WELDING RESEARCH ducted using pulsed laser irradiation. The laser spot was aimed at the drop neck and mid droplet in order to compare. The laser peak power is set at 75% (1200 W), and the laser peak duration fixed at 5 ms. The laser pulse frequency is 25 Hz. The laser incident angle is 60 deg. The torch orientation angle is zero. The images of metal transfer are selected from the highspeed sequence with 1 ms time interval. The first frame corresponds to the starting moment of the laser pulse emission. The typical metal transfer of experiment 2, with the laser applied at the mid-top of the droplet, and experiment 3, with the laser applied at the droplet neck, are shown in Figs. 8 and 9, respectively. It can be seen from both the figures the droplets are all successfully detached by the laser pulse. The detached droplet diameter has only approximately a 1.3 mm diameter. Laser Controllability on Metal Transfer Here, in experiments 2 and 3, since the current is only 80 A, the electromagnetic force is thus calculated to be approximately only 1.2 × 10–4 N; the detached droplet gravity is only approximately 1 × 10–5 N; and the plasma 98-s WELDING JOURNAL / MARCH 2016, VOL. 95 dragging force is also only at the order of 10–5 N. On the other hand, the surface tension that retains the droplet is as high as 3 × 10–3 N. It is quite clear that the resultant detaching force, not counting the laser recoil pressure, is much smaller than the retaining force. The fact is that droplets are all successfully detached by the laser pulses. The metal transfer frequency equals the laser pulse frequency: 25 Hz. The expected one droplet per pulse (ODPP) transfer mode is successfully achieved in the pulsed fiber laser-controlled GMAW. Hence, it is solidly confirmed that the laser recoil pressure plays the dominant role in the droplet detachment. The metal transfer here is quite close to current-independent transfer. Optimal Laser Incident Point on Droplet One can notice that the only difference between experiments 2 and 3 is the laser incident point on the droplet. In experiment 2, the laser is aimed at the mid-top of the liquid droplet body. The laser thus penetrates the liquid droplet and produces a breakdown and partial explosion. The droplet shape deforms significantly. When aimed at the droplet neck position, the laser actually is digging the semi-solid wire near the liquid-solid interface instead of the liquid body, such that the semisolid drop neck is increasingly melted and thinned by the pressure from the laser pulse and no explosion occurs during the droplet detachment. The metal transfer thus looks smooth and tame. The droplet shape stays regular. On the other hand, one can see from Fig. 8 that it takes about 7 ms for the laser pulse to detach the droplet in experiment 2. However, in Fig. 9, it is reduced to 5 ms in experiment 3. Furthermore, the detached droplet velocity in these two experiments, another variable evaluating the detaching ability of the laser pulse, also showed a significant difference. The average droplet velocity, measured in 1 ms after detachment in these two experiments, is measured and shown in Fig. 10. The droplet velocity with laser aimed at midtop of the droplet is 36% smaller. Overall, with respect to the short detaching time cost and much larger detached droplet velocity at same laser pulse energy, the droplet neck position is determined to be the optimal point at which the laser should be aimed. The laser incident point will be fixed in the following experiments. Droplet Detaching Mechanism In conventional GMAW, the droplet is mainly first dragged/elongated and then detached in the wire axial direction. The droplet gravity, electromagnetic force, plasma dragging force, and surface tension are all distributed on the droplet body. The droplet detaching behavior is more like a tensile break; that is, the physical essence of the force balance theory on metal transfer. However, Fig. 9 indicates the Fig. 12 — Lasercontrolled metal transfer at current = 40 A in experiment 4 (1 ms per frame). Fig. 13 — Typical metal transfer in lasercontrolled GMAWP (2 ms per frame). Fig. 14 — Illustration of doublesided lasercontrolled metal transfer.
Welding Journal | March 2016
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