# Fatigue of Friction Stir Welded Aluminum Alloy Joints: A Review
## 3 Factors Affecting Fatigue Performance
### Weld Defects  
The defects of FSW joints include flash, tunnel defects, surface defects, foreign bodies, porosities, kissing bond, zigzag curve, etc. [71,72]. The initiation process of fatigue cracks accounts for \(\sim\)90% of the total life. Existence of defects greatly accelerates the crack initiation process, which is one of the  
Figure 6: (**a**) Longitudinal and (**b**) transverse residual stresses as a function of lateral distance from the weld line [67].  
main causes of the sudden decline in the life of welded joints [24]. Figures 7 and 8 show the common welding defects.  
During the welding process, the material undergoes severe plastic deformation. The rotating tool shoulder, in contact with the surface of the material, causes materials to overflow at the edge to form the flash. Due to the tilt angle of the tool, the tailing edge of the tool shoulder presses the material and forms a circle of corrugated friction surface defects. The use of a stationary tool shoulder can effectively solve the friction surface problem, but it will impose certain requirements on the welding conditions or the geometric design of the tool. After the welding is completed, the keyhole will be left. It can be removed by cutting off the material containing the keyhole or using the special welding tool.  
Figure 8: Incomplete fusion paths in weld: (**a**) at the advancing side of the weldment; (**b**) towards the middle of the weldment; (**c**) upper embedded flaw; and (**d**) connectivity flaw [73].  
Figure 7: (**a**) Weld root flaw and (**b**) flash flaw [72; 73].  
The keyhole has no direct influence on the initiation or propagation of the fatigue crack. The main influence is to reduce the effective welding length of the plate [74]. [PERSON] et al. [75] simulated the FSW process and found that the fluidity of the material was significantly reduced due to inadequate pressure between the rear side of the tool and workpiece material. It was difficult for the material to form a dense joint from the retreating side to the advancing side, resulting in a tunnel defect. [PERSON] et al. [76] studied the fatigue lives of sound and flaw welds of an Al-Cu-Li alloy. The fatigue strength of defect-free welds is reduced by 10 to 15% at the lifetime of \(10^{5}\) cycles in contrast with the base metal. For the kissing bond (KB) weld with the weld root polishing, a reduction in fatigue strength by 17% was found comparing with defect-free welds [24]. It can be seen from Figure 9 that the fatigue lives of the 2024-T6 welded joints were greatly reduced [55; 77]. Some researchers found that the kiss defect has no significant effect on the fatigue strength of 5083-H321 [57; 78].  
Joint Line Remnant (JLR) is a flaw in the nugget region which is caused by the natural oxide layer on the workpiece surfaces before welding [76; 79], as shown in Figure 10. Joint Line Remnant (JLR) defect welds exhibited similar fatigue strength as defect-free welds and the Joint Line Remnant was not the crack initiation position [32; 80]. The hook-like defects (Figure 11) similar to the JLR seriously affected the material fatigue properties [81]. The fatigue strength of the FSW lap joint was less than 5.6% of the tensile strength of base metal [82].  
Figure 10: Optical micrograph of a kissing bond (KB)-bearing weld, the dotted line indicates the part of the KB that is removed by grinding of the surface treated specimens [76].  
Figure 9: Effect of kissing bond on material fatigue properties [70; 71; 77; 78].