WELDING RESEARCH ing effect due to the cold water environment, and the electromagnetic pinch force caused by the increasing current density in the arc column (Ref. 13). As shown in Fig. 8, if we zoom in on the images at 0.167 and 0.179 s, the droplets could be observed in more detail. Figure 8A clearly shows the droplet being pushed away from the arc and obviously deviating from the wire axis. The size was more than twice that of the wire diameter. The welding arc under the molten metal was strong and bright. As for Fig. 8B, the huge droplet was being lifted by complex forces. And the surrounding bubble was also growing larger. It can be inferred that the larger volume molten droplet extended the hightemperature field. Subsequently, the shorter distance from the previous bubble wall to the molten droplet resulted in more vaporized water. According to the research results of conventional GMAW, a possible contributor causing the repelled globular transfer mode was the cathode jet force on the droplets based on the static force balance theory (Ref. 16). Subsequently, it was deduced that the drifting cathode spot might cause varying force values and directions, which resulted in random flying paths of the droplets. In addition, the gas flow drag force was also considered an important part of the complex forces during welding (Ref. 25). Further research and analysis of the forces will be conducted in another paper. As previously mentioned, the weld appearance was uneven and the weld bead was oblique. This phenomenon could be explained and predicted by the complicated droplet transfer and welding arc behaviors. The captured repelled large droplet transfer mode especially caused random flying paths and uncertain landing locations. Large droplets also resulted in the intense fluctuation of the weld pool. In this case, the uneven and asymmetric weld bead was not an accident. Furthermore, the complex and periodically evolving bubbles surrounding the arc area had an inevitable impact on the arc and droplet’s behaviors. Some physic-chemical interactions provided extra forces on the droplets’ and finally affected their transfer paths and dimensions. Arc Drifting and Deviation Behaviors Analysis As shown in Fig. 9, the drifting arc cathode caused the deviation of the arc from the axis with a big angle, which was measured at about 30 deg. The amplitude was approximately 2.1 mm from the wire axis to the cathode 208-s WELDING JOURNAL / JUNE 2016, VOL. 95 spot. The value was about 1.3 times that of the wire diameter. According to the principle of minimum voltage, the welding arc always consumed the minimum energy along that path. The authors deduced that the oxide distribution and the cold cathode substrate caused this phenomenon. First, the oxides at the center of the weld pool were quickly consumed and removed. By contrast, more oxides existed at the edge of the weld pool, which were oxidized by the ionized oxygen. As a result, the cathode spot was attracted to the locations enriched with oxides. Second, the substrate was a typical cold cathode, which had high thermal conductivity due to the surrounding water environment. This typical arc always had a wandering arc and drifting cathode spot (Ref. 26). According to Fig. 6, although the welding arc cathode spot kept drifting with high frequency and amplitude, the arc length varied in a small range. As shown in Fig. 9, the arc length was about 3.05 mm, which indicated the wet FCAW was a typical short-arc welding method. Since the arc voltage was about 30 V, the average electric potential gradient of the arc was about 98.4 V/cm. Taking account of the anode and cathode voltages, which we assumed to be 1 and 10 V, respectively, the average electric potential gradient of the arc column could be calculated at about 63 V/cm. Compared with the values of other conventional gas metal arc welding methods (about 10 V/cm), the gradient was apparently much higher. This characteristic was in coincidence with the Gerdien arc (Refs. 26, 27), which was stabilized by a vortex of cold water generating higher current density and higher arc voltage gradient. First, the rapid cooling effect from the surrounding water was similar to the Gerdien arc. As a result, the welding arc was significantly compressed with higher current intensity and higher electric potential gradient. Second, with the impact of the surrounding water environment, vaporized and ionized water produced hydrogen and oxygen that take part in the ionization of the arc plasma (Refs. 13, 28). Note that a big proportion of the complex gases was H2 (62–95%) (Ref. 28). Due to different thermal conductivity and the thermal pinch effect, the arc column electric potential gradients in the hydrogen, vapor, and oxygen atmosphere were about 20, 8, and 4 times that in the argon atmosphere, respectively (Ref. 26). Therefore, the water environment and special mixed gas led to the high arc electric potential gradient. The welding current density in the arc column could be calculated based on the set current (measured as 205 A) and arc diameter. At the anode spot on the wire tip, the arc diameter was about 2.28 mm, and correspondingly, the arc diameter at the substrate was about 4.98 mm. Subsequently, the current density in the arc column has a maximum value about 5024 A/cm2 and a minimum value about 1053A/cm2 (near the substrate). Assuming the arc was a circular truncated cone, the average current density through the arc can be calculated according to the given equation: I = I VC / h = 205 1 3 (4.982 + 4.98 2.28 + 2.282)/400 = 1894 A/cm2 (1) where I meant the welding current, VC meant the assumed arc volume with a circular truncated cone shape, and h meant the arc length. Conclusions 1) During underwater wet FCAW, the bubbles, welding arc, and molten droplets continuously interacted with each other, and the very complex behaviors were captured by the developed visual sensing system. Two background light sources, i.e., a large dysprosium lamp and laser were employed to monitor the bubbles inside and outside, respectively. The bubbles grew violently in a pulsed way with varying speed rather than in a continuous, smooth way. And the bottom bubbles on the substrate surface could cover the weld pool.
Welding Journal | June 2016
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