Fig. 1. Structure diagram of this paper

Fig. 2. Characteristics of laser micro-welding. (a) Heat transfer^[25]; (b) melt pool flow^[18]; (c) deformation^[26]; (d) gap sensitivity^[13]

Fig. 3. Schematics of laser micro-welding modes. (a) Heat conduction welding; (b) deep penetration welding

Fig. 4. Study results of unstable transition zones in laser micro-welding. (a) Judging from width of weld, unstable transition zone is caused by alternating occurrence of heat conduction welding and deep penetration welding^[23]; (b) combination of welding plume and weld width proves that unstable transition zone is caused by instability of deep penetration welding ^[31]

Fig. 5. Effect of laser micro-welding mode on microstructure of weld^[31]. (a) Cross section and phase composition of heat conduction welding joint of stainless steel; (b) cross section and phase composition of deep penetration welding joint of stainless steel

Fig. 6. Effects of lasers on laser micro-welding. (a) Surface morphologies of copper welds welded by green laser and near-infrared laser^[37]; (b) surface and cross-section morphologies of welds in aluminum/copper dissimilar metal lap welding by ARM laser^[39]; (c) surface and cross-section morphologies of welds in aluminum/stainless steel butt welding by femtosecond laser^[40]

Fig. 7. Typical defects of laser micro-welding. (a) Incomplete penetration; (b) burn-through; (c) undercut; (d) spatter; (e) pore; (f) crack; (g) humping; (h) deformation

Fig. 8. Humping in welding. (a) Surface and cross-section morphologies of humping; (b) formation mechanism of humping

Fig. 9. Surface morphology and cross-section morphology of laser welded metal foil stacks. (a) Green laser welding of copper foil^[36]; (b) blue laser welding of copper foil^[35]

Fig. 10. Effects of laser power and beam diameter on weld formation. (a) Cross-section morphologies of welds at different powers when spot diameters are 34.8 μm and 17.4 μm, respectively^[62]; (b) surface morphologies of welds at different powers and speeds when spot diameters are 200 μm and 375 μm , respectively^[63]

Fig. 11. Effects of oscillation scanning on weld formation and molten pool. (a) Scanning images of cross sections of aluminum/copper dissimilar metal welds by energy disperse spectrometer (EDS)^[68]; (b) dynamic behaviors of molten pools during linear scanning welding and oscillating welding captured by high-speed photography^[70]

Fig. 12. Application of laser micro-welding of metal materials. (a) Pressure sensor^[71]; (b) bipolar plate for fuel cells^[72]; (c) blades for aerospace engines; (d) electronic component pins and copper printed circuit board^[73]; (e) tantalum satellite collimator^[76]; (f) cardiac pacemaker^[74]; (g)‒(i) lithium ion battery tabs^[79]

Table 1. Typical light sources and research results of laser micro-welding