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Wear characteristics of as-cast binary aluminium-silicon alloys
Wear, 54 (1979) 7 - 16 @ Elsevier Sequoia S.A., Lausanne
7 - Printed
WEAR CHARACTERISTICS SILICON ALLOYS
in the Netherlands
OF AS-CAST
BINARY
ALUMINIUM-
J. CLARKE Depurt~ent of mechanical Bolton (G t. Britain)
Engineering,
Bofton Z~stjtute of Technology,
Deane Road,
A. D. SARKAR John Dalton Faculty of Technology, Ml 5 GD (Gt. Britain) (Received
Manchester Polytechnic,
Chester Street, Manchester
April 28, 1978)
Summary
Binary as-cast ~uminium alloys with silicon content varying up to 21% have been slid against hard steel. Wear rates have been measured at various loads and it is shown that, from the point of view of wear and load-carrying capability, a near-eutectic alloy is the optimum. The hypereutectic alloys wear more but not by more than about a factor of 2 compared with the hypoeutectic alloy. It is concluded that the beneficial effect of silicon is to decrease the propensity to seizure. High silicon alloys, however, wear even hard steel counterfaces. Results of particle size analysis are included to show that although silicon modifies the mode of wear of these aluminium alloys, the distribution of particle size appears to be independent of silicon content and load. Iso-wear lines are plotted for various combinations of silicon and load and it is suggested that this form of presentation will be of help to design engineers if carried out on actual component, e.g. plain bearings.
1. Introduction
Both hypoeutectic and hypereutectic Al-Si alloys have been used for tribological components in dry and lubricated contacts for a long time. One typical application of the hypereutectic alloy is in the form of pistons in automobile engines possibly because of such advantages as lightness, high thermal conductivity and low cost. Addition of silicon, apart from reducing the coefficient of thermal expansion, produces an ~uminium alloy with good wear, casting, machining and corrosion characteristics. A number of hypoeutectic alloys are in general use which are by no means simple binary systems but have alloying additions of copper, magnesium and nickel together with trace amounts of iron, manganese and zinc. The hypereutectic
8
alloys have lower coefficients of thermal expansion and this may be the reason for their popularity as piston components in certain countries. Currently, there is a lot of interest in the friction and wear rates of AlSi alloys. An area of interest has been to investigate the role of microconstituents in terms of quantity, shape, size and general distribution mode of the silicon particles but definitive conclusions from these experiments have not been forthcoming. Of these, the effect of silicon content on the aluminiumbased alloys has been studied by a few workers from the point of view of friction and wear. The conclusions from these experiments are again contradictory. Thus results from a series of commercial alloys show that a high silicon alloy produces a wear-resistant material [l] whereas the contrary conclusion is arrived at from experiments on two age-hardening alloys [ 2} . Recently, a series of non-commercial alloys with a range of silicon content up to 20% have been studied [ 3] using a pin-on-disc machine. The authors imply that increasing the silicon content is beneficial from the point of view of wear resistance. Such controversy about the role of silicon content is unfortunate, but it should be appreciated that in one at least of the papers cited [ 21 the difference in the wear rates between hypoeutectie and hypereutectic alloys with about 21% silicon was small. In two of the previous studies [2,3] pins with diameters of the order of 6.25 mm are normally cast and used without any machining for sliding studies. It is very difficult to cast such small pins successfully without marked segregation of the silicon particles. One effect is that the amount of silicon content as obtained from chemical analysis is often open to doubt unless a wet method has been employed. Errors of this nature will obviously make meaningless an interpretation of the role of the silicon content in the pin samples. Since resistance to wear of these aluminium-based alloys is important if they are used as materials for tribological components, the present authors continued with a programme of investigation of the friction and wear characteristics of these materials begun about a decade ago. The experimental results regarding the wear of binary Al-Si alloy melts produced in the laboratory from commercially pure materials are discussed here, The results should be particularly useful because the silicon ranges chosen were similar to those reported by Shivanath et al. [3] in their work carried out independently of the present authors.
2. Experimental
procedure
All wear experiments were conducted in a pin-on-bush machine at room temperature and pressure. The machine was capable of measu~ng friction, although the results are not reported here, and wear was estimated by the weight loss technique. Commercially pure materials were melted in a laboratory electric furnace following approved melting techniques. The pins, which were 6.25 mm in diameter, were cast in sand moulds which were
9
warmed to avoid any chilling of the castings. A very rapid rate of heat extraction by chilling in a cold mould will result in a variation of the morphology of the cast structure compared with the case when the rate of cooling is slow. The pins were cleaned thoroughly but not machined except for the end faces which were polished to provide a consistent prior surface that rubbed against the bush. It was felt that in this way the tribological behaviour of the alloys in the as-cast state would be ascertained using the microstructure as the criterion. Each pin was run at a surface speed of 196 cm s-l on a fresh hard EN 31 steel bush suitably heat treated. For each alloy wear runs were made at intervals of 0.5 kg or less up to a top load of about 3 kg beyond which marked mechanical instability of the pins occurred.
3. Results Both the pins and the bushes were weighed at intervals and the weight losses were plotted against the appropriate sliding distance. The slopes of the steady state wear for the pins were taken to express the rates of wear in grams per cubic centimetre at a specific load. 3.1. Low silicon melts The wear rates of pure aluminium and two alloys with silicon contents of 1.8% and 6.3% respectively are shown in Fig. 1. The curves show a characteristic pattern where the wear rate increases with load at an accelerating rate. None of the pins could sustain a load higher than 2 kg. Even increasing the load by a further 0.5 kg caused gross seizure at the interface. A close examination of the pattern exhibited by the pure aluminium pin shows strikingly similar behaviour to earlier results obtained when aluminium slid on itself [4], i.e. there is a tendency for the wear rate to rise initially showing a peak at around 0.6 kg followed by a fall and then a rapid rise to destructive seizure of the pins on the steel bush. Some explanations were offerred in ref. 4 from indirect evidence. It was suggested that the initial rise was due to the load effect but it should be noted that, as before, the rate of change of wear does not follow the load linearly. A lowering of wear rate was attributed to the work hardening of the subsurface which is to be expected in a face-centred cubic metal like aluminium with so many possible slip systems. The rise after the fall was attributed to thermal softening as a result of increased mechanical energy. As silicon is added, the continuity of the metallic matrix is disrupted and a maximum followed by a fall is not apparent. The wear rate, however, remains high and continues to increase rapidly with load. 3.2. Higher silicon content The results in Fig. 2 show that the wear rates of the Al-Si alloys have a different general shape to those for low silicon melts. The wear rates again
Fig. 1. Wear rate against load of as-cast low silicon alloy pins sliding on a hard steel bush at a surface speed of 196 cm s-l. Silicon content: 0, 0%; A, 1.8%; 0, 6.3%. Fig. 2. Wear rate against load of as-cast silicon alloy pins sliding on a hard steel bush at a surface speed of 196 cm s-l. Silicon content: 0, ll%;n, 13%; q,15%; V, 16%; 6, 21%.
increase with load in a non-linear manner but at a diminishing rate. The maximum load which can be sustained without seizure increases but the magnitude of this is similar to other alloys, i.e. an alloy containing 21% silicon could not withstand a maximum load before seizure which was greater than that withstood by the alloy containing 11% silicon. The pattern of the curves and the maximum load capability of the alloys are similar to the two commercial age-hardening alloys reported earlier [ 21. 3.3. Wear rate and silicon con tent The effect of the silicon content on the wear rates at various loads is shown in Fig. 3. The beneficial effect of silicon from the wear resistance point of view is clear up to about 11% silicon, The wear rate then increases and this increase becomes fairly rapid beyond a silicon content of about 14%. At low loads the scatter of results is less but an interesting aspect of plotting the results in this manner is that there is a relation between the amount of silicon and the applied load as far as the minimum (Fig. 3) is concerned. This minimum load as estimated by eye is plotted against the silicon content in Fig. 4. The silicon content at which a given load gives minimum wear varies significantly from about 7% at a load of 1 kg to 11% at a load of 2.5 kg.
11
Fig. 3. Variation of wear rates of aluminium alloys sliding against a hard steel bush with silicon content. Loads: 0, 1 kg;n, 1.5 kg; 0, 2 kg;*, 2.5 kg. Fig. 4. Minimum wear rates of aluminium alloys sliding against a hard steel bush at a surface speed of 196 cm s-l as a function of load and silicon content.
3.4. The coun terface The counterface in this case was a very hard steel in the form of a bush. Both the pin and the bush were weighed at intervals and the balance was sensitive enough to follow the weight variations although, possibly, not as accurately as when the loss or gain was assessed by a radioactive tracer technique as has been done by previous workers on brass. Typically, some of the bushes continued to receive a deposit of aluminium with time owing to adhesion at the interface but the others lost weight. Plotting the variations in bush weight against the applied normal load shows certain definite trends in spite of the scatter in results (Fig. 5). A pure aluminium pin continues to deposit itself on the bush even at light loads as does the material with a small silicon content, i.e. 1.8%. The total amount of deposit increases with load as expected. The effect of silicon is decisive in that the bush wears progressively. Scatter is considerable but it could be stated with confidence that the silicon particles act as abrasives and cause wear of the bush. 3.5. Iso-wear lines With the normal load as the height coordinate, a series of lines were obtained such that each line represents one rate of wear at various combinations of load and silicon content (Fig. 6). Figure 6 was constructed from the curves drawn in Figs. 1 and 2 and not from actual wear data obtained from
12
. 0
.
Fig. 5. Rate of weight loss or gain of hard steel bushes as a function of normal load and the composition of the pin material (surface speed 196 cm s-l). Silicon content: 0, 0%; A, 1.8%; 0, 6.2%; V’, 12%;., 16%; x, 21%. Fig. 6. Iso-wear lines for various combinations of load and silicon content for aluminium alloy pins sliding on steel at a surface speed of 196 cm s-l. Wear rates (low9 g cm-‘): 0, 15;A,20;o, 25;., 30.
experiments. In spite of this, the iso-wear lines are useful because one could set a limit on the amount of permissible wear rate, e.g. in a plain bearing at the design stage. It will then be a matter of choosing the amount of silicon in the alloy for the specified design load. The curves show that a low wear rate dictates a light normal load on a component. This is nothing original, but Fig. 6 highlights one important point. This is that at any allowable wear rate the component will sustain a maximum load at a silicon content of the order of 11% for a binary melt consisting of aluminium and silicon. A heavy load can only be sustained if a high amount of wear can be tolerated. A near-eutectic alloy therefore provides a maximum load-carrying capacity when viewed in combination with favourable wear rates. 3.6. Particle size A particle size analysis of the wear debris was carried out with the aid of a Quantimet analyser and the results are shown in Table 1. The actual number of particles for the 21% silicon alloy rather than the percentage of particles is plotted in Fig. 7. This is chosen because all the three wear runs had a similar total sliding distance. The particle size is discussed later but, as expected, the particles of smallest size are the most
1
37.5 36.2 26.0 26.7 29.3 27.7
27.5 28.7 32.0
0.5 1.0 1.5 2.0 2.5 3.0
1.0 2.0 2.5
Si
Al-21%
machine
Si
N-12%
Pin-bush
Si
Al-6.5%
speed,
32.8 27.6 31.8 29.0 30.3
0.5 1.0 1.5 2.0 2.5
surface
33.3 29.8 31.0 28.8
0.5 1.0 1.5 2.0
31.0 32.8
196 cm s-l.
24.5 25.1 25.0
23.3 28.2 24.0 24.2 25.2 24.5
25.1 23.8 27.3 25.3 24.8
24.5 27.2 27.5 26.3
22.4 24.2
Mean particle 1.13 3.4
Al-2%
Si
loads for AI-B
22.7 23.3 22.4
18.0 20.6 22.7 23.2 23.0 21.8
20.9 22.1 23.5 23.4 21.7
20.4 23.7 21.9 24.3
19.8 22.8
size ( 10e2 6.8
16.5 15.5 13.8
13.6 10.6 18.1 18.2 15.7 15.9
14.3 17.9 12.2 16.5 13.6
15.1 14.1 13.1 14.2
16.7 13.2
mm) 13.6
with varying
silicon
7.1 6.0 5.0
6.9 3.6 8.1 6.5 5.7 7.6
5.4 6.5 3.8 5.1 6.3
5.6 4.3 4.8 5.7
8.2 5.2
27.2
1.40 1.10 1.20
0.58 0.62 0.64 1.00 0.90 2.30
1.20 1.40 1.00 0.50 2.30
0.80 0.70 1.20 0.30
1.65 1.35
54.3
0.14 0.14 0.26
0.00 0.00 0.30 0.06 0.05 0.13
0.20 0.40 0.25 0.10 0.90
0.13 0.03 0.10 0.20
0.12 0.25
111.2
of a given mean particle
alloys
Percentage of total number of particles for a material at a given load
0.5 2.0
(kg)
Load
of wear debris at various
Pure aluminium
Materials
Particle size distribution
TABLE
size
content
0.00 0.02 0.07
0.00 0.00 0.04 0.00 0.00 0.00
0.00 0.10 0.03 0.00 0.00
0.00 0.00 0.02 0.02
0.00 0.03
225.0
97.591 101.769 101.505
71.880 87.381 101.745 71.856 97.017 83.826
89.811 83.850 103.564 64.662 7.876
75.471 88.638 90.074 56.582
96.191 47.581
Total sliding (lo4 cm)
distance
Fig. 7. Number of wear fragments of various particle sizes for 21% silicon allay running on steel at 195 cm 5-l. Load: A, I.0 kg;*, 2.0 kg; X, 2.5 kg.
numerous. There is a variation of two to three orders of magnitude between the sizes of the smallest and the largest particles. The higher the load, the larger the number of particles, p~i~~l~ly the ones with small average diameters for this 21% silicon alloy. It should be noted here that this observation is not meant to be a general conclusion.
4. I3iscussion It can be stated with certainty that the rates of wear for these aluminium alloys are not linear, i.e. as for the age hardenable alloys [2] the adhesive or abrasive wear laws propounding direct proportionality of the wear rate with the applied normal load do not hold for these as-east metals. It is interesting that the low silicon alloys show an increasing rate of wear with load (Fig. 1). This can only be explained in terms of the inevitable severe wear when like metals are forced to undergo tribological interaction. Even for the pin with the highest amount of silicon, i.e. 6.3% in this group, the onset of motion deposits aluminium on the bush (Fig. S(a)), The pin then slides on similar material to itself and wear becomes severe effectively giving rise to a similar, although not as drastic, situation to aluminium sliding on an aluminium bush 141. The pin becomes mechanically unstable giving rise to gross seizure (Fig. 8(b)). A load of 2.5 kg on a pin 6.25 mm in diameter is well below the yield stress in tension for the aluminium alloys and therefore the term “mechanical instability” is perhaps not appropriate. A silicon content nearer the euteetic composition and over again produces a nonlinear rate of wear with load (Fig. 2) but the increase is at a diminishing rate. This is only true up to a load where the aluminium deposit on the bush is sparse or absent. In the case of the Al-13% Si pin (Fig. 2), after a load of 2 kg, when aluminium begins to be deposited again, the wear increases at an
15
(a)
(b)
Fig. 8. A 6.3% silicon alloy sliding on a steel bush: load, 2.5 kg; speed, 196 cm s-l. (a) Bush showing deposit of pin material; (b) the state of the pin at the moment of seizure.
rate and it is not easy to have a wear run beyond a load of about 3 kg even with a silicon content of 21%. The deposition process is clearly seen in Fig. 5. As the silicon increases even a hard steel bush wears and the wear is quite excessive at 21% silicon, being 16.5 X lo-’ and 35.2 X lo-’ g cm-l at loads of 1.5 and 2 kg respectively: Beyond a certain load, however, the aluminium fraction of the pin yields, and in some cases there is a perceptible weight gain but in the others the loss rate of the bush slows down because of the deposit. It is clear (Fig. 3) that up to about the eutectic composition silicon has a beneficial effect from the point of view of wear resistance. The hypereutectic alloys are inferior as concluded previously from the age-hardening alloys. However, the difference in the rates of wear between the minimum in Fig. 3 and that at 21% is small, being by only about a factor of a little over 2. However, the differences in the rates of wear with silicon content are decisive. The results from these experiments regarding the role of silicon in wear resistance are at variance with those of other workers [1,3] . The reason is not clear. The iso-wear lines plotted in Fig. 6 should provide convenient design data if similar results are obtained for actual components such as plain bearings. For any wear rate that is acceptable the maximum load is supported by a near-eutectic alloy. The load-bearing capacity of a 21% silicon alloy is nearly as low as that of an alloy with a negligible content. It is erroneous therefore to conclude that silicon increases the load-bearing capacity in proportion to its bulk in the alloy. What can be said with a certain amount of confidence is that the propensity to seizure diminishes with increasing silicon content. However, wear of the counterface occurs when the silicon content is high,
accelerating
16
4.1. Particle
size
There are two sources of error in examining particle size in these experiments. Firstly, some of the particles may be thrown clear of the collecting tray and second it is not easy to achieve complete separation on the slides during observation under a microscope.It is tempting to conclude from Fig. 7 that the number of particles of small size increases as the normal load increases. This should be expected since the effect of increasing the load is to enlarge the most favourable asperity contacts and then, as the compliance improves owing to squashing of the asperities, to form further contacts elsewhere which are usually small in size. Unfortunately examination of Table 1 shows that the load-dependence effect of the particle size of the wear debris does not hold very well with other alloys. Table 1 shows that there is no correlation between the size of the wear debris and the amount of silicon in the alloy. The apparently negative contribution of the particle size analysis is included, however, for the following reason. It is obvious that there is a fundamental difference between the mode of tribological interaction uis a’ uis the amount of silicon in a binary melt. When the matrix of the pin material is high in aluminium, material is deposited on the counterface, the amount of which increases with load at a given speed. This deposit is absent with the high silicon alloys and in fact the counterface wears at a rate which increases both with the amount of silicon and the applied normal load. There are thus two modes of wear, namely adhesive and two-body abrasion. It was therefore of interest to see whether or not the size distribution of wear debris will change with both load and silicon content. On the basis of the results obtained here, there is no significant correlation between the particle size and the load or silicon content.
5. Conclusions The present investigation shows: (1) the wear resistance of a binary Al-Si alloy improves with silicon content up to the near-eutectic composition; (2) the hypereutectic alloys wear more but by only a factor of about 2; (3) a high silicon alloy causes wear of the counterface even when this is a very hard steel; (4) the beneficial effect of silicon is that it minimizes the tendency for seizure; (5) a near-eutectic alloy has the best load-bearing capacity.
References 1 K. Okabayashi, Y. Nakatani, H. Notani and M. Kawamoto, Keikinzoku, 14 (1964) 2 A. D. Sarkar, Wear, 31 (1975) 331. 3 R. Shivanath, P. K. Sengupta and T. S. Eyre, Br. Foundryman, 70 (1977) 349. 4 A. D. Sarkar, Tribol. Int., (Aug. 1977) 235.
57.
7 - Printed
WEAR CHARACTERISTICS SILICON ALLOYS
in the Netherlands
OF AS-CAST
BINARY
ALUMINIUM-
J. CLARKE Depurt~ent of mechanical Bolton (G t. Britain)
Engineering,
Bofton Z~stjtute of Technology,
Deane Road,
A. D. SARKAR John Dalton Faculty of Technology, Ml 5 GD (Gt. Britain) (Received
Manchester Polytechnic,
Chester Street, Manchester
April 28, 1978)
Summary
Binary as-cast ~uminium alloys with silicon content varying up to 21% have been slid against hard steel. Wear rates have been measured at various loads and it is shown that, from the point of view of wear and load-carrying capability, a near-eutectic alloy is the optimum. The hypereutectic alloys wear more but not by more than about a factor of 2 compared with the hypoeutectic alloy. It is concluded that the beneficial effect of silicon is to decrease the propensity to seizure. High silicon alloys, however, wear even hard steel counterfaces. Results of particle size analysis are included to show that although silicon modifies the mode of wear of these aluminium alloys, the distribution of particle size appears to be independent of silicon content and load. Iso-wear lines are plotted for various combinations of silicon and load and it is suggested that this form of presentation will be of help to design engineers if carried out on actual component, e.g. plain bearings.
1. Introduction
Both hypoeutectic and hypereutectic Al-Si alloys have been used for tribological components in dry and lubricated contacts for a long time. One typical application of the hypereutectic alloy is in the form of pistons in automobile engines possibly because of such advantages as lightness, high thermal conductivity and low cost. Addition of silicon, apart from reducing the coefficient of thermal expansion, produces an ~uminium alloy with good wear, casting, machining and corrosion characteristics. A number of hypoeutectic alloys are in general use which are by no means simple binary systems but have alloying additions of copper, magnesium and nickel together with trace amounts of iron, manganese and zinc. The hypereutectic
8
alloys have lower coefficients of thermal expansion and this may be the reason for their popularity as piston components in certain countries. Currently, there is a lot of interest in the friction and wear rates of AlSi alloys. An area of interest has been to investigate the role of microconstituents in terms of quantity, shape, size and general distribution mode of the silicon particles but definitive conclusions from these experiments have not been forthcoming. Of these, the effect of silicon content on the aluminiumbased alloys has been studied by a few workers from the point of view of friction and wear. The conclusions from these experiments are again contradictory. Thus results from a series of commercial alloys show that a high silicon alloy produces a wear-resistant material [l] whereas the contrary conclusion is arrived at from experiments on two age-hardening alloys [ 2} . Recently, a series of non-commercial alloys with a range of silicon content up to 20% have been studied [ 3] using a pin-on-disc machine. The authors imply that increasing the silicon content is beneficial from the point of view of wear resistance. Such controversy about the role of silicon content is unfortunate, but it should be appreciated that in one at least of the papers cited [ 21 the difference in the wear rates between hypoeutectie and hypereutectic alloys with about 21% silicon was small. In two of the previous studies [2,3] pins with diameters of the order of 6.25 mm are normally cast and used without any machining for sliding studies. It is very difficult to cast such small pins successfully without marked segregation of the silicon particles. One effect is that the amount of silicon content as obtained from chemical analysis is often open to doubt unless a wet method has been employed. Errors of this nature will obviously make meaningless an interpretation of the role of the silicon content in the pin samples. Since resistance to wear of these aluminium-based alloys is important if they are used as materials for tribological components, the present authors continued with a programme of investigation of the friction and wear characteristics of these materials begun about a decade ago. The experimental results regarding the wear of binary Al-Si alloy melts produced in the laboratory from commercially pure materials are discussed here, The results should be particularly useful because the silicon ranges chosen were similar to those reported by Shivanath et al. [3] in their work carried out independently of the present authors.
2. Experimental
procedure
All wear experiments were conducted in a pin-on-bush machine at room temperature and pressure. The machine was capable of measu~ng friction, although the results are not reported here, and wear was estimated by the weight loss technique. Commercially pure materials were melted in a laboratory electric furnace following approved melting techniques. The pins, which were 6.25 mm in diameter, were cast in sand moulds which were
9
warmed to avoid any chilling of the castings. A very rapid rate of heat extraction by chilling in a cold mould will result in a variation of the morphology of the cast structure compared with the case when the rate of cooling is slow. The pins were cleaned thoroughly but not machined except for the end faces which were polished to provide a consistent prior surface that rubbed against the bush. It was felt that in this way the tribological behaviour of the alloys in the as-cast state would be ascertained using the microstructure as the criterion. Each pin was run at a surface speed of 196 cm s-l on a fresh hard EN 31 steel bush suitably heat treated. For each alloy wear runs were made at intervals of 0.5 kg or less up to a top load of about 3 kg beyond which marked mechanical instability of the pins occurred.
3. Results Both the pins and the bushes were weighed at intervals and the weight losses were plotted against the appropriate sliding distance. The slopes of the steady state wear for the pins were taken to express the rates of wear in grams per cubic centimetre at a specific load. 3.1. Low silicon melts The wear rates of pure aluminium and two alloys with silicon contents of 1.8% and 6.3% respectively are shown in Fig. 1. The curves show a characteristic pattern where the wear rate increases with load at an accelerating rate. None of the pins could sustain a load higher than 2 kg. Even increasing the load by a further 0.5 kg caused gross seizure at the interface. A close examination of the pattern exhibited by the pure aluminium pin shows strikingly similar behaviour to earlier results obtained when aluminium slid on itself [4], i.e. there is a tendency for the wear rate to rise initially showing a peak at around 0.6 kg followed by a fall and then a rapid rise to destructive seizure of the pins on the steel bush. Some explanations were offerred in ref. 4 from indirect evidence. It was suggested that the initial rise was due to the load effect but it should be noted that, as before, the rate of change of wear does not follow the load linearly. A lowering of wear rate was attributed to the work hardening of the subsurface which is to be expected in a face-centred cubic metal like aluminium with so many possible slip systems. The rise after the fall was attributed to thermal softening as a result of increased mechanical energy. As silicon is added, the continuity of the metallic matrix is disrupted and a maximum followed by a fall is not apparent. The wear rate, however, remains high and continues to increase rapidly with load. 3.2. Higher silicon content The results in Fig. 2 show that the wear rates of the Al-Si alloys have a different general shape to those for low silicon melts. The wear rates again
Fig. 1. Wear rate against load of as-cast low silicon alloy pins sliding on a hard steel bush at a surface speed of 196 cm s-l. Silicon content: 0, 0%; A, 1.8%; 0, 6.3%. Fig. 2. Wear rate against load of as-cast silicon alloy pins sliding on a hard steel bush at a surface speed of 196 cm s-l. Silicon content: 0, ll%;n, 13%; q,15%; V, 16%; 6, 21%.
increase with load in a non-linear manner but at a diminishing rate. The maximum load which can be sustained without seizure increases but the magnitude of this is similar to other alloys, i.e. an alloy containing 21% silicon could not withstand a maximum load before seizure which was greater than that withstood by the alloy containing 11% silicon. The pattern of the curves and the maximum load capability of the alloys are similar to the two commercial age-hardening alloys reported earlier [ 21. 3.3. Wear rate and silicon con tent The effect of the silicon content on the wear rates at various loads is shown in Fig. 3. The beneficial effect of silicon from the wear resistance point of view is clear up to about 11% silicon, The wear rate then increases and this increase becomes fairly rapid beyond a silicon content of about 14%. At low loads the scatter of results is less but an interesting aspect of plotting the results in this manner is that there is a relation between the amount of silicon and the applied load as far as the minimum (Fig. 3) is concerned. This minimum load as estimated by eye is plotted against the silicon content in Fig. 4. The silicon content at which a given load gives minimum wear varies significantly from about 7% at a load of 1 kg to 11% at a load of 2.5 kg.
11
Fig. 3. Variation of wear rates of aluminium alloys sliding against a hard steel bush with silicon content. Loads: 0, 1 kg;n, 1.5 kg; 0, 2 kg;*, 2.5 kg. Fig. 4. Minimum wear rates of aluminium alloys sliding against a hard steel bush at a surface speed of 196 cm s-l as a function of load and silicon content.
3.4. The coun terface The counterface in this case was a very hard steel in the form of a bush. Both the pin and the bush were weighed at intervals and the balance was sensitive enough to follow the weight variations although, possibly, not as accurately as when the loss or gain was assessed by a radioactive tracer technique as has been done by previous workers on brass. Typically, some of the bushes continued to receive a deposit of aluminium with time owing to adhesion at the interface but the others lost weight. Plotting the variations in bush weight against the applied normal load shows certain definite trends in spite of the scatter in results (Fig. 5). A pure aluminium pin continues to deposit itself on the bush even at light loads as does the material with a small silicon content, i.e. 1.8%. The total amount of deposit increases with load as expected. The effect of silicon is decisive in that the bush wears progressively. Scatter is considerable but it could be stated with confidence that the silicon particles act as abrasives and cause wear of the bush. 3.5. Iso-wear lines With the normal load as the height coordinate, a series of lines were obtained such that each line represents one rate of wear at various combinations of load and silicon content (Fig. 6). Figure 6 was constructed from the curves drawn in Figs. 1 and 2 and not from actual wear data obtained from
12
. 0
.
Fig. 5. Rate of weight loss or gain of hard steel bushes as a function of normal load and the composition of the pin material (surface speed 196 cm s-l). Silicon content: 0, 0%; A, 1.8%; 0, 6.2%; V’, 12%;., 16%; x, 21%. Fig. 6. Iso-wear lines for various combinations of load and silicon content for aluminium alloy pins sliding on steel at a surface speed of 196 cm s-l. Wear rates (low9 g cm-‘): 0, 15;A,20;o, 25;., 30.
experiments. In spite of this, the iso-wear lines are useful because one could set a limit on the amount of permissible wear rate, e.g. in a plain bearing at the design stage. It will then be a matter of choosing the amount of silicon in the alloy for the specified design load. The curves show that a low wear rate dictates a light normal load on a component. This is nothing original, but Fig. 6 highlights one important point. This is that at any allowable wear rate the component will sustain a maximum load at a silicon content of the order of 11% for a binary melt consisting of aluminium and silicon. A heavy load can only be sustained if a high amount of wear can be tolerated. A near-eutectic alloy therefore provides a maximum load-carrying capacity when viewed in combination with favourable wear rates. 3.6. Particle size A particle size analysis of the wear debris was carried out with the aid of a Quantimet analyser and the results are shown in Table 1. The actual number of particles for the 21% silicon alloy rather than the percentage of particles is plotted in Fig. 7. This is chosen because all the three wear runs had a similar total sliding distance. The particle size is discussed later but, as expected, the particles of smallest size are the most
1
37.5 36.2 26.0 26.7 29.3 27.7
27.5 28.7 32.0
0.5 1.0 1.5 2.0 2.5 3.0
1.0 2.0 2.5
Si
Al-21%
machine
Si
N-12%
Pin-bush
Si
Al-6.5%
speed,
32.8 27.6 31.8 29.0 30.3
0.5 1.0 1.5 2.0 2.5
surface
33.3 29.8 31.0 28.8
0.5 1.0 1.5 2.0
31.0 32.8
196 cm s-l.
24.5 25.1 25.0
23.3 28.2 24.0 24.2 25.2 24.5
25.1 23.8 27.3 25.3 24.8
24.5 27.2 27.5 26.3
22.4 24.2
Mean particle 1.13 3.4
Al-2%
Si
loads for AI-B
22.7 23.3 22.4
18.0 20.6 22.7 23.2 23.0 21.8
20.9 22.1 23.5 23.4 21.7
20.4 23.7 21.9 24.3
19.8 22.8
size ( 10e2 6.8
16.5 15.5 13.8
13.6 10.6 18.1 18.2 15.7 15.9
14.3 17.9 12.2 16.5 13.6
15.1 14.1 13.1 14.2
16.7 13.2
mm) 13.6
with varying
silicon
7.1 6.0 5.0
6.9 3.6 8.1 6.5 5.7 7.6
5.4 6.5 3.8 5.1 6.3
5.6 4.3 4.8 5.7
8.2 5.2
27.2
1.40 1.10 1.20
0.58 0.62 0.64 1.00 0.90 2.30
1.20 1.40 1.00 0.50 2.30
0.80 0.70 1.20 0.30
1.65 1.35
54.3
0.14 0.14 0.26
0.00 0.00 0.30 0.06 0.05 0.13
0.20 0.40 0.25 0.10 0.90
0.13 0.03 0.10 0.20
0.12 0.25
111.2
of a given mean particle
alloys
Percentage of total number of particles for a material at a given load
0.5 2.0
(kg)
Load
of wear debris at various
Pure aluminium
Materials
Particle size distribution
TABLE
size
content
0.00 0.02 0.07
0.00 0.00 0.04 0.00 0.00 0.00
0.00 0.10 0.03 0.00 0.00
0.00 0.00 0.02 0.02
0.00 0.03
225.0
97.591 101.769 101.505
71.880 87.381 101.745 71.856 97.017 83.826
89.811 83.850 103.564 64.662 7.876
75.471 88.638 90.074 56.582
96.191 47.581
Total sliding (lo4 cm)
distance
Fig. 7. Number of wear fragments of various particle sizes for 21% silicon allay running on steel at 195 cm 5-l. Load: A, I.0 kg;*, 2.0 kg; X, 2.5 kg.
numerous. There is a variation of two to three orders of magnitude between the sizes of the smallest and the largest particles. The higher the load, the larger the number of particles, p~i~~l~ly the ones with small average diameters for this 21% silicon alloy. It should be noted here that this observation is not meant to be a general conclusion.
4. I3iscussion It can be stated with certainty that the rates of wear for these aluminium alloys are not linear, i.e. as for the age hardenable alloys [2] the adhesive or abrasive wear laws propounding direct proportionality of the wear rate with the applied normal load do not hold for these as-east metals. It is interesting that the low silicon alloys show an increasing rate of wear with load (Fig. 1). This can only be explained in terms of the inevitable severe wear when like metals are forced to undergo tribological interaction. Even for the pin with the highest amount of silicon, i.e. 6.3% in this group, the onset of motion deposits aluminium on the bush (Fig. S(a)), The pin then slides on similar material to itself and wear becomes severe effectively giving rise to a similar, although not as drastic, situation to aluminium sliding on an aluminium bush 141. The pin becomes mechanically unstable giving rise to gross seizure (Fig. 8(b)). A load of 2.5 kg on a pin 6.25 mm in diameter is well below the yield stress in tension for the aluminium alloys and therefore the term “mechanical instability” is perhaps not appropriate. A silicon content nearer the euteetic composition and over again produces a nonlinear rate of wear with load (Fig. 2) but the increase is at a diminishing rate. This is only true up to a load where the aluminium deposit on the bush is sparse or absent. In the case of the Al-13% Si pin (Fig. 2), after a load of 2 kg, when aluminium begins to be deposited again, the wear increases at an
15
(a)
(b)
Fig. 8. A 6.3% silicon alloy sliding on a steel bush: load, 2.5 kg; speed, 196 cm s-l. (a) Bush showing deposit of pin material; (b) the state of the pin at the moment of seizure.
rate and it is not easy to have a wear run beyond a load of about 3 kg even with a silicon content of 21%. The deposition process is clearly seen in Fig. 5. As the silicon increases even a hard steel bush wears and the wear is quite excessive at 21% silicon, being 16.5 X lo-’ and 35.2 X lo-’ g cm-l at loads of 1.5 and 2 kg respectively: Beyond a certain load, however, the aluminium fraction of the pin yields, and in some cases there is a perceptible weight gain but in the others the loss rate of the bush slows down because of the deposit. It is clear (Fig. 3) that up to about the eutectic composition silicon has a beneficial effect from the point of view of wear resistance. The hypereutectic alloys are inferior as concluded previously from the age-hardening alloys. However, the difference in the rates of wear between the minimum in Fig. 3 and that at 21% is small, being by only about a factor of a little over 2. However, the differences in the rates of wear with silicon content are decisive. The results from these experiments regarding the role of silicon in wear resistance are at variance with those of other workers [1,3] . The reason is not clear. The iso-wear lines plotted in Fig. 6 should provide convenient design data if similar results are obtained for actual components such as plain bearings. For any wear rate that is acceptable the maximum load is supported by a near-eutectic alloy. The load-bearing capacity of a 21% silicon alloy is nearly as low as that of an alloy with a negligible content. It is erroneous therefore to conclude that silicon increases the load-bearing capacity in proportion to its bulk in the alloy. What can be said with a certain amount of confidence is that the propensity to seizure diminishes with increasing silicon content. However, wear of the counterface occurs when the silicon content is high,
accelerating
16
4.1. Particle
size
There are two sources of error in examining particle size in these experiments. Firstly, some of the particles may be thrown clear of the collecting tray and second it is not easy to achieve complete separation on the slides during observation under a microscope.It is tempting to conclude from Fig. 7 that the number of particles of small size increases as the normal load increases. This should be expected since the effect of increasing the load is to enlarge the most favourable asperity contacts and then, as the compliance improves owing to squashing of the asperities, to form further contacts elsewhere which are usually small in size. Unfortunately examination of Table 1 shows that the load-dependence effect of the particle size of the wear debris does not hold very well with other alloys. Table 1 shows that there is no correlation between the size of the wear debris and the amount of silicon in the alloy. The apparently negative contribution of the particle size analysis is included, however, for the following reason. It is obvious that there is a fundamental difference between the mode of tribological interaction uis a’ uis the amount of silicon in a binary melt. When the matrix of the pin material is high in aluminium, material is deposited on the counterface, the amount of which increases with load at a given speed. This deposit is absent with the high silicon alloys and in fact the counterface wears at a rate which increases both with the amount of silicon and the applied normal load. There are thus two modes of wear, namely adhesive and two-body abrasion. It was therefore of interest to see whether or not the size distribution of wear debris will change with both load and silicon content. On the basis of the results obtained here, there is no significant correlation between the particle size and the load or silicon content.
5. Conclusions The present investigation shows: (1) the wear resistance of a binary Al-Si alloy improves with silicon content up to the near-eutectic composition; (2) the hypereutectic alloys wear more but by only a factor of about 2; (3) a high silicon alloy causes wear of the counterface even when this is a very hard steel; (4) the beneficial effect of silicon is that it minimizes the tendency for seizure; (5) a near-eutectic alloy has the best load-bearing capacity.
References 1 K. Okabayashi, Y. Nakatani, H. Notani and M. Kawamoto, Keikinzoku, 14 (1964) 2 A. D. Sarkar, Wear, 31 (1975) 331. 3 R. Shivanath, P. K. Sengupta and T. S. Eyre, Br. Foundryman, 70 (1977) 349. 4 A. D. Sarkar, Tribol. Int., (Aug. 1977) 235.
57.