Fig. 1. Schematic of laser welding experiment. (a) Side keyhole observation experiment; (b) top keyhole observation experiment

Fig. 2. Laser energy distribution with different spot spacings. (a) 0 mm; (b) 0.225 mm; (c) 0.450 mm; (d) 0.675 mm

Fig. 3. Surface morphology of molten pool and keyhole with different spot spacings. (a) 0 mm; (b) 0.225 mm; (c) 0.450 mm; (d) 0.675 mm; (e) 0.900 mm; (f) 1.800 mm

Fig. 4. Surface morphology of keyhole at different times with spot spacing of 0.675 mm. (a) t₀; (b) t₀+2 ms; (c) t₀+4 ms; (d) t₀+6 ms; (e) t₀+8 ms

Fig. 5. Length and width of keyhole with different spot spacings

Fig. 6. Side morphology of keyhole with different spot spacings. (a) 0 mm; (b) 0.225 mm; (c) 0.450 mm; (d) 0.675 mm; (e) 0.900 mm; (f) 1.800 mm

Fig. 7. Variation curves of keyhole depth with different spot spacings

Fig. 8. Keyhole behavior with single and dual beams. (a) Single beam; (b) dual beam