*3.2. Microstructure Characterization*

Figure 4 shows the optical micrographs of the cross-sections of W1 to W9 laser welded joints. The joints exhibit slight defect of undercut. The undercut with a maximum depth (≈0.165 mm) was occurred in W9 sample, and the undercut imperfection is 0.055t (t is the thickness of the alloy plate), which is lower than the limit of for quality C level (0.1t) according to ISO 5817 standard. Thus, the laser-welded TC31 joints reach the weld quality C level (medium quality requirements). In addition, no misalignment defect was observed in these joints. Three distinct regions in the joints can be observed, i.e., the fusion zone (FZ), the heat-affect zone (HAZ) and the base material (BM). The FZ mainly consists of coarse prior β columnar grains. There are a few equiaxed grains at the junction of the FZ and the HAZ. The columnar grains in the middle of cross section grow horizontally and towards the weld centerline, while the columnar grains closed to the weld front and back surfaces grow from the weld fusion line to the weld surface. Figure 5 shows the magnification view of the fusion zone and heat-affected zone of the W5 joint. It can be seen that there is a large amount of staggered needle-shaped α phases inside the columnar crystal, which is the typical structure of martensite [28].

The phases in the FZ of the welded joints are examined by XRD. The examination is carried out on W2. As shown in Figure 6, the XRD pattern consists of the peaks corresponding to α -Ti phase, and no peak corresponding to β-Ti phase is observed. This result implies that the major phase in the weld is α -Ti phase. For further analyzing the microstructure of the joints, EPMA analysis is also conducted on the weld of W2. The backscattered electron image of the cross-section of the joint is shown in Figure 7. It is seen that the laser welded joint is free from obvious defects, such as voids and cracks. The BM consists of elongated α grain and intergranular β grain (Figure 7a). A large amount of acicular Ti and columnar prior β grain boundaries can be observed in the FZ as shown in Figure 7b,c, which implies that the prior β phase gradually transform into the secondary α phase [1,29].

**Figure 4.** Optical micrographs of cross sections of as the laser-welded TC31 joints: (**a**–**i**) represent W1 to W9 samples.

**Figure 5.** Magnification view of fusion zone and heat-affected zone of W5 joint: (**a**) fusion zone and heat-affected zone; (**b**) high-magnification scanning electron microscope image of the rectangular region in (**a**); (**c**) high-magnification scanning electron microscope image of (**b**).

**Figure 6.** X-ray diffraction pattern of W2 weld.

**Figure 7.** Backscattered electron images of the cross-section of W2 joint: (**a**) Base material (BM); (**b**) Heat-affect zone/Fusion zone (HAZ/FZ) transition zone; (**c**) Fusion zone (FZ).

The formation mechanism of the joint is discussed as follows. The microstructural transformation of the welds for TC31 laser-welded joints are influenced by the welding thermal cycle and the ingredient distribution of the bonding zone [13]. As illustrated in Figure 4, the size of prior grain boundaries significantly increased. This may be due to the growth of grain is sensitive to overheating. During the laser welding process, the unmelt BM closed to the bottom of welding pool and the top of heat-affecting zone is under the condition of ultra-high temperature reaching or exceeding the liquidus temperature of phase. The grain nucleates at the surface of partially molten base material, and then grow quickly towards the center of the weld. With the growth of columnar grain, the temperature gradient of the melt gradually flattens, and the solute concentration in melt increases, resulting in the constitutional undercooling in the front of the solid-liquid interface increasing and the equiaxed crystals forming

in the center of the weld. Due to the solidification behavior of weld center occur during the final stage of solidification, the growth of the equiaxed crystals is limited by the solidified columnar grains. Moreover, the columnar grains have an insulating effect on the equiaxed grains, which results in the coarse grains in the weld center. Near the surface of the weld, due to the change in heat dissipation conditions in the middle solidification of the weld, the columnar crystal grows toward the surface of the weld, and there is a large angle between the growth direction and the growth direction of the central columnar crystal. Laser welding is a rapid heating and cooling process, rapid quenching will cause martensite transformation of titanium alloy [1]. During the cooling process, the initial β columnar crystals of the weld metal generate α phase via shear transformation. It is due to the rapid cooling rate frustrating the atomic diffusion of the β phase. The α martensite grows and form one or several primary needle-like martensite parallel to each other, and then form a series of relatively fine secondary needle-like martensite. These secondary martensitic grains stop growing when encountering grain boundary or primary martensite, resulting in the formation of a typical staggered needle-like structure in the welding seam of the laser oscillating welds [29,30].
