U.S. patent application number 11/499165 was filed with the patent office on 2008-02-07 for wear-resistant aluminum alloy for casting engine blocks with linerless cylinders.
Invention is credited to Jose Alejandro Gonzalez-Villarreal, Andres Fernando Rodriguez-Jasso, Jose Talamantes-Silva, Salvador Valtierra-Gallardo.
Application Number | 20080031768 11/499165 |
Document ID | / |
Family ID | 39029354 |
Filed Date | 2008-02-07 |
United States Patent
Application |
20080031768 |
Kind Code |
A1 |
Valtierra-Gallardo; Salvador ;
et al. |
February 7, 2008 |
Wear-resistant aluminum alloy for casting engine blocks with
linerless cylinders
Abstract
An aluminum-silicon alloy composition is disclosed which meets
the manufacturing and performance conditions for linerless cylinder
engine block casting using low-cost casting processes such as
silica-sand molds. The alloy of the invention comprises in weight
percent: 13%-14% Si; 2.3%-2.7% Cu; 0.1%-0.4% Fe; 0.1%-0.45% Mn;
0.1%-0.30% Mg; 0.1%-0.6% Zn; 0.05%-0.11 % Ti; 0.4%-0.8% Ni;
0.01%-0.09% Sr; and and the rest being aluminum plus any
remainders. This alloy has very good machining characteristics,
giving a significantly improved surface finish in the cylinder
bores. The manufacturing cost of engine blocks is reduced in about
40% as compared with using current commercial alloys of the prior
art requiring iron liners. Any primary Si present is substantially
uniformly dispersed, and copper does not segregate during
solidification and cooling.
Inventors: |
Valtierra-Gallardo; Salvador;
(Coahuila, MX) ; Talamantes-Silva; Jose; (Santa
Catarina, MX) ; Rodriguez-Jasso; Andres Fernando;
(Santa Catarina, MX) ; Gonzalez-Villarreal; Jose
Alejandro; (Monterrey, MX) |
Correspondence
Address: |
FROMMER LAWRENCE & HAUG
745 FIFTH AVENUE- 10TH FL.
NEW YORK
NY
10151
US
|
Family ID: |
39029354 |
Appl. No.: |
11/499165 |
Filed: |
August 4, 2006 |
Current U.S.
Class: |
420/532 ;
148/549; 420/549 |
Current CPC
Class: |
C22C 21/04 20130101;
C22C 21/02 20130101 |
Class at
Publication: |
420/532 ;
420/549; 148/549 |
International
Class: |
C22C 21/04 20060101
C22C021/04 |
Claims
1. An abrasion resistant aluminum alloy comprising (in weight
percent): 13%-14% Si; 2.3%-2.7% Cu; 0.1%-0.4% Fe; 0.1%-0.45% Mn;
0.1%-0.30% Mg; 0.1%-0.6% Zn; 0.05%-0.11% Ti; 0.4%-0.8% Ni;
0.01%-0.09% Sr; and the balance being predominately aluminum plus
any remainders.
2. In a method for producing a complex aluminum engine linerless
cylinder block casting, the improvement comprising use of an Al--Si
alloy to form such casting having a composition according to claim
1.
3. The method according to claim 2, comprises forming said casting
in a silica sand mold with silica sand cores and wherein said
casting after solidification has a microstructure where any primary
Si present is substantially uniformly dispersed.
4. The method according to claim 3, wherein said molten alloy is
poured in said silica sand mold at a temperature between about
760.degree. C. to about 780.degree. C.
5. An abrasion resistant aluminum alloy comprising (in weight
percent): 13%-14% Si; 2.3%-2.7% Cu; 0.1%-0.4% Fe; 0.1%-0.45% Mn;
0.1%-0.30% Mg; 0.1%-0.6% Zn; 0.05%-0.11% Ti; 0.4%-0.8% Ni;
0.01%-0.09% Sr; and the balance being essentially aluminum.
6. A method for producing a casting of an Al--Si alloy having a
composition according to claim 5, for manufacturing an aluminum
alloy engine block with cylinder bores having a surface with
improved wear resistance made of the same aluminum alloy so as to
withstand the operation of said engine block without cylinder
liners; said method comprising: providing a silica sand mold with
silica sand cores and chill means for causing said alloy to
solidify in a controlled direction and solidification rate, such
that said casting after solidification has a microstructure wherein
any primary Si present is substantially uniformly dispersed;
introducing said alloy as a molten metal into said mold to form
said engine block casting.
7. The method according to claim 6, wherein said chilling means is
a metallic mass having a weight such that the ratio of chill weight
to casting weight is in the range between 1 to 5.
8. The method according to claim 6, wherein said cooling rate is in
the range from about 0.3 to 3.0.degree. C./s.
9. The method according to claim 7, wherein said cooling rate is in
the range from about 0.3 to 3.0.degree. C./s.
10. The method according to claim 6, wherein said molten alloy is
poured in said silica sand mold at a temperature from about
760.degree. C. to about 780.degree. C.
11. The method according to claim 7, wherein said molten alloy is
poured in said silica sand mold at a temperature between about
760.degree. C. to about 780.degree. C.
12. The method according to claim 8, wherein said molten alloy is
poured in said silica sand mold at a temperature between about
760.degree. C. to about 780.degree. C.
13. The method according to claim 9, wherein said molten alloy is
poured in said silica sand mold at a temperature between about
760.degree. C. to about 780.degree. C.
14. The method according to claim 6, wherein said molten alloy is
poured in said silica sand mold at a temperature between about
755.degree. C. and about 765.degree. C.
15. The method according to claim 9, wherein said molten alloy is
poured in said silica sand mold at a temperature between about
755.degree. C. and about 765.degree. C.
16. A cast article made from an aluminum-silicon alloy having
improved wear resistance characteristics suitable for manufacturing
engine blocks with cylinder bores having a surface with improved
wear resistance made of the same aluminum alloy so as to withstand
the operation of said engine block without cylinder liners; said
cast article having the following composition by weight: 13%-14%
Si; 2.3%-2.7% Cu; 0.1%-0.4% Fe; 0.1%-0.45% Mn; 0.1%-0.30% Mg;
0.1%-0.6% Zn; 0.05%-0.11% Ti; 0.4%-0.8% Ni; 0.01%-0.09% Sr; and the
balance being essentially aluminum.
Description
FIELD OF THE INVENTION
[0001] 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
[0002] 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.
[0003] 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.
[0004] 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.
[0005] Some relevant prior art patents found by applicants
regarding the alloy composition and the casting process are
described below:
[0006] U.S. Pat. No. 4,068,645 issued Jan. 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.
[0007] 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.
[0008] 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-permanent 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.
[0009] U.S. Pat. No. 4,434,014 issued Feb. 28, 1984 to David M.
Smith, et al. teaches that the properties of the cast articles
regarding wear resistance and machinability are obtained by 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.
[0010] 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%.
[0011] 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.
[0012] U.S. Pat. No. 4,648,918 issued Mar. 10, 1087 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.
[0013] U.S. Pat. No. 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.
[0014] U.S. Pat. No. 5,217,546 issued Jun. 8, 1993 to John A. Eady,
et al. discloses a cast hypereutectic Al--Si alloy having 12-15%
Si; ore than 0.10% Sr; more than 0.005% Ti; 1.5-5.5%
Cu;1.00-3.00Ni; 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 too expensive an alloy for competitive mass production of
engine blocks.
[0015] U.S. Pat. No. 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 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.
[0016] U.S. Pat. No. 5,484,492 issued Jan. 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% 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%
Cr; 0.001-0.1% Sr or Na; a maximum of 0.05% B; a maximum of 0.03%
Ca; a maximum 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).
[0017] To the best of applicants' knowledge, none of the last three
patent (assigned to Comalco) have every been commercialized.
[0018] U.S. Pat. No. 6,399,020 issued Jun. 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; 0.05-0.9 Ni;
0-1.0 Mn; 0.05-1.2 Ti; 0.12-1.2 Zr; 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%
Mg.
[0019] U.S. Pat. No. 6,592,687 issued Jul. 15, 2003 and U.S. Pat.
No. 6,918,970 issued Jul. 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 Cu/Mg ratio is
4-15. 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 Al.sub.3X having L1.sub.2 crystal structures, wherein X
stands for Ti, V or Zr.
[0020] U.S. Pat. No. 6,921,512 issued Jul. 26, 2005 and US Patent
Publication No. 2005/0199318 published Sep. 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%.
[0021] The desired microstructures in the Al--Si alloys are
produced by a right combination of growth rate during
solidification and temperature gradient.
[0022] 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
[0023] 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.
[0024] 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.
[0025] 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.
[0026] 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.
[0027] 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.
[0028] The alloy of the present invention comprises (in weight
percent): [0029] 13%-14% Si; [0030] 2.3%-2.7% Cu; [0031] 0.1%-0.4%
Fe; [0032] 0.1%-0.45% Mn; [0033] 0.1%-0.30% Mg; [0034] 0.1%-0.6%
Zn; [0035] 0.05%-0.11% Ti; [0036] 0.4%-0.8% Ni; [0037] 0.01%-0.09%
Sr; and [0038] 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
[0039] FIG. 1 shows a microphotograph of the microstructure (100
.mu.m) obtained from an unlined aluminum cylinder surface of an
engine block cast from the alloy of the present invention.
[0040] FIG. 2 shows a contrasting microphotograph of the
microstructure (100 .mu.m) obtained from an unlined aluminum
cylinder surface of an engine block cast from the alloy known as
A390.
[0041] FIG. 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
Durabore.TM. (a GM alloy understood to be exemplified by U.S. Pat.
No. 6,921,512).
DESCRIPTION OF PREFERRED EMBODIMENTS OF THE PRESENT INVENTION
[0042] 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.
[0043] 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.
[0044] 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.
[0045] 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.
[0046] 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.
[0047] 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 modern 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 180.degree.-200.degree. C.
[0048] The current challenge for the processes utilizing
hypoeutectic alloys is that machining high-silicon alloys means
greater wear of tools and high machining 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 microstructure of the castings of the
invention assures good machinability. Tool life is comparable to
machining an A356 alloy but with superior surface finish.
[0049] 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
modern engine blocks having unlined cylinders, while also
maintaining a competitive low manufacturing cost.
[0050] 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.
[0051] 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-US-00001 TABLE 1 Si Fe Cu Mn Mg Zn Ti Ni Sr B Alloy % % % % %
% % % ppm ppm A) 16.0-18.0 1.0 4.5 0.1 0.55 0.1 0.2 B) 13.0-15.0
0.3 2.0 0.5 0.5 1.0 0.1 2 2000 50 C) 10.6-11.5 0.5 2.5 0.6 0.3 0.4
0.11-0.15 250 25 D) 13-14 0.1-0.4 2.3-2.7 0.1-0.45 0.1-0.3 0.1-0.6
0.05-0.11 0.4-0.8 100-900 A) Hypereutectic Al-Si Alloys 390 and 391
B) Eutectic Alloy: 3HA C) Near Eutectic Alloys D) Alloy of the
present invention indicates data missing or illegible when
filed
[0052] 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.
[0053] 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.
[0054] Near eutectic alloys (C) do not have sufficient silicon
content to provide the required wear resistance.
[0055] 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 1% 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.
[0056] Referring to FIGS. 1 and 2, showing respectively a
microphotograph of the microstructure (100 .mu.m) obtained from an
unlined aluminum cylinder surface of an engine block cast from the
alloy of the present invention, and of the microstructure (100
.mu.m) 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 FIG. 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 FIG. 2.
[0057] 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.
[0058] With reference to FIG. 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 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 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.
[0059] The alloy and casting method of the present invention
present the following 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).
[0060] 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.degree. 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.
[0061] 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.
[0062] 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.
[0063] 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.
[0064] 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.
[0065] 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
[0066] 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):
[0067] Si=13.5% Sr=900 ppm; Fe=0.4%; Cu=2.5%; Ni=0.5%; Mn=0.4%;
Mg=0.35%; with the balance being essentially only aluminum (plus
minor amounts of any other essentially non-affecting elements,
hereinbefore referenced as the "remainders"). [0068] The alloy was
poured into the mold at a temperature of 750.degree. C. [0069] The
results were as follows: [0070] The microstructural segregation was
reduced. [0071] 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.
EXAMPLE 2
[0072] 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 C.degree..
[0073] 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.
[0074] 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.
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