Laser Surface Alloying Of Ultra-fine Ceramic Particles Reinforced Layer Onto Iron And Steels

The surfaces of metal materials are easily to be worn, corroded, oxidized or fatigue damaged in the external environment and the losses caused by those are huge. Moreover, sometimes the demands of internal and external in many parts are so different that they cannot be both considered with a whole treatment. Therefore, the surface reinforcing technology is paid great attention. As one of the most important way of surface reinforcing, laser surface alloying (LSA) is with great research value and application potential.In this study, Nd: YAG laser and CW CO₂ laser were used in LSA experiment on ductile iron, 9CrSi and 45# steel with sol coatings. Appropriate process parameters and influencing factors were discussed, and new LSA coatings were developed to get an ideal reinforcing result. The SEM, EDX, XRD, TEM, micro hardness, friction and wear properties of the samples were detected and the reinforcing mechanism is also discussed.The analysis of SEM revealed that the alloyed layer could be divided into alloyed zone and heat affected zone. The alloyed zone is consisted of cellular crystal, dentrite-cellular crystal and dentrite crystal because of different temperature gradient. The contents of the three kinds of crystals vary in different substrates. The alloyed zone mainly contains retained austenite, ledeburite and martensite, while the heat affected zone is consisted of retained austenite and martensite. The microstructure is affected by laser beam kind and process parameters. Due to the large metal absorptivity of Nd: YAG laser, a high temperature was reached in the molten pool during LSA process. Finer grain and fewer ledeburite was obtained for the high cooling speed. This could also be achieved by improving the laser scanning speed. But because of the low power, the alloyed layer obtained turns to be thin. Raising the contents of ceramics hard phases in the coating could cause the increase of ceramics hard phases in the alloyed layer, but the increasing speed slows down when the content get close to 30%. Silicon in the coating can hinder the transition of retained austenite.The microhardness of the alloyed layer is enhanced to as high as 1000HV. Higher microhardness of the surface could be obtained by reducing the laser power or increasing the laser scanning speed in a certain extent, with a thickness decrease of the alloyed layer. The addition of new type carbides and silicon is beneficial to the improvement of microhardness. The microhardness reinforcing mechanism contains the introduction of ceramics hard phases, the precipitation of hard carbides, martensitic transformation and microstructure refinement.The friction and wear properties of the surface are improved more than 10 times after LSA. The effect of parameters to friction and wear properties is similar with microhardness distribution. The addition of new type carbides reduces the friction coefficient of the alloyed layer. However, the wear property is not obviously influenced by the contents variation of the new type carbides. The main reasons of enhancement in friction and wear properties are the high hardness and fabricate function of the ceramics hard phases, solid solution strengthening, martensitic transformation and microstructure refinement.Due to the low thermal conduction coefficient, crack is easily generated in the alloyed layer on ductile iron. The amount of crack could be reduced by increasing laser power. The increase of laser scanning speed will cause a larger temperature gradient and increase the trend of crack. Silicon with a percent of 17.5% could reduce the amount of crack by 2/3 for its purification. The crack could also be avoided by pre heat treatment. Little crack was generated in the alloyed layers on 9CrSi and 45# steel. Alloyed layers of HG series and TH2A appear to be with good surface smoothness owing to the silicon and rare earth elements they contain.The new coatings are applied in Shuicheng steel enterprise and Tangshan machining station. Alloyed with HG02 coating, the medium roller of thread steel in Shuicheng steel enterprise is enhanced 80% in service life. Treated with HG03, the service lives of primary roller and medium roller are both enhanced 120%. In Tangshan machining station, LSA was processed on H-shape steel with HG03 and the service life of finish roller is enhanced 110%.