular oscillating laser, promoting dendrite fragmentation and increasing the number of nucleation cores in the molten pool. The grains of the weld seam refine, and the martensite size is reduced by approximately 13.6% compared with that by conventional hybrid welding.