*3.6. Tensile Properties*

The tensile results of the welded joints with different heat input are shown in Figure 9, while the fracture location of each welded joint is shown in Figure 10. Two tensile samples for each setting were used to present the fracture location on both face and root side, with the dotted yellow lines representing the weld metal region on both face and root sides. Figure 9b shows that the elongation of Test 1 (6.47%) and Test 3 (7.65%) is much lower than the other four samples although the ultimate tensile strength (UTS) remains at a high level. The fracture behaviour of these two samples is depicted in Figure 10a,b respectively. They both fractured from the face side undercut. Once the heat input was increased to more than 8.96 kJ/cm, as is the case in Tests 4–7 samples, the elongation reaches more than 10%. Although the Tests 4–7 tensile samples all fractured in the weld metal, the Tests 4 and 5 samples fractured in the weld metal from the root side fusion boundary (Figure 10c,d), while the fracture location of Tests 6 and 7 samples were randomly located in the weld metal (Figure 10e,f). It is worth noting here that the wavy ripples seem to have no evident effect on the tensile properties in Tests 6 and 7 samples.

**Figure 9.** Tensile properties; (**a**) tensile curves for welded joints with different heat input; (**b**) tensile results for welded joints with different heat input.

It is well accepted that the tensile strength is closely linked to the overall hardness of the welded joint. With the increase in heat input from 8.96 kJ/cm to 11.16 kJ/cm, the overall hardness decreases, as shown in Figure 8, which is probably a result of the change in cooling rate and microstructure. In addition, higher heat input also tends to produce wider weld metal and HAZ, which could increase the width of reduced hardness region, as is the case in this study. Both these two factors can contribute to the decrease in tensile strength with increasing heat input. The significant reduction in elongation in Tests 1 and 3 is a result of undercut defect, which is a well-known stress concentration point. This can also be confirmed by fractography, as shown in Figure 11a,c and e. Note that the left-hand side on the fracture surface is the face side of the tensile sample, whereas the right-hand side is the root side of the tensile sample. The fracture surface in Test 1 sample consists predominantly of delamination both across two planes and on a single plane, as indicated by yellow and red arrows, respectively in

Figure 11a, as well as small amount of radial zone on the left. This means that after quick fracture initiation and propagation, the tensile sample experienced delamination fracture mode along the vertical direction down to the root side, as shown in Figure 11b. The recovery of elongation in Tests 4–7 samples indicates that the mitigation or elimination of undercut effect is an effective way to improve the tensile properties by reducing or eliminating stress concentration. However, it is worth mentioning that although the tensile properties of Test 4 were not affected by slight undercut in this study, it is still highly desirable to eliminate it in real fabrication. Even in the absence of undercut, the fracture still presents two different routes. As can be seen in Figure 11c, after crack initiated from the root side fusion boundary (dark grey area on the right-hand side), quick crack propagation occurred until reaching the face side, as evidenced by the completely radial zone in the fracture surface. The schematic is shown in Figure 11d. This indicates that the fusion boundary on the root side can also act as a stress concentration point in the absence of appreciable backside reinforcement, although it is less harmful than undercut. Thus, it is recommended that post-weld surface treatment be carried out for the K-TIG welded joint if the workpiece root side is accessible. With the formation of appreciable reinforcement on the root side, as is the case in Test 6, evident fibrous zone is present in the centre region, as indicated by the dotted yellow circle in Figure 11e. This is a result of plastic deformation occurring before crack initiated. In addition, there is larger amount of shear lip formed on the fracture surface, indicating that this sample followed a regular fracture route in which the stress condition changed to shear from tension and resulted in a fracture around 45 degree relative to the maximum stress direction after crack reached the edge. The schematic of fracture path is shown in Figure 11f.

**Figure 10.** Fracture behaviour of welded joints; (**a**) Test 1 (HI = 7.78 kJ/cm); (**b**) Test 3 (HI = 8.42 kJ/cm); (**c**) Test 5 (HI = 8.96 kJ/cm); (**d**) Test 4 (HI = 9.18 kJ/cm); (**e**) Test 6 (HI = 10.22 kJ/cm); (**f**) Test 7 (HI = 11.16 kJ/cm).

**Figure 11.** Schematic of fracture mechanism; (**a**,**c**,**e**): Fractography of Test 1 (HI = 7.78 kJ/cm), Test 5 (HI = 8.96 kJ/cm) and Test 6 (HI = 10.22 kJ/cm) respectively. (**b**,**d**,**f**) The corresponding schematic of fracture mechanism for these three samples.

It is worth noting that the much-reduced elongation would make the prediction of service life ineffective and lead to premature failure. In addition, as the stress concentration in the root side fusion boundary may also lead to premature failure in highly stressed or fatigue conditions, it is highly desirable and necessary to eliminate any stress concentration and avoid the formation of undercut defect in the K-TIG welded joint. It is suggested that the heat input level be carefully monitored during the welding process through either arc behaviour or other feedback signals described in the literature, together with appropriate use of welding speed. The results presented in this study can be useful in the real fabrication process and could be used as a guidance of parameter optimisation and quality assurance for the K-TIG welding process.
