WELDING RESEARCH A B small as 30 deg, the laser was no longer aimed at the droplet neck but at the droplet main body after the initial droplet deformation/elongation occurred. The focused laser beam was thus surrounded by the relatively thick liquid droplet. Due to the effect of the surface tension and the gravitational force of the liquid metal above the laser penetrating path, the laser could not dig the droplet surface into a groove shape and then cut it off. Further, with a smaller laser incident angle, the droplet deflection tended to make the droplet move away from the laser irradiation, thus further weakening the laser digging effect in the laser incident direction. The radial component of the laser recoil force became the primary, which consequently increased the droplet deflection. On the other hand, the velocity of the detached droplet measured from Experiments 4–8 did not demonstrate significant 198-s WELDING JOURNAL / JUNE 2016, VOL. 95 differences. With respect to the smaller droplet deflection, the optimal range of laser incident angle was determined to be 45–60 deg. If not otherwise specified, the laser incident angle was fixed at 45 deg as the default in the experiments that follow. Laser Pulse Waveform After the optimal laser positioning parameters were determined, the next task was to determine the minimum laser peak power and duration that could achieve robust ODPP metal transfer. Given the laser spot diameter, the laser peak power determined the laser power density, thus determining the amplitude of the laser recoil pressure that determined if the droplet neck could be effectively dug, while the laser peak duration determined if the laser had an adequate time to penetrate the whole wire and cut off the droplet neck. Therefore, the effect of laser peak power and duration are analyzed in this subsection. Laser Peak Power. Experiments 9–11 used laser peak power of 90% (1400 W), 60% (950 W), and 45% (700 W), respectively. Other parameters were the same as those in Experiment 7. The result of Experiment 7 thus was used as the reference for comparison. High-speed images show the droplet could not be robustly detached when only 45% of the laser peak power was used. Approximately 25% of the droplets could not be detached by a single laser pulse. Hence, Fig. 8 only shows the metal transfer under 60% and 90% of the laser peak power. Droplet deflection and velocity are measured and shown in Fig. 9. It can be seen that droplet deflection under 90% and 75% of the laser peak power is quite close. However, when the power of the laser pulse was reduced to 60% of the peak power, the droplet deflection significantly increased by 147% to 42 deg; meanwhile, the droplet velocity also significantly decreased. Overall, the droplet velocity /deflection increased/decreased as the laser pulse power increased but the increase/decrease was getting slower. The preferred result is the combination of a moderate droplet velocity ensuring the detachment and relatively small deflections ensuring the bead formation. From this point of view, among the three levels of pulse power, 75% (1200 W) was considered moderate. Laser Peak Duration. In fact, one can see from Fig. 6 that almost all the droplets were fully detached from the wire tip after 5 ms of the laser pulse application. Even using 90% of laser peak power for ensuring definitely ad- Fig. 8 — Metal transfer under different laser peak powers: A — 90%; B — 60%. Fig. 10 — Typical metal transfer with different laser pulse frequences: A — 10 Hz; B — 20 Hz. Fig. 9 — Effect of laser peak power on drop A deflection and velocity. B Table 3 — Varying Parameters in Experiments 17–19 No. Current (A) Laser Pulse Frequency (Hz) 17 40 10 18 80 20 19 120 30
Welding Journal | June 2016
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