# Application of Pulsed Laser-TIG Hybrid Heat Source in Root Welding of Thick Plate Titanium Alloys
## 3 Results and Discussion  
### Back Appearance and Cross Section of Weld Bead  
Figure 5 shows the back surface and cross section of a weld seam of single TIG and hybrid welding. From Figure 5a, it can be found that the back surface of the weld seam produced by single TIG welding, presents incomplete fusion. The heat input is 390 J/mm. If the welding current is increased, and other parameters remain constant, the burn through on the back of the weld penetration can be seen, as shown in Figure 5b. The heat input is about 448 J/mm. During the laser-TIG welding process,  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Elements** & **Al** & **V** & **Fe** & **C** & **N** & **O** & **Ti** \\ \hline Wt\% & 5.50–6.75 & 3.50–4.50 & 0.30 & 0.08 & 0.05 & 0.20 & residual \\ \hline \hline \end{tabular}
\end{table}
Table 1: Chemical compositions of Ti-6 Al-4V (wt %).  
Figure 4: The size of specimen and groove shape.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameters** & **Value of Single TIG** & \multicolumn{2}{c}{**Laser-TIG Hybrid Welding**} \\ \cline{2-4} Power source type & DC & Pulse & DC \\ Electrode type & W-2\% ThO\({}_{2}\) & - & W-2\% ThO\({}_{2}\) \\ Diameter of electrode, (mm) & 3.2 & - & 3.2 \\ Angle of electrode (\(\theta\)), \({}^{\circ}\) & 45 & 90 & 45 \\ Welding current (\(I\)), A & 260, 300 & - & 200 \\ Laser pulse energy (\(Q\)), J & - & 20 & - \\ Laser pulse frequency (\(f\)), Hz & - & 20 & - \\ _Dla_, (mm) & - & & 2, 3.5 \\ Shielding gas type & Pure Ar & - & Pure Ar \\ Gas flow rate (\(q\)), L/min & 15 & - & 15 \\ Arc length (\(l\)), mm & 4 & - & 4 \\ Welding speed (\(v\)), mm/min & 400 & 400 & 400 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Welding parameters of laser-arc hybrid welding processes.  
adjusting the _Dla_, welding current of TIG, laser pulse energy, a sound fusion shape in butt welding of a thick plate root layer can be realized under a group of certain condition: \(Q=20\) J, \(f=20\) Hz, \(I=200\) A, \(v=400\) mm/min, \(Dla=2\) mm, as presented in Figure 5c. The heat input of hybrid welding is about 358 J/mm. Seen from the transverse section of hybrid welding, it is found that both sides are fusion sound. The back weld width 2 mm and root reinforcement 0.4 mm are moderate. Figure 5d presents the weld appearance of the hybrid weld samples under the condition of _Dla_ 3.5 mm, and other conditions are same as in Figure 5c. It is found that with the _Dla_ increase to 3.5 mm, the back weld will present an undercut defect. This means that the addition of pulse laser can promote the heat transfer to the depth of the penetration, and _Dla_ plays an important role in the heat distribution of the pool in the laser-TIG hybrid welding process. The sound and uniformed formation of the bottom can be realized by hybrid welding process because the laser makes appropriate heat distribution in the molten pool.  
After welding, the optical microstructure of the fusion zoom (FZ) and heat affected zoom (HAZ) with the parameters of Figure 5c are adopted to carry out the analysis, as shown in Figure 6a,b respectively. Due to the difference of the cooling rate in the FZ and HAZ, their weld microstructure presents different morphology. Because of the slower cooling in FZ, a number of bulky columnar crystals with the acicular phase \(\alpha^{\prime}\) phase and small amount of the primary \(\beta\) phase increased in FZ, as shown in Figure 6a. Due to the fast cooling in HAZ, the globular phase with the mixed pattern of the acicular martensite and the primary \(\alpha\) phase appeared in the HAZ, as shown in Figure 6b.  
Figure 5: Back surface and cross section of weld seam with single tungsten inert gas (TIG) and hybrid welding: (**a**) single TIG; (**b**) single TIG; (**c**) hybrid welding; (**d**) hybrid welding.  
Figure 6: Optical microstructure of weld: (**a**) fusion zoom (FZ); (**b**) heat affected zoom (HAZ).