Fig. 1. Schematics of laser welding and process observation system

Fig. 2. Pores inside magnesium alloy welded seam. (a) Hydrogen-induced pore; (b) keyhole-induced pore; (c) longitudinal section of weld; (d) marks of pores

Fig. 3. Cross sections of welds under different welding speeds and different modulation frequencies of laser power

Fig. 4. Longitudinal sections of weld centers under different welding speeds and different modulation frequencies of laser power

Fig. 5. Keyhole opening changes at different welding speeds. (a) 3.0 m/min; (b) 3.2 m/min; (c) 3.4 m/min; (d) 3.6 m/min; (e) 3.8 m/min; (f) 4.0 m/min

Fig. 6. Relationship between reflection time and absorptivity

Fig. 7. Weld penetration changing with laser power modulation frequency under different welding speeds. (a) 3.0 m/min; (b) 3.2 m/min; (c) 3.4 m/min; (d) 3.6 m/min; (e) 3.8 m/min ;(f) 4.0 m/min

Fig. 8. Energy variations caused by different modulation frequencies. (a) 50 Hz; (b) 100 Hz; (c) 150 Hz; (d) 200 Hz

Fig. 9. Weld process porosities under different welding speeds and laser power modulation frequencies