PROPERTY AND THERMOSTABLITY STUDY ON TC6 TITANIUM ALLOY NANOSTRUCTURE PROCESSED BY LSP

Wang Xuede,Li Yinghong,Li Qipeng,He Weifeng,Nie Xiangfan,Li Yuqin

(Science and Technology on Plasma Dynamics Laboratory,Air Force Engineering University,Xi′an,710038,P.R.China)

INTRODUCTION

In contrast to steel,the mechanical performance of titanium alloys is very attractive with combinations of strength, toughness,fatigue,and outstanding corrupting resistance.Titanium alloys have been widely used in aero industry,especially in aero-engines,which have already replaced the aluminum alloys and stainless steel to a certain extent[1].However,titanium alloys are very sensitive to minor defects and stress concentration,which results in the failure of the areoengine components[2-6].

Many surface treatment technologies such as laser-quenching,ion implantation and shot peening have been used to improve the fatigue abilities of titanium alloys[7-12].Among the above traditional surface treatment technologies,shot peening is the most effective and widely used.Shot peening is relatively inexpensive,uses robust process equipment,and can be used on large or small areas as required.However,the shot peening process has its limitations.In determining the produced compressive stresses,the process is semi-quantitative and depends upon a metal strip or gage called an Almen type gageto provide an indi-cation of shot peening intensity. Firstly,this gage does not guarantee that the shot peening intensity is uniform across the peened component.Secondly,the compressive residual stresses are limited in depth,usually not exceeding 0.25 mm in soft metals such as aluminum alloys and less in titanium alloys.Thirdly,the peening process results in a roughened surface,even sometimes minor cracks.This roughness needs to be removed before use in wear applications.The used typical processes remove the majority of the compressive layer[13].An alternative novel surface processing technology,namely laser shock peening(LSP),can induce greater depth of residual stress and produce strengthening nanostructure into metal surfaces using high-power, Q-switched laser pulses.

LSP used to shock TC4 titanium alloy has been vastly studied by researchers in domestic and abroad[14-19]. LSP induced compressive residual stress distribution in TC4 as well as its mechanism to prolong the target fatigue life was studied in detail[15,18].However,as one of the most important titanium alloys used in China,LSP of TC6 has not been studied yet.In this paper,the pattern of nanostructure,the characteristic of the enhanced microhardness and the the rmostablity of TC6 titanium alloy processed by LSP are studied in detail.

1 EXPERIMENTAL DETAILS

1.1 Principle and experimental procedure of LSP

During LSP,the laser beam is directed onto the treated surface,it passes through the transparent confining medium (water/glass) and strikes the sample.A short laser pulseis then focused onto the sample to immediately vaporize the ablation medium.The energy absorption at the confining medium/plasma interface leads to the formation of a shock wave which strikes the sample with an intensity of several Gigapascals.The high pressure against the surface of the sample causes a shock wave to propagateinto the material.The plastic deformation caused by the shock wave produces the compressive residual stress,nano-crystalization and hardness enhancement on the surface of the sample[17-19]. LSP has been proved effective in improving material fatigue strength in a number of alloys[20-24]. The use of LSPhas been proposed,and in someimplemented cases,as a localized surface treatment for airfoils in the fan sections of certain military aircraft engines that are particularly sensitive to foreign object damage(FOD)induced high cycle fatigue(HCF)failures[25].

1.2 Preparation of test samples

TC6 is primarily composed of the hexagonal close-packed(HCP)αphases and the body-centered cubicβphases.Both two kinds of phases are of equiaxed grains.Fig.1is the metallographic of the titanium without LSP,the white regions areαphases and the black regions areβ phases.The composition of TC6 titanium alloy is shown in Table 1[26].

Fig.1 Metallographic of original TC6 without LSP

Table 1 Composition of TC6 titanium alloy wt%

The test sampleis mounted on a five-coordinate tablecontrolled in x-y direction with floating water as confining medium.During LSP,the shock waves are induced by a Q-switched laser system based on a neodymium-doped glass and yttrium aluminum garnet(YAG)crystal lasing rod which operates in the near infrared with a wavelength of 1 054 nm and a pulse of around 20 ns.The LSP parameters to process the sample are listed in Table 2.

Table 2 LSP parameters

According to the previous experiments,the sample surface is laser processed with spots of 3 mm in diameter moving forward along the work piece.Samples are treated with three layers at the same power density(4.24 GW/cm2).The standard after LSPis that:Macro-deformation should not happen,dimension of samples should not change,and roughness with LSP should satisfy project application.Then,some of samples treated by LSParevacuum annealed at 623 K for 10h.

1.3 Measurement equipments and methods

The characteristics of TC6 layer before and after LSPare measured by Dimension-3100 atomic force microscope(AFM)and electron backscatter diffraction(EBSD).And the nanostructures induced by LSP with and without annealing are studied using JEOL/JSM-6360LV scanning electron microscope(SEM)and TEM-3010 transmission electron microscope(TEM).XRD-7000 X-ray diffraction(XRD)equipment is used to study the diffraction patterns.The Vickers microhardness(HV)measurements are made on a MVS-1000JMT2 microhardness tester using an indentation load of 500 g at cross section,a dwell time of 10 s.

2 RESULTSAND DISCUSSION

2.1 AFM observation

The typical deformed configuration of the shocked region is observed and measured using AFM,as seen in Fig.2.The deformation is approximately uniform along the shocked line,which is indicative of a 3-D deformation state.From both 3-D geometry and height information in Fig.2,the altitude variation is only 0.06μm.It is clear that LSP slightly increases the roughness,and there are no defects such as ablation and minor cracks produced by LSP.Since the surface integrality and the low roughness are significant to the wear and the fatigue performances of TC6 titanium alloy,the selected LSP parameters are beneficial to the titanium alloy.

Fig.2 3-D shapes without and with LSP

2.2 SEM and EBSDobservation

Fig.3(a)is the SEM morphology of thesample cross section without LSP. Over 50%equiaxedαphases evenly distributein thematerial and a quantity ofβphases unevenly distribute amongαphases.Fig.3(b)is the SEM morphology of the sample cross section with three LSPlayers at power density of 4.24 GW/cm2.In this picture,the severe plastic deformation(SPD)layer and substrate can be distinguished easily.The SPD layer is about 200μm.Theαphases are elongated by normal shock wave to the surface and theβphases are shattered in the SPD layer.Fig.3(c)is the SEM morphology of the sample cross section after annealing. Paired observing Figs.3(b,c),it can beseen that the depth of SPD layer does not decrease after annealing.Fig.3(d)is thepartial enlarged detail of Fig.3(c),the patterns ofαandβphases do not change either.

Fig.3 SEM morphologies of cross section

EBSD is a diffraction technique for obtaining crystallographic orientation with sub-micron spatial resolution from bulk samples or thin layers in SEM[27].

The crystallographic orientation of the surface microstructure is collected using EBSD,which provides information about the lattice rotation during LSP,as shown in Fig.4.The white regions correspond to theαphases and the black regions correspond to theβphases respectively.As can be seen in Fig.4(b),lattice rotation due to the high plasma shock wave leads to a dominance orientation distribution of the two kinds of phases,but there is no new produced phase.

Fig.4 EBSD of TC6 surface without and with LSP

The EBSD diffraction patterns at cross section without and with LSP are shown in Fig.5.Compared with Fig.5(a),the low diffraction quality of Fig.5(b)reveals that SPD happenes at the surface layer during LSP,and the affected layer is about 200μm. As we can see in Fig.5(b), theαphases are compressed and stretched in the strengthened layer,and a proportion ofαandβphases is shattered to smaller phas-es.The compressed phases induce compressive residual stress at the surface layer.The residual compressive stress with the strengthening phases together restrains initiation and propagation of the fatigue cracks during low cycle fatigue(LCF)and high cycle fatigue(HCF),thus improving the fatigue performance and increasing the fatigue life of the titanium alloy.

Fig.5 EBSD diffraction patterns at cross section

2.3 XRD observation

Since the shallow nanostructure produced by LSPand thebiggerθanglein routine XRD lead to the shortened distance of X-ray in the strengthened layer,a new film-XRD method is used to measure the diffraction patterns.Fig.6 is the XRD patterns with different layers.It can be seen that there is no additional peak with different LSP layers,which indicates that no phase transformation and no new crystalline phases are formed when high pressure laser shock wave transmits in the material.The main phases areαandβ.The second interesting featurein XRD patterns is that the peaks are broadened with LSP.The peak broadening indicates the grain size refinement and the introduction of lattice microstrains[28]. The main reason is that the high strain ratecan generate high density dislocation and cold work in the material.This consists with the phenomenon in nanocrystalline materials prepared via the other SPD methods.In the SPD zone,there are high internal stresses[29].As a result,XRD line broadening of these nc-materials is due to crystallite refinement,lattice microstrains and instrumental broadening.

Fig.6 XRD patterns of TC6 with different LSPlayers

2.4 TEM observation

The nanostructure changes of the current titanium alloy induced by LSP and annealing are shown in Fig.7.

Fig.7(a)presents the original features of the sample without LSP.The diffusive diffraction spots in the SAED pattern are identified asα phases.Fig.7(b)is the diffraction pattern and TEM micrograph near the surface processed by LSP,and TEM micrograph shows a very high density dislocation region and dislocation cells.The diffraction patterns reveal that dislocation cells are large and have thin walls composed of tangled dislocations in the area.Fig.7(c)reveals that three LSP layers are enough to generate nano-grain on the surface of the titanium samples. Surface nanocrystal induced by multiple LSPis similar to those caused by SPD methods.The underlying mechanism may include:deformation localized in shear bands consisting of an array of high density dislocations,dislocation,annihilation and recombination of small-angle grain boundaries separating individual grains,and a changein the direction of the grains with respect to their neighboring grains becoming completely random.

Fig.7 TEM photographs and diffraction patterns

Fig.7(d)shows the features of the LSPsamplevacuum annealing at 623 K for10h.Figs.7(bd)reveal that the dislocation density increases with increasing strain rate,but decreases after annealing.Furthermore,it can be seen that the nanocrystal sizes are nearly the same to those beforeannealing and the obscuregrain-boundary becomes distinct.Therefore,it is concluded that for the nanocrystal produced by LSP,the offered energy during annealing at 623 K is not high enough to lead nano-grain boundary migration. The nanocrystal presents a barrier for dislocation movements,thus causing the dislocations to pile up and counteract at the nanocrystal boundary.It seems reasonable to assume that the amount of precipitated nanocrystals has direct effect on the mechanical property of the current titanium alloy.Therefore,the nanocrystal produced by LSP of this titanium alloy is thermostable at 623 K.

2.5 M icrohardness test

The recorded mechanical property measurements indicate a significant increase in hardness and strength at surface layers with LSP. The hardness distribution is determined using microhardness at the cross section.Fig.8 shows the variation in microhardness as the depth from the treated surface increases.The original hardness of TC6 titanium alloy is about 334 HV0.5.At the LSP layer of top surface,the microhardness is 406 HV0.5,which is 72 HV0.5 higher than that of the matrix.The hardened depth affected by LSP is about 500μm.After annealing the surface microhardness decreases only 10 HV0.5,and the hardened depth remains stationary.Therefore,the microhardness distribution affected by LSPof the current titanium alloy is thermostable at 623 K.

The mechanism of the hardness enhancement induced by LSPincludes two factors.One is the actual increasein hardness,while the other is the introduction of residual compressive stress into the material during the nanocrystalization on the surface of the material.The residual stress partly releases during annealing,which leads to the minute decrease of microhardness.

Fig.8 Microhardness of different samples at cross section

3 CONCLUSIONS

In this paper,TC6 titanium alloy is laser shock processed and vacuum annealed at 623 K for 10 h.And the nanostructures and microhardness before and after annealing are examined.The conclusions areas follows:

(1)The processed surface AFM indicates that the selected LSPparameters are beneficial to the TC6 titanium alloy.The uniform deformation and lower roughness show that LSPis better than other traditional surface treatments.

(2)SEM and EBSD of the strengthened cross section layer show thatαphases are compressed and stretched at the strengthened layer,and a proportion ofαandβ phases is shattered to smaller phases.The strengthened layer is about 200μm in depth.

(3)The surface XRD indicates that there is no new produced phase during LSP,while the grain size refinement and the introduction of lattice micro-strains with LSPlead to the broadened peak.

(4)The TEM photographs and diffraction pattern indicate that the shock wave provides high strain rate deformation and leads to the formation of nanocrystal.Compared with the samples without annealing,the dislocation density is lower and the grain-boundary is more distinct in the annealed samples,but the size of the nanocrystal does not visibly grow larger after annealing.

(5)The microhardness measurement indicates that LSP improves the microhardness of TC6 for about 12.2% on the surface,and the hardness affected depth is about 500μm.The microhardness after annealing is 10 HV0.5lower,but the affected depth does not change.

(6)Tests after annealing indicate that hardness improvement and nanostructure produced by LSP are thermostable at 623 K,which is beneficial to improve the wear resistance and stay the generation of the fatigue cracks.The research production of this paper breaks the USA standard AMS2546 that titanium parts with LSPare subjected in subsequent processing within 589 K.

[1] Ren Xudong,Zhang Yongkang,Zhou Jianzhong,et al.Study of the effect of laser shock processing on titanium alloy[J].Huazhong Univ of Sci& Tech,2007,35(Z1):150-152.(in Chinese)

[2] Li Qipeng,Li Yinghong,He Weifeng,et al.Thermostablity study on microstructure and microhardness of laser shock processed Ti-6Al-2.5Mo-1.5Cr-0.5Fe-0.3Si[J].Materials Science Forum,2012,697/698:440-444.

[3] Lee D,Kim S.Effects of microstructural morphology on quasi-static and dynamic deformation behavior of Ti-6Al-4V alloy[J].M etallurgical and materials Trans A,2001,32A(2):315-318.

[4] Ocan′a JL,Molpeceres C,Porro JA,et al.Experimental assessment of the influence of irradiation parameters on surfacedeformation and residual stresses in laser shock processed metallic alloys[J]. Appl Surf Sci,2004,238(1/4):501-505.

[5] McCay M H,Hopkins JA.Laser surface processing of compressor blades for erosive environments[R].AIAA 2002-1300,2002.

[6] Martinez S A,Sathish S,Blodgett M P,et al.X-ray diffraction residual stress measurements on surface treated Ti-6Al-4V for aerospace applications[R].AIAA 2002-3733,2002.

[7] Golden P J,Hutson A,Sundaram V,et al.Effect of surface treatments on fretting fatigue of Ti-6AI-4V[J].Int JFatigue,2007,29(7):1302-1309.

[8] Vadiraj A,Karnaraj M.Effect of surface treatments on fretting fatigue damage of biomedical titanium alloys[J].Tribology Int,2007,40(1):82-91.

[9] Kubiak K,Fouvry S,Wendler B G.Comparison of shot peening and nitriding surface treatments under complex fretting loadings[J].Mater Sci Forum,2006,513:105-116.

[10]M ohrbacher H,Blanpain B,Celis J P.Friction and wear mechanisms on CVD diamond and PVD TiN coatings under fretting conditions[J].ASTM Special Technical Publication,1996,1278:76-88.

[11]Ganesh Sundara Raman S,Jayaprakash M.Influence of plasma nitriding on plain fatigue and fretting fatigue behavior of AISI304 austenitic stainless steel[J].Surf&Coat Techn,2007,201(12):5906-5912.

[12]Caron I,De Monicault JM,Gras R.Influence of surface coatings on titanium alloy resistance to fretting fatigue in cryogenic environment[J].Tribology Int,2001,34(4):217-236.

[13]M ontross C S,Tao Wei,Lin Ye,et al.Laser shock processing and its effects on microstructure and properties of metal alloys:A review[J].Int Jof Fatigue,2002,24(10):1021-1036.

[14]Sokol D W,Clauer A H.Applications of laser peening to titanium alloys[C]//ASME/JSME 2004 Pressure Vessels and Piping Division Conference.San Diego,CA:[s.n.],2004:36-45.

[15]Liu K K,Hill M R.Th eeffects of laser shock peening and shot peening on fretting fatigueof Ti-6AI-4V[C]//Fifth International Symposium on Fretting Fatigue.Montreal,Canada: [s.n.],2007:105-113.

[16]Hutson A L,Niinomi A L M,Nicholas T,et al.Effect of various surface conditions on fretting fatigue behavior of Ti-6AI-4V[J].Int Jof Fatigue,2002,24(12):1223-1231.

[17]Jin O,Mall S,Sanders J H,et al.Durability of Cu-Al coating on Ti-6Al-4V substrate under fretting fatigue[J].Surf Coat Techn,2006,201(3/4):1704-1712.

[18]Ruschau J J,Reji J,Thompson S R,et al.Fatigue crack nucleation and growth rate behavior of laser shock peened titanium[J].Int Jof Fatigue,1999,21(S1):199-209.

[19]Li Qipeng,Li Yonghong,He Weifeng,et al.Application of laser shock processing to improving the fatigue performanceof compressor blade[J].Advanced Materials Research,2010,135:184-189.

[20]Sano Y,Obata M,Kubo T,et al.Retardation of crack initiation and growth in austenitic stainless steels by laser peening without protective coating[J].Mat Sci&Eng,2006,417(1/2):334-340.

[21]Hatamleh O.Laser and shot peening effects on fatigue crack growth in friction stir welded 7075-T7351 aluminum alloy joints[J].Int Jof Fatigue,2007,29(3):421-434.

[22]Zhang Yongkang,Zhang Shuyi.Investigation of surface qualities of laser shock-processes zones and the effect on the fatigue life of aluminum alloy[J].Surf&Coat Techn,1997,92:104-109.

[23]Peyre P,Fabbro R.Laser shock processing of aluminium alloys: Application to high cycle fatigue behaviour[J].Mat Sci&Eng,1996,A210(1/2):102-113.

[24]Fairand B P,Wilcox B A,Gallaghtr W J,et al.Laser shock induced microstuctural and mechanical property changes in 7075 aluminum[J].Appl Phys,1972,43(9):3893-3895.

[25]William F,Bates Jr.Laser shock processing aluminum alloy[C]//Application of Laser in Material Processing.Washington D C:American Society for Metals,1979:56-60.

[26]"Aeronautical Manufacture Engineering Handbook"Edits Committee.Aeronautical manufacture engineering handbook[M].Beijing:Aerospace Industry Press,1997:119-126.(in Chinese)

[27]Youneng W,Kysar JW,Yao Y L.Analytical solution of anisotropic plastic deformation induced by micro-scale laser shock peening[J].Mechanics of Materials,2008,40(3):100-114.

[28]Rubio-Gonzalez C,Ocan′a JL,Gomez-Rosas G,et al.Effect of laser shock processing on fatigue crack growth and fracture toughness of 6061-T6 aluminum alloy[J].Mater Sci& Eng,2004,386(1/2):291-292.

[29]Clauer A H,Lahrman D F.Laser shock processing as a surface enhancement process[J].Key Eng Mater,2001,197:121-142.