## 3 Results and Discussion
### Weld Tensile Strength and Fatigue Durability
In comparison, fatigue fatal crack in the weld root failure mode initiated at the boundary between the columnar microstructure and equiaxed microstructure and propagates vertically into HAZ. Though differing from the failure location, fatal crack initiates at boundary of columnar grains in both failure modes. As the resistance to crack initiation in the high cycle fatigue (HCF) regime depends primarily on the resistance to dislocation motion, the HCF strength consequently relies on the yield strength which increases with reduced grain size, leading to a weak point at the columnar grain. The discrepancy in failure position between the tensile test and fatigue test for S4-F100 and S6-F150 may be attributed to different test conditions including the load level and strain rate. These differences are likely to amplify the impact of stress concentration, resulting from the geometrical nature of fillet lap joint [42], on the failure position during fatigue test, which may even overcome the weaken effect of micro cracks observed in the fusion zone (shown in Figure 12g-i). [PERSON] et al. [44] reported that  
Figure 11: (**a**) Maximum linear load during tensile test and; (**b**) weld zone throat radius as function of welding parameters. Figure 10: Maximum linear load during lap shear tensile test as function of welding parameters. a smoother weld external toe can greatly mitigate the stress concentration which makes the internal toe more favourable for stress concentration. Therefore, transition of the failure mode from weld root failure (S4-F100 and S6-F150) to fusion zone failure (S2-F50) is observed in this study as the root angle decreases (Figure 13). This type of transition of failure mode was also reported in a study by [PERSON] and [PERSON] [45]. Figure 12: Representative top view (**a,b**) and cross section view (**c,d**) of fatigue failure in fusion zone and weld root respectively with detailed view near the fatigue fatal crack in (**e,f**). (**g–i**) show the high magnification view of transverse micro crack in positions addressed in (**d**). (**a,c**) were obtained from S6-F150 at \(P_{B}\) = 5.57 kW. (**b,d**) were obtained from S2-F50 at \(P_{B}\) = 3.25 kW. Fatigue cycles, as functions of welding parameters and weld root angles, are plotted in Figure 14. Results indicate an improvement in fatigue durability when fracture occurs in weld root failure mode instead of fusion zone failure mode. Furthermore, for those welds in the weld root failure mode, a peak fatigue performance was observed at the root angle at around 55 degrees. Further increases of root angle reduced the fatigue durability due to excessive stress concentration in the external toe. To better understand significant difference in the fatigue durability between two failure modes, fatigue fractured surface was studied utilizing SEM. Figure 15a,c show the representative fracture surface of fusion zone failure and weld root failure, respectively. It is evident in the magnified image (Figure 15b) that micros crack and pores near the internal toe (as indicated by the large cavity) facilitate the rapid propagation of fatal crack failure the columnar grain boundary and consequently result in a brittle characteristic. In comparison, a smooth fracture surface was determined in the weld root failure, without obvious sign of micro cracks. It should be noted that in addition to the avoidance of micro crack near the fatal crack, the reduced size of the extruded grain structure in HAZ, compared to the columnar grain in the fusion zone, and the resultant higher localised material strength are also beneficial for the enhancement of fatigue durability [43].