Study On Salt Bath Vanadium-nitrocarburizing Of AZ31B Magnesium Alloy

Magnesium alloy is a metal engineering material with the greatest development potential in the trend of structural lightweight.It integrates the advantages of low density,good vibration and thermal conductivity and rich resources.It has broad development prospects in the fields of aerospace,transportation,electronics and so on.However,due to its low surface hardness,poor wear resistance and corrosion resistance,its application and development in industrial field are limited.This topic takes AZ31B magnesium alloy as the research object,studies a new magnesium alloy surface treatment process,salt bath carbonitriding vanadium co-osmotic,in order to form a high hardness carbonitriding vanadium layer on the material surface to improve the hardness,wear resistance,corrosion resistance and other surface properties of AZ31B magnesium alloy.The specific research contents are as follows:（1）In this paper,the parameters of the main influencing factors of salt bath vanadium-nitrocarburizing process were determined through preliminary exploration experiments:the carburizing temperature is 570℃～610℃,the carburizing time is 3 h～5 h,and the content of vanadium donor is 5%～10%.Based on the results of single factor inquiry test and taking microhardness as the index,the effects of carburizing temperature,time and vanadium content on the effect of carburizing were studied.Through the orthogonal test of three factors and three levels,the optimal process parameters of salt bath vanadium-nitrocarburizing process were obtained:the co-infiltration temperature is 590℃,the co-infiltration time is 4 h,and the content of vanadium supply agent is 10%.Under the optimal process parameters,the surface hardness of AZ31B magnesium alloy after treatment can reach 294.1 HV,which is 293%higher than that of Original samples.The range analysis shows that the co-infiltration temperature has the greatest influence on the co-infiltration process.（2）By observing and analyzing the surface morphology and cross-section morphology of the co-infiltrated samples under the optimal process parameters,it is concluded that the surface of the vanadium-nitrocarburizing samples is smoother and has fewer scratches than the Original samples.The outermost layer of the sample surface is carbon nitrogen vanadium compound layer,the middle is transition layer,and the interior is matrix.The total thickness of infiltration layer reaches 60～80μm.The thickness of compound layer is 20～30μm.Compared with the traditional metallization process,the change gradient of cross-section hardness is gentler,which is more conducive to improve the comprehensive properties of the surface.（3）According to the XRD and EDS analysis results of carbonitriding layer on the surface of vanadium-nitrocarburizing samples under the optimal process parameters,it is concluded that the phase composition of carbonitriding layer mainly includes VC,V₂C,VN,Mg₂C₃,Mg₃N₂ and other high hardness compounds,which greatly improves the surface hardness of AZ31B magnesium alloy.Combined with EDS element map and microhardness change curve,it is concluded that the diffusion and infiltration of V element is the main reason for the surface hardness of AZ31B magnesium alloy.The infiltration of V element begins after 4 hours at the co-infiltration temperature above 570℃.By studying the mechanism of carbonitriding,it is concluded that in the initial stage of vanadium-nitrocarburizing,C and N elements first penetrate into the surface through gap diffusion,subsequently,V elements infiltrate by vacancy diffusion and substitution diffusion.Meanwhile,the infiltration of C and N elements will make Mg element in an unstable state,which is conducive to the infiltration of V element.（4）Under the conditions of dry friction and oil lubrication,the friction and wear tests of vanadium-nitrocarburizing samples,carbonitriding samples and Original samples under different loads and sliding speeds were carried out.The friction and wear properties of the surfaces of the three samples were analyzed from the aspects of average friction coefficient,specific wear rate,two-dimensional and three-dimensional wear morphology.The results show that the average friction coefficient and specific wear rate of vanadium-nitrocarburizing samples under different working conditions are lower than those of carbonitriding samples and Original samples.Compared with Original samples and carbonitriding samples,vanadium-nitrocarburizing samples have the least surface wear.When the load is 20 N under dry friction condition,the average friction coefficient of carburizing vanadium carburizing sample decreases by 48.3%and 36.5%,respectively,compared with the original sample and carburizing sample.The wear rate decreased by 74.2%and 57.1%respectively.It is concluded that the vanadium-nitrocarburizing process can significantly improve the friction and wear properties of AZ31B magnesium alloy surface.（5）The corrosion resistance of Carbonitriding samples,carbonitriding samples and original samples were analyzed by electrochemical test,salt spray test and full immersion test.The results show that compared with the Original samples,the self-corrosion potential on the surface of the vanadium-nitrocarburizing samples is increased by 0.114 V,and the corrosion current density decreased by 74%,indicating that the vanadium-nitrocarburizing process reduces the self-corrosion tendency and corrosion rate of AZ31B magnesium alloy.At the same time,the corrosion products and corrosion area of vanadium-nitrocarburizing samples are smaller than those of Original samples.It is concluded that the process can effectively improve the corrosion resistance of AZ31B magnesium alloy surface.