WELDING RESEARCH Fig. 7 — Effect of laser incident angle on metal transfer. JUNE 2016/ WELDING JOURNAL 197-s A B C D E and needs more study. When the laser was aimed on the droplet midposition, one can see from Fig. 5C that the droplet was instantly broken through by the laser such that the droplet surface deformed intensively. The droplet was finally detached but with significant offset from the wire axial direction, similar to the repelled globular transfer in CO2 welding. In this way, the laser detaching mechanism no longer lies in the laser cutting effect, but the laser repelling effect. Nonuniform bead formation is typically expected in this case. When the incident point moves further downward to the droplet midbottom, the droplet is violently repelled by the laser pulse. However, since the incident point is too low, the repelled droplet may deviate from the laser irradiation, such that the laser recoil force no longer exists after an initial droplet deflection and the droplet thus cannot be detached. The experiment results of this subsection further prove that the droplet neck is the optimal laser incident point for obtaining the desired droplet detachment. Thereby, the laser pulse was aimed at the droplet neck as the default in the experiments that follow. Laser Incident Angle. Another important laser positioning parameter is the laser incident angle that determines the laser penetration path and direction inside the droplet. Experiments 4–8 were thus conducted to verify its effect on the metal transfer. The laser incident angle changed from 30 to 85 deg, as shown in Table 1. The 90- deg laser incident angle meant the laser beam was perpendicular with the wire. Typical metal transfer under these different laser incident angles is shown in Fig. 6A–E. Since the first part of this study verified the direction of the laser recoil force is coincident with the normal of the irradiated local surface of the droplet, the initial direction of the laser recoil force at the very starting moment of the laser pulse during Experiments 4–8 was believed to be the same. However, the laser digging and penetrating direction indeed changed with the incident angle. Once an initial groove formed at the droplet neck, the laser recoil direction consequently changed. As Fig. 6A, E shows, the droplet surface deformations under 85- and 30-deg incident angles are apparently different. It can be seen the laser tried to cut through the droplet main body rather than the droplet neck when a 30-deg incident angle was used. The metal transfer thus looks not sufficiently submissive, because the intensive laser-induced vaporization inside the liquid droplet resulted in a light explosion. In this sense, a too-small incident angle is not recommended for use. Figure 7 shows the measured droplet velocity and deflection from Experiments 4–8. It can be seen the droplet deflection first decreased before the incident angle reached 45 deg, because the radial component of the laser recoil force increased. However, when the laser incident angle was as Fig. 6 — Metal transfer under different laser incident angles: A — = 85 deg; B — = 75 deg; C — = 60 deg; D — = 45 deg; E — = 30 deg. Table 2 — Droplet Deflection and Velocity Measured from Experiments 14 and 15 No. Laser Pulse Frequency Droplet Deflection (deg) Droplet Velocity (mm/s) 14 10 19 326 15 20 37 463
Welding Journal | June 2016
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