AZ31 magnesium alloy is widely used in the aerospace, automotive, and electronic fields because of its low density, high tensile strength, good conductivity, and electromagnetic shielding effect. Laser welding has the advantages of high energy density, small heat input, easy automation, and good flexibility compared to other welding methods. Therefore, laser welding has significant potential in the field of magnesium alloy welding. However, magnesium alloys have a series of material characteristics, such as a high linear expansion coefficient, easy oxidation, low surface tension, and high reflectivity to lasers. Therefore, magnesium alloy welding joints are prone to defects such as poor weld formation and pores. Researchers have proposed methods to solve such defects. However, most of these methods require the adoption of other methods or processes. Recently, sinusoidal modulation laser welding has been proposed for copper laser welding, and researchers utilized this method for magnesium laser welding. In this study, a regression formula between welding parameters and weld depth is developed. However, limited research has been conducted on the effect of sinusoidal laser power modulation in magnesium alloys, and the relevant mechanism is not completely clear. In this study, the influence of sinusoidal laser power modulation on AZ31 magnesium alloy weld penetration and pores is studied to provide new ideas for the efficient and high-quality welding of magnesium alloys.

Sinusoidal modulation laser welding with a wavelength of 1080 nm is utilized for the AZ31 magnesium alloy with a thickness of 4 mm. The average laser power is 1500 W, and the modulation amplitude is 500 W at different frequencies. The welding speed increases from 3.0 m/min to 4.0 m/min with an interval of 0.2 m/min and the modulation frequency increases from 0 to 200 Hz with an interval of 50 Hz. Pure argon is used as the shielding gas at a flow rate of 20 L/min. During the welding process, the molten pool and keyhole are monitored using the combination of an illumination laser and a high-speed camera. After welding, the cross section and longitudinal sections of the weld are observed to obtain the weld depth, weld seam solidification profile, and porosity.

With the increase in the welding speed, the weld depth gradually decreases, whereas a slight fluctuation occurs in the weld width (Fig. 3). Under the test conditions used in this study, when there is no laser power modulation, the fluctuation frequency of the keyhole depth is approximately 160 Hz. After sinusoidal modulation laser power welding, the solidification contour period is close to the laser power modulation period (Fig. 4). When the welding speed is lower than 3.4 m/min, laser power modulation helps increase the penetration. However, when the welding speed is higher than 3.4 m/min, the penetration decreases at the power modulation frequency, indicating that less energy is absorbed by the keyhole (Fig. 7). From the perspective of porosity, when the welding speed is high, the number of process pores is not large, and laser power modulation does not cause a significant increase in porosity. When the welding speed is lower than 3.4 m/min, the porosity is significantly increased, particularly after laser power modulation, and the porosity is high. The average porosity is relatively lower under 150 Hz, which is close to the intrinsic fluctuation frequency of the keyhole, and it is highest at 50 Hz (Fig. 9).

As a highly reflective material with low laser absorption, the absorption of laser energy by the magnesium alloy depends on the reflection numbers of the beam in the keyhole. When the number of reflections is greater, the energy absorbed by the lower part of the magnesium alloy keyhole is significantly higher than that absorbed by the upper part. The higher the energy gathered at the bottom of the keyhole, the more significant the hysteresis effect of the keyhole depth change. Utilizing sinusoidal modulation laser welding for the AZ31 magnesium alloy in the deep penetration welding mode, the deep keyhole with hysteresis effect will be further deepened, although the depth increase is small. For the keyhole without the hysteresis effect, with the decrease in laser power, the keyhole rapidly shrinks and the energy absorptivity decreases. Therefore, the depth is difficult to recover when the power is in the first half cycle, and the weld penetration decreases and fluctuation increases. The magnesium alloy keyhole exhibits periodic opening and closing changes, and the instability of the keyhole is the main reason for the pore in the magnesium alloy weld. The statistical results show that sinusoidal laser power modulation interferes with the keyhole, particularly for a weld with a high depth width ratio, and the process porosity significantly increases. In terms of the modulation frequency, 50 Hz low-frequency modulation has the highest impact on the intrinsic period of the keyhole, and the stability of the keyhole is the worst. When the modulation frequency is 150 Hz, the fluctuation frequency is close to the intrinsic frequency of the keyhole. The porosity is similar to that of a weld with constant power.