pm aluminum camshaft belt pulleys for automotive engines

PM Aluminum Camshaft Belt Pulleys for Auto motive Eng i nes

G. Jangg, H. Danninger, K. Schroder, K. Abhari, H.-C. Neubing*, J. Seyrkammer**

Light and wear resistant Al camshaft belt pulleys for engines were produced from standard Al powder mixtures with addition of hard particles. Pre-screening tests of numerous inclusions showed 10 mass% ZrSi04 to be the best choice. Full size belt pulleys were produced by pressing, sintering at optimized parameters, and sizing. Wear testing was carried out on pill-disc testers and special belt pulley testbeds. Both dimensional and wear properties were found to be well comparable to those of conventional iron pulleys, with the exception of some attack on the belt by protruding particles after prolonged testing.

Sinteraluminium-Zahnriemenrader fur Kraftfahrzeugmotoren

Leichte und gleichzeitig verschleififeste Zahnriemenrader auf Al- Basis fur Motoren wurden aus ublichen Al-Pulvermischungen rnit Zusatz von harten Teilchen hergestellt. Beim Vergleich der Ein- lagerungen ergab sich der Zusatz von I0 Masse% ZrSi04 als optimal. Aus der entsprechenden Mischung wurden Zahnriemenrader gepreBt, unter optimierten Bedingungen gesintert und kalibriert. Fur die VerschleiRprufung wurden sowohl Stift-Scheibe-Gerate als auch speziell entwickelte Zahnriemenradprufstande verwen- det. Die Al-RBder waren sowohl von den Dimensionen als auch der VerschleiBbestBndigkeit her durchaus mit ublichen Sintereisen- radern vergleichbar, mit Ausnahme eines gewissen VerschleiRes des Riemens durch herauspolierte Teilchen nach sehr langen Priif- zeiten.

1 Introduction

In many modern automotive engines, the camshaft is rotated by the crankshaft via a belt drive. The belt pulleys are generally produced by powder metallurgical techniques. They are not heavily loaded mechanically in service, but there are strict requirements towards dimensional, in particular out-of-roundness, tolerances, typically 0.03 mm. Low al- loy PM iron of moderate density (SINTC) is usually employed, and sizing is done to attain the necessary dimensional tolerances. If necessary, abrasion resistance can be improved by steam treating.

Replacing the iron belt pulleys by such consisting of lighter material would result not only in gross weight saving. Also the rotating masses of the camshaft drive could be reduced which would be beneficial during each change of r.p.m.s, would improve the acceleration, and would reduce the stress exerted onto the belt. Pulleys pressed from steel sheet have not found large scale applications because of insufficient dimensional tolerances. With plastics pulleys, other problems such as in- tolerable wear, large thermal expansion and excessive noise have been encountered. Pulleys produced from PM aluminium seem to be a more promising alternative.

Sintering of precision parts from Al alloys can be regarded state-of-the art today, parts with dimensional tolerances com- mon in powder metallurgy being mass produced 11-31, However, P M aluminium alloys are sensitive towards abrasi-

Technische Universitat Wien, Institut fur Chemische Technologie anorganischer Stoffe, A- 1060 Wien, Austria * ECKART-Werke, D-90763 Furth, Germany. ** MIBA Sintermetall AG. A-4655 Vorchdorf, Austria.

ve wear due to their low hardness. The wear resistance of the surface has been improved by an anodizing treatment, but special P M powder mixtures had to be developed, and anodizing has been found to be a very expensive process. Thus, these anodized pulleys are employed only in sports cars in small numbers where they show excellent performance [4].

The wear resistance could also be improved by introduction of hard particles into the matrix. Experiments were carried out with “Tribaloy” powder [5 , 61, i.e. Co-Mo-silicide of high hardness and toughness and excellent abrasion resistance [7, 81. The price of Tribaloy was however found prohibi- tive. Subsequently, at TU Vienna in cooperation with the in- dustry experiments were carried out to select cheap abrasion resistant powders, to introduce them into the matrix by pressing and sintering and to check the respective properties [9].

2 Materials Selection

For selecting the most suitable combination of PM A1 matrix and hard particles, several requirements have to be met

- Sufficient hardness and abrasion resistance as well as shear and fracture strength of the inclusions

- good bonding and adhesion to the matrix - Only marginal deterioration of pressing and sintering

behaviour of the Al powder mictures by the inclusions, low tool wear during pressing

- Sufficient mechanical strength and wear resistance of the sintered composite material

- Hard powders should be available at low price and in con- sistent quality

- Low density of the inclusions

[lo]:

Mat.-wiss., u. Werk\tofftech. 27, 179-1 89 (1996) 0933-5137/96/0404-0179$10.00 + .25/0 179 0 VCH Verlagsgesellschaft mbH, D-6945 1 Weinheim, 1996

- It is generally agreed that particles of 50.. 100 pm diameter and spherical shape are most suitable. Too small inclusions are ineffective towards wear resistance, since fine particles are removed during wear loading together with the metallic fines and are not re-embedded into the surface to form hard layers. Too large particles are easily torn out of the matrix and thus deteriorate the wear resistance.

\ FeSi45 5 0 prn

3 Pre-screening of the Inclusion Materials

I

As stated above, sintering of Al materials without inclusions is well established today. Several ready-to-press powder mixtures are commercially available. For the work described here, Alumix 123 supplied by Eckart was used which is an Al-Cu-Mg-Si mixture containing 1.5 mass% pressing lubricant. The manufacturer recommends compaction at 250..350 MPa, de-waxing (lubricant burnout) at 420°C and then, after rapid heating, 20 min sintering at 595 & 2.5 "C. As-sintered (Tl ) tensile strength of 200 MPa at 3% elongation can be attained. By precipitation hardening (T6) strength can be raised to 300..350 MPa, at some loss of ductility.

Addition of hard particles strongly affects both the sintering behaviour, esp. dimensional change, and the properties. Volume fraction and particle size of the inclusions are critical parameters. For judging the suitability of various hard powders, appropriate sintering conditions had to be elaborated for each individual system.

Based on the criteria listed above, cheap and readily available hard powders were tested such as corundum, SIC (a com- mon component in particle reinforced A1 and also tested for pulleys [ 1 I]), glass spheres and glass fragments, zircon sand, steel powders 3 16L and 430L, and FeSi. Tribaloy was inclu- ded in the tests, too [ 12, 131. A main point of interest was the admixing of the hard phase to the Alitmix with respect to homogeneity and low tendency to segregation.

3.1 Pressingkintering behaviour and mechanical properties

The basic mixture Alumix 123 is well compactible. When pressing standard tensile test bars at 300 MPa, a green density of 2.63..2.64 g . cm13 is attained (i.e. 97.8% of theoretical if the lubricant is taken into account). Metallic inclusions affect the residual porosity of the green compacts less than ceramic ones (see Fig. l a , Ib) . As expected, the compactibility is adversely affected by larger amounts of hard phase. Fig. I in- dicates that fine particles are more detrimental than coarse ones and spattered or angular powders are more harmful than spherical ones.

Dimensional behaviour during sintering is particularly affected by the inclusions. It depends not only on the volume fraction but also on particle size and shape of the inclusions (Fig. 2, depicting dimensional change relative to the green compact). Generally the specimens exhibit higher expansion or lower shrinkage compared to inclusion-free reference samples. Metallic inclusions, esp. 430L and FeSi, result in large expansion (Table I ) which is caused by formation of diffusion zones as visible from Fig. 3. The brittle phases at the interfaces also deteriorate strength and especially ductility of the specimens (Table 2). Sintering at lower temperatures

9 8 - - FeSi95 150 prn

a \ 95 FeSi45 5 0 prn -

FeSi95 150 prn

150 prn

U

a 95

I I I I I I

Inclusion content wt% a 5 10 15 20

98

ae 96

x u) c m

c ._

94 > .- c - a, a

92

alass 50-100 urn

t 90 I I I I I I

5 10 15 20 25

b Inclusion content wt%

98 t ae x v)

a, U

c ._

m 96 .- c - m a

9 4 I I

5 10 15 20

C Inclusion content wt%

Fig. 1. Relative green density of Alumix 123 containing inclusions (Compacting pressure 300 MPa) Abb. 1. Relative Griindichte von Alumix 123 mit Einlagerungen (PreBdruck 300 MPa) la: Metallic inclusions (90-160 pm)

lb: Ceramic inclusions (63-90 pm)

lc: Glass/ZrSiO4

Metallische Einlagerungen (90-1 60 pm)

Keramische Einlagerungen (63-90 pm)

Glas/Zirkonsand

and for short times slows down the interfacial reaction and thus decreases the expansion but the thus insufficiently sintered matrix results in poor mechanical strength, either (Fig. 4). As will be shown below, also the wear resistance is adversely affected by the brittle interfacial layers. Metallic inclusions were therefore not proceeded with in the further work.

180 G. Jangg et al. Mat.-wiss. u. Werkstofftech. 27, 179-189 (1996)

+ 0.4 I I I -rounded 50-100 brn I

5 10 15 20 25

a Inclusion content wt%

.c - 0.2 s al

- 0.3 c

E 6 - 0.4

-125 pm

-160 pm

- 0.5 1 I I I I I 1 5 10 15 20 25

b Inclusion content wt%

I I I I I I

5 10 15 20 25 C Inclusion content wt%

Fig. 2. Linear dimensional change (relative to green compact) during sintering of compacts containing inclusions (compacted 300 MPa, sintered at 620 "C) Abb. 2. MaRanderung (bezogen auf das GrunlingsmaR) beim Sin- tern von PreBlingen mit Einlagerungen (PreRdruck 300 MPa, gesintert bei 620 "C) 2a: glass powder; sintered 25 min

Glaspulver; Sinterzeit 25 min 2b: ZrSi04; sintered 10 min

ZrSi04; Sinterzeit 10 min 2c: SIC; sintered 10 min

SIC; Sinterzeit 10 min

Table 1. Dimensional change (relative to green compact) during sintering of compacts containing metallic inclusions. (Alumix 123, compacted at 300 MPa, sintering time 10 min) Tabelle 1. MaBInderung (bezogen auf das GriinlingsmaB) beim Sintern von PreBlingen mit metallischen Einlagerungen (Alumix 123, PreBdruck 300 MPa, Sinterzeit 10 min)

I n c l u s i o n Type amount shape fraction

mass% Pm

Sintering relative dimensional temp density change "C % % linear

430L 7.6 316L 3.6

7.6 7.6

FeSi45 13.4 FeSi75 17.8 FeSi95 9.6

19.3

spattered spherical spherical spherical spherical spattered spattered spattered

125-160 125- 160 9@ I25

125-160 35-75

125-160 125-160 125-160

585 590 590 590 590 580 590 590

45.5 88.7 88.2 88.2 84. I 85.6 84.6 87.6

+ 13.5 + 0.33 + 0.22 + 0.12 + 0.52 + 2.06 + 2.30 + 2.40

Table 2. Properties of sintered Al containing metallic inclusions. (Alumix 123, compacted at 300 MPa, sintering time 25 min) Tabelle 2. Eigenschaften von Sinteraluminium mit metallischen Einlagerungen (Alumix 123. PreBdruck 300 MPa, Sinterzeit 25 min)

I n c l u s i o n Sintering Tensile strength Elongation Type amount shape fraction temp T1 T6 TI T6

mass% Pm "C MPa %

316L 3.8 spherical 125-160 590 170 23 1 1 .5 0.8

7.6 spherical 125-160 590 1 50 202 1 .5 0.8

FeSi75 17.8 spattered 125-160 580 88 123 2.3 0.7 FeSi95 9.6 spattered 125-160 590 91 81 2.3 0.4

19.3 spattered 125-160 590 85 59 0.4 0.4

7.6 spherical 90-125 590 176 224 1 .5 0.8

FeSi45 13.4 spherical 35-75 590 115 - 0.5 -

Mat.-wiss. u. Werkstofftech. 27, 179-189 (1996) Camshaft Belt Pulleys 181

Fig. 3. Metallographic sections of samples containing 3 I6L, 3a: sintered 5 min 580 T, 3b: sintered 5 min 6 10 "C Abb. 3. Schliffe von Proben mit 316L, 3a: gesintert 5 min 580"C, 3b: gesintert 5 min 610°C

When using non-metallic inclusions, the dimensional change can be kept low by appropriate sintering. The opti- mum parameters depend not only on the inclusion type but also on volume fraction, particle size and shape (Fig. 5). Ge- nerally, dimensional change is shifted towards expansion by larger inclusion content and lower particle size. Spherical particles are less effective than spattered ones. Compared to plain Alumix 123, mixtures containing inclusions should be sintered at higher temperatures to maintain dimensional stability and satisfactory mechanical properties. Nevertheless strength and ductility remain somewhat lower with the composites than with plain sintered A1 (Table 3).

3.2 Wear testing

Compared to plain sintered Al the composite materials should have much superior wear, i.e. abrasion, resistance to be suitable for belt pulleys. Generally, wear testing is proble- matic, since results really significant for a given application can be obtained only by practical tests or by such that simulate the actual wear conditions closely. However, for a first screening of the various sintered materials, a conventional pin-disc tester was regarded suitable. The disc was a resin bonded S i c grinding disc (Tyrolit C 250 BE 24); normal force was 10 N at a specimen cross section of about 1 cm2 and a sliding velocity of 0.58 m . s- ' [ 141. According to basic tribological rules, in the range of stable wear a wear coefficient K can be calculated

Table 3. Properties of Al sintered materials containing ceramic inclusions. (Alumix 123, compacted at 300 MPa, sintering time 25 min) Tabelle 3. Eigenschaften von Sinteraluminium mit keramischen Einlagerungen (Alumix 123, PreRdruck 300 MPa, Sinterzeit 25 min)

_____

Inclusions Sintering Fraction 25-50 Fm Fraction 50-100 p Type amount temp U T S elongation U.T S elongation

TI T6 TI T6 TI T6 T1 T6 macs% "C MPd YO MPa YO

Glass 5 590 151 224 2.2 I .o 153 226 2.1 I .6 spheres 620 129 190 4.0 1.8 140 204 2. I 0.8

10 590 I27 166 1.9 0.9 I36 172 1.6 1 .o 600 127 152 2.0 1.4 133 179 1.5 0.8 610 128 148 2.8 2.0 127 170 1.5 0.8 620 I20 147 4.0 2.2 122 I70 1.3 0.8 630 105 128 4.3 2.8 -

20 590 75 100 1.7 0.9 94 I27 I .o 1 .0 600 76 98 1.8 1.2 102 124 I .o 0.9 610 7s 96 1.9 1.3 90 121 I .2 0.9 620 73 89 2.3 1.3 85 I l l I .2 0.9 630 66 88 2.1 1.2

Glass 5 590 121 156 2.2 0.7 149 200 I .a 1 .0 620 99 153 2.3 0.7 129 I79 2.7 1.2

10 590 65 87 0.8 0.3 106 146 1.6 0.6 620 77 110 1.6 0.4 96 151 2.7 0.7

20 590 22 37 0.5 0.3 48 69 0.9 0.4 620 29 46 0.5 0.4 45 65 1.2 0.4

~ - -

- - - -


200 m n I

C, 150 a, C 2

= 100

c. 0

a, v) c F

50

a

~~

r l state

O \ - 0 - 4

/OAlumix 123 0,

Alumix 123 + 316 L

0 /

+ FeSi 75 \ Alumix 123 a 1

I I I 1 I

570 580 590 600 610 Sintering temperature O C

+ 0.4

+ 0.2 !i s .- -

0.0 (D a, c m u - 0.2 - m c 0 .-

- 0.4 a, E 0 .-

- 0.6

25-50 spherical --.

50-100 prn -

10 %

5%

- 5 %

- 0.8 1 V 10% \

300

250 a z 5

200 u) c

150 v)

a, - .- 2 100 s

50

b

I 1 1 I I I

590 600 610 620 630

r 6 state

\ Alumix 123

+ 316 L

1 Alumix 123 + FeSi 75

0

I

570 580 590 600 610

Sintering temperature OC

i- "\ 20 % \

. .O> 0,

\A \ . \ \ 10% \ \

\

\ \

\ , '10%

. -0 &hv- '+ 0 2 0 %

\ A-

'+* 5%

-- 25-50 prn l- 50-100 prn angular

t I I L I I I

590 600 610 620 630 Sintering temperature OG

K = Am/Q.F.S

K . . . wear coefficient [mm' . N-' . m-'1 Am . . . weight loss [mg] e . . . density of the material [g I crnP31 F . . . normal force [N] S . . . sliding distance [m]

Ideally, K is independent of the normal pressure and the sliding velocity. Preliminary tests showed that this condition is reasonably fulfilled here due to the moderate testing conditions which do not result in excessive heating of the specimens. A run-in period with initially increased wear was observed in some cases but did not exceed 10 min. This period

Fig. 4. Tensile strength of specimens containing 20 mass% metallic inclusions (90- 125 pm) as a function of the sintering temperature (Compacted 300 MPa, sintered 10 min) Abb. 4. Zugfestigkeit von Proben mit 20 Masse% metallischen Einlagerungen (90- 125 pm) in Abhlngigkeit von der Sinter- temperatur (PreBdruck 300 MPa, Sinter- zeit 10 min)

Fig. 5. Dimensional change during sintering of A1 containing glass inclusions as a function of sintering temperature and inclusion size Abb. 5. MaBlnderung beim Sintern von A1 mit Glaseinlagerungen in Abhangigkeit von Sintertemperatur und Glasfraktion

was ignored; the wear coefficient was calculated only from the period of stable wear.

Wear was found to decrease with increasing hardness and volume fraction of the inclusions (Fig. 6). In agreement with tribological experience, fine inclusions (< 20 pm) are ineffective, as are also very large ones (> 150 pm). In between, the particle size apparently is of marginal importance.

Based on the results gained, zircon sand of about 90 pm mean particle size was selected as the most suitable material for the inclusions. It is fairly hard but not too brittle and is near spherical which reduces tool wear during pressing. It can also be obtained in various fractions at low price. It can be easily admixed and after sintering adheres well to the matrix due to formation of very thin, microscopically invisible interfacial layers. Finally, the wear resistance obtained is well above the average.


1

1.5

e z 1.0

0 0

.!= m 0 0.5 E ._ D

0.0

Fig. 6. Wear coefficients measured o n a pin-disc tester. Different inclusions, resin bonded Sic disc, dry run, FN = 10 N, v = 0.58 m . s - ' , s = 2000 m Abb. 6. VerschleiRkoeffizient, gemessen am Stift-Scheibe-Pruf- stand im Trockenlauf gegen kunstharrgebundene Sic-Scheibe

.

.

.

.

4 Al camshaft belt pulleys

I

4.1 Sintering and mechanical properties of mixtures containing ZrSiO4

m n z

5

E - (u 5 0 '

100

m c c fn

._ ln c

t"

After the most suitable composite material had been found to be Alumix 123 containing 10 mass% ZrSi04 of about 90 pm particle size, sintering and mechanical properties were optimized using tensile test bars as well as full size belt pulleys (Fig. 7).

Sintering was done in a pusher furnace with a heating zone of 2 m length and locks at both ends to ensure clean atmo- sphere. High purity nitrogen (dew point < 50°C) produced from liquid nitrogen was used as protective gas. It was found advantageous to carry out de-waxing as a separate step (30 min at 380 ... 400 "C). After de-waxing the furnace was care- fully cleaned and wax residues were removed before sintering

'

Fig. 7. Sintered Al camshaft belt pulleys Abb. 7. Gesinterte Al-Zahnriemenrader

-

-

-

-

was carried out. De-waxing and sintering in one run, in con- trast, resulted in back-diffusion of the lubricant during sintering and, due to the shortness of the heating zone, in contami- nation of the specimens. This in turn caused higher expansion and poor mechanical properties.

Careful de-waxing was essential in particular with the large pulleys weighing approx. 160 g and containing 1.28 mass% HWC. It was checked in all cases by weighing the de-waxed specimens. Together with the pulleys tensile test bars were sintered in the same boat, and the effect of wax residues could thus be investigated (Fig. 8). Even small amounts of residual wax adversely affected expansion and mechanical properties.

Based on the experiments described above, the specimens were compacted at 300 MPa and sintered for 20 min. Com- pared to plain Alumix 123 the composites are less sensitive towards the sintering parameters. Sintering can be done in the range 60 5... 625 "C without any marked change of tensile strength and elongation (Fig. 9). At higher temperatures the expansion decreases, and dimensional stability thus can be adjusted by varying the sintering temperature. Only at temperatures above 635 "C strength and elongation start to fall off, pronounced expansion occurs, and rounding of the edges and loss of shape are observed because of too much liquid phase being present.

Sintering time and volume fraction of ZrSi04 are fairly un- critical, too (Figs. 10 and 11). The strength of the material can

2.0

* 1.5

E - c 1.0 5

0 7

0.5

0 B ko

O O\

I I I I I I

I I I I I 01 1 0.0 0.8 0.9 1.0 1 . 1 1.2 1.3 1.4

b Weight loss %

Fig. 8. Dimensional change, tensile strength and elongation of tensile test bars as a function of de-waxing effectivity Abb. 8. MaRanderung, Zugfestigkeit und Bruchdehnung von Zug- proben in Abhangigkeit vom Entwachsungsgrad


be increased by precipitation hardening, i.e. water quenching from the sintering temperature and subsequent natural or ar- tificial aging; however the tensile strength seems to be of mi- nor importance for belt pulley materials (although precipitation hardening might change the wear resistance of the matrix).

240

a 220

200 !ii f

E 0

180 c

6 + 0.6

ae

m

0 + 0.2 - E - E

- .- - -

+ 0.4 - -

m S

0.0 -

Ts 605OC -

O - O - ~ ~ T ~ - /

/ o

I '- T4 0 4 0 - -

/' -

\

\ O\

120 t \ I

1.2 t-

0 7 5 10 12.5

ZrSi04 content wt%

Fig. 11. Dimensional change, tensile strength and elongation of tensile test bars as a function of the ZrSi04 content (sintered 20 rnin 605 "C, TI) Abb. 11. MaBanderung, Zugfestigkeit und Bruchdehnung von Zugproben in Abhlngigkeit vom ZrSi04-Gehalt (Sinterung 20 min bei 605 "C, TI)

590 600 610 620 630 640 650 Sintering temperature O C

Fig. 9. Dimensional change, tensile strength and elongation of tensile test bars as a function of the sintering temperature (Sintering time 20 mip) Abb. 9. MaBlnderung, Zugfestigkeit und Bruchdehnung von Zug- proben in Abhlngigkeit von der Sintertemperatur (Sinterzeit 20 min)

15 20 25 30 35 40

Sintering time rnin.

Fig. 10. Tensile strength of specimens containing 10 mass% ZrSi04 as a function of the sintering time Abb. 10. Zugfestigkeit von Proben mit 10% ZrSi04 in Abhangig- keit von der Sinterzeit

4.2 Dimensional change of the pulleys during sintering

Since the dimensional requirements for belt pulleys are very strict, iron pulleys are usually sized. For A1 pulleys sizing is still more important, since their dimensional stability during sintering is inherently inferior to that of iron ones. Even to enable sizing, the sintered Al pulleys have to meet reproducible tolerances. In order to optimize the sintering temperature with respect to dimensional stability and sizeab- ility, 10 pulleys each were sintered at different temperatures and precisely measured (schematically depicted in Fig. 12). The dimensional values obtained and the respective scatter are given in Fig. 13, the dimensional change here being based on the die dimensions.

It showed that higher sintering temperature increases the shrinkage but also the distortion and out-of-roundness. It was also observed that the pulleys tend to deform conically due to gravity distortion. For obtaining the necessary dimensional stability, 20 min sintering at about 605 "C was found optimal. At this temperature I40 MPa tensile strength and 1.5% elongation are measured while distortion is still moderate.

Various measures such as adjusting different green density at perimeter and center, covering the boats to improve temperature homogeneity, or inserting the green pulleys into moulds did not improve the sintering behaviour. The dimensional stability of conventionally - i.e. lying flat on tiles - sintered pulleys was sufficient to enable sizing despite the slight conicity found even after sintering at 605 "C. The elongation of the Al-


Fig. 12. Characteristic dimensions of the pulleys D,: diameter at the center of toothed rim

(die: Dm = 113.8 mm) Do: diameter at upper edge of toothed rim Du: diameter at lower edge of toothed rim AD,,,: dimensional change during sintering, measured at D,, re-

lative to die dimension D,-D": difference in diameter between upper and lower edge

(conicity) AD: Out-of-roundness Abb. 12. Wesentliche Abmessungen der Zahnriemenrgder D,,,: Durchmesser in der Mitte des Zahnkranzes

(Matrize: D,,, = 1 13.8 mm) Do: Durchmesser am oberen Rand des Zahnkranzes Du: Durchmesser am unteren Rand des Zahnkranzes AD,,,: Mal3anderung beim Sintern, gemessen an D,,, bezogen auf

das MurrizrnmalJ D,-D,: Differenz des Durchmessers oben und unten (Konizitat) AD: Unrundheit

0.0 _... - 0.6

- 0.5

- 0.4

\

- 0.0

r ._ s

E 6 - 0.6 -

I I 1 - 0.8 1 ' I + 0.1

I I , /

595 600 615 625 630 635 Sintering temperature OC

Fig. 13. Dimensional change (relative to die) during sintering of belt pulleys as a function of the sintering temperature Abb. 13. MaRanderung (bezogen auf das IVfatrizenmaR) beim Sin- tern von Zahnriemenradern in Abhangigkeit von der Sintertempe- ratur

ZrSi04 composites, although low, still enables sizing without cracks being formed.

For use in the belt pulley tester (see below) also small pulleys were produced from Alumix- 10% ZrSi04 mixtures (Fig. 7). After 20 min sintering at 605 "C these pulleys, too, could be sized in the pressing die without problems.

4.3 Wear testing

In order to simulate the actual wear loading of the pulleys within the engine a wear testbed was developed (Fig. 14). Two pairs of one small and one large pulley each connected by a commercial belt could be tested (Sketch Fig. 14a). Compara- ble to service conditions, the large pulleys rotated at 2820revdmin and the small ones at double that number. The small pulleys were connected by a torsion spring that exerted a torque of 2 N.m onto the belt, which is also close to service loading. (At a torque of 4 N.m the belts were rapid- ly destroyed by tooth wear). Usual test duration was 1000 hrs which corresponds to 80.000 to 100.000 km at 80 km/h.

The tests were in part performed in air. In a second testbed that simulated more severe conditions the pulley pairs were enclosed in a box each which contained 1 g fine (< 45 pm) iron oxide powder. As reference samples conventional steam treated iron pulleys were also tested.

Wear was checked by various means both on Fe and Al- ZrSi04. After testing slight wear marks were visible as indi- cated in Fig. 15 which were measured using a depth gauge (Sketch Fig. 16). Furthermore, the roughness was measured along the wear marks. The results of the extensive tests are listed in Tuble 4.

The wear resistance of the A1 pulleys was found to be slightly lower than that of the iron ones, but both edge wear and decrease of radius were still well tolerable. With both materials, wear steadily increased with increasing test duration.

The most critical effect was found to be wear of the belt and not of the pulleys. The surface roughness of the A1 pulleys was initially higher than that of the Fe ones and increased with

Fig. 14. Camshaft belt pulley testbed Abb. 14. Zahnriemenrad-Priifstand

186 G . Jangg et al. Mat.-wiss. u. Werkstofftech. 27, 179-189 (1996)

Table 4. Wear data and surface roughness of belt pulleys Tabelle 4. VerschleiRdaten und Oberfliichenrauhigkeit von Zahnriemenriidern

Test duration, hrs Iron pulleys A1 pulleys 0 1000 1800 2500 0 1000 1800 2500

Out-of-roundness, min 0.03 0.10 - - 0.05 0.35 - - decrease of radius h. 0.0 0.03 0.05 - 0.0 0.04 0.08 - mm Edge, wear a, min 0.0 0.08 0.12 - 0.0 0.10 0.18 -

Roughness, prn Rt 12.5 17.6 18.9 21.5 25.5 27.5 57.2 81.4 R,,,,, 11.0 16.9 17.9 21.2 24.3 25.8 51.7 81.4 R, 9.0 13.2 13.0 13.2 16.4 18.7 31.0 42.5 R;, 0.4 I .4 1.9 2.0 1.5 1.9 4.2 7.2

a

/ \ test pulleys Fe

motor drive n = 1410 r.p.m

n =

t- belt t k i';

'adjustin'

>test pulleys AI

bolt

longer testing, since the matrix around the ZrSi04 particles was abraded and the particles protruded more and more. (The roughness of Fe hardly changed during testing). The protruding hard particles (see Fig. 1 7 ) tended to attack the belt. Both with A1 and Fe pulleys the first set of belts survived about 1000 hrs and then was replaced; during further testing using the same pulleys the belts for the Al pulleys had to be replaced in progressively shorter periods (600, 400, 300 hrs respectively) while with the iron pulleys the belts survived about 700 hrs each. Wear resulted also in some out-of-roundness which for the Al pulleys was more pronounced than for the Fe ones.

,original profile Ah

Fig. 15. Wear marks at the tooth flanks of tested pulleys Abb. 15. VerschleiRmarken an den Zahnflanken von gepriiften Ra- dern

Addition of abrasive iron oxide powder did not markedly affect the wear behaviour. Apparently the powder does not influence either wear rate or surface roughness.

Field testing of the A1 pulleys in taxicabs yielded similar results and confirmed that the A1 pulleys are quite comparable to iron ones with the exception of belt wear at long service times (> 1000 hrs) caused by protruding ZrSi04 particles. Here, using finer particles might be helpful since investigati- ons have shown [ 131 that Al containing finer ZrSi04 results in much smoother wear surfaces. The fine particles protrude only slightly and thus should attack the belt less than coarse ones, thus permitting the use of the pulley for the entire lifetime of

measuring line for ah

Fig. 16. Measurement of pulley wear 2 x Ah =decrease of pulley diameter Aa = wear of tooth flank Abb. 16. Messung des ZahnriemenradverschleiBes 2 x Ah =Abnahme des Raddurchmessers Aa = ZahnflankenverschleiB

/original profile maximum wear +,/i

aa J T y w o r n profile


Fig. 17. Protruding ZrSi04 particle at tooth flank Abb. 17. Herauspoliertes Zirkonsand-Teilchen an der Zahnflanke

the car. In any case, A1 P/M alloys reinforced with ZrSiO4 particles can be regarded as cheap and wear resistant materials offering distinct weight advantages, in particular for automotive applications.

5 Conclusions

A1 based sintered camshaft belt pulleys are interesting mainly for sports car engines or boat motors, possessing lower weight and better corrosion resistance than the conventional sintered iron pulleys. They can be produced from standard A1 powder mixtures containing abrasion resistant ZrSi04 particles of about 90 pm mean diameter. Addition of 10 mass% ZrSi04 results in sufficient wear resistance. The lower shrinkage or higher expansion during sintering caused by the hard phase can be compensated for by slightly increasing the sintering temperature. Tensile strength values of 140 MPa at 1.5% elongation can be attained, which seems to be sufficient for camshaft belt pulleys. The wear resistance is only slightly lower than with steam treated iron pulleys. Service-related laboratory tests and field tests in automobiles revealed some difficulties with belt wear but in principle pro- ved the suitability of the A1 camshaft belt pulleys.

6 Acknowledgement

This work was in part supported by the Austrian Fonds zur Forderung der wissenschufllichen Forschung (FWF Project P 5801 J.

Al camshaft belt pulleys: Specification

Powder mixture: ECKA-Alurnix 123 + 10 mass% ZrSi04 WTC (mean particle size 90.. 100 pm), mixture contains 1.28 mass% pressing lubricant HWC (weight loss during de-wa- Xing).

Manufacturing: Compaction at 300 MPa, separate de-waxing 20 ... 25 min at 380..400 “C in high purity nitrogen, control of weight loss during de-waxing.

Sintering in nitrogen (dew point < 40 “C) in pusher furnace equipped with locks. Rapid heating of the de-waxed compacts, sintering 20..25 min at 600..610 “C.

Sintering behaviour and properties

Sintered density: 2.78 g . cmp3

Dim. change during sintering: - 0.1 % linear relative to die dimension (large VOLVO pulley) slight distortion, out of roundness, conicity, at 600..610 O still tolerable, sintered samples can be sized (in the pressing tool)

Tensile strength TI (as sintered) 140 MPa T4 (naturally aged) 170 MPa T6 (artificially aged) 220 MPa Elongation 1 S... 2%

Wear behaviour

(Belt pulley testbed)

1000 hrs testing (i.e. approx. 80.000 ... 100.000 km driving in a car)

Decrease of diameter 0.04 mm Wear at tooth flank 0.10 mm Out-of-roundness after test 0.35 mm

Roughness prior to test after test Rt, Pm 25.5 27.5 Rln,,. Ccm 24.3 25.8 R,, pm 16.4 18.7 R,, pm 1.5 1.9

7

I

2

3

4

5

References

H. J . Dudus, C. B. Thompson, “Improved sintering procedures for A1 PIM parts”. Modern Dev. in Powder Met. 5 (1971) 19. G. Jangg, H. C. Neubing, “Sintering of Aluminium Parts: The State-of-the-Art”. Metal Powder Rep. 42 [ 121 (1987) 354. W! Kehl, H. F: Fischmeirtec “Liquid Phase Sintering of Al-Cu Compacts”. Powder Metall. 23 (1980) No. 3, 113. W! J . Huppmunn, H. Kirschsiepec W Hade, G. Schlieper, “Sintered aluminium parts for automotive applications”. Proc. 7. Int. Leichtmetalltagung Leoben-Wien, F. Jeglitsch ed., Leoben (1981) 236. E. M. Duvec D. P. Ferris, “Wear Resistant Aluminium PA4 Parts: Fabrication and Properties”. Modern Dev. in Powder Met. 10 ( 1977) 357.

6. E. Mosca, C. Pino, “Mechanical and Tribological Properties of Sintered Aluminium Alloys”. Metal Powder Rep. 35 (1980) 199.

7. C. B. Cunzeron, D. f! Ferris, Cobalt 3 (1974) 47 8. D. P. Ferris, J. Wulruedt, “Blended Powders Upgrade Wear

Resistance”. Int. J. Powder Met. I 1 (1975) 179. 9. MIBA Sintermetall AG, Europ. Pat. Appl. 868 902 17.2-2106

(24. 7. 1986; Austrian priority 25. 7. 1985).


10. H . Darzninger; G. Jungg, K. Sckriider; J . Seyrkamrner; H. C. Neubing, "Sintered Aluminum Camshaft Belt Pulleys." Adv. Powder Metall. & Partic. Mat. l Y Y 2 Vol. 6, 1.

13. M. A. Seidi, Ph D Thesis TU Vienna (1987). 14. K. Ahhari, Ph D Thesis TU Vienna (1990).

~ ~~ ~

11. J . E Fuure, C. Brunel, "New P/M Aluminium Alloys for Wear and sliding ~ ~ ~ l i ~ ~ ~ i ~ ~ ~ ~ ~ ~ . " ~ ~ ~ i ~ ~ ~ ~ powder ~ ~ ~ ~ 1 1 , " proc, PM '86 ~ u ~ ~ ~ l d ~ ~ f , W, A, Kaysser, W. J. H~~~~~~~ eds,, ver- lag Schmid. Freiburg (1986) Vol. 11. 985.

Anschrift: Dr. H. Danninger; Institut fur Chemische Technologie anorganischer Stoffe, Technische Universitat Wien, Getreidemarkt 9/161, A-1060 Wien

12. KrSchriider; Ph D fhesis TU Vienna (1985). [T 7611

Forfsetzung der Buchhesprechung von S. 164

Er schreibt dort (siehe oben): ,,Das Buch behandelt die Grundlagen der Ermudungsfestigkeit in einer fiir Konstrukteure, Berechnungs- und Versuchsingenieure geeigneten Darstellung". Der Titel seines Buches heiBt dagegen: ,,Ermudungsfestigkeit - Grundlagen fur Leichtbau, Maschinenbau und Stahlbau" und gleich der erste Satz seines Vorwortes lautet: ,,Das vorliegende Fachbuch, das sich an Konstrukteure, Berechnungs- und Versuchsingenieure so- wie an Forscher, Hochschullehrer und Studenten wendet (behandelt die theoretischen und praktischen Grundlagen der Gestaltung und Dimensionierung und Optimierung ermudungsfester Konstruk- tionen)".

Dabei konnte es keinen grBReren Unterschied geben, als den zwi- schen Konstrukteur und Forscher. Der Konstrukteur mu8 heute, jetzt, Entscheidungen treffen. fur die er technisch und finanziell, ggfs. sogar moralisch, voll verantwortlich ist, ohne daR er immer alle notwendigen Fakten kennt, und wobei er noch unter wirtschaft- lichen Zwangen steht. Der Forscher dagegen kann sein halbes oder ganzes Forscherleben einem einzigen Problem(chen) widmen, alle zwei Jahre dafur eine neue, der vorherigen diametral entgegenge- setzte Hypothese aufstellen und dafur tragt er hochstens die wissen- schaftliche Verantworthng, d. h. schlimmstenfalls finden seine Kol- legen das seltsam.

2. Die ungleiche, um nicht zu sagen falsche Gewichtung der verschiedenen Themen und Parameter, die die Schwingfestigkeit beeinflussen. So umfaljt das Kapitel KurzriDbruchmechanik, das fur die Anwendung in der Industrie, laut Aussage des Verfassers, (noch) nicht wesentlich ist, etwa 20% des ganzen Buches!

3. Die unqualifizierte Aneinanderreihung moglichst aller bekannten Verfahren und Hypothesen zu einem bestimmten The- ma. Die obige Aussage von Herrn Radaj, es gabe in der Wissen- schaft keine ,,richtigen", ,,physikalisch unhaltbaren" oder ,,abwe- gigen" Hypothesen finde ich erstaunlich. Die Literatur iiber Schwingfestigkeit und Bruchmechanik ist voll von geradezu unsinnigen Hypothesen, wie ein Blick in eine beliebige Ausgabe einer der vielen einschlagigen technischen Zeitschriften zeigt. Ebenso ist die Literatur voll von unsinnigen Versuchsprogrammen, die langst bekanntes wiederholen, a ~ i s vie1 zu wenigen Versuchsergeb- nissen vie1 zu allgemeine Schlusse ziehen oder- noch schlimmer, aus ungeeigneten Ergebnissen, z. B. rnit ungekerbten Probesta- ben, falsche Schliisse ziehen. Auch der Rezensent war an solchen Programmen beteiligt!

Von einem Fachmann ist zu fordern, daB er hier die Spreu vom Weizen trennt und sich auch kritisch mit bekannten und allgemein als ,,gut" bezeichneten Hypothesen oder Formeln auseinandersetzt. Z. B. hatte es Paris schon 1960 klar sein mussen, daB die Mittel- spannung einen EinfluB auf den Rinfortschritt hat, da13 man den nicht rnit einer ,,Konstanten" berucksichtigen kann, daR dieser Ein- fluR bei verschiedenen Werkstoffen verschieden ist, und daR es eine Art ,,bruchmechanische Dauerfestigkeit" geben muR. Das wurde Paris iibrigens wenige Jahre spater von Liu, einem anderen Bruch- mechaniker, auch vorgehalten.

Selbst einen Forscher interessiert n u r gelegentlich, wieviele ver- schiedene Formeln zur Berechnung der Formzahl Ctk es gibt. ein Konstrukteur mochte aber nur wissen, wie groR die Formzahl c(k seines Bauteiles ist, wie er sie als Kerbwirkungszahl p k in seine Berechnungen einsetzen mu6 und welche Ungenauigkeiten er dabei zu berucksichtigen hat.

Beim erneuten Durcharbeiten des Buches ist mir noch ein Man- gel aufgefallen, den allerdings alle mir bekannten Bucher zum The- ma Schwingfestigkeit auch haben: Das Problem der Ubertragbar- keit von Probestabdaten auf das Bauteilverhalten wird uberhaupt nicht behandelt. Dabei hat das Schwingfestigkeitsverhalten von ungekerbten Probestaben nichts mit dem Verhalten des Bauteiles aus dem nur nominell gleichen Werkstoff zu tun. Selbst bei gekerbten Probestaben ist eine Ubertragbarkeit mit sehr groBen Schwierigkei- ten verbunden (bauteiliihnliche Probestabe, z. B. SchweiB-, Niet- und Schraubverbindungen sind hier schon wesentlich besser geeig- net). Dieses Problem wird bewuMt oder unbewul3t totgeschwiegen, bewuMt z. B. von den Verfechtern gewisser Methoden der Lebens- dauervorhersage, die prinzipiell gar nicht funktionieren konnen, wenn man nicht blindlings eine Ubertragbarkeit annimmt.

Auch die Bruchmechanik hat hier ihre Probleme: Man nimmt j a an, daB bei gleichem AK die RiBfortschrittsrate gleich hoch ist, egal wie der Probestab und das zu berechnende Bauteil aussehen. Bei Aluminiunischmiedestucken stimmt diese Annahme ubrigens uberhaupt nicht, wie ich selbst durch mehrere groBe Versuchspro- gramme feststellen munte, deren Ergebnisse auch veroffentlicht sind.

DaB sich ein Verfasser, der keine jahrelangen, eigenen experimentellen Erfahrungen hat, schwerer tut als ein Fachmann, dafiir liefert das beiliegende Buch selbst viele Beispiele. Aus Platzgrunden sei hier nur eines genannt: Die Abbildung 93, auf Neuber zuriickge- hend: Danach ware eine AICuMg-Legierung mit einer 0,'L-Grenze von 400 N/mm' weniger kerbempfindlich als ein Stahl gleicher Streckgrenze, was natiirlich nicht richtig ist. Das Bild ist noch aus einem zweiten Grunde Bunerst zweifelhaft. Es gibt keine Al- CuMg-Legierung rnit einer FlieBgrenze von nur 200 N/mm2, die in der Praxis eingesetzt wird. Ein Fachmann, der sich durch die Be- wertung/Beurteilung von experimentellen und rechnerischen Ver- fahren auszeichnen sollte, hatte ein solches Bild nicht abgedruckt.

Was das von Herr Radaj monierte Fehlen einer Inhaltsangabe angeht: Wo steht denn geschrieben, daB jede Rezension zunachst eine Inhaltsangabe enthalten muR? Mir ging es bei meiner Rezen- sion um prinzipielle Probleme eines solchen Buches und nicht um das Aufzahlen von Kapiteluberschriften. Das ware namlich vergleichbar dem Aufzahlen von Hypothesen, ohne sie zu beurteilen!

Nach meinen zugegebenermal3en strengen Kriterien existieren derzeit in der Bundesrepublik (West/Ost) nur zwei Personen, die zufriedenstellende Biicher uber die ganze Schwingfestigkeit schrei- ben konnen; beide haben es llngst getan. Nach eigener Einschat- zung gehore ich ubrigens nicht zu diesem sehr begrenzten Perso- nenkreis!

Zufriedenstellende Biicher uber Teilgebiete gibt es dagegen in groBerer Anzahl, wofur etwa die Bucher von Herrn Radaj iiber Schweiliverbindungen gute Beispiele liefern. Noch positiver als bei der ersten Durcharbeitung fand ich diesmal den klaren Auf- bau, die gute Didaktik und das ausfiihrliche Stichwortverzeichnis.

DaR diese Kontroverse den offenbar schon guten Absatz des Bu- ches noch weiter steigern wird, ist mir schon klar. Da gibt es ja gerade in jiingster Zeit einen Prazedenzfall, den berechtigten Ver- riR des schlimmen neuen Buches von Gunther Grass durch Marcel Reich-Ranicki.

Auch die Antwort von Herrn Reich-Ranicki auf die Aussage eines deutschen Autors ,,Man erwartet eine neues Buch von mir" ,,Nein, man befurchtet es" pa& in diesem Zusammenhang gut.

W. Schutz, Ottobrunn


pm aluminum camshaft belt pulleys for automotive engines

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