Shot Peening and Ball-Burnishing to Improve HCF Strength of the New Titanium Alloy TIMETAL-54M

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1 Shot Peening nd Bll-Burnishing to Improve HCF Strength of the New Titnium Alloy TIMETAL-54M K. Zy 1, Y. Shn 1, Y. Kosk 2, L. Wgner 1 1 Institute of Mterils Science nd Engineering Clusthl University of Technology Clusthl-Zellerfeld Germny 2 Henderson Technicl Lbortory TIMET, Henderson, NV USA Abstrct TIMETAL-54M (Ti-5Al-4V-.6Mo-.5Fe) is new (α+β) titnium lloy which ws developed by TIMET, Henderson, NV (USA). The ddition of smll mounts of the β-stbilizers Mo nd Fe nd the reduction in Al-content reduce the β-trnsus temperture reltive to Ti-6Al-4V nd offer improved formbility due to lower working tempertures nd working stresses s well s better mchinbility (incresed cutting speed nd/or improved cutting tool life). Thus, TIMETAL-54M is supposed to provide cost benefit for prts tht require extensive mchining. Shot peening () nd bll-burnishing () were performed on vrious well defined microstructures (fully equixed, fully lmellr, duplex) using wide vrition in process prmeters to study potentil improvements in HCF performnce. The shot peening nd bll-burnishing-induced chnges in the surfce nd ner-surfce properties were evluted by mesurements of surfce topogrphy, micro-hrdness-depth profiles nd residul stressdepth profiles. The results indicte mrked influence of the microstructure on the observed HCF strength enhncements. Keywords: TIMETAL-54M, HCF performnce, shot peening, bll-burnishing. Introduction The most common titnium lloy is Ti-6Al-4V, which ccounts for more thn 5% of titnium lloy production. Recently, TIMET hs developed n lloy clled TIMETAL-54M (Ti-54M), which provides improved mchinbility nd formbility [1, 2]. The lloy with nominl composition (Ti-5Al-4V-.5Mo-.4Fe) is produced through Electron Bem Single Melt Process (EBSM) s the lloy cn tke mixed scrp such s from Ti-6Al-4V nd other lloys contining Mo, V nd Fe. In ddition to flexibility in rw mteril, it ws found tht mchinbility of Ti-54M ws superior to Ti-6Al-4V [3]. Ti-54M will sve mchining costs of prts tht require extensive mchining. Thus, the better formbility nd mchinbility of Ti- 54M might be driving force for replcing Ti-6Al-4V in mny erospce s well s biomedicl pplictions. For these pplictions, good high-cycle ftigue (HCF) performnce is of utmost importnce. This cn be chieved by microstructurl optimiztion nd in prticulr, by suitble mechnicl surfce tretments such s nd. In generl, these mechnicl surfce tretments led to severe plstic deformtion nd to the development of residul compressive stresses [4, 5]. Residul compressive stresses re well known to enhnce the ftigue performnce nd corrosion resistnce by retrding or even suppressing micro-crck growth from the surfce into the interior [6, 7]. The present study ims t investigting the influence of vrious microstructures of Ti-54M on the chnges in surfce nd ner-surfce properties such s surfce topogrphy, micro-hrdness-depth nd residul stress-depth profiles s induced by nd. The HCF performnce fter nd of the vrious microstructures will then be compred to the electropolished bseline ().

2 Experimentl Methods Mteril ws received s squre (38 x 38 mm) br stock in mill-nneled condition. 5 mm long blnks were β-nneled t 11 C for 3 min followed by wter-quench (WQ). Blnks were (α+β) uni-directionlly rolled t either C or C to deformtion degree of ϕ = 1.2. From the rolled pltes, specimen blnks (1 x 1 x 5mm) were tken in trnsverse direction (TD). The subsequent het tretments were performed to develop fully lmellr (FL), fully equixed (EQ) or duplex (D) microstructures. To generte FL microstructures blnks were bet nneled t 11 C for 1 hour followed by WQ or ir cooling (AC). The EQ microstructures were produced by heting rolled blnks t C for 1 hour followed by WQ or AC. The D microstructures were generted by nneling rolled blnks t 94 C for 1 hour followed by WQ or AC. All mteril ws finlly het treted t C for 24 hours. Tensile tests were performed using threded cylindricl specimens hving gge lengths nd dimeters of 25 nd 5mm, respectively. Young s moduli were mesured with strin gges ttched to the gge lengths of the specimens. Initil strin rtes were 6.7 x 1-4 s-1. HCF tests were performed in rotting bem loding (R = -1) on hour-glss shped specimens hving minimum dimeter of 4mm. All specimens were electrolyticlly polished (1μm surfce removl from s-mchined surfce) to exclude ny mchining effects tht could msk the results. ws performed using sphericlly conditioned cut wire (SCCW14) hving n verge shot size of.36 mm.the Almen intensity ws vried from.5 to.33 mma. Bll-burnishing ws done using conventionl lthe nd hydrostticlly driven tool from Ecoroll Compny working with hrd metl bll of 3mm (HG3). The burnishing pressure ws kept constnt t 3 brs. The process-induced chnges in the surfce lyer properties were chrcterized using profilometry, micro-hrdness-depth profiles nd residul stress-depth profiles. The residul stresses were determined by the incrementl hole drilling method. Ftigue crck nucletion sites were studied by opticl microscopy nd scnning electron microscopy (SEM). Results nd Discussion The microstructures of the vrious conditions re shown in Figure 1. () FL/WQ (b) EQ/WQ (d) (c) D15/WQ (e) EQ/AC Figure 1. Microstructures of TIMETAL-54M (f) D15/AC FL/WQ exhibits mrtensitic microstructure (Fig. 1). Reducing the cooling rte from WQ to AC, α-pltes nd thin β-pltes re clerly visible indicting nucletion nd growth

3 controlled forming mechnism in (Fig. 1d). EQ/WQ (Fig. 1b) nd EQ/AC (Fig. 1e) look very much the sme with n verge α grin size of 3 µm. D15/WQ (Fig. 1c) nd D15/AC (Fig. 1f) re chrcterized by 15% equixed primry α-phse. Similrly s in FL, the mtrix microstructure chnges from mrtensitic (D15/WQ, Fig. 1c) to lmellr (D15/AC, Fig. 1f). Tensile properties of the vrious conditions re illustrted in Tble 1. Tble 1. Tensile properties of the vrious microstructures in Ti-54M Microstructure Fully Lmellr (FL) Equixed (EQ) Duplex (D15) Cooling YS (MP) UTS (MP) El (%) ε F = ln (A o /A F ) WQ AC WQ AC WQ AC The cooling rte hs mrked effect on the strength properties in FL. Both YS nd UTS in FL/WQ re bout 2 MP higher thn in, this result being cused by the width of the mrtensite pltes (Fig.1) being much finer thn tht of the α-lmelle (Fig. 1d). No difference ws found in tensile properties between EQ/WQ nd EQ/AC (Tble 1) which corresponds to the similrity in microstructure (compre Fig. 1b nd 1e). The presence of lmellr mtrix in D15 leds to cooling rte effect on tensile properties very similr to tht observed in FL (Tble 1). Due to the much smller former β grin size in D15, ε F vlues re highly superior to those in FL. The bsence of lmellr portions in EQ nd the smll α-grin size re the reson for the highest vlues of ε F mong the vrious microstructures (Tble 1). The influence of Almen intensity in on the surfce roughness vlues of EQ/WQ is shown in Figure 2 indicting liner increse with Almen intensity. The effect of this vrition in Almen intensity on the ftigue life t σ = MP is shown in Figure 3. Strting with, the ftigue life first increses with n increse in Almen intensity up to bout.2mma (Fig. 3) followed by slight decrese. Thus,.2mmA ws used for estblishing S-N curves. Roughness [µm] (SCCW 14) Ry Almen intensity [mma] Rz R Figure 2. Surfce roughness vs. Almen intensity of EQ Cycles to filures, N F σ = MP Almen intensity [mma] Figure 3. Ftigue life vs. Almen intensity of EQ The influence of with.2mma on the micro-hrdness-depth profiles nd residul stressdepth profiles of EQ re illustrted in Figure 4 nd Figure 5, respectively. In ddition, results fter (3br) re lso plotted. As seen, not only leds to penetrtion depth of incresed hrdness mrkedly higher thn in (Fig. 4) but lso to higher nd deeper residul compressive stresses.

4 44 2 Microhrdness HV bulk hrdness (SCCW14,.2mmA) (HG 6, p = 3br) 2 Distnce from surfce (µm) Residul Sress (MP) ,2 (.22mmA) HG 6 (3br) EQ/WQ 2 Distnce from surfce (µm) Figure 4. Micro-hrdness profiles in EQ Figure 5. Residul stress profiles in EQ The S-N curves of the vrious microstructures re illustrted in Figure 6 compring nd with. FL/WQ Cycles to filure, N F Cycles to filure, N F ) FL/WQ b) D15/WQ D15/AC Cycles to filure, N F Cycles to filure, N F c) D15/WQ d) D15/AC EQ/WQ Cycles to filure, N F e) EQ/WQ Figure 6. S-N curves of the vrious microstructures: effects of nd The HCF strengths of the vrious microstructures in reflect the corresponding yield stress vlues (Fig. 7).

5 HCF strength, σ 1 7 (MP) D15/AC Yield stress, YS (MP) Figure 7. Dependence of HCF strength of bseline on yield stress EQ FL/WQ D15/WQ While the effect of cooling rte on HCF strength is similr between FL nd D15, FL results in considerble higher HCF strength vlues (Fig. 7). From prllel work it is known tht HCF crcks in D15/AC nuclete in the (α+β) lmellr mtrix indicting tht its strength is lower thn tht of FL [8]. Presumbly, this is cused by element prtitioning nd resulting higher strength of the primry-α-phse in D15. Micro-hrdness mesurements hve shown tht the strength of the primry-α-phse opposed to tht of the (α+β) lmellr mtrix is not ffected by cooling rte. As seen in Figure 6, both nd result in mrked enhncements of the HCF strengths of the vrious microstructures. These dt re replotted in Figure 8. σ (MP) 2 EQ D15/WQ FL/WQ 1 D15/WQ 1 5 D15/AC FL/WQ 5 D15/AC EQ HCF strength, σ 1 7 (MP) HCF strength, σ 1 7 (MP) ) b) Figure 8. HCF strength improvements σ (MP) 2 As seen in Figure 8, both (Fig. 8) nd (Fig. 8b) result in HCF strength improvements which increse with incresing HCF strengths of the bseline. Apprently, the degree of improvement depends on the microstructure. Chnging the cooling rte from AC to WQ in D15 not only mrkedly increses the HCF strength from 525 to 65 MP but lso the degree of further improvements by mechnicl surfce tretments. These mount to 25 nd 75 MP in cse of for D15/AC nd D15/WQ, respectively (Fig. 8). The corresponding vlues in cse of re 75 nd 125MP for D15/AC nd D15/WQ, respectively (Fig. 8b). After the sme vrition in cooling rte, the HCF strength of FL increses from to 74 MP while further improvements by mount to 5 nd 6 MP for nd FL/WQ, respectively Fig. 8). The corresponding vlues in cse of re 1 nd 11 MP for nd FL/WQ, respectively (Fig. 8b).

6 Conclusions The presented results on well defined microstructures of the new titnium lloy Ti-54M show tht YS nd HCF strength cn be enhnced in FL nd D15 microstructures by using wterquench insted of n ir cool from the prticulr solution temperture. This is clerly relted to the refinement of the width of the α-lmelle through mrtensitic phse trnsformtion in both microstructures. Becuse of the bsence of lmellr components in EQ, no cooling rte effects on microstructure or tensile properties were observed. Mechnicl surfce tretments nd improve the HCF strength of ll tested microstructures. While is generlly superior to, the degree of improvement of the HCF strength depends on both microstructure nd strength level. References [1] M. Armendi, A. Gry, L. M. Irirte nd P. J. Arrzol, Comprison of the mchinblity of Ti6Al4V nd TIMETAL 54M using uncoted WC-Co tools, Journl of Mterils Processing Technology 21 (21), pp 197. [2] V. Venktesh, Y. Kosk, J. Fnning, S. Nykn, Processing nd Properties of Timetl 54M, in: Proc. of the 11 th Int. World Conference on Titnium (JIMIC5), M. Niinomi, S. Akiym, M. Iked, M. Hgiwr, K. Mruym (Eds.), Kyoto, Jpn,( 27), pp 713. [3] S. L Nykn, J. C Fnning, D. W. Tripp, Chemicl homogeneity, structure, nd properties of Titnium lloys produced by Electron-Bem Single-Melting (EBSM), in: Proc. of the 11 th Int. World Conference on Titnium (JIMIC5), M. Niinomi, S. Akiym, M. Iked, M. Hgiwr, K. Mruym (Eds.), Kyoto, Jpn, (27), pp 749. [4] J. O. Almen, P. H. Blck, Residul stresses nd ftigue in metls, McGrw-Hill, New York, [5] V. Schulze, Chrcteristics of surfce lyers produced by shot peening, in: Proc. of the 8 th Int. Conference on Shot Peening, L. Wgner (Eds.), Grmisch-Prtenkirchen, Germny, (22), pp 145. [6] E. Mwd, H.-G. Brokmeier, M. Hofmnn, Ch. Genzel, L. Wgner, Stress distribution in mechniclly surfce treted Ti-2.5Cu determined by combining energy-dispersive synchrotron nd neutron diffrction, Mter. Sci. Eng. A 527 (21) pp [7] L. Wgner, M. Wollmnn, Shot peening of non-ferrous lloys to enhnce ftigue performnce, in: Proc. of the 1 th Int. Conference on Shot Peening, K. Tosh (Eds.), Tokyo, Jpn, (28), pp 355. [8] L. Wgner, K. Zy, M. Wollmnn, Y. Kosk, Thermo-mechnicl tretments nd mechnicl surfce tretments on property improvements in the new (α+β) Titnium lloy TIMETAL 54M, PFAM VXIII, Sendi, Jpn, (29)