*3.2. The Behavior Characteristics of Weld Pool in Double-Pulsed GMAW*

In the process of DP-GMAW, the thermal and pressure acting on the weld pool of TP and TB are significantly different, the dynamic behavior of weld pool were varied with thermal frequency (F). For a better understanding of the influence of DP-GMAW current waveform parameters on the dynamic behavior of austenite stainless steel weld pool, the profile and oscillation characteristics under different current waveform parameters were recorded and extracted. In order to simplify the analysis process, two typical current waveform parameters (No.3 and No.12) were selected to summarize the weld pool profile.

The heat and the mass transferred to the weld pool during the period of a single current pulse can hardly affect the shape of weld pool significantly. In P-GMAW, the heat input and the droplet impingement force acting on weld pool are constant during welding process, and the profile of the weld pool caused by them remains stable, as shown in Figure 12a. In DP-GMAW, the length and the width of weld pool in the base period of thermal phase were obviously smaller than that in the peak period, as shown in Figure 12b,c. The smaller heat input and less metal deposition lead to rapid shrinkage of pool size during base period. The pool trailing edge shrank and separated from the solidified bead boundary of the weld, as shown in Figure 12c. This is the main factor that forms bead surface ripple. The shrinkage and expansion of the weld pool outline is mainly affected by the fluctuation of heat transfer and mass transfer in Tp and Tb period of thermal pulse.

**Figure 12.** The variation of weld pool profile in thermal pulse: (**a**) P-GMAW (No.12); (**b**) peak period of thermal pulse of DP-GMAW (No.3); (**c**) Base period of thermal pulse of DP-GMAW (No.3).

The length variation of the weld pool under different parameters of the current waveform are shown in Figure 13. Figure 13a shows the weld pool length at Tp and Tb with different TPF. During the P-GMAW welding process, the weld pool can be regarded as a constant. The blue and red curves of Figure 13a are the curve of weld pool length at Tp and Tb, respectively, with the thermal frequency. The descending trend of blue curve and the ascending red curve indicates that pool length at Tp is gradually shortened while that at Tb is gradually increased with the increase of heat pulse frequency. It indicates that with the increase of thermal frequency, the period of Tp decreases, and the heat accumulation and droplet transition acting on the weld pool gradually decrease, resulting in the decrease of the weld pool length at Tp. While the differences of the heat and droplet transfer

amount into the weld pool at Tp and Tb stage gradually decreases, the pool length difference between Tp and Tb gradually decreases but both close to the length of P-GMAW.

**Figure 13.** Variation of weld pool length with (**a**) heat thermal frequency; (**b**) duty cycle of the thermal pulse; (**c**) current amplitude of thermal pulse.

Figure 13b shows the pool length at Tp and Tb with different DTp. With larger DTp, the total heat input and droplet transition acting on the weld pool increase, the pool length at Tp and Tb both increasing. It has to be noticed that as DTp increases, the difference between the pool length at Tp and Tb always increases first and then decreases. Small DTp could cause longer Tb with fixed thermal pulse frequency, the heat accumulation in Tp was too small to significantly expand the size of weld pool. A larger DTp could cause smaller Tb, shorter low heat input time (Tb) could not cause a significant reduction in the size of the pool. Too large or too small DTp could result in small length difference.

Figure 13c shows the pool length at Tp and Tb with different ΔI. With larger ΔI, the total heat input and droplet transition acting on the weld pool decrease, the pool length at Tp and Tb both decreasing. Larger ΔI increases the differences of heat accumulation and amount of deposited metal between the Tp and Tb.

The oscillation process of weld pool was recorded by the method mentioned in Section 2.2. Two typical current waveform parameters (No. 3 and No. 11) were selected to summarize the oscillation process of the weld pool, as shown in Figure 14. The fluctuation in the amplitude of the pool oscillation during the P-GMAW welding process was constant (Figure 14a), while that during the DP-GMAW was obvious (Figure 14b). That is the oscillation amplitude of weld pool in Tp is much larger than that in Tb, since the droplet impingement force of P-GMAW and the size of weld pool were stable (Figures 11b and 13a), the amplitude of pool oscillation can be regarded as a constant.

**Figure 14.** Height of reference point A as a function of time: (**a**) P-GMAW(No.12); (**b**) DP-GMAW(No.3).

In DP-GMAW, the fluctuation process of oscillation amplitude is similar to the process of thermal and pressure on the weld pool, as shown in Figure 11. The droplet impingement force on the weld pool increases significantly when the high-frequency current pulses increase the volume of weld pool during Tp period, which is the main reason for the larger oscillation amplitude of the weld pool. The size of the weld pool and the droplet impingement force simultaneously resulting in a decrease

in the amplitude of oscillation during Tb period. The variation of oscillation amplitude of weld pool under different parameters of the current waveform are shown in Figure 15.

**Figure 15.** Variation of weld pool oscillation amplitude with (**a**) heat thermal frequency; (**b**) duty cycle of the thermal pulse; (**c**) current amplitude of thermal pulse.

The curves of the weld pool amplitude in Tp and Tb under different thermal pulse frequencies had a similar trend to the weld pool length, as shown in Figures 13a and 15a. Increasing the frequency of thermal pulses led to a reduction in the difference between the size of the weld pool at Tp and Tb. As for the oscillation amplitude at Tp, the decrease in the size of the weld pool can decrease the oscillation amplitude under the same droplet impingement force. The increase in the weld pool size at Tb can increase the oscillation amplitude, as shown in the red line of Figure 15a. However, there is no denying the fact that the average oscillation amplitude of DP-GMAW was greater than that of P-GMAW.

Larger duty cycle of the thermal pulse (DTp) can trigger greater oscillation amplitude with larger pool size and longer high-frequency droplet impingement. too large or too small DTp could result in small amplitude difference of the oscillation amplitude within Tp and Td, as shown in Figure 15b.

Larger ΔI will increase the difference between the pool size and the droplet impingement force in Tp and Tb, leading to a greater oscillation amplitude difference. However, the decrease of total heat input decreased the average amplitude of weld pool, as shown in Figure 15c.

So, as to what is known, DP-GMAW weld pool behavior is more complicated compared with P-GMAW. During switching from Tp to Tb, the pool size experiences "expanding–shrinking" variation, the change of the oscillation amplitude of weld pool is synchronized with the pool size.
