# Study on Penetration Sensing Method Based on Pool Oscillation and Arc Voltage during Pulsed GMAW
## 3 Results and Discussion
### The Link between the Voltage Signal and the Weld Pool Behavior  
High-speed photographic images showed that the weld pool oscillation changed the relative position between the tip of the welding wire and the surface of the weld pool. It affected the arc length and caused the voltage signal fluctuation during the base time, as shown in Figure 13. The shortest distance (L\({}_{\rm V}\)) and vertical distance (L\({}_{\rm min}\)) between the droplet and the pool during the base time were measured by image processing algorithm (as shown in Section 2.1), and the relationship between the pool behavior and the voltage was analyzed. Figure 12: Components of the weld pool oscillation signal at 40 Hz and 80 Hz with different penetration status. Full penetration: (**a**) V = 11 cm/min; (**b**) V = 16.5 cm/min. Critical penetration: (**c**) V = 19.25 cm/min. partial penetration: (**d**) V = 30.25 cm/min. Figure 13: The relative position between the tip of the wire and the weld pool during base time (V = 13.75 cm/min). As shown in Figure 13, two main factors affected the relative position between the wire tip and the pool surface: (1) wire extension variation; (2) changes in height and curvature of weld pool surface. During the base time, the wire feeding speed was constant, while the wire could not melt, and the length of stick-out wire increased. The increased length of stick-out wire could lead to the arc length compression and the voltage drop over the wire increase, but the conclusion given in papers [13; 19] shows that the change in the voltage drop over the wire extension during the base current time did little contribution to the voltage signal. It is commonly accepted that the arc voltage is proportional to the arc length. Each of the above exceptions could lead to arc voltage variation. Figure 14 shows the synchronous curves of the voltage signal, the minimum distance (L\({}_{\text{min}}\)) and the vertical dimension (L\({}_{\text{V}}\)). As shown in Figure 14, the voltage signal curve, the minimum distance (L\({}_{\text{min}}\)) curve and the vertical dimension (L\({}_{\text{V}}\)) curve all showed an obvious trough for all penetration status. It was caused by the initial wave peak of the weld pool during the base time. The vertical dimension curve is the direct response to the weld pool motion (as shown in Figure 2). The voltage signal curve had good consistency with the minimum distance curve. The starting and ending time of both troughs and shape of the curves were the similar as shown in Figure 14a,b,d,e. It illustrates that the change of the shortest distance between the weld pool surface and the tip of the wire, which is the effective length of the arc, is the major factor of voltage fluctuation. However, the end time of the trough of the minimum distance curves and the voltage curves acted lag behind which of the voltage signal curves. It was caused by the transfer process of wave peaks on the surface of the weld pool. With the impact of the molten drop, a surface wave peak was formed and moved to the rear part of the weld pool. When the wave peak moved backwards and the pool under the welding wire fall back, the vertical dimension increased, while the minimum distance did not increase synchronously, as shown in Figure 13. The minimum distance curve and the voltage signal curve lost the frequency characteristics of the pool oscillation due to the hysteresis of ending time of the trough, so the pool oscillation frequency cannot be extracted from the voltage signal. Figure 14: The synchronous curves of the voltage signal, the minimum distance (L\({}_{\text{min}}\)) and the vertical dimension (LV). Full penetration (V = 13.75 cm/min). (**a–c**). Partial penetration (V = 24.75 cm/min). (**d–f**). As shown in Figures 13 and 14, due to the curvature of the weld pool surface, there was some difference between \(\Delta\)Lmin and \(\Delta\)LV. while it could not be denied that \(\Delta\)Lmin had evident positive correlation with \(\Delta\)LV.