Although laser cladding has been rapidly developed in past years and widely applied in some industrial fields, there are still some technical problems unsolved.
① Bonding between cladding layer and substrate. Ideally, there should be tight, low-dilution and narrow interdiffusion belt between coating and substrate materials. Such metallurgical bonding mainly depends on the property of cladding alloy and substrate, in addition to laser processing craft and cladding layer thickness. Nice wettability and self-fluxing nature can bring about pleasing metallurgical bonding, but, it may fail because of their great difference in melting point. When laser cladding alloy has excessively high melting point, it is faced with small melting area, unpleasing surface smoothness and overburnt substrate surface that is likely to pollute cladding layer; on the contrary, coating overburning, alloying element evaporation, shrinkage rate increase and cladding layer destroy (texture and property) will take place. Meanwhile, substrate becomes refractory, interfacial tension rises, and it is hard to avoid opening and inclusion between coating and substrate.
During laser cladding, it is required to minimize dilution rate while making metallurgical bonding. Studies show that minimum dilution rates differ from each other when different substrate materials carry out alloying with surface. In general, the rate must be below 10%.
② Air pore. As for laser cladding layer, air pore is also a very harmful defect because it not only tends to result in cracks and cause great injury to the high air-sealed cladding layer, but also exerts a direct impact on the layer’s resistance to abrasion and corrosion. The reason the air pore happens is that coating powder is exposed to oxidization, dampening before laser cladding or some elements create oxidizing reaction at high temperature. Besides, air pore may also come out when gas fails to be exhausted in time during laser treatment’s rapid melting and solidification. The air pores also include overlapping holes in the process of multi-track overlapping cladding, solidification holes caused by solidifying contraction of laser cladding layer as well as bubbles evaporated from some substances. Generally speaking, it is hard to prevent laser cladding layer from such air pores, but compared with hot spray coating, laser cladding layer has obviously less air pores.
In the process of laser cladding, some measures may be taken to avoid air pores, such as, prevent alloy powder from oxidization while storage and transport, dry off and dispel dampness before use, keep off oxidization during laser cladding, or choose the proper technical parameters according to testing results. With that, air pores will be removed to the utmost extent.
③ Composition segregation of cladding layer. During laser cladding, uneven composition (namely, segregation) and texture may happen for following reasons. First of all, laser cladding is usually heated rapidly to give rise to temperature gradient from substrate to cladding layer, which will surely cause directional solidification while cooling. In view of knowledge of metallography, cladding layers that solidify at different times have different compositions. So, because of fast cooling speed after solidification and failure of homogenized thermal diffusion, the composition segregation takes place. Further texture unevenness and cladding layer injury arise. But, such phenomenon still can’t be addressed now. Secondly, it is blamed for molten pool convection arising from uneven laser radiation, which tends to result in macro homogenization of alloy elements in cladding layer. The reason is that substances in molten pool are mainly transferred through liquid flow. Meanwhile, composition microsegregation may also occur. In addition, considering that alloy properties like viscosity, surface tension and interaction among alloy elements will exert an influence on molten pool convection, they also affect composition segregation.
Although it is impossible to completely remove composition segregation from laser cladding, we can take following measures to suppress it just in order to get well-formed cladding layer and satisfy its designed performance, namely, adjust interaction duration between laser and cladding metal, or change laser beam type, replace entire convection with zonal mode, and alter technical parameters.
④ Crack and fracture. It is crack and fracture that impede the expansion of laser cladding. They are mainly caused by the difference of physical properties between laser cladding materials and substrate, and by the huge heat stress of cladding layer under the influence of fast-heating and rapid-cooling high-density laser beam. In a general way, heat stress inside cladding layer is a kind of tensile stress, hence, crack will take place when local tensile stress is beyond the limit of coating strength. Moreover, crack is more likely to happen on dendritic boundary, air pore and inclusion of laser cladding layer by reason of their low strength and concentrated stress. Adjusting composition of laser cladding powder may relieve the stress concentration in coating and decrease the number of fractures during laser cladding. Even so, crack, air pore and inclusion during alloy cladding still fail to be well solved. So, it is very necessary to develop proper materials for laser cladding.
When it comes to laser cladding technique, new technologies shall be further developed to make cladding layer better, such as, gradient coating. The method of hard phase content gradient coating helps to get crack-free gradient cladding layer that sees continuously variable hard phase content. Thermal treatment (preheating and laser remelting) ahead of or in the wake of laser cladding can also prevent cladding layer from crack and opening.
Now, laser cladding is ever-growing. Thanks to continuous optimization of cladding technology and well-targeted R&D of laser cladding powder, it has been widely applied in petrochemistry, aerospace, automobile, iron and steel, as well as machinery industry.
