脉冲激光填丝焊接薄板熔池流动行为分析

本文使用VOF方法对熔池自由表面进行追踪,将焊丝简化为熔滴,建立了脉冲激光填丝焊接薄板三维数值模型,揭示了脉冲激光填丝焊接0.5 mm厚Hastelloy C-276薄板熔池流动行为,通过焊缝余高尺寸及熔合线形貌验证了模型的可靠性.结果表明,熔滴作用前,熔池上表面区域主要存在由表面张力导致的熔池边缘向熔池中心的流动,最大速度出现在熔池中部且指向熔池下表面,达到了m/s量级;在熔池下表面区域的流动形式是表面张力及熔池中心流动共同作用的结果,在下部形成两个方向相同的涡流;焊丝的熔入对熔池流场有显著的影响,熔滴对熔池的冲击作用改变了熔池原有的对流方向,使表面张力主导的熔池对流特征消失;熔滴熔入过程中熔池最大速度出现在熔滴熔入位置,约为1.74 m/s.在熔滴冲击与熔池表面张力联合作用下,熔池表现出振荡特性,随后熔池内流动再次回到由表面张力驱动的对流形式.激光脉冲结束后,熔池下部固液界面继续向母材区域扩展约3-4 ms,这种现象与激光的脉冲作用、熔池内的流动及Hastelloy C-276的物性参数有关.

Flow characteristics of the molten pool in pulsed laser welding of thin sheet with filler wire

A 3D transient model has been developed to analysis the molten pool flow characteristic in pulsed laser welding thin sheet with filler wire.The free surface is tracked by VOF method,and the filler metal is simplified as droplets.The flow characteristics in welding of Hastelloy C-276 sheets with 0.5 mm in thickness are analyzed with this model.The calculated weld appearance fits well with the experimental one.Before the droplet contacts with the molten pool,the liquid metal driven by surface tension flows towards the center and downward at the upper surface.The max velocity at the center is about 0.97 m/s.At the trailing portion of lower surface,two eddies with similar spin direction are generated due to the outward surface flow and the downward flow.The filler metal has significant influence on the convection pattern in the molten pool,and results in the disappearance of flow characteristics dominated by surface tension.The filler metal flows downward and outward rapidly after the droplet contact with molten pool.The maximum velocity is about 1.74 m/s and appears at the location where the droplet enters into the molten pool.The droplet has an impact effect on the molten pool and the convection pattern is changed.The molten pool shows oscillation characteristic due to the combined effect of droplet impact and surface tension.The volume of the molten pool dose not decrease right after the laser pulse ends,the lower part of the molten pool expands for about 3-4 milliseconds.And this phenomenon is related to the pulse effect of the laser beam,the flow pattern of the molten pool and the thermal properties of the material.Due to high pulse frequency of the laser beam and low thermal conductivity of the material,the molten pool always exists during welding process.