WELDING RESEARCH also can be seen that the droplet is driven to a waving motion by the recoiling jet. The laser recoil effect is sufficiently confirmed. A slight frustration is that the laser recoil pressure generated by the 250-W laser beam still is not strong enough to detach the droplet. However, the laser-induced vapor jet and droplet waving observed do verify the feasibility of laser-controlled current-independent metal transfer, as long as the laser power density is high enough to generate an adequate recoil pressure. Along with the waving motion of the droplet, the laser incident point on the droplet also moves up and down. However, it is interesting to observe that the vector of the laser recoil jet always looks to be the same with the normal direction of the irradiated liquid surface, not behaving as in the previous assumption that the vapor jet vector would depend on the laser incident direction. The brightness distribution on the droplet reflects the temperature on the droplet to a certain extent. It can be seen there is a local area on the droplet that seems brighter, suggesting the presence of a higher temperature. However, the white hot spot on the droplet is relatively small compared with the whole liquid surface. It implies 96-s WELDING JOURNAL / MARCH 2016, VOL. 95 that the average temperature of the whole droplet may have not been obviously increased by the laser. Verification of CurrentIndependent Metal Transfer Indication of CurrentIndependent Transfer According to the dynamic force balance theory on the metal transfer (Ref. 26), the droplet detachment occurs when the resultant detaching force exceeds the retaining force, i.e., the following criterion is satisfied in the axial direction: Fem + Fd + Fg + Fin > F�� (1) where Fem represents the electromagnetic force, Fd the plasma drag force, Fg the droplet gravity, Fin the inertia force, and F is the surface tension, all in the axial direction. The surface tension F F�� = 2��rw�� (2) where is the surface tension coefficient, and for the used wire material, is 1.2 N/m. The electromagnetic force Fem is given by (Ref. 3) Fem(t ) = μ0I(t )2 4�� ln rd (t ) rw +�� �� ���� �� ���� (3) Ap = +(rd 2 =rw 2 ) (7) Fig. 6 — Observation of laserinduced vaporization. Fig. 5 — Metal transfer in lowcurrent GMAW. Fig. 7 — Experiment 1, 1 ms per frame. Metal transfer under 250W CW laser irradiation with current at 80 A. Fig. 8 — Experiment 2, 1 ms per frame. Metal transfer with laser aimed at drop midtop.
Welding Journal | March 2016
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