Fig. 1. Schematic of welding device

Fig. 2. Size of tensile specimen

Fig. 3. Cross-sectional morphologies of welds. (a) Without niobium; (b) niobium with a mass fraction of 0.74%; (c) niobium with a mass fraction of 1.36%

Fig. 4. Microstructures of weld fusion zone. (a) Without niobium, fusion zone upper part; (b) niobium with a mass fraction of 0.74%, fusion zone upper part; (c) niobium with a mass fraction of 1.36%, fusion zone upper part; (d) without niobium, fusion zone bottom part; (e) niobium with a mass fraction of 0.74%, fusion zone bottom part; (f) niobium with a mass fraction of 1.36%, fusion

Fig. 5. Microstructures of weld center. (a) Without niobium, weld center upper part; (b) niobium with a mass fraction of 0.74%, weld center upper part; (c) niobium with a mass fraction of 1.36%, weld center upper part; (d) without niobium, weld center bottom part; (e) niobium with a mass fraction of 0.74%, weld center bottom part; (f) niobium with a mass fraction of 1.36%, weld center bottom part

Fig. 6. EBSD of microstructures and grain statistical results. (a) Without niobium, EBSD of fusion zone; (b) without niobium, EBSD of weld center; (c) niobium with a mass fraction of 0.74%, EBSD of fusion zone; (d) niobium with a mass fraction of 0.74%, EBSD of weld center; (e) without niobium, grain statistical result of fusion zone; (f) without niobium, grain statistical result of weld center; (g) niobium with a mass fraction of 0.74%, grain statistical result of fusion zone; (h) niobium with a mass f

Fig. 7. SEM backscattered photos of weld and EDS results of precipitated phases. (a)(b) Without niobium, fusion zone and its partial enlargement; (c) niobium with a mass fraction of 0.74%, fusion zone; (d)(e) niobium with a mass fraction of 1.36%, fusion zone and its partial enlargement; (f) EPMA area scan result of niobium; (g) EDS of Al₂Mg₃Zn₃ precipitated phase; (h) EDS of MgZn₂ precipitated phase; (i) EDS of Al-Nb precipitated phase

Fig. 8. Microhardness distribution of the top and bottom of the weld. (a) Top; (b) bottom

Fig. 9. Tensile strength of welded joints

Fig. 10. Macroscopic fracture position and microscopic fracture of tensile samples. (a) Macro fracture location; (b)(c) without niobium, microscopic fracture and its partial enlargement; (d)(e) niobium with a mass fraction of 0.74%, microscopic fracture and its partial enlargement; (f)(g) niobium with a mass fraction of 1.36%, microscopic fracture and its partial enlargement