Journal of Materials Processing Technology, 21 ( 1990 ) 91-100 91 Elsevier

D Y N A M I C S T R A I N - A G E I N G OF U N A L L O Y E D T I T A N I U M D U R I N G C O M P R E S S I O N AT S T R A I N R A T E S OF 0 . 0 5 TO 32 s-1 A N D T E M P E R A T U R E S OF 3 0 3 TO 5 7 3 K

P. VENUGOPAL

Department of Metallurgical Engineering, Indian Institute of Technology, Madras (India)

S. VENUGOPAL* and V. SEETHARAMAN

Metallurgy Programme, Indira Gandhi Centre for Atomic Research, Kalpakkam (India)

(Received March 11, 1988; accepted in revised form January 13, 1989)

Industrial Summary

Dynamic strain-ageing is a well known phenomenon in metal working, which phenomenon, however, is objectionable from the formability point of view. Dynamic strain-ageing is dependent upon the chemical composition of the work-metal, the deformation, the deformation rate and the temperature. The loss in ductility and the variation of the flow stress, the strength coefficient and the strain-hardening exponent, with variation in temperature, characterise the onset of dynamic strain-ageing. A sudden increase in the work-metal temperature due to adiabatic heating can also contribute to the occurrence of dynamic-strain ageing.

In this work the interaction of: (i) the true stress; (ii) the work-hardening rate (O=da/d~); and (iii) the strain-rate sensitivity index (~ = Aa/Aln ~ ), with temperature, has been analysed to ascertain the onset of dynamic strain-ageing in the compression testing of commercially pure titanium.

Introduction

The low heat capacity and the low thermal conductivity of titanium promote a higher rise in temperature due to work of deformation when tested under compression, the rise in temperature being expected to be greater at higher strain-rates due to lesser contact time between the work-metal and the tools.

The work of Ref. [ 1 ] has revealed that the rise in temperature in the work- metal being deformed can introduce dynamic strain-ageing and Refs. [2,3] have confirmed the onset of the above phenomenon in the temperature range of 500 to 800 K for commercially pure titanium at the relatively very low strain- rate of 10 -4 s -1.

Industrial forming operations are characterised by a high speed of defor-

*This work has been carried out during the author's deputation period as Research Scholar at IIT, Madras, as a part of his M.S. programme.

0924-0136/90/$03.50 © 1990 Elsevier Science Publishers B.V.

92

mation: however, the strain rates are such as can be accommodated by stan- dard compression tests. The present work envisages the manifestations of dy- namic strain-ageing at initial test temperatures of from 373 to 573 K in compression testing at strain rates of from 0.07 to 32 s - 1.

In addition to the references cited, the reader may consult the following for general background reading on t i tanium and its alloys: the paper "Dynamic strain ageing" by Baird, published in The Inhomogeneity of Plastic Deforma- tion, ASM, 1973, pp. 191-222; the review by Timothy, published in Acta Me- taUurgica, 1987; and the book by Sumiation and Jonas, Flow Localisation.

Experimental

The composition of the unalloyed titanium chosen for the present investi- gation is presented in Table 1. In the literature [4 ] it is noted that, generally, the warm-working range of temperature for almost all materials is from 573 to 773 K. Three types of machines, namely a hydraulic press, a friction screw press and an eccentric press, were available within the Metal Forming Labo- ratory, IIT, Madras, affording mean strain-rates of 0.07, 0.11, 8.5 and 32 s-1; a temperature range of 303 to 573 K in steps of 50 K was employed for the solid and ring-compression testing on the unalloyed titanium at these strain rates. In-situ furnaces developed at the laboratory were used to heat the work-metal, the upsetting being carried out within the furnace itself. The recommended lubricant, MoS2 [5], was employed in the testing.

The solid compression test specimens had a ratio of height to diameter of

TABLE 1

Specification and room-temperature properties of the work-material

Material specification Material: Manufacturer: Nominal size: Condition: Chemical composition:

Commercially Pure Titanium Grade Titan 12 Mishra Dhatu Nigam Ltd., Hyderabad, India 31 mm dia and 20 mm dia rods Hot rolled, annealed a, pickled and skin peeled Carbon 0.0107% Iron 0.043% Hydrogen 26.3 ppm Oxygen 977 ppm Nitrogen 52.1 ppm Titanium balance

Room-temperature properties (as received) 0.2% Yield strength 200 MPa Ultimate tensile strength 350 MPa Elongation 57% Reduction in area 80% Hardness 139 VHN

a923 K for 45 minutes, air cooled.

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1.5 whilst the ring specimens conformed to outer diameter: inner diame- ter: thickness being in the proportions of 6: 3: 2. Both the solid and the ring specimens had groves for inserting chromel alumel thermocouples for sensing the temperature. The dynamic force and the stroke were sensed by a load cell and a Hottinger Baldwin Messtechnik LVDT respectively, all of the quantities being recorded on a moving-coil galvanometer recorder.

Results and discussions

From the force-stroke recordings pertaining to solid compression testing, the flow stresses were evaluated, these uncorrected stresses being termed the "working" stresses. The ideal flow stresses were then evaluated by incorporat- ing a correction for workpiece-platen friction: the friction factors were inferred from the results of the ring tests whilst Siebers equation [6] was used in es- t imating the ideal flow stress. Figure 1 presents a typical true stress-strain plot at a strain rate of 0.07 s - 1.

Preliminary inferences The specimens tested at a strain rate of 8.5 s - 1 and at temperatures of 423,

473 and 573 K fractured after reaching a strain of 0.45. Similarly, specimens tested at a strain rate of 32 s - 1 and at temperatures of 373 K and 423 K frac-

1200 MATERIAL; CP T i 11oo-,o - - y f 1000

u 900 /

,,, 500 f / o /,/ ~- 400 -- - ~

,oo / • 373K

20O ,J" ~ &23K 473K

lO0 0 ~B, 523K 573K

0 0.2 0.4 0'.6 0-8 TRUE STRAIN

Fig. 1. Variation of true stress with strain for different initial test temperatures at a mean strain-

rate of 0.07 s -I (hydraulic press: manual mode).

94

tured under strains of 0.55 and 0.4 respectively, while two out of five tested at 0.11 s-1 and at 473 K displayed cracks. A typical microstructure showing the existence of localised flow is presented in Fig. 2.

The well-known Holloman equation had been shown to describe adequately the stress-strain curves in the temperature range of 198 to 573 K at strains greater than 0.25 [2,7-9]. The variation of the Holloman constants K and n with initial test temperature can be expected to provide some preliminary in- formation concerning brittle behaviour due to dynamic strain-ageing. The variations of K and n with test temperatures at different strain rates are pre- sented in Figs. 3 and 4 respectively, from which the following can be inferred:

(a) The value of n is observed to increase with initial test temperatures for all the strain rates, except for the strain rate of 32 s - 1.

(b) A peak in the value of strength coefficient K is encountered for strain

~, ~,i~, i J i ~̧ !i!~'~!~

Fig. 2. Typical micrograph showing the localised flow in T i 99.9, upset in the friction screw press: ~-= 8.5 s -~ (×200) .

I ¢ 0 0

o.

. 1 2 0 0

1 0 0 0

E 8 0 0

x 6 0 0

z w

4 0 0

2OO

o

O

t I

E- : 8 .5 s - I

k E : 0 . 0 7 s - I

~ = 0 . I ! s " !

HYDRAULIC PRESS (NANUAL) ~ : 32 S - I o HYDRAULIC PRESS (AUTO) ~'-

9 ECC~HTR,C ,PRESS, i I 2 8 0 3 2 0 3 6 0 4 0 0 4 4 0 4 8 0 5 2 0 5 6 0 6 0 0

Fig. 3. Variation ofthe strength coefficientwith initial testtemperaturefordifferentmean strain- rates.

95

1.2

1 . 0 O Q.

,,X, 0 . 8

Z ~o.~ w ,,..,

, , r

~ 0 . 2

~ o

~ HYDRAULIC PRESS (MANUAL) O O HYDRAULIC PRESS (AUTO)

~ FRICTION SCREW PRESS D D ECCENTRIC PRESS

~ : 8 - 5 s "1

/ ~ = 0 .07 s - t _ _

e .~ ~- - ~ E : 0 . I I s - I - - ~ ~__....- ~ 0 - '~"

I ~_ ~ : 3~ s-'

2 8 0 3 2 0 360 & 0 0 ~&O & 8 0 5 2 0 5 6 0 6 0 0 I N I T I A L TEST T E M P E R A T U R E ~ K

Fig. 4. Variation of the strain-hardening exponent with initial test temperature for different mean strain-rates.

rates of 8.5 and 32 s-Z: K is observed to be independent of temperature over the temperature ranges of 423 to 523 K and 303 to 523 K for strain rates of 0.07 and 0.11 s -z respectively.

The fracturing of the specimens, the localised flow, and the variation of K and n with initial test temperature are all indicative, that the unalloyed tita- nium is influenced strongly by dynamic strain-ageing over the temperature and strain rate ranges where the material showed fracture.

Supporting evidence of dynamic strain ageing In order to confirm the occurrence of the dynamic strain-ageing phenome-

non and to examine the effect of adiabatic heating on the phenomenon, various physical manifestations of dynamic strain-ageing [10] have been examined. Manifestations such as: (i) a peak in the flow stress, a, with forming temper- ature; (ii) a peak in the variation of rate of work-hardening O= da/de; (iii) a peak in the variation of the Hall-Petch slope K+ with temperature; (iv) a min- imum in the variation of ductility with temperature; (v) a minimum in the rate of strain-rate sensitivity, with ~ = Aa/Aln ~ becoming negative in the temper- ature range of dynamic strain ageing; were thought to be supportive in ascer- taining the onset of dynamic strain-ageing in the present work. These mani- festations are presented schematically in Fig. 5, after Ref. [10]. The types of serrations normally observed in the load-stroke diagrams at different temper- ature regions are also indicated in Fig. 5.

Serrations in the load-stroke diagram In the present investigations, testing under friction screw press and eccen-

tric press in the temperature range of 373 to 523 K, serrations were observed in the load-stroke diagrams. Due to the damping action of the hydraulic oil, serrations on the load-stroke diagrams for testing under hydraulic press were not appreciable.

96

~ K

II I TRUE 5TRES c.

1 \ i~ETcx \ i /

\ 2" " r ~0"

~ I_ _ I . . . . . .

~" . . . . . UE SIRESSj -'~' i IE ,IRUE $]RAIN ,

o ~,~ 'i I ~ i °-'~ J /

o o,~ ~1 ~ I o-,{ NO IDSA REGION~_ NO

"SERRATIONS" "TYPES OF -I ~ERRA11ONS SERRAT ONS I rA A*B C I l DIE "J

TEMPERATURE,K Fig. 5. S c h e m a t i c i l l u s t r a t i o n o f t h e v a r i o u s m a n i f e s t a t i o n s o f d y n a m i c s t r a i n - a g e i n g ( a f t e r Ref.

[10]).

Variation of true stress with forming temperature In order to account for the rise in temperature during forming, the variation

of true stress with the actual temperature of the specimen was evaluated and is presented in Figs. 6 (a ) - (d ) . The peak in a is indicative of the onset of dy- namic strain-ageing in the strain-rate range of 0.07 to 32 s - 1. The temperature at which the peak occurs is influenced strongly by the strain rate. As can be seen from these figures, the generally greater is the strain rate, the lower is the temperature at which the peak is encountered. The rise in temperature due to adiabatic heating is greater at higher strain-rate due to lesser heat loss from the work-metal to the surroundings: this leads to manifestations of dynamic strain-ageing at relatively lower initial forming temperatures.

From the above figures it can be observed also that the peak is not associated with the initial test temperatures of 303 and 573 K. Though the rise in tem- perature of the specimen is expected to be more at 303 K, the residence time of the specimens in the dynamic strain-ageing regime is low in this situation. At a higher temperature of 573 K the work done on the sample is low (due to the low flow stress); thereby the rise in temperature during forming is less appreciable. The in-situ temperature measurements also confirmed the above logic. The microstructures of the samples upset at 303 and 573 K also sup-

97

1200 1100

~900 U~

70(]

tn SO0

300

100 280 320

~ " ' ~ iH'tl)RAU'LIC PRESS ~ 0.S ~ ~ ANUAL (~ "0"07 $" O---'O~ 0.30"4

~ " " 0.2 , _ , o.,

'~ ~ . ~ O-OS.

~ " " ~ ~ ~'~---" ~ ~--"~ a

360 400 440 480 $20 560 600 6/,0 ADIABATIC TEMPERATURE ~K

1~oo , JBY;)RAU;.IC PRESS 1100

| AU10 ~,0.11 s "~ g.

7oe

w 500 Q: ~" 300

100

,~---"--~ O. 5 0.4

D - - - o 0.3 d b - - d ~ 0.2 ~ 0 . 1 x x O-OS

~ h d 280 320 360 400 ~.~.0 460 520 560 600 640 360 &O0 ~,40 400 520 560 600 640

ADIABATIC TEMPERATURE, K ADIABATIC T.EMPERATURE. K

200 I FRICll0N SCREW PRESS ,', .~ 0.t,S 100 I ~ . 0.5 s-I o 00.35

I I I I u o o.2s 900 I --" " 0.15

J : = O.OS

500 - -

300

, o o __Lc 320 360 t, O0 l.l,O 1.80 520 560 600 640 680

ADIABATIC TEMPERATURE, K 800 0-,~

o . ~ o 0.3

700 ~ ~ ~ ~ e ~ e 0,10"2

600 / ~

Soo

30C ECCENTRIC PRESS J

2oc ~',,z2 s-', , 280 320

Fig. 6. Variat ion of true stress wi th tempera ture for different s t ra ins at a mean s t ra in-ra te of: (a) 0.07 s-Z; (b) 0.11 s-X; (c) 8.50 s - l ; and (d) 32 s -1.

20:

~ ,~ , r " x / . ,- x

=< ,o

~¢ 6 a 250 350 450 550 650

ADIABATIC TEMPERATURE,K

22

~'o 2° ~'o o N 18 ~ ,___~x z ~:--7 z. / \

: :¢ ' , .~#1o X l J 12 X l ; n, re O O ~¢ 10 3:

b 8

2SO 3 5 0 4 5 0 5 5 0 6 5 0

A D I A B A T I C T E M P E R A T U R E ~ K

24

22 f 20

16

14

12 ...M

111 c 250 150 450 550 650

ADIABATIC TEMPERATURE,K

14

12

10

8 "% p ' x _

\ / "

o d 2S0 350 #.50 550 650

ADIADA11C I E M P E R A T U R E ,K

Fig. 7. Variation of work-hardening rate with adiabatic temperature in: (a) the hydraulic press (manual mode, ~=0.07 s-l); (b) the hydraulic press (auto mode, ~=0.11 s-l); (c) the friction screw press (~=8.5 s-Z); and (d)the eccentric press (~=32 s-l).

98

20G

0

c

~ 0 0 b

- - 6 0 0 - • 473K o 523K o 573K

- 6 0 0 0 011

• 373K ~ /~ /,23K

\ k

0.2 0.3 0.4 TRUE STRAIN

6O x 373K -"- 423 K

"4 40 .~73K a 5 2 3 K

<~ O573K L,-

J

0 0 0.1 0.2 0.3 0.4

TRUE STRAIN

30O

a 200

. .

10(] ~( "" \"

o %\ }

- 1 0 0 , -~ -...~ ~ ".. ",,.

- 20C " " ~ 7 - r

b -3oc I [ o o . , o . ,

TRUE STRAIN

x 373K ~23K

• 473K o 5Z3K o 573K

c 0.S

Fig. 8. Variation of Aa/Aln ~ with true strain over the strain-rate range of: (a) from 0.07 to 0.11 s - l ; (b) from 0.11 to 8.5 s-I ; and (c) from 8.5 to 32 s -].

ported the above, in terms of the absence of localised flow. Thus it appears that the dynamic strain-ageing is predominant within the initial test temperature range of 373 to 523 K.

Variation of rate of work hardening with temperature Figures 7 (a ) - (d ) represent the variation of work-hardening rate as a func-

tion of the actual forming temperature for all of the strain-rates tested, the peak of which (indicative of the onset of dynamic strain-ageing) can be seen clearly at all strain rates. Generally the temperature at which the peak is ob- served is around 500 K. The tensile-test data of Refs. [2,3] showed evidence of a peak at 500 to 800 K.

Adiabatic heating at large strain-rates is seen, therefore, to have a strong influence on dynamic strain-ageing during compression testing.

Variation of strain-rate sensitivity index (},) with temperature An attempt has been made to examine the variation of 7 as a function of

strain at various temperatures, using the present data. The parameter F is eval- uated in the strain-rate range of 0.07 to 0.11 s -1, 0.11 to 8.5 s -1 and 8.5 to 32 s - 1. Even though the strain-rate variation in the above ranges is large, a mean-

99

ingful inference can be made with respect to ~. Figures 8 (a ) - (c ) give the plot of ~ as a function of true strain at different temperatures. In the strain-rate ranges of 0.07 to 0.11 s -1 and 8.5 to 32 s -1, ~ is negative and supports the existence of dynamic strain-ageing at lower initial test temperatures. In the strain-rate range of 8.5 to 32 s -~, it is expected that ~ would become negative below a strain of 0.1.

Conclusions

The dynamic strain-ageing phenomenon is observed in commercially pure t i tanium at lower test temperatures (within the range 373 to 523 K) in the compression test. The rapid increase in specimen temperature due to heat of deformation is the main cause of the onset of dynamic strain-ageing in the compression test. The temperature and the strain at which the dynamic strain- ageing occurs depend on the strain rate and the initial test temperature. It is found that the higher is the strain-rate the lower will be the strain and the temperature at which dynamic strain-ageing occurs. At 303 K, the rapid rise in temperature is high but the residence time is low, while at 573 K the rise in temperature in the specimen is not high: thus dynamic strain-ageing does not manifest itself at these temperatures for the strain-rates investigated.

Acknowledgements

The authors are grateful to Dr. Placid Rodriguez, Head, Metallurgy Pro- gramme and Materials Science Laboratory, Indira Gandhi Centre for Atomic Research, Kalpakkam, for his keen interest and encouragement during the course of this investigation.

References

1 V. Ramachandran, D.H. Baldwin and R.E. Reed Hill, Metall. Trans., 1 (1970) 3011. 2 S.N. Monterio and R.E. Reed Hill, Metall. Trans., 4 (1973) 1011-1015. 3 Abdul-Sulam M. Elechie, in: H. Kimura and O. Izumo (Eds.), Titanium 1980, Proc. 4th Int.

Conf. on Titanium, May 19-22, 1980, Kyoto, Japan, The Metallurgical Society of AIME, Warrendale, 1980, pp. 831-840.

4 M. Hirschwogel, J. Mech. Work. Technol., 2 (1979) 317-331. 5 H.W. Wagener and K.H. Tampe, Proc. 5th Int. Conf. on Titanium, September 10-14, 1984,

Munich, F.R.G.; Titanium Science and Technology, Vol. 1, Deutsche Gesellschaft fiir Me- tallkunde, Oberursel, F.R.G., 1985, pp. 577-584.

6 K. Lange (Ed.), Hand Book of Metal Forming, McGraw Hill, New York, 1985. 7 F.C. Holden, H.R. Ogden and R.I. Jaffee, Am. Soc. Test. Mater., Spec. Tech. Publ., 204

(1957) 14-31. 8 A.W. Brown and P.G. Partridge, in: R.I. Jaffee and H.M. Burete (Eds.), Titanium Science

and Technology, Proc. 2nd Int. Conf. on Titanium, Massachusetts, May 2-5, 1972, Plenum Press, New York, 1972, pp. 1021-1031.

100

9 K. Okazaki and H. Conrad, Acta Metall., 21 (1973) 1117-1129 10 P. Rodriguez, Bull. Mater. Sci., 6 (4) (1984) 653-663. 11 S. Venugopal, MS Thesis, IIT, Madras, India, 1986.