form, wire feed speed, and laser pulse waveform are all controlled by a singleboard controller. The arc variables and metal transfer are synchronously recorded at 5 kHz by a data acquisition system and a high-speed camera. All the welding experiments are conducted as bead-on-plate welding with 3 mm/s travel speed and 15 L/min pure argon shielding gas. The base metal is mild steel. The wire is ER70S-6 with 0.8 mm diameter. The distance from the contact tip to workpiece is set at 13 mm. The fiber laser works in either continuous wave (CW) mode or pulsed mode. In CW mode, the nominal max laser power is 250 W. In pulsed mode, the max pulse power is 1500 W with 10-ms duration. The focused red guide laser spot diameter is 0.5 mm. In actual operation of the laser, the laser power is set with a percentage of the max pulse power of 1500 W. Laser Setup Parameters The key parameters in laser installation for enhanced GMAW (shown in Fig. 2) are the following: 1) The laser incidence angle , defined as the angle between the laser beam and z direction, which may affect the dynamic laser recoil pressure on the droplet and the reaction of the droplet to the laser. 2) Laser incidence point. The distance from the incident point to workpiece Ld is fixed at 6 mm approximately, while the droplet position, i.e., the arc length, is adjusted to adapt to this distance and thus guarantee the laser has been aimed at the desired position on the droplet. 3) Torch orientation: determined by between the torch axis and z axis. A multiaxis manual stage with 0.1-mm resolution is designed to adjust these parameters in experiments, as shown in Fig. 3. The y-axis slide is on duty to aim the wire. The z-axis slide changes the laser incident point. The rotation around y axis adjusts the laser incident angle. Metal Transfer in LowCurrent GMAW In order to clearly illustrate the capability of laser-enhanced metal transfer in achieving a desirable currentdependent metal transfer, the metal transfer in conventional GMAW with low currents is first investigated/measured for later comparison. The arc voltage is intentionally set as high as 30 V to guarantee the droplet has an adequate space to grow and finally be detached in free-flight mode. The current changes from 30 to 100 A. (Currents with further reduced amperage would not maintain the arc stably.) It is apparent from Fig. 4 that the metal transfer is in the drop globular mode, which is characterized by large droplets with diameters much greater than that of the wire. The detached droplet diameter and transfer frequency in these series of experiments are measured from the high-speed images and are plotted in WELDING RESEARCH Fig. 5. It can be seen that the metal transfer frequency at 80-A current is only approximately 1.2 Hz, and the droplet diameter is several times that of the wire diameter. Such low transfer frequency tends to result in discontinuous and irregular bead formation and the arc is unstable. Intentional use of short-circuiting transfer in lowcurrent range by shortening the arc length generates severe spatters and the wire tends to dip into the workpiece/ weld pool as solid and then extinguish the arc to terminate the welding process. Thereby, conventional GMAW is typically not performed with currents in tens of amps. LaserInduced Vaporization The essence of the method that applies a laser on the droplet to control the metal transfer is believed to be lying in the laser-induced vaporization and the resultant laser recoil pressure. To verify, the laser-induced vaporization is first observed/analyzed in order to help better understand the principle of the laser-controlled metal transfer process. As shown in Fig. 6, the laser incident angle is 90 deg. In order to clearly observe the laser-induced vapor jet, no arc is established and the laser is the only heating source to melt the wire and vaporize the liquid droplet. The droplet position is controlled by regulating the wire feed speed based on vision feedback. The position control guarantees the laser spot is always aimed at the droplet. The laser works in CW mode and the laser power is set at 250 W. As can be seen from Fig. 6, the laserinduced vapor jet is clearly observed. It MARCH 2016 / WELDING JOURNAL 95-s Fig. 3 — Installation of laser focusing head. Fig. 4 — Typical drop globular transfer in lowcurrent GMAW (current = 90 A, 6 ms per frame).
Welding Journal | March 2016
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