## 3 Experimental Results and Analysis  
### 6082 Aluminum Alloy MIG-Welding T-Joint Microstructure  
The solidification of the weld metal generally begins at the boundary of the weld pool, i.e., it solidifies from the solid-liquid interface. Solidification ways are plane solidification, cell solidification, and dendritic solidification. The way of solidification depends on temperature gradient (G), crystallization rate (R), and the supercooling of components [20].  
In the 6082 aluminum-alloy MIG-welded T-joint, as shown in Figure 3a, when crystallization at the boundary of the molten pool begins, the components are supercooled to form cell crystals, and it can be observed that these grains adhere to the unmelted base material and they start to grow (in the yellow box). These grains have many different orientations during initial growth, but the grains along the direction of the temperature gradient compete for growth, and grains grown along other directions are inhibited and stop growing, thereby forming a length of about 100 \(\upmu\)m. Cell crystal: since the solubility of the solid phase of the aluminum alloy is less than the solubility of the liquid phase, as the solid phase formed by solidification increases, solute concentration in the liquid phase increases, so the subcooling of the component increases, and the cell crystal appears to laterally form a cell shape. Dendritic (blue elliptical area): Figure 3b shows the microstructure of the weld center. At the center of the weld, the degree of the components' supercooling continues to increase, and the molten metal receives little restraining force around the core and can be nucleated from the inside of the liquid phase during crystallization. A snowflake-shaped equiaxed crystal (within a yellow circle) having a diameter of about 50 \(\upmu\)m is formed. However, in the process of equiaxed crystal growth, due to the agitation of the arc during the welding process, the equiaxed crystals are broken. If the agitation is small, then the incompletely fractured equiaxed crystals forms larger dendrites (blue in the color box); when large agitation is applied, a plurality of fine crystal grains having a diameter of 10-20 \(\upmu\)m are formed to achieve the effect of refining the crystal grains. Refining the grains reduces the tendency of the liquefaction crack of the welded joint and improves the crack-propagation resistance of the welded joint.  
Figure 4a shows the microstructure of the 6082 aluminum alloy base material, and the base material grains are fibrous along the rolling direction of the sheet. A large number of second phases are dispersed on the \(\alpha\)(Al) aluminum substrate, and there are more black second phases on the base material. In addition to the melting of the filler metal during the welding process, the base material undergoes recrystallization under the action of thermal cycling to form a heat-affected zone (HAZ), as shown in Figure 4b. By observing the HAZ of the welded joint, the number of particles of the second phase is significantly reduced, and it can be observed that the color of the HAZ is brighter than that  
Figure 3: Microstructure of weld joint of MIG-welded T joint of 6082 aluminum alloy. **(a)** Grain-boundary growth mode of weld pool; and, **(b)** grain structure of weld center.  
of the base metal, which is mainly due to the original dispersion distribution in the base metal when it is affected by the heat source. A large amount of fine second phases diffuse into \(\alpha\)(Al) to form a supersaturated solid solution, so that the content of second phases is greatly reduced and the color becomes lighter. By comparing the microstructure of the base material and the HAZ, it can be observed that the grains in the HAZ are significantly grown due to the influence of the welding thermal cycle, and the fibrous structure does not change in length, but the width is significantly increased.