Modulated fiber laser welding of high reflective AZ31

Abstract

Power sine modulation fiber laser welding (FLW) of AZ31 magnesium alloy was conducted. When the diameter of transmission fiber and focal spot was 400 μm and 0.4 mm, respectively, the average laser powers for penetrating a 2.7-mm-thick AZ31 plate at a welding speed of 5 m/min were reduced by about 33 % by using modulated FLW. The relationship between modulation parameters (i.e., amplitude, frequency, and average laser power) and weld depth which was closely related to the transfer efficiency of laser energy was studied through partial penetration laser welding test on 8-mm-thick AZ31 plate based on quadratic regression orthogonal design. It was found that influence of power modulation on laser welding of AZ31 alloy was highly dependent on laser power density. In the low power density range, laser energy coupling efficiency could be significantly improved by combining low amplitude with high frequency or high amplitude with low frequency. With the increasing of laser power density, the optimum frequency corresponding to maximum weld depth decreased, and the positive effect of the favorable combination of high amplitude and low frequency on laser energy coupling continuously weakened. When laser power density was high enough, power modulation had hardly positive effect on weld depth and energy coupling efficiency. It was argued that improvement of energy coupling efficiency in laser welding of AZ31 by using power modulation was due to the reduction in the portion of energy lost into surroundings. Finally, laser butt welding was conducted on 2.7-mm-thick AZ31 under the condition of high beam quality and the tensile strength of both butt-welded joint and base metal that was tested.