WELDING RESEARCH JUNE 2016/ WELDING JOURNAL 195-s laser power density. Shao continued the effort in this direction, but the focus was to track the droplet in real time in order to apply the laser pulse at the right time to the right location (Refs. 23, 24). As a further significant evolution in this direction, the first part of this study used a pulsed fiber laser and focused it into a 0.5-mm-diameter spot to irradiate the droplet. Currentindependent metal transfer was successfully obtained by applying a 1200-W laser pulse for 5 ms (Ref. 25). Although the welding current was only 40 A, the detached droplet diameter was approximately only 1 mm. Since the sum of the electromagnetic force, plasma dragging force, and droplet gravitational force was much smaller than the retaining force, i.e., the surface tension, it is apparent that the detachment was dominated by the laser pulse rather than the welding current. The droplet detachment was thus independent of the current. The metal transfer frequency was exactly the same with the laser pulse frequency. The metal transfer mode was exactly ODPP. Although current-independent metal transfer has been successfully obtained by applying a high-powerdensity laser pulse, the parameters were experimentally determined without systematic understanding of how relevant parameters and resultant transfer are related and how the parameters should be determined. As such, this second part of the study is devoted to gaining such a systematic understanding through experiments and analysis. The major relevant parameters to be studied and analyzed include the following three types: Laser Positioning Parameters, including the laser incident point on the droplet and laser incident angle . They are the two most important parameters since they influence the laser recoil force and droplet reaction. Laser Waveform Parameters, including the laser peak power, peak duration, and pulse frequency. They directly determine if the droplet can be detached by the laser pulse. The minimum laser peak power and duration need to be determined to ensure the detachment will be successful while still minimizing the needed laser pulse energy. Welding Arc Parameters, including the welding current, gun angle, and arc length. These parameters all significantly influence the welding process of conventional GMAW. It is thus also crucial to understand how they affect the laser-droplet interaction behavior. Experimental System Figure 1 shows the experimental configuration — the same as the one used in the first part of this study. The Fig. 1 — Sketch of experimental system. Fig. 2 — Illustration of laser setup parameters. Fig. 3 — Illustration of laser incident point on droplet. Fig. 4 — Metal transfer with laser aimed at droplet neck, 60deg incident angle.
Welding Journal | June 2016
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