Microstructure And Properties Of The Coating Produced By Laser Cladding With Ni-based Composite Powder

Ball valve has been used in harsh working conditions for a long time,so wear marks and pits often appear on the surface,which will lead to the failure of the entire ball valve.In order to improve the service life of ball valve,wear-resistant and corrosion-resistant coating is often prepared on its surface in industry.As an advanced technology of surface treatment,laser cladding has been widely used in the preparation of wear-resisting and corrosion-resistant coatings.In recent years,more and more researchers have carried out related research on WC-reinforced Ni-based composite coating by laser cladding,and found that the laser cladding composite coating has greatly improved the surface properties of the matrix material.However,it is easy to produce pores,cracks and spallation in the cladding process due to the large thermal transformation stress in the laser coating and the great difference in thermal properties between WC particles and Ni-based alloy,which has become a technical problem to be solved and a research hotspot.In this paper,the influence mechanism of WC content on the flow direction of the molten pool,microstructure and metallurgical behavior of Ni-based coating was further explored,and through the established regression model,the process parameters were optimized and the method of obtaining high-quality laser coating was explored.For the WC composite coating,the formation mechanism of zero-cracking coating and the strengthening mechanism of the coating were explored by the concoction measures of LaB₆ and ZrW₂O₈ respectively.The main research works are as follows:?1?The influence mechanism of WC content on crack sensitivity,WC particle distribution,microstructure,metallurgical behavior,microhardness,wear resistance and corrosion resistance of Ni-based coating was explored.The results showed that when the WC content of coating increased from 0 to 20wt.%,the coating cracked and its crack rate basically unchanged,but when the WC content continued to increase,the crack sensitivity increased.The melting width and dilution rate of the coating increased first and then decreased sharply,and the flow direction of the molten pool changed with the increase of WC addition.When the WC content was higher than 20wt.%,WC particles were dispersed in the composite coating driven by Marangoni force.The main phases of Ni60/WC composite coating are??Ni,Fe?,FeNi₃,Cr₄Ni₁₅W,Ni₄B₃,M₂₃C₆,M₇C₃,Fe₃Ni₃B,Cr₅B₃,CrB,WC and W₂C.As WC content increased,dendrites were gradually refined,carbide hard phase changed from Cr-rich carbide to W-rich carbide,white and gray hard phases were obviously increased,and the number of eutecticum between white and gray phases was correspondingly reduced.In addition,the addition of WC particles changed the atomic concentration distribution of Fe,W,Cr,B and C in each region of the coating,which increased the bonding strength of the coating.The microhardness of the coating increased gradually,its wear resistance and corrosion resistance increased first and then decreased with the increase of WC content,and when the WC content was 30wt.%,the wear resistance and corrosion resistance of the coating were the best.?2?The regression model of cladding process parameters was constructed and process parameters were optimized.According to the melting width,melting height,dilution ratio,surface forming condition,crack number,hardness and microstructure uniformity of the coating,the technological evaluation index was established,and the regression model of the correlation between hardness,melting width,melting height,dilution rate,quality evaluation index and process parameters was established.Simultaneously,the process parameter window for preparing crack-free Ni60/WC coating was obtained,and the process parameters were optimized.According to the optimization results,the optimal cladding condition was obtained.When the laser power was 1.7kW,the powder feeding voltage was 7.2V,and the scanning speed was 2mm/s,the hardness of the cladding layer was 59.6HRC,the dilution ratio was 6.7%,and the quality evaluation index was 89.2,which met the optimization requirements.The experimental verification of the regression model showed that the predicted value of regression was close to the experimental value,and the relative error was within 10%,indicating that the established regression model can predict the relationship between cladding characteristics and process parameters.Compared with the coating prepared by high power and low powder feeding voltage,the optimized coating had no crack and high hardness,but its wear resistance and corrosion resistance are reduced.Therefore,it is the key to fabricate the coating with low cracking tendency and high quality is to make WC fully dissolve to strengthen the coating by properly matching power,powder feeding voltage and scanning speed.?3?The influence mechanism of LaB₆ on the crack sensitivity of the coating and the strengthening mechanism of LaB₆ on the coating were revealed by studying the microstructure and properties of Ni60/WC coating with different content of LaB₆.The results showed that with the increase of the content of LaB₆ powder,the cracking tendency of the coating decreased,its melting width increased gradually and the melting height first increased and then decreased,while the dilution decreased first and then increased.At the same time,the carbides changed from dendritic,block and strip M₂₃C₆ to diamond M₇C₃,and the size gradually decreased.With the increase of LaB₆ content,the average microhardness of the coating increased first and then decreased,and the fluctuation range of microhardness decreased gradually.Under the influence of the number of intermetallic compounds,the precipitation of boride hard phases and the microscopic segregation,the wear resistance of coating decreased with the increase of LaB₆ content.In addition,the corrosion resistance of the coating with 0.6wt.%and 0.9wt.%LaB₆ had better corrosion resistance,but the corrosion resistance of the coating with LaB₆ was lower than that of Ni60/WC coating.Therefore,according to regular pattern between the content of LaB₆ and the crack sensitivity,wear resistance and corrosion resistance of the coating,the amount of LaB₆ should not be too much in order to prepare crack-free and high-quality coating,and the optimal amount was 1.2wt.%.At this time,a coating of the lowest cracking tendency can be obtained,of which comprehensive performance was better than that of the Ni-based coating.?4?The influence mechanism of ZrW₂O₈ on the crack sensitivity of the coating and the strengthening mechanism of ZrW₂O₈ on the coating were explored by studying the formation,crack sensitivity,microstructure,microhardness,wear resistance and corrosion resistance of Ni60/WC coating with different content of ZrW₂O₈.The results showed that the melting width gradually increased,the melting height gradually decreased while the dilution rate changed on the contrary,in addition to decreasing crack sensitivity as ZrW₂O₈ content increased.Each coating had??Ni,Fe?,Cr₄Ni₁₅W,Ni₄B₃,Cr₅B,WC and W₂C phases,the phases of FeNi₃,Fe₃Ni₂ and M₂₃C₆ disappeared,as well as Ni₁₇W₃,FeNi,M₃C₂ and ZrO₂phases were in situ synthesized and increased with the increase of ZrW₂O₈ content.Accompanied with the change,the proportion of dendrites and equiaxed crystals in the microstructure of those coating gradually increased along with the increase in ZrW₂O₈content,the grains gradually refined and the microstructure became more uniform and denser.While the microhardness of the coatings fluctuated more slightly and its average value gradually decreased.An increase in ZrW₂O₈ content increased wear resistance and corrosion resistance significantly except for the coatings with 2%and 4%ZrW₂O₈ contents.However,the coating prepared should not be too low in height.When the content of ZrW₂O₈ was10wt.%,the melting height of the coating was suitable.At this time,the coating exhibited the most compact microstructure without any defects such as pores and cracks,and the best wear resistance and corrosion resistance.Therefore,the coating with high suitability,low cracking tendency,high wear resistance and high corrosion resistance can be prepared by adding appropriate ZrW₂O₈.