U.S. patent application number 09/740708 was filed with the patent office on 2002-08-15 for titanium aluminide material resistant to molten aluminum.
Invention is credited to Chandley, George D., Crowell, Nathan C., Ruffini, Raymond F..
Application Number | 20020108679 09/740708 |
Document ID | / |
Family ID | 24977706 |
Filed Date | 2002-08-15 |
United States Patent
Application |
20020108679 |
Kind Code |
A1 |
Chandley, George D. ; et
al. |
August 15, 2002 |
Titanium aluminide material resistant to molten aluminum
Abstract
A titanium aluminide alloy and tooling made therefrom for use in
contact with molten aluminum and its alloys where the titanium
aluminide alloy includes a rare earth element in an effective
amount to prolong resistance to attack of the alloy and tooling by
the molten aluminum and its alloys.
Inventors: |
Chandley, George D.; (Lake
Wales, FL) ; Ruffini, Raymond F.; (Mt. Vernon,
NH) ; Crowell, Nathan C.; (Milford, NH) |
Correspondence
Address: |
Mr. Edward J. Timmer
Walnut Woods Centre
5955 W. Main Street
Kalamazoo
MI
49009
US
|
Family ID: |
24977706 |
Appl. No.: |
09/740708 |
Filed: |
December 19, 2000 |
Current U.S.
Class: |
148/281 ;
148/421 |
Current CPC
Class: |
C22C 14/00 20130101;
C22C 1/02 20130101 |
Class at
Publication: |
148/281 ;
148/421 |
International
Class: |
C22C 014/00 |
Claims
We claim
1. A titanium aluminide alloy for use in contact with a molten
material comprising aluminum, said titanium aluminide alloy
including a rare earth element in an effective amount to prolong
resistance to attack of said alloy by the molten material.
2. The alloy of claim 1 wherein the rare earth element comprises
Y.
3. The alloy of claim 2 wherein said Y is present in an amount of
about 1.5% to about 5.5% by weight of the alloy.
3-1. The alloy of claim 1 which comprises predominantly gamma
TiAl.
4. The alloy of claim 1 that includes a surface oxide formed
in-situ thereon.
5. The alloy of claim 4 wherein said surface oxide is formed
in-situ by heating said alloy in an oxygen bearing atmosphere.
6. The alloy of claim 4 wherein said surface oxide is formed by
cooling a hot casting comprising said alloy in air.
7. The alloy of claim 1 comprising TiAl that includes one or more
additional alloying elements.
8. Tooling for use in contact with molten material comprising
aluminum, wherein said tooling comprises the titanium aluminide
alloy of any one of claims 1-7.
10. A method of increasing the service life of a titanium aluminide
alloy in contact with a molten material comprising aluminum,
comprising including in the titanium aluminide alloy a rare earth
element in an effective amount to prolong resistance to attack of
the alloy by the molten material.
11. The method of claim 10 wherein said rare earth element is
included in a predominantly gamma TiAl alloy.
12. The method of claim 10 wherein said rare earth element
comprises Y included in an amount of about 1.5% to about 5.5% by
weight of the alloy.
13. The method of claim 10 including forming a surface oxide
in-situ on the alloy.
14. The method of claim 13 wherein the surface oxide is formed by
cooling a hot casting comprising said alloy in air.
15. The method of claim 13 wherein the surface oxide is formed
in-situ by heating said alloy in an oxygen bearing atmosphere.
16. A method of prolonging resistance of a titanium aluminide alloy
to a molten material comprising aluminum, comprising contacting the
alloy for a time with said molten material, removing the alloy from
the molten material, heating the alloy in an oxygen-bearing
atmosphere at elevated superambient temperature to form a surface
oxide thereon, and re-contacting the alloy having the surface film
thereon in the molten material.
17. The method of claim 16 including prior to first contacting the
alloy with the molten material, heating the alloy in an
oxygen-bearing atmosphere at elevated temperature to form a surface
oxide thereon.
18. The method of claim 16 including providing a rare earth element
in the alloy.
19. The method of claim 18 wherein the rare earth element is
provided in a predominantly gamma TiAl alloy.
20. The method of claim 18 wherein the rare earth element is Y.
21. In a method of die casting a molten material comprising
aluminum, wherein said molten material is introduced into a die
from a shot sleeve using a plunger in the shot sleeve, the
improvement comprising providing one or more of said die, shot
sleeve, and plunger as a titanium aluminide alloy including a rare
earth element in an effective amount to prolong resistance to
attack of said one or more of said die, shot sleeve and plunger by
the molten material.
22. The method of claim 21 wherein said titanium aluminide alloy
includes Y.
23. The method of claim 22 wherein said Y is present in said alloy
in an amount of about 1.5% to about 5.5% by weight of said
alloy.
24. The method of claim 21 wherein a core element is disposed in
the die and comprises said titanium aluminide alloy.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to materials, tooling and
methods for use in contact with molten aluminum and its alloys in
manufacture of products therefrom.
BACKGROUND OF THE INVENTION
[0002] In the making of metal matrix composites (MMC's) comprising
fine ceramic (e.g. alumina) reinforcement particles dispersed in a
matrix comprising aluminum or its alloys, a semi-solid slurry (a
thixotropic liquid/solid mixture) of the molten aluminum matrix
material is formed in a refractory crucible, and the ceramic
reinforcement particles are introduced into the partially molten
aluminum slurry and mechanically mixed therein by a rotating mixing
blade immersed in the slurry. Introduction of the ceramic
reinforcement particles into the slurry enables a high volume
percentage, such as 30-40 volume %, of reinforcement particles to
be dispersed in the aluminum matrix of the final MMC. Such an MMC
process is described in the Flemings U.S. Pat. No. 3,948,650.
[0003] The mixing blade immersed in the partially molten slurry is
subjected to abrasive action from the ceramic particles as they are
dispersed in the partially molten slurry. Expensive flame sprayed
alumina coated stainless steel (Type 304) mixing blades used in the
past typically exhibit catastrophic wear after only 30 minutes such
that replacement with a new mixing blade is required.
[0004] There is a need for improved materials and tooling for use
in contact with molten aluminum and its alloys as well as partially
molten slurries thereof.
[0005] An object of the present invention is to satisfy this
need.
SUMMARY OF THE INVENTION
[0006] The present invention provides in one embodiment a titanium
aluminide alloy for contact with molten aluminum and its alloys
wherein the titanium aluminide alloy includes a rare earth element
in an effective amount to prolong resistance of the alloy to attack
by the molten aluminum and its alloys.
[0007] An illustrative embodiment of the invention provides tooling
for use in contact with molten aluminum and its alloys where the
tooling comprises a titanium aluminide alloy including yttrium in
an amount of about 1.5 to about 5.5 weight % of the alloy to
prolong resistance to attack of the tooling to molten aluminum and
its alloys.
[0008] In another illustrative embodiment of the invention, the
titanium aluminide alloy or tooling is heated in an oxygen-bearing
atmosphere (e.g. air) at elevated temperature to form a passivating
surface oxide film in-situ thereon prior to contact with molten
aluminum and its alloys. The titanium aluminide alloy or tooling is
periodically removed from service in contact with the molten
aluminum or aluminum alloy, cleaned and reheated in an
oxygen-bearing atmosphere at elevated temperature to prolong its
service life.
[0009] Tooling pursuant to the present invention can comprise a
mixing blade for making MMC's in the manner described above and
also can comprise other tooling, such as die casting machine
components including a die, core element, shot sleeve, and plunger
for the die casting of aluminum and its alloys. In a method of die
casting pursuant to the invention, one or more of the die, core
element, shot sleeve and plunger is/are made from the rare
earth-bearing titanium aluminide alloy described above.
[0010] The above and other objects and advantages of the present
invention will become more readily apparent from the following
drawings taken in conjunction with the following detailed
description.
DESCRIPTION OF THE DRAWINGS
[0011] The Figure is a schematic view of apparatus for making an
MMC by mixing ceramic reinforcement dispersoids in a semi-solid
slurry comprising aluminum using a titanium aluminide alloy mixing
blade pursuant to the invention.
DESCRIPTION OF THE INVENTION
[0012] The present invention provides a titanium aluminide alloy
and tooling made therefrom for contact with molten aluminum and its
alloys, such as for immersion in molten aluminum and its alloys.
Molten aluminum and its alloys include commercially pure aluminum
metal and alloys of aluminum with other metals and/or elements
where aluminum is present as a majority of the alloy.
Representative aluminum alloys include, but are not limited to,
alloys 319, 320 and others. The titanium aluminide material and
tooling made therefrom comprise a titanium aluminide alloy that
includes a rare earth element in an effective amount to prolong
resistance of the material and tooling to attack by the molten
aluminum and its alloys. The titanium aluminide alloy comprises an
intermetallic compound comprising Ti and Al. For example, the
titanium aluminide alloy typically comprises a predominantly gamma
phase TiAl alloy, although other titanium aluminide alloys may be
used in practicing the invention. Titanium aluminide alloys
comprising about 30% to about 35% by weight Al, 55% to 65% by
weight Ti, one or more alloying elements such as W, Nb, Cr, Si, B,
V and others, and a rare earth element in effective amount can be
used to practice the invention depending on particular temperatures
and stresses to be encountered in service.
[0013] Preferably, the titanium aluminide alloy comprises a
predominantly gamma phase TiAl intermetallic compound containing a
small amount (e.g. up to 15 volume %) of alpha Ti.sub.3Al phase.
The rare earth element can be selected from the group consisting of
yttrium (Y) and rare earth elements of atomic number 57 to 71 of
the Periodic Table. For example, the titanium aluminide alloy can
include Y, one or more of the rare earth elements including
mischmetal, and combinations of Y and one or more rare earth
elements.
[0014] In an illustrative embodiment of the invention offered for
purposes of illustration and not limitation, the material comprises
predominantly gamma phase TiAl alloy including yttrium in an amount
of about 1.5 to about 5.5 weight % of the alloy.
[0015] For example, a gamma TiAl alloy with different amounts of Y
therein was evaluated for resistance to molten aluminum in
immersion testing. In particular, a particular predominantly gamma
phase TiAl alloy (base alloy) was modified to include 1.5%, 3.5%,
5.5% and 7.5% by weight Y. The Y-modified alloy was made by melting
charges of a gamma TiAl base alloy comprising, in weight %, 33.6%
Al, 0.5% Cr, and 0.1% Nb and adding Y to respective charges to
achieve the above 1.5%, 3.5%, 5.0% and 7.5 weight % Y
concentrations. Each Y-modified alloy was melted and
cast/solidified in ceramic shell molds to form cylindrical bars
having a diameter of {fraction (1/4 )}inch and a length of 4 inches
and also blocks having dimensions of 2 inches by 1 inch by 3
inches. The cast cylindrical bars were used as specimens. In
addition, the cast blocks were cut into specimen slabs having
dimensions of 1 inch by 2 inches by {fraction (1/2 )}inch.
[0016] The specimens included a surface oxide film or layer formed
by heating the specimens in a furnace in air to 1830 degrees F
(1000 degrees C.), holding at that temperature in air for 18 hours,
and cooling in the furnace to below 500 degrees F.
[0017] Specimens of each Y-modified alloy were immersed in molten
aluminum alloy 380 at 700 degrees C. (1300 degrees) and removed
periodically for examination for reaction with the melted aluminum
alloy. The molten aluminum alloy was still (i.e. not stirred)
during immersion of the specimens therein. The Table below sets
forth the number of days that the various specimens and unmodified
base alloy (0 weight % Y) did not exhibit visible attack or
reaction with the molten aluminum,:
1 TABLE Y Weight % Time Without Visible Attack 0 3 days 1.5 7 days
3.5 7 days 5.0 14 days 7.5 7 days
[0018] From the Table, it is apparent that the specimens of the
base alloy devoid of Y (0 weight % Y) exhibited visible attack by
the molten aluminum within 3 days. In contrast, the specimens of
the base alloy including 1.5%, 3.5% and 5.0% Y by weight pursuant
to the invention exhibited resistance to the molten aluminum for
prolonged times. The inclusion of Y in the base alloy appeared to
increase the resistance of the surface oxide film on the specimens
to attack by the molten aluminum, although Applicants do not wish
to be bound by any theory in this regard. The inclusion of Y in the
base alloy in an amount of 7.5% by weight Y was not further
beneficial in that the surface oxide film or layer was observed to
spall off of the 7.5 weight % Y base alloy specimens in the molten
aluminum.
[0019] Furthermore, specimens of the base alloy including 3.5% by
weight Y were heated in air at 1000 degrees C. for 18 hours prior
to immersion in the molten aluminum to form in-situ a passivating
surface oxide film or layer. These specimens were left immersed in
the molten aluminum alloy 380 for 7 days and then removed and
cleaned by mechanical scraping to remove adherent aluminum alloy
and expose a fresh surface of the titanium aluminide alloy
specimen. The cleaned specimens then were again heated in air at
1000 degrees C. for 18 hours prior to reimmersion in the molten
aluminum alloy. Repetition of these heat treatment/cleaning steps
was discovered to be effective to prolong the life of the specimens
for many weeks (e.g. 3 weeks) before visible attack of the
specimens was observed. Only very minor dimensional changes of the
specimens were observed after these heat treatment/cleaning
steps.
[0020] FIG. 1 illustrates schematically apparatus for making a
metal matrix composite (MMC) by mixing ceramic reinforcement
dispersoids in a semi-solid slurry comprising aluminum using a
passivated rare earth-bearing titanium aluminide alloy mixing blade
pursuant to an embodiment of the invention. For example, the
apparatus is shown comprising a mixing container 10, such as a
refractory crucible, from which the semi-solid slurry mixed with
ceramic reinforcement dispersoids is countergravity cast into a
mold (not shown) as described, for example, in U.S. Pat. No.
5,042,561, the teachings of which are incorporated herein by
reference. The mixing container 10 is disposed on a rotary ceramic
table 12 in a vacuum chamber 14 connected by conduit or port VP to
a conventional vacuum pump P. The rotary table is rotated by a
motor 16 and belt 19 through a vacuum sealed bearing 18. An
induction coil 20 is positioned in the chamber 14 about the
container 10 to inductively heat a solid charge comprising aluminum
or aluminum alloy (hereafter referred to as aluminum charge). The
induction coil 20 receives electrical power via electrical cables
(not shown) that pass through electrical power port EP. The solid
aluminum charge is positioned in the container 10 comprising
silicon carbide ceramic material substantially non-reactive with
the molten aluminum charge and is melted in air by energization of
the induction coil 20. After the aluminum charge is melted, a lid
or cover 14a of the vacuum chamber 14 is lowered and vacuum tight
sealed on the chamber 14. A relative vacuum (subambient pressure)
then is drawn in chamber 14 by actuation of the vacuum pump P. A
typical vacuum level of 0.050 to 0.080 torr is provided in chamber
14.
[0021] Energization of the induction coil 20 then is controlled in
a manner to cool the melted aluminum charge M to form a semi-solid
aluminum slurry comprising a partly solid/partly liquid charge
(i.e. thixotropic slurry). For aluminum alloy 356 (nominally
comprising 7 weight % Si, 0.3 weight % Mg and balance aluminum),
the aluminum charge is melted (melting temperature of 1135 degrees
F.) and is cooled while stirring with mixer blade 30 to 1110
degrees F. by controlled de-energization/energization of the
induction coil 20 to form the semi-solid slurry in the container
10. As the melted aluminum charge is cooled, it is mixed by
rotation of the table 12 and mixer blade 30 to provide a
thixotropic slurry comprising about 30% to 40% by volume solid
phase and balance liquid or molten phase, although these
percentages of solid/liquid phases in the slurry are offered only
for purposes of illustration and not limitation. The mixer blade 30
is fastened to and rotated by shaft 32 extending through the lid
14a and a drive train 33 coupled to a suitable motor 34 outside of
the chamber 14. The temperature of the semi-solid aluminum slurry
is determined by thermocouple T.
[0022] Preheated ceramic reinforcement powder 50 is introduced from
a hopper 40 outside of chamber 14 through a supply tube 42
extending into the chamber 14 to overlie the container 10. The
hopper 40 is evacuated to the same vacuum level as is provided in
the chamber 14 before the preheated reinforcement powder 50 is
introduced into the crucible 10. A conventional pinch valve 52
between the hopper 40 and the supply tube 42 is opened to supply
the preheated powder to the crucible. The preheated reinforcement
powder is introduced under the same vacuum as in chamber 10 to
ensure that the powder is dry and flows smoothly onto the top of
the aluminum slurry in the container 10.
[0023] An illustrative reinforcement powder for use in making MMC's
comprises alumina powder having particle size in the range of 5 to
20 microns diameter, although any particular reinforcement powder
or any particular particle size range may be used. Other
reinforcement powder which can be used in making MMC's include, but
are not limited to, silicon carbide and other ceramic particles.
The reinforcement powder is rapidly mixed into the semi-solid
aluminum slurry in the container 10 by combined rotation of the
table 12 and the mixing blade 30. After thorough mixing, the
aluminum/particle slurry is heated by induction coil 20 to the
liquid temperature range for casting the alloy. The liquid melt
having the reinforcement powder mixed therein then is cast from the
container 10 by removing lid 14a and immersing a suction tube (not
shown) in the liquid aluminum/dispersed powder charge in the
crucible and countergravity casting the charge into an evacuated
ceramic shell or other casting mold positioned thereabove as
described in U.S. Pat. No. 5,042,561, whose teachings are
incorporated herein by reference to this end.
[0024] In accordance with an embodiment of the invention, the mixer
blade 30 can comprise a titanium aluminide alloy including a rare
earth element in an effective amount to prolong resistance of the
mixer blade to attack and degradation by the slurry. For example,
the mixer blade 30 can be made of investment cast predominantly
gamma phase titanium aluminide alloy including a rare earth
addition as described above. The mixer blade 30 can be passivated
by forming a surface oxide film thereon. The surface oxide film is
formed in a thickness range of about 1 micron to 100 microns and
passivates the titanium aluminide alloy mixer blade 30. The
passivating surface film can be formed on the mixer blade 30 by
cooling an as-investment cast blade while hot to ambient
temperature in air and using the mixer blade in the as-cast and
oxidized (passivated) condition. Alternately, the surface film can
be formed by machining the cast mixer blade to desired
configuration followed by heating the machined mixer blade to an
elevated temperature such as from about 800 degrees F., for example
at 1000 degrees F. and above, in air or other oxygen bearing
atmosphere for a time (e.g. 18 hours in air at 1830 degrees F.)
effective to form the passivating surface film.
[0025] Tooling pursuant to the present invention can comprise the
mixer blade for making MMC's in the manner described above. Tooling
also can comprise other components, such as conventional die
casting machine components for die casting aluminum and its alloys
including a die, a core element disposed in the die, a shot sleeve
communicated to a die cavity defined in the die and a plunger
movable in the shot sleeve to move molten material in the shot
sleeve into the die cavity. In the die casting of aluminum and its
alloys pursuant to a method of the invention, the molten aluminum
or aluminum alloy is introduced into the shot sleeve and the
plunger in the shot sleeve is moved to introduce the molten
aluminum or alloy into the die cavity where an optional core
element is present and where the molten aluminum or alloy at least
partially solidifies to form a die cast article. Pursuant to the
invention, one or more of the die, core element, shot sleeve, and
plunger are made of the above-described titanium aluminide alloy
including a rare earth element, such as Y, in an effective amount
to prolong resistance to attack as described above.
[0026] The passivated titanium aluminide intermetallic tooling of
the invention is advantageous in that such tooling is used without
any need to coat the tooling with a bulk protective coating of any
kind. That is, the oxide surface film formed in-situ on the tooling
comprises the passivating surface film on the tooling of the
invention and is advantageous to yield more dimensional precise
parts or components.
[0027] Although the invention has been described in detail above
with respect to certain embodiments, those skilled in the art will
appreciate that modifications, changes and the like can be made
therein without departing from the spirit and scope of the
invention as set forth in the appended claims.
* * * * *