Process for the production of bearing materials

Bearing materials and a process for the manufacture thereof are described. The bearing alloys are aluminium-based and comprise the following compositions expressed in weight percent; 8 to 35 tin, 1 to 3 copper, 2 to 10 silicon and remainder aluminium apart from incidental impurities. The alloys are preferably bonded to steel. The composite material is then raised in temperatrue preferably within the range 450.degree. to 500.degree. C. for a total time between 10 minutes and 90 seconds and then cooled at a cooling rate of at least 50.degree. C./min. for at least part of the temperature drop to ambient temperature. The composite material may optionally be additionally heat-treated thereafter to further improve the properties.

The present invention relates to bearing materials and processes for the 
manufacture thereof and particularly to aluminium-based bearing materials 
having alloying additions including copper, silicon and a relatively soft 
phase such as tin, for example. 
A bearing material based on aluminium and comprising approximately 20 wt % 
Sn and 1 wt % Cu is well-known and widely used in, for example, the 
automotive industry in plain journal bearing applications. Whilst the 
fatigue resistance and compatibility i.e. the ability to resist local 
welding between the bearing alloy and rotating shaft, of this material is 
entirely adequate in most applications there have been fatigue problems 
when using such material in highly rated engines. Furthermore, the 
compatibility of such materials when used in conjunction with cast-iron 
shafts is somewhat less than desirable. Generally the surface finishes 
attainable on the journals of cast-iron shafts are inferior to those 
attainable on steel shafts. The result of this is that cast-iron shafts 
tend to be more abrasive than steel shafts. 
Another well-known aluminium-based material having a higher fatigue 
strength than Al-Sn20-Cu1 is Al-Si11-Cu1. Ths silicon is usually present 
as a uniform distribution of particles throughout the matrix. Whilst the 
fatigue strength of this material is high, due to the relatively hard 
nature of the matrix, its conformability is relatively poor. To overcome 
the problem of conformability that is, the ability of the bearing alloy to 
accommodate small misalignments between itself and a rotating shaft, this 
latter material is often operated with an electrodeposited overlay of, for 
example, Pb-Sn10 with an interlayer of nickel between the overlay and the 
bearing alloy. The soft overlay provides both conformability and dirt 
embeddability. 
Dirt embeddability is becoming increasingly important as diesel engines are 
tending more and more to be operated on less refined fuels. The debris 
emanating from less refined diesel fuels produces both erosion and 
corrosion of soft overlays which result in a shortened operating life. 
Furthermore, where the overlay is worn away and large areas of nickel 
interlayer are exposed there is some evidence to suggest that the risk of 
bearing seizure increases. 
The rate of overlay wear is further increased in automotive engines which 
have cast-iron shafts. 
Extensive research has shown that improved fatigue strength and improved 
conformability and compatibility with cast iron shafts may be obtained by 
the incorporation of silicon into an aluminium-based bearing material 
whilst still retaining a soft phase within the matrix. My co-pending 
patent application GB No. 2,144,149 describes aluminium-based bearing 
materials having inter alia 8 to 35 wt % tin, 1 to 11 wt % silicon and 0.2 
to 3 wt % copper. These materials possess fatigue strength and 
compatibility against a cast-iron shaft superior to Al-Sn20-Cu1 and 
conformability superior to unplated Al-Si11-Cu1. The fatigue strength of 
these alloys, however, is not as high as the latter material. 
A further problem with the Al-Si11-Cu1 material additional to the need to 
overlay plate and which also leads to a more expensive production route is 
the need to turn or otherwise machine the final bearing surface to be 
plated as distinct from the cheaper alternative of bore broaching. 
It is an object of the present invention to provide a bearing material 
having greatly improved fatigue strength and improved compatibility over 
Al-Sn20-Cu1 against cast-iron shafts. It is a further object to provide a 
bearing material having fatigue strength in unplated form comparable to 
that of Al-Si11-Cu1 whilst being able to be bore broached to finish size. 
It has now been unexpectedly discovered that alloys lying within the ranges 
disclosed in GB No. 2,144,149 may, by suitable thermal processing, achieve 
the desired objects stated above. Furthermore, by adjusting the thermal 
processing within defined limits the properties of the resulting bearing 
material may be controlled to suit specific applications. 
I have found that a process for the production of a bearing material, 
generally applicable to the material in monolithic form, having a 
composition lying within the following ranges expressed in weight per 
cent; 8 to 35 tin, 1 to 3 copper, 2 to 10 silicon and remainder aluminium 
apart from incidental impurities may comprise the steps of casting the 
desired alloy in suitable form, raising the temperature of the alloy to a 
temperature in excess of 400.degree. C. but less than 525.degree. C. and 
subsequently cooling the alloy at a cooling rate of at least 50.degree. 
C./min. for at least part of the temperature drop to ambient temperature. 
Preferably the tin content may lie within one of the two ranges from 9 to 
13 wt % and from 15 to 25 wt % and similarly the copper content preferably 
lies within the range 1.5 to 2.5 wt %. Where the tin content lies within 
the range 9 to 13 wt % the silicon content may preferably lie within the 
range 3 to 5 wt %. Where, however, the tin content lies within the range 
15 to 25 wt % the silicon content may preferably lie within the range 2 to 
4 wt %. 
Preferably the alloy is cooled at a cooling rate of at least 50.degree. 
C./min. to a temperature below 200.degree. C. whereupon the cooling rate 
may be altered if desired. 
The process given above is generally applicable to alloy in monolithic 
form. The alloy, however, is most advantageous when used for bearing 
material in bimetal form i.e. where the bearing alloy is bonded to a 
strong backing material such as steel, for example, and used to produce 
so-called thin-walled bearings. 
Aluminium-based alloy bonded to a backing material such as steel is 
generally produced by a continuous or semi-continuous production process 
wherein large coils of the bimetal material are produced for further 
processing eventually into individual bearings. 
One consideration which is of paramount importance when the thermal 
treatment of aluminium-based alloys bonded to ferrous substrates is 
undertaken is that of the possible formation of intermetallic compounds at 
or near the interface between the aluminium and steel. The formation of 
such compounds may have a catastrophic effect on the durability of the 
bond between the bearing alloy and the steel and, moreover, this 
catastrophic effect may occur before any intermetallic compound formation 
becomes visible under the optical microscope. 
It is essential, therefore, that any thermal treatment avoids the formation 
of intermetallic compounds of iron and aluminium whether visible under the 
optical microscope or otherwise. It has now been discovered that 
temperatures which heretofore were thought impractical with 
aluminium-based alloys bonded to steel because of such brittle 
intermetallic compound formation may in fact be utilised. This is 
providing that the rate of heating to that temperature is high and the 
dwell time at temperature is sufficiently short. Furthermore, it has also 
been discovered that the high heating rates and short times referred to 
above are able to bring into solution sufficient of the copper and silicon 
to allow the resulting bearing material to achieve the objectives of 
fatigue strength and broachability etc. of the invention. 
According to an aspect of the present invention a process for the 
production of an aluminium-based bearing material having a steel backing 
and a composition lying within the following ranges expressed in weight 
percent; 8 to 35 tin, 1 to 3 copper, 2 to 10 silicon and remainder 
aluminium apart from incidental impurities comprises the steps of 
producing a desired alloy composition in suitable form, bonding the alloy 
to steel, raising the temperature of the bonded material to a temperature 
of at least 400.degree. C. but less than 525.degree. C. and wherein the 
aggregate time to heat to temperature and the dwell time at temperature 
lies within the range from 240 minutes to 60 seconds and subsequently 
cooling the bonded material at a cooling rate of at least 50.degree. 
C./minute for at least part of the temperature drop to ambient 
temperature. 
Preferably, the temperature to which the material is heated may lie in the 
range 425.degree. C. to 500.degree. C. and the time within the range from 
120 seconds to 10 minutes. More preferably the temperature may lie in the 
range from 450.degree. C. to 490.degree. C. 
In a preferred embodiment of the present invention the material may further 
include an interlayer of, for example, aluminium between the bearing alloy 
and the steel backing. Such an interlayer may be produced by cladding of 
the alloy billet by, for example, roll-pressure bonding prior to bonding 
to the steel backing. 
It has been further discovered that an additional benefit of the inventive 
process is that the high temperatures and short times are sufficient to 
also produce reticulation of the tin phase within the worked structure and 
to fully consolidate the bond between the alloy and the steel. It is 
believed that the unexpectedly high performance of these bearing materials 
may possibly only be obtained with a reticular structure. 
Thermal treatment of the bonded material according to the invention may, 
however, be carried out subsequent to other heat treatments known in the 
art, a typical example of which is heating at about 350.degree. C. for 
around 3 hours. It will be appreciated by those skilled in the art that 
the opportunity to delete such long and expensive heat treatment stages 
may result in considerable economic savings in the production of bearing 
materials. 
The time of the heating period may depend upon the temperature to which the 
material is raised. For example, where the eventual maximum temperature 
lies close to 450.degree. C. the total aggregate heating time prior to 
cooling may be near to 10 minutes whereas if the maximum temperature 
attained lies close to 500.degree. C. the total time may be closer to 120 
seconds. 
The post-heating cooling rate will have an effect on the properties of the 
alloy. For example, where the cooling rate is about 75.degree. C./min. 
some of the copper and silicon will precipitate out of solution. Where the 
cooling rate is more rapid such as, for example, about 150.degree. C. to 
300.degree. C./min more copper and silicon will be retained in solution. 
It is envisaged that the thermal treatment of the bonded material may be 
accomplished on a continuous strip basis where the strip first passes 
through rapid heating means and subsequently through rapid cooling means. 
In order to make such a process economically viable it is necessary for 
the strip to travel at reasonable speeds. Therefore, relatively high 
temperatures for short times are more desirable. Although at 400.degree. 
C. the alloy system is capable of taking into solution significant 
quantities of copper and silicon the reaction is slow and is difficult to 
accomplish as a continuous strip process. Whilst on a batch process basis 
it would be feasible to heat a coil of material at 425.degree. C. for 
three or four hours, for example, it would be difficult to achieve the 
necessary cooling rate to retain in solution the copper and silicon 
without a form of quenching of the entire coil which process may be 
awkward and unweildy. 
An optional additional heat treatment may be undertaken whereby the copper 
and silicon held in solution is precipitated out in a controlled manner. 
Such a heat treatment may involve a heat treatment period of, for example, 
between 1 and 72 hours desirably at a temperature above the anticipated 
operating temperature of the bearing. A suitable temperature may be 
between 150.degree. C. and 230.degree. C., for example, a more preferred 
temperature range, however, may lie between 180.degree. C. and 220.degree. 
C. and the corresponding times may lie in the range from 2 to 24 hours.

In order that the process of the present invention may be more fully 
understood some non-limiting examples will be described by way of 
illustration only. 
EXAMPLE 1 
An alloy having a composition Al-Sn11-Si4-Cu2 was continuously cast into 
billet form of 25 mm thickness. The billets were homogenisation annealed 
for 16 hours at 490.degree. C. and then machined to a thickness of 19 mm. 
The billets were then rolled using several passes and a final annealing 
heat treatment to 7.6 mm thickness. The rolled strip was then clad on one 
side using roll-pressure bonding with 0.8 mm prepared aluminium foil. 
After cladding the strip was rolled down to 0.89 mm, the foil side cleaned 
and abraded and the strip roll-pressure bonded to prepared 2.5 mm thick 
steel strip. The resulting bimetal strip then possessed a steel backing 
having a thickness of 1.5 mm and the alloy/foil lining having a total 
thickness of 0.5 mm. The bearing alloy after roll-pressure bonding 
possessed a hardness of approximately 76 Hv. 
The bimetal strip so produced was then heat-treated in an air circulating 
oven on a cycle which equated to 3 hours at 350.degree. C. The bearing 
alloy hardness after such heat treatment was approximately 37 Hv. 
The heat-treated bimetal strip was further heat-treated by rapidly heating 
in a fluidised bed to 475.degree. C. for a total cycle time of 160 
seconds. The bimetal strip took approximately 40 seconds to attain 
420.degree. C. and the remaining 120 seconds comprised the temperature 
rise from 420.degree. to 475.degree. C. and dwell at 475.degree. C. The 
bimetal strip was then cooled at a cooling rate of approximately 
150.degree. C./minute. The hardness of the strip at this stage was 
approximately 47 Hv. From the strip produced test bearings of length 30 mm 
and diameter 53 mm were produced. Test bearings were also produced from 
the input strip i.e. strip prior to fluidised bed heat treatment. 
The bearings so produced were fatigue tested on a test rig under the 
following conditions: 
Shaft speed 2800 rev/min 
Initial load 62 MPa 
Load increased after 20 hours at each load by 7 MPa until failure 
Oil temperature 80.degree. C. 
Sinusoidal load pattern 
The bearings were tested against Al-Sn20-Cu1 material for comparison. The 
results were as shown in Table 1. 
TABLE 1 
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Material No. of Tests. Failure at Indicated Load (MPa) 
Mean Fatigue 
Composition 
62 
69 
76 
83 
90 
97 
103 
110 
117 
124 
131 
Rating (MPa) 
__________________________________________________________________________ 
Al--Sn20-- Cu1 
1 2 6 6 4 4 83 
Al--Sn11--Si4--Cu2 
1 1 1 1 93 
Without fluidised 
bed treatment 
Al--Sn11--Si4--Cu2 2 4 2 5 5 2 114.5 
After fluidised 
bed treatment 
__________________________________________________________________________ 
Material was also made into bearings for seizure testing. The conditions of 
testing are given below. 
1. The size of bearings used for fatigue testing were machined to half 
length to facilitate the use of higher specific loads than can normally be 
obtained. 
2. The lubricating oil (SAE10) was preheated to 120.degree. C. 
3. The rig was run for 1 hour at 100 MPa. 
4. The load was increased by 20 MPa and the rig run for 10 minutes at the 
new load. This procedure was repeated until seizure occurred or the back 
of the bearing temperature rose rapidly to about 160.degree. C. 
The load at which seizure occurred or the test terminated due to rapid 
temperature rise is the seizure rating. 
The results are shown in Table 2. The test bearings were run against mild 
steel and cast iron shafts to evaluate compatibility and 
seizure-resistance for the different materials. 
TABLE 2 
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Seizure at indicated loads (MPa). 
Material No. of tests. 
Composition 
Shaft 100 
120 
140 
160 
180 
200 
220 
240 
260 
__________________________________________________________________________ 
Al--Sn20--Cu1 
Mild steel 2 3 
Al--Sn20--Cu1 
Cast iron 
2 2 1 
Al--Sn11--Si4--Cu1 
Mild steel 2 4 
Untreated 
Al--Sn11--Si4--Cu1 
Cast iron 1 1 1 
Untreated 
Al--Sn11--Si4--Cu2 
Mild steel 3 
Fluidised bed 
treated 
Al--Sn11--Si4--Cu2 
Cast iron 2 2 1 1 
Fluidised bed 
treated 
__________________________________________________________________________ 
It should be noted in the above test results in Table 2 that 260 MPa 
represents the highest loading attainable with the particular test rig 
used and that of the three tests giving 260 MPa ratings for Al-Sn20-Cu1, 
one in fact did not seize and would have given a higher rating. Of the 
three tests giving 260 MPa ratings for the material made by the process of 
the present invention none of these bearings in fact seized and all three 
would have given higher ratings. 
It is clear from Table 1 that the fatigue resistance of material made by 
the process of the present invention is clearly superior to both the 
Al-Sn20-Cu1 material and to the material of the type disclosed in GB No. 
2,144,149 but which does not have the subsequent thermal treatment. It may 
be seen that the mean fatigue rating for Al-Sn20-Cu1 is approximately 83 
MPa whereas the comparative fatigue rating for the Al-Sn11-Si4-Cu2 
material is around 93 MPa before thermal treatment. After processing in 
accordance with the invention, however, the mean fatigue rating is 
improved to 114.5 MPa, an improvement in fatigue strength of 23% due to 
the process of the present invention. 
From Table 2 it may be observed that the improvement in compatibility with 
cast iron shafts over Al-Sn20-Cu1 is maintained in the present alloys. The 
mean seizure rating of the Al-Sn20-Cu1 material is 132 MPa. The mean 
seizure rating of the inventive alloys is 203 MPa. Furthermore, the 
seizure performance of the inventive alloys is also superior when used in 
conjunction with a steel shaft. 
Broaching tests were also carried out on bearings produced from the bimetal 
strip. Bearings were produced from strip having had the fluidised bed 
thermal treatment and strip prior to such thermal treatment. The tests 
comprised the removal of approximately 0.025 mm of alloy using a tungsten 
carbide cutter and a steel cutter for comparison and both having a 
30.degree. rake. Surface roughness measurements were taken on the broached 
bearings and the results are given in Table 3. 
TABLE 3 
______________________________________ 
Surface Roughness (.mu.mR.sub.A) 
Cutter Fluid bed treated 
Untreated 
______________________________________ 
Steel 0.31-0.35 0.42-0.51 
Carbide 0.27 0.32-0.89 
______________________________________ 
It may be seen from Table 3 that material produced by the process of the 
invention has a finer more uniform finish than known alloys and moreover 
is capable of being broach finished as a production process. This is 
contrasted to Al-Si11-Cu1 which is not capable of being broach finished 
owing to its high matrix strength and lack of soft phase. 
EXAMPLE 2 
Alloy of the same composition was produced and processed into bimetal up to 
and including roll-pressure bonding of the alloy to the steel as in 
Example 1. 
The bimetal so produced was then rapidly heated in a fluidised bed to 
475.degree. C. for a total time of 4 minutes after which it was cooled at 
a cooling rate of approximately 300.degree. C./min On examination the bond 
between the steel and alloy was found to be free of inter-metallic 
formation and this was confirmed by testing which demonstrated a high 
integrity bond comparable to that of Example 1. The bearing alloy was also 
found to possess a fine reticular structure. 
EXAMPLE 3 
Alloy was produced and processed as in Example 2 except that the resulting 
bimetal was heated to 500.degree. C. in a total time of 2 minutes. Again 
the alloy to steel interface was of high integrity and confirmed by bond 
testing and the structure was again reticular. 
EXAMPLE 4 
Samples of bimetal were produced as in Example 1 up to and including the 
stage of heat-treating in air for a cycle equating to 3 hours at 
350.degree. C. Pieces of bimetal were then heated to 450.degree. C. over a 
cycle of 180 seconds and then water-spray quenched which gave a cooling 
rate in excess of 1000.degree. C./min. The resulting alloy hardness was 52 
Hv. Pieces of this material were then heat-treated at times ranging 
between 1 and 24 hours at 200.degree. C. A maximum hardness of 60 Hv was 
achieved after about 16 hours. 
EXAMPLE 5 
Samples prepared as in Example 4 were taken and teat-treated for between 1 
and 24 hours at 220.degree. C. A maximum hardness of 58 Hv was achieved 
after about 6 hours and which gradually declined to 55 Hv after about 24 
hours. 
The rapid heating was undertaken in the Examples above by means of 
fluidised bed heating. Any means may, however, be used provided that the 
heating rate is sufficiently rapid. Alternative methods may include, for 
example, induction heating, high intensity radiant heating, plasma heating 
or any method known in the art. Cooling of the strip may be undertaken by 
gas impingement or any other method down to, for example, 200.degree. C. 
and thereafter by conventional water cooling jackets around a muffle, 
other methods including, for example, fluidised bed cooling, liquid spray 
cooling, or passing the strip through a quench bath may be used. 
The alloys of the present invention, therefore, provide substantial 
improvements in both fatigue and seizure resistance over known alloys 
which in some cases require expensive overlays. Furthermore, the alloys 
produced by the process of the invention may be broach finished. It is an 
optional step, however, to provide an overlay on bearings produced by the 
inventive process if it is so desired. 
In some instances it may be desirable, however, to provide a bearing having 
increased conformability and yet higher fatigue strength, for example, in 
turbocharged engines or high speed diesels. In such applications the 
bearing alloy may be coated with an overlay chosen from the group 
including tin, lead/tin, lead/tin/copper, tin/copper, tin/antimony, 
tin/copper/antimony and lead/tin/copper/antimony. Furthermore, there may 
be interposed between the alloy lining and the overlay coating an 
interlayer chosen from the group including nickel, iron, silver, cobalt, 
copper, zinc and copper/tin. 
Alternatively, bearings produced from alloy according to the present 
invention may be provided with an overlay of the type disclosed in 
co-pending European Patent Application No. 85309180.9 wherein the overlay 
comprises a thin layer of tin which is sacrificial in nature.