**2.2. Laser beam welding**

The welding equipment consisted of an 8‐kW continuous‐wave ytterbium fibre laser YLS‐8000‐ S2‐Y12 (IPG Photonics Corporation) integrated with an IXION ULM 804 CNC‐controlled universal laser machine. A collimation lens of 120 mm, a focal length of 300 mm and a process fibre with a diameter of 600 µm were employed to produce a focal spot diameter of approxi‐ mately 700 µm. The centre wavelength of the fibre laser was 1070 nm. The divergence half‐ angle of the focused multimode beam was 30.3 mrad, and the resulting beam parameter product BPP = 11.3 mm\*mrad.

Prior to welding, the faying edges of the specimens were machined, ground and then thor‐ oughly cleaned with ethanol to remove any surface oxides and contaminants. Preliminary welding experiments with small coupons were conducted to identify the optimal combination of welding parameters and obtain a good weld quality and an appropriate weld shape. The parameters finally chosen and employed for welding of coupons are listed in **Table 3**. The specimens were fixed in an open plastic box filled with Ar to protect the weld bead from air during the LBW process. The uniform Ar flow around the weld bead was provided by the injection of the shielding gas through the porous Al plate at the bottom of the box. Kashaev et al. reported based on hot gas extraction analysis that this shielding technique was very effective [5]. The welding direction was perpendicular to the rolling direction of the material.


**Table 3.** Laser beam welding process parameters.

#### **2.3. Postweld heat treatment**

Postweld heat treatment was conducted using Workhorse vacuum furnace, Centorr Vacuum Industries, USA, at a vacuum degree of 0.1 Pa. The parameters of different types of PWHT are given in **Table 4**. Annealing temperatures did not exceed the β transus, which is nearly 995°C for the Ti‐6Al‐4V alloy [1, 2]. During heat treatment, the welded plates were hung using molybdenum wire to prevent any contact between the specimens and furnace wall. No significant distortion was observed after heat treatment. Cooling to room temperature was performed in Ar atmosphere. All heat‐treated specimens were then machined from both sides before extracting the fatigue specimens.


**Table 4.** Conditions of postweld heat treatment.

**2.2. Laser beam welding**

114 Study of Grain Boundary Character

product BPP = 11.3 mm\*mrad.

Laser power 5500 W

Welding speed 4.0 m/min

Focal position ‐3.0 mm

Filler wire feed rate 3.0 m/min

**Table 3.** Laser beam welding process parameters.

before extracting the fatigue specimens.

**2.3. Postweld heat treatment**

Shielding Argon, 15 l/min

Filler wire Ti Grade 5, Ø 1.0 mm

The welding equipment consisted of an 8‐kW continuous‐wave ytterbium fibre laser YLS‐8000‐ S2‐Y12 (IPG Photonics Corporation) integrated with an IXION ULM 804 CNC‐controlled universal laser machine. A collimation lens of 120 mm, a focal length of 300 mm and a process fibre with a diameter of 600 µm were employed to produce a focal spot diameter of approxi‐ mately 700 µm. The centre wavelength of the fibre laser was 1070 nm. The divergence half‐ angle of the focused multimode beam was 30.3 mrad, and the resulting beam parameter

Prior to welding, the faying edges of the specimens were machined, ground and then thor‐ oughly cleaned with ethanol to remove any surface oxides and contaminants. Preliminary welding experiments with small coupons were conducted to identify the optimal combination of welding parameters and obtain a good weld quality and an appropriate weld shape. The parameters finally chosen and employed for welding of coupons are listed in **Table 3**. The specimens were fixed in an open plastic box filled with Ar to protect the weld bead from air during the LBW process. The uniform Ar flow around the weld bead was provided by the injection of the shielding gas through the porous Al plate at the bottom of the box. Kashaev et al. reported based on hot gas extraction analysis that this shielding technique was very effective

[5]. The welding direction was perpendicular to the rolling direction of the material.

Postweld heat treatment was conducted using Workhorse vacuum furnace, Centorr Vacuum Industries, USA, at a vacuum degree of 0.1 Pa. The parameters of different types of PWHT are given in **Table 4**. Annealing temperatures did not exceed the β transus, which is nearly 995°C for the Ti‐6Al‐4V alloy [1, 2]. During heat treatment, the welded plates were hung using molybdenum wire to prevent any contact between the specimens and furnace wall. No significant distortion was observed after heat treatment. Cooling to room temperature was performed in Ar atmosphere. All heat‐treated specimens were then machined from both sides

#### **2.4. Microstructural characterization**

Transverse cross sections were cut from the stable middle region of the joint for metallographic examination and microhardness testing. After sectioning, the samples were mounted, ground and polished using an oxide polishing suspension (OPS) compound. Microstructural obser‐ vations were performed using both inverted optical microscopy (OM) Leica DMI 5000M and scanning electron microscopy (SEM) JEOL JSM‐6490LV. Prior to light microscopy, the speci‐ mens were etched by Kroll's reagent (3% HF, 6% HNO3, 91% distilled water) to unveil the microstructural features. For SEM investigations, a mirror‐like OPS polished surface was used. SEM microstructure observations and texture analysis of the joints were conducted using secondary electrons images and electron backscatter diffraction (EBSD). The EBSD measure‐ ments were performed for a specimen area of 135 µm × 135 µm at an acceleration voltage of 30 kV, a spot size of 4.7 nA, an emission current of 75 µA, a working distance of 13 mm, a step size of 0.3 µm and a sample tilt angle of 70°. For the orientation calculation, the generalized spherical harmonic series expansion (GSHE) method was applied based on triclinic sample symmetry. The average grain size was measured using the OIM software and the results of EBSD measurements. EDX spectroscopy was used for the local chemical composition deter‐ mination. For EDX analysis, SEM was operated at an acceleration voltage of 15 kV, a working distance 10 mm and a live time 150 s. The data obtained were calculated based on the standard ZAF method of correction.

#### **2.5. Microhardness testing**

Transverse cross sections of the samples for microhardness testing were prepared in the same manner as discussed for the microstructural evaluations. The Vickers microindentation hardness test was carried out using a Zwick/ZHU0,2/Z2,5 universal hardness testing machine and testXpert software. The samples were tested with a 500‐g load applied for 15 s according to ASTM E384‐11 [10]. The indentation spacing was 200 µm to provide the minimum recom‐ mended distance between test points [10]. This resulted in 61 indentations for each line. To investigate thickness gradients, microhardness profiles were measured at three testing positions: radiation exposure side (RES), middle of the weld (M) and the weld root side (RS) (illustrated in **Figure 9**). The distances from RES and RS lines to the edges of the specimen were 200 µm each.
