WELDING RESEARCH A B C D fiber laser is operated at the pulsed mode and focused into a 0.5-mm-diameter spot. The power source runs at the constant-current (CC) mode. Other fixed experimental parameters include the arc length (6 mm), contact tip to work-piece distance (13 mm), wire (0.8-mm- diameter ER70S-6 wire), shield gas (pure argon, 15 L/min), and travel speed (3 mm/s). The laser installation is shown in Fig. 2. Given the laser position, the laser incident point on the droplet can be open-loop controlled by matching with the arc length. A machine-vision system was used to monitor the droplet position and, thus, to double ensure the laser was aimed/applied at the desired position. Unless otherwise specified, a 1200 W × 5–ms laser pulse was applied to the droplet neck at 25 Hz while the tilting angle of the gun was zero; the welding current was constant at 80 A; and metal transfer images are shown with a 1-ms time interval. The first frame of each figure corresponds exactly to the starting moment of the laser pulse. Results and Discussion Laser Positioning Parameters Laser Incident Point. The optimal laser incident point should be the first to determine. The laser droplet interaction 196-s WELDING JOURNAL / JUNE 2016, VOL. 95 mode would be totally different when the laser is aimed at different positions on the droplet, as shown in Fig. Table 1 — Laser Incident Angle in Experiments 4–8 No. Laser Incident Angle (deg) 4 85 5 75 6 60 7 45 8 30 3. The first part of this study conducted a preliminary comparison on the resultant metal transfer with two different incident points: droplet neck and midtop position. Figure 4 shows the desired submissive metal transfer can be achieved by aiming the laser at the droplet neck. Figure 5A, already presented in Ref. 23, shows the metal transfer becomes violent and a slight droplet burst is induced when the laser incident point is slightly moved downward to the droplet mid-top. The optimal laser incident point has already been determined to be the droplet neck in the first part of this study. Only when the laser spot is aimed at the droplet neck can the metal transfer be submissive without laser-induced explosion, and the detaching ability of the given laser pulse is maximized. For this paper, the laser aiming point was moved upward/downward to further examine/analyze the effect of the incident location. Experiments 1–3 were thus conducted with the laser incident angle set at 60 deg for three additional incident locations — wire tip, droplet mid, and mid-bottom positions. Figure 5B–D shows the typical metal transfer in Experiments 1–3 with the laser being aimed at these additional locations. One can see in Frame 4 of Fig. 5B that the laser pulse aimed at the wire tip does dig a shallow groove on the solid wire but cannot penetrate the wire. It is interesting to note that the laser impulse to the wire seems to have been conducted to the liquid droplet and thus excited the droplet into oscillation. In particular, Frame 7 corresponds to the moment the droplet reaches its maximum elongation, after which the droplet springs back to the wire tip and the oscillation starts. The detailed scientific mechanism on such excitation of droplet oscillation is currently not quite clear Fig. 5 — Metal transfer with the laser aimed at different positions on the droplet. A — Laser aimed at droplet midtop position; B — laser aimed at droplet wiretip position; C — laser aimed at droplet mid position; D — laser aimed at droplet midbottom position.
Welding Journal | June 2016
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