## 4 Discussion  
### T-Type Joints Produce Pores  
By observing the weld joint of the 6082 aluminum alloy T-joint bilateral welding, we can find that there was basically no porosity in the center of the weld and the pores mainly existed in two cases, as shown in Figure 10. The first case was that the pores presented in the penetration were near the pores near the weld line at the root of the weld; the second case is that the pore was near the weld line. The common cause of the pores in these two cases is that, in the 6082 aluminum-alloy T-welding, there was a large amount of surface oxide film at the joint between the vertical plate and the horizontal plate at the root of the weld. The surface of these oxide films adsorb moistures. The following reaction occurs during welding:  
\[3\mathrm{H_{2}O}+\mathrm{Al}=\mathrm{Al_{2}O_{3}}+6\mathrm{H} \tag{4}\]  
According to Formula (4), it is known that a large amount of hydrogen is generated in the presence of moisture, and these hydrogen elements are generated at the bottom of the molten pool; a small portion of these hydrogen elements dissolve into the aluminum alloy in the liquid phase. However, most of the hydrogen element is present as hydrogen. However, the solubility of hydrogen in solid aluminum alloy is 0.036 mL/100 g, while the solubility of hydrogen in a liquid-phase aluminum alloy is 0.69 mL/100 g and the dissolution rate of hydrogen in the liquid phase and solid phase changed greatly. During the MIG-welding cooling process, there was supersaturated hydrogen in the liquid phase. When the temperature dropped to the solidus line, a large amount of hydrogen was  
Figure 10: Porosity distribution in T-joint. Figure 9: (a) T-type joint weld fatigue fracture; (b) weld crack propagation area; and, (c) weld transient area. precipitated. This precipitated hydrogen provided the conditions for bubble nucleation. On the other hand, the surface energy required for bubble nucleation is very low, so it is very easy to nucleate at the dendritic interface when the MIG is welded to the aluminum alloy. This provided conditions for bubble nucleation at the junction of the 6082 aluminum-alloy T-weld at the root of the weld and the transverse plate. In the case of aluminum-alloy thick-plate T-type MIG welding, the case of incomplete penetration at the joint occurred, and, in the case of no penetration, the oxide film on the surface of the nonmelted base material provided an attachment point for the nucleation of the bubble. It can be seen from Figure 10 that most of the base metal in the T-joint did not melt, so bubbles that are generated near the fusion line tended to adhere to these oxide films, and the bubbles needed to satisfy the conditions in which they grow:  
\[\mathrm{P_{h}}>\mathrm{P_{a}}+\mathrm{P_{c}}+\mathrm{P_{s}}=1+\frac{2\sigma}{ \mathrm{r}} \tag{5}\]  
P\({}_{\mathrm{h}}\) represents the pressure in the bubble, \(\sigma\) is the surface tension of the liquid metal and the bubble, the surface tension of the liquid aluminum \(\sigma\) is 0.9 N/2 cm, and r is the bubble radius. At the beginning of bubble nucleation, the radius is extremely small, and the inside of the bubble needs a lot of pressure to grow. However, from Figure 10, we can observe that the pores generated at the T-joint were insect-like pores, and the length of the pores was long. This indicates that the bubble length was very long when the bubble is formed at the bottom of the molten pool, resulting in a large bubble radius r. It indicates that, during MIG welding, there is a large amount of bubble nucleation at the root of the T-joint along the welding direction. These bubbles come into contact with each other and fuse into insect-like long bubbles. This causes the internal pressure to be small, enabling the bubbles to continue to grow, which provides conditions for the growth of the bubble at the root of the T-joint.