| At present,ordinary low carbon steel has been widely used in various fields of industrial production due to its excellent comprehensive performance and low price.However,it is still difficult to meet the harsh working environment in special circumstances.because its material properties are limited.Laser cladding technology can produce excellent coatings on the surface of ordinary steel,which have high hardness,high wear resistance,corrosion resistance and high temperature oxidation resistance,etc.This coating can replace precious and rare metal materials,and expand the application scope of common materials.The laser cladding process is similar to the welding,in that the coating material melts and cools at an extremely fast rate.The extreme temperature gradient and composition segregation lead to the occurrence of porosity and crack defects after solidification,which seriously affects the use and promotion of the technology.In this paper,a nickel-based WC reinforced coating is prepared on the Q235 steel substrate by laser cladding technology.Firstly,nickel-coated WC composite powder with “core-shell” structure was used as cladding material to study the effect of different WC content on the structure and properties of the coating.On this basis,the optimal WC content ratio was selected and the subsequent tests were carried out.Then,two auxiliary processes of ultrasonic vibration and mechanical vibration are simultaneously applied in the laser cladding process.The effects of ultrasonic vibration power and mechanical vibration frequency on the microstructure and defects of the coating,element distribution,phase composition,microhardness and wear resistance were studied by adjusting the vibration parameters and using related characterization methods(Such as scanning electron microscope,energy spectrometer,X-ray diffractometer,microhardness tester,etc.),and the mechanism of action in the laser melting pool and solidification process was analyzed.The results show that the coatings with 15%,25% and 35% WC are obtained by laser cladding,and there is a white band formed by plane crystal in the interface bonding zone,the central organization consists mainly of dendrites and columnarcrystals.The coatings phase composition are mainly composed of γ-Ni(Fe),Ni3 Fe,WC,M23C6 and M7C3(M=Fe,Cr),etc.WC,M23C6 and M7C3 are hard phases,which play a key role in improving the hardness and wear resistance of coatings.The average microhardness of the coatings with 25% WC is about 831.HV,which is 3.2 times higher than that of the matrix,the hardness curve fluctuates most smoothly.The wear rate is14.5×10-3mm3,and there are fewer pore and crack defects.The quality and comprehensive performance of the coatings are better.The air-load ultrasonic vibration assistant technology can improve the macro-smoothness of the coating,and effectively reduce the defects of gas holes and cracks.With the increase of ultrasonic vibration power,the grain morphology of the coatings change from dendrite to fine dendrite and columnar crystal,the orientation of the bottom grain decreases,and the quality of the coating is obviously improved.Compared with the corresponding coatings with different ultrasonic vibration power(P=150W,250 W,350W),it is found that when the ultrasonic vibration power is 350 W,the length of the dendrite trunk is only 34-48μm,the distribution of Ni,Cr,W,Fe and Co in the scanning region is more uniform.The average microhardness of the coating is increased to 937.1HV,and the wear rate is only 9.5×10-3mm3,which is 34.5% lower than that of the non-vibration assisted coating.In addition,the phase analysis map shows that the diffraction peaks at diffraction angles of 44.1°,51.5°,and 75.8° are broadened,and the diffraction intensity is enhanced,indicating that ultrasonic vibration can refine grains and improve crystal structural integrity.With the increase of mechanical vibration frequency,the bonding zone increases from 3μm at 200 Hz to 8μm at 400 Hz,and the grain size of the coating decreases from45-55μm to 20-25μm.The uniformity of element distribution increases gradually,and the single element aggregation,porosity and crack defects are improved.When the auxiliary vibration frequency f=400Hz,the coating quality and mechanical properties are the best.The average microhardness of the coating is 978.5HV,which is 19.1%higher than that of the non-vibration-assisted coating.The average friction coefficient is only 0.42,the wear rate is 8.3×10-3mm3,and the wear mechanism is slight abrasive wear.The coating exhibits good wear and abrasion resistance. |
PDF Full Text Request