WELDING RESEARCH 17–19. The presented high-speed image sequences show the droplet deflection tended to decrease with the welding current, given the laser pulse energy and droplet size. The droplet deflections in Experiments 14–19 were measured and are shown in Fig. 12. As can be seen, the droplet radial deflection was almost negligible when the droplet was surrounded by the 120-A current arc. This is explainable because the droplet was actually surrounded and restrained by the arc-related forces, mainly the electromagnetic force and plasma dragging force. The electromagnetic force is proportional to the square of the welding current. The higher the current is, the more axially restrained the droplet becomes; therefore, the smaller the deflection will be. Gun Orientation. During GMAW, the arc, reflected by the arc shape, affects the distribution of the arc forces. In laser pulse-controlled GMAW, droplet detachment and flying trajectory may be affected by changing the arc deflection. In Experiments 20 and 21, the gun was tilted right and left, respectively, for 15 deg. In particular, the laser incident angle was changed to 75 deg to avoid blocking of the laser beam when tilting the gun. The results of Experiment 5, where is zero and the other parameters were the same, are also referred for comparison. Figure 13 illustrates tilting of the gun and Fig. 14 shows typical metal transfers in Experiments 20 and 21. It can be seen that droplet deflection was effectively 200-sWELDING JOURNAL / JUNE 2016, VOL. 95 reduced by intentionally tilting the gun against the laser beam, because the arc forces in this case would push the droplet against the laser impulse to a certain extent even though the arc forces generated by an 80-A welding current are relatively small. Arc Length. In the experiments presented previously, the arc length was controlled to be stable at 6 mm. Experiment 22 switched to a shorter arc length, 4 mm, to examine the effect of the arc length on the resultant metal transfer behavior. The welding current was 80 A, the laser pulse was 1200 W × 5 ms, and the pulse frequency was 25 Hz. The laser incident angle was 60 deg. Figure 16 shows the typical metal transfer in Experiment 22. No obvious droplet deflection was observed. The relatively low welding current used and slow travel speed resulted in relatively thick/high weld pool elevation behind the arc, which together with the short arc length determined that the arc tended to be forward deflected rather than wire-axial symmetrical, as shown in Fig. 17. The distribution of the arc plasma, as well as the related arc forces, were consequently changed. In this case, the arc forces tended to push the droplet against the laser impulse and thus contributed to reducing the droplet deflection. On the other hand, using short arcs could also reduce the flying time of the droplet, resulting in a smaller radial offset distance even if the droplet deflection angle was the same. Conclusions In the first part of this study, the ideal current-independent metal transfer was successfully achieved by using a high-power-density laser pulse to irradiate the droplets. The current paper further experimentally verified the effect of key process parameters on the droplet detachment behavior, especially the droplet deflection after being detached. The major parameters include the laser positioning parameters, laser pulse waveform parameters, and arc parameters. Their effects on the resultant metal transfer can be summarized as follows: 1) The optimal laser incident position was determined to be the droplet A B Fig. 15 — Effect of gun orientation on droplet deflection. Fig. 17 — Illustration of arc deflection in Experiment 22. Fig. 14 — Metal transfer under different gun orientations: A — Gun tilting left; B — gun tilting right. Fig. 16 — Laser pulsecontrolled metal transfer with short arc.
Welding Journal | June 2016
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