*3.1. Surface Formation*

Tables 5 and 6 illustrate both the face side and root side surface formation for the weldments with different parameters. It can be observed that when the travel speed was fixed at 34.2 cm/min, undercut was experienced for all the weldments, as marked by red arrows and shown in Table 5. When the travel speed was reduced to 30 cm/min, the undercut defect disappeared, together with smooth surface formation on both sides, as shown in Table 6. A further decrease in travel speed led to wavy ripples along the face side weld edges, as highlighted by red circles, although undercut was not experienced. Also, even irregular bead face was produced on the root side when the travel speed was reduced to 24 cm/min in Test 7 sample.




#### **Table 6.** Weld surface formation with different travel speed.

It is known that high welding speed is easy to cause undercut formation. This is because the molten metal experiences much stronger backwards momentum at higher travel speed. The mass flow rate is the highest along the centreline of the weld, which tends to draw molten metal towards the centreline of the flow and induces lack of melt on the side walls on the face side [21]. Thus, undercut was consistently observed when high travel speed was used. Considering that the Tests 4 and 5 samples possess similar heat input and different travel speed, the presence of undercut in Test 4 may further indicate that travel speed plays a part in the formation of undercut. In addition, as reported by Tomsic and Jackson [22], high heat input makes the keyhole widen, more molten metal displaced by the keyhole flows back to the trailing region for a longer distance and a uniform bead is hard to obtain. This could be the reason for the formation of wavy ripples on the face side and irregular bead face on the root side. As the mitigation of undercut defect was observed with increasing arc current, it is reasonable to believe that the heat input also plays certain role in the formation of undercut in the K-TIG welding process. The effect of heat input on the undercut formation is discussed in following section.
