激光熔池流动模式和熔池流动特征决定了焊缝成形和焊接缺陷的分布, 因此研究不同焊接位置对激光焊接流动行为的影响对优化焊接位置和焊接参数具有重要意义。采用1.5 kW板条型CO2气体激光进行了平焊和立焊向上、立焊向下等不同焊接位置下的焊接试验, 并采用高速摄像对其熔池小孔特征进行了监测, 再采用粒子图像测速(PIV)方法对熔池流动特征进行了定量表征和分析。明确了激光焊接时的熔池流动模式, 在熔池区域存在着两个大的涡流区域, 分别在熔池周围和在熔池的尾部。与立向上焊相比, 激光立向下焊的熔池形状呈扁圆形, 宽度增加而长度减少, 小孔的面积增加, 其熔池流动的脉冲特征减弱, 熔池流动的稳定性增加。这与立向下焊时熔池上方液态金属受重力影响向下流动, 与下方液态金属受蒸汽反冲压等驱动向上流动, 两者相遇产生的强烈搅拌作用有关。这为理解不同焊接位置不同的焊缝成形和气孔率差异提供了依据。

Flow behavior and flow mode of molten pool are the essential characteristics in laser welding which dominate shapes of welding joints and the formation of welding defects including undercut and porosity. It is of great significance to investigate the effect of different welding positions on flow behavior in laser welding for better quality of welding joints. A series of welding experiments were carried out with a 1.5 kW slab CO2 laser at flat position and vertical up, vertical down positions when the molten pools' images were captured by a high-speed camera. Such images were quantitatively analyzed by using PIVlab tool which based on particle image velocimetry algorithm. The results showed that the molten metal flows around the keyhole and two convection loops situates the rear of the molten pool at the three positions. Compared with vertical up position, the weld pool in laser welding at vertical down positon shows an oblate shape with the increased width and the shrunk length, while the keyhole area grows and the stability of the molten pool is rising with a pulse characteristic of the molten pool flow is weakened. This phenomenon is caused by the changed flow mode at vertical down position where the liquid metal of the top position in molten pool flows downward driven by gravity, and collide with the stream driven by keyhole plasma eruption, which give rise to the strong stirring effect of molten pool. This lays a basis for understanding the difference between weld formation and porosity rate in different welding positions.