Wear, 140 (1990) 83-92

The effect of lead content on the wear characteristics stir-cast Al-Pb alloy

Sunil Mohau, Vijaya Agarvvala and Subrata Ray*

83

of a

Metallurgical E?@ne&w Department, University of Roorkee, Rowhze 247667 (India)

(Received August 2, 1989; revised December 13, 1989; accepted February 13, 1990)

Abstract

The present investigation was carried out to determine the effect of lead content in stir- cast Al-Pb alloys. It was found that dispersion of about 8-20 wt.% Pb in commercial aluminum modifies the wear characteristics of alummum by introducing a transition from mild to severe wear with increasing sliding velocity. Beyond 20 wt.% Pb, the bulk wear was found to increase gradually.

1. Introduction

Amongst the ~~~-b~ed alIoys, Alan alloys are wide& used in industrial bearings. The replacement of tin by lead is of interest because, besides being cheaper, lead ensures a better interfacial 6l.m of lubricant than tin does [ 11. However, the production of Al-Pb alloys by conventional techniques poses problems of segregation owing to mutual immiscibiily and density differences [2]. Further, it is reported [ 1 ] that the resistance to deformation decreases from 115 to 100 MPa when the lead content is increased from 10 to 40 wt.%. It is also seen that the ultimate tensile strength (UTS) and hardness of Al-Pb alloys decrease with increasing lead content.

The present investigation was undertaken to determine the effect of lead content in stir-cast Al-Pb alloys with a view to identifying the desired levels of lead which offer good wear resistance to the alloy with respect to the applied load and the sliding velocity, The wear mech~ was studied through optical and scanning electron microscopy examinations of the wear track and wear debris.

2. Experimental details

2.1. stir mah7.g The schematic diagram of the experimental set-up shown in Pig. 1 has

been used by other investigators [3, 41. The 0.7 kg of the commercially pure al-m is charged in the graphite crucible kept ln the resistance-

*Present addressz Department of MatedaIs, College of Engineering and Applied Sciences, University of W~~o~~-~~ee, WI 63201, U.S.A.

0043-1648/90/$3.60 (B Elsevier Sequoia/Printed in The NetherIauds

Fig. 1. Schematic diagram of the experimental set-up.

Fig. 2. Photograph of the test block and cup used in the wear test.

heated mufBe furnace. When the molten metal reached 1173 K, deoxidizers were added and the preheated baffles pushed into the crucible through the molten metal. The desired amounts of lead shot were added through an alumimun foil hopper and the melt agitated at 53 S’ with a preheated ceramic-coated four-bladed flat stirrer. Meanwhile the furnace was switched off to allow natural cooling of the melt. When the melt reached 973 K, the graphite stopper plug in the hole at the bottom of the crucible was removed and the turbulent melt poured into a copper mold surrounded by an ice-brine solution. The chemical compositions of the stir-cast alloys were determined by wet chemical analysis and by the inductively coupled plasma (ICP) method for trace elements. The densities of the alloys were dete~ed by the weight loss method.

2.2. Wear testing Polished block specimens 12 mm x 12 mm x 20 mm were used to test

the wear characteristics of the stir-cast Al-Pb alloy against the surface of a standard test ring of carburized steel hardened to a Rockwell C hardness of 60-62 HRC as provided in a Timken wear and lubricant testing machine. Figure 2 shows the typical ring-and-block specimens used in the wear test. The blocks are polished up to 4/O grade emery paper to arrive at the standard size and surface finish. The loss in weight of the block specimen during the wear test is a measure of wear loss. The bulk wear is the weight loss of the specimen in sliding a 77 m distance.

The wear tracks were examined using twmning electron microscopy by observing the condition of the contacting surfaces of the test piece.

3. Resdts

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3.1. Chemical composition and microstrwture The chemical composition of the stir-cast Al-Pb alloys was determined

by wet chemical analysis and found to contain minor apples of about 0.69 wt.% Fe and 0.05 wt.% Si. The amount of lead varied from about 4 to about 56 wt.% and consequently the density of the stir-cast alloys changed as given in Table 1. Figure 3(a) shows a scanning electron micrograph of stir-cast Al-Pb alloy with about 56 wt.% Pb. The lead particles are mostly spherical in shape as shown in F&. 3(b) for stir-cast AI-PI, alloy with about 10 wt.% Pb.

3.2. Wear behavior The bulk wear varied with load at a given sliding velocity of 25.67 X lo-*

m s’ for the block specimens of stir-cast Al-Pb alloys. The variation in bulk

TABLE 1

Lead content and the density of the stir-cast Al-Pb alloys

ScmpL

1 ‘2 3 4

5 6 7

Pb content Dsnsdty

Mt.%) Cg cm-9

3.69 2.78 8.04 2.88

10.39 2.93 20.09 3.19 30.18 3.51 47.57 4.23 55.83 4.70

Fig. 3. Scanning electron micrographs Al-lOwt.%Pb.

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wear for commercially pure auburn has also been plotted in these figures for reference as shown in F’ig. 4. This figure shows that the bulk wear increases with increase in load. The same data when replotted as in Fig. 5 show that, between a lead content of 8 and 20 wt.%, the bulk wear has very little variation with lead content but increases gradually beyond a lead content of about 20 wt.%. It may also be noted that the bulk wear of pure lead is about 2.5-3 times higher than that of commercially pure alumimun.

12 -. _ .._..... _~

o 47. 56 Wt % Pb B 30. 16 Wt % Pb

t 10 m 20.09 Wt % Pb o IO 39 Wt % Pb o 6. OL Wt % Pb

ID o 3.69 Wt% Pb ‘5? *

x

,”

:6

z 1 ;

mL

01 I 1 I I ,

1L 16 16 20 22 24 2

Load, kgf -

Fig. 4. The variation in bulk wear with load in the case of stir-cast Al-Pb alloys and pure ahlminum.

0 I I I I I

0 10 20 30 LO 50 c

Wt % Lead -

F’ig. 5. The variation in buik wear witk lead content of stir-east Al-Pb alloys at dii%rent loads~

Figure 6 shows that the bulk wear decreases with increasing sliding velocity, passes through a minimum and then increases at higher sliding velocities. Between sliding velocities of 20 X 10S2 and 50 X low2 m s-’ there is very little change in wear in alloys containing about 20 wt.% Pb. It is also interesting to note that the wear reduces as lead content is increased up to about 20 wt.% but then it increases with further addition of lead. The wear of the samples with more than 20 wt.% Pb increases gradually with reduction in sliding velocity contrary to that observed for alloys with lead content lower than 20 wt.%. Figure 7 shows the variation in the minimum bulk wear and the corresponding sliding velocity at a load of 19.4 kgf in samples with different lead contents. The sliding velocity increases linearly with an increase in lead content but there is a change in the trend in variation of minimum wear at around 20 wt.% Pb.

F’igures 8(a), 8(b), 8(c) and 8(d) show the scanning electron micrographs of the wear tracks in samples of pure ahuninum and stir-cast Al-Pb alloys containing 3.69, 20.09 and 55.83 wt.% Pb respectively and tested at a load of 19.4 kgf and sliding velocity of 38.5X 10e2 m s-l.

Figure 9 shows the secondary-electron image along with the backscattered electron image of the debris obtained in a particular region, during the wear test in stir-cast AI-20.09 wt.% Pb alloy at a sliding velocity of 25.67X 10e2

3.69 Wt % Pb

8.OL Wt % Pb 10.39 Wt ?‘a Pb

1 I I I I I 1 I , (bl

10 15 20 25 30 35 LO $5 50 5

Slidkng velocity, m/s x 10 -

Fig. 6. The variation in bulk wear with sliding velocity in the case of stir-cast Al-Pb alloys and pure akmdnum.

88

” l

V 101 I I I I I I

0 5 10 15 20 25 30 35

Wt % Lead -t

Fig. 7. The variation in minimum bulk wear and the corresponding sliding velocity in the case of stir-cast Al-Pb alloys.

cc> Cdl

Fig. 8. Scanning electron micrographs of the wear track in the case of stir-cast Al-Pb alloys and pure aluminum: (a) pure aluminum; (b) AMwt.%Pb; (c) Al-2Owt.%Pb; (d) Al-&hvt.%Pb.

89

Fig. 9. Seconds-elec~n and backscattered electron images of wear debris in the case of Al-lOwt.%Pb alloy.

m s-l. Fine oxide particles and elongated large metallic particles stretching out of the wear track are observed.

4. Discussion

A large miscibility gap and a large difference in specific gravities of lead and ~~~ makes the production of Al-Pb alloys very difficult [ 1, 2, 51. To disperse lead in ahuninum, vigorous stirring is employed during casting and a copper mold cooled by an ice-brine solution is used for rapid solidification to prevent segregation of molten lead particles. The distribution of lead particles obtained by these techniques is shown in Fig. 3.

The results of the wear tests shown in Fig. 4 reveal that the bulk wear continuously increases with increasing load as can be seen in the case of stir-cast Al-Pb alloys with different lead contents along with commercially pure aluminum. The same phenomenon has already been observed by other workers [ 6-111. According to Archard [ 121, wear W is directly proportional to load P under dry sliding conditions as given by the following equation:

KPS w=-_- (1)

P,

where s is the sliding distance, p, is the hardness of the material under test and K is a constant which has a larger value for severe wear than for mild wear. When sliding takes place between two surfaces, repeated welding and fracture through the weaker metal causes the wear. Thus it is apparent that at higher loads the enhanced rate of fracture will result in a larger amount of wear [ 13, 14f. Further, at higher loads, the temperature attained by the specimen during sliding is also higher because of an increased amount of heat generation owing to friction. Therefore welding becomes more pronounced and extensive and results in a higher wear rate. However, contrary to the

90

linear rate of wear given by eqn. (1), the increase in the bulk wear with load is non-linear. Figure 5 shows the variation in bulk wear with lead content at Merent loads. At a given load, the bulk wear decreases with increasing lead content in the alloy up to 20 wt.% owing to the smearing of a thin layer of lead over the al&run matrix as shown in Pigs. 8(b), 8(c) and 8(d). The smeared area of lead in the matrix increases with lead content up to 20 wt.% Pb but beyond this level there is thickening of the smeared lead layer, resulting in deep wear tracks and fragmentation of the lead layer as shown in Pig. 8(d). When the lead layer is thin, as shown in Pig. 8(c), it provides a solid lubricant layer [ 1 ] and the wear tracks are formed in and resisted primarily by the alu~~ below it. When the thickness of the lead layer is increased, the wear tracks are within the layer of lead and the wear resistance in these areas is similar to that in pure lead. Consequently the rates of wear are much higher at higher lead contents above 20 wt.% in stir-cast Al-Pb alloy. Similar variations in wear behavior with film thickness has been observed by other workers [ 15, 161 for different soft metals when coated on hard substrate.

Figure 6 shows that the bulk wear decreases initially with increase in sliding velocity and then starts to increase as observed earlier by other workers 16, 8, 17j. There is a minimum in bulk wear which shifts towards higher sliding velocities when the lead content is increased as can be seen in Pig. 7. This behavior may have resulted from the relative domination of the process by the spreading lead layer which reduces the wear and the process of enhanced rate of heat generation, leading to an increase in wear. The trend of the variation in bulk wear with sliding velocity in the range (12.83-51.33) x lo-’ m s’ is reversed when the lead content is greater than 20 wt.% as shown in Pig. 6. This fact indicates a basic change in the nature of surface. The wear tracks shown in Fig. 8(a) reveal an uneven terraced surface in commercial aluminum caused by localized fracture. This unevenness of the surface has largely disappeared with the addition of about 4 wt.% Pb in the stir-cast alloy as shown in Fig. 8(b). However, wear tracks are relatively deep in darker regions of the surface compared with those in the relatively white regions of smeared lead. The wear track in samples with about 20 wt.% Pb has a relatively thin but more uniformly spread layer of lead as shown in Pig. S(c) and the wear tracks are quite superficial. When the lead content of the stir-cast Al-Pb alloy is raised to about 56 wt.%, there is a thicker layer of lead which has deeply engraved wear tracks. This thick layer of lead has become a load-bearing constituent and has cracked extensively owing to the lower strength of lead as shown in Pig. 8(d). The oxide particles formed at sliding velocities below that for minimum wear, as shown in Fig. 8(d), are indicative of mild wear at least in certain areas of the surface whereas the elongated metallic particles seen in Pig. 9 are formed by severe wear at higher sliding velocities exceeding that corresponding to minimum wear. The behavior of wear with sliding velocity has also been observed by other workers [ 11, IS-201. It is interesting to note that this transition from mild to severe wear has not been observed in pure auburn but, with the

91

addition of lead beyond 8 wt.%, this transition has appeared in stir-cast Al-Pb alloys. X-ray analysis of the oxide particles indicates the formation of Pb2A1205, a complex oxide formed because of the presence of both lead and alumimnu on the surface. Further, the transition shifts to higher sliding velocities as the lead content increases. The mild wear, as evident from Pig. 8(d), is due to extensive oxidation of thick layers of lead and has been confirmed by an X-ray diffractometer study of the debris in the mild wear region. At higher sliding velocities, predominantly metallic debris has been observed.

5. Conclusions

The present investigation leads to the following conclusions. (i) Dispersion of 8 wt.% Pb and above in commercial ahuuinum modifies

the wear characteristics of aluminum by introducing mild wear at least on part of the leaded alumina surface at lower sliding velocities.

(ii) As the lead content of stir-cast Al-Pb alloy is increased, the bulk wear decreases up to 20 wt.% Pb, beyond which the bulk wear increases gradually owing to widespread cracking and removal of thick layers of lead.

Acknowledgment

The investigators are grateful to the Council of Scientific and Industrial Research, India, for providing financial assistance for this investigation.

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