Energy Coupling Behavior in Modulated Fiber Laser Welding of High Reflectivity AZ31 Mg Alloy

Previous researches show that appropriate power modulation methods can increase the weld penetration and decrease the drawbacks of splatters and pores in laser welding for the high reflectivity materials such as magnesium alloy, aluminium alloy and copper alloy. This paper conducted tentative explorations to the underlying physical mechanism of this phenomenon. The sine modulated laser welding test of AZ31 was developed based on ternary quadratic regression design and the influence of average power (P_A), modulation amplitude (A) and frequency (F) on welding joint cross-sectional fusion zone area (A_FZ) were studied. The results show that power modulation can obviously improve energy coupling in the lower P_A welding process, while this improvement will be weakened and even disappear with P_A increasing. Both "small amplitude + high frequency" and "large amplitude + low frequency" can increase the cross-sectional A_FZ when welding in the case of low P_A. The longitudinal-sectional morphology of "stainless steel (reflectivity: 60%) + Mg alloy (reflectivity: 80%) " bimetallic specimen welded joint obtained in the sine modulated welding of 8 Hz was compared. It is demonstrated that when the power decreases from the peak value, the weld depth in lower reflectivity materials 2205 decreases synchronously, but the decreasing moment of weld depth in high reflectivity materials AZ31 delays about 0.036 s (about 30% of a sine period). The phenomenon may be attributed to that a deeper keyhole could be formed by the instantaneous peak power value, which increases the number of reflection of laser beam in keyhole. Then, in spite of the declining of transient laser power, the high aspect ratio keyhole will be maintained for about 1/3 of a sine period due to the enhanced energy coupling efficiency.