Investigation On Microstructure And Properties Of Fe-based Coatings Prepared By Extreme-High-speed Laser Cladding

With the development of industrialization,the requirements for the performance and service life of key components are getting higher.However,many failure of the key components forms occur due to the terrible and complicated service environment.Thus,the research on surface strengthening and remanufacturing technology has become a hot topic in the industry.Conventional laser cladding（LC）can be used to strengthen or repair the material surface,but the industrial application of LC technology is restricted due to its low cladding efficiency and high dilution ratio.As a novel surface modification technology,extreme-high-speed laser cladding（EHLA）has attracted extensive attention and expectations due to the advantages of high efficiency,flexibility and economy,which has the potential to replace hard chrome plating（HCP）technology and play an important role in the field of surface modification and repairing.However,the systematic of research of EHLA technology has not been explored.Therefore,in this research,Fe-based coatings were deposited by LC and EHLA at different cladding speed,respectively.By comparing the coatings prepared by different processing technologies,the forming mechanism of EHLA deposition was expounded,and the internal strengthening mechanism of wear and corrosion resistance were also explained.In addition,the influence mechanism of the integrated process of laser remelting（LR）and EHLA on the cladding quality and microstructural evolution.The specific research contents and conclusions are as follows:（1）Fe-based coatings were prepared on 45 steel substrate with different cladding speeds by EHLA technology.Comparing with the coatings prepared by routine LC technology,the influence of different processing parameters on the surface morphology,phase composition,element distribution and microstructural evolution of EHLA coatings were analyzed.When the cladding speed varies from 30 m/min to 60 m/min,the thickness of the EHLA coating is reduced from 150μm to 64μm.Meanwhile,the surface of EHLA coating presents uneven and irregular morphology,and the surface roughness range from 10 to 20μm.The microstructure of EHLA coating is finer compared to that of LC coating,and extreme-fine grains and nano-grains can be even found in the top region of EHLA coating.As the microstructure of EHLA coating is closely related to the cladding speed,the growth rate R increases and the temperature gradient G decreases with the increase of cladding speed,which leads to the decrease of G/R.Since the change of R is more significant than that of G,the value of G×R（cooling rate）increased,which leads to the solidification structure transformed into fine equiaxial dendrite region.When the cladding speed is 60 m/min,the cooling rate of top region reaches4.02×10⁶ K/s,which is much higher than that of the bottom region.In addition,the phase composition of Fe-based coatings prepared by LC and EHLA are mainlyα-Fe,while other phases are less.Compared with LC coating,the EHLA coating presents more uniform element distribution and lower segregation,which contributes to uniform and stable mechanical performance.（2）The corrosion behaviors and wear resistances of the EHLA coatings with different cladding speed were analyzed by electrochemical corrosion test and friction and wear test.Besides,the mechanism of the EHLA coatings on corrosion and wear resistance were further discussed.The average micro-hardness of LC coating is about 500 HV,while the micro-hardness of EHLA coating is improved and reaches 560-635 HV.EHLA coating shows the same wear mechanism as LC coating,namely abrasive wear,adhesive wear and oxidation wear.The friction coefficient and wear rate of the traditional LC coating are0.6 and 1.235×10^-2 mm³N^-1m^-1,respectively,while that of the EHLA coatings are 0.368and 4.54×10^-3 mm³N^-1m^-1,which are decreased by 38.6%and 62%,respectively,presenting a better wear performance.Compared with LC coating,EHLA coatings have significantly higher corrosion potential and lower corrosion current density.According to the EIS fitting results,the EHLA coatings have higher charge transfer resistance R_t,which indicates that EHLA coatings have better corrosion resistance and lower corrosion rate.（3）Single-layer and multi-layer Fe-based coatings were fabricated by the integrated process of EHLA and LR（EHLA-LR）technology,and their surface and internal quality,microstructure,surface micro-hardness and corrosion resistance were analyzed.With the increase of the number of deposited layers,the fluctuation and defects of the top surface also increase.The EHLA-LR process can effectively improve the surface morphology and surface quality of the coating.The surface roughness of single layer and multi-layer Fe based coatings are decreased by 65.78%and 57.89%after LR treatment.For multi-layer coatings,the interlayer defects of adjacent layers are greatly eliminated via LR layer by layer.The interlayer metallurgical bonding and internal quality are significantly improved.Meanwhile,the density of monolayer Fe-based coating can increased from 99.7245%to 99.8362%.LR technology promotes a more regular and uniform structure with same orientation and refined size of grains.EHLA-LR coatings possessed a better corrosion resistance due to the uniformity of ultrafine grains.The electrochemical test results show that there is little difference in the self-corrosion potential(E_corr)between the two kind of cladding coatings.However,the self-corrosion current density(I_corr)of the EHLA-LR coating is significantly reduced.Compared with EHLA coating,the average hardness of EHLA-LR coatings are slightly increased,but the wear properties have little difference.This research on the experiment and exploration of EHLA technology and its integrated technology（EHLA-LR）provides a certain theoretical guidance for the experimental exploration and industrial application of other high-performance EHLA coating.The theoretical guidance and practical significance of EHLA technology is of great significance for further promotion in practical industrial production.