where 0 is the magnetic permeability, I is the welding current, rw the wire radius, and is the half angle subtended by the arc root at the center of the droplet. does not change significantly when the half angle ranges from 90 to 150 deg (Ref. 3), so that the selection of the half angle would not significantly influence the involved calculating results and the half angle is fixed at 120 deg for ease of calculation. The droplet gravity Fg is given where rd is the droplet radius and ' represents the density of liquid droplet, which is smaller than that of the solid wire. The plasma drag force is given by (Ref. 3) Fig. 10 — Effect of laser incident point. where Cd is the aerodynamic drag coefficient, Ap is the area of the drop seen from above, and p and vp are the density and velocity of the arc plasma. Little attention needs to be paid to the plasma dragging force since it is generally at the order of 10–5 N. The inertia force is involved by taking the droplet oscillation into account, Fin = –ma, the a represents the acceleration of the droplet. In a case without an external active excitation, the droplet oscillation will be very weak and thus the inertia force can be ignored in this study. Based on the force balance theory on metal transfer, it can be inferred that the indication of the realization of current-independent would be that the droplet be detached at sufficiently low current at a relatively small size. Low current means small electromagnetic force and small droplet size means small droplet gravitational force. The droplet detachment in this condition must be accomplished by a new detaching effort/force. CW LaserControlled Metal Transfer Although pulsed laser irradiation is believed to be optimal for controlling the metal transfer, the transfer behavior under CW laser mode is still of great interest and thus first verified by experiment 1. The laser power keeps constant at 250 W. The welding current is 80 A. The metal transfer is shown in Fig. 7. The first frame shows a surface deformation on the droplet, which illustrates the laser incident point at the middle of the droplet. Thus the recoil force repels the droplet and pushes it to wave and the droplet finally got detached. The transfer mode is a kind of repelling drop globular. The detached droplet diameter is measured approximately 3 mm. As has been measured in the previous section, the droplet diameter under 80-A current without laser irradiation is 3.7 mm. The reduction of the detached droplet size does demonstrate a certain enhancement on the drop detachment due to the continuous laser irradiation. However, droplet gravity still plays the main role in detaching the droplet. Such enhancement is still quite limited and is far away from the ultimate goal: current independent metal transfer. Pulsed LaserControlled Metal Transfer Experiments 2–4 were then con- �� = ln(sin��) �� 1 4 �� 1 1�� cos�� + 2 (1�� cos��)2 ln 2 1+cos�� (4) Fd = 1 2 Cd Ap��pvp 2 (6) Fg (t ) = 4 3 ��rd (t )3��' g (5) Ap = π( rd2 − rw2 ) (7) WELDING RESEARCH MARCH 2016 / WELDING JOURNAL 97-s Fig. 9 — Experiment 3, 1 ms per frame. Metal transfer with laser aimed at droplet neck. Fig. 11 — Bead formation in experiment 3 (left segment) in comparison with a bead made without laser enhancement.
Welding Journal | March 2016
To see the actual publication please follow the link above