**2. Experimental**

The base metal is high-temperature titanium alloy TC31 in this study, and its composition and mechanical properties are listed in Tables 1 and 2, respectively. TC31 alloy was machined into plates with a dimension of 200 mm × 100 mm × 3 mm. The plates were polished with SiC sandpaper, acid pickled in HF solution and ultrasonically cleaned with acetone and ethyl alcohol.


**Table 1.** The composition of high-temperature titanium alloy TC31 (wt%).

**Table 2.** Mechanical properties of alloy TC31 with the thickness of 3 mm at room temperature and high temperature 923 K.


Butt welding experiments were carried out on the base material using laser oscillating welding method. The laser welding parameters for two welding methods are listed in Table 3. In laser oscillating welding experiments, the laser beam oscillated in a figure-eight manner with the weaving frequency of 200–400 Hz and the weaving amplitude of 0.1–0.5 mm. The laser power was in the range of 1900–2100 W, and the welding speed is in the range of 1200–1800 mm/min. Since titanium alloy is easily reacted with ambient gases, argon was used as shielding gas with the flow rate of 15 L/min during welding process. The laser focused on the workpiece surface with a diameter of 0.45 mm, defocus amount of 0 and a working distance of 247 mm. A D50 wobble seam oscillating head (IPG Laser GmbH, Burbach, Germany) was utilized to adjust the oscillating frequency and amplitude.


**Table 3.** Laser oscillating welding parameters.

After welding, the internal quality of the welds was examined by X-ray nondestructive testing instrument (NDT, MU2000-D) (YXLON International GmbH, Hamburg, Germany) using a tube voltage of 120 kV and a tube current of 3.4 mA. Due to the limitation of capability of equipment, the nondestructive testing was conducted twice on different parts for each sample. The microstructure of the weld was studied by a GX53 metallurgical microscope (Olympus Corporation, Tokyo, Japan) after being etched with Kroll's reagent: 2 mL HF, 6 mL HNO3 and 92 mL H2O. X-ray diffraction (XRD, D/MAX-2500) (Rigaku Corporation, Tokyo, Japan) using Cu-Kα radiation was employed to examine the structure of the welds. For X-ray diffraction measurements, a diffractometer with the X-ray tube operating at 40 kV and 200 mA target current was used. The microstructure of the joints was also studied using an electron probe microanalyzer (EPMA, JXA-8100) (JEOL Ltd., Tokyo, Japan). To determine the mechanical properties of the joints at room temperature and high temperature 923 K, the weld plate samples were machined into joint samples with dimensions depicted in Figure 1 for measurement (as GB/T 228.1-2010 standard and GB/T 228.2-2015 standard). The samples were cut along the direction perpendicular to the weld. Tensile tests were performed, respectively, on three joint samples by using a universal testing machine at a tensile speed of 0.5 mm/min. After the tensile test, fracture surfaces of the weld joints were observed by a scanning electron microscope (SEM, JSM-7001F) (JEOL Ltd., Tokyo, Japan) with an energy dispersive spectrometer (EDS, Pegasus XM2 EDS) (EDAX Inc., Mahwah, NJ, USA) system.

**Figure 1.** Dimensions of the specimens in tensile testing: (**a**) sample design for room temperature testing; (**b**) sample design for high temperature testing.
