*3.1. Weld Appearance and Internal Quality*

High-temperature titanium alloy TC31 plate samples were weld under different laser welding conditions listed in Table 3. The front and back appearances of the butt weld joints are shown in Figure 2. As shown in Figure 2, the welds are continuous in the surfaces without cracks, and uniform fish-scale patterns are observed. The samples except samples W5 and W9 display the front appearance as yellow or silvery white. With increasing the laser power up to 2100 W or decreasing welding speed to 1200 mm/min, the front appearances of the welds present a color of green, purple or blue, which indicates that the oxidation of the front appearances is further deepened [27]. By contrast, the back appearance of all the welds is silvery-white in color, indicating an effective protection for the joints through gas shielding. Besides, as can be seen from Figure 2e,i, the back weld width of W5 and W9 samples are obviously larger than those of other samples, indicating the higher heat input.

**Figure 2.** The front and back appearances of the laser-welded TC31 joints: (**a**–**i**) represent W1 to W9 samples.

For further investigating the internal quality of the welds, the X-ray non-destructive testing was conducted and the result is shown in Figure 3. Except that porosity defects are observed in W8, other joints exhibit sound bonding without lack of fusion, slag or hot cracks.

**Figure 3.** Images of the laser-welded TC31 joints in X-ray detection.
