Causes and preventive measures of pores and cracks in laser cladding layer

       The pores in the laser cladding layer are caused by the presence of gas in the laser melting process, and it is too late to escape the surface during the rapid solidification process. Generally, the reaction pores are formed due to the reaction of carbon in the metal with oxygen or the reduction of metal oxides by carbon. However, there are also non-reactive pores such as volatilization of solid substances and evaporation of moisture. The existence of pores is easy to become a gathering place for crack initiation and propagation, so controlling the pores in the cladding layer will also be one of the important measures to prevent cracks in the cladding layer. The control of pores is mainly considered from two aspects: one is to take preventive measures to limit the source of gas, such as drying and dehumidification of powder before use, and using inert gas to protect the molten pool during laser cladding. The second is to adjust the process parameters to slow down the cooling and crystallization rate of the molten pool to facilitate the overflow of gas.



In the process of laser cladding, the temperature rises quickly, the cooling rate is high, the thermal stress of the cladding layer is large, and the crack sensitivity is strong, which is easy to produce cracks. The main methods of suppressing crack generation are as follows: one is to use preheating and slow cooling to reduce the possibility of crack generation and relaxation stress. Preheating is to heat the whole or surface of the substrate to a certain temperature, reduce the temperature gradient between the cladding layer and the substrate material, and ease the thermal stress. Slow cooling is to prevent the structural stress induced by the transformation of the cladding layer structure (such as martensite). Second, the stepped cladding layer can be designed, and the transition cladding layer is selected between the base material and the surface cladding layer. The performance of the transition layer is between the base material and the surface layer material, so that the stress in the cladding layer is distributed in steps. To achieve the purpose of relaxing stress and reducing cracks.