Journal of Materials Processing Technology, 22 ( 1990 ) 163-175 163 Elsevier

I N F L U E N C E OF S T R A I N RATE ON THE COLD E X T R U S I O N OF COMMERCIALLY P U R E TITANIUM

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 1, 1988; accepted in revised form April 22, 1989 )

Industrial Summary

Titanium and its alloys demand a high magnitude of forming stresses in cold extrusion, which stresses can be reduced by utilising the deformation heat generated during cold forming. The deformation heat can be utilised effectively at high strain-rates, as under these conditions the heat loss by conduction is less, due to lesser contact time between the work-metal and the tool surface. In this investigation an attempt has been made to utilize deformation heat in the cold extrusion of commercially pure titanium. Extrusion tests, both solid and hollow forward, have been under- taken on cold commercially pure titanium, using both a hydraulic press (a low strain-rate ma- chine) and an eccentric press (a high strain-rate machine) to evaluate the interaction of the deformation ratio and the strain rate with the adiabatic heating that effects lower forming stresses. The results show that at higher rates of working the forming stress is reduced. Post-extrusion tests have also been undertaken to examine the properties of the extrudes, in terms of standard tensile, hardness and corrosion tests, to establish their suitability for engineering applications.

Introduction

Titanium and its alloys are graining importance in industry due to their high strength-to-weight ratio and their excellent corrosion resistance. The raw-ma- terial cost of these alloys can be compensated for by adopting economical form- ing routes. Recently, considerable interest has been shown in the manufacture of t i tanium components by cold-extrusion techniques, which can lead to effi- cient utilisation and conservation of these expensive alloys [ 1 ]. The cold forg- ing of t i tanium and its alloys generally demands a high magnitude of forming stresses. Extensive tribological studies by Wagener et al. [ 2 ] on commercially pure titanium, based on the backward can-extrusion test, have revealed that MoS2-based lubricants give sound extrudes with minimal tool galling and op-

*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.

164

timal punch stress in cold working. These stresses are likely to be accentuated while cold extruding titanium alloys, which are more difficult to extrude, and may necessitate the use of expensive carbide punches for the backward can- extrusion of thin-walled components of titanium and its alloys.

One possible approach in reducing the tool stresses is to adopt moderately high working temperatures. Generally, the higher the temperature, the lower is the forming stress. However, it is necessary also to contend with problems such as the formation of oxides, the depletion of lubricants, dynamic strain- ageing [3], etc., while forming at elevated temperatures.

A portion of the forming energy is converted into heat, commonly termed adiabatic heat. The low heat capacity and the low thermal conductivity of ti- tanium have a strong influence on adiabatic heating [2], and it is possible to make use of the latter in terms of reduced forming stresses and of the associ- ated tool stresses during cold/warm working. The low heat capacity and the low thermal conductivity can give rise to a greater increase in the temperature of the billet and thereby to a reduction in the forming stresses. If the heat generated during deformation has to be retained for effectively reducing the forming stresses, it becomes necessary to examine the time-elements charac- terising the loss of heat from the work metal to the tool surface by conduction, for a given deformation ratio. Whilst higher strain-rates will ensure less time for heat loss, excessive instant heat-generation may introduce dynamic strain- ageing and its attendent problems.

The main objective of this investigation is to study the cold-extrusion be- haviour of commercially pure titanium in both solid-forward and hollow-for- ward (Hooker) extrusion and to establish the interaction of the deformation ratio and the strain rate with the degree of adiabatic heating, so as to be able to predict the conditions leading to lower forming stresses in the cold-extrusion operation.

Experimental

The chemical composition of the work material is given in Table 1. Solid- forward and hollow-forward extrusion has been performed at room tempera- ture using special tooling [4 ], which can be adapted for either a hydraulic press or an eccentric (crank) press. Figure 1 presents the typical experimental tool set-up for the extrusion test. The extrusion-strains investigated are: (i) 0.32, 0.5, 0.65 and 0.82 in solid-forward extrusion; and (ii) 0.39, 0.61, 0.76 and 1.1 in Hooker extrusion. The tooling made it possible to measure also the actual temperature in the deforming zone during cold extrusion. The extrusion billets were machined from rod stock to the dimensions given in Fig. 2 and heat- treated after applying the lubricant MoS2 as per the recommendations in Ref. [2]. The forces, displacements and temperatures were measured with wire

TABLE 1

Materials specification and room temperature properties of work material

165

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 Hydrogen 26.3 ppm; Oxygen Nitrogen 52.1 ppm; Titanium

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

0.043% 977 ppm balance

a923 K for 45 minutes, air cooled.

Fig. 1. Experimental tooling for the solid-forward cold-extrusion process, mounted in an eccentric press: ( 1 ) press frame; (2) slide; (3) load cell; (4) LVDT assembly; (5) dummy block; (6) punch; (7) billet; (8) container; (9) container shrink-ring; {10) die; (11) die shrink-ring; (12) die lo- cating-ring; ( 13 ) die support-disc; (14) build-up block; (15) tool housing/bed; ( 16 ) thermocouple.

166

a

~bD

z~.3:3:ff ~0.00

19.5 -o.ol

+0.00 18-1 -o.o t

:8:8~ 16.7

C D *0.00

21.3 - ° ' ° I +0.00

19.5-o.ol

~'0.00 18.1-o ~ol

*0.00 16.7-o.oi

C : - - - ---.:"

30 ..o.oz

/,S ° o,2 Ho,. ~ for t h e r m o -

J___[i

b

+0-02

_ ~ r _ ( _ / ~ l _ _ ~ V _ f o r t h e r m o -

\\ _/ii o.pl,

30"-0.02

Fig. 2. Details of the billets for: (a) forward extrusion; (b) Hooker extrusion.

strain-gauge load cells, a LVDT and Ni-Cr thermocouples and recorded using appropriate devices. A view of the extrudes is furnished in Fig. 3.

Post extrusion test such as microstructural studies, a hardness survey, ra- diography, a dye-penetrant test, the tensile test, the impact test and corrosion tests were carried out on the extrudes.

R esu l t s and d i s c u s s i o n

E x t r u s i o n s t ra in-ra te In extrusion, the conical shape of the die increases the velocity of the ma-

terial flow as the reduction proceeds, this increase in velocity, in turn, causing an increase in the strain rate as the reduction increases in the deformation zone. The instantaneous strain rate at any section (for any diameter) in the deformation zone is given by the following equation (see [3,5] )

~= (2 V D 2 tan ol ) / d 3

where ~ is the instantaneous extrusion strain-rate; V is the velocity of the ram; c~ is the semi-angle of the die; D is the diameter of the billet or the punch; and d is the diameter of the extrude at any section in the deformation zone.

The instantaneous strain rate for each reduction in the solid- and hollow- forward processes was plotted against strain and the mean strain-rate was then

167

a

b

Fig. 3. Photograph of forward extrudes: (a) solid-extrusion billets with a thermocouple (1st from left) and extrudes at strains of 0.32, 0.5, 0.65 and 0.82 (from left to right); and (b) Hooker- extrusion billet with a thermocouple ( 1 st from left ) and extrudes at strains of 0.39, 0.61, 0.84 and 1.1 (from left to right).

evaluated on the average-area concept, using the following equation: ~max

~= 1 f ~d~ ~max

0

where •max is the maximum extrusion strain. The mean extrusion strain rates so evaluated are entered into Table 2.

168

TABLE 2

Mean extrusion strain-rates for different s t ra ins in the hydraulic press and in the eccentric press, for the solid forward and Hooker processes

Process Strain Mean s t ra in-rate (s -1)

Hydraulic press Eccentric press

Solid forward

Hooker (hollow forward)

0.32 0.63 35.6 0.5 0.74 38.9 0.65 0.69 34.9 0.82 0.84 26.5

0.39 0.99 75.5 0.61 1.17 59.1 0.84 1.51 114.9 1.1 1.92 101.4

The mean strain-rate is higher in hollow (Hooker) forward extrusion than it is in solid-forward extrusion, for any given strain level and ram velocity, as can be seen from Table 2. The mean strain-rate is inversely proportional to the aspect ratio (i.e. the ratio height /width ) of the deforming material, vide ~ = V !

h, where V is the velocity of the ram and h is the height of the deforming material. Hollow-forward extrusion, where the formed component is hollow (i.e. is of lesser volume), requires a billet of lesser height than is required for solid-forward extrusion, for a given outside diameter of the billet.

6 0 0 . * . E R , * L CPT~ 6 0 ( . *TER,*L:CP ,*

5 o

5 0 0 - ~ s° 50(

~(oo o ~ / ' ~ . ~ _ c = 0 . 8 2 _ _ _ _ _ ~ ° ~ ~ u_ 300 ~ ~ e=o.6~s ~ EL 30 ( 4

[ ~ ~=o.5 z ~ ~ ~oo.6, z Z 200 / i E ~ 20( r~ =0.32 r,., ~ E =0.39 °

" 100 10C

i i o i I a 6 [ I b 0 5 10 15 20 25 30 0 5 10 15 20 25 30

PUNCH " I ' R A V E L , m m PUNCH T R A V E L , m m

Fig. 4. Force-travel diagram for: (a) solid-forward extrusion; and (b) Hooker extrusion.

169

8 Q O I, IATERIAL: CP I i 02S il~

700

g. " P m a x

Z ~ 6 0 0 ~ =.VORAUUC

r~ ~ ~CRANK PRESS

500~ o___o HVOR.ut , _ i l l PR[SS "~ ~,- - ~ [CCEmRt

o. . . . . . 7 / z 4 0 0 - - - - ° ~ ~ ~ ,o. ~ - oc 300 / X /

/ 200

s I00 - ~

40 110(3

100(

~ 9oo

Z

~ 700

_~ ~ 600

2o~ ~ ~ 500

15~ ~ 4oo

Z w 300 10'"

loo

0 a 0 0 0 0.2 0.4 0.6 0.8 1.0 0

EXTRUSION STRAIN

M A T E R J A L : C P Ti

Ji! / 45 ~ °'° / "~

P m a x 4 0 w~ HYDRAULIC PRESS ~ . ~

,,e,,, / 35 (~----Ct HYDRAULIC pRESS ~ "

/ 25 w Gg

/ 15 a.

w

~" tO

c bD 0.2 0.4 0.6 0.8 1.0 1.2

EXTRUSION STRAIN

Fig. 5. Effect of the extrusion strain (area reduction) on the punch pressure and the temperature rise in: (a) the solid-forward extrusion process; and (b) the Hooker extrusion process.

Fig. 6. Presenting a photograph of sectioned Hooker extrudes, showing: 1st from left, an unex- truded billet; 2nd to 5th from left, Hooker extrudes at strains of 0.39, 0.61, 0.84 and 1.1 respectively.

170

E ~ 0 . 3 2 E • 0 ' 5

171

E = 0 - 3 9 £ ~ 0 - 6 1

C E = 0 - 8 ~ £:= 1.1

Fig. 7. P r e s e n t i n g m i c r o g r a p h of: ( a ) the ' as - rece ived ' ma te r i a l ( X 200); (b ) t he T i 99.9 solid- forward ext rudes , ex t ruded in t he hydrau l ic press, au to mode i.e. ~---- 0.11 s - ] ( X 200 ); a n d (c) the T i 99.9 H o o k e r ext rudes , ex t ruded in the hydrau l ic press, au to mode i.e. i-= 0.1 ] s - 1 ( X 200 ).

Effect of strain rate on the peak extrusion-pressure The experimentally measured force-travel diagrams for solid-forward extru-

sion for the strains investigated are given in Figs. 4(a) and (b). The peak extrusion pressure and the temperature rise of the billet per mm of stroke travel are given in Fig. 5 (a) for the solid-forward extrusion process in the hy- draulic press (a lower strain-rate) and in the eccentric press (a higher strain- rate). The extrusion pressure is seen to increase with strain for both machines. For the hydraulic press, the temperature-rise increases up to a strain of 0.5, then remains constant up to a strain of 0.65 and then increases again with further strain. In the eccentric press, the temperature rise is observed to in- crease continuously with increase in strain. Figure 5 (a) reveals, therefore, that the punch pressure is less in the eccentric press than in the hydraulic press, at

172

all strains, the difference in the punch pressures increasing with increase in strain. As the strain increases, the rise in temperature becomes relatively greater in the eccentric press, indicating the effect of adiabatic heating in reducing the punch pressure.

Figure 5 (b) provides a similar inference in the hollow-forward extrusion process. At the maximum strain (1.1 in the present investigation) a large dif- ference in extrusion pressure between that for the hydraulic press and that for the eccentric press is noted. Although the heat transfer area is large in hollow- forward extrusion, a significantly reduced force is obtained with the eccentric press, due to the greater strain-rate which is associated with hollow-forward extrusion.

At the maximum strain of 1.1 (hollow forward), the force reduction in the eccentric press with respect to the hydraulic press is about 22% and at the minimum strain of 0.32 (solid forward) it is about 9.8%. Thus working at higher strain-rates reduces the forming stress due to the effective utilisation of adi- abatic heating in the cold working of commercially pure titanium. Further, the higher rate of working of the eccentric press yields higher productivity.

Post-extrusion tests The sectioned hollow extrudes (Fig. 6) reveal that both the concentricity

and the straightness of the bore are good. Further, radiographic, ultra-sonic and dye-penetrant examinations show that the extrudes are sound and free from both internal and external defects.

The micrographs of the as-received material, of the solid-forward and of the hollow-forward extrudes are presented in Figs. 7(a), (b) and (c) respectively. With increase in strain, there is a gradual increase in the deformation of the grains: this is evidenced by the appearence of slip-like wavy lines. Further, more grains tend to rotate and become aligned with the direction of extrusion, with increase in the strain values. The figures indicate also that a very strong texture is developed in the process; when the strain level is greater than unity, the texture development appearing to be more pronounced in the hollow-for- ward extrusion process than in the solid, for identical levels of strains. Rela- tively, the extent of deformation of the grains seem to be governed by the actual stress patterns that exist in the material. The microstructures of the extrudes that were extruded in the eccentric press are similar to those of the extrudes produced in the hydraulic press, at identical levels of strains, and thus there is not a change in structure arising from this change of strain rate. Since the structures are identical, the mechanical properties should be same, for the ex- trudes from the eccentric and the hydraulic presses. In view of this argument, mechanical testing was planned for the samples extruded in the hydraulic press only.

Figure 8 gives the room temperature mechanical properties of the extrudes, the values at zero strain referring to the "as-received" material. The ultimate

173

700 = e O .2 " / .Y IELO STRENGTH = = PERCENTAGE AREA REOUCIION 0 - " - " - - 0 PERCENIA6E IOTAL ELONGATION

6 0 0 - = = . A e O N ( S S I V H N ) O ' ~ IMPACT ( N e R S Y

~ z 500

40C ~ z u~

oo,

200'

100

S /

v

0.2 0.4 0.6 0.8 EXTRUSION STRAIN

100

80

60 t - .

~o ~

20

0 1.0

Fig. 8. Showing the variation of the mechanical properties of commercially pure t i tanium as a function of the extrusion strain (area reduction ).

tensile strength, the yield strength and the hardness (VHN) are found to in- crease and the ductility and the impact strength are found to decrease, with increase in the strain, due to the work-hardening effect. These properties, how- ever, are adequate for engineering purposes.

The results of hardness tests on the extrudes indicate that some localised deformation could have occurred. The plots of hardness at different locations of the samples are given in Fig. 9, and indicate that: (i) the material near to the surface is strained more than that of the inner portion; (ii) the surface strain is greater at larger reductions; and (iii) the strain at the central axis is independent of the reduction, beyond a value of strain of 0.5. Since the strain is greater at the surfaces, compressive residual stresses are built up at the sur- faces. A good surface finish in combination with this surface compressive re- sidual stress, should therefore enhance the fatigue properties of these extrudes.

The results of the corrosion tests using a boiling medium of 11% H2SO4 + 3% HC1 + 1% FeC13 + 1% CuC12 for 24 hours [6], reveal that the resistance to uni- form corrosion is excellent and unaffected by cold work. Similarly, the results of the pitting- and crevice-corrosion tests in an 0.5 M NaC1 medium at 303 K in an argon atmosphere [7] indicate that the properties of the extrudes are satisfactory and unaffected by cold work. The results of the stress corrosion tests using a medium of methyl hydrochloric acid (methyl alcohol containing 1.13 vol% of a 36% aqueous solution of hydrogen chloride) for 160 hours [8] show that cold extrusion has not affected the stress-corrosion resistance of the titanium.

174

230

210

d 19o

170 a ~e .~ 150 :z / M I D D L E ZON

1 3 ( 0 1 ; 2'0 3'0 /.'0 5'0 6 ; D I S T A N C E ~ m m

D I S I A N C E , m m 10 2O 3O 40 50

130 , ,

z 150 k B O U N D A R Y ZONE

> 170 '

U~ , / / ' " 19(] z Q ~" 210 <~

-1- 230

25C

75

60 70

DIE ENTRY Z O N E

~o ,; o, 1'o ~o R A D I A L D I S T A N C E , r n m

I

I R A D I A L D I S T A N C E , m m

20 10 0 10 20,

t / / • \ \

DIE E X I T ZONE

E = 0 . 8 2 . . . . . E = 0 . 5

E = 0 . 6 5 . . . . . E = 0.32

Fig. 9. Illustrating the distribution of hardness after solid forward-extrusion.

Conclusions

(i) The extrusion of unalloyed titanium in an eccentric press offers a con- siderable reduction in the forming force due to the effective utilisation of adi- abatic heating.

(ii) The extrudes are sound and quite satisfactory for engineering application.

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.

175

R e f e r e n c e s

1 H.W. Wagener and K.H. Tampe, Seminar on New Trends in Metal Forming Techniques and Machinery, Department of Mechanical and Production Engineering, National University of Singapore, 1984.

2 H.W. Wagener and K.H. Tampe, VDI Verlag Nr. 101, Dtisseldorf, F.R.G., 1985. 3 K. Lange (Ed.), Hand Book of Metal Forming, McGraw Hill, New York, 1985. 4 P. Venugopal, Report of Short Study Visit to University of Kassel, F.R.G., Department of

Metallurgical Engineering, IIT, Madras, 1985. 5 P.L.B. Oxley, in J. Harding (Ed.), Proc. Conf. Mechanical Properties of Materials at High

Rates of Strain, Oxford, April 2-4, 1974, Inst. Physics, London, 1974, pp. 359-389. 6 P.E. Manning, Corrosion - NACE 39 (3) (1983) 98-101. 7 T.R. Beck, Proc. Int. Corrosion Conf. Series, NACE-3 (1971) 644-651. 8 K. Ebtehaj, D. Hardie and R.N. Parkins, Corrosion Sci., 25 (6) (1985) 415-429. 9 S. Venugopal, MS Thesis, IIT, Madras, India, 1986.