*3.3. Mechanical Properties*

The tensile strength of the welded specimens at room temperature are listed in Figure 8, and the photographs of W1 and W2 joints after the tensile test are shown in the insets of Figure 8. It is observed that the average tensile strength of W2 reaches 1200 ± 10 MPa, which is equal to the tensile strength of TC31 alloy. Meanwhile, the average tensile strength of W1, W6, W7 and W9 samples also exhibit high tensile strength, reaching 1183 ± 4 MPa, 1185 ± 31 MPa, 1184 ± 63 MPa and 1190 ± 1 MPa, respectively. The fracture of two specimens of W1 joint occurred at the base material, while the ruptures of W2 specimens occurred in the HAZ. As seen in Figure 8, the W8 sample exhibits relatively low mechanical properties, which is caused by the porosity defects in the weld [31,32].

**Figure 8.** Tensile strength of welded TC31 specimens by wave laser oscillating welding method at room temperature. (The insets show the photographs of W1 and W2 joints after the tensile test.).

Given that the working temperature of TC31 alloy can reach from 923 K to 973 K, the mechanical properties of the joints at high temperature conditions is also essential. The tensile strength of the welded specimens at 923 K are also tested and listed in Figure 9. As seen in Figure 9, the tensile strength of W2 joint reaches 635 ± 3 MPa. Moreover, the highest tensile strength for TC31 specimens by laser oscillating welding at 923K can reach up to 638 ± 6 MPa for W1 joint. The fracture of W1 joint specimens at 923 K occurred at the BM away from the weld seams.

**Figure 9.** Tensile strength of welded TC31 specimens by laser oscillating welding method at 923 K. (The inset show the photograph of the W1 joint after the high-temperature tensile test.).

In order to investigate the mechanism for the high strength of the joints by laser oscillating welding, the fracture surfaces of the welded joints were examined by SEM. Figure 10 presents the magnified view of the fracture surface of W2 after the tensile test performed at room temperature. A large number of dimples densely distribute on the fracture zone, which is a typical feature of ductile failure. The formation of the dimples implies plastic deformation in the micro regions, which is suggested to be favorable for improving mechanical properties of the joint. Figure 11 presents the magnified view of the fracture surface of W1 joint after the tensile test performed at 923 K. It can be seen from Figure 11 that the area where the fracture is close to the surface of the weld (Figure 11b) and the middle part of the welded joint (Figure 11c,d) show distinct features. Region i fracture morphology shows the characteristics of tough dimple fractures. The fracture surface is rock sugar-like. There are a large number of small dimples on the grain boundaries of the fracture surface and show slip characteristics. This results from nucleus, growth, and connection. Meanwhile, there are also a small number of dimples concentrated in some areas (red dotted area in Figure 11b). As can be seen from Figure 11c,d, there are a large number of dimple structures in Region ii in the middle part of the welded joint. The above results indicate that the joint undergoes ductile fracture under high temperature conditions.

The TC31 joint fabricated by the laser oscillating welding method exhibits high tensile mechanical properties at room temperature, which is exceeding or approaching the mechanical properties of the base alloy. On the one hand, as can be seen from Figures 4 and 7, no defects, such as cracks, inclusions, or unwelded joints, are generated except W8 sample. Laser welding is a rapid melting and solidification process, and it is difficult for the pores formed in the molten pool to escape, which becomes the stress concentration point of the joint. Compared with pulsed laser welding method [12], the content of pores in the weld seam obtained by wave laser oscillating welding is significantly reduced, and the depth-to-width ratio of the weld seam is relatively small. This is due to the adoption of wave laser oscillating welding method, the reciprocating swing of the laser beam on the weld seam causes part of the weld to remelt repeatedly, prolonging the residence time of the molten metal in the weld pool. The deflection of the laser beam also increases the heat input per unit area, reduce the depth-to-width ratio of the weld, which is favorable for the escape of bubbles. Besides, the oscillation of the laser beam causes the air holes to oscillate, which can also provide the stirring force for the welding molten pool, increase the convection and stirring of the molten pool and then eliminate the air holes. This is a favorable factor for improving the mechanical properties of the joint [33–35].

**Figure 10.** Fracture surface of W2 welded joint after tensile test performed at room temperature, (**b**) is the magnified view of (**a**).

**Figure 11.** Fracture surface of W1 welded joint after tensile test performed at 923 K: (**a**) macroscopic appearance of fracture; (**b**) high magnification magnification of Region i; (**c**) high magnification magnification of Region ii; (**d**) high magnification magnification of Region iii.

On the other hand, the TC31 high temperature titanium alloy forms a martensite structure in the weld after laser welding. Martensite α is a supersaturated solid solution of alloying elements inα phase. During the cooling of the weld from the β-phase region to room temperature at a rapid cooling rate,

the atoms have no time to diffuse, and only a small needle-shaped and unevenly distributed martensite structure can be precipitated by shearing. α grows inside the initial β columnar crystals, forming one or several parallel primary α firstly, and extending through the entire grain over a long distance, stopping at the grain boundary. Then a series of relatively fine secondary needles α are formed, which stops at the grain boundaries or primary martensite, resulting in the formation of a typical basket structure in the weld. This structure has good comprehensive properties, such as plasticity, creep resistance and high-temperature endurance strength [1].
