## 3 Experimental Results and Analysis
### T-Joint Fatigue Fracture and Fatigue Fracture  
The fatigue-fracture specimen of the 6082 aluminum-alloy T-weld joint was observed. As shown in Figure 8, it was found that the fatigue crack originated from the root of the T-welded joint, spreading toward the weld, while the cracking that occurred at the T-weld joint was due to there being pores in the root of the 6082 aluminum alloy T-weld joint. Pores reduce the stress area of the welded joint, which reduces the mechanical properties of the joint. At the same time, under the action of the fatigue-test cyclic load, stress concentration occurs at the pore boundary, which causes the fatigue cracks to crack from the pores. It can be observed from Figure 8 that the T-shaped welded joint between the vertical plate and the horizontal plate was not welded at the left side of the air hole, which is equivalent to the existence of a preset crack before the fatigue test, which greatly reduces the fatigue life of the 6082 aluminum alloy T-weld joint and the maximum stress that can be carried. More unfortunately, there were pores at the nonfused edges, which caused cracking on the side of the pores close to the base metal, and cracking was more likely to occur on the weld side due to stress concentration. Therefore, pores and incomplete penetration are the main causes of fatigue fracture. The mechanisms of pores and cracking are described in detail further in the article.  
Figure 7: 6082 S–N curve of T-welded joint of aluminum alloy.  
A fatigue test of the 6082 aluminum alloy T-welded joint was carried out. The fatigue fracture of the T-welded joint was analyzed by SEM. It was found that the fatigue source area was the pore of the weld root, and the fatigue crack was cracked by the pore. Crack stress-intensity factor \(\Delta\)K was near the threshold value \(\Delta\)Kth, and the crack was in the slow-expansion phase (Figure 9a). In Figure 9a, it was found that there were many secondary microcracks perpendicular to the fracture surface on the fracture surface. Figure 9b is an enlarged view of the yellow square of Figure 9a, in which there was a distinct ductile-fracture characteristic, where a large number of tearing ribs were present, which were caused by crack propagation in different planes. It can be observed in Figure 9b that the two tear edges eventually merged to form one tear edge (shown by a yellow arrow), which reflected the direction in which the crack extended from bottom to top. It can also be observed in Figure 9b that the cleavage-like step also illustrates the presence of cleavage-fracture characteristics at the fracture. Figure 9c shows the instantaneous break zone in the fatigue fracture, and it can be observed from the figure that semielliptical tear-shaped dimples are composed entirely of the instantaneous break zone, which is why these dimples are tear-shaped. At the same time, it can be observed that the size of the dimples is small, because they are due to fatigue cracks that are generated during the fracture process through the second-phase particles. The size of the second-phase particles in the weld of the 6082 aluminum alloy MIG welding was also small, which caused a smaller size-tearing dimple. It can be observed from the figure that the direction of the semicircular dimple opening was mainly in the direction indicated by the arrow, that is, the direction in which the main crack extended.  
Figure 8: Fatigue crack in T joint of 6082 aluminum alloy.  
Figure 9: _Cont._