# Study on Penetration Sensing Method Based on Pool Oscillation and Arc Voltage during Pulsed GMAW
## 3 Results and Discussion  
### Oscillation Characteristics of Weld Pool in P-GMAW  
P-GMAW has the advantages of directional metal transfer and no spatter over wide ranges of heat and mass input levels. It uses a low base current to maintain the arc and a high peak current to melt the electrode wire and detach the droplet. The impact force of the droplet was the main factor to trigger the oscillation behavior of the weld pool. Figures 4 and 5 show the dynamic behavior of the weld pool during base current under full and partial penetration. Figures 4 and 5 show that the weld pool surface below the wire was depressed due to the impact of the droplet, then the surface of the weld pool bounces to form waves. The red arrow of Figures 4 and 5 show the positions of wave peaks on the weld pool surface at different times. Under the same pulse parameters, the impacts of the droplets on the pool were similar [15], while the fully penetrated weld pool and the partly penetrated weld pool have a distinct difference in the oscillation behavior. Compared with the oscillation amplitude of the partial penetrated pool (according to the \(\Delta\)H = 20 pixels in Figure 4), the oscillation amplitude of the fully penetrated weld pool was larger (according to the \(\Delta\)H = 10 pixels in Figure 5). The difference in the oscillation behavior of the weld  
Figure 4: Combination of voltage signal waveform and fully penetrated weld pool oscillation process: High-speed photographs of weld pool at different times:(a)–(j). (welding travel speed: 13.75 cm/min). Figure 3: Different penetration status. Welding travel speed (cm/min): (a) 16.5; (b) 19.25; (c) 24.75.  
pool leads to the difference in the arc length, which resulted the difference from the voltage signals (as shown from the blue boxes in Figures 4 and 5). To further study the relationship between penetration status and oscillation behavior of the weld pool, the welding joints with different penetration status were manufactured under different welding travel speeds with the same welding parameters. Analysis of the weld pool dynamics was carried out by measuring the Y-coordinate of the reference point A on the pool surface with time, as described in Section 2.1. The results of the analysis for weld pools with different penetration status are presented in Figure 6a-g respectively. The start time in Figure 6 is the moment at the droplet entered the pool completely. The test was repeated 3-4 times to verify the repeatability of the weld pool oscillation behavior for each parameter. The weld pool was agitated, and waves were generated due to the impact of the droplets after the droplet impacts the weld pool. In this paper, the first wave peak was called the initial wave peak, the amplitude of each curve was measured in this paper, as shown in Figure 6a. The energy of which was derived from the momentum carried by the droplet. After the liquid surface falls back, the amplitude of the subsequent wave peak was smaller than that of the first wave peak due to the energy loss of the oscillation [11]. The weld pool penetration gradually changed from full penetration to critical penetration with the welding speed increased, and the amplitude of the wave peak decreased gradually while the number of subsequent wave peak increased, as shown in Figure 6a-d. However, only one wave peak occurred on the curves for the partly penetrated weld pools, as shown in Figure 6e-g. In this paper, all weld joints were manufactured under the same parameters, welding speed was the only variable parameters, so the impact force applied to all weld pools were consistent. The geometry and boundary conditions of the weld pool under different penetration status were the main factors causing different motion behaviors. Figure 7 shows the geometry of the weld joints under different penetration status, and the effective throat thickness and the width of the back weld were calibrated according to ISO-17695.