U.S. patent application number 10/891480 was filed with the patent office on 2004-12-30 for dispersion hardenable al-ni-mn casting alloys for automotive and aerospace structural components.
Invention is credited to Belov, Nicholas A., Glazoff, Michael V., Lin, Jen C., Murtha, Shawn J., Zolotorevsky, Vadim S..
Application Number | 20040261916 10/891480 |
Document ID | / |
Family ID | 27668816 |
Filed Date | 2004-12-30 |
United States Patent
Application |
20040261916 |
Kind Code |
A1 |
Lin, Jen C. ; et
al. |
December 30, 2004 |
Dispersion hardenable Al-Ni-Mn casting alloys for automotive and
aerospace structural components
Abstract
An aluminum casting alloy includes at least about 0.5 wt % Ni
and 1-3 wt % Mn. It further includes zirconium or scandium for
precipitation hardening during T5 heat treatment.
Inventors: |
Lin, Jen C.; (Export,
PA) ; Zolotorevsky, Vadim S.; (Moscow, RU) ;
Glazoff, Michael V.; (Pittsburgh, PA) ; Murtha, Shawn
J.; (Monroeville, PA) ; Belov, Nicholas A.;
(Moscow, RU) |
Correspondence
Address: |
ECKERT SEAMANS CHERIN & MELLOTT, LLC
ALCOA TECHNICAL CENTER
100 TECHNICAL DRIVE
ALCOA CENTER
PA
15069-0001
US
|
Family ID: |
27668816 |
Appl. No.: |
10/891480 |
Filed: |
July 15, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10891480 |
Jul 15, 2004 |
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10325561 |
Dec 20, 2002 |
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6783730 |
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60345182 |
Dec 21, 2001 |
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Current U.S.
Class: |
148/549 ;
148/415; 420/550 |
Current CPC
Class: |
C22C 21/00 20130101 |
Class at
Publication: |
148/549 ;
148/415; 420/550 |
International
Class: |
C22C 021/00 |
Claims
We claim:
1. An aluminum alloy wherein Ni is the most abundant alloying
ingredient.
2. An aluminum alloy, according to claim 1, wherein the
concentration of Ni is about 4%.
3. An aluminum alloy, according to claim 1, wherein Mn is the
second most abundant alloying ingredient.
4. An aluminum alloy, according to claim 3, wherein the
concentration of Ni is about 4% and the concentration of Mn is
about 2%.
5. An aluminum casting alloy comprising: about 0.5-6 wt % Ni, about
1-3 wt % Mn, and at least one element for precipitation hardening
during artificial ageing.
6. An aluminum casting alloy, according to claim 5, wherein the
concentration of Ni is about 2-5 wt %.
7. An aluminum casting alloy, according to claim 6, wherein the
concentration of Ni is about 4 wt %.
8. An aluminum casting alloy, according to claim 5, wherein the
concentration of Mn is about 1.5-2.5 wt %.
9. An aluminum casting alloy, according to claim 5, wherein the
concentration of Mn is about 2 wt %.
10. An aluminum casting alloy, according to claim 5, wherein the at
least one element for precipitation hardening includes Sc.
11. An aluminum casting alloy, according to claim 10, wherein the
concentration of Sc is up to about 0.3 wt %.
12. An aluminum casting alloy, according to claim 5, wherein the at
least one element for precipitation hardening includes Zr.
13. An aluminum casting alloy, according to claim 12, wherein the
concentration of Zr is up to about 1 wt %.
14. An aluminum casting alloy, according to claim 12, wherein the
concentration of Zr is about 0.6%.
15. An aluminum alloy casting comprising: about 0.5-6 wt % Ni,
about 1-3 wt % Mn, and at least one element for precipitation
hardening, a microstructure of the casting comprising aluminum
dendrites in a matrix comprised of a eutectic of aluminum versus an
AlNiMn intermetallic compound, and a relatively fine dispersion
comprised of the at least one element for precipitation hardening,
at least some of the dispersion disposed in the aluminum
dendrites.
16. An aluminum alloy casting, according to claim 15, wherein the
at least one element for precipitation hardening includes Sc.
17. An aluminum alloy casting, according to claim 16, wherein the
concentration of Sc is less than about 1 wt %.
18. An aluminum alloy casting, according to claim 17, wherein the
concentration of Sc is about 0.3 wt %.
19. An aluminum alloy casting, according to claim 15, wherein the
at least one element for precipitation hardening includes Zr.
20. An aluminum alloy casting, according to claim 19, wherein the
concentration of Zr is less than about 1 wt %.
21. An aluminum alloy casting, according to claim 20, wherein the
concentration of Zr is about 0.6 wt %.
22. An aluminum alloy casting, according to claim 15, wherein the
aluminum alloy casting is a structural component of an aerospace
product.
23. An aluminum alloy casting, according to claim 15, wherein the
aluminum alloy casting is a structural component of a motor vehicle
product.
24. A method of making an aluminum alloy casting, the method
comprising: preparing a molten aluminum alloy comprising 0.5-6 wt %
Ni, 1-3 wt % Mn, Zr in a range from about 0.3-1 wt %; heating the
molten aluminum alloy to a temperature of at least above the
liquidus temperature; casting the molten aluminum alloy in a mold;
artificially ageing the casting.
25. A method, according to claim 24, wherein a concentration of the
Zr is about 0.6 wt % and the temperature is at least about 825
C.
26. A method, according to claim 24, wherein a concentration of the
Zr is about 0.7 wt % and the temperature is at least about 850
C.
27. A method, according to claim 24, wherein a concentration of the
Zr is about 0.8 wt % and the temperature is at least about 875
C.
28. A method, according to claim 24, wherein a concentration of the
Zr is about 0.9 wt % and the temperature is at least about 900
C.
29. A method, according to claim 24, wherein a concentration of the
Zr is about 1 wt % and the temperature is at least about 925 C.
30. A method, according to claim 24, wherein the molten aluminum
alloy further includes at least one grain refiner.
31. A method, according to claim 30, wherein the at least one grain
refiner includes Ti, TiB2, and TiC.
32. A method, according to claim 31, wherein the concentration of
Ti is about 0.15 wt %.
33. A method, according to claim 24, wherein the step of
artificially ageing the casting comprises maintaining a temperature
of the casting at about 400 C for about 20 hours.
34. A method, according to claim 24, wherein the step of
artificially ageing the casting comprises maintaining a temperature
of the casting at about 330 C for about 3 hours and then at about
450 C for about 2 hours.
35. A method, according to claim 24, wherein the step of casting
the alloy is at least one of high pressure die casting, permanent
mold casting, dry sand casting, green sand casting, investment
casting and other shaped casting processes.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] The present invention is a Continuation-In-Part Application
of the allowed U.S. patent application entitled "An Al--Ni--Mn
Casting Alloy for Automotive and Aerospace Structural Components",
Ser. No. 10/325,561.
FIELD OF THE INVENTION
[0002] The present invention relates to aluminum casting
compositions, particularly to aluminum alloys containing Ni and Mn
which have good properties in T5 temper.
BACKGROUND OF THE INVENTION
[0003] The aerospace and automotive industries continually seek
components which can be produced from light metal alloys such as
aluminum alloys. The Aluminum Association casting alloy 356.0
("A356.0") is one such alloy. Alloy A356.0 contains by wt. %
6.5-7.5 Si, 0.12 Fe, 0.10 Cu, 0.05 Mn, 0.30-0.45 Mg, 0.05 Zn, 0.20
Ti, with the balance being aluminum and incidental impurities. It
is noted that the recitation by the Aluminum Association of a
single value for composition, rather than a range of values,
indicates an upper limit to the amount of that element. Thus the
alloy A356.0 does not require Fe, Cu, Mn, Zn, nor Ti.
[0004] In order to obtain the desired strength and ductility for
such cast aluminum alloys, the alloys generally are thermally
treated. The alloy temper is determined by the properties required
in the alloy. A T6 temper is generally necessary for alloy A356.0
to maximize the strength and ductility of the cast product. To
achieve a T6 temper, the cast product is subjected to a solution
heat treatment and quench followed by an artificial aging process.
Typical solution heat treatment involves heating the cast product
to temperatures in the range of 450.degree.-560.degree. C. so that
soluble alloying elements within the cast product diffuse evenly
throughout the product in a solid solution.
[0005] Large cast components may require up to 20 hours of solution
heat treatment to achieve a uniform concentration of the soluble
elements in the solution. Artificial aging, which follows the
solution heat treatment, involves heating the component to a
temperature in the range of 120.degree. to 200.degree. C. to
achieve a controlled fine dispersion of precipitates within the
cast alloy.
[0006] The high temperature solution heat treatment needed to
obtain a T6 temper is quite expensive and, furthermore, introduces
distortions into the cast product. It may, therefore, be necessary
for a cast component in T6 temper to be straightened or machined in
order to obtain precisely controlled dimensions.
[0007] In recent years, the automotive industry's demand for large
aluminum castings for structural components has increased
tremendously. These large components include A, B and C posts,
engine cradles, door frames, and the like. Due to their size and
complexity, it is very difficult, if not impossible, to apply known
straightening practices to these castings. As a result, the cost
for producing these components using an alloy requiring solution
heat treatment and straightening would be very high.
[0008] One non-heat-treatable alloy is taught in U.S. Pat. No.
6,132,531. That alloy was developed for castings requiring high
ductility (>15%) and crushability. Such properties are useful in
the manufacture of nodes for a vehicular space frame. A major
drawback of that alloy is that it contains beryllium which poses a
health hazard during production, and greatly complicates the
recycling process.
[0009] A T5 temper involves cooling a cast product from an elevated
temperature to the lower temperatures used for artificial aging
and, thus, requires less energy to produce than a product in the T6
temper, which requires a higher range of temperatures. It will be
appreciated that solution heat treatment (T6 temper) is a costly
and often a lengthy process step in the production of cast aluminum
alloy products. Any attempt to minimize or eliminate solution heat
treatment can increase the efficiency and economics of producing
cast aluminum alloy products. However, cast products in the T5
temper, generally, lack the strength and toughness of the same
products produced in the T6 temper.
[0010] While some work has been performed to develop cast alloy
compositions that would obviate the need for solution heat
treatment, such alloys have not exhibited the level of strength and
toughness of conventional cast alloys such as A356.0 in the T6
temper that calls for high temperature solution heat treatment.
[0011] Accordingly, a need remains for an aluminum casting alloy
which displays good castability, low tendency for hot cracking,
good strength and toughness in the as-cast condition and which
achieves its desired strength by conditioning to a T5 temper
without solution heat treatment.
SUMMARY OF THE INVENTION
[0012] In one aspect, the present invention is an aluminum alloy
wherein nickel is the most abundant alloying ingredient.
[0013] In another aspect, the present invention is an aluminum
casting alloy comprising:
[0014] about 0.5-6 wt % Ni,
[0015] about 1-3 wt % Mn, and
[0016] at least one element for precipitation hardening during
artificial ageing.
[0017] In an additional aspect, the present invention is an
aluminum alloy casting comprising:
[0018] about 0.5-6 wt % Ni,
[0019] about 1-3 wt % Mn, and
[0020] at least one element for precipitation hardening,
[0021] a microstructure of the casting comprising aluminum
dendrites in a matrix comprised of a eutectic of aluminum versus an
AINiMn intermetallic compound, and a relatively fine dispersion
comprised of the at least one element for precipitation hardening,
at least some of the dispersion disposed in the aluminum
dendrites.
[0022] In a further aspect, the present invention is a method of
making an aluminum alloy casting, the method comprising:
[0023] preparing a molten aluminum alloy comprising 0.5-6 wt. % Ni,
1-3 wt. % Mn, and Zr in a range from about 0.3-1 wt. %;
[0024] heating the molten aluminum alloy to a temperature above the
liquidus;
[0025] casting the molten aluminum alloy in a mold; and
[0026] artificially ageing the casting.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is a phase diagram showing the liquidus of the
aluminum-zirconium system at the aluminum rich side of the
diagram;
[0028] FIG. 2 is a composition map of a sample of an AlNiMnZr
alloy, according to the present invention, cast from a temperature
of 700 C;
[0029] FIG. 3 is a composition map of a sample of the alloy shown
in FIG. 2, according to the present invention, cast from a
temperature of 750 C;
[0030] FIG. 4 is a composition map of a sample of the alloy shown
in FIG. 2, according to the present invention, cast from a
temperature of 800 C; and
[0031] FIG. 5 is a plot illustrating the development of hardness
during artificial ageing of the present invention for two different
temperature regimes.
DETAILED DESCRIPTION OF THE INVENTION
[0032] The present invention includes aluminum casting alloys
containing nickel and manganese, with additions of elements that
cause a fine grained precipitate during T5 heat treatment.
Zirconiumand/or Scandium are particularly contemplated in this
capacity. The composition, further, may also include grain refiners
such as Titanium.
[0033] When referring to any numerical range of values herein, such
ranges are understood to include each and every number and/or
fractions between the stated range minimum and maximum. A range of
about 0.5-6 wt. % nickel for example, would expressly include all
values of about 0.6, 0.7 and 0.8 wt. % nickel all the way up to,
and including, 5.7, 5.8 and 5.9 wt. % nickel. The same applies to
each other numerical property and/or elemental range set forth
herein.
[0034] A typical zirconium level is about 0.3 wt %. Table A
provides suitable broad and narrow ranges of the weight percent of
the components of the aluminum casting alloy of the present
invention. It is noted that Scandium is a particularly costly
element, so for many applications, zirconium is preferred.
1 TABLE A Element Broad (wt. %) Narrow (wt. %) Ni 0.5-6 2-5 Mn 1-3
1-3 Sc 0.5 max 0.1-0.3 Zr 1.0 max 0.4-0.6 Ti (B) 0.3 max 0.3 max Fe
0.3 max 0.3 max Si 0.2 max 0.2 max Aluminum, Balance Balance
incidental elements and impurities
[0035] Zirconium and/or scandium are included in the alloy of the
present invention to maximize the effect of dispersion hardening in
the T5 temper so as to achieve mechanical properties that are
similar to the T6 temper of casting alloy A356.0. Consequently,
production of the casting alloy of the present invention can be
made more economical. Furthermore, castings made of the alloy, in
T5 temper will exhibit better dimensional control than castings in
T6 temper.
[0036] Additions of iron or silicon to a casting aluminum alloy
typically result in the formation of various phases (i.e.
constituent particles) that can negatively affect the
microstructure of the cast product. Iron may enter phases as
Al.sub.9FeNi or Al.sub.6FeNi thereby concentrating nickel in these
ternary compounds with concomitant reduction in the volume fraction
of Al.sub.3Ni and coarsening of the microstructure. Silicon forms
intermetallic compounds such as Al.sub.15Mn.sub.2Si.sub.3. Such
silicon bearing phases can be in equilibrium with Al.sub.3Ni or
Al.sub.16Mn.sub.3Ni and participate in low temperature eutectic
reactions thereby broadening the solidification range and reducing
the castability and hot cracking index of the alloy. Accordingly,
it is recommended that the levels of iron and silicon in the alloy
be controlled to very low levels in order to avoid these
effects.
[0037] Although the invention has been described generally above,
the following particular examples give additional illustration of
the product and process steps typical of the present invention.
EXAMPLES
Example 1
[0038] The impact on castability and mechanical properties of
nickel levels in as-cast aluminum alloys containing about 2 wt. %
manganese was tested. Alloys containing various levels of nickel
and a control alloy of A356.0 (T6 temper) were cast into 22 mm
diameter ingots. The hot cracking index (HCI) is measure of
castability and was measured by a series of pencil probe molds.
Mechanical properties of ultimate tensile strength (UTS) and
elongation (El) were determined in the as-cast state and after
exposure to a corrosive environment, 24 hours in an aqueous
solution of NaCl and H.sub.2O.sub.2. The results are reported in
Table 1.
2 TABLE 1 Before corrosion After corrosion test test Alloy/ HCI,
UTS, UTS, Temper Wt. % Ni mm Mpa E1, % Mpa E1, % 1/F 0 12 98 36 101
-- 2/F 0.5 4 121 9 -- -- 3/F 1 4 146 13 141 16 4/F 2 4 170 -- -- --
5/F 4 4 201 8 191 7 A356.0/F 0 4 186 -- 169 6
[0039] These data shows that a minimum of about 0.5 wt. % Ni is
required to achieve good castability (hot cracking index of no more
than 4 millimeters). In addition, the overall corrosion resistance
was not significantly affected by the total Ni content.
Example 2
[0040] The influence of ancillary additions of various elements on
castability (hot cracking index) and mechanical properties of
aluminum alloys containing 4 wt. % Ni and 2 wt. % Mn was tested.
Table 2 lists the ancillary additions and results of the testing
along with data for A356.0 alloy. All the alloys tested exhibited
acceptable castability according to hot cracking testing. Titanium
addition (Alloy 7) improved elongation over the base alloy (Alloy
6), while Alloy 9 containing scandium addition exhibited
significantly increased strength over all alloys following a low
temperature heat treatment. The column labeled HB refers to
Brinnell Hardness.
3 TABLE 2 Before corrosion After corrosion Wt. %, test test Alloy/
ancillary HCI, UTS, UTS, Temper Element mm Mpa E1, % HB Mpa E1, %
6/F -- 4 201 8 59 191 7 7/F 0.1 Ti(B) 4 218 13.3 64 213 11 8/F 0.3
Zr 4 210 6.5 65 194 5 9/T5 0.3 Sc.sup.(*.sup.) 4 283 2.2 100 302 --
A356.0/F na 4 186 -- -- 169 6 *Alloy subjected to 3 hour heat
treatment at 300.degree. C.
Example 3
[0041] Additional alloy compositions set forth in Table 3 were
prepared as well as alloy A356.0 in the F temper (as fabricated).
Alloys 10, 11, and 13-15 were tested in the F temper, and Alloys 12
and 16 were produced in a T5 temper (3 hour treatment at
300.degree. C.). Testing was performed on 22 mm diameter castings.
The mechanical properties, UTS, yield strength (YS) and elongation
(El), of the alloys were determined using 6 millimeter samples cut
from the castings. Hot cracking index was determined using a pencil
probe. The improved elongation exhibited by Alloys 10 and 11 over
A356.0, with nearly similar strength properties, is believed to be
due in part to the formation of primary crystals of nickel
aluminides. Alloy 12 exemplifies the alloy composition of the
present invention by including 0.3 wt. % Sc and exhibiting high
strength and elongation in the T5 temper. A comparison of Alloy 10
with
[0042] Alloy 13 shows that iron and silicon may be included in the
alloy of the present invention without significant impact on the
mechanical properties. Alloys 10, 11 and 14 show the as-cast
strength is mainly a function of Ni content. Alloys 13 and 15
indicate the ductility decreasing with Fe content when Ni content
is 5 wt % but not when the Ni content is 2 wt %. The inclusion of
scandium and zirconium was tested in alloy 16 which achieved
mechanical properties similar to those of A356.0.
4 TABLE 3 Before corrosion After corrosion test test Alloy Sample
UTS, YS, UTS, YS, (Temper) Composition No. Mpa Mpa E1, % Mpa Mpa
E1, % AA356.0 7Si--0.3Mg 1 193 98 5.7 184 96 5.0 (F) 2 193 106 5.7
170 112 4.0 3 192 105 6.0 164 103 4.7 4 185 94 6.7 168 98 4.7 Avg.
191 101 6.0 172 102 4.6 10 (F) 2Ni--2Mn-- 1 157 82 20.0 148 79 17.0
0.1Ti(B) 2 154 81 20.7 151 84 22.7 3 152 79 24.3 154 83 20.7 4 153
79 20.7 152 84 19.7 Avg. 154 80 21.4 151 83 20.0 11 (F) 4Ni--2Mn--
1 174 103 17.3 170 98 15.0 0.1Ti(B) 2 173 97 18.0 171 95 17.3 3 177
95 15.6 169 91 13.0 4 172 95 15.0 170 101 16.0 Avg. 174 98 16.5 170
96 15.3 12 (T5) 2Ni--2Mn--0.3Sc 1 244 189 11.0 237 186 13.0 2 242
189 11.0 239 188 9.3 Avg. 243 189 11.0 238 187 11.2 13 (F)
2Ni--2Mn-- 1 168 81 18.3 159 79 15.3 0.1Ti(B)--0.2Fe-- 2 163 81
18.3 159 94 17.7 0.1Si 3 168 84 19.7 153 82 13.3 4 159 81 16.0 155
81 15.7 Avg. 165 82 18.1 157 84 15.5 14 (F) 3Ni--1.7Mn-- 1 157 88
20.7 0.1Ti(B) 2 154 84 19.3 3 153 82 15.7 4 158 82 16.0 Avg. 155.5
84 17.9 15 (F) 5Ni--2.2Mn-- 1 189 97 4.0 0.1Ti(B)-- 2 166 101 4.0
0.2Fe--0.1Si 3 182 111 5.0 4 173 95 3.7 Avg. 177.5 101 4.2 16 (T5)
4Ni--Mn-- 1 256 146 6.0 0.1Ti(B)-- 2 252 155 5.7 0.3Zr--0.15Sc 3
245 151 5.7 4 239 133 4.7 Avg. 248 146 5.5
Example 4
[0043] Alloys compositions 8 and 9 were also cast in the form of
bars machined out of metallic bold castings and having a cross
section of 15.times.30 millimeters. Mechanical properties for these
bars appear in Table 4 and indicate that these castings have more
dispersed microstructure. In particular, Alloy 9 with the additions
of zirconium and scandium in the T5 temper achieves mechanical
properties similar to alloy AA356.0 in the T6 temper.
5TABLE 4 Alloy UTS TYS (Temper) Composition No. MPa MPa E1 % 8 (F)
4Ni--2Mn-- 1 199 104 20 0.1Ti(B) 2 200 108 13 3 204 107 16 4 193
100 18 9 (T5) 4Ni--2Mn--0.3Zr-- 1 279 198 6 015Sc--0.1Ti(B) 2 280
193 9 3 276 198 8 4 279 198 7
Example 5
[0044] An alloy with zirconium as the only agent for precipitation
hardening was cast in the form of bars machined out of metallic
mold castings and having a cross section of 15.times.30
millimeters. The alloy composition had 4 wt % Ni, 2 wt % Mn and 0.5
wt % Zr. Mechanical properties for these bars in T5 temper are
shown in Table 5, along with values for A356.0 in T6 temper. This
table shows that these castings have properties similar to those of
alloy A356.0 in T6 temper.
6TABLE 5 Alloy TYS, MPa UTS, MPa E % HCI, mm Al--Ni--Mn--Zr 182 270
10 2-4 T5 (avg) A356-T6 210 280 7 2 (Typical)
[0045] Attention is now directed to the figures which provide
information relating to the use of zirconium for dispersion
hardening during T5 heat treatment.
[0046] FIG. 1 is a portion of the aluminum-zirconium phase diagram
10, showing the vicinity of the liquidus 19 at the extreme
aluminum-rich side of the diagram. The abscissa, 11 is zirconium
concentration, which, in this figure, ranges from 0 to 1 wt. %. The
ordinate 12 is the temperature.
[0047] The region denoted 13 is the liquid. Region 14 is solid
aluminum containing a small amount of zirconium dissolved therein.
Region 16 is a mixture of the liquid and the intermetallic
compound, Al.sub.3Zr. Region 18 is a mixture of solid aluminum and
the intermetallic compound Al.sub.3Zr.
[0048] The phase diagram 10 is employed to select a temperature for
the molten aluminum alloy prior to casting. The melt temperature
should be sufficiently high that the alloy will be above the
liquidus of the phase diagram, so that there are no solid phases,
notably Al.sub.3Zr, present in the melt. Then, the melt is
introduced into a mold and chilled quickly to minimize the time
spent in region 16, where Al.sub.3Zr is thermodynamically stable,
and in which Al.sub.3Zr tends to precipitate.
[0049] In region 18, Al.sub.3Zr is thermodynamically stable, but
because diffusion in the solid is very slow, precipitated particles
of Al.sub.3Zr grow very slowly. During artificial ageing, a
fine-grained precipitate of Al.sub.3Zr is developed in the aluminum
alloy. This fine grained precipitate increases both strength and
ductility.
[0050] FIGS. 2, 3 and 4 are composition maps of samples of a cast
aluminum alloy containing 2 wt % Ni, 2 wt % Mn and 0.5 wt % Zr. The
scale of these figures is indicated by the line segment 30, which
has a length of 10 microns. These figures show aluminum dendrites
23 surrounded by eutectic mixture 24. The white areas 26 are
particles of Al.sub.3Zr.
[0051] The sample shown in FIG. 2 was cast from a temperature of
700 C, the sample shown in FIG. 3 was cast from a temperature of
750 C, and the sample shown in FIG. 4 was cast from a temperature
of 800 C.
[0052] Scaling from FIG. 1 indicates that for an alloy containing
0.5 wt % Zr, the liquidus temperature is about 780 C. Hence, for
the sample shown in FIG. 2, which was cast from a temperature of
700 C, the alloy was in region 16 of the phase diagram shown in
FIG. 1. As expected, large particles of Al.sub.3Zr are seen in this
figure. The large Al.sub.3Zr particles are still present in the
sample shown in FIG. 3, which was cast from a temperature of 750 C.
However, such large particles of Al.sub.3Zr are absent from the
sample shown in FIG. 4, which was cast from a temperature of 800
C.
[0053] A person skilled in the art will recognize that large
intermetallic particles generally cause embrittlement, whereas a
fine dispersion of intermetallic particles has the potential to
increase both strength and toughness. Accordingly, to obtain a
casting with good mechanical properties, it should be cast from a
melt which is heated above the liquidus for the particular Zr
concentration present.
[0054] For an alloy containing 0.5 wt % Zr, a melt temperature of
at least about 800 C is recommended. For an alloy containing 0.6 wt
% Zr, a melt temperature of at least about 825 C is recommended.
For an alloy containing 0.7 wt % Zr, a melt temperature of at least
about 850 C is recommended. Likewise, for an alloy containing 0.8
wt % Zr, a melt temperature of at least about 875 C is recommended,
for an alloy containing 0.9 wt % Zr, a melt temperature of at least
about 900 C is recommended, and for an alloy containing 1 wt % Zr,
a melt temperature of at least about 925 C is recommended.
[0055] In order to achieve a fine dispersion of Al.sub.3Zr,
artificial ageing is recommended. Two temperature regimes were
tested. Schedule 1 consisted of holding the sample at a temperature
of 400 C. for 20 hours. Schedule 2 consisted of holding the sample
for 3 hours at 330 C, and then at 450 C. for 2 hours. No
dimensional changes were seen after these heat treatments.
[0056] FIG. 5 is a plot 40 showing the development of strength
during heat treatments according to schedule 1 and schedule 2. The
abscissa 42 is time in hours. The ordinate 44 is Brinnell hardness,
which is employed as a measure of strength. A number of samples
were placed in an oven, and the temperature of the oven was
controlled according to either schedule 1 or schedule 2. Individual
samples were then withdrawn from the oven at various times. The
samples were cooled to room temperature, and Brinnell hardness was
measured. Each data point in the figures represents a sample. Curve
46 is for samples treated according to schedule 1 and curve 48 is
for samples treated according to schedule 2.
[0057] Regarding the samples treated according to schedule 1 (curve
46), the hardness is seen to increase monotonically until the value
of 86 is attained after 20 hours at a temperature of 400 C.
[0058] The samples treated according to schedule 2 (curve 48)
developed hardness more quickly than those treated according to
schedule 1. For these samples, the maximum hardness, about 88, was
attained in about 2 hours at a temperature of 330 C, after which
time the hardness stabilized at about 86. Hence, both schedule 1
and schedule 2 resulted in the same value for hardness. Schedule 2
appears to be more economical to implement.
[0059] Alloys of the present invention may be made into shaped
castings by high pressure die casting, permanent mold casting, dry
sand casting, green sand casting, investment casting and other
shaped casting processes.
[0060] While the presently preferred embodiments of the invention
have been discussed above, it is to be understood that the
invention may be otherwise embodied within the scope of the claims,
which follow.
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