## 3 Results and Discussion
### Microstructure and Chemical Composition  
Figure 11 shows the microstructure observed for each welding pass of every weld. The average grain sizes of the first pass (Figure 11a) and the second pass (Figure 11b) were 39.0 \(\upmu\)m and 39.7 \(\upmu\)m in the W1 weld, respectively. The average grain sizes of the first pass (Figure 11c) and the second pass (Figure 11d) were 40.1 \(\upmu\)m and 40.8 \(\upmu\)m in the W2 weld, respectively. The average grain sizes of the first pass (Figure 11e) and the second pass (Figure 11f) were 35.7 \(\upmu\)m and 36.3 \(\upmu\)m in the W3 weld, respectively. Thus, the grain sizes of the W1 weld and the W2 weld were almost at the same level within the error range, and the W3 weld had the smallest grain size, which was about 10% lower than that of the W1 weld and the W2 weld. According to the Hall-Petch relation [17, 18], the smaller the grain size, the more grain boundaries that interfere with the dislocation movement, so greater stress is required for deformation.  
Figure 10: Vickers micro-hardness of aluminum 5083 alloy GMA weld: (**a**) W1; (**b**) W2; (**c**) W3.  
Table 4 shows the results of analyzing the chemical composition of the welds. As a result of measuring the chemical composition of the welds, the W1 weld and the W2 weld showed that the magnesium content of the filler wire was different, but the amount of magnesium remaining in the deposited metal zone after welding was not significantly dissimilar. In the filler wire, the magnesium contents of W1 and W2 were different by about 1 wt.%. Still, the content of the deposited metal zone after welding was significantly reduced by about 0.16 wt.% in the first pass and about 0.19 wt.% in the second pass. In the GMA welding process with a high current, the magnesium in the aluminum alloy filler wire is vaporized more than the amount diluted in the deposited metal zone. The added magnesium of the filler wire is vaporized in a large amount during welding, and the magnesium of the base metal is diluted in the deposited metal zone. Similarly, in the W1 weld and the W2 weld, which have almost no manganese content, the measured manganese in the deposited metal zone was diluted from the base metal. The W3 weld had a magnesium content similar to that of the other welds, but the manganese content was over 60% higher. When the tensile strength results (Figure 8) and chemical composition results are analyzed together, it can be seen that the manganese in the filler wire has a  
Figure 11: Microstructure by electrolytic etching of aluminum 5083 alloy GMA welds: **(a)** W1-first pass; **(b)** W1-second pass; **(c)** W2-first pass; **(d)** W2-second pass; **(e)** W3-first pass; **(f)** W3-second pass.  
positive effect on the improvement of the weld strength after GMA welding and the results of the best tensile strength are shown.  
Figure 12 shows the regression analysis results on the magnesium content of filler wire and tensile strength of welds in aluminum 5083 alloy GMA welding. The W1 weld, with almost no manganese (0.003 wt.% or less) and 7.33 wt.% of magnesium, was compared with the W2 weld, with almost no manganese (0.004 wt.%) and 6.38 wt.% of magnesium, and analyzed. The coefficient of determination (R-squared) of the regression formula was 0.05. When the magnesium content of the aluminum alloy filler wire was 6 wt.% or more, the tensile strength and the magnesium content were not linearly proportional. However, as shown in the results of the W2 weld and the W3 weld of Figure 8, when the magnesium content of the aluminum alloy filler wire is higher than that of commercial products (6 wt.% or more), the strength of the weld is improved according to the addition of manganese.