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In this paper, fibre laser-cold metal transfer arc (CMT) and oscillating fibre laser-CMT hybrid welding processes were adopted for A7204P-T4 aluminium alloy sheets with thickness of 6 mm. Furthermore, the effect of Nb content on microstructure and mechanical properties, the metallurgical behaviour, and influence mechanism were investigated. Oscillating laser effectively reduced the weld porosity and increased the joint’s elongation after fracture by 81%. With the addition of Nb, the heterogeneous nucleation significantly refined weld grains and removed columnar and dendritic structures which attributed to the solute segregation of Nb and primary phase NbAl3. The average tensile strength of joints with 0.43%, 0.74%, and 0.83% Nb mass fractions to 336, 334.5, and 341 MPa, respectively., In this paper, fibre laser-cold metal transfer arc (CMT) and oscillating fibre laser-CMT hybrid welding processes were adopted for A7204P-T4 aluminium alloy sheets with thickness of 6 mm. Furthermore, the effect of Nb content on microstructure and mechanical properties, the metallurgical behaviour, and influence mechanism were investigated. Oscillating laser effectively reduced the weld porosity and increased the joint’s elongation after fracture by 81%. With the addition of Nb, the heterogeneous nucleation significantly refined weld grains and removed columnar and dendritic structures which attributed to the solute segregation of Nb and primary phase NbAl3. The average tensile strength of joints with 0.43%, 0.74%, and 0.83% Nb mass fractions to 336, 334.5, and 341 MPa, respectively.