WELDING RESEARCH space; the acoustic radiating force results in a certain enhancement on drop globular transfer. The diameter of detached droplets is thus reduced compared to that without ultrasonic wave. However, the ultrasonic wave also shortens the arc and thus increases the tendency of short-circuiting transfer. Higher arc voltage is thus needed to increase the arc length to avoid the lessdesired short-circuiting transfer. While aforementioned methods have achieved significant improvements toward more desirable metal transfer, they are all still far away from the ultimate goal: current-independent metal transfer. While searching for an innovative method, bringing a new and sufficiently large detaching force while not disturbing the arc stability is still of great significance. The laser-enhanced GMAW, a recent innovative solution, was thus first proposed and preliminarily verified at the University of Kentucky (Ref. 23). A direct diode laser line (max 862 W, 14 × 1-mm focused stripe) was adopted to irradiate the droplet. An auxiliary recoil force was believed to be generated and enhance the droplet detachment (Refs. 24, 25). However, while the relatively long laser stripe ensured the droplet was being irradiated by applying the laser beam along the wire droplet, the intensity of the laser power was significantly reduced. As a result, the recoil pressure is too small to provide a sufficient detaching force. As such, a substantial, although reduced, electromagnetic force is still needed such that the current cannot be reduced 94-s WELDING JOURNAL / MARCH 2016, VOL. 95 to a level that can be referred to as “any” in the reasonable range that can maintain the arc. Actually, there is still doubt about the laser’s effect since no obvious laser effect was observed. Another possible reason is the reduction of surface tension due to droplet temperature increase under laser irradiation. The latest published progress is that a 50-W fiber-feed laser was used to control the metal transfer. The laser beam was focused to a 1-mm-diameter spot, and the droplet detachment looks enhanced but still quite limited (Ref. 26). The laser intensity is still too small. However, as can be foreseen, the laser irradiation method does have the potential to fully decouple the droplet detachment from the welding current. The essence likely lies in the laser power density. Thus, a pulsed fiber laser with relatively high peak power but relatively low average power may most likely facilitate the next evolution in laser-enhanced GMAW. Sufficiently high laser peak power can be applied only at the moment it is needed. That is, once the droplet grows to the desired size, a laser pulse can be applied to detach the droplet. As such, the resultant metal transfer is a kind of one-drop-per-pulse, similar to pulsed GMAW (Ref. 5). Therefore, a pulsed fiber laser not only avoids waste of laser energy but also achieves the desired controllability. A compact pulsed single-mode fiber laser, QCW150/1500, manufactured by IPG Photonics, is adopted in this study. The laser pulse was expected to generate adequate laser recoil pressure and dominate/complete the detachment of the droplet. The desirable metal transfer, i.e., fully currentindependent metal transfer, may be realized for the first time. For metal transfer controllability, the establishment of this capability may be regarded as a milestone. This paper presents the system and experimental verification for the establishment of the capability for fully current-independent metal transfer. The metal transfer under reasonably low current range without the application of a laser will be first measured for later comparison. The laserinduced vaporization will then be verified before using the laser to control the metal transfer. Then the metal transfers under continuous and pulsed laser irradiation will be both experimentally studied and compared together with the metal transfer without laser to experimentally verify the success in achieving the fully currentindependent metal transfer. Experimental System and Preliminary Experiments Experimental System Figure 1 shows the experimental configuration for this study. The welding power source works in the constant current (CC) mode. The electromagnetic force is thus approximately constant such that the force analysis on the droplet is relatively simple. The arc length is stabilized at 6 mm by adjusting the wire feed speed based on the arc voltage feedback. The current wave- Fig. 1 — Sketch of experimental system. Fig. 2 — Illustration of laser setup parameters.
Welding Journal | March 2016
To see the actual publication please follow the link above