Note: Descriptions are shown in the official language in which they were submitted.
WO 2011/124590 PCT/EP2011/055318
ALUMINUM DIE CASTING ALLOY
FIELD OF THE INVENTION
The present invention relates to aluminum alloys that can be processed by
conventional high pressure die casting and are dispersion-strengthened, age-
hardenable, and have useful mechanical properties at temperatures up to at
least 300 C.
BACKGROUND OF THE INVENTION
Aluminum alloys are one of the most important groups of light materials
employed in the automotive industry, mainly because of their high specific
strength. Most of the traditional aluminum casting alloys are based on the
aluminum-silicon eutectic system because of its excellent casting
characteristics. Unfortunately the solidus in this system does not exceed 550
C,
and consequently the maximum working temperature of aluminum-silicon alloys
is limited to about 200 C. In addition, the major alloying elements in
traditional
aluminum-based alloys (i.e., zinc, magnesium, and copper) have high
diffusivity
in the aluminum solid solution. Therefore, while these elements enhance the
room temperature strength of the alloy, they compromise the alloy's thermal
stability. For example, aluminum alloys based on the AI-Zn-Mg, the Al-Cu-Mg,
and the Al-Li systems are able to achieve very high tensile strength (up to
about
700 MPa); however their mechanical properties rapidly degrade when they are
used at high temperature. In many applications, stability of mechanical
properties at high temperature - not high strength - is the primary need.
Therefore traditional aluminum alloys are not useful in such applications, and
there is a need for a light-weight, thermally-stable material.
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PRIOR ART
Attempts have been made in the prior art to provide aluminum casting alloys
with enhanced thermal stability. Notable among these attempts are those that
utilize the aluminum-nickel system with minor additions of zirconium. The
following journal articles represent these attempts:
N.A. Belov, "Structure and Strength of Cast Alloys of the System Aluminum-
Nickel-Zirconium," Metallov., No. 10, pp. 19-22, 1993.
N.A. Belov, "Principles of Optimizing the Structure of Creep-Resisting Casting
Aluminum Alloys using Transition Metals," Journal of Advanced Materials, Vol.
1, No. 4, pp. 321-329, 1994.
N.A. Belov, V.S. Zolotorevsky, S. Goto, A.N. Alabin, V.V. Istomin-Kastrovsky,
and V.I. Mishin, "Effect of Zirconium on Liquidus and Hardening of Al-6%Ni
Casting Alloy," Metals Forum, Vol. 28, pp. 533-538, 2004.
The preceding journal articles teach that an optimum structure for an aluminum
alloy that exhibits stability at high temperature can be produced on the basis
of
a eutectic composition consisting of an aluminum solid solution (a-aluminum)
phase that is alloyed with at least 0.6 % by weight zirconium; and a second
phase that has high creep strength, namely nickel tri-aluminide (AI3Ni).
The preceding journal articles also teach that objects made from these alloys
are obtained by melting the carefully weighed solid alloy ingredients
(aluminum,
aluminum nickel master alloy, and aluminum zirconium master alloy) at about
900 C. This relatively high melting temperature is necessary in order to
dissolve
the high zirconium content (>_ 0.6 % by weight zirconium) into aluminum and
obtain a homogeneous aluminum-nickel-zirconium melt. In addition, the
preceding journal articles teach that the aluminum-nickel-zirconium melt must
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be cooled at a cooling rate that is faster than 10 C/second in order to
solidify it
and retain a homogeneous super saturated solid solution of zirconium in a-
aluminum at room temperature. Furthermore, the preceding journal articles
teach that as the material cools from the melt temperature, it may be shaped
into the desired object form by casting it in a mold. Said mold must permit
the
material to cool from the melt temperature to room temperature at a rate that
exceeds 10 C/second. Finally, the preceding journal articles teach that the
cast
solid object may be aged at a temperature between 350 C and 450 C in order
to precipitate fine zirconium tri-aluminide (Al3Zr) particles that harden the
alloy.
When properly processed, the alloys represented in the preceding journal
articles have better mechanical properties at elevated temperature than
traditional aluminum casting alloys. However, hardening will not occur in the
alloys represented in the preceding journal articles unless the zirconium
content
of the alloy is in excess of 0.4 % by weight, and significant hardening will
not
occur unless the zirconium content of the alloy is at least 0.6 % by weight.
Smaller amounts of zirconium will not result in a volume of second phase
particles (in this case AI3Zr) that is sufficient to induce significant
hardening of
the a-aluminum solid solution. Fig. 1 depicts the amount of solid present in
the
melt as a function of temperature for an alloy of the prior art. The Figure
shows
that the alloy is completely molten only at temperatures above 850 C. Such
high melt temperature does not allow the alloys represented in the preceding
journal articles to be processed into shaped objects by conventional high
pressure die casting since the temperature of the melt that may be introduced
into the shot sleeve of a traditional high pressure die casting machine should
not exceed 750 C.
A high cooling rate - in excess of 10 C/second - is necessary for retaining
0.6
% by weight zirconium in solid solution in a-aluminum at room temperature.
With the exception of high pressure die casting, such a fast cooling rate
cannot
be attained in most objects that are cast by conventional casting processes.
Accordingly, with the exception of casting very small objects in graphite or
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copper molds, the alloys represented in the preceding journal articles cannot
be
processed into shaped objects by conventional casting processes.
DISCLOSURE OF THE INVENTION
This invention relates to a class of aluminum alloys which (i) are dispersion-
strengthened, (ii) can be age-hardened for improved mechanical properties, and
(iii) can be processed by conventional high pressure die casting to produce
shaped articles that have useful mechanical properties at temperatures up to
at
least 300 C.
It is an object of the present invention to provide light-weight, wear-
resistant,
and corrosion-resistant materials that are castable via the conventional high
pressure die casting process and that are thermally-stable up to at least 300
C.
The foregoing object is achieved according to the invention by an aluminum die
casting alloy comprising
2 to 6 % by weight nickel,
0.1 to 0.4 % by weight zirconium,
0.1 to 0.4 % by weight vanadium,
optionally up to 5 % by weight manganese,
optionally up to 2 % by weight iron,
optionally up to 1 % by weight titanium,
and aluminum as the remainder with impurities due to production total max. 1 %
by weight.
A preferred nickel range is 4 to 6 % by weight, a preferred zirconium range is
0.1 to 0.3 % by weight, and a preferred vanadium range is 0.3 to 0.4 % by
weight.
The alloys of the present invention have the general chemical composition:
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aluminum-nickel-zirconium-vanadium and their chemical composition is
optimized such that their liquidus temperature is less than 750 C.
Upon solidification from the melt, nickel and aluminum form a eutectic
structure
comprised of a solid solution of nickel in aluminum (referred to as the a-
aluminum phase) and a second phase comprised of nickel tri-aluminide (AI3Ni).
Alloys with a eutectic component in their microstructure have a narrower
solidification range, and therefore are less prone to hot tearing, than alloys
without a eutectic component in their microstructure. The AI3Ni phase is in
the
form of thin rods whose diameter is in the range of 300 to 500 nanometers. If
cooling from the melt temperature to room temperature is performed fast
enough (i.e., at a rate that exceeds 10 C/second), then also dissolved in the
a-
aluminum phase will be zirconium and vanadium. Upon subsequent controlled
thermal aging of the solid alloy, zirconium and vanadium combine with
aluminum via a solid-state reaction to form a strengthening precipitate phase
of
the chemical composition AI3ZrxV1_x. The sub-micron size meta-stable
AI3ZrxVi_x
particles have the L12 cubic crystal structure and are uniformly distributed
in the
a-aluminum solid solution.
The alloys of the present invention may also include up to 5 % by weight
manganese and up to 2 % by weight iron. In addition to forming metal
aluminides, which can further strengthen the alloy, iron and manganese are
useful ingredients in high pressure die casting alloys as they tend to
mitigate
soldering of the alloy to the die components.
The alloys of the present invention may also include up to 2 % by weight
magnesium, up to .2 % by weight hafnium, up to 1 % by weight titanium, up to I
% by weight molybdenum, up to 1 % by weight chromium, up to 0.5 % by
weight silicon, up to 0.5 % by weight copper and up to 0.5 % by weight zinc.
The alloys of the present invention preferably include substantially uniformly
dispersed particles of AI3ZrxV1_x, where x is a fraction of unity that depends
on
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the ratio of Zr : V in the alloy, the particles having an equivalent diameter
of less
than about 50 nm and preferably less than about 30 nm.
The alloys of the present invention preferably include particles of AI3Ni
having
an equivalent diameter of less than about 500 nm, preferably less than about
300 nm, particularly less than about 100 nm.
The alloys of the present invention may include substantially uniformly
dispersed particles of manganese aluminide having an equivalent diameter of
less than about 50 nm and preferably less than about 30 nm.
The alloys of the present invention may include substantially uniformly
dispersed particles of iron aluminide having an equivalent diameter of less
than
about 50 nm and preferably less than about 30 nm.
A feature of the alloys of the present invention which distinguishes them from
prior art aluminum alloys which contain nickel and zirconium but without
vanadium (described in the journal articles by N.A. Belov) is that the alloys
of
the present invention have a much lower liquidus temperature (typically less
than 750 C as opposed to more than 850 C for the prior art alloys). The lower
liquidus temperature permits the alloys of the present invention to be
processed
into shaped objects. by conventional high pressure die casting whereas the
alloys of the prior art cannot be processed into shaped objects by
conventional
high pressure die casting and are thus limited to the casting of small objects
in
graphite molds.
Another feature of the alloys of the present invention which distinguishes
them
from the prior art aluminum alloys containing nickel and zirconium but without
vanadium is that the precipitation hardening particles in the alloys of the
present
invention are Al3ZrrV1_X particles (compared to AI3Zr particles in the alloys
of the
prior art). Because of the smaller size of the vanadium atom (0.132 nm)
compared to the zirconium atom (0.159 nm), the AI3ZrrV1_X lattice has a
lattice
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parameter that is smaller than that of the AI3Zr lattice and which more
closely
matches the lattice parameter of the a-aluminum matrix. For this reason,
aluminum-nickel alloys that are hardened with AI3ZrxV1_x precipitates are more
thermally stable than aluminum-nickel alloys that are hardened with AI3Zr
precipitates.
The foregoing and other features and advantages of the present invention will
become more apparent from the following detailed description and
accompanying drawings.
DESCRIPTION OF THE DRAWINGS
Figure 1 is a computer-generated solidification path for aluminum - 6 % by
weight nickel - 0.6 % by weight zirconium alloy;
Figure 2 is a computer-generated solidification path for aluminum - 6 % by
weight nickel - 0.1 % by weight zirconium - 0.4 % by weight
vanadium alloy.
DETAILED DESCRIPTION OF THE INVENTION
Dispersion strengthening of aluminum alloys relies on the creation of
dispersed
particles in the alloy's matrix. This strengthening mechanism is typified by
alloys
based on the aluminum-nickel system. Hypo-eutectic and eutectic aluminum-
nickel alloys solidify in a structure that contains a fine dispersion of
nickel tri-
aluminide (AI3Ni) particles in a matrix comprised of a solid solution of
nickel in
aluminum (a-aluminum). Since nickel tri-aluminide is essentially insoluble in
aluminum up to about 855 C, aluminum-nickel alloys are more stable at
elevated temperatures than aluminum-silicon alloys. However, aluminum-nickel
binary alloys do not posses adequate mechanical properties for most
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automotive applications as their room temperature tensile yield strength does
not exceed 80 MPa; and therefore additional strengthening of these alloys is
necessary.
Precipitation strengthening is a well-known mechanism of strengthening
aluminum alloys as typified by alloys based on the aluminum-copper system. In
these alloys precipitation of copper aluminide particles in an a-aluminum
matrix
is thermally controlled in order to produce effective strengthening of the
alloy
matrix.
The present invention combines characteristics of both types of the hardening
mechanisms previously described in order to obtain aluminum alloys with
sufficient elevated temperature mechanical strength for most automotive
applications. The alloys of the present invention contain a fine dispersion of
creep-resistant nickel tri-aluminide particles and a strengthening precipitate
that
is based on zirconium and vanadium, namely AI3ZrxV1_x.
In the prior art alloys, which contain nickel and zirconium but without
vanadium
(described in the journal articles by N.A. Belov), a strengthening phase with
the
chemical composition AI3Zr is formed. In the invention alloy, the
strengthening
phase is also based on the AI3Zr structure but with vanadium atoms
substituting
for some of the zirconium atoms. The accurate representation of the
strengthening phase in the invention alloy is thus AI3ZrxVi_x with x being a
fraction of unity whose magnitude depends on the ratio of zirconium to
vanadium. The role that vanadium plays in the invention alloy is important in
allowing the alloy to be processed into articles by high pressure die casting.
The
extent of strengthening induced by a precipitate is related to both the volume
fraction of the precipitate and the size of the precipitate particles. A large
volume fraction of small size particles is essential for strengthening. The
prior
art alloys employ a minimum 0.6% by weight zirconium in order to create about
0.83% by volume of the AI3Zr strengthening phase. This amount is shown to be
sufficient for significant strengthening of the alloy. However, examination of
Fig.
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1 shows that the liquidus temperature of an alloy with 0.6% zirconium is over
850 C. This relatively high melt temperature is prohibitive for conventional
high
pressure die casting, and therefore alloys of the prior art cannot be mass
produced by high pressure die casting operations. A preferred version of the
invention alloy employs only 0.1% by weight zirconium and 0.4% by weight
vanadium. This mixture creates about 0.84% by volume of the AI3ZrxV1_x
strengthening phase. The main benefit of employing vanadium in the invention
alloy is that the liquidus temperature of the invention alloy is only about
730 C -
see Fig. 2, which permits the use of conventional high pressure die casting in
manufacturing shaped articles with the invention alloy.
A broad description of the invention material after optimum processing is that
it
is an a-aluminum (a very dilute solid solution of nickel in aluminum) matrix
which contains about 0.8-1.0% by volume of a uniformly distributed
strengthening phase that is based on zirconium and vanadium and that has a
structure represented by the chemical formula AI3ZrxV1_x, and about 1-10% by
volume nickel tri-aluminide particles uniformly dispersed in the alloy matrix.
In a
material of this invention that has been processed to have maximum strength,
the AI3ZrxV1_x strengthening particles are meta-stable, have the L12 cubic
structure, are coherent with the a-aluminum matrix, and have an average
diameter of less than about 25 nm.
The production of such a structure requires: (1) fast cooling from the melt
temperature, and (2) controlled thermal aging of the solidified article.
Fast cooling from the melt temperature is necessary to ensure that zirconium
and vanadium are retained in solution in the a-aluminum matrix at room
temperature; i.e., at room temperature the alloy contains the Al3Ni eutectic
phase and a second phase that is a super saturated solid solution of zirconium
and vanadium in a-aluminum. For the invention alloy, a cooling rate that
exceeds 10 C / second is necessary to obtain a super saturated solid solution
of zirconium and vanadium in a-aluminum. One of the advantages of the
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invention alloy over prior art alloys is that it is designed so that it can be
processed into shaped articles by conventional high pressure die casting
wherein the molten alloy at about 750 C is introduced directly into the shot
sleeve of the die casting machine. It is then injected under high pressure
into a
steel die; the pressure is maintained on the alloy until solidification is
complete,
and then the solidified article is ejected. It is known that cooling rates in
conventional high pressure die casting operations typically exceed
C/second. Therefore the casting process which shapes the article also
provides the quenching that is necessary for obtaining a homogeneous super
10 saturated solid solution of the strengthening elements (zirconium and
vanadium) in a-aluminum.
Controlled thermal aging of solidified cast articles made with the invention
alloy
is necessary in order to precipitate the meta-stable L12 cubic AI3ZrxVi_x
strengthening particles in the a-aluminum solid solution. This may be
accomplished by an optimized thermal aging schedule. One such schedule
includes holding the solidified cast article at a temperature between 250 C
and
350 C for between two and six hours followed by holding it at a temperature
between 350 C and 450 C for between two and six hours. A preferred thermal
aging schedule includes holding the solidified cast article at 350 C for three
hours followed by holding it at 450 C for an additional 3 hours. Simultaneous
with precipitating the AI3ZrxV1_x strengthening particles in the a-aluminum
solid
solution, the prescribed thermal aging schedule fragments and changes the
shape of the Al3Ni eutectic rods into submicron size particles. This
fragmentation and globularization of the AI3Ni eutectic rods enhances the
overall ductility of the cast article.
Although this invention has been shown and described with respect to detailed
embodiments thereof, it will be understood by those skilled in the art that
various changes in form and detail thereof may be made without departing from
the spirit and scope of the claimed invention.
