Journal o f Mechanical Working Technology, 14 (1987) 137--147 137 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

O N T H E A P P A R E N T S T R E N G T H - C O E F F I C I E N T , S T R A I N - H A R D E N I N G

E X P O N E N T A N D B O U N D A R Y - F R I C T I O N C O E F F I C I E N T O F S I N T E R E D

P / M C O P P E R C O M P A C T S

P. VENUGOPAL, S. ANNAMALAI, VIJAY JAGANNATHAN and V. VENKATRAMANI

Department o f Metallurgical Engineering, Indian Institute of Technology, Madras-600 036 (India)

(Received January 24, 1985; accepted in revised form February 20, 1986)

I n d u s t r i a l S u m m a r y

In forming calculations (such as in the estimation of force) in the selection of the machine, the capacity of the motor, the tool and the tool material, and in the estimation of the manufacturing tolerances of formed parts, basic data describing the flow properties of the work metal in the form

y = K .e n

are of great value. The values of K and n are different (from those of wrought parts), when extended to

sintered P/M parts on account of matrix hardening and densification and are commonly known as the apparent strength-coefficient, Ka, and the apparent strain-hardening ex- ponent, n a.

Compacts with wide variation in K a and na with respect to equivalent wrought parts would give rise to difficulties while forming. It is to be expected that compacts prepared at an appropriate green compacting load and pressing rate followed by optimal con- trolled sintering would give values of K a and n a nearer to those comparable with equiv- alent wrought parts: such values could then be regarded as an index of good sound flow properties of the compacts.

Cold forming of P/M parts rivals the expensive hot isostatic pressing and hydrostatic pressing processes from the conservation of energy and easy adaptability points of view. However, cold forming is limited by tool stresses and forces. The friction associated with cold forming is responsible, to a large extent, for the increased tool working stresses. Further, friction influences the densitification of the compacts, as a result of which the values of K a and n a are affected. Higher strain rate and optimal lubrication will reduce the role played by friction.

The present paper is concerned with the estimation of Ka, n a and p for sintered copper preforms by means of the standard ring-compression test.

1. I n t r o d u c t i o n

T h e c o l d f o r m i n g r o u t e f o r m a n u f a c t u r i n g P / M pm' t s a p p e a r s t o be a v e r y

a t t r a c t i v e p r o p o s i t i o n d u e t o t h e a s s o c i a t e d c lo se t o l e r a n c e s a n d f r o m the

e n e r g y c o n s e r v a t i o n p o i n t s o f v iew. T h e w o r k o f ref . [1] i n d i c a t e s t h a t t h e r e

0378-3804/87/$03.50 © 1987 Elsevier Science Publishers B.V.

138

is a 26.5% saving in the cost of manufacturing commuta tor sleeves by the cold forming of sintered iron preforms, with respect to the cold extrusion of wrought parts: the method suggested in the above work [1] is the cold Hooker Extrusion Technique [ 2].

While substantial deformat ion is generally secured by the cold extrusion technique [3], (due to all the three principal stresses being compressive), this method seems to be limited by the tool working stresses and forces. Thus critical examination of means of reducing the tool working stresses during the manufacturing operation deserves attention. In order to be able to assess the forces required to deform a P/M compact , basic quantities such as the strength coefficient, the strain-hardening exponent and the frictional co- efficient are needed: such data can be fi t ted into appropriate equations for the estimation of force during various forming operations.

The strength coefficient and the strain-hardening exponent estimated for P/M compacts account for both matrix strain-hardening and densification (void closure) [1,4]. These quantities, when associated with P/M compacts , are designated as Ka and na. It is to be expected that if P/M compacts are well prepared, the void-closure phenomenon would be minimised and the values of Ka and na would also be close to those of identical wrought parts. Figure 1, from the work of ref. [ 1 ] , shows the values of K a and na of sintered iron P/M compacts obtained by the ring-compression test at room temperature. It was indicated in this work [ 1] that compacts of condi- tion "a" gave sound extrusions since the value of na was close to that of equivalent wrought material. Thus, it would seem advisable that sintered P/M preforms should be tested for their values of Ka and na to afford prior knowledge of the probabil i ty of their forming sound components.

Generally, compacts are prepared at high load fol lowed by controlled sintering. It would be reasonable to think in terms of minimising these two aspects, based on concern for high tool stresses and energy. One of the parameters responsible for increased load during compact ion is the asso- ciated friction, which influences the uniformity of densification across the length of the compact . Non-uniform densification is termed "thinning" and can cause serious problems during subsequent forming. The simplest way to alleviate this situation would be by means of lubrication, but the latter has its own problems. The standard recommended " m o l y k o t e " paste lubricant can cause difficulty during sintering in that its expulsion can occur. Dry lubricants are thus preferable: however, dry lubricants result in a higher friction coefficient.

The other way of reducing the frictional environment and the consequent difficulties, is to think in terms of time of pressing. It is to be expected that increased rate of pressing will ensure less contact t ime between the asperity points of mating surfaces (work metal-- tool metal interface) and increased relative velocity will ensure a lower friction coefficient.

139

600

500

~E 4OO E

Z (...

>- 300

2 0 0 _

Ring compress ion da ta . Lub : Molykote

DETAILS

a

C

d

e- - ~ ~

Sta r t i ng preform densi ty , gms/cc

CURVE o b C

d e

6'75 6"64 6"46 6 "27 5 "82

Compacted at a load of kN

350 350 250 250 150

/ W /

App.strength Icoefft. Ka in

N/ mm 2

610 562 556 501 4B4

App.strain hard. exp.

nQ

0.27 0.28 0 29 0 '30 0 3 9

100 01 0 2 0 3 0 4 0 5 0 6 07 0B 09

True s t ra in 6

Fig. 1. Logarithmic plot of flow curves of sintered iron compacts (after reference 1).

2. Objectives of the present work

2.1 Compaction Based on the aforementioned concepts, an attempt has been made in the

present work to prepare sintered copper preform compacts with different compacting pressures, pressing rates and lubricants so that a higher preform density and minimal thinning can be secured.

2.2 Ring compression test Sliced ring specimens have been tested at room temperature by the stan-

dard ring-compression test, using different pressing rates and lubricants, to estimate the apparent strength-coefficient, the strain-hardening exponent and the boundary-friction coefficient.

3. Experimental details

3.1 Copper powder Electrolytic copper powder has been used for the present investigation.

140

Samples of powder were taken and uniformly mixed in a double-cone mixer along with admixed 2% Zinc Stearate lubricant. Compacts were then made using this material. The chemical composi t ion of the powder is (wt %): Copper 99.0 min; Oxygen 0.5 max; acid insoluble 0.03%; whilst its particle distribution (wt %) in terms of Seive BSS Mesh sizes of +100, +200, +325 and - 3 2 5 is 5, 40--50, 20--30 and 20--30 respectively.

The weight of the green powder taken in all the experiments ranged from 150 to 180 g, for a single piece.

3.2 Compacting tool A floating-type compact ion tool was used, having a hollow cylindrical

container (outer diameter (OD) 50 mm, inner diameter (ID) 30 mm, length 165 mm) with a ground and lapped inner surface, and fit ted with a con- centrically-secured core rod. The hollow top punch (OD 30 mm, ID 14.5 mm, length 160 mm) moves within the container to compact the metal powder, whilst the hollow bo t tom punch (OD 30 mm, ID 14.5 mm, length 130 mm) stays fixed. The container itself is supported by a spring, the compression of which affords relative movement of the container and thus reduces the friction effect o f the container wall.

The tooling was assembled within the platens of a 1000 kN hydraulic deep-drawing press. The known weight of powder was placed in the cylinder bore by hand. A load cell was employed to measure the compacting force and the ejection of the compacts was effected by the main ram itself.

3.3 Sintering Sintering of the green compacts was carried out in a cracked-ammonia

atmosphere. Expulsion of the binder was effected by holding at 500°C, after which sintering was carried out for 1 hour at 900°C.

3.4 Thinning The sintered compacts were sliced along their length by a diamond saw

into 5-mm thick slices. The weight and volume of each slice was measured and consequently the densi ty variation along the length was determined to obtain a "thinning plot" .

3.5 Ring-compression test The ring~compression test consists of upsetting the standard specimen

between two flat dies whilst registering the force and stroke diagrams on a suitable recorder, from which latter are evaluated the true stress and natural strain. Typical diagrams are presented as Fig. 2.

Since the concept of volume constancy cannot be extended to P/M- compact compression (due to void closure), the concept o f constancy of weight has been taken as a basis for computing the instantaneous area. Stage- wise upsetting has been carried ou t on the ring specimens to ob t a in the

141

400 [ i I J I I "-T Co'mp.loocl I ~

kN I ~ ' ~ ~/~_jH :250kNi | 300 ~--~ L_ l D ~)-'j / Comp. time _.4

200

2 100

0 0 1 2 3 mm 5

Punch stroke

4°°1 I t r t

kN t l Compact ][ Iood : 250 kN

300 - Compact time | • " 50 secs

200

o. 100

0 1 2 3

Punch stroke O.D X T,D X H = 30 X 14"5 X 10

I 4 mm

= 30 X 1/,'5 X 10 ! (dimns in mm) (dimrls in ram) . . . .

Start ing preform dens i ty : 7.34 g/cc - - - : ?720 g)cc I Pressing time: 50 secs : 5 secs

Lubricant used: No lub. : No lub i

Fig. 2. Typica l fo rce - - s t roke diagrams for the cold r ing-compress ion of P /M compac t s .

7"8

E

7.6

7.4

7.2

P f

I ~ 1 '5v ~] ! - -

le= o ~

0'C4 0'08 0.12

s tagewise upse l t ing i

. . . . . . e - ; IC 1 '62 IE: n

I

.'-d

vsC on I )ecimens o~ [ o lubrica nt [ - - - ~ec S /

0.16 0 2 0 h 0'24 0"2B 0 3 2 0"36 C = log e

0 40

Fig. 3. Dens i f ica t ion curves.

142

densification occurring with strain: Figure 3 presents typical densification curves. These curves are used for indicating the instantaneous density at different strains. From this instantaneous density the strain, the known weight and the instantaneous cross-sectional area can be evaluated. Consequently, with the help of punch force- stroke diagrams, the true stress can be computed. Figure 4 shows a typical log plot of true stress-natural strain curves obtained this way.

? o o I ~ - ~,:~,,,,,+,-,'m:' 0 . 7 8 - - ~ -t- - - - - ~:---~+~ 600 ---- B - - 398 0"71 ~ . . . . . . . : . . . . . ~" .. . . . ~ +oop_+ + _ , + , o . + o - + . . . . . . . . . . . . . .

4ooL_ o_ +o+ i o.,+ i -+ + - + ~ . + ~ - ~

- - - + + +

-- / compacted oll ~ s~cs?°:o:~r l : : : i i : : t~+ ' t :3:Pt°~ t Is::ds:ncJ 175 kN. B. o--o For no lubricant, compocted with 175kN

50 at 50 secs+ compress ion tested at S0secs. C. ~ For zinc s tearo te l u b r i c a t e d , c o m p o c l e d

with 175 kN at 50 secs jcompress ion tesled at 5 secs. D. x-x For zinc sleorole lubricated, compacted

with 175 kN at 50 secs~ compression tested at 50 sees.

01 0"2 0"3 0"4 0"5 0"6 0.7 0-8 0"9 True s t ra in E

Fig. 4. Typical logarithmic plots of flow curves.

4 . R e s u l t s

4.1 Thinning Figures 5 and 6 show the thinning data obtained for different compaction

loads and pressing rates. The combined effect of higher compaction load and increased pressing rate in minimising thinning will be noted: even for the lowest compacting load of 175 kN, the thinning effect becomes less with increased pressing rate.

143

8-0

u u

E 7'0

>k

( -

OJ a

6 ' C J

Curve. a

curve , b c u rve . c

compacted at a

do do

load of 175 kN (_+ 2-0)

210 kN ( ' t -3 .2 ) ",~ -'. 250 kN (~" 2"1) X X

A

X

O

Compact dimns.

O. D = 3 O m m I . D=15 mm

L : 4 O m m

o - m e a n dens i t y 7.10 gms/cc _ _ b - m e a n dens i t y 7-43gms/cc c - m e a n dens i t y ?.,47gms/cc

- - X c

. . . . ~ J

O

-#

0 5 10 15 20 25 30 35 40

D is tance from punch face, mm

Fig. 5. Density variation along the length ( " th inn ing" curves) for a pressing t ime of 5 s.

8 - 0

u

E O~

.7"0

t -

OJ

6-0

Curve a - c o m p a c t e d at a load of 175 k N (2"4 -+) o o

Curve b do 210 kN (3"3- + ) t, .',

Curve c do 250 kN (1-0 "1") X X

X

___/ C 0

I

c mean dens i tyT.6

. ~ a n dens i ty i-O

a mean density6-63

x

o

V / / / / / / / / / / A I

---~inl~red c~m~c t 15. '30 ' - pa j / / / / / / / / / / J

L 4 0 f I r ....

15 20 25 30 D i s t a n c e f r om punch f a c e . m m

10 35 40

F igs in b racke ts i n d i c a t e p e r c e n t a g e v a r i a t i o n of d e n s i t y over mean d e n s i t y across the length.

Fig. 6. As for Fig. 5 but for a pressing t ime o f 50 s.

144

Although the percentage variation of thinning for 210 and 250 kN load at a pressing time of 5 s appears to be larger than for 175 kN load at a pressing time of 50 s, the higher mean preform<lensities of the compacts under these conditions can assure a sound crack-free post-forming operation. As outlined in the introduction, a lower pressing rate at all compaction loads promotes a trend towards a greater thinning effect and a lower mean preform density, due to higher friction constraints, see Fig. 6.

Based on the desirability for a lower pressing load and a higher mean preform-density (overlooking the marginal f luctuation of percentage varia- t ion of thinning that would arise from minor variations in the compacting loads, see Fig. 5), the optimal compact within the present experimental range would be tha t corresponding to a pressing load of 210 kN at a pressing time of 5 s.

4.2 True s tress--natural strain curves and values o f Ka and n a A representative diagram of a logarithmic plot of flow curves is presented

in Fig. 4. All the values of K a and n a determined for different experimental condit ions have been entered into Table 1. Figure 7 from the work o f ref. [5] is presented for comparison of values of Ka and n a with values of K and n for equivalent wrought parts: strikingly, the values of Ka and n a of P/M copper compacts are the larger, but this is to be expected, due to the densification occurring during compression testing, as outlined in the Intro- duction. Such wide differences have also been encountered in the work o f

5O0

E400 E

3oo >."

250 QJ t-

u~

o U_

100

Mater ia l :Wrought copper(electrolytic) ~ J ~ , = ~ - ~ ' Lubricants: o-o Zinc s tearate

x--x No lub r i ca t ion _ ~ . . ~ " ~

Temperature: 32°C (R T) r - - Strain rate: 0"2 ~1 T ~ Z ~

(- 5 secs ) t j>,,'~ ~ i

ZINC NO S t rength STEARATE LUSRICATIo N Coefficient K 436 N/ram 2 z~3 N/turn2

I by Strain harde- L solid com-Ii nincj expo- L pression I nenl : n 0.22 0 ,25

L ~ a f ter VenugopaI,P i I i I 1 ( 5 )

50 - t - - i i . . . . . L T )'02 0"03 0"05 0"1 0"2 0"3 0"4 0"6 0'8

True S t ra i n E

Fig. 7. Logarithmic plot of the flow curve for wrought copper (after reference 5).

TABLE 1

Experimental values of Ka, n a and tL (vide ring compression test)

145

Slice Compaction Compaction Compression Lubricant n a K a number load time time (N/mm 2 )

(kN) (s) (s)

1 175 50 50 NL 0.71 398 0.21 2 175 50 5 NL 0.78 513 0.19 3 175 50 50 ZnSt 0.73 309 0.15 4 175 50 5 ZnSt 0.80 363 0.14 5 175 5 50 NL 0.71 457 0.16 6 175 5 5 NL 0.71 537 0.19 7 175 5 50 ZnSt 0.71 457 0.13 8 175 5 5 ZnSt 0.70 479 0.11 9 210 50 50 NL 0.63 468 0.21

10 210 50 5 NL 1.00 501 0.18 11 210 50 50 ZnSt 0.61 380 0.14 12 210 50 5 ZnSt 0.75 562 0.13 13 210 5 50 NL 0.77 550 0.16 14 210 5 5 NL 0.75 631 0.17 15 210 5 50 ZnSt 0.85 468 0.13 16 210 5 5 ZnSt 0.69 501 0.11 17 250 50 50 NL 0.74 525 0.16 18 250 50 5 NL 0.80 692 0.17 19 250 50 50 ZnSt 0.75 562 0.12 20 250 50 5 ZnSt 0.82 617 0.10 21 250 5 50 NL 0.73 589 0.16 22 250 5 5 NL 0.75 617 0.16 23 250 5 50 ZnSt 0.73 513 0.12 24 250 5 5 ZnSt 0.74 562 0.12

NL -- No Lubricant ZnSt - Zinc Stearate

ref . [ 1 ] , see Fig. 8. Whi l s t the va lues of K a n d n for 0 .1% c a r b o n s teel in

w r o u g h t f o r m are a p p r o x . 250 N / m m 2 and 0 .2 r e spec t ive ly , t he va lues o f K a a n d na for s i n t e r ed i r o n P /M c o m p a c t s a p p e a r t o be 490 N / m m 2 a nd 0 . 3 8 r e spec t ive ly .

4.3 Boundary friction coefficient t~ T h e u p s e t r i n g - s p e c i m e n s were m e a s u r e d for change in i n t e r n a l d i a m e t e r

a n d r e d u c t i o n in h e i g h t f o r t he e s t i m a t i o n o f p in c o n s u l t a t i o n w i t h s t a n d a r d

t~ c a l i b r a t i o n cha r t s [6] , see Fig. 9. Based o n the l owes t va lue o f a p p a r e n t s t r a i n - h a r d e n i n g e x p o n e n t na ( the

lowes t va lue be ing a n i n d i c a t i o n t h a t the c o m p a c t s are c loser t o e q u i v a l e n t w r o u g h t ma te r i a l s ) t he c o m p a c t s p r e p a r e d w i t h 2 1 0 kN press ing l oad a n d a c o m p a c t i o n t i m e o f 5 s a p p e a r t o be the o p t i m u m . R e f e r e n c e to T a b l e 1 i n d i c a t e s t h a t the l o w e s t va lue o f t he f r i c t i o n c o e f f i c i e n t o f 0 .11 is o b t a i n e d for these c o n d i t i o n s .

146

('M

E E

>,-

O

h

I L , I I IMati:Sintered iron i I | I

5 0 0 1 - - - - - - - - - - [ ~ p o w d e r - - 1 -- i --~-~

I ! , r L... I , ITemp: 32°C (R T ) [ i ~ ! I IS t ra in r u e : ~ 50 s e c s J ~ ! '

] / 1 i ] I J I ~ . ~ " ~ a, f ter V e n k a t r a m a n , S . ( 1 )

, ] I : , , I , 1 1 i J ~ I A p p a r e n t s t r e n g t h _

. . . . . . J c o e f f i c i e n t K o - :490N/n ' lm z ~ oy r~ng com- . . . . J p ress ion J , A.pp.arent s t ra in . I 1 ] harde, . ing exponen t %:0'36

0'1 0"2 0"3 0'4 0"5 0"6 0 '7 O0 0-9 True strain C

Fig. 8. Logarithmic plot of the flow curve for a sintered iron-powder compact (after reference 1).

It is to be expected that compacts prepared with a compaction load of 210 kN and a pressing time of 5 s would ensure least thinning. Such com- pacts, if cold formed with Zinc Stearate lubrication, would ensure low tool stresses and crack-free post forming: work to confirm this supposition is currently in progress.

5. Conclusions

(1) The best condition in the present work -- based on least thinning and higher mean preform density -- for sound post forming appears to be 210 kN compaction force with a time of pressing of 5 s.

(2) The optimal strain rate and boundary lubrication environment to yield values of Ka and na compatible to those of equivalent wrought material is a compaction load of 210 kN at a pressing time of 5 s. The lowest value of p has been obtained with Zinc Stearate, compression tested at 5 s pressing time. These conditions give an apparent strength-coefficient Ka of 501 N/mm 2, a strain-hardening exponent na of 0.69 and a value of p of 0.11: these observations are complimentary to those of COnclusion (1), It is recommended that post forming should be carried out on such compacts at this rate of deformation.

(3) Values of Ka, na and p for sintered copper compacts have been tab- ulated for application.

147

70 I { ' ' l

EXPERIMENTS

60 • RINGS UPSET OF ROOM & / ) , 0 TEMPERATURE

• RINGS UPSET OF 4276c

50 THEORY: ~ = CONSTANT

w u

IIJ

~" 3 0 -

o 20

m X

"~ I0 ~-

~ o - z 0.15

~o o.,o

20

I I

0 10 20 30 ~0 50 60

REDUCTION IN HEIGHT t PERCENTAGE

70

Fig. 9. Theoretical calibration curves for the standard '6 :3:2 ' ring, for an ideally plastic material, together with results from ring-compression tests.

R e f e r e n c e s

1 S. Venkatraman, Hooker Extrusion of Sintered Iron Preforms, M.S. Thesis, IIT, Madras, 1983.

2 C.I.R.P. Production Engineering Dictionary, Vol. 5, Cold Forging and Cold Extrusion, Verlag W. Girardet, Essen, Definition Number 5126.

3 K. Lange, Lehrbuch fur Umform Technik (Textbook for Metal Forming), Vol. II, Springer Verlag, Berlin, Heidelberg, New York, 1974 (in German).

4 K. Obara, Y. Nishino and Y. Saito, The cold forging of ferrous P/M preforms, Modern Developments in Powder Metallurgy, Metal Powder Industries Federation, Princeton, N.J., 1974, Vol. 7, p. 423.

5 P. Venugopal, Effect of Friction and Temperature on the Combined Forward and Backward Can Extrusion, Ph.D. Thesis, IIT, Madras, 1982.

6 A.T. Male and M.G. Cockcroft, A method of determination of the coefficient of friction of metals under conditions of bulk deformation, J. Inst. Met., 93 (1964- 65) 38 -46 .