基于熔池流动分析的高速激光焊驼峰焊道形成过程研究

INVESTIGATION OF HUMPING FORMATION BASED ON MELT FLOW ANALYSIS IN HIGH-SPEED LASER WELDING PROCESS

利用高速摄像机观察了高速激光焊熔池的流动行为,分析了驼峰焊道的形成过程及焊接速度对驼峰倾向的影响,并探究了驼峰焊道的抑制措施.研究发现,驼峰的形成经历了形核、长大、凝固3个阶段.形核阶段,环小孔流淌的侧向金属流在小孔后方的汇聚造成了液态金属的起伏,成为驼峰形成核心:长大阶段,液态金属起伏的波谷率先冷却凝固并阻止波峰处液态金属继续向熔池尾部流动,波峰则被从其前方向熔池尾部流动的液态金属填充长大:凝固阶段,长大了的波峰自下而上凝固,固-液界面凸起,接触角减小,波峰处液态金属难以铺展.焊接速度增大,小孔周围的2个侧向金属流在小孔后方的汇聚点与激光热源距离增大,小孔后方液态金属温度降低,表面张力增大,液态金属起伏难以铺展.驼峰易于形核,驼峰倾向增大.小光斑直径激光能够缩短激光热源与汇聚点的距离,双光束激光的副光束的后热作用使得小孔后方液态金属的冷却速率降低,这2种措施均减小了小孔后方液态金属的表面张力,增加了液态金属的铺展能力,从而抑制了驼峰的形核.

The high-speed camera was used to observe the behavior of the melt flow in the high- speed laser welding process. The humping formation and the effect of the welding speed on the humping tendency have been discussed, and the suppression methods of humping have also been studied. The results show that the humping formation undergoes three stages： nucleation stage, growth up stage and solidification stage. During the nucleation stage, the two side streams flow around the keyhole and subsequently converge at the rear of the keyhole. Finally, the wave motion of the liquid metal, which means the nucleus of the humping, is formed in the converging process. During the growth up stage, the valley of the wave is given priority to solidify, which prevents the peak liquid metal from flowing to the rear of the molten pool. The peak of the wave is filled to expand by the front liquid metal. During the solidification stage, the grown up peak solidifies from the bottom to top, which lifts the solid-liquid interface upward, declines the contact angle, and makes it difficult for the peak liquid metal to spread. With increasing the welding speed, the distance between the converging position at the rear of the keyhole and the laser beam increases, the temperature of the liquid metal at the rear of the keyhole decreases, the surface tension goes up, the peak liquid metal is hard to spread, the humping is apt to nucleate, and humping tendency is increased. Adopting the laser with small spot diameter is helpful to shorten the distance between convergence position and laser beam, and using the trailing beam ofthe dual beam laser is propitious to increase the temperature of the liquid metal in the rear of the keyhole. These two approaches are effective in decreasing the surface tension, and suppressing the nucleation of humping.