**4. Discussions**

The electrical oscillograms in Figure 2 show that the standard GMAW experiences some sort of oscillating pattern. It was noticed that this pattern corresponds to the instant the arc eroded the sidewalls. Moreover, the author pointed out that the arc attached to sidewall such that the current flows in the shortest path, thus the arc potential in the arc column remains at the minimum possible level.

This phenomenon in constant voltage (CV) leads ultimately to the melting of the contact tip and interruption of the welding processes. However, it was noted that, in constant current (CC), this process did not occur since the reduction in arc length, induced by the arc attachment to the sidewall, caused a small reduction in arc length in comparison the reduction in current, thus the power source reached a new equilibrium point.

On the contrary, in CW-GMA welding, the current path was shortest to the cold wire, which caused the arc to climb to it. This phenomenon prevented the arc attachment to the sidewalls, and consequently prevented sidewall erosion. Moreover, one observes that the arc was much more stable in CW-GMA welding (Figure 3) than in standard GMAW (Figure 2). The cyclogrammes for the the two cases confirmed this assertion. The root pass performed using CW-GMAW (Figure 5a) was more stable than in the conventional GMAW (Figure 4a), which had a tail that points to short-circuits caused by the sidewall erosion (dashed square).

Regarding the cap pass, one can observe that, in conventional GMAW, the arc tended to climb towards the contact, as indicated by the higher values of voltage (increase in arc length) and current in Figure 4c. However, in the CW-GMAW, such phenomenon did not occur as systematically, with only occasional values of current and voltage tending to the high voltage region. High speed images were used to investigate the arc dynamical behavior. The images show that the arc attached to the sidewalls in standard GMAW because of the internal regulation of the source in CV, as discussed above (Figure 6). In CW-GMAW, as expected from the electrical signals, no sidewall erosion was detected, as can be seen in Figures 6 and 7.

Figure 8 shows the macros of the joint manufactured using conventional GMAW. One can observe that some fusion points are lacking. These were caused by the arc climbing causing over-melts in one direction, but leaving a gap and causing the lack of fusion. On the contrary, in CW-GMAW, one can notice that, as the arc is pinned to the cold wire, this causes a more stable melting pattern avoiding incomplete fusion across the joint (Figure 9).

The macros, taken from the middle of joints (Figure 10), show the difference in productivity. The standard GMAW joint was completely filled with three passes while the the CW-GMAW was filled with two passes. However, one observes that, in relation to the cold wire joint, there was a lack of penetration due to the arc pinning to the cold wire, which limits the penetration and dilution.

Figure 11 compares the HAZ for the two processes: standard GMAW and CW-GMAW. The difference in their size can be attributed to differences in thermal signature of CW-GMAW in comparison to GMAW. The difference in ICHAZ might be linked to higher thermal gradient in CW-GMAW than in conventional GMAW. This thermal gradient might result from an improved melting efficiency in CW-GMAW for some conditions.

Figures 12 and 13 show the hardness maps over the complete macro and the root pass for both conventional GMAW and CW-GMAW, respectively. The Vickers hardness values in CW-GMAW (Figure 12) point to shorter cooling time (higher cooling rate), which might be linked to the higher thermal gradient caused by the possibly higher melting efficiency in CW-GMAW. This might explain the higher hardness in CW-GMAW compared to the standard GMAW. Regarding the root pass, the difference in hardness was likely due to the larger gradient formed by the larger joined area in CW-GMAW root. This led to higher cooling rates in the reheated root of the weld metal, with higher hardness.
