WELDING RESEARCH JUNE 2016/ WELDING JOURNAL 199-s A B C equate laser recoil force, it still took at least 4 ms to fully detach the droplet, as can be seen from Fig. 8A. These results imply that 5 ms should be the minimum laser peak duration to ensure a robust droplet detachment. However, it was still in the authors’ interest to know what would happen if a shorter laser pulse was used. Experiments 12 and 13 were thus conducted with 3- and 4-ms laser pulse duration. The laser pulse power was set at 90% to guarantee the laser recoil pressure would be sufficiently high. Experimental results show almost no droplets could be detached by a laser pulse with 3-ms duration. Instead, the droplets could be detached by the laser pulse with 4-ms duration with approximately 75% rate. Hence, 5-ms laser pulse duration was confirmed to be the minimum to ensure a stable current-independent droplet detachment for the wire used. Laser Pulse Frequency. Since the metal transfer mode in pulsed lasercontrolled GMAW is actually ODPP (one drop per laser pulse) transfer, the laser pulse frequency exactly determines the metal transfer frequency. Given the welding current, the laser pulse frequency actually determines the time for the droplet to grow. Hence, the effect of the laser frequency on the metal transfer is actually the effect of the droplet mass at the detaching moment. Experiments 14–16 used 10-, 20-, and 30-Hz laser pulse frequency, respectively. The welding current was fixed at 40 A. It is found that the metal transfer in Experiment 16, with 30-Hz laser pulse frequency, became unstable. Typical metal transfers in the Experiments 14 and 15 are shown in Fig. 10A–C. It can be seen that the detached droplet was getting smaller and the droplet deflections look increased when the laser pulse frequency was increasing. The droplet velocity and deflection in Experiments 14 and 15 are measured and shown in Table 2 for quantitative analysis. It can be seen that the droplet deflection and velocity both increased significantly when the laser pulse frequency was increased from 10 to 20 Hz, as can be predicted by the theorem of momentum, given the laser pulse, the smaller droplet mass, the larger droplet velocity, and thus the larger deflection. Welding Arc Parameters Welding current, gun orientation, and arc length are the three crucial parameters in conventional GMAW. It was expected they might also influence the laser pulse-controlled metal transfer to a certain extent. Therefore, a series of experiments was conducted in this subsection to explore the effect of these parameters on the metal transfer, step by step. Welding Current. The welding current determines the electromagnetic force acting on the droplet. Although the electromagnetic force under a low current is not sufficient to detach the droplet by itself, it does produce certain effects on the laser-dominated metal transfer process. Table 3 shows the welding current used in Experiments 17–19. The laser pulse frequency changed with the current in order to control the droplet growing period such that the initial droplet size at the laser pulse emitting moment was approximately the same in these three experiments. Figure 11A–C shows typical metal transfer behaviors in Experiments Fig. 11 — Metal transfer under different welding currents: A — Welding current I = 120 A, laser pulse frequency 33 Hz; B — welding current I = 80 A, laser pulse frequency 20 Hz; C — welding current I = 40 A, laser pulse frequency 10 Hz. A B Fig. 12 — Droplet deflection under different welding currents. Fig. 13 — Illustration of weld gun orientation: A — Gun tilting left; B — gun tilting right.
Welding Journal | June 2016
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