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• Successfully joined magnesium and aluminum dissimilar metals by nanosecond pulsed laser welding technology. • The laser power has a significant effect on the weld appearance, interface organization, fracture morphology and mechanical properties. • The heat transfer mechanism of the molten pool is described, and two paths of heat transfer are proposed. • At a laser power of 25 W, the maximum tensile force of the joint is the largest, which is 76.5 N. • The temperature field during the welding process was simulated. AZ31B magnesium alloy and 6061 aluminum alloy of 0.2 mm thickness were joined by nanosecond pulsed laser welding technology, and the effects of nanosecond pulsed laser power on the formation of weld seam, interface microstructure and mechanical properties were investigated, and numerical simulation of the temperature field during the welding process was carried out. The results show that when the laser power is 50 W, more defects such as cracks, pores and sags appeared in the joints due to the high heat input, and these defects resulted in a smaller effective joint area between the metals, which is very unfavorable to the strength of the joints. As the laser power decreases, at a laser power of 25 W, fewer defects are exhibited and the effective joint area becomes larger. As the laser power continues to decrease, the low heat input makes metal melting and joining very difficult. At low laser power, the Mg element intrudes backwards into the aluminum, causing the joint boundary interface to take on a reverse "V" shape. According to the results of the tensile shear experiments, the maximum tensile force of the joint was the highest at a laser power of 25 W, which reached 76.5 N. In addition, it was observed that the effective joint area, the number of oxides and their inclusions, and the degree of angular deformation had a very important effect on the mechanical properties of the joints. The results of temperature field numerical simulation experiments show that the heat is accumulated from outside to inside during the laser scanning process, and whether the temperature in the center of the molten pool reaches the melting point of the alloy can be used as an important basis for analyzing the degree of welding. [ABSTRACT FROM AUTHOR]