E F F E C T O F MACHINING VARIABLES O N 'THE SURFACE AND STRUCTURAL INTEGRITY O F TITANIUM

Dr . Willinl>> P. K o s t c r , Director of Metallurgical Engineering

Dr. Michael Fie ld , Pres iden t

both: Metcut Resea rch Associates Inc. , Cincinnati, Ohio 45209

ABSTRACT

It i s wel l es tabl ished that conditions of m e t a l removal m a y exe r t a m a r k e d influence on the engineering proper t ies of machined components. Varia t ions of fatigue s t rength a s g rea t a s 5: 1, f o r example, have been repeatedly observed a s a function of var ia t ions in the grinding p r o c e s s a s applied to a l a rge number of s t ruc tu ra l a l loys , This paper s u m m a r i z e s the sur face integri ty behavior of be ta rol led Ti- 6A.l-4V, a typical s t ruc tu ra l titanium alloy. Ma te r i a l removal p r o c e s s e s included i n this survey a r e su r f ace grinding, hand grinding, end mill ing, e l ec t r i ca l d i scharge machining, e l ec t rochemica l machining, and chemical machining.

The effects of these p r o c e s s e s and var ia t ions i n their p a r a m e t e r s a r e summar i zed i n t e r m s of mic ros t ruc tu re , microhardness , r e s idua l s t r e s s prof i le in the machined surface, and fatigue behavior of the alloy.

INTRODUCTION

During recen t y e a r s t h e r e has been continuous activity in many a r e a s of the t ranspor ta t ion industry to develop m o r e efficient, l ighter weight s t ruc tu re s . These goals have been m e t i n t h r ee a r e a s of t e m p e r a t u r e and environment through development of nickel ba se and s e m i r e f r a c t o r y a l loys upable i n exces s of 2000°F and also in the

development of high s t rength s t ee l s suitable fo r s t ruc tu ra l use i n ambien t t e m p e r a t u r e s . Between these ex t r emes , considerable attention h a s a l so been d i r ec t ed a t titanium alloy development fo r use in 500 - 1000°F region.

In the s e r v i c e l ife of m a n y types of engineering s t ruc tu re s , dynamic loading i s a c r i t i c a l fac tor . Whenever dynamic loading i s involved, operating s t r e s s e s a r c f requent ly limited by the fatigue charac te r i s t i cs of the

F r oln: Proceedings of North American Metalworking Research Conference, McMaste r Univcr si ty, Narnilton, Ontario, Canada, May 14- 1 5 , 1973 ,

I I (Hamilton: McMaster Universi ty, 1 9 7 3 ) , Vol. 2 -Metal Cuttlng anti I E l e c t r ~ c a l Machining, pp. 67-87 .

INTRODUCTION (continued)

ma te r i a l s being used. In many situations, fatigue failures originate a t sur faces of components, indicating the possibil i ty that fatigue behavior i s sensit ive to surface condition. Experirnentnlly, numerous ma te r i a l s engineering studies have demonstrated that , in fact , fatigue b e h a v i ~ r of ma te r i a l s i s surface dependent. Surface integrity studies, relating the effect of variations in component n)achining and finishing to fatigue behavior, have fur ther indicated the importance of the surface condition in s t ruc tu ra l components.

Surface integrity i s becoming an in tegra l pa r t of design and application considerations for many of these m a t e r i a l s . Attention has been drawn to the importance of sur face integr i ty by a varie ty of manufacturing and se rv i ce difficulties. Development of sur face integrity data, relating the effects of surface a l terat ions produced during machining to component per formance , has emphasized the importance of surface integrity controls in the manufacture of high quali ty hardware.

Quantitatively, the sur face integr i ty of m a t e r i a l s has been evaluated by studying the effects on p rope r t i e s produced by different meta l removal p r o c e s s e s and changes in var iab les within these p roces ses . Surface conditions have been evaluated in t e r m s of surface topography and surface meta l lu rgy . Fatigue and, i n some c a s e s , s t r e s s corrosion have been considered a s the pr incipal a spec t s of mechanical behavior of concern i n studying sur face integr i ty - effects. Considering: 1) the nature of advanced ma te r i a l s offering improved capabil i t ies; 2 ) the difficulty in machining and finishing these higher s t rength mater ia l s ; and 3) the sensi t ivi ty to component sur face condition inherent in many operational modes to which these p a r t s will be subjected, the need for paying careful at tention to the sur faces of finished hardware i s brought cr i t ical ly into focus. In machining any component, i t i s necessary to sat isfy the surface quali ty o r sur face integr i ty requi rements . Surface integrity has two dis t inct and important aspec ts . The f i r s t i s surface topography which desc r ibes sur face roughness and o ther fea tures of the geometry of the sur face . The second i s the su r f ace meta l lu rgy of the layer produced in machining, including the effect of any al terat ions with respect to the b a s e o r m a t r i x me ta l which may b e presen t .

F o r this repor t , data available on beta rolled Ti- 6A1-4V has been selected fo r presentat ion. The informat ion developed on this m a t e r i a l i l lus t ra tes the magnitude of the range of p rope r t i e s that can be expected in high s t rength a l loys a s influenced by m e t a l removal method. Space available does not p e r m i t inclusion of a l l of the detai ls of t es t cut p a r a m e t e r s and re la ted a r e a s of in te res t . These effor ts a r e , however, well documented i n o the r sou rces . (112, 3 )

TEST h4hTERLAL

Ti- 6A1-4V was obtained a s beta- rolled plate in accordance w i t h an a i r f r a m e specification developed by the Boeing Company, X U M S 7 - 174. Details concerning the analysis and charac te r i s t ics of this alloy a r e noted below:

The specified me l t chemistry of this m a t e r i a l i s a s follows:

Titanium Aluminum Vanadium I ron Carbon Hydrogen Oxygen Nitrogen Other Impuri t ies

Remainder 5 .5 - 6.75 3.5 - 4 .5 0.25 max. 0.08 max. 0.0125 max. 0. 175 max. 0.03 max. 0.40 max.

The heat t r ea tmen t used for this m a t e r i a l was a s follows

1450" F/ 15 minutes / a i r cooled (production annealed)

The tensi le p roper t ies measu red a f t e r heat t rea tment were:

UTS, psi: 130, 000 minimum 144, 000 0.27'0 Y . S., psi: 120,000 minimum 131, 000 Elongation, 7'0 in 4D: 10.0 minimum 12.5

The Boeing specifications fur ther indicated that the micros t ruc ture was to consis t of 1007'0 acicular alpha phase, which was confirmed. The ha rdness of the ma te r i a l was recorded a s Rc 31- 33.

PROCEDURE

Mate r i a l was procured a s 114 in. plate i n the proper ly heat t rea ted condition. Suitable coupons for both res idua l s t r e s s and fatigue specimens w e r e cut f r o m the plate and machined by low s t r e s s grinding to approximately .040 in. in e x c e s s of the final t e s t dimension. This remaining stock was removed by the t e s t cutting procedure to be evaluated. In the ca se of this study on Ti- 6A1-4V, the metal removal methods studied included:

PROCEDURE (continued)

su r f ace grinding hand grinding end mill ing - end cutting pe r i phe ra l end mill ing d r i l l ing e l e c t r i c a l d i s cha rge machining (EDM) e l ec t rochemica l machining (ECM) chemica l machining (CHM).

In a l l of t he se t e s t cutt ing p rocedu re s , the purpose is to va ry the range of p a r a m e t e r s s o a s to expose the t e s t m a t e r i a l to process ing e x t r e m e s which m a y be reasonab ly expected to occu r in ac tua l production operat ions . In the c a s e of grinding, f o r example , low s t r e s s grinding involves a re la t ive ly low speed, 2000 ft. p e r minute , and a soft wheel such a s a J h a r d n e s s . Abusive grinding is done a t 6000 ft. p e r minute with a n M o r N wheel . In c a s e of mi l l ing, gentle mi l l ing involves s h z r p c u t t e r s while abusive mi l l ing involves dul l cu t t e r s . Other parameters i n chip r e m o v a l opera t ions w e r e invest igated but i t has been shown that t he cu t t e r s h a r p n e s s h a s a predominan t effect i n the resul t ing sur face condition. Dri l l ing s e v e r i t y i s a l s o va r i ed by changing d r i l l sha rpness . In the c a s e of EDM, the var ia t ion t e s t ed i s between roughing operat ions a t high c u r r e n t densi ty and f inishing opera t ions a t a much lower c u r r e n t dens i ty . Significant va r ia t ions i n ECM and CHM p a r a m e t e r s involve the u se of s t anda rd and off-standard e lec t ro ly te solutions.

EVALUATION

A s t anda rd su r f ace in tegr i ty evaluat ion cons i s t s of both a definition of t h e s u r f a c e condition resul t ing f r o m the m e t a l r emova l operation under s tudy a s we l l as a n a s s e s s m e n t of the effect of th i s condition on significant m e c h a n i c a l p r o p e r t i e s . The evaluat ions used in connection with th i s s tudy w e r e as follows:

Metal lography: Study of s u r f a c e roughness , m ic ro s t ruc tu r e , and both m a c r o - and m i c r o h a r d n e s s .

Res idua l S t r e s s : De te rmina t ion of su r f ace s t r e s s profile by l a y e r r emova l technique.

Fa t igue Strength: De te rmined b y can t i l ever bending constant f o r c e t e s t s a t 1800 cyc l e s p e r minu te , room t empe ra tu r e .

SUMMARY O F RESULTS

Metallography

A metal lographic review of typical su r f ace conditions studied f o r be ta - ro l led Ti-6A1-4V i s p resen ted in F i g u r e s 1 through 7 . Sur faces exhibited by t i tanium produced by su r f ace grinding under gentle, conventional, and abus ive conditions a r c shown in F igure 1. Notice in the ca se of abus ive and conventional su r face grinding that a deg ree of p las t ic deformat ion o r d is tor t ion i s visible a t the su r face . Notice a l s o that the n l i c roha rdnes s of the ex t r eme sur face l aye r i s lower than the ba se condition, indicating v e r y shallow softening of a few points Rockwell C. Th is h a r d n e s s d rop is probably due to overaging of the t i tanium a s a r e su l t of highly local ized su r f ace heating during grinding. The gently ground sample does not exhibit th i s su r f ace softening. It i s l ikewise f r e e of v i s ib le p las t ic deformat ion. The hand ground samples a s shown in F igu re 2 both exhibit s m a l l amounts of p las t i c deformat ion. They a l s o show evidence of su r f ace softening and s i m i l a r re la t ively rough sur face f in i shes .

Mic ro s t ruc tu r e s showing typical c r o s s sect ions of su r f ace s cut by a b road range of mi l l ing conditions a r e shown in F igure 3. Var ious d e g r e e s of p las t i c deformat ion a t the su r face can be seen in t he se pho tomicrographs . In genera l , the l a r g e r deg ree of p las t ic deformat ion is assoc ia ted with dull tooling. Dull tools w e r e defined a s those condit ions fo r which the nominal wrearland was indicated a s . 0 18 to . 020 in. In con t r a s t , a so-cal led s h a r p tooling condition was employed whe re the wear land was speci f ied a s . 0 0 3 in. max imum. Detai led m i c r o h a r d n e s s s tud ies w e r e run on sec t ions of su r f ace s produced by a l l 18 of these mi l l ing conditions. No significant o r consis tent m i c r o - h a r d n e s s var ia t ion, however, w a s found to ex i s t fo r any of the 18 condit ions. Since no m i c r o h a r d n e s s va r ia t ions w e r e observed , it was fe l t unneces sa ry to p r e s e n t th i s in format ion in g raph ica l f o rm .

Pho tomic rog raphs of the c r o s s sec t ions of t i tanium su r f ace s cut by EDM, ECM, and CHM a r e shown i n F i g u r e s 4, 5, and 6. The EDM of t i tanium (F igu re 4) shows the typ ica l r e c a s t l aye r which i s m o s t pronounced a s a r e su l t of cutting under roughing conditions. The re la t ive ly high hea t input a s soc i a t ed with roughing conditions gene ra l ly c a u s e s deeper r e c a s t l a y e r s a s a resu l t of EDM. In th i s sample , evidence of su r f ace hardening, up to .002 in. deep, is a l s o observed . Th i s phenomenon i s a l s o a s soc i a t ed with high local ized su r f ace heating dur ing abusive EDM.

SUMMARY O F RESULTS (continued)

Metallography (continued)

ECM (F igu re 5) typically produces no disce rnable mic ros t ruc tu r e o r h a r d n e s s change. The abusive mode, however, causes a marked sur face roughening and a l s o resu l ted in pronounced selective etching. CHM (F igu re 6) a l s o r e su l t s in no m i c r o s t r u c t u r e distort ion but a marked su r f ace roughening under abusive conditions. The CI-IM su r f ace s a l so exhibit s u r f a c e softening. This l a t t e r condition has been observed on ninny a l loys a s a resu l t of C H M process ing .

Rc.sidua1 S t r e s s

Typical r e s idua l s t r e s s prof i les obtained on th is titanium alloy a r e shown i n F i g u r e s 7, 8, and 9. As may b e s een i n F igure 7, the peak res idua l s t r e s s i s approx imate ly 35 - 40 k s i i n tension a s a resul t of gentle surface grinding. In the c a s e of abusive grinding, the peak s t r e s s i s much higher , approaching 100 ks i . The r e su l t s f o r hand ground s ample s of t h i s a l loy w e r e s i m i l a r in magnitude of s t r e s s although the depths of the a f fec ted a r e a w e r e considerably l e s s .

The r e s idua l s t r e s s p rof i l es assoc ia ted with the s eve ra l mill ing conditions s tudied w e r e qui te va r i ed and lacked a specific o r consistent t rend, s e e F igu re 8. In a number of situations, higher peak tensile s t r e s s e s we re found to b e a s soc i a t ed with the u s e of dull mi l l ing cu t te r s . T h e r e we re o the r s i tuat ions , however, where th is re la t ion s imply did not hold; thus, making it un rea l i s t i c to r each a gene ra l conclusion in th is a r e a .

Res idua l s t r e s s produced i n the su r f ace of beta-rolled Ti-6A1-4V b y CHM under both abusive and gentle conditions i s shown in F igure 9. It i s s ignificant to note that the peak s t r e s s of 40 k s i in tension was found under both of t he se CHM conditions. I t i s a l so significant that t h i s l eve l of r e s idua l s t r e s s ex i s t s a s a r e su l t of the CHM operation i n that no rma l ly CHM i s thought of a s a s t r e s s - f r e e procedure which r e s u l t s i n so-ca l led neu t r a l or z e r o s t r e s s su r f ace s .

Fat igue

The fat igue behav ior of beta-rolled Ti-6A1-4V due to var ia t ions in gr inding and chemica l mill ing i s shown in F igure 10. Most significant h e r e i s the e x t r e m e depress ion i n fatigue s t reng th ( f rom 62 to 13 k s i ) c aused by a n abusive v e r s u s gentle sur face grinding operation. It is

i n t e r e s t i ng to note that gentle hand grinding exhibits an endurance l imi t of 57 k s i , c l o se to the l eve l assoc ia ted with gentle surface grinding. The abus ive hand gr inding depressed the endurance s t rength to 30 ks i . While

SUMA4ARY O F RESULTS (continued)

Fat igue (cont inued)

th i s was not a s estrer .ne a s the dep re s s ion to 13 k s i associa ted with abus ive s u r f a c e grinding, i t i s v e r y log ica l that the hand held grinding, e \ ren though abus ive , would be l e s s s c v e r e than abusive sur face grinding as in?posed under conditions in a. r igid n)achine tool with posit ive mechan i ca l infeed.

Also indicated in F igu re 10 a r e fa t iguc c u r v e s assoc ia ted with gentle and abusive CHM. Note that the endurance l imi t s , 51 and 45 ks i , r e spec t ive ly , a r e quite c lose toge ther . Notice that th is situation e x i s t s i n sp i t e of a v e r y l a r g e d i f fe rence in su r f ace finish between the gentle and abusive CHM samp le s , 20 and 165 micro inches AA, respect ively .

Fa t igue t e s t s w e r e run on spec imens mi l l ed under 18 s e t s of different cutt ing condit ions: 9 e ach end m i l l - end cutting and end m i l l - per iphera l cutting. As a gene ra l t r end , end cut s amp le s showed a slight i nc r ea se in fa t igue s t r eng th a s the mil l ing cu t t e r was allowed to become dull; va lues ranged f r o m 64 up to 77 k s i . In con t ras t , the use of a dull p e r i p h e r a l end rnill resu l t ed i n a degrada t ion in fatigue, hence sur face i n t eg r i t y p r o p e r t i e s . Fat igue s t r eng th s i n pe r i phe ra l end mill ing ranged f r o m 64 down to 32 k s i , The r ange of fat igue behavior associa ted with t h i s mi l l ing p r o g r a m and in compa r i son to fatigue data obtained on Ti- 6A1-4V a s su r f ace ground i s shown in F igure 11. I t i s significant t o note tha t t h e r e is o v e r a two-fold di f ference in fat igue s t rength or' m i l l e d t i tanium, resu l t ing en t i r e l y f r o m dif ferences in m e t a l r emova l condi t ions .

SUMMARY

It h a s been demons t r a t ed that a b r o a d range of fatigue s t rengths can be ach ieved in a m a t e r i a l a s a r e s u l t of va r ia t ions i n m e t a l r emova l method. I t m u s t a l s o b e emphas ized that t h e s e var ia t ions a r e in no way due to o r capab le of c o r r e l a t i o n with changes in su r f ace roughness . They a r e i n s t ead the r e s u l t of a l t e r a t i ons induced in the sur face of the m a t e r i a l a s the r e su l t of the m e t a l r emova l operat ion. A s u m m a r y of the high cyc l e fat igue data obtained on Ti-6A1-4V i s shown in Figure 12.

I t h a s often been sugges ted that r e s i d u a l su r f ace s t r e s s i s a p r i m a r y and p e r h a p s so l e f a c to r i n p r ede t e rmin ing the fatigue s t rength o r su r face i n t eg r i t y l e v e l of a m a t e r i a l . T o examine th i s , a plot of the fatigue s t r e n g t h data f r o m a l l of the t e s t s covered in this paper v e r s u s the peak l e v e l of s t r e s s in the cor responding r e s idua l s t r e s s profi le i s shown i n F i g u r e 13. At f i r s t glance, a g e n e r a l t r end showing the endurance l imit

SUMMARY (cont inued)

to i n c r e a s e a s the peak rcs idun l s t r e s s m o v e s f r o m tens i l e to co rnpress va lues i s indicated. T h i s genera l t rc,nd cannot b e denied. If, howevcr , we examine the region of + L O k s i r e s i d u a l s t r e s s ( c o n ~ p r e s s i v c ) , we not that in th is region, fat igue s t r eng th v a r i e s f r o m 40 t o o v e r 70 ks i . L ikewise , i f we look a t the a r e a w h e r c t h e fat igue s t r eng th range i s 60 - 65 ks i , peak r e s i d u a l s t r e s s e s a r c s e e n to v a r y f r o m t 30 k s i

ive

C

( c o m p r e s s i v e ) to - 2 0 k s i ( tens ion) . It m a y b e concluded, t h e r e f o r e , tha t r e s i d u a l s t r e s s m e a s u r e m e n t s c a n only be used a s a v e r y rough guide i n approx imat ing s u r f a c e in teg r i ty behav io r .

As m o r e da ta i s developed i n t h i s a r e a , i t b e c o m e s i n c r e a s i n g l y evident tha t a n adequa te a s s e s s m e n t of s u r f a c e i n t e g r i t y cannot be achieved th rough m e a s u r e m e n t of roughness , r e s i d u a l s t r e s s , o r the u s e of o t h e r s u r f a c e inspec t ion techniques . The in t roduct ion of adequate i n - p r o c e s s con t ro l is s e e n a s the n e c e s s a r y solu t ion to achieving a d e s i r e d s u r f a c e i n t e g r i t y level .

R E F E R E N C E S

..- i . K o s t e r , w . F. ; F i e i d , ~ i c h a e i ; F r i t z , L.J . ; e t a i . i970. S u r f a c e i n t e g r i t y of mach ined s t r u c t u r a l components . A F M L - T R - 7 0 - 11. Cincinnati , Ohio: Metcut R e s e a r c h A s s o c i a t e s Inc.

2 . K o s t e r , W. P. ; e t a l . Apr i l 1972. Manufactur ing methods f o r s u r f a c e i n t e g r i t y of m a c h i n e d s t r u c t u r a l components . A F M L - T R - 7 1-258. Cincinnati , Ohio: Metcut R e s e a r c h A s s o c i a t e s Inc.

3. K o s t e r , W . P . ; F r i t z , L. J . a n d Kohls, J . B . 1971. Sur face i n t e g r i t y in mach in ing of 4340 s t e e l and Ti-6A1-4V. P a p e r No. IQ7 1-237. D e a r b o r n , Michigan: Socie ty of Manufacturing E n g i n e e r s .

F i g u r e 2

F i g u r e 3

F i g u r e 4

Gentle Condit ions - Slight s u r f a c e (b) Abus ive Condit ions - Signif icant s u r f a c e r o u g h n e s s i r r e g u l a r i t y bu t wi thout o t h e r m e a s u r - but without m e a s u r a b l e h a r d n e s s change.

a b l e e f fec t s . S u r f a c e F i n i s h : 205 AA lOOOX S u r f a c e Finish: 18 AA lOOOX

Depth B e n e a t h S u r f a c e , Inches

SURFACE CHARACTERISTICS O F TITANIUM 6A1-4V (Be ta Ro l l ed , 32 R,)

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G E N T L E S U R F A C E G R I N D 6 2 G E N T L E H A N D G R I N D 5 7 G E N T L E ( ; i l l 1 i i

A B U S I V E H A N D G R I N D 3 0

A I 3 U S I V E S U R F A C E G R I N I ) 1 3

F A T I G U E CHARACTICRISTICS OF B E T A R O L L E D TITAI'i1U.M 6Al-4V SURFACES 13110DUCED

B Y E N D MILLING A N D GRTNT)TNG

10 6

CYCLES TO F A I L U R E

Z B U S I V E 3RrND

Figure 11

HIGH C Y C L E F A T I G U E

I-1 G E N T L E

I I A B U S I V E

END C U T 77

ENDURANCE LIMIT, KSI.

SUMMARY OF HIGH CYCLE BEHAVIOR

O F Ti- 6Al-4V (BETA ROLLED, R, 32)

Figure 12

RESIDUAL STRESS VS. ENDURANCE LIMIT 1 Ti- 6A1-IV B E T A R O L L E D S O U R C E A F I M L - T R - 7 0 - 11

I I I 1 I G E N T L E ABUSIVE

E N D M I L L - E N D C U T 0 E N D M I L L - P E R I P H E R A L C U T 0 CHEMICAL MILLING S U R F A C E G R I N D

0 n

HAND G R I N D b

ENDURANCE LLMIT - KSI