In this study, EN AW 6061-T6 Al alloy having a thickness of 6 mm was joined with a 5000 W fiber laser welding machine using different welding speeds (20, 25, and 35 mm/s) and shielding gas compositions (100% Ar, % 50 Ar + % 50 He, % 25 Ar + 75 He). In order to reduce the strength decrease in the weld zone, 5356 filler metal with a diameter of 1.2 mm was used with the cold wire feeding method at a constant speed of 2 m/min. Structural analyses of the joints were performed by macroscopic and microscopic examination, and hardness profiles and strength values of the joints were determined by mechanical tests. The findings showed that (i) the width of the seam increased due to the increase in the amount of He in the gas composition, and the number of pores decreased accordingly, (ii) although no significant change was observed in the hardness profiles, the highest tensile strength value (242 MPa) was obtained using a gas composition of 50% He + 50% Ar and welding speed of 20 mm/s, (iii) shear failure was the dominant fracture, and the porosities played a decisive role in the fracture.