# Application of Pulsed Laser-TIG Hybrid Heat Source in Root Welding of Thick Plate Titanium Alloys
## 4 Discussion
### The Effect of Pulsed Laser in Welding Process  
In this research, during pulsed laser-arc welding, the state of the melting pool is closely related to the periodical behavior of the liquid metal. Figure 13 shows the relative location of the arc and the molten pool in different welding processes. As illustrated in Figure 13a, during single TIG welding, because of the arc stiffness caused by the high-power jet from the tungsten electrode, the arc always completely covers the molten pool and arc force acts on the surface of the molten pool along the extension of the tungsten electrode. Due to lower energy density of single TIG welding, the incomplete penetration may be caused with higher linear energy. However, a small increase in linear energy requires an increase in arc current. According to Formula (6), the arc pressure is proportional to the square of the currents, so the increase of current will result in the increase of arc pressure, arc stiffness and size of the molten pool, ultimately causing the weld to be burnt through [28; 29].  
\[F=\frac{\mu}{4\pi}\times\frac{I_{a}^{2}}{\pi R^{2}}(R^{2}-r^{2}) \tag{6}\]  
where \(I_{a}\) is the arc current, \(\mu\) is the arc magnetic permittivity, \(F\) is the arc static pressure, \(R\) is the radius of the arc, \(r\) is the distance from arbitrary points to the axial of arc.  
A balance state of four-balance forces (surface tension, arc force, gravity and adhesive force) is difficult to achieve with liquid titanium alloys with lower and changeable surface tension at the melting point. It can be found that TIG welding with single type and poor tolerance is difficult to control in the welding process.  
Figure 13b presents the schematic diagram of the arc and molten pool in hybrid welding, in comparison with Figure 13a, it can be seen that the molten pool along the welding direction can be divided into front molten pool area I named the hybrid heat source action area and rear molten pool area II named the arc action area, and this differs from TIG welding in which a single molten pool is formed. This is mainly because the original arc is attracted and enhanced by laser induced plasma during laser action, and the arc column is compressed, deflected and attracted to the laser action point over the top of the molten pool. Meanwhile, in the case of high energy density gradient, arc energy is transferred along the direction of penetration. After laser action, the arc restores to its original state with low energy density and acts on the molten pool. Under the action of the pulsed laser, the  
Figure 13: Schematic diagram of molten pool in different welding process: (**a**) single TIG; (**b**) hybrid welding.  
enhanced arc alternates as the original arc acts on the pool area I and II, respectively. When the laser acts on area I, metal in area I is melted forming a molten pool. However, without arc action, the bottom of the molten pool in area II would solidify into a weld seam due to a very large amount of heat being taken away from the molten pool by shielding gas. After laser action, the molten pool is completely covered by a low energy density arc and the arc heat is not enough to fully melt the bottom of the weld seam into molten pool, so the bottom of the molten pool is still solid. In this case, the top of the liquid metal will be held by the bottom weld seam. Meanwhile, the length of the molten pool is shortened along the welding direction. In this situation, the four forces make it easy to achieve balance and a stable molten pool is obtained.