A 10 mm narrow gap TC4 titanium alloy welded joint by laser welding with filler wire was obtained by different welding heat inputs. The microstructures of the welded joint were analyzed by OM, SEM, XRD and EBSD. The mechanical properties of the welded joint were analyzed by microhardness test and tensile test. The results show that with the increase of laser welding heat input, the average grain size and width of the equiaxed crystal zone in the center of the weld increases, the grain angle in the columnar crystal zone gradually tends to be perpendicular to the center of the weld, and the width of the heat affected zone (HAZ) increases without obvious grain coarsening. The HAZ of the welded joint is softened, and its microhardness value is lower than that of the weld and the base metal. The tensile strength of the welded joint is slightly higher than those of the base metal, but the elongation after fracture is less than that of the base metal, and it decreases with the increase of laser welding heat input.

Through in situ observation at high temperature confocal laser scanning microscope, the microstructures and phase transformation of Ti-6Al-4V titanium alloy welded joint by laser welding with filler wire during heating, heat preservation and cooling process in simulating welding thermal cycle were studied. When the weld microstructure was heated to 380 °C, obvious grain growth was observed. After heating to 1 050 °C, weld microstructures were mostly composed of coarse β-columnar grains. During the heat preservation process of the weld metal, it could be found that the β phase boundary moved slowly. In the cooling process of weld microstructures, with the increase of the cooling rate, the transition temperature for new phase precipitation decreased gradually, and that for ending the precipitation increased gradually. The microstructures after cooling to room temperature at a rate of 0.5 °C/s and 5 °C s⁻¹ were composed of α phases. At 20 °C s⁻¹, acicular α′ martensite appeared in the microstructures. At 80 °C s⁻¹, precipitated phases were all composed of acicular α′ martensite.The Ti-6Al-4V titanium alloy weld microstructure by laser welding with filler wire is completely hexagonal close packed (HCP) crystal structure.The hardness of the weld microstructures was improved with the cooling rate getting faster.

A 20 mm thick TC4 titanium alloy plate was welded by ultranarrow gap laser welding with filler wire with Ar and He as shielding gas, respectively. A characterization analysis of the microstructures and mechanical properties of the welded joint was conducted with OM, SEM, XRD, and EBSD and through the microhardness test and tensile test. The results showed that HAZ of the welded joint formed with Ar as shielding gas was much wider than that with He, and weld microstructure composition with the two shielding gases was basically consistent; phase boundary of the weld metal obtained with Ar was clearer, with a larger misorientation between the laths; α′ martensite lath in weld metal prepared with He showed obvious preferred orientation distribution, and α′ martensite microstructure was much finer; the misorientation of α′ phase grain boundary of weld microstructure prepared with Ar was slightly less distributed in high angle grain boundary than that with He; tensile property of the welded joint prepared with He was better than that with Ar; the hardness of each zone of welded joint prepared with He was less fluctuated and the hardness value measured was slightly higher than that with Ar.