Note: Descriptions are shown in the official language in which they were submitted.
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Title
Wear-resistant Aluminum Alloy for Casting Engine Blocks with Linerless
Cylinders
Field of the Invention
The invention relates to aluminum alloys that can be cast into high-quality
aluminum
cylinder blocks, utilizing a low-cost low pressure sand casting process, for
automotive
engines having good mechanical properties and wear and scuffing resistance; so
that
according to the present invention the engine blocks can be manufactured
without the need
for insertion of iron (or costly aluminum) liners in order to have effective
cylinder walls.
Background of the Invention
" Most of the automotive and aviation cylinder engine blocks made out of
aluminum
alloys are currently manufactured by casting the block body in silica sand
molds using sand
cores and inserting a set of cast iron liners to form the cylinder-piston
contact surfaces. Other
processes for casting blocks have included gravity semi-permanent molds, high
pressure die
casting, low pressure die casting, the lost foam process and the zircon sand
package molds;
and the liners can be either inserted as "cast-in" or "pressed-in". More
recently, in a few
high-end aluminum engine blocks liners made of aluminum have been substituted
for cast
iron liners. However, the high cost of the currently-available Al alloy needed
to meet the
requirements for such aluminum cylinder liners prevents such alloy from also
being used to
cast the remainder of the aluminum engine block (as do also some negative
physical
attributes if it were to be used in the remainder of the block). The cost of
such Al alloy, even
when limited to use as a liner, has also prevented it form being universally
adopted to replace
iron liners in spite of the lower weight and greater cooling advantages.
This practice of utilizing liners however requires a number of process and
material
measures that, if able to be eliminated without the indicated drawbacks, would
provide many
advantages to block manufacturers. For example, the inventory of liners would
be
eliminated, the scrap rate of blocks due to poor bonding between the aluminum
body and the
liners would decrease, the energy consumption for preheating the liners would
also be
eliminated, and the casting process would be simplified. Currently, preheating
the liners is
done by electrical induction and consumes time as well as adding complexity to
the overall
casting process. All of the foregoing is especially true relative to iron
liners. The need exists
therefore for an aluminum alloy composition and casting process which
eliminates the need
for liners in an aluminum engine casting thereby overcoming such technical and
economic
disadvantages of the prior art.
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It is known from the patent and technical literature; that silicon added to
aluminum
beyond the eutectic composition increases hardness of the alloy and
consequently increases
the wear resistance of its surfaces. However the sole increase of Si
concentration in the alloy
does not provide all the desired properties to the cast blocks (concerning
wear-resistance,
machinability, castability and other mechanical properties). Such desired
properties are
governed by the type of microstructure formed in the solidified casting.
Another process
problem posed by Si, when alloyed with aluminum, is that it adds a greatly
increased capacity
for heat that must be dissipated from the alloy during solidification. This
results in uneven
cooling, especially in large complex castings such as automotive engine
blocks, causing
problems in properly developing the often competing desired properties of the
bulk casting
relative to the cylinder surface.
Some relevant prior art patents found by applicants regarding the alloy
composition
and the casting process are described below:
US Patent 4,068,645 issued January 17, 1978 to David Charles Jenkinson,
teaches that
the microstructure of a hypereutectic Al-Si alloy can be modified with
strontium and/or
sodium for obtaining Brinell hardness in the range of 70 -150 by including
magnesium up to
about 4 wt.%. This patent teaches that the desired microstructure must avoid
the formation of
primary aluminum or primary silicon phases and that there must be a high-
volume fraction of
finely dispersed eutectic silicon which provides the wear resistance to the
cast article.
According to this patent, the 'desired microstructures are provided by careful
selection
and combination of four parameters: (a) silicon content, (b) modifier content,
(c) growth rate
during solidification and (d) temperature gradient at the solid/liquid
interphase during
solidification.
Several combinations of the above four parameters are disclosed which provide
the
desired microstructure. The teachings of this patent however are applicable to
permanent and
semi-perrnanent mold casting processes where a controlled temperature gradient
may be
achieved by programming the cooling rate of the mold at different zones, but
it is not
applicable to silica-sand molds casting processes (where conventionally the
solidification rate
is only able to be modified by the addition of thermal cores which absorb heat
from the liquid
aluminum in the mold). This patent clearly teaches away from chill-casting in
order to obtain
the desired absence of primary Si and primary Al phases.
US patent 4,434,014 issued February 28, 1984 to David M. Smith, et al. teaches
that
the properties of the cast articles regarding wear resistance and
machinability are obtained by
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a composition comprising 12-15% Si; 0.001 - 0.1 Sr; 0.1 - 1.0 Fe; 1.0 - 3.0
Ni; 0.1 - 0.8%
Mn; and other components.
This patent teaches also that Ni, Fe and Mn are interchangeable with each
other, being
the ranges as follows: Fe + Mn between 0.2 and 1.5%; Fe + Ni between 1.1 and
3.0%; and
Fe + Ni + Mn between 1.2 and 4.0%.
Titanium is added to improve castability and the mechanical properties of this
alloy.
This alloy however has a high cost due to the high content of Ni, in contrast
with the alloy of
the present invention having less than about 0.4 - 0.8%Ni. The lower
concentration Ni thus
particularly makes the alloy of the present invention more competitive.
US patent 4,648,918 issued March 10, 1987 to Kasuhiko Asano, et al. teaches an
abrasion-resistance aluminum alloy having a composition comprising: 7.5 -15%
Si; 3.0 -
6.0% Cu, 0.3 - 1.0% Mg, 0.25 - 1.0% Fe; 0.25 - 1.0% Mn; and a balance of Al
and other
components. The alloy of this patent is directed to improve the extrudability,
forgeability and
mechanical properties of ingots. The Cu content is higher than the alloy of
the present
invention and the heat treatment and final processing of this alloy are far
different from the
sand-casting process of the present invention.
US patent 5,019,178 issued May 28, 1991 to John Barlow et al. discloses a
production method of an aluminum-silicon liner produced from a melt consisting
essentially
of 14 - 16% Si; 1.9 - 2.2% Cu; 1.0 -1.4 Ni; 0.4 - 0.55 Mg; 0.6 - 1.0% Fe; 0.02
- 0.1% Sr;
and 0.3 -0.6 Mn. The alloy of this patent is formed into cylinder liners under
pressure during
the solidification stage of the casting process. This patent does not teach or
suggest that the
whole engine block be made of the claimed alloy in a low-pressure sand-casting
process.
US patent 5,217,546 issued June 8, 1993 to John A. Eady, et al. discloses a
cast
hypereutectic Al-Si alloy having 12 - 15% Si; more than 0.10 1o Sr; more than
0.005% Ti;
1.5 - 5.5% Cu; 1.00 - 3.00 Ni; 0.1 - 1.0 Mg; 0.1 - 1.0% Fe; and other
components.
According to this patent, the microstructure obtained is such that any primary
Si formed is
substantially uniformly dispersed and is substantially free of segregation,
with the
microstructure predominantly comprising an eutectic matrix. The alloy of this
patent however
relies on Ti and an excessive amount of Ni, which makes it tqo expensive an
alloy for
competitive mass production of engine blocks.
US patent 5,316,070 issued May 31, 1994 to Kevin P. Rogers, et al. teaches a
process
for controlled casting of a hypereutectic Al-Si alloy in permanent molds.
Permanent molds
can be fully equipped with cooling systems and with precise temperature
control so that a
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pre-established solidification program can be implemented and therefore the
desired
microstructure of the cast article may be achieved. The teachings of this
patent can not be
applied to sand-casting processes.
US patent 5,484,492 issued January 16, 1996 to Kevin P. Rogers et al.
discloses a
hypereutectic Al-Si alloy essentially having at least one element selected
from a first group of
elements consisting of 0.005 0o up to 0.25% of Cr, Mo, Nb, Ta, Ti, Zr, V and
Al; at least one
element selected from a second group of elements consisting of 0.1 to 3.0 %
Ca, Co, Cr, Cs,
Fe, K, Li, Mn, Na, Rb, Sr, Y, Ce, elements of the Lanthanide series and
elements of the
Actinide series; and a third group of elements consisting of: 12 - 15% Si; 1.5
- 5.5 Cu; 1.0 -
3.0% Ni; 0.1 - 1.0% Mg; 0.1 - 1.0% Fe; 0.1- 0.8% Mn; 0.01 - 0.1 Zr; 0- 3.0%
Zn; 0-
0.2% Sn; 0- 0.2% Pb; 0- 0.1 fo Cr; 0.001 - 0.1 % Sr or Na;; a maximum of
0.05% B; a
maximum of 0.03% Ca; a maxirnum of 0.05% P; and others with a maximum of
0.05%. The
casting microstructure is such that any primary Si present is substantially
uniformly dispersed
and predominantly comprises a eutectic matrix. The present invention in
contrast uses a
different and lower range-of Ni (0.8% maximum).
To the best of applicants' knowledge, none of the last three patent (assigned
to
Comalco) have every been commercialized.
US patent 6,399,020 issued June 4, 2002 to Jonathan A. Lee et al. discloses an
aluminum alloy suitable for high-temperature applications, such as pistons and
other internal
combustion engines applications, having the following composition: 11.0 -
14.0% Si; 5.6 -
8.0% Cu; 0 - 0.08 Fe; 0.5 - 1.5 Mg;0.05-0.9Ni;0-1.0Mn;0.05-1.2,Ti;0.12-1.2Zr;
0.05 - 1.2 V; 0.05 - 0.9 Zn; 0.01 - 0.1 Sr; with the balance Al. In this alloy
the ratio of Si/Mg
is 10 - 25, and the ratio of Cu/Mg is 4- 15. The alloy of the applicants'
invention differs
from the alloy composition disclosed in this patent, mainly in the Si/Mg ratio
and in the
amount of Sr. Since Sr is an expensive element, the alloy of the present
invention is more
cost-competitive. In addition, the present invention does not include Zr or V
and has a
maximum of 0_3 1o Mg.
US patent 6,592,687 issued July 15, 2003 and US patent 6,918,970 issued July
19,
2005, both to Jonathan A. Lee et al. disclose an aluminum-silicon alloy having
the following
composition in weight percent: 14 - 25.0 Si; 5.5 - 8.0 Cu; 0.05 - 1.2 Fe; 0.5 -
1.5 Ni; 0.05 -
0.9 Mn; 0.05 - 1.2 Ti; 0.05 1.2 Zr; 0.05 - 1.2 V; 0.05 - 0.9 Zn; 0.001 - 0.1
P; and with the
balance being Aluminum. The '970 patent's alloy has an extended range of Si
(6.0 - 25.0%)
plus Sr (with a range of 0.001-0.1). The Si/Mg ratio is 10 - 25 and the Cu1Mg
ratio is 4-15.
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This alloy has as key elements Ti, V and Zr that modify the lattice parameters
of the
aluminum matrix by forming compounds of the type A13X having L 12 crystal
structures,
wherein X stands for Ti, V or Zr.
US patent 6,921,512 issued July 26, 2005 and US Patent Publication No.
5 2005/0199318 published September 15, 2005, both appearing in the name of
Herbert William
Doty, disclose an aluminum alloy suitable for casting and machining cylinder
blocks for
automotive engines. The alloy comprises by weight, 9.5 - 12.5 % Si; 0.1 -1.5 %
Fe; 1.5 -
4.5% Cu; 0.2 - 3% Mn; 0.1 - 0.6% Mg; 2.0% maximum Zn; 0 - 1.5% Ni; 0.25%
maximum
Ti; up to 0.05% Sr; with the balance being aluminum. An important feature of
this Patentee's
invention is the proportion of Mn to Fe. The weight ratio Mn/Fe is between 1.2
to 1.75 or
higher when the Fe content is equal to or greater than 0.4% and the weight
ratio Mn/Fe is at
least 0.6 to 1.2 when the Fe content is less than 0.4% of the alloy. In
contrast, the Si range of
the present invention is 13-14%.
The desired microstructures in the Al-Si alloys are produced by a right
combination of
growth rate during solidification and temperature gradient.
Documents cited in this text (including the foregoing patents), and all
documents cited
or referenced in the documents cited in this text, are incorporated herein by
reference.
Documents incorporated by reference into this text or any teachings therein
may be used in
the practice of this invention.
Summary and objects of the Invention
It is an object of the present invention to provide a new hypereutectic Al-Si
alloy
suitable for low pressure casting processes utilizing silica-sand molds and
cores to cast an
engine block having the required combination of machining, casting and wear
resistance
properties so as also not to require wear liners.
It is another object of the present invention to provide such a new Al-Si
alloy for
manufacture of aluminum engine blocks with unlined cylinders that are
competitive with
current mass produced aluminum engine blocks with iron liners .
It is a further object of the present invention to provide a new Al-Si alloy
which
produces improved engine block castings with mechanical properties that avoid
the necessity
for cylinder liners made from a different alloy or metal, and that also are
easier to machine
than engine block castings made from existing hypereutectic Al alloys of the
prior art.
Other objects of the invention will be pointed out or will be evident from the
following description of the preferred embodiments and the accompanying
drawings.
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The proposed invention herein described and claimed is,an aluminum-silicon
alloy
composition which, when cast, meets the manufacturing and performance
conditions required
for cylinder engine blocks and further can be cast using low-cost casting
processes such as
silica-sand molds.
The alloy of the present invention comprises (in weight percent):
13% - 14% Si;
2.3% - 2.7% Cu;
0.1 % - 0.4% Fe;
0.1%-0.45 foMn;
0.1%-0.30 1oMg;
0.1%-0.6%Zn;
0.05% - 0.11 % Ti;
0.4% - 0.8% Ni;
0.01 !o - 0.09% Sr; and
the balance being aluminum (apart from a minor amount of any trace elements,
impurities,
residuals, and other ingredients which in the aggregate are known as the
"remainders" and are
present in amounts insufficient to substantially affect the efficacy of this
alloy for its intended
purpose, including its wear resistance).
Brief description of the drawings
Figure 1 shows a microphotograph of the microstructure (100 m) obtained from
an
unlined aluminum cylinder surface of an engine block cast from the alloy of
the present
invention.
Figure 2 shows a contrasting microphotograph of the microstructure (100 m)
obtained from an unlined aluminum cylinder surface of an engine block cast
from the alloy
known as A390.
Figure 3 is a schematic phase diagram of Al-Si alloys showing the preferred
range of Si
content for the alloy of the invention as contrasted to prior art alloys known
as A380, A390,
A413, and DuraboreTM (a GM alloy understood to be exemplified by U.S. Patent
No.
6,921,512).Description of nreferred embodiments of the present invention
Although the invention is herein described as applied to an aluminum alloy
cylinder
engine block casting through a low pressure sand casting process it will be
understood that in
its broader aspects it may also be applicable to other types of castings
requiring similar
properties and also to other casting processes.
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It is known that increasing the concentration of silicon in an alloy of the
type utilized
for automotive engines casting generally increases the hardness and wear
resistance of the
resulting casting, and that the final properties thereof depend on the cooling
rate of the
casting.
The traditional sand-casting processes featuring low-pressure mold filling,
for
example the Cosworth process (and also the non-commercialized Comalco
process), cannot
produce good-quality blocks utilizing alloys having a high concentration of
silicon, primarily
due to the difficulties posed by the sand molds and cores for controlling the
solidification
rate, and therefore the microstructure of the castings. When utilizing the
aluminum alloys of
the prior art with high Si contents, the intricate geometry of the cylinder
engine blocks
combining thick and thinner sections cause the formation of primary silicon
phases with
undesirable grain and size distribution of the primary silicon phase, as well
as a high porosity
level of the casting.
Another problem related to the utilization of high Si concentration alloys is
that their
heat of fusion is high as compared with hypoeutectic alloys, therefore, the
sand molds must
be able to cope with and dissipate the high heat release during the
solidification process.
The aluminum alloy blocks to be manufactured demand strictly controlled
characteristics and mechanical properties in order to perform as expected in
modern vehicles.
Blocks without liner inserts must have high wear resistance in the running
surfaces and
withstand high pressures on the order of 100 to 200 bar in those engines
having high peak
firing pressures. The porosity level must be below 1% and the maximum pore
size must be
below 500 microns in the running surfaces;
It is necessary also that the aluminum alloy has a high thermal conductivity
in order to
sustain high heat transfer rates from the hot areas of the engine to the
cooling liquid of the
engine cooling system, as well as having good corrosion resistance to the
cooling media. The
high-efficiency modem engines also demand that the alloys from which the
engine blocks are
cast show high strength and high resistance to fatigue and creep at elevated
temperatures, in
the range of 280 -200 C.
The current challenge for the processes utilizing hypoeutectic alloys is that
machining
high-silicon alloys means greater wear of tools and high rriachining cost, as
in the case of the
A390 alloy. In the process of the invention, primary silicon formation is
suppressed resulting
in a fully eutectic microstructure despite its high silicon content. This
characteristic of the
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microstructure of the castings of the invention assures good machinability.
Tool life is
comparable to machining an A356 alloy but with superior surface finish.
The alloy of the present invention is based on the Al-Si-Cu-Mg-Ni-Mn-Fe system
to
enhance maximum wear resistance. It provides the required characteristics
demanded by
modem engine=blocks having unlined cylinders, while also maintaining a
competitive low
manufacturing cost.
The casting process of the invention utilizes a thermal core (or massive
chill) in
combination with silica-sand cores and molds. The chill provides the right
direction of the
solidification process as well as the necessary solidification rate which
results in high fatigue
properties of the castings.
The alloy of the present invention is particularly suited for the production
of linerless
aluminum alloy blocks at a lower cost than the currently used alloys. The
following table 1
compares the typical concentration of the elements of the prior-art alloys
with the
composition of the present invention.
Table 1
A) Hypereutectic Al-Si Alloys 390 and 391
B) Eutectic Alloy: 3HA
C) Near Eutectic Alloys
D) Alloy of the present invention
Altoy Si Fe Cu Mn Mg Zn Ti Ni Sr B P
% ,% % % % % % % ppm ppm ppm
16.0-
A) 18.0 1.0 4.5 0.1 0.55 0.1 0.2 200
13.0-
B) 15.0 0.3 2.0 0.5 0.5 1.0 0.1 2 2000 50 30
10.6- 0.5 2.5 0.6 0.3 0.4 0.11-
C) 250 25
11.5 0.15
0.1- 2.3- 0.1- 0.1- 0.1- 0.05- 0.4- 100-
D) 13-14
0.4 2.7 0.45 0.3 0.6 0.11 0.8 900
Alloy 390 (A) is the historical choice for wear-resistance cast motor
elements, but as
discussed above it is not applicable for sand casting processes.
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Alloy 3HA (B) is also an alloy of choice for those applications, but its cost
is high
because of its high content of nickel (2%). The high concentration of Ni
increases the alloy
cost by 35% ($15,000 US/Ton of Ni), and the 2000 ppm of Sr further combines to
make it
even more expensive.
Near eutectic alloys (C) do not have sufficient silicon content to provide the
required
wear resistance.
Despite it being known that high Ni content would improve the wear resistance
of the
casting surfaces, the high cost of Ni discouraged its utilization, since about
each 10!0 of Ni
content increases by about 15% the cost of the cast block. Nickel also helps
in avoiding Cu
segregation during solidification and therefore some of the prior art alloys
nevertheless tend
to increase the nickel content. Therefore applicants have looked for a better
new alternative.
They found a new alloy composition containing no more than 0.8 %Ni and 900
ppm's of Sr,
which produces large complex castings with the desired microstructure and
mechanical
properties capable of manufacture by a sand casting process.
Referring to Figures 1 and 2, showing respectively a microphotograph of the
microstructure (100 X) obtained from an unlined aluminum cylinder surface of
an engine
block cast from the alloy of the present invention, and of the microstructure
(100 X) obtained
from an unlined aluminum cylinder surface of an engine block cast from the
alloy known as
A390, it is evident that the alloy of the present invention shown in Figure 1
provides a
microstructure where primary Si phase grains are very small and uniformly
dispersed as
compared with the microstructure of the prior art alloy shown in Figure 2.
Additionally, the challenge faced by applicants in developing a new alloy
which
overcomes the disadvantages of the alloys of the prior art when used in
combination with a
silica sand casting process was to find a composition such that despite the
high heat release
and low cooling rate of the silica sand process intermetallic segregation and
porosity in the
casting are minimized.
With reference to Figure 1, applicants have represented in a phase diagram of
an Al-
Si alloy system the position of some of the prior art alloys and the distinct
position of the
alloy of the present invention. It can be seen in this phase diagram that
hypoeutectic and
eutectic alloys are easier to handle in silica sand casting processes since
these alloys are
liquid at lower temperatures than hypereutectic alloys. In view of this
property of the Al-Si
alloys, increasing Si content requires that the molten alloy be poured in the
sand molds at a
higher temperature and therefore more heat needs to be dissipated from the
solidifying metal
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through the sand molds and cores. The alloy of the present invention provides
sufficient Si
content for achieving the desired wear resistance in the casting surfaces and
the other
components of the alloy make it suitable for its casting in silica sand molds
having relatively
lower heat dissipation properties than molds of other casting processes. At
the same time, the
5 alloy of the present invention is less expensive than other prior art alloys
having similar wear
resistance particularly because of its lower Ni content. The alloy of the
present invention
provides a cost competitive process for massive engine blocks casting without
the need of
cylinder liners, particularly when cast in silica sand molds and cores.
The alloy and casting method of the present invention present the following
10 advantages:
The wear resistance provided by the alloy avoids the necessity of inserting
iron liners in the
cylinder bores. Consequently, the manufactured blocks are smaller and lighter,
(saving the
weight and cost of iron liners) and can increase the engine capacity without
increasing engine
size (for example from 2.3 to 3.0 liters).
The alloy of the invention has better thermal characteristics regarding heat
dissipation
(particularly with the absence of iron cylinder liners). Applicants' blocks
run about 10 C
cooler than currently used aluminum blocks having iron liners blocks, due to
the fact that the
interface between the iron liners and block is eliminated.
The alloy also allows for tighter clearances because the thermal expansion
coefficients of both pistons and the blocks are similar (in contrast with the
greater
differentiation of thermal expansion coefficients between the piston aluminum
alloy and the
iron liners). This advantage provides a quieter engine operation and makes the
engines
environmentally cleaner.
There is no need for liner inventory and handling. Therefore there are
important
savings in the manufacturing process, not only due to avoiding the cost of
iron liners but also
because there is no need of preheating such liners by electric induction. The
same is true of
the more rarely used aluminum liners, which in addition are made from a more
expensive
alloy than the alloy of the reminder of the engine casting block.
The linerless engines made from the alloy of the present invention are also
easier to
recycle, since no separation of iron cylinder liners from aluminum is
required.
The alloy of the invention further provides very good machining
characteristics, and
although the tool life is comparable and similar to machining of the currently-
known A356
.alloy, the surface finish in the cylinder bores is significantly better.
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The manufacturing cost of unlined engine blocks is reduced by about 40% by
using
the alloy and method of the invention as compared with the manufacturing cost
when using
the known alloys of the prior art.
Example 1
An Al-Si alloy was prepared according to the present invention and a block was
cast
in silica sand molds and cores. The alloy had the following composition (in
weight percent):
Si =13.5 % Sr = 900 ppm; Fe=0.4 %; Cu=2.5 %; Ni=0.5%; Mn=0.4%; Mg=0.35 !0;
with the balance being essentially only aluminum (plus minor amounts of any
other
essentially non-affecting elements, hereinbefore referenced as the
"remainders").
The alloy was poured into the mold at a temperature of 750 C.
The results were as follows:
The microstructural segregation was reduced.
Modified eutectic cells were more evenly distributed, and the primary aluminum
was
reduced. Primary silicon particles were still observed, but they comprised
less than 1% of the
total silicon.
Examnle 2
In order to test the wear resistance of the alloy of the invention, a series
of single stage 20
hour duration tests were carried out using a Plint TE77 testing machine. The
test set-up
provides a reciprocating line contact between a dowel and a plate. The
hardened dowel is
used to simulate the piston ring while a flat ground plate is used to simulate
the cylinder liner.
The oil used was a commercially available automotive petrol engine mineral oil
heated to 100
Co.
Three different materials were evaluated: (1) cast iron liners for diesel
applications,
(2) a hypereutectic aluminum-silicon alloy (of the type currently being used
as expensive
liners in high performance engines; where the primary wearing resistance phase
was a phase
of primary silicon), and (3) the alloy of the present invention. Results
indicate that
qualitatively the wear scars obtained on all there materials have been similar
and do not
appear to be significantly different in magnitude between the materials
tested.
It is of course to be understood that the invention has been specified in
detail only
with respect to certain preferred embodiments thereof, and that a number of
modifications
and variations can be made without departing from the spirit and scope of the
invention
which is defined by the following claims.
