*3.2. Weld Longitudinal Cross-Section*

The weld longitudinal cross-section and arc behaviour are shown in Figure 4. After arc was extinguished, the weld pool was suddenly frozen. The profile inside the keyhole channel was retained. When lower heat input was used, as in Test 1 sample, strong arc plasma was experienced on the face side, together with very high deflection angle relative to the horizontal line, as shown in Figure 4a. As the heat input increased, the amount of arc plasma on the face side decreased significantly, along with the increase in the slope of keyhole leading wall, as shown in Figure 4b. What is more, the deflection angle of plasma cloud relative to the horizontal line decreased with increasing heat input.

As can be seen from the schematic in Figure 4, when lower heat input was used, there was insufficient heat deposited on the lower half of the keyhole leading wall. Thus, the keyhole leading wall exhibited a severely curved shape. Only small amount of the arc plasma can be ejected via the keyhole exit, leaving majority of them guided towards the face side. As the heat input was increased, more heat was deposited on the keyhole bottom region. The bottom region (A zone) of the keyhole leading wall in Figure 4a was melted away into liquid and displaced to the rear part to form the weld, which makes the keyhole exit deviate less from the welding torch. Therefore, larger amount of the arc plasma could be easily ejected through the keyhole exit, leaving very small amount of them guided towards the face side. In addition, the increase in heat input further decreased the slope of keyhole rear edge, which allowed the plasma cloud to be deflected at a very small angle.

**Figure 4.** Weld longitudinal cross section and arc shape; (**a**) Test 1; (**b**) Test 7.

### *3.3. Weld Transverse Cross-Section*

The weld macrographs are depicted in Figure 5. It can be seen that both the face side and root side width increased with increasing heat input. More importantly, there existed slight hump in Tests 1 and 2 samples, as shown in Figure 5a,b. The hump decreased and even disappeared with increasing heat input, as shown in Figure 5c,d. This is consistent with the mitigation or elimination of undercut defect.

As mentioned above, the heat input is also believed to be a controlling factor in the formation of undercut defect. Also, the variation trend of heat input, hump and undercut formation are consistent. Thus, it is necessary to consider the interaction among them. As mentioned above, when a high heat input was used, the keyhole front wall was less curved, majority of the plasma gas jet spurted out via the keyhole exit and very small amount of them was reflected by the keyhole front wall towards the rear keyhole wall, as indicated in Figure 6a. Therefore, the face side weld formation was smooth with the absence of humping and evident undercut. While if the heat input was reduced, the keyhole wall on the front side bent against the travel direction and the distance between the welding torch and the keyhole exit would increase, as demonstrated previously in Figure 4a. In this case, the plasma

gas flow field changed dramatically as depicted in Figure 6b. Almost all of the plasma jet was guided towards the lower region of the keyhole wall on the backside. As the welding current used in K-TIG welding process was relatively high, the pressure associated with the plasma jet flow could overwhelm the surface tension coming from the weld pool on the backside. Therefore, the trailing weld pool was severely deformed. Certain amount of the weld pool was pushed towards the face side, and hump appeared in the weld centre. This kind of hump would reduce the amount of liquid metal on the edges of the weld pool on the face side, which may facilitate the formation of undercut. This implies that the heat input level is also an important parameter in determining the undercut formation.

**Figure 5.** Macrographs of four typical welds with different heat input; (**a**) Test 1 (7.78 kJ/cm); (**b**) Test 2 (8.1 kJ/cm); (**c**) Test 4 (9.18 kJ/cm); (**d**) Test 7 (11.16 kJ/cm).

**Figure 6.** *Cont.*

**Figure 6.** Schematic of plasma gas flow inside keyhole; (**a**) high heat input; (**b**) low heat input.
