## 3 Results and Discussion  
### The Welding Pool Oscillation Process  
Multiple consecutive high-speed photographic images during one droplet transition period of the conventional short-circuit transition process is selected to observe the oscillation process of the weld pool, as shown in Figure 2a. Figure 2b is the oscillation signal of the reference point B on the surface of the weld pool. The position of the wave crest is shown by the arrow in Figure 2a. As shown in Figure 2a, due to the impact of the electric explosion, the liquid metal of the weld pool under the welding wire forms a surface wave under pressure and transmits to the surrounding, and the surface wave at the back of the welding wire moves to the rear edge, which did not pass the reference point, as shown by Arrow 1 at 28.5 ms in Figure 2a. Since the forward-moving surface wave is hindered by the front wall of the weld pool, it has a higher peak, which is defined as the main wave in this paper. The main wave causes a drastic height change at the reference point, as shown by Arrow 2 at 30.7 ms in Figure 2a. The two wave crests merge into one wave crest during the backward movement, as shown by the blue arrow at 32.9 ms in Figure 2a. The amplitude of the secondary wave, formed after the main wave passes through, gradually decreases under the action of oscillation damping, as shown by the blue arrow at 44.2 ms in Figure 2a. Usually, when the main wave crest propagates towards the trailing edge of the weld pool, another liquid traveling wave will be triggered at the next droplet transition. The continuous superposition and attenuation of these traveling waves constitute the undulating liquid level of the weld pool. Figure 3 shows the oscillation signal of the partially penetrated weld pool with different wire feeding speeds (2.4 m/min, 2.7 m/min, 3.0 m/min, 3.3 m/min). It can be found that the oscillation amplitude increases with the increase of the wire feeding speed in the S-GMAW process. The heat input and amount of metal transition acting on the weld pool is low with low wire feeding speed, which results in the weld pool with smaller volume and less deformation, as shown in Figure 4a. With the increase of wire feeding speed, the melting amount of welding wire increases per unit time, which results in more deposition of molten metal and increase of the weld pool size; the amount of metal pushed away by the impact of electric explosion increases, and the deformation of weld pool and the amplitude of weld pool also increase, as shown in Figure 4b. Figure 2: (**a**) The oscillation process of partially penetrated S-GMAW weld pool (wire feeding speed is 3 m/min, plate thickness is 4 mm); (**b**) The oscillation signal at reference point of partially penetrated S-GMAW weld pool. Due to the existence of oscillation damping, the oscillation amplitude of the weld pool would gradually decrease during one droplet transition period, as shown in the peak amplitude of the oscillation after the main wave in the green box in Figure 3. [PERSON] et al. [11] indicated that, when the main wave of the weld pool propagates in the fluid for a distance \(x\), the relationship between the oscillation amplitude \(A\) and the energy decay factor \(\alpha\) is shown in Equation (1):  
\[A=A_{0}\times\mathrm{e}^{-\alpha x} \tag{1}\]  
where, \(A_{0}\) is the initial oscillation amplitude of the weld pool and \(\alpha\) is the energy attenuation factor, which is related to the viscosity of the weld pool fluid. Figure 4: Schematic diagram of the influence of the wire feed speed on the pool oscillation: (**a**) Low wire feed speed with small volume of welding pool and weak external impact; (**b**) High wire feed speed with large volume of welding pool and strong external impact. Figure 3: The oscillating signal at the reference point on the surface of the molten pool of the S-GMAW process with different wire feeding speeds. (**a**) 2.4 m/min; (**b**) 2.7 m/min; (**c**) 3.0 m/min; (**d**) 3.3 m/min. [PERSON] et al.