Fig. 1. Schematics of experimental devices. (a) Bead-on-plate welding experiment; (b) “sandwich” welding experiment

Fig. 2. Typical plume shape

Fig. 3. Variation of swing angle of the bottom swing plume with time

Fig. 4. Variation of narrow plume rising speed with time

Fig. 5. Morphology of keyholes and plumes at different time

Fig. 6. Variation of plume tilt angle and bright spot depth of the front wall of the keyhole with the time

Fig. 7. Keyhole shape in “sandwich” welding experiment

Fig. 8. Plume shape under different defocusing amounts

Fig. 9. Keyhole size before and after plume removal under different defocusing amounts

Fig. 10. Percentage increase of penetration after plume removal at different defocusing amounts

Fig. 11. Typical keyhole shape and “sandwich”welding keyhole shape

Fig. 12. Schematic of narrow plume formation