## 3 Experimental Results and Analysis
### Hardness Analysis  
Figure 5a shows the hardness distribution of the welded joint of the 6082 aluminum alloy. The hardness test point is shown in Figure 5b (from point A to point B), and the measuring point spacing is 0.2 mm. The medium hardness test points are divided into seven parts of the base material areas I, II, III, IV, V, VI, and base material (BM), including the weld zone and the heat-affected zone. During the welding process, the base metal changes due to the thermal cycle of the weld, and its mechanical properties change. This is called the heat-affected zone. In the 6 XXX series aluminum alloy, the strength of the base metal mainly depends on the second phase of the failure-strengthened aluminum alloy. These second phases mainly include the GP zone, \(\beta^{\prime}\), \(\beta^{\prime}\), and \(\beta\) (Mg2 Si), under the action of thermal cycling. These second phases dissolve and precipitate. The maximum temperature that can exist in the GP zone is 150 \({}^{\circ}\)C; the temperature of the \(\beta^{\prime}\) phase dissolution and precipitation is about 240-250 \({}^{\circ}\)C; precipitation temperature of \(\beta^{\prime}\) phase is about 250-320 \({}^{\circ}\)C; precipitation temperature of \(\beta\) phase is about 450 \({}^{\circ}\)C; and, the dissolution temperature is approximately 550 \({}^{\circ}\)C [21]. Therefore, when the HAZ is subjected to different welding thermal cycles, different second-phase particles in different parts are precipitated and dissolved, so the hardness of the HAZ also greatly varies. From the hardness diagram, we can see that the hardness of Zone I is the highest and the highest value even reaches 130 HV. After the weld zone of Zone II, hardness drops sharply. It can be seen that the hardness of the weld zone is about 77-83 HV and the hardness reaches 70 of the base metal. The hardness increases rapidly to about 105 HV after passing through the weld zone. The hardness of the HAZ in Zone III is basically the same as that of the base metal; as it moves away from the center of the weld, the hardness curve begins to fall in Zone IV, the heat affected zone. This is a softening zone, as the hardness in Zone IV is about 75 HV, which almost reaches the lowest hardness value of the whole welded joint; when entering Zone V, the whole curve shows an upward trend, but the curve is very unstable between each test point. The change is very large: when the curve enters Zone VI, the hardness curve is stable at around 100 HV. After that, the curve rises steadily and finally reaches the hardness of the base material of about 110 HV, indicating that the hardness point has hit the base material (BM). Figure 4: 6082 microstructure of T-welded joint of aluminum alloy: (**a**) base material (BM); and, (**b**) heat-affected zone (HAZ). In Figure 5a, Zones I and III are both quenching zones in the heat-affected zone and Zone I is the zone of the T-joint neutral plate. It can be seen from the figure that the hardness is the highest in this zone and the hardness can be averaged at 124 HV. Zones I and III are the areas closest to the weld and have the highest heat temperature. Therefore, second-phase particles are dispersed in the base metal dissolve into \(\alpha\)(Al) at high temperature to form a supersaturated body. The effect of dissolution strengthening led to an increase in the hardness of Zones I and III. Zone I is a vertical plate in the T-veld joint, and, during the welding process, it is subjected to two thermal cycles of welding. More second-phase particles are dissolved into \(\alpha\)(Al), which greatly increases hardness. Average hardness reaches 125 HV; Zone IV is the softening zone of the heat-affected zone of the aluminum alloy, and hardness in this zone was at least 70 HV.