*2.2. Experimental Design*

The principle of the PPG-VPPAW process is shown in Figure 2. When the solenoid valve was closed, the plasma gas flow was blocked by the solenoid valve and gathered at the entrance of the solenoid valve. When the solenoid valve was opened, the blocked plasma gas flow was released and input into the plasma arc at a velocity greater than the set value at that moment. Then, the plasma gas flow returned to the set value and waited the next closure of the solenoid valve.

**Figure 2.** The principle of PPG-VPPAW process.

In order to study the effect of pulsed plasma gas acting on the plasma arc, the arc electric signals and arc image were acquired from PPG-VPPAW and compared with those in VPPAW and DP-VPPAW. The current and plasma gas flow waveforms of the VPPAW, DP-VPPAW, and PPG-VPPAW processes are shown in Figure 3. The VPPAW process has the variable polarity square current wave with unequal straight and reverse polarity time intervals, and the plasma gas flow rate remains constant, as shown in Figure 3a. For the DP-VPPAW process, the current wave has been decreased periodically, the total current wave presents two periodically varying pulses (i.e., a high-frequency variable polarity pulse and a low-frequency pulse), and the plasma gas flow rate is the same as for the VPPAW process, as shown in Figure 3b. In the PPG-VPPAW process, the current waveform is the same as for the VPPAW process while the plasma gas flow rate is input intermittently in the form of a pulse, as shown in Figure 3c. Based on a large number of trials, the process parameters in this study were selected in Table 1 to make the comparison clear. In Table 1, *IEN* was the electrode negative current; *IEP* was the electrode positive current; *IB* was the basic current in DP-VPPAW; *IP* was the pick current in DP-VPPAW. The arc image capture rate was 3000 fps and the signal sampling rate was 10,000 Hz. The plasma gas and the shielding gas both were pure argon.

**Figure 3.** Schematic diagram of current–plasma gas flow waveform. (**a**) VPPAW; (**b**) DP-VPPAW; (**c**) PPG-VPPAW.

˄ ˅



In order to study the effect of pulsed plasma gas on the molten pool in the keyhole welding process, the weld-forming experiments were carried out. The weld bead geometries and porosity distribution from the VPPAW, DP-VPPAW, and PPG-VPPAW processes were investigated. In order to study the influence of the pulsed plasma gas on the molten pool, the filling wire was not applied for avoiding the effect of filling material on molten pool behavior and simplifying the experimental model. In these experiments, 5 mm thick 5A06 aluminum alloy was selected as the work piece. The plasma gas and the shielding gas both were pure argon. Based on a large number of trials, the parameters in this part were selected to obtain a good weld bead geometry, as shown in Table 2.


**Table 2.** Parameters for weld-forming experiments.

### **3. Results and Discussion**

### *3.1. Variation in Arc Electrical Signal*

The welding electrical signals of VPPAW (Experiment 1-1) and DP-VPPAW (Experiment 1-2) are shown in Figure 4a,b, respectively. It can be observed that the current waves well fit the preset parameters. The root mean square (RMS) and absolute mean (AM) values of current are shown in Table 3. In comparison with traditional VPPAW, the current of PPG-VPPAW was almost unchanged. Figure 4c,d show the welding electrical signals of PPG-VPPAW under different plasma gas flow pulse frequencies with the same plasma flow rate; the plasma gas flow pulse frequencies were 4 Hz (Experiment 1-3) and 20 Hz (Experiment 1-4), respectively. Compared with Figure 4a,c, it can be observed that the current wave had a negligible effect on the pulsed plasma gas, while the arc voltage

fluctuated periodically with the pulsed plasma gas frequency. The electrode negative period voltage (*UEN*) in the VPPAW process and solenoid valve on state of the PPG-VPPAW process was 24.97 V on average, and the electrode positive period voltage (*UEP*) was 35.49 V on average. When the solenoid valve was off state and the gas flow pulse frequency was 4 Hz, the *UEN* and *UEP* decreased by 5.8 V and 9.8 V, respectively. When the gas flow pulse frequency was 20 Hz, the *UEN* and *UEP* decreases were 5.1 V and 8.9 V, respectively. The above results showed that the lower the frequency of plasma gas, the more distinct the influence on the arc voltage waveform. At a certain plasma gas pulse frequency, the pulsed plasma gas has a greater influence on the electrode positive polarity voltage.

**Figure 4.** Welding current–voltage synchronizing wave form. (**a**) VPPAW; (**b**) DP-VPPAW; (**c**) PPG-VPPAW (4 Hz); (**d**) PPG-VPPAW (20 Hz).


**Table 3.** The root mean square (RMS) and absolute mean (AM) values of current. ˄˅ ˄˅

When the plasma gas injects into the arc column, it requires more energy to keep enough ionized particles that let the current through. When the plasma gas is shut off, the required energy which was used to ionize the gas decreases, and the arc voltage also decreases. This decrease relates to the shut-off time of plasma gas and has a maximum value. With the frequency of plasma gas increased, the shut-off time decreases with lack of time to let the arc voltage decrease to the maximum value. In the EP phase of the PPG-VPPAW process, due to the cathodic cleaning phenomenon [15], the size of the arc profile is larger than that in the EN phase, which lets it be more affected by the plasma gas shut-off.

Figure 5 displays more details about the arc voltage of PPG-VPPAW with the plasma gas flow pulse frequencies of 4 Hz and 20 Hz. The pulse signals of "1" and "0" indicate that the solenoid valve is in the on state and off state, respectively. It can clearly be seen that with the plasma gas flow pulse frequency of 4 Hz, the arc voltage is dropped immediately to the minimum voltage when the plasma gas is shut off, which costs 14 ms, and the voltage decrease rate is 0.44 V/ms. When the plasma gas is turned on, it takes 28 ms to return the average voltage, and the voltage increase rate is 0.20 V/ms accordingly. Once the plasma gas flow pulse frequency increases to 20 Hz, the voltage decrease rate is nearly same as that in 4 Hz (0.46 V/ms), but it is hard to find a stable minimum voltage due to the lack of shut-off time of the plasma gas. When the plasma gas is turned on in 20 Hz situation, the voltage recovery time is 22 ms, and the voltage increase rate is 0.23 V/ms accordingly. The above results show that the plasma arc needs more time to recover the effect of plasma gas closure, and the plasma gas flow pulse frequency has less effect on the voltage decrease rate.

**Figure 5.** The arc voltage of PPG-VPPAW. (**a**) PPG-VPPAW (4 Hz); (**b**) PPG-VPPAW (20 Hz).

When the plasma gas is shut off, there is no plasma gas injecting immediately and the arc zone could be considered as a relative closed environment and establish balance easily. Compared with it, when the plasma gas is turned on, the plasma gas continuously injects into the arc column and makes it harder to establish balance, so the arc voltage needs more time to stabilize after the plasma gas is turned on. Furthermore, the higher the plasma gas frequency is, the lower the plasma gas accumulation rate is, so the arc voltage needs less time to stabilize after the plasma gas is turned on. The voltage decrease rate is correlated with arc characteristic, but there is no significant correlation

with the frequency of plasma gas. However, when the frequency of plasma gas is above a certain value, the plasma gas is turned on again when the arc voltage is not yet decreased to the stable voltage without plasma gas, so the recovery speed of the voltage becomes faster.
