Study On High-pressure Waterjet Peening Surface Self-nanocrystallization And Ion Implantation Hybrid Modification

Researches have shown that surface modification is a good way to meet the higher or special requirements of service condition on the properties of material surface,such as the strength,hardness,friction and electrochemical performance,and physics preoperties like heat,electricity and magnetic.Specificly,having introduced the nano-material conception into the surface engineering,the modified nanocrystal-srtucture surface layer fabricated with mechanical methods can effectively improved the comprehensive mechanical performance of materials.To far,there have been kinds of surface self-nanocrystallization technologies being systematically studied in many metal systems.While their practical application has been limited due to lower processing efficiency,technically complexity and poor corrosion resistance of the treated surface.For this purpose,this study sets out to explore a novel technology using high-pressure waterjet peenging to fabricate nano-crystalline layer on surfaces.To test it,we compared and stuied the materials micro-structue evolution and grain refinement mechanism using the different method,and also investigated properties.For the newly-proposed technology,we also explore the of the influence of pressure,transverse speed and the distance on surface qualities in waterjet peening process.And,MEVVA has been ultilized to further optimize the friction and electrochemical performances of the nano-structure surfaces.The main contents and results are summarized as follows:(1)The impact of the pressure,the transverse speed of nozzle,the stand-off-distance on the surface qualities in high-pressure waterjet peening surface nanocrystallization process has been systematically studied.(2)Two methos were ultilized to fabricate nano-layer on metal materials.Firstly,fast multiple rotation rolling(FMRR)has induced a severe deformed nanocrystallized layer of 100μm thick on the surface of PC titanium.When the treatment duration was 60min,grains on the top surface were refined and average size reduced to 50nm.Furthermore,by adjusting the parameters,including pressure and stand-off-diatance,high-pressure waterjet peening successfully fabricated nanocrystalline layer on surface:when the pressure P0=200MPa,transver speed of nozzle VTr=400mm/min,the severe-deformed depth is relatively shallow and the average grain size was about 46nm,and meanwile α’-martensite phase transformation happened in the severe plastic deformateion process;when P0=100MPa,VTr=400mm/min,the average grain size was reduced to 44nm on the pure titanium surface.(3)316L stainless steel is a kind of metal materials with medium stacking fault energy(SFE)and there exists twinning as well as dislocation motion in plastic deformation.During the grain refinement process,with the increase of the strain,dislocation accumulated and stress concentration caused mechanical twinning.These twinnings were then segmented by high-density dislocations and the divided sub-grains boundaries orientation got bigger with increasing strain,and finally nanocrystalline grains formed.While at room temperature,twinning is the main mode in small strain deformation of pure titanium.With deformation proceeding,dislocations hindered by twin boundaries accumulated and divided the twinnings.(4)The influence of the two SNC treatment on the properties like hardness,strength,friction and corrosion of pure titanium and 316L stainless steel was studied.After FMRR-SNC treatment for 60min,the hardness of top surface of titanium can be 278Hv and the tensile strength σb=423MPa(33MPa higher than that of original sample,when thickness of the tensile sample d=2mm);while after WJP-SNC,the hardness of the surface of pure titanium was relatively lower(261Hv)with nearly no different in surface grain size,and tensile strength σb=475MPa.As for 316L stainless steel treated after WJP-SNC,the surficial hardness was 325Hv(1.5 times that of the matrix hardness)and the tensileand yieldstrength was 650MP(ab)and 580MPa(σ0.2),respectively,better than that of FMRR-SNC sample(σb=608MPa,σ0.2=365MPa).Under same tensile condition,the tensile and yield strength of samples WJP-treated were better than that of the FMRR treated,but decrease of elongation was more obvious.The improvement in hardness and strength after WJP-SNC treatment was federatively caused by fine-grain strengthening,work hardening and phase transformation.(5)Self-nanocrystallizetion treatment reduced the surficial friction coefficient of pure titanium.The friction coefficient declined from 1.1 of original sample to 0.6 of 60min-FMRR-SNCed sample,to 0.7 of WJP-SNCed sample,and the main reason is about the increase of hardness.The poorer surface quality,increasing grain boundaries,martensite phase transformation and high-density dislocations after SNC treatment caused the unsatisfactory corrosion resistance performance for 316L stainless steel.After FMRR-SNC treatment for 60min,the breakdown potential decreased from 0.5V of original sample to 0V,and the passive current increased remarkably from～10-6A/cm2 to 10-5A/cm2;the changing situation of FMRR-SNC was same with WJP-SNC.(6)The FMRR-SNC treatment for 316Lstainless steel effectively increase the injection depth and concentrations of Al and Ti ion.It was found that Ti implantation had the best improvement effect on corrosion resistance for 316L,which optimized the electrochemical performance as good as original level.The subsequent Al ion implantation after SNC treatment for pure titanium can further raise it top surface hardness up to 310Hv,which kept the surfacial lower friction coefficient in the range of 0.15～0.2.This study has proposed a novel high-pressure waterjet peening surface self-nanocrystallization technology for processing of complex-shape surfaces with high efficiency and no pollution introduced,and this new method was also with simple procedure.And also a preliminary study on SNC/ion implantation hybrid modification was implemented to explore the facilitation effect that SNC had on the ion implantation and the corrosion performance,which has theoretical and practical significance for expanding the use of SNC’s application in industrial productionThis work was financially supported by the Natural Science Foundation of the People’s Republic of China(grant no.81171463).