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Objectives As a heattreatable highstrength aluminum alloy with low density, good mechanical properties, easy forming, and corrosion resistance, Al -Mg -Si alloys have been widely used in highspeed railroads, automobiles, aerospace, and other fields. The addition of Cu to Al -Mg -Si alloys is more economical and applicable. It improves the mechanical properties and agehardening efficiency at peak aging, resulting in ultimate tensile strength of 400 MPa at peak aging for highstrength Al -Mg -Si -Cu alloys, which is significantly higher than that of existing commercial Al -Mg -Si alloys. Softening the heataffected zone during welding is a major problem that limits the use of heattreatable aluminum alloys. Laser -CMT composite welding (CMT, cold metal transfer) is a promising highperformance, lowwelding distortion technology that utilizes the advantages of laser and CMT welding to overcome the shortcomings of both singlelaser and traditional arc welding, while increasing productivity and improving welding quality. Therefore, this study investigates the effects of laser power on the macroscopic morphology and tissue properties of the welded joints of highstrength Al -Mg -Si -Cu alloys and evaluates the possibility of applying laser -CMT composite welding technology to highstrength Al -Mg -Si -Cu alloys.Methods In this study, a high strength Al -Mg -Si -Cu alloy with a thickness of 2 mm and filler wire with a diameter ofcharacteristics of the welded joint are altered by changing the laser power. Full penetration joints are obtained when the laser power is higher than 3. 6 kW. The weld width increases and the reinforcement height decreases slightly with increasing laser power. The weld consists of columnar and equiaxed crystals, and the columnar zone of the weld widens with increasing laser power (from 134 μm at 3. 6 kW to 232 μm at 4. 8 kW). Increasing the laser power also increases the weld dilution and Mg burnout; this decreases the Si and Mg concentrations in the weld, reducing their solid solution strengthening effects. Consequently, the weld hardness and strength decrease with increasing laser power. Overall, the best mechanical properties of the welded joint are obtained at a laser power of 3. 6 kW, with the average hardness and tensile strength of the weld reaching 65% (85 HV) and 64% (271 MPa), respectively, of the base material.

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