激光-电弧复合焊在坡口中的等离子体耦合行为

Plasma Coupling Behavior of Laser-Arc Hybrid Welding in Groove

研究了在坡口中进行复合焊时激光和电弧的耦合行为,结果表明:当光丝间距为2 mm时,激光和电弧能够持续耦合,但熔池表面波动剧烈,焊接飞溅较大,表面成形较差;当光丝间距为4 mm时,激光和电弧等离子体出现周期性耦合,熔池表面波动较小,焊缝表面成形较好。出现周期性耦合主要是因为焊丝熔化后先填充坡口,造成电弧下方液体堆积,激光作用区域和电弧作用区域的高度不在同一水平面上,使得实际光丝间距大于4 mm,激光和电弧不耦合;当堆积液体达到一定高度后,在重力和表面张力作用下液体向激光作用区域流淌,液面趋于水平,实际光丝间距接近4 mm,激光和电弧等离子体连接在一起,再次发生耦合。当光丝间距大于6 mm时,激光和电弧不发生耦合。

Objective BG890 QL low-alloy high-strength steel is widely used in the field of construction machinery because of its high strength and low-temperature toughness. The conventional arc welding method, on the other hand, results in a low welding efficiency of medium thickness plate BG890 QL. Many scholars use the laser hybrid welding method to study the medium thickness plate welding to improve the welding efficiency of medium thickness plate BG890 QL low alloy high strength steel. A large body of literatures have been written about laser hybrid welding on flat plates, and many research have been conducted on the arc and laser coupling mechanism, droplet transfer morphology, weld formation, and weld performance in the welding process. However, to increase the penetration of laser-arc hybrid welding, the upper part is generally grooved, and the welding depth is increased by laser while the arc covers the surface to fill, which can ensure the weld depth and weld formation, resulting in laser-arc hybrid welding being widely used in the field of medium thickness plate welding. When the laser-arc hybrid welding is carried out in the groove, with the fluctuation of the molten pool, the liquid in the molten pool fills the groove, resulting in that the arc and laser are not in the same horizontal plane, and the coupling effect of laser and arc plasma also changes, which is different from that of laser and arc plate welding. It is necessary to study the coupling mechanism of laser and arc plasma in the groove.Methods Laser-arc hybrid welding was performed on 16-mm thick BG890 QL low-alloy high-strength steel with V-groove(the groove angle was 50° and the depth of groove bottom is 8 mm). The laser-arc hybrid welding parameters were as follows: laser power is 7 k W, arc welding current is 240 A, welding voltage is 22 V, and welding speed is 0.5 m/min. High-speed camera was used to capture the morphology of arc and laser-plasma, as well as the flow behavior of a molten pool. The optical microscope Zeiss-Axio Cam MRc5 was used to examine the microstructure.Results and Discussions According to the morphology of arc and laser-plasma and the flow behavior of the molten pool(as shown in Figs. 3-8), the coupling mechanism of laser and arc in the groove with the laser-arc distance of 4 mm is obtained, as shown in Fig. 9. The figure shows a longitudinal section on the left and a cross-section on the right. Fig. 9(a) depicts when the laser and arc begin welding, they form a common molten pool in which liquid accumulates. The laser and the arc plasma are both at the same height and in the same horizontal plane, but the plasma only partially overlaps. The two are in the weak coupling, and the molten pool fluctuation is not severe. As the welding heat source moves, the liquid in the front of the weld pool fills the bottom of the groove [as shown in Fig. 9(b)], the surface of the weld pool remains in the laser action area, and the liquid level in the arc area at the back of the weld pool remains high, resulting in that the arc and the laser are not in the same horizontal plane, the coupling between the laser and the arc is insufficient, and the composite effect becomes worse. It is the same as single arc welding, which is advantageous for weld forming. Then, as the heat source moves and the molten pool liquid level fluctuates, the liquid level in the laser action area rises, and the arc is in the same horizontal plane [as shown in Fig. 9(c)]. The two are coupled again. The heat source then moves, the liquid in the laser area refills the groove, the liquid level drops, the laser and arc plasma is separated, and the coupling is separated [as illustrated in Fig. 9(d)].Conclusions Laser-arc hybrid welding is carried out in the groove, when the laser-arc distance is 2 mm, the arc and laser-plasma are strongly coupled. From the beginning of the welding process to the end of the welding process, the arc and laser-plasma are inextricably linked. The plasma brightness is high, the spatter is increasing, and the weld pool is violently fluctuating;the results show that weld penetration is high but weld surface forming is poor. The laser and arc plasma are periodically coupled when the laser-arc distance is 4 mm. With the increase of the filling height at the bottom of the groove, the laser and the arc are not in the same horizontal plane, the distance between the laser and arc increases, and the arc and laser-plasma are separated;When the liquid at the bottom accumulates to a certain height, it flows to the laser action area due to gravity and surface tension;the liquid level tends to be horizontal, and the laser-arc distance decreases;the laser and the arc begin to couple, and the molten pool is joined together;the weld surface is well-formed, and the weld depth is slightly reduced. When the laser-arc distance exceeds 6 mm, there is no coupling between the laser and the arc in the groove. During laser-arc hybrid welding in the groove, the liquid level of the whole weld pool is inclined due to the accumulation of liquid in the weld pool. The range of the best coupling distance of laser and arc is narrow(~4 mm), and the weld penetration is high and the surface forming is good.