# Joint Properties of Aluminum Alloy and Galvanized Steel by AC Pulse MIG Braze Welding
## 3 Results and Discussion
### Effect of Process Parameters on Wettability
#### 3.1.2 Variation in Welding Current and EN Ratio  
In order to reveal the effects of welding current and EN ratio on wettability, parametric analysis was performed under the following welding parameters: welding current of 50-70 A with EN ratio of 0-20%, welding voltage of 19 V, and welding speed of 500 mm/min. Figure 2 illustrates the typical cross-sectional bead shape of dissimilar braze-welded joints. The results show that an increase in welding current exhibited an increase in the amount of metal deposition and bead width due to the higher heat input. Compared to 10%, 20%, and 0% (from 50 to 70 A), the 10 and 20% cases of cross-sectional bead morphology were more convex. This is related to the wire melting behavior during brage welding. When the EN ratio increased, the filler wire was directly heated and further resulted in a higher metal deposition because the wire melting speed increased. On the other hand, the heat input applied to base metal could increase at an EN ratio of 0% [6].  
\begin{table}
\begin{tabular}{c c c c} \hline
**Torch Aiming Position (mm)** & **Bead Appearance** & **Cross-Section** & **Wetting Angle (\({}^{\circ}\))** & **Wetting Length (mm)** \\ \hline \(-1\) & Not braze-welded & - & - \\ \(0\) & & 26.5 & 7.6 \\ \(1\) & & 21.1 & 7.5 \\ \(2\) & & 45.1 & 7.4 \\ \hline \end{tabular}
\end{table}
Table 4: Bead appearance and cross-sectional view of braze-welded joints.  
As shown in Figure 3, a welding defect concentrated in the root area was observed. These defects appeared with an irregular size and location in the root area with no tendency of variations in welding current and EN ratio. Welding defects, such as pores, hot cracks, unfused roots, and the lack of fusion can frequently form in the root area, especially in dissimilar braze-welding using Zn-coated steel [1, 12, 13, 14]. This defect formed in the root area may be pores due to the evaporation of Zn from the GI steel sheet because dissimilar metal joining was carried out in a lap joint without a gap between the Al alloy and steel sheet. For the formation mechanism of these welding defects, the Zn coating on the steel surface rapidly vaporized due to a low vaporization point (906 \({}^{\circ}\)C) and expansion with a strong vapor pressure during braze-welding. Although Zn evaporation tried to emit through the root and toe area, the root area was covered in a molten pool, and it finally remained as pores in the root area during solidification. Especially, the root area, where the Al alloy and steel sheets were tightly joined, had a high thermal conductivity, resulting in the faster solidification, causing sufficient heat input to not transfer properly. Therefore, pores and the lack of fusion were easily formed in the root area [15, 16].  
The comparison of wetting angle and wetting length obtained with different welding currents and EN ratios are shown in Figure 4. The wetting length increased with welding current ranging from 50 to 70 A, and it also increased with EN ratio. However, the wetting angle increased despite the increase in wetting length. In terms of geometrical bead morphology, a low wetting angle and long wetting length exhibited a good spreadability and wettability. In the present study, the wetting angle increased with the amount of metal deposition, as the torch aiming position was set to 2 mm during braze-welding.