激光熔覆过程中熔池对流运动对熔覆层气孔和元素分布的影响

激光熔覆时熔池对流运动对熔覆层成分和气孔缺陷分布具有较大影响。通过在灰铸铁和45钢两种基体上进行激光熔覆NiCuFeBSi 系合金的工艺试验，对熔池的对流运动形式进行了机理分析与试验验证。结果表明，受熔池强对流运动影响，熔覆层气孔呈均匀弥散化分布趋势；且自熔覆层底部向顶部方向合金元素分布均匀，无缓慢过渡特征。根据YAG 激光光斑能量的高斯分布特征，获得了熔池对流运动的驱动力是熔池温度分布不均形成的熔池表面张力梯度。而且熔池表面张力梯度与激光能量密度，熔池温度，熔体对流运动速度是彼此密切相关的。熔池对流运动机理分析表明，热流密度越高，温度越高，表面张力梯度越大，熔体对流运动速度越大。

Effect of molten pool convection on pores and elements distribution in the process of laser cladding

The molten pool convection has a great impact on elements and pores distribution during the laser cladding process. In this paper, the form of molten pool convection was investigate in terms of laser cladding NiCuFeBSi alloys on the substrate of gray cast iron and 45 steel. Results indicated that the distribution forms of pores and elements in the clad layer are uniform and dispersive due to the molten pool convection. Moreover, element distribution is also uniform without any slow transition from the bottom of clad layers to the top by means of electron probe microanalysis (EPMA). And according to the Gaussian distribution of energy in the YAG laser beams, it can be concluded that the driving force of convection is the surface tension gradient due to the non-uniform distribution of molten pool temperatures. Those of the laser thermal density, the molten pool temperature, the convective speed and the surface tension gradient are closely related with each other. It indicates that the temperature increases as the laser thermal density rising. The higher temperature is, the larger surface tension gradient will be as well as the convective speeds.