# 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
Figure 5: Combination of voltage signal waveform and partly penetrated weld pool oscillation process: High-speed photographs of weld pool at different times:(**a**)–(**j**) (welding travel speed: 24.75 cm/min). Figure 6: The height of the reference point A on the weld pools surface as a function of time. Full penetration: (**a**) V = 11 cm/min; (**b**) V = 13.75 cm/min; (**c**) V = 16.5 cm/min. Critical penetration: (**d**) V = 19.25 cm/min. Full penetration: (**e**) V = 24.75 cm/min; (**f**) V = 30.25 cm/min; (**g**) V = 35.75 cm/min. Figure 7: Transverse sections of weld joints with different welding travel speed. Full penetration: (**a-c**); critical penetration: (**d**); full penetration: (**e-g**). As shown in Figure 7, the cross-sectional area and penetration depth of the weld decreased with the acceleration of the welding speed. As for full penetration status, the effective thickness (d) of the weld was 2 mm, and the width of the back weld (w) decreases with the increase of the welding speed as shown in Figure 7a-c. The effective thickness of the critical penetration was 2 mm, and the width of the back weld (w) was 1.35 mm as shown in Figure 7d. As for partial penetration. Effective weld thickness (d) decreased with the increase of welding speed, as shown in Figure 7e-g. Figure 8 shows the amplitude of the initial oscillation peak and the geometry of the weld joints under different penetration status. Figure 8 shows the curves of oscillation amplitude and the geometry of weld joints with different welding speeds. The oscillation amplitude decreased with increasing welding speed and an abrupt change occurred in the amplitude of weld pool oscillation with the transition from full to partial penetration status. Different boundary conditions between partly and fully penetrated weld pool led to different oscillation modes and amplitude. Figure 9 shows the oscillation mode with different penetration status. For partly penetrated weld pool, the bottom of the weld pool was supported by the solid material, which obstructed the downward movement of the molten metal. It led to a greater loss of energy in the process of oscillation, resulting in a lower amplitude. As for critical penetration status, the main part of the pool was supported by the solid metal with relatively small W, leading to the similar oscillation behavior as the partial penetration, as shown in Figure 6d,e. As for full penetration, W was relatively larger. The weld pool was only supported by the surface tension of the molten metal within the range of W. In this case, a new free surface and degree of freedom were generated in the weld pool, which led to a change in the oscillation mode of the weld pool. [PERSON] and [PERSON] established an analytical model based on the theory of stretch membrane and studied the oscillatory behavior of the fully penetrated pool [16]. It was found that the oscillation amplitude of the stationary full penetration pool was more intense than that of the partial and critical penetration due to the lack of solid boundary constraint at the weld pool bottom. The change of oscillation mode leads to the different oscillation frequency characteristics of the weld pool. The location information of reference point A cannot be continuously extracted because the droplet impinged on the weld pool there. In this experiment, the measured height of reference point B (H\({}_{\text{B}}\)) for partial and full penetration status are plotted as a function of time in Figure 10.  
Figure 8: The amplitude of initial oscillation peak and geometry of the weld joints under different penetration status. The oscillation signal of the weld pool was a nonlinear time-varying signal. The Continuous Wavelet Transform (CWT) can analyze time-varying signals in the time domain and frequency domain simultaneously.