## 3 Results and Discussion
### Weld Tensile Strength and Fatigue Durability  
To evaluate the effect of micro crack on weld mechanical performance, tensile lap shear test was performed along the transverse direction. Fusion zone failure was determined in all welding conditions and the failure initiates from the internal toe between the two parts and grows through the top part at an angle of around 45 degree (see Figure 9a,b). Interestingly, the failure position agrees with previous work [41] on fillet lap RLW joints of AA5182 sheet metal components. However, those welds were not prone to micro cracks because of the nature of the AA5182 alloy. This indicates that the transverse micro crack is not the root cause of failure position at the internal toe and instead it is the localised strain concentration at the internal toe that promotes the initiation of failure. The strain concentration is believed to be attributed to two factors: (i) loss of material strength in the fusion zone due to the large columnar grain structure according to the Hall-Petch relationship and evolution of the strengthening precipitates [22]; and (ii) stress concentration at the edge of bonding area resulting from the structural nature of fillet lap configuration during the tensile shear loading [42]. The representative tensile fractography from three individual processing window is shown in Figure 9c,e,g. Large cavities observed on each fractography confirm the existence of micro cracks. Furthermore, a non-uniform fracture morphology was captured (Figure 9d,f,h), where small dimples in Region 1 (highlighted by dashed box) indicates the ductile fracture features and facets in Region 2, (highlighted by solid box) confirms the cleavage fracture near micro cracks. These fracture features revealed that transverse micro crack facilitates the rapid propagation of tensile failure and degrades the joint strength. Though reducing welding speed shows no obvious impact on the mitigation of micro crack, Figure 8, there is still an apparent improvement in joint tensile strength as shown in Figure 10. This is attributed to two phenomena: firstly, an increase in the throat thickness of fusion zone results in a larger effective bonding area (black box in Figure 11b and hence, greater resistance to the tensile shear. Secondly, the refinement of the columnar grains  
Figure 8: Distribution of (**a**) total length of all cracks and (**b**) average length of individual crack as function of welding parameters, measured by SEM on cross-sections with three replications at different positions along the welding directions for each welding condition. Grey dashed line represents the averaged level of individual processing window. (red triangle in Figure 11b) adjacent to the failure position improves the localised material strength [43]. Figure 9: Representative (**a**) top view and (**b**) cross-section view of the failure position during tensile test by optical microscopy; representative fracture morphology of welds by SEM in the processing window of (**c**) S2-F50, (**e**) S4-F100 and (**g**) S6-F150, with the high magnification view in (**d**,**f**,**h**), respectively. Fatigue test was then employed to investigate the effect of micro crack on joint fatigue durability. An averaged maximum linear load of welds among three processing windows was first calculated and then both 40% and 50% of this averaged maximum linear load were used as the peak load in the fatigue test with load ration of R = 0.1. Overall, weld root failure was observed in the processing window of S4-F100 and S6-F150 while fusion zone failure was found in the process window of S2-F50. Figure 12a-d shows the representative top view and cross-section view of the fatigue fractured section in each failure mode. For the fusion zone failure, the fatigue fatal crack originally initiated in the fusion zone adjacent the internal toe, which composed of columnar microstructure as shown in magnified figure (Figure 12e) and propagates along the grain boundary.