The excellent comprehensive mechanical properties of Ti-6Al-2Zr-3Nb-Mo titanium alloy have obvious advantages in ship and marine engineering. The research on high-power fiber laser welding of Ti-6Al-2Zr-3Nb-Mo titanium alloy is still in its infancy, and it is of great theoretical and practical significances on improving the development of marine titanium alloy equipment. The research on high-power laser welding of Ti-6Al-2Zr-3Nb-Mo titanium alloy is carried out, and high power fiber laser welding can achieve good results of one-time penetration and one-side welding or two-side forming. It belongs to the leading level in China. The morphology, microstructural distribution and properties of laser welded Ti-6Al-2Zr-3Nb-Mo joints are studied in detail, which provides a theoretical basis for subsequent engineering applications.

The high-power laser welding of Ti-6Al-2Zr-3Nb-Mo titanium alloy is studied by using high-power fiber laser equipment. The welded joints with excellent quality and beauty are obtained. The residual stress on the surface of the welded joint is tested by using the XL2118A16(U) static resistance strain gauge with the blind hole method. The round bar tensile test is carried out on the STNTECH20/G material testing machine. The side bending test is carried out on the BHT5106 electro-hydraulic servo bending tester. The standard V-shaped impact test is carried out on the ZBC2302-C pendulum impact tester. The microhardness test is carried out on the automatic Vickers hardness tester with a load of 5 kg and the loading time of 30 s. The microstructure of the joint is analyzed by the OLYMPUS GX71 optical microscope. The microstructure of the joint is analyzed under the Quanta650 scanning electron microscope. The fine microstructure of the welded joint is observed by the JEM-2100 transmission electron microscope.

For the 16 mm thick Ti-6Al-2Zr-3Nb-Mo titanium alloy sheet, high power fiber laser welding can achieve good results of one-time penetration and one-side welding or two-side forming. The overall surface morphology of the welded joint is good with silver white or light yellow. And there are no pores, slag, unmelting, incomplete penetration, cracks, and other defects in the weld (Table 4). The microstructure of the base metal is a typical α and β duplex structure, which is composed of primary α phase and lamellar α transformed β phase. The microstructural distribution is uniform (Figs. 5-7). The weld has a columnar crystal structure. The coarse original grain boundary of β phase is retained. The internal metallographic structure is composed of acicular martensite α′ phase and a small amount of intergranular α phase. The acicular martensite α′ phase is cluster-like and arranged in a certain direction. The microstructure is fine and dense. The spacing is small, which is characterized by "Widmanst?tten structure" or "basketweave" (Figs. 8-10). The metallographic structure of the heat affected zone is mainly composed of coarse block α and lamellar α transformed β phase. The lamellar structure is composed of needle-like α phase, lamellar β phase, and a small amount of needle-like martensite α′ phase. The lamellar structure is cut and interlaced with each other, which shows the characteristic of "basketweave" (Figs. 11 and 12). The residual stress of the welded joint is far lower than the yield strength of the material, mainly concentrated in the range of 5 mm near the weld and the heat affected zone. Its distribution characteristics are in line with the characteristics of high energy density and low deformation of laser welding (Fig. 14). The welded joint has stable mechanical properties, ductile fracture, and tensile strength of up to 936 MPa, higher than that of the base metal (Table 4 and Fig. 15). The microhardness of the weld root is slightly low, and those at other positions are almost the same. There is little difference in room temperature impact properties (the maximum difference of 6 J), where the weld impact energy absorption followed by that of the heat affected zone is the highest, and that of the base metal is the lowest (Table 5 and Fig. 19).

The laser welding of Ti-6Al-2Zr-3Nb-Mo alloy is studied. The single-side penetration welding and double-side forming welding process of 16 mm thick Ti-6Al-2Zr-3Nb-Mo alloy test plate is obtained. The microstructure, mechanical properties and process properties of the laser welded joint of Ti-6Al-2Zr-3Nb-Mo alloy are tested. The results show that the surface of laser welded joint of Ti-6Al-2Zr-3Nb-Mo alloy is well formed. The surface is silver white or light yellow, and there is no defects in the weld. The weld seam has a columnar crystal structure, and the original grain boundary of the coarse β phase is retained. The internal metallographic structure is composed of the acicular martensite α′ phase transformed by the whole lamellar β phase and a small amount of intercrystalline α phase. The acicular martensite α′ phase is arranged in a certain direction and has a cluster shape. The microstructure is fine and the spacing is small. The microstructure of the heat affected zone is mainly composed of coarse block α and lamellar α transformed β phase. The lamellar microstructure is composed of needle-like α phase, lamellar β phase, and a small amount of needle-like martensite α′ phase. The lamellar microstructure is cut and crisscrossed with each other. The residual stress distribution of welded joints is mainly concentrated in the weld and the heat affected zone. The mechanical properties of welded joints are stable, and the tensile strength is higher than that of the base metal. The tensile fractures are ductile rupture. The impact absorbing energy of the weld and the heat affected zone are higher than that of the base metal. High power laser welding of titanium alloy has a good application prospect.