U.S. patent application number 12/863148 was filed with the patent office on 2011-02-24 for high-strength aluminum casting alloys resistant to hot tearing.
This patent application is currently assigned to QUESTEK INNOVATIONS LLC. Invention is credited to Herng-jeng Jou, Charles Kuehmann, Abhijeet Misra.
Application Number | 20110044843 12/863148 |
Document ID | / |
Family ID | 41120131 |
Filed Date | 2011-02-24 |
United States Patent
Application |
20110044843 |
Kind Code |
A1 |
Misra; Abhijeet ; et
al. |
February 24, 2011 |
HIGH-STRENGTH ALUMINUM CASTING ALLOYS RESISTANT TO HOT TEARING
Abstract
An aluminum casting alloy resistant to hot tearing includes, in
wt %, about 4.0 to about 6.9 Zn, about 2.0 to about 3.5 Mg, about
0.6 to about 1.2 Cu, about 0.38 to about 0.57 Sc, about 0.18 to
about 0.28 Zr, and the balance Al and impurities, substantially
excluding Fe, Mn, and Si, said alloy characterized by a freezing
range of less than about 150.degree. C., solidus temperature above
about 490.degree. C., and eutectic phase fraction above about 5% at
the late stages of solidification. The alloy is processed to form a
dispersion of L1.sub.2 particles inoculating fcc grains with a
grain diameter of about 40 to about 60 .mu.m, and .eta.'-phase
precipitates enabling an ambient yield strength from about 410 MPa
to about 540 MPa.
Inventors: |
Misra; Abhijeet; (Evanston,
IL) ; Kuehmann; Charles; (Riverwoods, IL) ;
Jou; Herng-jeng; (Wilmette, IL) |
Correspondence
Address: |
BANNER & WITCOFF, LTD.
TEN SOUTH WACKER DRIVE, SUITE 3000
CHICAGO
IL
60606
US
|
Assignee: |
QUESTEK INNOVATIONS LLC
Evaston
IL
|
Family ID: |
41120131 |
Appl. No.: |
12/863148 |
Filed: |
January 16, 2009 |
PCT Filed: |
January 16, 2009 |
PCT NO: |
PCT/US09/31251 |
371 Date: |
July 15, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61021503 |
Jan 16, 2008 |
|
|
|
Current U.S.
Class: |
420/532 |
Current CPC
Class: |
C22C 21/10 20130101;
C22F 1/053 20130101 |
Class at
Publication: |
420/532 |
International
Class: |
C22C 21/10 20060101
C22C021/10 |
Goverment Interests
GOVERNMENT INTERESTS
[0001] Activities relating to the development of the subject matter
of this invention were funded at least in part by United States
Government and thus may be subject to license rights and other
rights in the United States, specifically contract number
FA8650-05-C-5800.
Claims
1. An aluminum casting alloy with anti tear characteristics
comprising, in wt %, about 4.0 to about 6.9 Zn, about 2.0 to about
3.5 Mg, about 0.6 to about 1.2 Cu, about 0.38 to about 0.57 Sc,
about 0.18 to about 0.28 Zr, and the balance Al and impurities,
substantially excluding Fe, Mn, and Si, said alloy characterized by
a dispersion of L1.sub.2 particles inoculating fcc grains, and
.eta.'-phase precipitates.
2. The alloy of claim 1 wherein the mean grain diameter of the fcc
grains is about 40 to 60 .mu.m.
3. The alloy of claim 1 wherein the .eta.'-phase precipitates have
a mean radius of less than about 5 nm.
4. The alloy of claim 1 having less than about 0.0075 weight
percent Fe, less than about 0.2 weight percent Mn and less than
about 0.03 weight percent Si.
5. The alloy of claim 1 having the following constituents in weight
percent: about 5.8-6.8 Zn, 2.9-3.5 Mg, 1.0-1.2 Cu, 0.52 Sc, and
0.20 Zr.
6. The alloy of claim 1 having the following constituents in weight
percent: about 4.8-5.8 Zn, 2.7-3.3 Mg, 1.0-1.2 Cu, 0.55 Sc, and
0.25 Zr.
7. The alloy of claim 1 having the following constituents in weight
percent: about 4.0-5.0 Zn, 2.0-2.6 Mg, 0.52-0.72 Cu, 0.42 Sc and
0.25 Zr.
8. An aluminum casting alloy with anti tear characteristics
comprising, in wt %, about 4.0 to about 6.9 Zn, about 2.0 to about
3.5 Mg, about 0.6 to about 1.2 Cu, about 0.38 to about 0.57 Sc,
about 0.18 to about 0.28 Zr, and the balance Al and impurities,
substantially excluding Fe, Mn, and Si, said alloy characterized by
a freezing range of less than about 150.degree. C., solidus
temperature above about 490.degree. C., and eutectic phase fraction
above about 5% at the late stages of solidification.
9. An aluminum casting alloy with anti tear characteristics
comprising, in wt %, about 4.0 to about 6.9 Zn, about 2.0 to about
3.5 Mg, about 0.6 to about 1.2 Cu, about 0.38 to about 0.57 Sc,
about 0.18 to about 0.28 Zr, and the balance Al and impurities,
substantially excluding Fe, Mn, and Si, said alloy characterized by
a freezing range of less than about 150.degree. C., solidus
temperature above about 490.degree. C., and eutectic phase fraction
above about 5% at the late stages of solidification and a
dispersion of L1.sub.2 particles inoculating fcc grains, and
.eta.'-phase precipitates.
10. An aluminum casting alloy with anti tear characteristics
comprising, in wt %, about 4.0 to about 6.9 Zn, about 2.0 to about
3.5 Mg, about 0.6 to about 1.2 Cu, about 0.38 to about 0.57 Sc,
about 0.18 to about 0.28 Zr, and the balance Al and impurities,
substantially excluding Fe, Mn, and Si, said alloy characterized by
a freezing range of less than about 150.degree. C., solidus
temperature above about 490.degree. C., and eutectic phase fraction
above about 5% at the late stages of solidification, a dispersion
of L1.sub.2 particles inoculating fcc grains with a grain diameter
of about 40 to about 60 .mu.m, and .eta.'-phase precipitates, and
ambient yield strength from about 410 MPa to about 540 MPa.
Description
BACKGROUND OF THE INVENTION
[0002] The 7XXX wrought Al--Zn-based alloys are commonly used in
structural applications demanding high specific strength. Compared
to wrought alloys, castings decrease the fabrication cost and
associated logistics lead time, because castings enable
near-net-shape products. However, the known 7XXX alloys are
susceptible to hot tearing during solidification and therefore not
optimal for casting. The hot tearing is caused by a relatively high
thermal expansion coefficient and significant volumetric difference
between liquid and solid.
[0003] Senkov et al. [U.S. Pat. No. 7,060,139 (incorporated by
reference herein)] disclose a high-strength aluminum alloy with a
nominal composition of Al--6.0.about.12.0 Zn--2.0.about.3.5
Mg--0.1.about.0.5 Sc--0.05.about.0.20 Zr--0.5.about.3.0
Cu--0.10.about.0.45 Mg--0.08.about.0.35 Fe--0.07.about.0.20 Si, in
wt %. The alloy by Senkov et al. shows high tensile strength while
maintaining high elongation in ambient temperatures and cryogenic
temperatures. The freezing range of the alloy by Senkov et al. is
about 164 to about 195.degree. C., the solidus temperature about
422 to about 466.degree. C., and the eutectic phase fraction about
1.1 to about 1.5%. However, the alloy shows poor casting
characteristics. Thus, there has developed a need for new 7XXX
aluminum casting alloys that are resistant to hot tearing. Such
alloys would be useful for articles of manufacture such as hydrogen
turbo pump housing or other aerospace materials.
SUMMARY OF THE INVENTION
[0004] In a principal aspect, the present invention comprises
high-strength aluminum casting alloys that are resistant to hot
tearing. The yield strength of the casting alloys ranges from about
410 MPa to about 540 MPa, at room temperature. The invented alloys
are Al--Zn-based and comprise the major alloying elements Sc, Zr,
Mg, and Cu. The amounts of Sc and Zr are optimized to produce
primary L1.sub.2-phase particles which refine the grain size and
improve the hot-tearing resistance as well as fatigue resistance
and toughness. The amounts of Zn, Mg, and Cu are optimized for high
resistance to hot-tearing and high strength. The amounts of Fe, Mn,
and Si are kept low and at a minimum because these elements have a
detrimental effect on strength and hot-tearing resistance.
[0005] To produce primary L1.sub.2-phase particles, the solvus
temperature of the L1.sub.2 phase must be above the solvus
temperature of the fcc phase. The solvus temperatures can be
computed with thermodynamic database and calculation packages such
as Thermo-Calc.RTM. software version N offered by Thermo-Calc
Software. Alternatively, in the composition space of the alloys,
the solvus temperatures can be approximated by the following
equations:
L1.sub.2
solvus=87.01.times.wp.sub.Sc+157.89.times.wp.sub.Zr-243.43.time-
s.wp.sub.Sc.times.wp.sub.Zr+267.06.times.wp.sub.Sc.sup.0.14+769.51.times.w-
p.sub.Zr.sup.0.05
fcc
solvus=-1.76.times.wp.sub.Zn-5.14.times.wp.sub.Mg-0.005.times.wp.sub-
.Zn.times.wp.sub.Mg+139.13.times.wp.sub.Zn.sup.0.002+792.11.times.wp.sub.M-
g.sup.0.0002
where wp.sub.Sc, wp.sub.Zn, wp.sub.Zr, and wp.sub.Mg are the weight
percentages of Sc, Zr, Zn, and Mg, respectively. These equations
are based on the best fit for solvus temperatures.
[0006] Additionally, the amount of Zr is kept below about 0.3 wt %
to minimize the formation of Al.sub.3Zr which has a D0.sub.23
crystal structure. As shown by Hyde in Al--0.5Sc--0.4Zr (wt %),
D0.sub.23 particles quickly grow too large [Hyde, K. 2001. The
Addition of Scandium to Aerospace Casting Alloys. Ph.D. diss.,
University of Manchester (incorporated herewith)], and are not very
effective for refining the fcc grain size. In the discovered
alloys, small Al.sub.3(Sc, Zr) particles with an L1.sub.2 crystal
structure are employed instead to inoculate small fcc grains during
melt cooling. Because Zr is an inexpensive substitute for Sc in
L1.sub.2, the alloys of the invention use as much Zr as possible,
about 0.25.+-.0.05 wt %. However, where cost is not a limiting
factor, as little as 0.15 wt % Zr can be used in combination with a
larger amount of Sc.
[0007] The amounts of Sc and Zr in the casting alloys are optimized
for cooling rates up to about 100.degree. C. per second. The
L1.sub.2-Al.sub.3(Sc, Zr) particle size distribution depends on the
melt cooling rate. Casting into a sand mold results in a cooling
rate of about 0.5.degree. C. per second. Higher cooling rates are
accessible through direct-chill casting where the billet is cooled,
for example, with water during solidification. Cooling rates above
about 100.degree. C. per second are accessible through casting
methods such as the Continuous Rheoconversion Process (CRP).
[0008] As shown in FIG. 1, large primary L1.sub.2 particles will
result in fcc grains larger than 200 .mu.m in diameter. To achieve
an fcc grain diameter of about 40 to about 60 .mu.m at cooling
rates up to about 100.degree. C. per second, the mean radius of
primary L1.sub.2 particles should be less than about 2 .mu.m and
its phase fraction should be less than 1% by weight.
[0009] FIG. 1A shows the amounts of Sc and Zr which enable the
required L1.sub.2 particle size. Because the amount of Zr is kept
below about 0.3 wt %, the amount of Sc is kept above about 0.4 wt
%, up to about 0.6 wt %.
[0010] Because hot tearing is caused substantially by a thermal
contraction during solidification, resistance to hot tearing can be
improved by decreasing the freezing range and increasing the
solidus temperature below which the aluminum alloy is completely
solid. It is also helpful to increase the eutectic phase fraction
formed at late stages of solidification, because the eutectic phase
solidifies completely at one temperature and reduces the amount of
melt contracting over the freezing range.
[0011] Solidification parameters such as the freezing range, the
solidus temperature, and the eutectic phase fraction can be
computed with thermodynamic database and calculation packages such
as Thermo-Calc software. To compute solidification parameters of
complex alloy systems with Thermo-Calc software, the Gibbs free
energy of relevant phases must be assessed following the CALPHAD
(CALculation of PHAse Diagrams) approach. One such relevant phase
is the metastable .eta.' phase, because the 7 XXX wrought alloys
employ q' phase precipitates for strengthening. For efficient
strengthening, the mean radius of .eta.' precipitate should be less
than about 5 nm.
[0012] The .eta.' phase precipitation kinetics can be simulated
with PrecipiCalc.RTM. software version 0.9.2 offered by QuesTek
Innovations LLC after assessing the thermodynamic description. The
predicted particle size distribution can be used as input to a
mechanistic model of the yield strength, which comprises
contributions from precipitation strengthening, grain-size
strengthening, solid-solution strengthening, and dislocation
strengthening. The amounts of Zn, Mg, and Cu of the alloys are
chosen to optimize the solidification parameters at various yield
strength levels.
[0013] The amounts of Fe, Mn, and Si are kept as low as possible
because these elements otherwise form large insoluble constituent
particles of Al.sub.13Fe.sub.4, Al.sub.7Cu.sub.2Fe, Mg.sub.2Si, and
Al.sub.6Mn which negatively affect the toughness, fatigue, and SCC
resistance. The amount of Fe is preferably kept below about 0.0075
wt %, Mn below about 0.2 wt %, and Si below about 0.03 wt %.
[0014] In order to avoid incipient melting during homogenization or
solution treatment, the homogenization or solution treatment
temperature should be below the final solidification temperature,
preferably with a safety margin of about 10 to 30.degree. C. A
two-step treatment distinguishing the homogenization from the
solution treatment, as shown in FIG. 3, can introduce an additional
safety factor to avoid incipient melting. The calculated final
solidification temperature is about 493.degree. C. Thus, in one
embodiment, the homogenization and solution treatment should be at
about 460 to 480.degree. C. The time of such treatments should be
long enough to eliminate the majority of as-cast segregation. As
shown in FIG. 4, homogenization simulations show that a
homogenization at 460.degree. C. for 2 hours followed by a solution
treatment at 480.degree. C. for 1 hour should be sufficient to
eliminate the majority of as-cast elemental segregation. This
simulation was conducted with the kinetic software DICTRA.TM.
(DIffusion Controlled TRAnsformations) version 24 offered by
Thermo-Calc Software.
[0015] The subject matter of the invention is applicable to
aluminum 7XXX alloys in particular, but the invention is not
necessarily so limited. Thus, one benefit of the invention is to
eliminate, or substantially eliminate, hot tearing of cast aluminum
alloys.
[0016] Further benefits, advantages and features of the invention
are set forth herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] In the detailed description which follows references will be
made to the drawing comprising the following figures:
[0018] FIGS. 1A and 1B respectively are graphs depicting the
simulated primary L1.sub.2 particle radius and simulated grain size
as a function of the alloy Sc and Zr;
[0019] FIGS. 2A, 2B, and 2C respectively are graphs depicting
strength and solidification parameter contours as a function of Zn,
Mg, and Cu content wherein the following legends are utilized:
[0020] ______ Yield strength iso-contours (ksi)
[0021] % Eutectic (Scheil)
[0022] ______ Scheil Freezing range (.degree. C.)
[0023] - - - Scheil solidification temperature (.degree. C.)
[0024] Star: High strength solution (YS.about.80 ksi)
[0025] Triangle: Medium strength solution (YS.about.70 ksi)
[0026] Square: Low strength solution (YS.about.60 ksi)
[0027] FIG. 3 is a time-temperature diagram illustrating the
processing steps for processing an embodiment of the alloy of the
invention; and
[0028] FIG. 4 is a homogenization simulation of the examples of the
invention.
[0029] FIG. 5 is a micrograph of alloy A of the invention. The
micrograph is typical of the examples of the invention.
DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0030] Following are specific examples of the invention.
Example 1
Alloy A
[0031] A melt was prepared comprising Al--6.3 Zn--3.2 Mg--1.1
Cu--0.52 Sc--0.20 Zr, in wt %. The exemplary alloy preferably
includes a variance in the constituents in the range of plus or
minus ten percent of the mean value. The alloy was cast through the
CRP reactor into a sand-casting mold at measured cooling rates of
50.about.100.degree. C./second. As shown in FIG. 3, the optimum
processing condition was to apply hot isostatic pressing,
homogenize and solutionize at 460.degree. C. for 2 hours and
480.degree. C. for 1 hour, quench with water, hold at room
temperature for 24 hours, and age at 120.+-.10.degree. C. for 20
hours. The ambient yield strength in this condition was 521.+-.12
MPa. The grain diameter was about 50 .mu.m, or an ASTM (American
Society for Testing and Materials) grain size number of about 5.7.
The calculated freezing range is 136.degree. C., solidus
temperature 493.degree. C., and the eutectic phase fraction formed
at late stages of solidification is 10%.
[0032] A rectangular panel of alloy A was cast successfully without
hot tearing. The melt was degassed with argon for 45 minutes at
700.about.720.degree. C. and then reheated to 740.degree. C. just
prior to mold pouring. The mold measured about 1 cm in depth. The
pouring time to fill the mold was approximately 20 seconds. The
mold filled successfully, producing a panel suitable for
characterization. Following the breakout from the mold, removal of
all gating and cleaning, the panel was shipped to UES, Inc. at the
Wright Patterson Air Force Base for characterization. FIG. 5 shows
the microstructure of alloy A, where pores from casting, an
exemplary L1.sub.2 particle, and the eutectic phase are marked as
a, b, and c, respectively.
Example 2
Alloy B
[0033] A melt was prepared comprising Al--5.3 Zn--3.0 Mg--1.1
Cu--0.55 Sc--0.25 Zr, in wt %. The exemplary alloy preferably
includes a variance in the constituents in the range of plus or
minus ten percent of the mean value. The alloy was cast through the
CRP reactor into a sand-casting mold at a measured cooling rate of
100.degree. C./second. As shown in FIG. 3, the optimum processing
condition was to apply hot isostatic pressing, homogenize and
solutionize at 460.degree. C. for 2 hours and 480.degree. C. for 1
hour, quench with water, hold at room temperature for 24 hours, and
age at 120.+-.10.degree. C. for 20 hours. The ambient yield
strength in this condition was 482.+-.6 MPa. The grain diameter was
about 54 .mu.m, or an ASTM grain size number of about 5.5. The
calculated freezing range is 139.degree. C., solidus temperature
494.degree. C., and the eutectic phase fraction formed at late
stages of solidification is 9%. A rectangular panel of alloy B was
cast successfully without hot tearing in accord and otherwise
generally with the protocol of alloy A.
Example 3
Alloy C
[0034] A melt was prepared comprising Al--4.5 Zn--2.3 Mg--0.62
Cu--0.42 Sc--0.25 Zr, in wt %. The exemplary alloy preferably
includes a variance in the constituents in the range of plus or
minus ten percent of the mean value. The alloy was cast through the
CRP reactor into a sand-casting mold. As shown in FIG. 3, the
optimum processing condition was to apply hot isostatic pressing,
homogenize and solutionize at 460.degree. C. for 2 hours and
480.degree. C. for 1 hour, quench with water, hold at room
temperature for 24 hours, and age at 120.+-.10.degree. C. for 15
hours. The calculated ambient yield strength in this condition is
410.+-.40 MPa. The calculated grain diameter is about 50 .mu.m or
an ASTM grain size number of about 5.7. The calculated freezing
range is 145.degree. C., solidus temperature 494.degree. C., and
the eutectic phase fraction formed at late stages of solidification
is 6%. Two panels were successfully cast from one heat of alloy C
without hot tearing and otherwise generally in accord with the
protocol used for alloy A.
[0035] Table 1 summarizes the compositions of the examples set
forth above and sets forth the general range of the constituents
for the practice of the invention in weight percent:
TABLE-US-00001 TABLE 1 Range Alloy A Alloy B Alloy C Zn 4.0-6.9
5.8-6.8 4.8-5.8 4.0-5.0 Mg 2.0-3.5 2.9-3.5 2.7-3.3 2.0-2.6 Cu
0.6-1.2 1.0-1.2 1.0-1.2 0.52-0.72 Sc 0.38-0.57 0.52 0.55 0.42 Zr
0.18-0.28 0.20 0.25 0.25 Al Balance Balance Balance Balance Fe
<0.0075 <0.0075 <0.0075 <0.0075 Mn <0.2 <0.2
<0.02 <0.2 Si <0.03 <0.03 <0.03 <0.03
[0036] Table 2 summarizes the information with respect to the
microstructural elements of the examples set forth above and
considered relevant to the range of the constituents in the
practice of the invention.
TABLE-US-00002 TABLE 2 Solidus Temp About 490.degree. C. or higher
Freezing Range About 150.degree. C. or lower Eutectic Phase
Fraction About 5-15% Phases fcc, L1.sub.2 < 1% by weight, and
.eta.' fcc Grain Size About 40-60 .mu.m Mean Particle Size (.eta.')
Less than about 5 nm Yield Strength About 410-540 MPa
[0037] While embodiments of the invention have been disclosed, the
scope thereof is not so limited and the invention is to be limited
only by the following claims and equivalents thereof.
* * * * *