4.4. Microstructure
The 6063-T6 alloy is an Al-Mg-Si series aluminum alloy, which is a heat-treatable strengthened aluminum alloy. The main strengthening phase within the matrix is the β-phase, and the grains of the base material are fibrous. The strengthening phase Mg
2Si (black particles) is distributed in the base material matrix, as shown in
Figure 14.
Although the number of different welding currents under the grain boundary eutectic organization does not vary significantly, a large difference exists in its morphology, as shown in
Figure 15. When the welding current was large, the grain boundary eutectic structure was distributed in a coarse grid; when the welding current was small, the eutectic structure had a discontinuous rod-like distribution, and a large amount of eutectic structure was dispersed in the grain. The phase composition of 6063-T6 aluminum alloy weld metal is mainly α-Al solid solution. When the welding pool cools down, the liquid-solid phase transformation begins, forming a small amount of α-Al solid solution. As the temperature decreases, the volume fraction of α-Al solid solution increases.
During the welding process, the cooling rate of 6063-T6 aluminum alloy is faster, which will inhibit the second phase precipitation. When the welding current was 75 A, the strengthening phase was point-like or rod-like and the distribution was not uniform. When the welding current was increased to 80 A, due to the increase of welding heat input, the second phase had enough time to precipitate, but the shape was too coarse, which had an adverse effect on the quality of the weld. When the welding current was 85 A, the second phase shape rule was complete. When the welding current reached 90 A, due to excessive welding heat input, the precipitated strengthening phase remelted into the matrix, resulting in the deterioration of the mechanical properties of the welded joint.
When the welding current was small, the high-temperature residence time was short, the welding heat input was small and the aluminum alloy solidification effect was general while the weld was rapidly cooling, resulting in the second phase being too late for uniform precipitation, and the welded joint grain size was relatively small; with the increase in welding current, the size of the grains appeared to have a growth trend. This is mainly because the higher heat input increases the temperature of the molten pool of metal, allowing time for grain growth, and the grains grow rapidly.
4.5. Electrochemical Corrosion Properties
The polarization curves of the weld zone of the joint at different welding currents in a 3.5% NaCl solution under deoxygenation conditions are shown in
Figure 16. From the figure, it can be understood that the polarization curve obtained when pitting repair is performed is divided into two types: one is a smooth transition curve, such as the curve at a welding current of 85 A. The other is the curve with inflection points, such as the welding current of the 75 A, 80 A, and 90 A curves. The Tafel extrapolation method was used to obtain the results of the fitting of the polarization curve as shown in
Table 6. From the fitting results it can be seen that when the welding current was 90 A, the highest self-corrosion potential of the joint occurred. From the shape of the polarization curve, the specimens subjected to welding currents of 75 A, 80 A, and 90 A appeared as a passivation zone, whereas the specimens subjected to a welding current of 85 A did not show obvious signs of a passivation phenomenon occurring. This shows that when the welding current was 85 A, although the sample had a high corrosion potential, its passivation effect was not good. This is mainly related to the welding heat input, and the welding current was relatively small, resulting in a decreased over-age precipitation phase. Alloying elements in the matrix help to improve the corrosion potential, but the formation of passivation film may not be dense due to the influence of alloying elements, and thus no obvious passivation interval can be seen on the polarization cross-section.
When the corrosion potential in the dynamic potential polarization curve was more negative, the reaction equilibrium constant was smaller, and the resistance of the aluminum alloy in the electrochemical reaction process was smaller, leading to a higher likelihood of a corrosion reaction. When the welding current was 85 A, the corrosion potential at this time was the lowest, and thus this condition provided the best corrosion resistance. Overall, this is mainly related to the welding heat input. With different welding heat inputs, Mg2Si phase precipitation morphology, size and distribution are different. In the precipitation phase, due to the existence of potential differences between the collective, the process of corrosion will occur in the microcell reaction, thus changing the corrosion rate.
The main elements added to the 6063-T6 aluminum alloy are Mg and Si, and a small amount of Fe is also present. Because of the high content of elemental Si, the second phase in the specimen consists mainly of the precipitated phase, the Si phase and the Al(Fe)Si phase. Since only the corrosion potential of the precipitated phase is less than that of Al and the corrosion potential of the other second phase is greater than that of Al, two possibilities occur when the pitting of the material occurs. When the precipitated phase and the aluminum alloy matrix form a primary cell, the corrosion potential of the precipitated phase is lower, and pitting corrosion occurs in the precipitated phase. When the aluminum alloy matrix and another second phase with higher corrosion potential form a primary cell, pitting corrosion occurs in the aluminum alloy matrix.
The corrosion current and corrosion potential in the dynamic potential polarization curve reflects the kinetic parameters of corrosion rate and the thermodynamic parameters of material corrosion tendency, respectively. A lower corrosion current density indicates that the material is more resistant to corrosion and a lower corrosion potential indicates that the material is more susceptible to corrosion. After MIG welding of 6063-T6 aluminum alloy, the microstructure of the weld area of the joint changes significantly, which leads to a change in its corrosion resistance.
When the welding current is small, small holes appear on the surface of the weld area of the joint, and these holes are more susceptible to the erosion of Cl
− in the NaCl solution, resulting in a decrease in the corrosion resistance of the welded joint. It has been shown that the dissolution of Al, Mg and Si elements in aluminum alloys is due to the difference in pitting potential [
34].
As shown in
Figure 17, different degrees of pitting occurred on the surface of welded joints. The pitting degree gradually deepened and spread to all sides with the increase of welding current, and the pits also gradually grew larger. Compared with the other three groups of parameters, the joint corrosion at 85 A welding current was not serious: only a small number of pits appeared, and the joint had good corrosion resistance, which is also echoed in the previous research results.
Figure 18a,c show the morphological characteristics of the pits and their surroundings. There are white corrosion residues of different shapes in the pits, and the black precipitation phase is predominant near the pits and corrosion cracks also appear around them, so the presence of the precipitation phase in the welded joint will affect the corrosion of the joint.
Figure 18b,d show the results of EDS analysis, through which it can be found that the black precipitates are more abundant in Mg and Si elements, and the white precipitates are more abundant in Fe elements. The pitting behavior of welded joints essentially occurs on the surface of Mg
2Si particles, and the black precipitation phase Mg
2Si dominates in the pits. With the elevation of welding current, the pitting corrosion gradually increases, and the self-corrosion potential of Mg
2Si is lower than that of other phases, and thus it will be the first to corrode, and the presence of Mg
2Si phase will seriously deteriorate the corrosion resistance of welded joints. The Si phase will remain in the crater, and the self-corrosion potential of the Al substrate is higher than that of the Si phase, so it will again accelerate the corrosion of the surrounding Al substrate [
35].