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Wear characteristics of Al-Si alloys
49
Wear, 172 (1994) 49-58
Wear characteristics of Al-Si alloys H. Torabian,
J.P. Pathak and S.N. Tiwari
Department of Metallurgical Engineering, Institute of Technology, Banaras Hindu University,Varanasi - 221 005 (India)
(Received June 7, 1993; accepted October 14, 1993)
AbStraCt
The wear characteristics of Al-Si alloys containing 2-20 wt.% have been studied using a pin-on-disc type wear testing machine at room temperature. The effects of alloy composition, sliding distance, sliding speed and load on wear rate of Al-Si alloys have been investigated. It has been found that the wear rate is strongly dependent on alloy composition, applied load and sliding speed. The wear rate decreases and the load-bearing capacity of the alloy increases with increasing silicon content. The nature of the wear process changes with alloy composition and experimental conditions.
Among the materials of tribological importance, aluminium-silicon alloys have received considerable attention for practical as well as fundamental reasons. Both the hypoeutectic and hypereutectic Al-Si alloys have been in use for the tribological components of internal combustion engines in dry and lubricated contacts for a long time. The addition of silicon, apart from reducing the coefficient of thermal expansion, produces an alumini~ alloy with good wear, corrosion, casting and machining characteristics. A significant quanti~ of research work has been done in recent past years to study the wear behaviour of Al-Si alloys [l-lo]. However, out of these studies, only a few have been devoted to investigating systematically the effect of silicon on the wear and frictional characteristics of aluminium. Moreover, the conclusions derived from the above work are also contradictory. Vandelli [2] carried out experiments under reciprocating sliding conditions on three hypereutectic alloys (14.5%, 17% and 25% Si alloys) and concluded that wear resistance was probably governed by the dist~bution of the silicon particles and alumini~, as 17% Si alloy had the best wear resistance. Okabayashi et aL [3] have studied the wear and friction characteristics of Al-Si alloys and concluded that the hypoeutectic Al-Si alloys had a higher wear resistance than the hypereutectic alloys. In a further work [4] they showed that the refinement of primary silicon scarcely improved the wear resistance of hypereutectic Al-Si alloys and the amount of primary silicon crystals was a more important factor than their size for wear resistance. Shivanath ef al. f7f have studied ~3-1~8~4/$07.~ 0 1994 Elsevier Sequoia. Alt rights reserved SSDI 0043-1648f93106366-C
a series of Al-Si alloys, ranging in silicon content from 4% to 20% Si, and they found that the increase in the silicon content of the alloy led to an increase in the wear resistance of aluminium, except in the case of eutectic composition, which showed a high rate of wear. Sarkar [8,9] conducted experiments on a series of Al-Si alloys- and found that the wear resistance of Al-Si alloys improved with silicon content up to only near-eutectic composition, and the hypereutectic alloys wore more than the hypoeutectic alloys. Hence it is obvious from the above that there is still no clear agreement in the literature regarding the role of size, dist~bution and the amount of silicon particles on the wear resistance of Al-Si alloys. The present work therefore has been undertaken to investigate systematically the effect of silicon content over a wide range of composition on the wear resistance of Al-Si alloys and to study in detail the topography of the worn surfaces and sub-surfaces to elucidate the wear mechanisms that operate.
2. Experimental details
Simple binary Al--$ alloys varying in silicon content from 2 to 20 wt.% Si were chosen for the present investigations. The above alloys were prepared by a special foundry technique (impeller-mixing and chillcasting), the details of which are described elsewhere [ll]. This technique consists of preparing a homogeneous alloy by vigorous mixing in the molten state followed by rapid freezing in a metallic mould. Briefly,
TABLE
1. Chemical
composition
of AI-Si
alloys
Al-Si alloys for their wear behaviour against a high carbon-chromium steel disc (heat treated to a hardness of 61-64 HRC). The disc was driven by a d.c. motor which had a speed range of 25-2500 rev min ‘. Cylindrical test pins of 0.04 m length and 0.008 m diameter were machined from the ingots of the alloys prepared. The flat surface of both the test specimen and the steel disc were ground to a constant surface finish (centreline average of about 0.5 pm) and they were thoroughly degreased and dried before the commencement of each wear test. Specimens were initially weighed on a single-pan electrical balance capable of reading down to lo-’ kg. At the end of each wear test, specimens were wiped clean of wear particles, degreased and reweighed. The difference in the weights of the test pin before and after the test gave the weight loss from which the wear volume was calculated. The latter was studied as a function of the sliding distance (600-15 000 m), normal load (5-150 N) and sliding velocity (0.25-3 m s-l) for all the alloy compositions. A new specimen and fresh wear track on the disc were used for each wear run. All the wear experiments were carried out under unlubricated conditions at a relative humidity of about 60% and a room temperature of 303 K.
-_~__Alloy
Si (wt.%)
Fe (w.t%)
Cu (wt.%)
AI-2Si AklSi AI-6Si AI-8Si Al-l 1.6Si AI-12SSi AI-15Si AI-17Si AI-20Si
1.93 3.95 6.1 7.98 115 12.5 15.2 16.9 19.87
0.177 0.183 0.192 0.2 0.214 0.22 0.23 0.236 0.25
0.026 0.0285 0.03 0.033 0.037 0.0381 0.041 0.043 0.047
the weighed amount of commercially pure aluminium and Al-20 wt.% Si master alloy were charged into a fire clay-coated steel crucible and heated well above the liquidus temperature of the alloy desired. The impeller mixer was lowered into the melt and rotation began at the required speed. Impeller mixing of the molten alloy was carried out for about 5 min at a constant temperature and a constant speed of rotation. Cylindrical ingot castings were prepared by pouring the alloy, while mixing continued, into an iron mould placed beneath the crucible. The above procedure was repeated for all the alloy compositions and cylindrical ingot castings of 0.025 m diameter and 0.25 m length were made in the above runs. The chemical compositions of the prepared alloys are given in Table 1. In order to evaluate the mechanical properties of the alloys prepared in the above manner, specimens of the standard dimensions [12] required for tensile testing were machined from the ingots. All the tests were carried out at room temperature for different composition of alloys using an instron testing machine. Vickers hardness tests were performed on all specimens under a load of 49 N. The mechanical properties of the different alloys are shown in Table 2.
2.3. Metallographic examination In order to analyse the mode of wear during the test runs, various techniques of standard metallographic examination were used besides the visual examination. They consisted of optical and/or scanning electron microscopic examination of the worn test pin surfaces, longitudinal taper section of the wear pin, wear tracks and wear debris. The latter were studied for their colour, sizes and shapes, while X-ray diffraction analysis of the wear particles provided information about the phases formed/present during interaction of the mating surfaces.
2.2. Wear testing A pin-on-disc wear testing machine was used (details of which are described elsewhere [13]) to evaluate the TABLE
2. Mechanical
properties
of impeller-mixed,
Alloy composition (wt.%)
Ultimate tensile strength (MN m-‘)
0.2% proof (MN
AI-2%Si Al-I%Si Al-6%Si AG3%Si Al-l 1.6%Si Al-12.5%Si Al-lS%Si Al-17%Si AI-2O%Si
127.3 142.2 155.7 169.6 185.4 189.0 183.25 175.8 172.4
52.6 58.3 64.8 71.5 80.0 82.5 71.7 73.7 72.0
chill-cast tensile stress m-‘)
AI-Si
alloys
0.2% compressive proof stress (MN m-‘)
Elongation
Hardness
Density
(%)
(WN)
(kg m -3x
63.2 66.8 73.3 77.0 87.0 90.0 85.5 83.4 81.0
12.4 10.2 9.6 7.2 5.8 5.4 4.7 3.0 2.5
39.5 47.3 55.6 61.6 67.0 70.0 72.5 76.6 81.0
2.68 2.67 2.65 2.62 2.59 2.57 2.55 2.53 2.50
103)
H. Torabian et al. I Wear charactetdics
3. Results and discussion 3.1. Structure and mechanical properties Figures l(a)-(c) show representative photomicrographs of some of the Al-Si casting alloys, observed under an optical microscope. The effect of impellermixing and chill-casting of Al-Si alloys shows refinement of the eutectic silicon along with the dendrites of the primary aluminium-rich phase (a). Figure l(a) shows an optical micrograph of Al-8% Si alloy, and it may be seen that more-or-less rounded particles of cy-dendrites (light areas) are crystallized, which are surrounded by fine eutectic silicon. The rounded morphology of the primary aluminium-rich phase may be due to breaking of dendrite arms as a result of the stirring action still present in the liquid freezing at a fast rate in the metallic mould. The photomicrograph of Al-12.5% Si alloy (Fig. l(b)) shows not only the refinement of the eutectic silicon particles but also the change in its
of Al--& alloys
51
mo~holo~ (from the conventional needle shape to a nearly rounded shape) as a result of the high rate of cooling during freezing. However, it may be seen (Fig. l(c)) that the degree of refinement of the eutectic silicon decreased as the silicon content of the alloy increased beyond the eutectic com~sition. It may also be observed (Fig. l(c)), that the primary dendrites, along with the eutectic silicon particles, grow in size and the primary silicon phase appears as the silicon content is increased to 15% Si. Further, the primary silicon particles are surrounded by udendrites, suggesting a shift of the eutectic ~m~sition to higher silicon content and the late nucleation of primary silicon phase over the a-dendrites, which may be a consequence of a large undercooling resulting from the high rate of heat extraction in the chill mould. The effects of silicon on the mechanical properties of AI-Si alloys are well known [14,15]. The mechanical properties depend on the shape, size and distribution of eutectic and/or primary silicon particles. Small, spherical, uniformly distributed silicon particles improve the strength properties of Al-Si alloys. Table 2 shows the effect of composition on the mechanical properties of Al-Si alloys. It may be seen that as the amount of silicon increases, the strength properties of Al-Si alloys also increase up to the eutectic composition, after which they show a decline with further increase in the silicon content. However, the hardness increases and the elongation (%) decreases continuously with increasing silicon content of the alloy. The above observation may be largely attributed to the size, shape and distribution of silicon particles in the as-cast structures up to the eutectic composition. Silicon is present as fine particles and is uniformly distributed in the structure, and hence the strength properties increase. However, when the primary silicon appears as coarse polyhedral particles, the strength properties decrease with increasing silicon content, but the hardness goes on increasing because of the increase in the number of silicon particles. 3-2. Wear c~aracie~ti~~ 3.2.1. Efect
Fig. 1. Optical photomicrographs of Al-Si alloy: (a) Al-8 wt.% Si; (b) Al-12.5 wt.% Si; (c) Al-15 wt.% Si.
of sliding distance
To investigate the wear characteristics, the first phase of the experimental work is usually an evaluation of the change in wear volume with increasing sliding distance. Figure 2 shows results for such an evaluation, and the wear volume VS. sliding distance curves were found to be similar for all the compositions studied. It is obvious that initially there is a short period of running-in wear, which is followed by a period of steady state wear during which there is a linear relationship between the wear volume and sliding distance. The latter is in accordance with the pattern for most metallic materials, as derived theoretically as well as observed experimentally [16].
52
H
Crabian
et al. i Wear churacterd.~tics of’ Al-S
alky
30r
IX,
0
3
,
,
6
9
,
,
(
,
12 15 18 21 SLlDlNG DISTANCE (m x 600)
I
25
Fig. 2. Effect of sliding distance on wear volume of Al-$ alloys sliding at a surface speed of 1 m s-l under applied load of 2X9.81 N. Silicon content (wt.%): 0, 2; A, 4; 0, 8; 13, 12.5; A, 15 and X, 20.
3.2.2. Effecr of load Wear rates i.e. wear volume per unit sliding distance were computed for different alloy compositions by taking the slopes of their respective steady state wear curves. Such wear rates for different alloys have been investigated as a function of increasing load and the results are shown in Fig. 3. It may be seen that the wear rate is strongly dependent on the applied load and it increases linearly with load in three distinct regions in al1 the alloys. These regions may be referred to as mild wear, inte~ediate wear and severe wear, which are separated by sharp transition points. The above wear regimes are a function of alloy composition and applied load and the transition points are indicative of changes in the wear mechanism operating during a particular region. Mild wear is observed at low loads and shows a greater duration (occurs over a wider range of load) for higher silicon alloys. The intermediate and severe wear regions are distinguished from the mild region by higher rates of increase in the wear rate per unit weight. The latter values are 2 to 4 times higher in the severe wear region than those of the mild wear. It may be seen that the transition load at which the change from one wear region to another takes place increases with increasing silicon content of the alloy (Fig. 4). Furthermore, the load at which the seizure takes place (where the test pin and the disc become jammed) also increases with increasing silicon content of the alloy, to the extent that the hypereutectic alloys do not seize even at the maximum load used in the present investigation (Fig. 3). Shivanath ec al. [7] have identified at least two of the above types of wear region in their work, but their wear rate dependence on bad as well as the com~sition
0
4
2
6 8 10 LOAD fNx 9.81)
12
14
16
Fig. 3. Effect of load on wear rate of AI-Si alloys sliding at a surface speed of 1 m SC’. Silicon content (wt.%): l , 2; A, 4; 0, 8; II, 12.5; A, 15 and X, 20. S indicates seizure. 14 r 12 -
8-
6-
4-
2-
I 0
I
2
I
t
4 6 SILICON
I
I
*
I
8 10 12 14 CONTENT (wt W
Fig. 4. Effect of silicon content
I
I,
16
16
on the transition
20
Ioad.
was different from the present work. Dwarkadasa [17] and Kadhim and Dwarkadasa [18] in their subsequent work on different materials have also identified the mild and intermediate wear regions and showed in each case a linear dependence of wear rate on applied load, similar to the present work. 3.2.3. Efict of sliding velocity Figure 5 shows the variation of wear rate with sliding velocity at constant applied load for different alloys.
H. Torabian et al. f Wear characteristics
of Al-S
53
alloys
130
1
60 -
110 t
0
0.5
1.0 Speed
1.5
2.0
2.5
9
Y
3.0
ms-’
Fig. 5. Effect of sliding speed on wear rate of AI-Si alloys sliding under a load of 2X9.81 N. Silicon content (wt.%): 0, 2; A, 4; 0, 8; Cl, 12.5; & 15 and X, 20.
I 0
It may be observed that the wear rate initially decreases slightly with increasing sliding speed up to a certain value, beyond which there is a sharp rise in the wear rate, irrespective of the alloy composition. It may be observed that the speed at which the sharp increase in wear rate occurs increases with increasing silicon content up to the eutectic composition, beyond which there is not much change. 3.2.4. Effect of alloy composition The effect of silicon content on the wear rate of aluminium is shown in Fig. 6. It is seen that the wear rate of aluminium is strongly dependent on the silicon content of the alloy as the wear rate decreases continuously with increasing silicon content of the alloy under a variety of conditions of applied load. However, it may be noticed that this effect is more pronounced up to 15 wt.%, silicon as the decline in wear rate is relatively small beyond this composition. The morphology of silicon particles changes significantly (Fig. l(c)) as the silicon content exceeds eutectic composition. However, there is no clear evidence to believe that the size, shape and distribution of silicon particles play any significant role in determining the wear rate of Al-Si alloys. It is, therefore, the silicon content of the alloy, i.e. the total amount of silicon phase present in the structure, which appears to govern the wear rate of Al-Si alloys and the latter decreases with increasing silicon content. The findings of this work are therefore in agreement in the above respect with those of Dewhurst [5] and Clegg et al. [6], and disagree with that of Sarkar [8,9], who found that the wear rates of hypoeutectic alloys were lower than those of hypereutectic alloys.
1
II
2
4
’
11
1
“I
6 14 16 8 10 12 SILICON CONTENT (wt ‘k)
18
20
Fig. 6. Variation of wear rates of Al-Si alloys with silicon content under different loads (NX9.81): 0, 1; A, 2; A, 4; 0, 6; n , 8 and V, 10.
3.3. Iso-wear lines With the applied load on the y-axis and silicon content on the x-axis, a number of lines were obtained from the wear data of the previous sections such that each line represented one rate of wear at various combinations of load and silicon content. Figure 7 shows such iso-wear lines, which indicate that a low wear rate dictates a light applied load on a component. Further, as the silicon content increases, the loadcarrying capacity of the alloy increases. Such iso-wear lines are therefore useful for design engineers because one could set a limit on the amount of permissible wear rate, i.e. at any allowable wear rate, how much load can be born by a particular alloy composition. It is obvious that a heavy load can only be sustained if a high amount of wear can be tolerated. 3.4. Metallographic evidence There have been several attempts [19,20] at correlating the wear behaviour with the surface topography. It has been also recognized that surface wear is influenced by sub-surface deformation [21], which has been effectively used to show the occurrence of delamination and fracture of material resulting in material removal. Therefore in the present study analysis of debris particles produced during the test run and SEM examination of worn surfaces and sub-surface regions were carried out. At low loads, the formation of fine black powder is observed (Fig. 8(a)), and X-ray examination of this
to severe plastic deformation
in
the direction of sliding
is obvious from the Fig. lo(c). The formation ofmctallic debris by cracking and its propagation beneath the
2oxlz3
OXlP
SILICON CONTENT
(wt %I
Fig. 7. &o-wear lines for various combination content
for Al4
of load and silicon alloys sliding at a speed of 1 m s-l.
dark coloured material showed the presence of oxides, such as alumina, silica and iron oxide. SEM examination of the worn pin surface in the mild wear region shows (Fig. 9(a)) cracking and spalling of the oxide layers. The steel disc surface in the wear track was found to be almost scratch-free and smooth. The oxide debris so formed was transferred to the steel disc and was swept by the pin to form a heap at the edges of the wear track. With a further increase in load in the region of intermediate wear, a relatively coarse mixture of black powder and tiny metallic particles was produced (Fig. S(b)) and the latter increased in amount with increasing load. X-ray examination of this mixture confirmed the presence of the above types of oxides, as well as aluminium and silicon particles. Traces of iron were also detected by passing a magnet over this powder. SEM examination of worn pin surface in this region of wear shows some deformation of the surface layers, in addition to removal of oxide particles (Fig. 9(b)). The longitudinal taper section of the worn pm shows surface grooves formed by removal of the metallic debris from the edges and break-up of the primary silicon particles in the interior section of the pin (Figs. 10(a) and (b)). The formation of metallic wear particles and the appearance of the worn pin surface subjected to high loads (in the severe wear region) are shown in Figs. S(c) and 9(c). The massive scale of damage and plastic flow of the material are quite evident. The cracking and distortion of primary silicon particles due
worn edges as a result of general and localized plastic deformation of the matrix under severe conditions of loading is ah.0 obvious {Fig. 10(d)). The debris generated at high loads contained (Fig. 8(c)) several large particles exhibiting totally metallic lustre. The disc surface showed some degree of scoring by hard particles, particularly in the tests run against hypereutectic alloys, whereas in the case of low silicon hypoeutectic alloys, part of the debris adhered firmly to the disc surface and could only be removed with difficuhy. The appearance of the wear debris and topographical features of the worn surfaces during the study of the effect of sliding speed did not show any significant change from that found as shown above when increasing the load. The wear particles produced at low speeds were of uniformly fine size and dark in colour and the worn surface showed the formation of oxide layers (Fig. 9(d)). As the speed was increased the fine particles were compacted and delaminated as flakes occurred (Fig. 8(d)). At high speed, the wear fragments consisted of mostly coarse metallic particfes which were formed as a result of delamination of the plastically deformed surface layer (Figs. 8(e) and 9(e)). 3.5. Modes of wear Based on visual and metallographic evidence and wear results, it may be said that the wear of Al-Si alloys under the influence of increasing load occurs in three distinct regions: mild or oxidative wear under low loads, severe or metallic wear under high loads and a combination of oxidative and metallic wear under an inte~ediate range of loads. The mild wear is purely oxidative because of the ability of the surface material to oxidize under ambient conditions. As the surface oxide is removed, the fresh metal exposed is further oxidized, and cracking, along with spalling of the surface layers, generates oxidized debris and thus the wear in this region is controtled by fracture and removal of oxide. As supporting evidence, deformation in the subsurface regions was not observed. Smooth wear tracks were produced and low wear rates were obtained, because oxide layers produced on the pin and disc surfaces prevented adhesive wear. Furthermore, the wear debris produced during mild wear consisted of dark-coloured powder of much smaller size then the metallic particles formed under adhesive wear conditions. In addition, X-ray diffraction analysis indicated the composition of the powder to be made only of different oxides. The intermediate region of wear was characterized by the formation of a mixture of oxide and metallic debris particles. In this region, wear also occurred by
55
(e)
Fig. 8. Stereo micrographs of the wear debris of AI-Si alloys: (a) black fine particles at low loads; (b) mixture of fine black and metallic particles at ‘int&mediate loads; (c) large metallic particles at high loads; (d) fine compacted particles at low speeds; and (e) coarse metallic particles at high speeds.
some localized deformation of the substrate and breaking of the silicon particles. The wear track and the rough surface produced by scoring of hard particles became smoother by oxide filling the valleys during the course of sliding. Therefore, in this regime of combined oxidative-cum metallic wear, wear took place by a process of spalling of oxide layers, which occurred fairly irregularly over the surface, and also by plastic deformation and delamination of sub-surfaces, as at any one time only a part of the surface was in contact with the disc counterface. The latter process of wear dominated over the oxidative wear with increasing applied load, as evidenced by an increase in the quantity of metallic particles in the wear debris.
Severe wear occurred at high loads by the failure of the pin material at the interface generating entirely metallic debris, and in the extreme situations resulted in seizure. The deformation of the surface occurred on a fairly massive scale as the yield strength of the pin material was exceeded. The deformation in subsurface layers distorted and led to cracking of the silicon particles. The severe wear was thus controlled by the processes of adhesion and delamination, as well as abrasion by hard particles of the debris. With an increase in sliding speed, wear of Al-Si alloys occurs largely by the processes of adhesion and delamination. The change in wear rate with increasing sliding speed is based on the competing effects of the
H.
Tornbian
e
Fig. 9. Scanning electron micrographs of transverse sections of the test pin surface: (a) cracking and spalling of oxide layer at fow loads; (b) spailing of oxide layer and some substrate deformation at intermediate loads; (c) ploughing and shearing of the surface at high loads; (d) worn pin surface at low speeds; and (e) worn pin surface at high speeds.
temperature rise and strain rate. It is well known that the hardness of a material depends on the rate of indentation. At low sliding speeds, there will be an increase in strain rate which, in turn, increases the hardness or flow strength of the pin material. The temperature is developed only at the contact points which make up the true area of contact, and thus all the above factors contribute to the initial decrease in wear rate at low sliding speed. However, as the sliding speed increases, greater frictional heat generated at or below the asperity contacts is transferred to the other side of the pin, which softens the pin material, resulting in an increased true area of contact. This, in turn, leads to a higher wear rate. It may be thus said that
the effect of work hardening is more prominent than the temperature effect up to a critical sliding speed, above which the temperature effect dominates over the former, and this accounts for the increase in the wear rate at high sliding speeds.
4. Conclusions The following conclusions, can be drawn from the present work. 1. For a given load and constant sliding speed, the nature of the wear curve changes with an increase in sliding distance. Initially there is a period of
H. Torabian et al. I Wear characteristics of AI-S
alloys
Fig. 10. Optical photomicrographs of longitudinal taper section of the worn pins: (a) grooves at the intersection of the wear surface; (b) localized deformation of the surface and breaking up the silicon particles; (c) distortion and fracture of primary silicon particles; and (d) propagation of cracks and delamination.
2.
3.
4.
5.
6.
running-in wear, and this is followed by a linear relationship between wear volume and sliding distance for all the alloys studied. Wear rate increases linearly with increasing applied load in three distinct regions. All the alloys exhibit oxidative wear at low loads, combined oxidative-cum metallic wear at moderately high loads and metallic wear at high loads. The transition load changes with change in silicon content of the alloy. It increases with increasing silicon content of the alloy. The wear rate of Al-Si alloys initially decreases with increasing sliding speed up to a certain value, beyond which there is a rise in wear rate, irrespective of alloy composition. The wear rate is strongly dependent on alloy composition. It decreases continuously with increasing silicon content of the alloy. The load-bearing capacity of the alloy increases with increasing silicon content. The latter minimizes the tendency for seizure.
7. The nature of the wear process changes with change in alloy composition and experimental conditions. Wear fragments are produced by cracking and spalling of oxides, adhesion, abrasion and/or delamination of surface layers.
References 1 E.E. Stonebrook, Mod. Cast., 38 (1960) 111. 2 G. Vandelli, Aluminio, 37 (1968) 121. 3 K. Okabayashi, Y. Nakatani and H. Notani, J. Jpn. Inst. Light Metak, 14 (1964) 415. 4 K. Okabayashi and M. Kawamato, Bull. Univ. Osaka Prefecture A, 17 (1968) 199. 5 E.V. Dewhurst, Br. Found., 59 (1966) 1. 6 A.J. Clegg and A.A. Das, Br. Found., 70 (1977) 56. 7 R. Shivanath, P.K. Sengupta and T.S. Eyre, Br. Found., 70 (1977) 349. 8 A.D. Sarkar, Wear, 31 (1975) 331. 9 A.D. Sarkar and J. Clarke, Wear, 54 (1979) 7. 10 KM. Jasim and E.S. Dwarakadasa, Wear, I19 (1987) 119.
58
H. Torahian et al. I Wear characteristics of AL.5
11 J.P. Pathak, H. Torabian and S.N. Tiwari, As-cast structures and mechanical properties of impeller mixed chill-cast leaded aluminium silicon alloys, /. Mater. Sci., in press. 12 R.P. Lukens, Annual Book of ASM Standards, 10 (1978) 106. 13 J.P. Pathak, S.N. Tiwari and S.L. Malhotra, Wear, I22 (1986) 341. 14 L.F. Mondolfo, in Aluminium Alloys: Structure and Properties, Buttenvorths, London, 1976, p. 352.
alloys
15 N.R. Pillai, J. Sci. Indust. Hex, 26 (1967) 466. 16 A.D. Sarkar, Wear of Metalr, Pergamon, Oxford, 1976, p. 46. 17 ES. Dwarkadasa and R.S. Yaseen, Wear, 84 (1983) 375. 18 M.J. Kadhim and E.S. Dwarakadasa, Wear, 82 (1982) 377. 19 A.D. Sarkar and J. Clarke, Wear, 75 (1982) 71. 20 B.N. Pramila Bai, SK. Biswas and N.N. Kumtekar, Wear, 87 (1983) 237. 21 N.P. Suh, Wear, 25 (1973) 111.
Wear, 172 (1994) 49-58
Wear characteristics of Al-Si alloys H. Torabian,
J.P. Pathak and S.N. Tiwari
Department of Metallurgical Engineering, Institute of Technology, Banaras Hindu University,Varanasi - 221 005 (India)
(Received June 7, 1993; accepted October 14, 1993)
AbStraCt
The wear characteristics of Al-Si alloys containing 2-20 wt.% have been studied using a pin-on-disc type wear testing machine at room temperature. The effects of alloy composition, sliding distance, sliding speed and load on wear rate of Al-Si alloys have been investigated. It has been found that the wear rate is strongly dependent on alloy composition, applied load and sliding speed. The wear rate decreases and the load-bearing capacity of the alloy increases with increasing silicon content. The nature of the wear process changes with alloy composition and experimental conditions.
Among the materials of tribological importance, aluminium-silicon alloys have received considerable attention for practical as well as fundamental reasons. Both the hypoeutectic and hypereutectic Al-Si alloys have been in use for the tribological components of internal combustion engines in dry and lubricated contacts for a long time. The addition of silicon, apart from reducing the coefficient of thermal expansion, produces an alumini~ alloy with good wear, corrosion, casting and machining characteristics. A significant quanti~ of research work has been done in recent past years to study the wear behaviour of Al-Si alloys [l-lo]. However, out of these studies, only a few have been devoted to investigating systematically the effect of silicon on the wear and frictional characteristics of aluminium. Moreover, the conclusions derived from the above work are also contradictory. Vandelli [2] carried out experiments under reciprocating sliding conditions on three hypereutectic alloys (14.5%, 17% and 25% Si alloys) and concluded that wear resistance was probably governed by the dist~bution of the silicon particles and alumini~, as 17% Si alloy had the best wear resistance. Okabayashi et aL [3] have studied the wear and friction characteristics of Al-Si alloys and concluded that the hypoeutectic Al-Si alloys had a higher wear resistance than the hypereutectic alloys. In a further work [4] they showed that the refinement of primary silicon scarcely improved the wear resistance of hypereutectic Al-Si alloys and the amount of primary silicon crystals was a more important factor than their size for wear resistance. Shivanath ef al. f7f have studied ~3-1~8~4/$07.~ 0 1994 Elsevier Sequoia. Alt rights reserved SSDI 0043-1648f93106366-C
a series of Al-Si alloys, ranging in silicon content from 4% to 20% Si, and they found that the increase in the silicon content of the alloy led to an increase in the wear resistance of aluminium, except in the case of eutectic composition, which showed a high rate of wear. Sarkar [8,9] conducted experiments on a series of Al-Si alloys- and found that the wear resistance of Al-Si alloys improved with silicon content up to only near-eutectic composition, and the hypereutectic alloys wore more than the hypoeutectic alloys. Hence it is obvious from the above that there is still no clear agreement in the literature regarding the role of size, dist~bution and the amount of silicon particles on the wear resistance of Al-Si alloys. The present work therefore has been undertaken to investigate systematically the effect of silicon content over a wide range of composition on the wear resistance of Al-Si alloys and to study in detail the topography of the worn surfaces and sub-surfaces to elucidate the wear mechanisms that operate.
2. Experimental details
Simple binary Al--$ alloys varying in silicon content from 2 to 20 wt.% Si were chosen for the present investigations. The above alloys were prepared by a special foundry technique (impeller-mixing and chillcasting), the details of which are described elsewhere [ll]. This technique consists of preparing a homogeneous alloy by vigorous mixing in the molten state followed by rapid freezing in a metallic mould. Briefly,
TABLE
1. Chemical
composition
of AI-Si
alloys
Al-Si alloys for their wear behaviour against a high carbon-chromium steel disc (heat treated to a hardness of 61-64 HRC). The disc was driven by a d.c. motor which had a speed range of 25-2500 rev min ‘. Cylindrical test pins of 0.04 m length and 0.008 m diameter were machined from the ingots of the alloys prepared. The flat surface of both the test specimen and the steel disc were ground to a constant surface finish (centreline average of about 0.5 pm) and they were thoroughly degreased and dried before the commencement of each wear test. Specimens were initially weighed on a single-pan electrical balance capable of reading down to lo-’ kg. At the end of each wear test, specimens were wiped clean of wear particles, degreased and reweighed. The difference in the weights of the test pin before and after the test gave the weight loss from which the wear volume was calculated. The latter was studied as a function of the sliding distance (600-15 000 m), normal load (5-150 N) and sliding velocity (0.25-3 m s-l) for all the alloy compositions. A new specimen and fresh wear track on the disc were used for each wear run. All the wear experiments were carried out under unlubricated conditions at a relative humidity of about 60% and a room temperature of 303 K.
-_~__Alloy
Si (wt.%)
Fe (w.t%)
Cu (wt.%)
AI-2Si AklSi AI-6Si AI-8Si Al-l 1.6Si AI-12SSi AI-15Si AI-17Si AI-20Si
1.93 3.95 6.1 7.98 115 12.5 15.2 16.9 19.87
0.177 0.183 0.192 0.2 0.214 0.22 0.23 0.236 0.25
0.026 0.0285 0.03 0.033 0.037 0.0381 0.041 0.043 0.047
the weighed amount of commercially pure aluminium and Al-20 wt.% Si master alloy were charged into a fire clay-coated steel crucible and heated well above the liquidus temperature of the alloy desired. The impeller mixer was lowered into the melt and rotation began at the required speed. Impeller mixing of the molten alloy was carried out for about 5 min at a constant temperature and a constant speed of rotation. Cylindrical ingot castings were prepared by pouring the alloy, while mixing continued, into an iron mould placed beneath the crucible. The above procedure was repeated for all the alloy compositions and cylindrical ingot castings of 0.025 m diameter and 0.25 m length were made in the above runs. The chemical compositions of the prepared alloys are given in Table 1. In order to evaluate the mechanical properties of the alloys prepared in the above manner, specimens of the standard dimensions [12] required for tensile testing were machined from the ingots. All the tests were carried out at room temperature for different composition of alloys using an instron testing machine. Vickers hardness tests were performed on all specimens under a load of 49 N. The mechanical properties of the different alloys are shown in Table 2.
2.3. Metallographic examination In order to analyse the mode of wear during the test runs, various techniques of standard metallographic examination were used besides the visual examination. They consisted of optical and/or scanning electron microscopic examination of the worn test pin surfaces, longitudinal taper section of the wear pin, wear tracks and wear debris. The latter were studied for their colour, sizes and shapes, while X-ray diffraction analysis of the wear particles provided information about the phases formed/present during interaction of the mating surfaces.
2.2. Wear testing A pin-on-disc wear testing machine was used (details of which are described elsewhere [13]) to evaluate the TABLE
2. Mechanical
properties
of impeller-mixed,
Alloy composition (wt.%)
Ultimate tensile strength (MN m-‘)
0.2% proof (MN
AI-2%Si Al-I%Si Al-6%Si AG3%Si Al-l 1.6%Si Al-12.5%Si Al-lS%Si Al-17%Si AI-2O%Si
127.3 142.2 155.7 169.6 185.4 189.0 183.25 175.8 172.4
52.6 58.3 64.8 71.5 80.0 82.5 71.7 73.7 72.0
chill-cast tensile stress m-‘)
AI-Si
alloys
0.2% compressive proof stress (MN m-‘)
Elongation
Hardness
Density
(%)
(WN)
(kg m -3x
63.2 66.8 73.3 77.0 87.0 90.0 85.5 83.4 81.0
12.4 10.2 9.6 7.2 5.8 5.4 4.7 3.0 2.5
39.5 47.3 55.6 61.6 67.0 70.0 72.5 76.6 81.0
2.68 2.67 2.65 2.62 2.59 2.57 2.55 2.53 2.50
103)
H. Torabian et al. I Wear charactetdics
3. Results and discussion 3.1. Structure and mechanical properties Figures l(a)-(c) show representative photomicrographs of some of the Al-Si casting alloys, observed under an optical microscope. The effect of impellermixing and chill-casting of Al-Si alloys shows refinement of the eutectic silicon along with the dendrites of the primary aluminium-rich phase (a). Figure l(a) shows an optical micrograph of Al-8% Si alloy, and it may be seen that more-or-less rounded particles of cy-dendrites (light areas) are crystallized, which are surrounded by fine eutectic silicon. The rounded morphology of the primary aluminium-rich phase may be due to breaking of dendrite arms as a result of the stirring action still present in the liquid freezing at a fast rate in the metallic mould. The photomicrograph of Al-12.5% Si alloy (Fig. l(b)) shows not only the refinement of the eutectic silicon particles but also the change in its
of Al--& alloys
51
mo~holo~ (from the conventional needle shape to a nearly rounded shape) as a result of the high rate of cooling during freezing. However, it may be seen (Fig. l(c)) that the degree of refinement of the eutectic silicon decreased as the silicon content of the alloy increased beyond the eutectic com~sition. It may also be observed (Fig. l(c)), that the primary dendrites, along with the eutectic silicon particles, grow in size and the primary silicon phase appears as the silicon content is increased to 15% Si. Further, the primary silicon particles are surrounded by udendrites, suggesting a shift of the eutectic ~m~sition to higher silicon content and the late nucleation of primary silicon phase over the a-dendrites, which may be a consequence of a large undercooling resulting from the high rate of heat extraction in the chill mould. The effects of silicon on the mechanical properties of AI-Si alloys are well known [14,15]. The mechanical properties depend on the shape, size and distribution of eutectic and/or primary silicon particles. Small, spherical, uniformly distributed silicon particles improve the strength properties of Al-Si alloys. Table 2 shows the effect of composition on the mechanical properties of Al-Si alloys. It may be seen that as the amount of silicon increases, the strength properties of Al-Si alloys also increase up to the eutectic composition, after which they show a decline with further increase in the silicon content. However, the hardness increases and the elongation (%) decreases continuously with increasing silicon content of the alloy. The above observation may be largely attributed to the size, shape and distribution of silicon particles in the as-cast structures up to the eutectic composition. Silicon is present as fine particles and is uniformly distributed in the structure, and hence the strength properties increase. However, when the primary silicon appears as coarse polyhedral particles, the strength properties decrease with increasing silicon content, but the hardness goes on increasing because of the increase in the number of silicon particles. 3-2. Wear c~aracie~ti~~ 3.2.1. Efect
Fig. 1. Optical photomicrographs of Al-Si alloy: (a) Al-8 wt.% Si; (b) Al-12.5 wt.% Si; (c) Al-15 wt.% Si.
of sliding distance
To investigate the wear characteristics, the first phase of the experimental work is usually an evaluation of the change in wear volume with increasing sliding distance. Figure 2 shows results for such an evaluation, and the wear volume VS. sliding distance curves were found to be similar for all the compositions studied. It is obvious that initially there is a short period of running-in wear, which is followed by a period of steady state wear during which there is a linear relationship between the wear volume and sliding distance. The latter is in accordance with the pattern for most metallic materials, as derived theoretically as well as observed experimentally [16].
52
H
Crabian
et al. i Wear churacterd.~tics of’ Al-S
alky
30r
IX,
0
3
,
,
6
9
,
,
(
,
12 15 18 21 SLlDlNG DISTANCE (m x 600)
I
25
Fig. 2. Effect of sliding distance on wear volume of Al-$ alloys sliding at a surface speed of 1 m s-l under applied load of 2X9.81 N. Silicon content (wt.%): 0, 2; A, 4; 0, 8; 13, 12.5; A, 15 and X, 20.
3.2.2. Effecr of load Wear rates i.e. wear volume per unit sliding distance were computed for different alloy compositions by taking the slopes of their respective steady state wear curves. Such wear rates for different alloys have been investigated as a function of increasing load and the results are shown in Fig. 3. It may be seen that the wear rate is strongly dependent on the applied load and it increases linearly with load in three distinct regions in al1 the alloys. These regions may be referred to as mild wear, inte~ediate wear and severe wear, which are separated by sharp transition points. The above wear regimes are a function of alloy composition and applied load and the transition points are indicative of changes in the wear mechanism operating during a particular region. Mild wear is observed at low loads and shows a greater duration (occurs over a wider range of load) for higher silicon alloys. The intermediate and severe wear regions are distinguished from the mild region by higher rates of increase in the wear rate per unit weight. The latter values are 2 to 4 times higher in the severe wear region than those of the mild wear. It may be seen that the transition load at which the change from one wear region to another takes place increases with increasing silicon content of the alloy (Fig. 4). Furthermore, the load at which the seizure takes place (where the test pin and the disc become jammed) also increases with increasing silicon content of the alloy, to the extent that the hypereutectic alloys do not seize even at the maximum load used in the present investigation (Fig. 3). Shivanath ec al. [7] have identified at least two of the above types of wear region in their work, but their wear rate dependence on bad as well as the com~sition
0
4
2
6 8 10 LOAD fNx 9.81)
12
14
16
Fig. 3. Effect of load on wear rate of AI-Si alloys sliding at a surface speed of 1 m SC’. Silicon content (wt.%): l , 2; A, 4; 0, 8; II, 12.5; A, 15 and X, 20. S indicates seizure. 14 r 12 -
8-
6-
4-
2-
I 0
I
2
I
t
4 6 SILICON
I
I
*
I
8 10 12 14 CONTENT (wt W
Fig. 4. Effect of silicon content
I
I,
16
16
on the transition
20
Ioad.
was different from the present work. Dwarkadasa [17] and Kadhim and Dwarkadasa [18] in their subsequent work on different materials have also identified the mild and intermediate wear regions and showed in each case a linear dependence of wear rate on applied load, similar to the present work. 3.2.3. Efict of sliding velocity Figure 5 shows the variation of wear rate with sliding velocity at constant applied load for different alloys.
H. Torabian et al. f Wear characteristics
of Al-S
53
alloys
130
1
60 -
110 t
0
0.5
1.0 Speed
1.5
2.0
2.5
9
Y
3.0
ms-’
Fig. 5. Effect of sliding speed on wear rate of AI-Si alloys sliding under a load of 2X9.81 N. Silicon content (wt.%): 0, 2; A, 4; 0, 8; Cl, 12.5; & 15 and X, 20.
I 0
It may be observed that the wear rate initially decreases slightly with increasing sliding speed up to a certain value, beyond which there is a sharp rise in the wear rate, irrespective of the alloy composition. It may be observed that the speed at which the sharp increase in wear rate occurs increases with increasing silicon content up to the eutectic composition, beyond which there is not much change. 3.2.4. Effect of alloy composition The effect of silicon content on the wear rate of aluminium is shown in Fig. 6. It is seen that the wear rate of aluminium is strongly dependent on the silicon content of the alloy as the wear rate decreases continuously with increasing silicon content of the alloy under a variety of conditions of applied load. However, it may be noticed that this effect is more pronounced up to 15 wt.%, silicon as the decline in wear rate is relatively small beyond this composition. The morphology of silicon particles changes significantly (Fig. l(c)) as the silicon content exceeds eutectic composition. However, there is no clear evidence to believe that the size, shape and distribution of silicon particles play any significant role in determining the wear rate of Al-Si alloys. It is, therefore, the silicon content of the alloy, i.e. the total amount of silicon phase present in the structure, which appears to govern the wear rate of Al-Si alloys and the latter decreases with increasing silicon content. The findings of this work are therefore in agreement in the above respect with those of Dewhurst [5] and Clegg et al. [6], and disagree with that of Sarkar [8,9], who found that the wear rates of hypoeutectic alloys were lower than those of hypereutectic alloys.
1
II
2
4
’
11
1
“I
6 14 16 8 10 12 SILICON CONTENT (wt ‘k)
18
20
Fig. 6. Variation of wear rates of Al-Si alloys with silicon content under different loads (NX9.81): 0, 1; A, 2; A, 4; 0, 6; n , 8 and V, 10.
3.3. Iso-wear lines With the applied load on the y-axis and silicon content on the x-axis, a number of lines were obtained from the wear data of the previous sections such that each line represented one rate of wear at various combinations of load and silicon content. Figure 7 shows such iso-wear lines, which indicate that a low wear rate dictates a light applied load on a component. Further, as the silicon content increases, the loadcarrying capacity of the alloy increases. Such iso-wear lines are therefore useful for design engineers because one could set a limit on the amount of permissible wear rate, i.e. at any allowable wear rate, how much load can be born by a particular alloy composition. It is obvious that a heavy load can only be sustained if a high amount of wear can be tolerated. 3.4. Metallographic evidence There have been several attempts [19,20] at correlating the wear behaviour with the surface topography. It has been also recognized that surface wear is influenced by sub-surface deformation [21], which has been effectively used to show the occurrence of delamination and fracture of material resulting in material removal. Therefore in the present study analysis of debris particles produced during the test run and SEM examination of worn surfaces and sub-surface regions were carried out. At low loads, the formation of fine black powder is observed (Fig. 8(a)), and X-ray examination of this
to severe plastic deformation
in
the direction of sliding
is obvious from the Fig. lo(c). The formation ofmctallic debris by cracking and its propagation beneath the
2oxlz3
OXlP
SILICON CONTENT
(wt %I
Fig. 7. &o-wear lines for various combination content
for Al4
of load and silicon alloys sliding at a speed of 1 m s-l.
dark coloured material showed the presence of oxides, such as alumina, silica and iron oxide. SEM examination of the worn pin surface in the mild wear region shows (Fig. 9(a)) cracking and spalling of the oxide layers. The steel disc surface in the wear track was found to be almost scratch-free and smooth. The oxide debris so formed was transferred to the steel disc and was swept by the pin to form a heap at the edges of the wear track. With a further increase in load in the region of intermediate wear, a relatively coarse mixture of black powder and tiny metallic particles was produced (Fig. S(b)) and the latter increased in amount with increasing load. X-ray examination of this mixture confirmed the presence of the above types of oxides, as well as aluminium and silicon particles. Traces of iron were also detected by passing a magnet over this powder. SEM examination of worn pin surface in this region of wear shows some deformation of the surface layers, in addition to removal of oxide particles (Fig. 9(b)). The longitudinal taper section of the worn pm shows surface grooves formed by removal of the metallic debris from the edges and break-up of the primary silicon particles in the interior section of the pin (Figs. 10(a) and (b)). The formation of metallic wear particles and the appearance of the worn pin surface subjected to high loads (in the severe wear region) are shown in Figs. S(c) and 9(c). The massive scale of damage and plastic flow of the material are quite evident. The cracking and distortion of primary silicon particles due
worn edges as a result of general and localized plastic deformation of the matrix under severe conditions of loading is ah.0 obvious {Fig. 10(d)). The debris generated at high loads contained (Fig. 8(c)) several large particles exhibiting totally metallic lustre. The disc surface showed some degree of scoring by hard particles, particularly in the tests run against hypereutectic alloys, whereas in the case of low silicon hypoeutectic alloys, part of the debris adhered firmly to the disc surface and could only be removed with difficuhy. The appearance of the wear debris and topographical features of the worn surfaces during the study of the effect of sliding speed did not show any significant change from that found as shown above when increasing the load. The wear particles produced at low speeds were of uniformly fine size and dark in colour and the worn surface showed the formation of oxide layers (Fig. 9(d)). As the speed was increased the fine particles were compacted and delaminated as flakes occurred (Fig. 8(d)). At high speed, the wear fragments consisted of mostly coarse metallic particfes which were formed as a result of delamination of the plastically deformed surface layer (Figs. 8(e) and 9(e)). 3.5. Modes of wear Based on visual and metallographic evidence and wear results, it may be said that the wear of Al-Si alloys under the influence of increasing load occurs in three distinct regions: mild or oxidative wear under low loads, severe or metallic wear under high loads and a combination of oxidative and metallic wear under an inte~ediate range of loads. The mild wear is purely oxidative because of the ability of the surface material to oxidize under ambient conditions. As the surface oxide is removed, the fresh metal exposed is further oxidized, and cracking, along with spalling of the surface layers, generates oxidized debris and thus the wear in this region is controtled by fracture and removal of oxide. As supporting evidence, deformation in the subsurface regions was not observed. Smooth wear tracks were produced and low wear rates were obtained, because oxide layers produced on the pin and disc surfaces prevented adhesive wear. Furthermore, the wear debris produced during mild wear consisted of dark-coloured powder of much smaller size then the metallic particles formed under adhesive wear conditions. In addition, X-ray diffraction analysis indicated the composition of the powder to be made only of different oxides. The intermediate region of wear was characterized by the formation of a mixture of oxide and metallic debris particles. In this region, wear also occurred by
55
(e)
Fig. 8. Stereo micrographs of the wear debris of AI-Si alloys: (a) black fine particles at low loads; (b) mixture of fine black and metallic particles at ‘int&mediate loads; (c) large metallic particles at high loads; (d) fine compacted particles at low speeds; and (e) coarse metallic particles at high speeds.
some localized deformation of the substrate and breaking of the silicon particles. The wear track and the rough surface produced by scoring of hard particles became smoother by oxide filling the valleys during the course of sliding. Therefore, in this regime of combined oxidative-cum metallic wear, wear took place by a process of spalling of oxide layers, which occurred fairly irregularly over the surface, and also by plastic deformation and delamination of sub-surfaces, as at any one time only a part of the surface was in contact with the disc counterface. The latter process of wear dominated over the oxidative wear with increasing applied load, as evidenced by an increase in the quantity of metallic particles in the wear debris.
Severe wear occurred at high loads by the failure of the pin material at the interface generating entirely metallic debris, and in the extreme situations resulted in seizure. The deformation of the surface occurred on a fairly massive scale as the yield strength of the pin material was exceeded. The deformation in subsurface layers distorted and led to cracking of the silicon particles. The severe wear was thus controlled by the processes of adhesion and delamination, as well as abrasion by hard particles of the debris. With an increase in sliding speed, wear of Al-Si alloys occurs largely by the processes of adhesion and delamination. The change in wear rate with increasing sliding speed is based on the competing effects of the
H.
Tornbian
e
Fig. 9. Scanning electron micrographs of transverse sections of the test pin surface: (a) cracking and spalling of oxide layer at fow loads; (b) spailing of oxide layer and some substrate deformation at intermediate loads; (c) ploughing and shearing of the surface at high loads; (d) worn pin surface at low speeds; and (e) worn pin surface at high speeds.
temperature rise and strain rate. It is well known that the hardness of a material depends on the rate of indentation. At low sliding speeds, there will be an increase in strain rate which, in turn, increases the hardness or flow strength of the pin material. The temperature is developed only at the contact points which make up the true area of contact, and thus all the above factors contribute to the initial decrease in wear rate at low sliding speed. However, as the sliding speed increases, greater frictional heat generated at or below the asperity contacts is transferred to the other side of the pin, which softens the pin material, resulting in an increased true area of contact. This, in turn, leads to a higher wear rate. It may be thus said that
the effect of work hardening is more prominent than the temperature effect up to a critical sliding speed, above which the temperature effect dominates over the former, and this accounts for the increase in the wear rate at high sliding speeds.
4. Conclusions The following conclusions, can be drawn from the present work. 1. For a given load and constant sliding speed, the nature of the wear curve changes with an increase in sliding distance. Initially there is a period of
H. Torabian et al. I Wear characteristics of AI-S
alloys
Fig. 10. Optical photomicrographs of longitudinal taper section of the worn pins: (a) grooves at the intersection of the wear surface; (b) localized deformation of the surface and breaking up the silicon particles; (c) distortion and fracture of primary silicon particles; and (d) propagation of cracks and delamination.
2.
3.
4.
5.
6.
running-in wear, and this is followed by a linear relationship between wear volume and sliding distance for all the alloys studied. Wear rate increases linearly with increasing applied load in three distinct regions. All the alloys exhibit oxidative wear at low loads, combined oxidative-cum metallic wear at moderately high loads and metallic wear at high loads. The transition load changes with change in silicon content of the alloy. It increases with increasing silicon content of the alloy. The wear rate of Al-Si alloys initially decreases with increasing sliding speed up to a certain value, beyond which there is a rise in wear rate, irrespective of alloy composition. The wear rate is strongly dependent on alloy composition. It decreases continuously with increasing silicon content of the alloy. The load-bearing capacity of the alloy increases with increasing silicon content. The latter minimizes the tendency for seizure.
7. The nature of the wear process changes with change in alloy composition and experimental conditions. Wear fragments are produced by cracking and spalling of oxides, adhesion, abrasion and/or delamination of surface layers.
References 1 E.E. Stonebrook, Mod. Cast., 38 (1960) 111. 2 G. Vandelli, Aluminio, 37 (1968) 121. 3 K. Okabayashi, Y. Nakatani and H. Notani, J. Jpn. Inst. Light Metak, 14 (1964) 415. 4 K. Okabayashi and M. Kawamato, Bull. Univ. Osaka Prefecture A, 17 (1968) 199. 5 E.V. Dewhurst, Br. Found., 59 (1966) 1. 6 A.J. Clegg and A.A. Das, Br. Found., 70 (1977) 56. 7 R. Shivanath, P.K. Sengupta and T.S. Eyre, Br. Found., 70 (1977) 349. 8 A.D. Sarkar, Wear, 31 (1975) 331. 9 A.D. Sarkar and J. Clarke, Wear, 54 (1979) 7. 10 KM. Jasim and E.S. Dwarakadasa, Wear, I19 (1987) 119.
58
H. Torahian et al. I Wear characteristics of AL.5
11 J.P. Pathak, H. Torabian and S.N. Tiwari, As-cast structures and mechanical properties of impeller mixed chill-cast leaded aluminium silicon alloys, /. Mater. Sci., in press. 12 R.P. Lukens, Annual Book of ASM Standards, 10 (1978) 106. 13 J.P. Pathak, S.N. Tiwari and S.L. Malhotra, Wear, I22 (1986) 341. 14 L.F. Mondolfo, in Aluminium Alloys: Structure and Properties, Buttenvorths, London, 1976, p. 352.
alloys
15 N.R. Pillai, J. Sci. Indust. Hex, 26 (1967) 466. 16 A.D. Sarkar, Wear of Metalr, Pergamon, Oxford, 1976, p. 46. 17 ES. Dwarkadasa and R.S. Yaseen, Wear, 84 (1983) 375. 18 M.J. Kadhim and E.S. Dwarakadasa, Wear, 82 (1982) 377. 19 A.D. Sarkar and J. Clarke, Wear, 75 (1982) 71. 20 B.N. Pramila Bai, SK. Biswas and N.N. Kumtekar, Wear, 87 (1983) 237. 21 N.P. Suh, Wear, 25 (1973) 111.