For transportation, aviation, aerospace, and defense military equipment components, the need for lightweighting is particularly urgent. Magnesium alloys have the benefits of high specific strength and stiffness, damping and vibration reduction, electromagnetic shielding, remarkable machining performance, and easy recycling. With the continuous research and innovation of new materials and technologies of magnesium alloys, their application potential will be infinite. Researchers suggested an approach of applying a tunable ring spot in laser processing that has attracted a lot of attention in the field of laser processing. Although domestic and foreign scholars have performed a lot of research investigations on magnesium alloy laser welding and have attained good findings. There is still a big gap between the existing research investigations and practical engineering applications, and it is crucial to develop novel laser welding technology.
In this study, AZ31B magnesium alloy butt joints with a thickness of 5 mm are welded by fiber laser with an adjustable ring spot. Based on ensuring the total power of 3 kW, pure center laser, center laser/ring laser, and pure ring laser are employed to weld magnesium alloy joints. The surface shapes, microstructures, and mechanical properties of the joints are discovered and examined.
In terms of weld formation, first, compared with that when using pure center laser welding, the melting width in the upper part of the weld is considerably increased when using ring laser welding, and the combination of center laser and ring laser has a greater impact on the weld cross-sectional shape. Second, when the ring laser power is greater than the center laser power, the formations of the upper and lower surfaces of the welds become unstable (Figs. 3 and 4). In terms of metallographic structure, during pure center laser welding, the columnar crystal region near the fusion line of the weld is very narrow, and the grains in the center of the weld are finer, but due to the fast cooling rate, there are small pores and cracks around the weld [Figs. 5(a1)-(a4)]. When the center is welded with a ring laser, the convection heat transfer time is extended in the molten pool’s upper part, increasing the grain size in the center of the weld. Even in pure annular laser welding, a wide columnar grain region is formed near the weld fusion line [Figs. 5(c1)-(c3)]. The weld zone hardness is higher than that of the base metal, and as the ring laser power increases, the weld center hardness decreases. When the central laser power is 2000 W and the ring laser power is 1000 W (sample 2#), the tensile strength and elongation of the welded joint are the largest, reaching 215 MPa and 14.0%, respectively, which are 79.6% and 96.6% of those of the base metal [Fig. 7(a)]. The joint fracture is a brittle-ductile mixed fracture.
The existence of a ring laser has a considerable influence on the weld formation. Based on the center laser, the ring laser application can substantially increase the fusion width in the upper part of the weld. The combination of center laser and ring laser has a great influence on the cross-sectional shape of the weld, the best weld formation is generated when the center laser power is 2000 W and the ring laser power is 1000 W. The existence of ring laser has a certain effect on the microstructure of the weld. Only in the central laser action area, there is no clear heat-affected zone or columnar grain zone near the weld fusion line, and the equiaxed grains are relatively fine. In the ring laser action area, there are heat-affected and columnar crystal zones near the weld fusion line, and the equiaxed crystal grains are coarse. Further, with the ring laser power increasing, the hardness value of the central area of the weld decreases. The ring laser beam can enhance the elongation of laser welded joints of the magnesium alloys, and the welded joints have better tensile strength produced under the optimized process parameters.
