Fatigue response of various titanium alloys to shot peening

Transcription

1 Fatgue response of varous ttanum alloys to shot peenng T. Dorr and L. Wagner Char ofmaterals Technology and Physcal Metallurgy Techncal Unversty ofbrandenburg at Cottbus, Germany P.O.Box , Cottbus, Germany e-mal: doerr(a)tu-cottbus. de Abstract The fatgue response of the varous ttanum alloy classes (a, a+p and (3) to shot peenng s evaluated. Results on the a-alloy T-2.5Cu show marked lfetme mprovements compared to the electropolshed reference. In contrast, the fatgue lfe mprovements for the p-alloys T-10V-2Fe-3Al and T-3Al-8V-6Cr-4Mo- 4Zr are rather small. Intermedate lfe mprovements were observed on the (oc+p)-alloy T-6Al-7Nb. Subsurface fatgue crack nucleaton stes were observed on the a- and (a+p)-alloys whle cracks nucleated at the surface on the P-alloys. The dfferent response of the varous alloy classes s explaned by dfferences n cyclc deformaton behavor. The results on cyclc hardenng/softenng correlate wth the observed crack nucleaton stes.

2 350 Surface Treatment Introducton Shot peenng s a cold workng process n whch the surface of a component s bombarded wth small round partcles of metal,.glass or ceramc. Shot peenng s regularly and wdely used on ttanum components for aerospace and ndustral applcatons to provde enhanced fatgue resstance and reduced susceptblty to gallng,frettngand stress corroson crackng (Fuchs [1]). Shot peenng leads to ncreases n surface roughness and near-surface dslocaton densty (cold work) and develops a characterstc resdual stress profle n nearsurface regons (Vohrnger [2]). Whle a hgh surface roughness accelerates crack nucleaton because of notch effects due to local stress concentratons, t has no sgnfcant effect on mcrocrack propagaton, as cracks grow out of the stress feld. Cold work retards crack nucleaton owng to the hgher local yeld stress but accelerates crack propagaton caused by the low resdual ductlty. Resdual compressve stresses have only slght effects on crack nucleaton, but can drastcally retard crack propagaton. To what extent the process-nduced resdual compressve stresses affect fatgue performance depends on ther cyclc stablty. The present nvestgaton s amed at correlatng the fatgue performance of the varous ttanum alloy classes after shot peenng to ther cyclc deformaton behavor. Expermental The ttanum alloys T-2.5Cu, T-6Al-7Nb, T-10V-2Fe-3Al and T-3Al-8V-6Cr- 4Mo-4Zr (Beta C) were taken to represent the a, (a+ (3), solute-lean and soluterch metastable-p alloy classes, respectvely. Thermal and/or thermomechancal treatments utlzed to vary mcrostructural parameters are descrbed n detal elsewhere (Kese [3], Holzwarth [4], Lndemann [5], Berg [6]). Resultng mcrostructures for the varous alloys and tensle propertes are lsted n Table 1 and Table 2, respectvely. Low cycle fatgue (LCF) tests were performed on cylndrcal specmens (gage length 20 mm) usng a servohydraulc testng machne under load control n fully reversed loadng at a frequency of 0.3 Hz. Cyclc deformaton behavor was characterzed by plottng the plastc stran ampltude recorded by an extensometer vs. number of cycles. For estmatng the cyclc deformaton behavor n the shot peened surface layer, addtonal tests were performed on cold rolled materal. Hgh cycle fatgue (HCF) tests were performed on hour-

3 Surface Treatment 351 glass shaped specmens n rotatng beam loadng (R=-l) at a frequency of about 60 Hz. Shot peenng was performed usng an njector type system for low and medum Almen ntenstes (< 0.30 mma) and a pressure blast system for hgh Almen ntenstes (> 0.30 mma). Sphercally condtoned cut wre (SCCW 14) wth an average shot sze of 0.36 mm and a vckers hardness HV 0.2 of 610 was used. Almen ntenstes were wdely vared n order to determne optmum fatgue performance. Specmens wth an electropolshed surface (EP) served as reference. Fracture surfaces were studed by SEM. Table 1: Heat treatments and mcrostructures of the varous ttanum alloys Alloy T-2.5Cu T-6Al-7Nb T-10V-2Fe-3Al T-3Al-8V-6Cr- 4Mo-4Zr (SHT) Alloy class a a+p metastable P (solute lean) metastable P (solute rch) Heat treatment Ih 805 C/WQ 8h400 C+8h475 C Ih 1000 C/AC 24h 500 C Ih 775 C/AC 8h 480 C Ih 927 C/AC Mcrostructure Equaxed a Equaxed ocp n lamellar (a+p) matrx Equaxed ctp n P matrx Equaxed P Table 2: Tensle propertes of the varous ttanum alloys T-2.5Cu + 25% cw T-6Al-7Nb T-10V-2Fe-3AI T-3Al-8V-6Cr- 4Mo-4Zr (SHT) + 25% cw E [GPa] (?0,2 [MPa] UTS [MPa] El [%] SF = In AO/AF

4 352 Surface Treatment Results and dscusson Optcal mcrographs of the varous mcrostructures are shown n Fg. 1. The mcrostructure of T-2.5Cu conssts of slghly elongated a grans separated by the eutectod component (oc+t^cu) (Fg. la). T-6Al-7Nb exhbted a duplex mcrostructure contanng 20% otp (Fg. Ib). Annealng T-10V-2Fe-3Al below the p-transus at 775 C results n a (3 graned structure wth an a? content of 15% (Fg. Ic). Soluton heat treatng T-3Al-8V-6Cr-4Mo-4Zr n the p phase feld results n a coarse graned p-mcrostructure wthout prmary a (Fg. Id). /.- ^rv- -~o^. * a) T-2.5Cu b) T-6Al-7Nb C) T-10V-2Fe-3Al U) T-3Al-8V-6Cr-4Mo-4Zr Fg. 1: Mcrostructures of the varous ttanum alloys

5 Surface Treatment 353 Examples of the dfference n cyclc deformaton behavor between the a- and P- alloy classes are shown n Fg. 2. Cyclc hardenng was observed n T-2.5Cu (Fg. 2a), whereas pronounced cyclc softenng was found n T-3Al-8V-6Cr- 4Mo-4Zr (Fg. 2b). As also seen n Fg. 2, pror cold work, whch markedly ncreases the yeld stress for T-2.5Cu and to a much lesser extent for T-3Al-8V- 6Cr-4Mo-4Zr (Table 2), dd not change ths characterstc dfference n cyclc deformaton behavor between T-2.5Cu (Fg. 2a) and T-3Al-8V-6Cr-4Mo-4Zr (Fg. 2b). '10 10' 10* Cycles N a) T-2.5Cu: Cyclc hardenng o 10 < 1 2 Io.H 10 f r* all. ' S>25%cw/ -^K ',X A ^, f 3%^ T-3 M-8V-6C,r-4Mo-4Zr 10' 10* Cycles N b)t-3al-8v-6cr-4mo-4zr: Cyclc softenng 103 Fg. 2: Cyclc deformaton behavor of the dfferent ttanum alloy classes - Effect of pror cold work The effect of shot peenng on the fatgue lfe (da = const.) of the varous ttanum alloys s shown n Fg. 3. Startng wth the electropolshed reference (Np ~ 10*), the fatgue lfe frst markedly ncreases wth Almen ntensty for all alloys followed by a saturaton n fatgue lfe as Almen ntensty s further ncreased. However, the degree of lfe tme mprovement s very dfferent for the varous alloys. Best results were observed on the a-alloy T-2.5Cu (Fg. 3a) followed by the (ct+p)-alloy T-6Al-7Nb (Fg. 3b) and the metastable solute lean p-alloy T- 10V-2Fe-3Al (Fg. 3c). Lowest mprovements were found on the metastable solute rch p-alloy T-3Al-8V-6Cr-4Mo-4Zr (Fg. 3d). As seen n Fg. 3, shot peenng mproves the fatgue lfe for the gven stress ampltudes by as much as 2 orders of magntude for T-2.5Cu, somewhat more than 1 order of magntude for T-6Al-7Nb, roughly 1 order of magntude for T-10V-2Fe-3Al und even less than that for T-3Al-8V-6Cr-4Mo-4Zr.

6 354 Surface Treatment 106 T-6AI-7Nb SP a, = 750 MPa o Almen ntensty [mma] T-2.5Cu SP a, = 575 MPa 0.6 oc/> u 5 10* Almen ntensty [mma] 0.6 a) T-2.5Cu 1 n? '" T-10V-2Fe-3AI SP ^ Z o, = 850 MPa j Z &.~6 *> " [g I [m o o b) T-6Al-7Nb/AC Beta C, SHT SP o, = 575 MPa 0 ; / /"^ 0 I 1 Tl T ' Almen ntensty [mma] Almen ntensty [mma] 0.6 c)t-10v-2fe-3al d) T-3Al-8V-6Cr-4Mo-4Zr Fg. 3: Fatge lfe of the varous ttanum alloys after shot peenng - Effect of Almen ntensty (c^ ~ const.) These results can be explaned by the dfferences n the cyclc stablty of the process-nduced resdual stresses. If the materal cyclcally hardens as observed on the ct-alloy T-2.5Cu (Fg. 2a), the process-nduced resdual compressve stresses wll be cyclcally qute stable and thus, wll benefcally affect fatgue lfe (Fg. 3a). On the contrary, fatgue lfe wll hardly be mproved, f resdual stresses wll easly relax due to cyclc softenng as observed on the solulte rch p- alloy (Fg. 3d, compare Fg. 3d wth Fg. 2b).

7 Surface Treatment 355 The S-N curves (Fg. 4) confrm the dfferent response of the varous ttanum alloy classes to shot peenng. Almen ntenstes were chosen for each alloy whch lead to the greatest ncrease n fatgue lfe. The mprovement n fatgue performance due to shot peenng s most marked for the a-alloy T-2.5Cu (Fg. 4a). Lowest mprovements are observed for the metastable p-alloys T-10V-2Fe- 3A1 (Fg. 4c) and T-3Al-8V-6Cr-4Mo-4Zr (Fg. 4d). As expected, ntermedate lfe mprovements are observed for the (a+p) alloy T-6Al-7Nb (Fg. 4b). It s known that (ot+p) alloys generally exhbt cyclc softenng (Wagner [7]), however, ts degree s much lower than observed on the metastable p-alloy class. 800; er s ampltude, a, [l\ (0 CD 4UU ; CO /JXN.-V 1C )3 a) T-2. 5Cu a N Tr sp[r< T-2. 5Cu " 105 1Q6 10^ Cycles to falure, Np m ouu - o. ; 5 ynn - o" : CD" ± co ; eo to /nn 4UO W onn " 1 : \ KJI S 1 1 \w EP I \ SP y 10^ 10" 10^ 1QG Cycles to falure, Np b) T-6Al-7Nb T-6A l-7nb \ 4^^ I _ Q_ TS III EetaC:, SHT SP S. ' TM\~^ d* EP JL «? ; ^ CD" ^ ff "O N SF> 'P E -j co ; [5 g ^ % 400 ;,f TH TJ-10Vf-2Fe-3AI ^ 500- I I r ' ouu 1 Q3 10" 10% 10 10^ 10^ 10" 10^ Cycles to falure, Np Cycles to falure, N c)t-k>v-2fe-3al d)t-3al-8v-6cr-4mo-4zr Fg. 4: S-N curves of the varous ttanum alloys - Effect of optmum shot peenng

8 356 Surface Treatment The fracture surfaces of the shot peened fatgue specmens tested n the HCF regme revealed subsurface crack nucleaton for the a- and (ct+p)- alloy classes, whle fatgue cracks nucleated at the surface for the metastable (3-alloys. Examples of HCF fracture surfaces are shown n Fg. 5 ndcatng subsurface and surface fatgue crack nucleaton for shot peened T-2.5Cu (Fg. 5a) and T-3Al- 8V-6Cr-4Mo-4Zr (Fg. 5b), respectvely. Subsurface fatgue cracks nucleate n regons of resdual tensle stresses whch balance the outer resdual compressve stress feld (Wagner [8]). In ttanum alloys, whch are generally free of nclusons or other defects, the tendency for nternal fatgue crack nucleaton wll prmarly depend on the amount and cyclc stablty of the resdual tensle peak stress, the tensle mean stress senstvty of the materal and ts fatgue strength n vacuum (t s presumed that condtons comparable to that of a vacuum preval n the nteror of a materal). Snce both a- and (3- alloys show a normal mean stress senstvty of the fatgue strength and no marked dfferences wth regard to envronmental effects on fatgue strength, t s argued that subsurface fatgue cracks n the a- and (ot+p)-alloy classes are presumably the result of cyclcally stable resdual tensle stresses. The correspondng resdual compressve stresses n turn effectvely retard or even stop mcrocrack growth from the surface nto the nteror (Fg. 5a). On the other hand, early resdual stress decay as expected n a materal that cyclcally softens wll prevent nternal fatgue crack nucleaton. Thus, cracks wll start at the surface (Fg. 5b) as they also do n the resdual stress free electropolshed reference. a) T-2.5Cu: Subsurface b) T-1 OV-2re-3Al: Surface Fg. 5: Fatgue crack nucleaton stes n shot peened specmens

9 Surface Treatment 357 The authors would lke to thank the Deutsche Forschungsgemenschaft for fnancal support through Wa 692/10-1,2 References [1] H. O. Fuchs, Optmum peenng ntenstes, Int. Conf. on Shot Peenng 3 (H. Wohlfahrt, R. Kopp and O. Vohrnger, eds.) , [2] Vohrnger, O., Changes n the state of the materal by shot peenng, Int. Conf. on Shot Peenng 3 (H. Wohlfahrt, R. Kopp and O. Vohrnger, eds.) , [3] J. Kese and L. Wagner, Fatgue behavor n T-10V-2Fe-3Al: Mcrostructural effects on crack nucleaton and mcrocrack growth, Fatgue 96' (G. Lutjerng and H. Nowack, eds.) DVM, , [4] U. Holzwarth, J. Kese and L. Wagner, Effect of Surface Fnshng on Fatgue Performance of the Surgcal Implant Alloy T-6Al-7Nb, Ttanum Fatgue Symposum, Materals Week '98, 1998 (n press). [5] J. Lndemann and L. Wagner, Mean stress senstvty n fatgue of a, (a+p) and P ttanum alloys, Materals, Scence and Engneerng A , , [6] A. Berg, J. Kese and L. Wagner: Mcrostructural gradents n T-3 A1-8V- 6Cr-4Zr-4Mo for excellent HCF-strength and toughness, Materals, Scence and Engneerng A243, , [7] L. Wagner, Fatgue Lfe Behavor of Ttanum Alloys, ASM Handbook Vol. 19, Fatgue and Fracture, , [8] L. Wagner and G. Lutjerng, Influence of Shot Peenng on the Fatgue Behavor of T-Alloys, Shot Peenng, Pergamon Press, , 1981.