WELDING RESEARCH JUNE 2016 / WELDING JOURNAL 205-s tions and refractions. Third, the amount of weld fumes floating in the bubbles and surrounding water result in light intensity loss and blurred observations. An imaging system was designed to solve the problems. As shown in Fig. 1, underwater welding experiments would be conducted in the water tank (400 700 500 mm), which was thin in the vertical direction to reduce the light intensity attenuation. The water tank was placed on a platform driven by a step motor to move linearly along the welding direction, and meanwhile, the torch would be kept stationary. Subsequently, a high-speed camera and a background light source were placed outside the tank and kept at constant locations with certain angles according to the welding torch. Note that the bubbles were assumed regular round. And the lens diameter of the light source was required to be bigger than the bubbles’ diameters to fully cover the welding area. Furthermore, based on the observation by the authors, the bubbles’ diameters seemed generally smaller than 20 mm. Optical lenses, including a sharp cut-off filter, a narrow bandpass filter, and dimmer glass, were employed to reduce the light distortions and solve the disturbances from the strong arc radiation. Note that a big volume of water was beneficial to the dilution of the welding fumes in the water. Therefore, the tank was almost full before the experiments. The selection of background light source was very important to overcome the strong arc radiation disturbance. For this research, two background light sources were required to monitor the bubbles inside and outside separately; one light source should be capable of penetrating the bubble walls to observe the droplet and welding arc, while the other had to have a bigger diameter to observe the bubbles’ behaviors from the macro perspective. For the latter one, a dysprosium lamp with a diameter of about 0.5 m was selected and placed at the other side of the tank opposite to the camera. However, selecting the former one was a much more challenging job. Since the specific wavelength and intensity needed to be determined, it was proposed that the spectral radiation from the welding process be collected and analyzed to acquire the real distribution curve at different wavelengths (Ref. 13). The wavelength at which the welding arc had a relatively lower intensity radiation could be adopted to choose suitable background light. In this case, the required laser power would be lowered to surpass the arc intensity at the specific wavelength. Based on the authors’ former research results and the introduced spectral analysis method (Ref. 13), the collected spectrum curve is shown in Fig. 2. The characteristics of the distribution at different bands can be obtained in Fig. 2. The ultraviolet band (200–370 nm) had short wavelengths, and its relative intensity was much lower than others. Visible light (380–760 nm) from the welding area had the most intensive radiations, especially at the green and yellow light bands (520–610 nm). The red light intensity seemed stable within a certain range. At the band 761–840 nm, a downward tendency of the near-infrared light intensity can be observed with the mounting wavelength. Accordingly, the red light and nearinfrared light had lower and more stable relative intensity. In addition, longer wavelengths could reduce the loss when traveling either in the water or air. Therefore, the background laser should be elected with the wavelength in this band to easily eliminate strong arc disturbances. Taking account of the light attenuation in the water and air, a laser was selected with an 808-nm wavelength, and its rated power was 30 W. In addition, the laser lens diameter was chosen as approximately 20 mm. The imaging system was then developed based on a high-speed camera (Optronis Cam- Record 5000 2) with a frequency of 2000 fps and resolution 512 512. Underwater welding experiments were conducted to visually sense the welding process. The dynamic behaviors of the bubbles, metal transfer, and arc behaviors were investigated, and general weld appearance characteristics were described. Results Typical Bubble Evolution Underwater wet FCAW is a selfshielded welding method in nature, which is shielded by the continuously generated slag and gases. On one hand, the welding arc burns in the generated bubbles, which are full of gases. On the other hand, the slag on the bead surface protects the weld pool from the water environment. The two protections provide the possibility to obtain a stable welding process and high-quality weld joints. The mechanism of how the reac- Fig. 5 — The metal transfer process during underwater wet FCAW. Fig. 6 — The underwater welding arc behaviors.
Welding Journal | June 2016
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