# Influence of Welding Parameters on the Weld Pool Dimensions and Shape in a TIG Configuration
## 4 Discussion
### Effect of the Nature of the Shielding Gas  
Case n\({}^{\circ}\)3 (Arcal.37) and case n\({}^{\circ}\)4 (He) allow the study of the influence of the nature of the shielding gas. Figure 5a describes the weld pool width evolution from 1 to 20 s in the case of the interaction of a three-millimeter Arcal.37 and a three-millimeter helium plasma with a welding current of 100 A.  
Figure 4: **(a)** Effect of the arc length on the evolution of the weld pool width; **(b)** Effect of the arc length on the evolution of the weld pool shapes; **(c)** Effect of the arc length on the depth-to-width ratio; **(d)** Mechanism of the weld pool expansion: thermal diffusivity.  
The expansion of the weld pool retains the same behaviour. Diameters evaluated in case n\({}^{\circ}\)4 (Helium gas) are wider but the main difference is with the depths. The comparison of the weld pool shapes for both cases from 1 to 20 s is presented Figure 5b.  
Figure 5: \(Cont.\)  
Analysis of these profiles shows that the fusion shape changes with the shielding gas. Bulges observed in case n\({}^{\circ}\)3 disappear in case n\({}^{\circ}\)4. The weld pool depth is significantly higher with the use of helium. A stagnation of the depth is observed in case n\({}^{\circ}\)3 whereas the depth keeps rising in case n\({}^{\circ}\)4. Maximal width reaches 7.94 \(\pm\) 0.02 mm (respectively 7.16 \(\pm\) 0.02 mm) and maximal depth gets to 4.57 \(\pm\) 0.02 mm (respectively 3.40 \(\pm\) 0.02 mm) in case n\({}^{\circ}\)4 (respectively case n\({}^{\circ}\)3).  
In gas mixtures containing helium, such as Arcal.37, the resulting drop voltage for the same current intensity is higher than in pure Ar [20]. Thus, we can assume that for the same current intensity I = 100 A and the same distance between the two electrodes (L = 3 mm), the applied energy is higher in pure helium than in Arcal.37 gas, where the helium proportion is lower leading to a higher flux transfer. The depth evolution in pure helium is therefore greater, even at t = 1 s.  
Figure 5c describes the evolution of depth-to-width ratio. A stabilization of this ratio is observed in case n\({}^{\circ}\)3 around 0.5, while it keeps increasing in case n\({}^{\circ}\)4. In these configurations, changing the nature of the shielding gas leads to deeper welds. For the same configuration, helium plasma created a more energetic environment than Arcal.37 plasma.  
The weld pool surface temperature distribution is thus different and the direction of Marangoni forces is modified [21]. This leads to an inward flow resulting in a deeper weld pool. Moreover, movements resulting from the Lorentz forces increase. Some studies showed that the maximal velocity of the flow due to Lorentz forces rises when helium is used instead of argon [13]. The ratio between the energy used to fuse the volume of metal (_Efusion_) and the energy involved in the process (_Esupplial_) is given in Figure 5d. A diminution of this ratio with time is observed in both cases with values slightly higher in case n\({}^{\circ}\)4. This indicates that the radiation, conduction or evaporation losses are less significant with the use of helium. The process would therefore be more efficient in case n\({}^{\circ}\)4 (He). The nature of the shielding gas is a fairly significant parameter on the weld dimensions and on the process efficiency.