Titanium alloys are widely used in aerospace, automotive manufacturing, and marine engineering because of their high strength, low density, and excellent corrosion resistance. Laser welding, owing to its high energy density, high precision, and small heat-affected zone, provides robust support for connecting critical titanium-alloy components in industrial applications. However, during the laser welding of titanium alloys, the involved high-density Gaussian laser energy can generate defects easily, e.g., pores in the weld seam, thus adversely affecting the performance of the welded joint. In laser manufacturing, the emergence of novel lasers offers new possibilities for suppressing such defects. Fiber-diode laser hybrid welding enables the efficient and high-quality welding of highly reflective materials and has garnered widespread attention from academia and industry. Nevertheless, the effects of diode laser power and welding speed on titanium-alloy welding properties in fiber-diode laser hybrid welding remain unclear, thereby hindering theoretical guidance and process optimization for industrial applications. Therefore, this study investigates the effects of diode laser power and welding speed on the formation, microstructure, and mechanical properties of joints realized via fiber-diode laser hybrid welding to determine the optimal process window.

In this study, the effects of diode laser power (1.0, 1.5, and 3.0 kW) and welding speed (30, 60, and 120 mm/s) on the formation, microstructure, and mechanical properties of 4 mm thick TC4 titanium-alloy joints welded via fiber-diode laser hybrid welding were investigated. Welding experiments were performed using a fiber laser with a wavelength of 1080 nm, a core diameter of 34 μm, and a spot diameter of 45 μm, and a diode laser with a wavelength of 915 nm, a core diameter of 600 μm, and a spot diameter of 1.2 mm. High-purity (volume fraction of 99.99%) Ar gas flowed at a rate of 30 L/min was used as the shielding gas during welding. The experimental parameters are listed in Table 2.

Figure 2 shows the macrosectional formations of the welds at different diode laser powers. As the diode laser power increases, the weld morphology transforms from a “Y” shape to a “goblet” shape, with the upper weld width increasing by approximately 60% (from 1.8 mm to 2.9 mm). Simultaneously, the width of the heat-affected zone increases, whereas that of the lower weld remains relatively unchanged. This indicates that the diode laser energy preferentially conducts heat laterally, thereby modulating the transient thermal convection within the molten pool. As depicted in Fig. 3, the grains in the hybrid-laser action zone are much coarser compared with those in the single-laser action zone, which is attributable to the higher heat input. In terms of the mechanical properties, as shown in Fig. 4, the tensile strength decreases with increasing diode laser power (from 918 MPa to 907 MPa), whereas the elongation after fracture remains relatively constant (average of approximately 6%), with fracture occurring in the weld seam. The fracture morphology shows an increase in the number of keyhole-type pores in the weld as the diode laser power increases. This suggests that excessive diode laser power destabilizes the keyhole, thus deteriorating the mechanical properties of the joint.

Figure 6 shows the macrosectional formations of the welds at different welding speeds. As the welding speed increases, the weld formation changes from an “X” shape to a “Y” shape, and the width of the heat-affected zone decreases due to reduced heat input. As shown in Fig. 7, the grain size decreases with increasing welding speed (from 2.99 μm to 2.87 μm). This reduction in grain size causes an increase in the number of grains and thus an increase in the proportion of high-angle grain boundaries (from 80.5% to 84.0%). In terms of the mechanical properties, as illustrated in Fig. 10, both the tensile strength and elongation after the fracture of the joints first increase and then decrease with increasing welding speed, with the maximum tensile strength reaching 927 MPa and the elongation being 9%. Fractures occur in the base material. As shown in Fig. 11, an increase in the welding speed reduces the heat input, refines the grain size, and enhances the mechanical properties of the joint. However, when the welding speed further increases to 120 mm/s, the solidification rate of the molten pool decreases significantly, thus preventing bubbles from escaping and deteriorating the mechanical properties of the joint.

During laser hybrid welding, the energy of the diode laser tends to propagate laterally, which is conducive to stabilizing the keyhole and reducing the porosity in the weld seam. However, when the power of the diode laser exceeds a certain range, an increase in the laser power can paradoxically reduce the stability of the optically induced keyhole, thus deteriorating the mechanical properties of the joint. As the welding speed increases, the heat input decreases, thus refining the grain size considerably and increasing the number of high-angle grain boundaries. Nevertheless, the values of the mechanical properties first increase and then decrease as the welding speed increases. This is because, when the welding speed exceeds a certain threshold, the solidification time of the molten pool shortens, thus preventing bubbles from escaping in time. Consequently, the porosity of the weld seam increases and the mechanical properties of the joint deteriorates.