Journal o/Mechanical Working Technology, 16 (1988) 231-242 231 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

ON THE P R O P E R T I E S A N D ECONOMICS OF S I N T E R E D IRON P O W D E R M E T A L L U R G I C A L E X T R U D E S

P. VENUGOPAL*, S. VENKATRAMAN**, R. VASUDEVAN* and K.A. PADMANABHAN*

*Metallurgical Engineering Department, I.I. T., Madras (India) **General Technical Manager, M/S Heat Tech. Engineers, 112 Kamaraj Avenue, A dyar, Madras (India)

(Received January 27, 1986; accepted in revised form March 19, 1987)

Industrial Summary

Cold Hooker extruded iron compacts have been evaluated for density, mechanical and metal- lurgical properties, in the as-extruded, annealed and re-sintered conditions and the results com- pared with those for an equivalent wrought composition. The extent of densification occurring due to cold deformation of the P/M compacts has also been examined.

The commercial viability of the process of cold extruding a P/M preform has been analysed by considering the manufacture of a suitable component by the above process and comparing it with manufacture by the existing conventional process. This evaluation gives an indication of the eco- nomics of the Hooker extrusion process of P/M preforms for the manufacture of a similar family of products.

1. Experimental

Cold Hooker extrusions of sintered iron preforms were performed on pre- forms of different initial densities, different extrusion reductions and different die included angles, using MoS2 paste as the lubricant. The extruded iron pre- forms were examined for various properties.

1.1 Density The density and density variation along the length and across the thickness

of defect-free P / M extruded samples were evaluated. The determination of the density was on a dry basis, computed by dividing the weight of the compact by the calculated volume, the latter being obtained from the O.D., the I.D. and the height of compact. The densities along the length of the extrudes were measured by machining away 5 mm slices and measuring the density of the remaining portion, by which means the mean density of the 5 mm portion which had been sliced off could be calculated. The density variation across the thickness was determined similarly by grinding the O.D. in steps.

0378-3804/88/$03.50 © 1988 Elsevier Science Publishers B.V.

232

1.2 Mechanical properties Defect-free extrudes were evaluated for ultimate tensile strength, yield

strength and percentage reduction in area from Hounsefield tensometer spec- imens, machined from extrudes. The tests were conducted for as-extruded, annealed and re-sintered conditions.

Annealing was carried out at 700 °C for 30 minutes in a protective nitrogen atmosphere. Re-sintering was carried out in the furnace under parameters identical to those for the sintering done on the preforms. (Sintering was done in a continuous mesh belt brazing furnace under a neutral atmosphere (cracked ammonia) under conditions of temperature 1140°C and time 30 minutes.) The sintering and re-sintering of extrudes were carried out at M/S Lucas, T.V.S., Madras, India.

The hardness was measured on a longitudinal section of the extrudes by the Vickers hardness test, using a 2.5 kg load.

The compressive yield strength of the extruded samples was determined from ring-compression tests, the rings being machined from the extrudes. These tests were carried out using a 1000-kN hydraulic press.

1.3 MetaUographic and physical examination All metallographic examination of the P / M preforms and extrudes was car-

ried out within the Metallurgical Laboratory at M/S Sundaram Fasteners, Madras, India (the scanning electron micrograph of the iron powder used was obtained within I.I.T. ). The Magnaflux Crack Detector of this company was used to examine all extrudes for cracks and all the extrudes were evaluated for both surface finish and dimensions within the company's Standards Room.

2. Results and discussion

2.1 Density Figure 1 shows the variation of density of a P / M extrude, the preform having

been compacted at 350 kN load to an initial density of 6.6 g/cm 3. It will be noted that the variation of density along the length of the extruded sample is less than that for the preform. The compact (preform) had a density variation of 1.2% along its length for an LID ratio of 1.5, whilst after extrusion the vari- ation is reduced to 0.5% (ignoring the discard and the un-extruded but t ) .

Figure 1 also details the density variation across the thickness, showing min- imal fluctuation in density.

Table 1 indicates the densities of the extrudes, results being averaged over a large number of samples. A slight increase in the as-extruded densities was observed for P / M preforms having initial densities of 6.6 g/cm 3 as compared to that for those of 6.35 g/cm a. The extent of densification was also found to increase slightly with increasing deformation ratio. No appreciable difference

233

E

k/ 10 20 30 ~0 SO 60 Distance (ram}

Fig. 1. Density of the P/M extrude: the work material is a sintered iron preform compacted at a load of 350 kN to an initial preform density of 6.6 g/cm 3.

in e x t r u d e d d e n s i t y wa s o b s e r v e d for e x t r u s i o n s ca r r i ed o u t w i t h die ang les = 45 ° to 60 °. S ince e x t r u d e d dens i t i e s equa l t o 99% o f t h e t h e o r e t i c a l d e n s i t y were a t -

t a i n e d in s o m e s a m p l e s e x t r u d e d w i t h e = 0.916, scope for a t t a i n i n g dens i t i e s in excess o f 99% exis ts , p r o v i d e d h i g h e r d e f o r m a t i o n ra t ios (e) a re u s e d a n d

TABLE 1

Densities of samples Hooker-extruded from P/M preforms

POr. (g/cm 3) 5.9 6.35 6.6 -- 0.916 0.693 0.916

a ( ° ) 45 -- 7.5-7.6 7.6-7.7

7.7-7.8 60 -- - 7.6-7.7

7.7-7.8

234

Hv me(in

30C

200

100

e=0.916 K=45 ° i ~ {r u , U.T.S. Preform ~ Proof stress density :6"6 g lcm 3 Extrusion I Elongation on G.L = 5"65 ~ , % e density :7 "7-7 .Sg lcm 3 ~'~ Hardness(2"Skg load) H v

• ,. ~ x . s l ~ A . : ! ~ - - B ~ c - - ~ r ~ - - o • N/ram I / 1:::130

] L b ~ I .e

3oo'- - - S ~- - : - - -

2ool- - ~ / _ , _

\ _ kS i i \ : / -- "., ~

--2 - 100 - - Z ---v J ~ - - \ - ~ /

~1 / = M I ! \- v, / \- 5"

I punch

ExptL set-up

)

A-Extruded B-As annealed (700°C 30 rain) C-Sin tered after extrusion(11~O°C. 30 rain) D- Mak (0.05 C steel) extruded and annealed (?O0°C 30 min).

30* r r t 5 , ~

" ( i i rl"

L 2 2 ~ I__ * J exlr MSion preform/

bil let

Fig. 2. M e c h a n i c a l p rope r t i e s o f H o o k e r e x t r u d e d s i n t e r e d i ron p r e fo rms .

better grades of powder (where the hydrogen loss is less than that of the pres- ent electrolytic grade of iron powder) are used.

It is possible to obtain very large LID ratios by the cold Hooker extrusion process, such as would be difficult to obtain by the conventional P / M process- ing route without the occurring of a large density variation along the length of the extrudes: the sole limiting factor in such cases is the loading on the tooling.

2.2 Mechanical properties The hardness, percentage elongation, yield strength and ultimate tensile

strength of a P / M extrude (e=0.916, pore=6.6 g/cm 3, a = 4 5 ° ) in the as-ex- truded, annealed and re-sintered condition are shown in Fig. 2. Annealing of the extruded preform causes a drop in the hardness and the strength and an

235

t ~5 J

o L n . . . . . .

\ ' , . . \ \ \ \ \ \ \ \ \ \ \

0 1 2 3

4$ ,, -)

J a b

L@

Z n

300

20(]

100

50

L O

3

2 3

®

Distance

Fig. 3. Results of a hardness traverse on samples Hooker-extruded where ~=0.916 and o~=45 °, and also of a traverse on the corresponding preform, compacted at 350 kN to a preform density of P0m ---- 6.6 g/cm3: A - - as extruded; B - - extruded and sub-critically annealed at 700 ° C for one hour; C - - extruded and re-sintered at 1140°C for 40 minutes; D - - sintered preform.

increase in the percentage elongation. Re-sintering of the extrudes increases appreciably the ultimate tensile strength and the percentage elongation with- out effecting an appreciable increase in the hardness. A notable feature is the increase in the percentage elongation from 2% in the as-extruded condition to 14% in the re-sintered condition: this is due to the considerable rounding of the pores in the re-sintered condition, as can be seen in Fig. 5 (h) .

The mechanical properties in the re-sintered condition compare very fa- vourably with those of an equivalent wrought 0.05% C steel in the extruded and annealed condition, except for the greater percentage elongation in the case of the wrought material.

236

250

z 200

150 ._~

a .

E 100 o

U

50

6 = 0 . 6 9 3

Yc : 578 N lmm 2

0 0 0 0.5 1.0 0 015 1.0

(ram) (ram)

C r o s s - h e a d traverse

Fig. 4. Results of compression tests on extruded specimens: for e = 0.916 the I.D. = 15 mm and the O.D. -- 22.5 mm, whilst for e -- 0.693 the I.D. -- 23.5 mm; Yc is the compressive yield strength.

The hardness-traverse plots along the length of the P / M extruded sample for the as-extruded, annealed and re-sintered condition are shown in Fig. 3, together with the results of the traverse of a preform. The as-extruded sample (curve A) shows a very large increase in hardness, from 65 HV (preform: curve D ) to 260 HV. After annealing, the hardness drops to around 130 HV and after re-sintering to around 115 HV.

The compressive yield strength of the extruded material was observed, Fig. 4, to be 825 N/ram 2 and 578 N / m m 2 for e -- 0.916 and 0.693 respectively when an initial preform density of Po~ = 6.6 g/cm ~ was used. These results compare very favourably with those obtained by Dower and Miles [ 1,2 ].

2.3 MetaUographic examination The as-sintered microstructures of P / M preforms compacted to loads of 150,

250 and 350 kN, corresponding to initial preform densities of 5.9, 6.35 and 6.6 g/cm 3, can be seen in Figs. 5 (a) to (c) whilst the as-extruded microstructures are presented in Figs. 5 (d) to (f). Fig. 5 (g) shows the microstructure of a P / M extrude from a 350 kN P / M preform (po~=6.6 g/cm 3, e=0.916, o~=45 ° ) which has been annealed, whilst the microstructure of the same sample in the

237

~ ~

i ~

i~i ̧

!~ ~

i~

~ i

!~i ~

~ ~

~ L

~

239

Fig. 5. Micrographs, showing: (a) sintered iron preform where Po = 5.9 g/cm3; (b) as for (a) for po=6.35 g/cm3; (c) as for (a) for po=6.6 g/cm3; (d) as-extruded sample wherepo=6.6 g/cm 3 and

=0.916; (e) as for (d) where po=6.6 g/cm 3 and e=0.693; (f) as for (d) where po=6.35 g/cm 3 and e=0.916; (g) extruded and annealed sample where p=6.6 g/cm 3 and e=0.916; (h) extruded and re-sintered sample where Po = 6.6 g/cm 3 and e = 0.916.

re-sintered condition is shown in Fig. 5 (h) . The extent of rounding of the pores, which has contributed to the increased ductility in the re-sintered con- dition, can be seen in Fig. 5 ( h ).

2.4 Dimensions and surface finish The crack-free extruded P / M samples were found to maintain uniformly

consistent dimensions. The variation in wall thickness of the samples pro- duced was within 0.05 mm, the concentricity 0.1 mm, and the surface finish varied from 2 to 2.5/~m. These results compare favourably with those for wrought cold extrusions, but it must be noted tha t the above results are based

240

on the laboratory samples produced and in no case can reflect the results of actual production runs.

3. Cost comparison between a wrought cold extrude and a P/M preform cold extrude

A typical cold-extruded component used in the automotive starter assembly, i.e. a commutator sleeve, has been analysed for costs, based on the conven- tional cold-extrusion route and on the cold extrusion of a powder metallurgical preform by the Hooker process. The part drawing can be seen in Fig. 6.

Table 2 compares the various cost elements. This preliminary analysis in- dicates a cost benefit for the Hooker extrusion route of 26% over the conven- tional cold-extrusion route. The indirect costs have not been considered in this comparison, only the direct cost having been taken. The cold-extrusion (con- ventional wrought) route consists of 12 processing steps as against only 6 steps in the case of the P / M preform extrusion route. Thus the P / M Hooker process is cheaper and provides an attractive economic alternative, provided that the route can be established for commercial production.

3,:8:,~ -0.15

,J

,0.05

22 .5~

; 7 .0 .00

d i m e n s i o n s a r e in m m

Fig. 6. Commutator sleeve.

241

TABLE 2

Cost comparison between a wrought cold-extruded sleeve and a P/M preform extruded sleeve a

Cold-extruded sleeve Hooker extrusion from P/M preform

No. Processing steps Cost/1000 No. Processing steps Cost/1000 (Rs.) (Rs.)

1. Shear 200 1. Blend 50 2. Phosphate coat and 115 2. Compact 200

lubricate 3. Upset 500 3. Sinter 1000 4. Anneal 250 4. Hooker extrude 1500 5. Phosphate coat and 115 5. Coin 500

lubricate 6. Backward extrude 1500 6. Machine to maintain 200

overall length 7. Tool costs 1200

(amortised over 50,000 for compacting tool, 5000 for extrusion tool, 10,000 over coining tool)

8. Raw material at Rs. 2000 10/kg, {blank wt.200 g)

7. Pierce 250 8. Anneal 250 9 Phosphate coat and 115

lubricate 10. Forward extrude 1500 11. Coin 500 12. Machine to 200

maintain length 13. Tool costs 1500

(amortised over 10,000 parts)

14. Raw material at 2000 Rs.8/kg, (blank wt.250 g)

Total cost for 1000 8995 Total cost for 1000 6650 parts parts

aComponent: Commutator sleeve (Fig. 6), finished weight: 230 g; material: low-carbon steel.

4. Conc lus ions

(1) I t is poss ib le to achieve dens i f i ca t ion in excess of 98 -99% of the theo- ret ical dens i ty in a co ld -ex t ruded iron powder meta l lu rg ica l c o m p a c t by the H o o k e r ex t rus ion process , wi th m i n i m a l v a r i a t i o n of dens i ty a long the length. T h e inf luencing p a r a m e t e r s for the above are the ini t ial p r e f o r m dens i ty a n d the ex t rus ion reduct ion .

T h e ex t ruded length depends p r i m a r y u p o n the tool ing and it is poss ib le by the H o o k e r - e x t r u s i o n t echn ique to ob ta in LID ra t ios wi th ve ry smal l va r i a t ion in dens i ty a long the length: th i s is ve ry diff icul t to achieve b y conven t iona l P / M process ing.

242

( 2 ) The iron P / M extrudes have a strength comparable to tha t of the equiv- alent wrought material, but their ductility is far less. In the re-sintered condi- tion, the strength and the ductility of P / M extrudes increase substantially.

(3) There appears to be a decided cost advantage for the P / M Hooker ex- trusion route, in comparison with the conventional cold-extrusion process, for the manufacture of components.

( 4 ) The cold Hooker extrusion of P / M preforms appears to offer an attrac- tive manufacturing route.

References

1 R.J. Dower and G.H. Miles, The cold extrusion of mild steel billets produced by powder metallurgical techniques, Mod. Dev. Powder Met., 7 (1974) 175-201.

2 R.J. Dower and G.H. Miles, The production of mild steel rings by a combined powder metal- lurgy and cold forging process, Powder Metall., 3 (1976) 141-152.

3 S. Venkatraman, Studies on Cold Extrusion of Sintered Iron Preforms by the Hooker Pro- cess, M.S. Thesis, I.I.T., Madras, India, Jan. 1983.