U.S. patent application number 13/516799 was filed with the patent office on 2012-10-11 for copper aluminum alloy molded part having high mechanical strength and hot creep resistance.
This patent application is currently assigned to RIO TINTO ALCAN INTERNATIONAL LIMITED. Invention is credited to Michel Garat, Danny Jean, James Frederick Major.
Application Number | 20120258010 13/516799 |
Document ID | / |
Family ID | 42122814 |
Filed Date | 2012-10-11 |
United States Patent
Application |
20120258010 |
Kind Code |
A1 |
Garat; Michel ; et
al. |
October 11, 2012 |
COPPER ALUMINUM ALLOY MOLDED PART HAVING HIGH MECHANICAL STRENGTH
AND HOT CREEP RESISTANCE
Abstract
The subject of the invention is a cast part with high static
mechanical strength, and high hot creep strength, made of aluminum
alloy of chemical composition: Si: 0.02-0.50%, Fe: 0.02-0.30%, Cu:
3.5-4.9%, Mn: <0.70%, Mg: 0.05-0.20%, Zn <0.30%, Ni:
<0.30%, V: 0.05-0.30%, Zr: 0.05-0.25%, Ti: 0.01-0.35%, other
elements in total <0.15%; and 0.05% each, the remainder being
aluminum. It more particularly relates to cylinder heads for
supercharged diesel or gasoline internal combustion engines.
Inventors: |
Garat; Michel; (Moirans,
FR) ; Major; James Frederick; (Kingston, CA) ;
Jean; Danny; (La Baie, CA) |
Assignee: |
RIO TINTO ALCAN INTERNATIONAL
LIMITED
Montreal
QC
|
Family ID: |
42122814 |
Appl. No.: |
13/516799 |
Filed: |
December 7, 2010 |
PCT Filed: |
December 7, 2010 |
PCT NO: |
PCT/FR2010/000812 |
371 Date: |
June 18, 2012 |
Current U.S.
Class: |
420/535 ;
164/131 |
Current CPC
Class: |
C22C 21/14 20130101;
B22D 21/007 20130101; C22C 21/16 20130101; C22C 21/12 20130101 |
Class at
Publication: |
420/535 ;
164/131 |
International
Class: |
C22C 21/14 20060101
C22C021/14; C22C 21/16 20060101 C22C021/16; B22D 29/00 20060101
B22D029/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 22, 2009 |
FR |
09062218 |
Claims
1. Cast part with high static mechanical strength at ambient and
hot temperatures and high creep strength at high temperature, cast
in aluminum alloy with a chemical composition comprising, expressed
as percentages by weight: Si: 0.02-0.50% Fe: 0.02-0.30% Cu:
3.5-4.9% Mn: <0.70% Mg: 0.05-0.20% Zn: <0.30% Ni: <0.30%
V: 0.05-0.30% Zr: 0.05-0.25% Ti: 0.01-0.35% other elements in total
<0.15%; and less than 0.05% each, the remainder being
aluminum.
2. Cast part according to claim 1, characterized in that the
magnesium content of the alloy is between 0.07 and 0.20%.
3. Cast part according to claim 1, characterized in that the
magnesium content lies between 0.08 and 0.20%.
4. Cast part according to claim 1, characterized in that the copper
content lies between 3.8 and 4.9%.
5. Cast part according to claim 1, characterized in that the
vanadium content is between 0.08-0.25%.
6. Cast part according to claim 1, characterized in that the
zirconium content lies between 0.08 and 0.20%.
7. Cast part according to claim 1, characterized in that the
titanium content lies between 0.05 and 0.25%.
8. Cast part according to claim 1, characterized in that the
silicon content lies between 0.02 and 0.20%.
9. Cast part according to claim 1, characterized in that the iron
content lies between 0.02 and 0.20%.
10. Cast part according to claim 1, characterized in that the
manganese content lies between 0.20 and 0.50%.
11. Cast part according to claim 1, characterized in that the zinc
content is less than 0.10%.
12. Cast part according to claim 1, characterized in that the
nickel content is less than 0.10%.
13. Cast part according to claim 1, having undergone T7 or T6 type
heat treatment.
14. Insert comprising a cast part according to claim 1.
15. Insert according to claim 14, characterized in that said insert
is essentially made up of the cast part.
16. Cast part according to claim 1, wherein the cast part is a
cylinder head.
17. Method for casting cylinder head according to claim 16,
comprising: providing a mold formed from an aggregate and a
water-soluble binder; casting the alloy in the mold; spraying water
on the mold so as to break up the mold and cool the cylinder
head.
18. Cast part according to claim 1, characterized in that the
magnesium content lies between 0.09 and 0.13%.
19. Cast part according to claim 1, characterized in that the
copper content lies between 4.0 and 4.8%.
20. Cast part according to claim 1, characterized in that the
vanadium content is between 0.10-0.20%.
21. Cast part according to claim 1, characterized in that the
titanium content lies between 0.10 and 0.20%.
22. Cast part according to claim 1, characterized in that the
silicon content lies between 0.02 and 0.06%.
23. Cast part according to claim 1, characterized in that the iron
content lies between 0.02 and 0.12%.
24. Cast part according to claim 1, characterized in that the iron
content lies between 0.02 and 0.06%.
25. Cast part according to claim 1, characterized in that the zinc
content is less than 0.03%.
26. Cast part according to claim 1, characterized in that the
nickel content is less than 0.03%.
Description
FIELD OF THE INVENTION
[0001] The invention relates to parts cast in copper aluminum alloy
subject to high mechanical stresses and working, at least in some
of their zones, at high temperatures, in particular cylinder heads
of supercharged diesel or gasoline engines.
BACKGROUND OF RELATED ART
[0002] Unless otherwise stated, all the values relating to the
chemical composition of the alloys are expressed as a percentage by
weight.
[0003] Alloys commonly used for cylinder heads of mass-production
automotive vehicles are essentially silicon alloys (5 to 10% of Si
in general) often containing copper and magnesium in order to
improve their mechanical properties, in particular when hot. The
main types used are: AlSi7Mg, AlSi7CuMg, AlSi (5 to 8) Cu3Mg,
AlSi10Mg, AlSi10CuMg. These alloys are used with different methods
of heat treatment: sometimes in state F without treatment,
sometimes in state T5 with simple aging, sometimes in state T6 with
solution heat treatment, quenching and aging at peak hardeness or
slightly below, and often in T7 state with solution heat treatment,
quenching and over-aging or stabilization.
[0004] The reason aluminum silicon alloys are used is their
superior casting properties, particularly absence of hot tearing,
high flowability, and good ability to feed the shrinkage cavities.
Only those alloys with a silicon content greater than or equal to
5% are appropriate for shell molding, by gravity or low pressure,
which is the dominant process for mass-produced motor cylinder
heads.
[0005] For manufacturing short production runs usually made using
sand casting, such as cylinder heads for high performance vehicles
or parts for working at high temperature for the arms and
aeronautics industries, copper alloys of the type AlCu5 are also
sometimes used, with the addition of elements to promote high
temperature resistance, such as Ni, Co, Ti, V and Zr: in this
category AlCu5NiCoZr and AlCu4NiTi are to be noted. These alloys
are highly resistant to heat, especially at 300.degree. C. where
they significantly outperform aluminum silicon mentioned above, but
suffer from two serious weaknesses: their high hot tearing
susceptibility, together with poor ability to feed shrinkage
cavities, which makes them very difficult to shell-mold for mass
production, and also the mediocrity of their mechanical properties
at ambient temperature: in particular they have very low
elongation, making them fragile and inefficient in terms of
mechanical fatigue. Table 1 summarizes the properties at ambient
temperature of both alloys, sand-cast and heat-treated in state T7
(Rp0.2 (or 0.2% TYS) being the yield strength in MPa; Rm (or UTS)
the tensile strength in MPa, and A (or E) the elongation at
fracture as a percentage):
TABLE-US-00001 TABLE 1 Alloy Rp0.2 (MPa) Rm (MPa) A (%) AlCu4NiTi
Not measurable 343 0.11 AlCu5NiCoZr 270 295 1
[0006] There is also an alloy previously standardized by the
Aluminum Association (subsequently designated as "AA" for
convenience) under number 224, which is of the AlCu5MnVZr type. It
was declared "inactive" by this association, who withdrew it years
ago from its regularly updated document "Designations and Chemical
Composition Limits for Aluminum Alloys in the Form of Castings and
Ingot." This alloy 224 does not contain magnesium (this element
coming within the category of impurities, with a maximum set at
0.03% each and 0.10% in total), and old characterization results on
sand-cast plates showed T7 state properties described in Table
2:
TABLE-US-00002 TABLE 2 Alloy Rp0.2 (MPa) Rm (MPa) A (%) 224 280 360
4.8
The Problem
[0007] Given that in future common rail diesel and turbocharged
gasoline engines the combustion chambers of the cylinder heads, and
especially the valve bridges, reach or even exceed, 300.degree. C.,
and will undergo higher pressures than in previous generations of
engines in service today, the use of aluminum copper alloys, is a
"break-out" solution in relation to the incremental progress made
by optimizing aluminum silicon alloys.
[0008] But it still remains to find an alloy in this family that
combines: [0009] high mechanical properties at ambient temperature,
[0010] high mechanical properties in the range 250-300.degree. C.,
[0011] and high creep strength at 300.degree. C., the temperature
particularly characteristic of valve bridges, parts that are
especially subject to thermo-mechanical strain.
[0012] Conventional AlCu5Mg alloys such as AlCu5MgTi (designated as
204 by the AA), and A206 and B206 (according to the AA), for parts
working at ambient or moderate temperatures do not meet these
requirements, particularly at 300.degree. C.
[0013] Alloys AlCu4NiTi and AlCu5NiCoZr (203 according to the AA)
mentioned above are themselves too weak and brittle at ambient
temperature.
[0014] AlCu5MnVZr (formerly 224 according to the AA) for parts
operating at high temperatures have a more interesting combination
of properties but still lack yield strength at ambient temperature
when compared to the desired improved properties: in state T7 it
has a yield strength of Rp0.2=280 MPa, compared with 275 MPa for
AlSi7Cu0.5 Mg0.3 T7 and 311 MPa for AlSi5Cu3Mg T7 (values measured
by the applicant and published in the articles "Alliages
d'aluminium ameliores pour culasses Diesel" (Hommes et
fonderie--February 2008--N.degree. 382) and "Aluminium Casting
Alloys for Highly Stressed Diesel Cylinder Heads", (3.
internationales Symposium Aluminium+Automobil); Dusseldorf; FRG;
3-4 Feb. 1988, pp. 154-159, 1988) respectively.
[0015] We therefore sought to obtain a considerable improvement
over the former 224 in terms of yield strength and ultimate
strength from ambient temperature up to 250-300.degree. C. We also
sought to improve the creep strength at 300.degree. C. of this
former alloy.
Subject of the Invention
[0016] The invention therefore relates to a cast part with high
static mechanical strength at ambient and high temperatures and
high creep strength at high temperature, especially at 300.degree.
C. and above, cast in aluminum alloy with the following chemical
composition, expressed as percentages by weight:
[0017] Si: 0.02-0.50%, preferably 0.02-0.20%, preferably still
0.02-0.06% Fe: 0.02-0.30%, preferably 0.02-0.20%, preferably still
0.02-0.12% and better 0.02-0.06%,
[0018] Cu: 3.5-4.9%, preferably 3.8-4.9% and preferably still
4.0-4.8%,
[0019] Mn: <0.70%, preferably 0.20-0.50%
[0020] Mg: 0.05-0.20%, preferably 0.07-0.20%, and preferably still
0.08-0.20% and finally very preferably 0.09-0.13%,
[0021] Zn: <0.30%, preferably <0.10% and preferably still
<0.03%,
[0022] Ni: <0.30%, preferably <0.10% and preferably still
<0.03%,
[0023] V: 0.05-0.30%, preferably 0.08-0.25%, and preferably still
0.10-0.20%,
[0024] Zr: 0.05-0.25%, preferably 0.08-0.20%,
[0025] Ti: 0.01-0.35%, preferably 0.05-0.25%, and preferably still
0.10-0.20%
[0026] other elements in total <0.15%; and 0.05% each,
[0027] the remainder being aluminum;
DESCRIPTION OF THE FIGURES
[0028] FIG. 1 shows a cluster of four shell-mold test specimens
made by Rio Tinto Alcan of diameter 1/4'' (6.35 mm).
[0029] FIG. 2 shows differential enthalpic analysis curves for
alloys AlCu4.7MnVZrTi with a magnesium content of 0%, 0.09% and
0.13%.
[0030] FIG. 3 shows the results of creep tests at 300.degree. C. on
T7 treated alloys AlCu4.7MnVZrTi and AlSi7Cu3.5MnVZrTi also T7
treated with a magnesium content varying from 0% to 0.13% and 0.1%
to 0.15% respectively.
DESCRIPTION OF THE INVENTION
[0031] The invention is based on the finding by the applicant that
it is possible to make very significant improvements to the
properties mentioned above of the former alloy 224 (according to
the AA), thus solving the problem, in particular by the addition of
a limited amount of magnesium.
[0032] The addition of a small quantity of magnesium, of the order
of 0.10 to 0.15%, considerably increases the yield strength and
resistance of the alloy not only at ambient temperature but also
when hot, in particular at 250-300.degree. C. and above.
[0033] It is at ambient temperature that the relative gain is the
greatest: as explained in the following examples and tables 6, 7,
8, the yield strength increases from about 190 MPa without
magnesium to about 340 MPa with only 0.09% and then to more than
390 MPa with 0.13%. Considering the average of results obtained
with 0.09% and 0.13% magnesium, the gains in terms of yield
strength and resistance at ambient temperature are remarkable: +96%
and 29% respectively in relative terms. Elongation is substantially
reduced by half but still retains an adequate level of 6 to 8%.
[0034] At high temperatures, 250 and 300.degree. C., the gains from
the addition of magnesium remain even though they decrease. The
gains observed in terms of yield strength and resistance are 35 and
13% in relative terms at 250.degree. C., and 27 and 8% in relative
terms at 300.degree. C. respectively. Far from harming the hot
stability of hardening phases as might be thought, the addition of
magnesium remains beneficial at least up to 300.degree. C., and
especially since the loss of elongation fades away at these high
temperatures.
[0035] Furthermore, the addition of magnesium considerably improves
the creep resistance at high temperature, reducing by approximately
2 the deformation observed after 300 h at 300.degree. C. with a
strain of 30 MPa. The addition of magnesium is not detrimental to
hot stability, contrary to the philosophy that led to the
definition of conventional alloys AlCu5NiCoZr (203 according to the
AA) and AlCu5MnVZr (224 according to the AA) that are devoid of
magnesium.
[0036] It is interesting to locate the average performance of the
alloy according to the invention (for simplicity we assigned the
average characteristics of alloys with 0.09% and 0.13% magnesium to
the alloy designated "AlCu4.7MnMg.sub.avVZrTi") compared to some
cylinder head alloys based on aluminum silicon. Table 3 summarizes
the mechanical properties.
TABLE-US-00003 TABLE 3 heat treatment Ambient T.degree. 250.degree.
C. 300.degree. C. Alloy Rp 0.2 Rm A % Rp 0.2 Rm A % Rp 0.2 Rm A %
AlCu4.7MnMg.sub.avVZrTi T7 369 451 7.4 182 226 9.8 125 158 14.8
AlSi5Cu3Mg F 172 237 2.1 107 133 5.8 60 86 12 AlSi7Mg0.3Ti T7 257
299 9.9 55 61 34.5 40 43 34.5 AlSi7Cu0.5Mg0.3Ti T7 275 327 9.8 66
73 34.5 40 44 34.6 AlSi7Cu3.5Mg0.15MnVZrTi T7 306 392 5.2 101 115
27 60 70 31
[0037] With regard to creep resistance at 300.degree. C., the T7
treated alloy according to the invention can be compared to
AlSi7Cu3.5Mg0.15MnVZrTi also T7 treated, which was also developed
by the applicant and is, to his knowledge, the most creep-resistant
of the series of aluminum silicon alloys considered in the previous
table. The curve in FIG. 3 shows the great superiority of
AlCu4.7MnMgVZrTi, which substantially deforms four times less in
the same conditions.
[0038] It therefore appears that the "break-out" goal of progress
in relation to existing alloys is achieved by adding magnesium to a
base of the type AlCu5MnVZrTi.
[0039] Although the addition of magnesium gradually lowers the
incipient melting temperature out of equilibrium, it remains
possible to subject the alloy to a solution heat-treatment at
525.degree. C. or 528.degree. C. as is done fairly conventionally
with alloys A206 and B206. A stepwise treatment will ultimately
make it possible to treat the alloy at a slightly higher final
temperature but this stepwise treatment is not necessary given the
very high results obtained with isothermal treatment below
incipient melting temperature.
[0040] The magnesium content can be increased beyond the area
already tested in the examples. If one is looking only for very
high strength and hardness, with low ductility requirements, a
maximum level of 0.38% may be considered, given that the incipient
melting temperature will be lowered and the heat treatment must be
adapted accordingly. The minimum for a significant hardening effect
is of the order of 0.05%. A more restricted range is 0.07% to 0.30%
and the preferred range, corresponding to the
strength-ductility-creep compromise quantified in the examples,
while having an industrially acceptable width, is 0.08-0.20% or
even 0.09 to 0.13%.
[0041] Regarding the other elements making up the type of alloy
according to the invention, their contents are justified by the
following considerations:
[0042] Silicon: generally detrimental to ductility and may lower
the incipient melting temperature. However, it improves the foundry
properties and in particular is likely, even at low levels, to
reduce hot tearing susceptibility, as described in the ASM
Handbook, Volume 15, 2008 edition. A minimum level of 0.02% is
necessary. A maximum level of 0.50% is a possibility for parts that
are solidified very quickly requiring little or no elongation, but
generally less than 0.20% or 0.06% is to be preferred.
[0043] Iron: detrimental to ductility but decreases hot tearing
susceptibility, as is also described in the ASM Handbook, Volume
15, 2008 edition. Furthermore, limiting it to a very low level
obviously increases the cost of the part. A minimum level of 0.02%
is therefore advantageous. A maximum level of 0.30% is a
possibility for parts that are solidified very quickly requiring
little or no elongation, but generally less than 0.20% is to be
generally preferred for large production runs for the automotive
industry, or even 0.12% or 0.06% for parts under significant
strain.
[0044] Copper: hardens the alloy, increasing yield strength and
resistance but decreases elongation. The range of the former alloy
224 was 4.5 to 5.5%. The experience gained by the applicant with
B206 indicates that it is a good idea to limit copper to a maximum
of 4.9% because above this it is very difficult to dissolve all the
copper. As the present results, obtained with copper from 4.7 to
4.8%, show that the strength at ambient temperature obtained with
the addition of magnesium is very high but elongation is reduced
compared to the old 224 alloy without magnesium, it seems logical
to allow for the possibility of lowering copper to below 4.5%, and
especially down to 3.5%. The applicant performed work on the B206
alloy, the results of which can be transferred to the alloy of the
invention, and show that lowering copper from 5.0% to 4.0% leads to
a significant saving in elongation at the expense of strength, but
that the latter remains greater than 400 MPa From the perspective
of some cylinder heads, it is even conceivable to accept a somewhat
larger decrease in strength so as to favor elongation and reduce
copper down to 3.5%. Sub-ranges may be chosen between 3.5% and 4.9%
depending on the compromise of properties aimed at for the specific
part. In general, sub-ranges centered on 4.3% or 4.4% such as
3.8-4.9% and better 4.0-4.8% lead to a fairly balanced
compromise.
[0045] Manganese: this element should not exceed 0.70% at the risk
of forming coarse intermetallic phases. As it usually improves
mechanical properties, particularly when hot, a range of 0.20-0.50%
similar to that of alloys of type 206 is preferred.
[0046] Zinc: this is an impurity that at high levels may decrease
the mechanical properties and make the liquid bath more oxidable.
It is conceivable to tolerate up to 0.30% in order to facilitate
the use of recycled metal, but less than 0.10% is preferred, and,
better, less than 0.03% for high-performance parts.
[0047] Nickel: contributes in general to the mechanical strength
when hot but significantly reduces elongation. As hot strength is
provided in the invention by the addition of other
elements--copper, magnesium, zirconium and vanadium--nickel is
considered here as an impurity, which is kept down to a maximum of
0.30% in order to facilitate the use of recycled metal, and
preferably 0.10% and most preferably 0.03% for high performance
parts.
[0048] Vanadium: This peritectic element particularly improves the
high temperature creep strength. The applicant observed that in
another alloy base containing silicon, the creep strength was
significantly improved between 0 and 0.05%, then improved more
gradually from 0.05% to 0.17% and was stable above 0.17% at an
excellent level. Limiting the maximum level of vanadium to 0.15% as
in the former 224 does not therefore seem desirable. In the alloy
according to the invention, a level of 0.05 to 0.30% is planned,
which may be restricted to sub-domains closer to 0.08-0.25% and
preferably 0.10-0.20%.
[0049] Zirconium: this peritectic element also especially improves
high temperature creep strength, and its effect is additive to that
of vanadium. A content of 0.05-0.25% and preferably 0.08-0.20% is
chosen.
[0050] Titanium: this peritectic element has two different effects:
firstly, it is often used as a grain-refining element, often in
combination with the addition of a master alloy or salt adding
titanium and boron. However, there are other refining practices
consisting of adding only products introducing titanium and boron,
or even boron alone, and in the latter case the presence of
titanium is not favorable. Secondly, titanium contributes to good
high temperature creep strength, though less strongly than vanadium
and zirconium, as noted by the applicant. We therefore chose a
maximum content of 0.35%, but generally prefer an addition of 0.05
to 0.25% and more preferably from 0.10 to 0.20%.
[0051] The other elements are considered as impurities. In order to
facilitate recycling, for some parts a total maximum level of 0.50%
can be tolerated, but preferably for parts undergoing strain maxima
of 0.15% overall and 0.05% each will be adopted.
Examples
[0052] In a 35 kg electric furnace a series of three alloy
compositions described in Table 4 was produced. All elements
expressed as a percentage by weight.
TABLE-US-00004 TABLE 4 Ref. Si Fe Cu Mn Mg Ti V Zr 0 Mg 0.09 0.14
4.83 0.34 0.00 0.18 0.21 0.14 0.09 Mg 0.08 0.14 4.74 0.33 0.09 0.22
0.17 0.13 0.13 Mg 0.09 0.14 4.81 0.33 0.13 0.20 0.17 0.13
[0053] These alloys were refined by the addition of AlTi5B (30 ppm
titanium added) and degassed by a 10-minute treatment using a
graphite impeller rotating at 300 r.p.m. with an argon flow of 5
liters/minute, all covered by an MgCl.sub.2 60%-40% KCl washing
flow.
[0054] Shell-mold test specimens diameter 1/4'' (6.5 mm) were then
cast, of the Rio Tinto Alcan type shown in FIG. 1 designed for
tensile testing and shell-mold test specimens ASTM B108 diameter
1/2'' (12.7 mm) designed to serve as blanks for creep specimens of
4 mm in diameter. FIG. 1 shows in particular a cluster 10 of four
shell-mold specimens 11 by Rio Tinto Alcan with a stem diameter
1/4'' (6.35 mm). This cluster 10 uses, at a scale of 1/2, the
design of the ASTM B108 test specimen.
[0055] We first determined the incipient melting temperature of
different compositions by performing differential enthalpic
analyses (DEA) on pellets machined from the test specimens cast.
The rate of temperature rise was 20.degree. C./minute. DEA curves
are shown in FIG. 2. The incipient melting temperatures observed
corresponding to melting peaks obviously depend on the magnesium
content as shown in Table 5:
TABLE-US-00005 TABLE 5 Mg content (%) Incipient melting temperature
(.degree. C.) 0 542.7 0.09 538.2 0.13 533.9
[0056] The incipient temperature gradually shifts to lower
temperatures when the Mg content increases from 0% to 0.09% and
then 0.13%.
[0057] These three alloys were then heat-treated by applying
solution heat treatment comprising a preliminary stage for 2 hours
at 495.degree. C. and then a main stage of 12 hours at 528.degree.
C., followed by water quenching at 65.degree. C. and aging for 4
hours at 200.degree. C. This produces a state T7 alloy.
[0058] Prior to this heat treatment, blanks for the creep tests
underwent hot isostatic pressing at 1000 bar at 485.degree. C. for
2 hours to remove any microporosity that could seriously affect the
tests given the small diameter of the specimen.
[0059] The static mechanical properties were measured at ambient
temperature and at 250.degree. C. and 300.degree. C. In the latter
two cases, the specimens were preheated for 100 hours at that
temperature before being stretched.
[0060] The results are shown in Tables 6, 7 and 8:
TABLE-US-00006 TABLE 6 mechanical properties at ambient temperature
Alloy Rp0.2 Rm A Mg (%) MPa MPa % 0 187.8 349.3 15.3 0.09 344.5
435.0 8.2 0.13 393.4 466.4 6.6
TABLE-US-00007 TABLE 7 mechanical properties at 250.degree. C.
Alloy Rp0.2 Rm A Mg (%) MPa MPa % 0 134.7 199.5 10.7 0.09 172.2
223.7 7.3 0.13 191.4 228.8 12.2
TABLE-US-00008 TABLE 8 mechanical properties at 300.degree. C.
Alloy Rp0.2 Rm A Mg (%) MPa MPa % 0 98.3 147.1 14.5 0.09 130.2
167.2 11.2 0.13 120.0 149.4 18.3
[0061] Creep tests were conducted at 300.degree. C. in the
following conditions:
[0062] The test specimens, 4 mm in diameter in the active area and
machined from blanks of diameter 12.7 mm, were first pre-heated for
100 hours at 300.degree. C. in a separate furnace, then placed on
the creep-testing machine and stabilized again for 1/2 hour at
300.degree. C. before being placed under a constant load of 30 MPa.
Deformation as a percentage is then recorded continuously for a
period of 300 hours at 300.degree. C. The main criterion used for
the interpretation of the deformation tests is obtained after 300
hours.
[0063] Table 9 summarizes the results:
TABLE-US-00009 TABLE 9 Creep at 300.degree. C. at 30 MPa Mg content
(%) Deformation (%) after 300 h 0 0.26 0.09 0.13 0.13 0.14
[0064] These results are plotted in FIG. 3 which also shows, as a
reference, the results obtained by the applicant with a series of
AlSi7Cu3.5MnVZrTl type alloys with different Mg contents.
[0065] A part can then be cast from the advantageous alloy defined
above; this part may in particular be a cylinder head or an insert
of a cylinder head or other parts which require high static
mechanical strength at ambient temperature and at high temperature
and high creep resistance at high temperature, especially at
300.degree. C.
[0066] The part is advantageously T7 treated, although T6 treatment
is also possible.
[0067] A new foundry process called "Ablation Casting" has recently
been introduced in North America. This process was described in the
article "Ablation Casting" by J. Grassi, J. Campbell, M. Hartlieb
and F. Major presented at the TMS 2008. This process consists of
first casting the part in a fairly insulating mold of sand+binder,
and then when it has reached a sufficient solid fraction at least
locally, spraying the mold with one (or more) water jets that
instantly dissolve the sand binder, causing the mold to collapse.
The part being solidified is then directly exposed to the impact of
the water which extracts the heat very quickly (in a similar way to
that observed, for example, during continuous vertical casting of
aluminum billets). This leads to a very rapid solidification of the
alloy and gives fine structures with high mechanical properties,
equal to, or even greater than those obtained by shell molding with
a metal mold.
[0068] Ablation casting is particularly suitable for casting alloys
with high hot tearing susceptibility. Initially, this is sand
casting that has little adverse effect on shrinkage, and then after
ablation of the mold the end of the solidification process takes
place without any rigid mold at all. In addition to providing a
high solidification rate, the process also leads to high
temperature gradients because spraying is usually gradual, starting
on selected areas and advancing towards end-of-solidification
points where it is possible to attach feeders. This advantageously
also promotes the use of alloys with a low ability to feed
shrinkage cavities, such as aluminum copper alloys, including the
alloy according to the invention.
[0069] The invention also therefore relates to a method for molding
a part from the alloy according to the invention, in particular an
insert or a cylinder head, comprising stages of: [0070] providing a
mold formed from an aggregate and a water-soluble binder; [0071]
casting the alloy in the mold; [0072] spraying water on the mold so
as to break up the mold and cool the insert or cylinder head to
accelerate solidification of the alloy.
[0073] The implementation of this method advantageously allows the
mass production of cast parts with the alloy according to the
invention having much higher hot mechanical properties than
aluminum silicon alloys. [0074] The prospects for using copper
aluminum alloys with high strength at high temperatures are not,
however, restricted to the ablation process: there are other ways
in which conventional sand casting, possibly combined with metal
coolers, and shell molding with a metal mold, possibly with
modifications to the design of parts making it possible to accept
the inferior foundry properties of this family of alloys.
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