WELDING RESEARCH MARCH 2016 / WELDING JOURNAL 99-s high-power-density laser-controlled droplet detachment is more like a shear break. The laser recoil force is focused at a small spot on the droplet. The high laser recoil pressure acts more as a cutting effect rather than an axial drag effect to detach the droplet. The criterion for droplet detachment in laser-enhanced GMAW as assumed in the previous study (Ref. 21): Fem + Fd + Fg + Fin + Frecoil > F thus no longer work in this study. The dominant criterion of droplet detachment here should be that the laser recoil pressure is high enough to dig the droplet neck and the pulse duration is long enough to allow the laser pulse to fully cut through the neck. Future efforts are needed to build a theoretical model predicting the transition laser power density and the laser cutting speed on the droplet neck. Droplet Deflection and Process Robustness Beside the shearing effect, the droplet is also axially accelerated because the laser recoil pressure also has an axial component. On the other hand, since the droplet is flexibly pendant at the wire tip, the radial component of the laser recoil force not only digs the droplet neck, but also accelerates the droplet radially. That is why the droplet flying trajectory shows a deflection from the wire axial direction rather than along it. It is not suitable to judge if such a deflection would benefit or harm the welding process. As long as the deflection is not too severe and the angle is relatively stable, the deflection will not affect the continuity and uniformity of the bead formation. As shown in Fig. 11, the bead formation of experiment 3, the first segment of the bead shown on the right side of the figure, is produced without laser irradiation and it is quite irregular and looks poor. After the laser is applied, the metal transfer is much more frequent and robust. The bead formation looks fine and uniform. The continuous and uniform bead formation also implies a good robustness of the laser-controlled metal transfer in two aspects: 1) the complete weld bead proves that the ODPP laser detachment is adequately stable, and the droplets are not detached occasionally by the laser pulse; 2) the uniformity of the bead formation indicates that the droplet deflections during the transfer cycles don’t drift significantly, but quite robustly. Minimal Current In experiment 4, the welding current is further reduced to 40 A while the laser parameters are unchanged. In this way, the electromagnetic force is minimized to be at the order of 10–6 N. It is so small it could be ignored. The exciting result is the desired ODPP metal transfer is still obtained. Since the metal transfer frequency does not change and the welding current is reduced, the detached droplet diameter is even reduced to 1 mm approximately, as shown in Fig. 12. The detached droplet gravity is thus reduced to be at the order of 10–6 N. Thereby, one can conclude that the metal transfer is fully decoupled from the welding current. It is confirmed that current-independent metal transfer is indeed realized by the pulsed laser control method. As long as there is a pendant droplet, no matter which kind of current waveform is being used to melt the wire and grow the droplet, it could be effectively detached by the laser pulse. The less desirable thing is the droplet deflection looks increased. This is easy to understand since the laser energy input is the same while the droplet mass is smaller. In this sense, the droplet diameter is slightly larger than the wire diameter; for example, a 1.2-mm droplet for a 0.8-mm wire is recommended for the single-side lasercontrolled process to avoid severe deflections. Laser Detachment in GMAWP Experimental investigation has confirmed that the pulsed laser irradiation method is an effective way to produce the desirable currentindependent metal transfer. It means the current waveforms can now be freely designed or optimized for specific requirements that may vary from application to application. The current waveform is responsible for growing the droplet, while the laser is to detach it. The flexible combination of current waveform and laser detachment brings much more complete controllability to the GMAW process. For example, the main issue of metal transfer in ultralow current GMAW is the droplet grows too slow. The laser needs to wait until the droplet has grown to a desired size. Thus a simple combination is to use the laser detaching method in pulsed GMAW. In the last subsection, one already has noticed that the 40-A currents produce small heat input to base metal, but the droplet grows too slowly. Since slightly larger droplet diameter than the wire diameter is more appreciated in single-laser-controlled droplet detachment, continuous 40-A current waveform could be reshaped to a pulsed current waveform. Experiment 5 uses a base of 30 A with 30 ms duration and a peak of 90 A and 10 ms duration. The laser pulse is emitted at the end moment of the current pulse. The laser pulse frequency is thus still 25 Hz. Figure 13 shows the typical metal transfer. The droplet size and droplet deflection are well balanced. The peak current here is to increase the melting rate rather than produce a high electromagnetic force to detach droplets. In practical implementations, the laser peak energy and current peak could be free configured per the application demands. Future Modification of Laser Detachment Current-independent metal transfer produced by a pulsed laser has been demonstrated in this study. The difference from traditional drop spray transfer is that the droplet deflection is not avoidable when using a single-side laser irradiation. In fact, the deflection actually acts as a cushion to the laser recoil impulse. The laser cutting effect on the droplet neck is thus significantly weakened. The laser pulse energy is thus wasted to some extent. Possible modification of the laser detaching process is to use double-sided laser irradiation, as shown in Fig. 14. The deflecting effects can be balanced out. The laser cutting effect on the droplet neck would be maximized. The further thinking is that the power and phase match of the two laser beams may enable a free manipulation of the droplet flying trajectory in a certain state space. The edges would be the deflected trajectories under single-side laser irradiation. The metal transferring tra-
Welding Journal | March 2016
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