*3.3. Variation in Welding Formation*

Figure 10 shows the front and back of the PPG-VPPA (20 Hz) weld joint. It can be clearly seen that the keyhole is completely penetrated at the end of the weld joint. The weld joint formed as shown in Figure 10 is due to the lack of metal filling and the molten pool flow toward the front of the weld. Figure 11 shows the appearance of the weld surface with different welding methods. A smooth weld joint profile of VPPAW is shown in Figure 11a. As shown in Figure 11b–d, both DP-VPPAW and PPG-VPPAW form a fish-scale pattern on the surface of the weld joint due to the stirring action on the welding pool caused by the periodical oscillation of the arc pressure. The ripples formed on the weld joint surface show that the fluidity of the welding pool has been enhanced, and benefits the welding quality. Compared with Figure 11b,c, it is clearly demonstrated that the plasma gas frequency has a stronger effect acting on the welding formation than current frequency under the premise of acceptable welding formation. Compared with Figure 11c,d, the denseness of ripple profile on the weld joint surface is directly proportional to the frequency of the plasma gas.

**Figure 10.** The front and back of the PPG-VPPA (20 Hz) weld joint.

**Figure 11.** Morphology of weld surface. (**a**) VPPAW; (**b**) DP-VPPAW; (**c**) PPG-VPPAW (20 Hz); (**d**) PPG-VPPAW (40 Hz).

The preparation method of the sample is shown in Figure 12. Three samples were selected in every weld joint. We observed that the characteristics of the three samples of the same weld joint were basically the same. Therefore, a sample of each weld joint was randomly selected for further analysis. The cross-sections of the weld joint with different welding methods are exhibited in Figure 13a. The average weld reinforcement and width are measured as shown in Table 4. The VPPAW process and PPG-VPPAW process have nearly the same current wave, so the weld width from these two processes are similar. For the DP-VPPAW process, due to the decrease of current in the low frequency, the weld width also decreases. The distribution of porosity in the weld fusion line region with different welding methods is shown in Figure 13b, where WZ is weld zone, FZ is fusion zone, and HAZ is heat-affected zone. It shows clearly that the porosity has appeared both in fusion zone and weld zone from the VPPAW process, and the porosity could be observed in the fusion zone from the DP-VPPAW process. In contrast, there is no observable porosity in the cross-section from the PPG-VPPAW process. The results of arc electrical characteristics and arc profile show that PPG-VPPA periodically fluctuates due to the periodic variation of plasma gas flow rate. The molten pool oscillates periodically under the oscillating arc. Therefore, the pulsed plasma gas has a stronger effect than the pulsed current wave which enhances the fluidity of the molten pool and the spillover probability of the porosity from the molten pool increases [20,21].

**Figure 12.** The preparation method of the sample.

**Figure 13.** Macrographs of cross-section and the distribution of pores in weld fusion line region. (**a**) Macrographs of cross-section; (**b**) the distribution of pores in weld fusion line region.


**Table 4.** The weld reinforcement and width.
