| Ti6Al4V(TC4)is a strategic alloy in aviation,aerospace and ocean industries for its outstanding properties such as high strength to density ratio,excellent corrosion resistance.However,it exhibits poor tribological properties which limit its application.Plasma nitriding has been introduced to harden the alloy surfaces and improve tribology properties.It is an attractive process for the formation of titanium nitride,compatibility with the substrate,high nitriding speed and less torsions.So far,the normal temperature of nitriding TC4 alloy is from 600? to 900?.High temperature makes grain size growth,T-S and fatigue decreasing.In Nitriding blow 600?,the diffusion speed is slow and little nitriding layer can be gotten,so the surface hardening is limited.In this paper,a new integrated process,which based on deformation acceleration,is purposed to improve the low temperature nitriding for Ti6A14V.The process is solid solution strengthening--cold deformation—plasma nitriding at low temperature.The process is designed to have integrated effects:1.Deformation speeds up the velocity of nitriding below 600? to make the nitriding speed matching the speed at high temperature;2.and as specmen has been treated in solid solution and nitriding temperature is set in the ranges of aging of Ti6A14V,precipitation in matensite phase will occur to strengthen the substrate inside at the same time.Solid solution trials at different temperature were carried out.The ductility result is the criterion to select suitable solid solution temperature and 820? was selected as the temperature of solid solution process.Multi factor experiments were designed and temperature,deformation rate and nitriding time was set as varing factor.Different levels for factors were set and the level of temperature are 400?,500? and 540?,in aging temperature range of Ti6A14V.The specmens were treated at 820? and rolled by roll machine,and then nitrided in the furnace in accordance with experiment design.Aging trial was carried out as benchmark at 500? and 540?,with same setting of temperature,time and deformation with nitriding process.Gibbs free energy,Ti-N phase diagram and Ti-Al-N phase diagram were calculated by software and the blalnce phases were analyzed.The microscope,XRD,micro indention test,wear test,SEM,EDS,and nano-indentation test was used to analyze the nitriding layer,substrate and tribology properties.After nitriding process,the nitriding layer has 2 parts:titanium nitride layer and diffusion zone.The thickness of titanium nitride layer is no more than 1?m and keeps stable in different processes of the designed experiments.Thichness of diffusion zone varies with different process ·The results indicate that the impact of deformation to nitriding layer and surface properties is significant.The impact of deformation to thickness of diffusion zone is significant.With process of 540?/16h,specmen of 10%deformation gets diffusion zone of 40?m,while specmen of 50%deformation gets diffusion zone of 60?m,which increases by 50%?Hardness increases of deformation accelerated integrated nitriding process,represented as ?H2,is higher than that of nitriding process,represented as ?H1.The ratio of ?H2/?H1 is 231%when nitriding temperature is 500?.The integrated nitriding process at low temperature gets the approach which matchs the result at high temperature.With process of 540?/40%/16h(nitriding 16h at 54?,40%deformation),the hardness of surface is HV661,which is similar to that of HV630 with process of 700?/10h of normal nitriding.The impact of deformation to wear resistance is significant.With 540?/8h process,wear rate of 50%deformation specmen decreases 60%than that of non deformed.The integrated process improves the tribological properties.The biggest wear rate decrease between after and before integrated nitriding at 540?,is 84%?The substrate of specimens have been aged after nitriding,and metallogragic analysis exibits aged phase with core hardness increasing.The nitrided microsturcture are directional in terms of rolling deformation and the grain size of microstructure and precipitated phase decreases,as well as the final hardness of substrate increases.Both nitriding process and pure aging process strengthen substrate and the effects are similar.With process of 540?/22h/50%,the TiAl4V specmen gets surface hardness of HV602,and a substrate hardness of HV398.The wear rate of specmen decreases 84%than that of cold deformation and is the lowest in all process.This is the best process we get in Multi factor experiments.Above all,the impact of deformation accelerated is significant and the integrated deformation accelerated nitriding process strengthen the surface and substrate simultaneous in a process.Therefore the results is helpful to extend the application scope of Ti6A14V and have application prospect in engineering application. |
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