U.S. patent application number 09/750002 was filed with the patent office on 2002-07-04 for processing of aluminides by sintering of intermetallic powders.
Invention is credited to Deevi, Seetharama C., Gedevanishvili, Shalva.
Application Number | 20020085941 09/750002 |
Document ID | / |
Family ID | 25016119 |
Filed Date | 2002-07-04 |
United States Patent
Application |
20020085941 |
Kind Code |
A1 |
Deevi, Seetharama C. ; et
al. |
July 4, 2002 |
Processing of aluminides by sintering of intermetallic powders
Abstract
A sintering process for producing an aluminide by reacting a
first powder with a second powder, the first powder comprising
M.sub.xAl.sub.y wherein M is Fe, Ni or Ti, x.gtoreq.1, y.gtoreq.1
and x>y or y>x and the second powder comprises pure M or M
alloy powder. Iron aluminides such as Fe.sub.3Al, FeAl or alloys
thereof can be made by reacting powders of one or more of
Fe.sub.3Al, FeAl.sub.3, FeAl.sub.2, Fe.sub.2Al.sub.5 or alloys
thereof with pure iron or an iron alloy. Nickel aluminides such as
Ni.sub.3Al or NiAl or alloys thereof can be made by reacting
powders of one or more of NiAl.sub.3, Ni.sub.2Al.sub.3,
Ni.sub.3Al.sub.2, Ni.sub.5Al.sub.3 or alloys thereof with pure Ni
or a Ni alloy powder. Titanium aluminides such as Ti.sub.3Al, TiAl
or alloys thereof can be made by reacting one or more of
TiAl.sub.3, TiAl.sub.2 or alloys thereof with pure Ti or Ti alloy
powder. The process provides a more dense product by solid state
reaction of an intermediate intermetallic compound with a component
of the final aluminide compact. As a result of the process, the
final density can be increased to at least 98% of the theoretical
density. Products which can be made by the process include worked
products such as rolled sheet, extruded shapes such as tube, drawn
products such as wire or bar, or molded/forged products such as
fuel injection nozzles.
Inventors: |
Deevi, Seetharama C.;
(Midlothian, VA) ; Gedevanishvili, Shalva;
(Richmond, VA) |
Correspondence
Address: |
Peter K. Skiff
BURNS, DOANE, SWECKER & MATHIS, L.L.P.
P.O. Box 1404
Alexandria
VA
22313-1404
US
|
Family ID: |
25016119 |
Appl. No.: |
09/750002 |
Filed: |
December 29, 2000 |
Current U.S.
Class: |
419/6 ;
419/5 |
Current CPC
Class: |
B22F 3/225 20130101;
B22F 2998/00 20130101; B22F 2998/10 20130101; C22C 1/0491 20130101;
B22F 3/225 20130101; F02M 61/166 20130101; B22F 2998/00 20130101;
F02M 61/168 20130101; B22F 2998/10 20130101; B22F 1/0003 20130101;
B22F 3/22 20130101; B22F 3/23 20130101 |
Class at
Publication: |
419/6 ;
419/5 |
International
Class: |
B22F 007/00 |
Claims
What is claimed is:
1. A method of manufacturing a non-porous aluminide compact by a
powder metallurgical technique, comprising steps of: forming a
powder mixture comprising a first powder comprising M.sub.xAl.sub.y
wherein M is Fe, Ti or Ni, x.gtoreq.1, y.gtoreq.2, x>y or
y>x, and a second powder comprising M or M alloy; heating the
powder mixture so as to react the first powder with the second
powder to form the non-porous aluminide compact.
2. The method of claim 1, wherein the heating step is carried out
in a vacuum environment and the non-porous aluminide compact
produced during the heating step has a porosity of less than
1%.
3. The method of claim 1, wherein the aluminide is FeAl, Fe.sub.3Al
or alloy thereof, the first powder is one or more of
Fe.sub.2Al.sub.5, FeAl.sub.3, FeAl.sub.2, Fe.sub.3Al and the second
powder is pure Fe or iron base alloy powder.
4. The method of claim 1, wherein the aluminide is NiAl, Ni.sub.3Al
or alloy thereof, the first powder is one or more of
Ni.sub.2Al.sub.3, Ni.sub.3Al.sub.2, Ni.sub.5Al.sub.3, NiAl.sub.3
and the second powder is pure Ni or Ni base alloy powder.
5. The method of claim 1, wherein the aluminide is TiAl, Ti.sub.3Al
or alloy thereof, the first powder is one or more of TiAl.sub.2 or
TiAl.sub.3 and the second powder is pure Ti or Ti base alloy
powder.
6. The method of claim 1, wherein the first powder is
Fe.sub.2Al.sub.5, FeAl.sub.2, FeAl.sub.3, Fe.sub.3Al or alloy
thereof and the second powder is pure Fe or Fe base alloy.
7. The method of claim 1, wherein the first powder is
Ni.sub.2Al.sub.3, Ni.sub.3Al.sub.2, Ni.sub.5Al.sub.3, NiAl.sub.3 or
alloy thereof and the second powder is pure Ni or an Ni base
alloy.
8. The method of claim 1, wherein the first powder is TiAl.sub.2 or
TiAl.sub.3 or alloy thereof and the second powder is pure Ti or Ti
base alloy.
9. The method of claim 1, wherein the powder mixture is free of
pure aluminum powder.
10. The method of claim 1, wherein the aluminide is FeAl,
Fe.sub.3Al, NiAl, Ni.sub.3Al, TiAl, Ti.sub.3Al or alloy
thereof.
11. The method of claim 1, wherein the iron aluminide is iron
aluminide or an iron aluminide alloy, the first powder is
Fe.sub.2Al.sub.5 and the second powder comprises pure iron or an
iron base alloy, FeAl or Fe.sub.3Al being initially formed as a
layer between the pure iron or iron base alloy and the
Fe.sub.2Al.sub.5 during the heating step.
12. The method of claim 1, wherein the powder mixture is
binder-free.
13. The method of claim 1, wherein the powder mixture is heated at
a heating rate of less than 15.degree. C./minute during the heating
step.
14. The method of claim 1, wherein the sintered compact is heated
sufficiently to increase the density of the sintered compact to
over 98% of the theoretical density.
15. The method of claim 1, further comprising injection molding the
powder mixture into a shaped article or working the powder mixture
to form a continuous product.
16. The method of claim 1, wherein the powders comprise reaction
synthesized, water or gas atomized powder.
17. The method of claim 1, wherein the powder mixture comprises an
atomized powder and the method further comprises a step of sieving
the powder and blending the powder without a binder prior to a
consolidation step.
18. The method of claim 1, wherein the heating step comprises
heating the powder mixture at a temperature of 1200.degree. C. to
below the melting point of the powders in a vacuum atmosphere.
19. The method of claim 1, wherein the sintered compact has a grain
size of 10 to 50 .mu.m.
20. The method of claim 1, wherein the sintered product contains
oxides in an amount sufficient to inhibit grain growth and/or
enhance creep resistance of the sintered product.
21. The method of claim 1, wherein the step of forming the powder
mixture comprises mixing powders having an average particle size of
0.1 to 150 .mu.m.
22. The method of claim 1, wherein the powders include nanorized
powders in an amount sufficient to enhance packing of the
powders.
23. The method of claim 1, wherein the aluminide comprises an iron
aluminide alloy having, in weight %, .ltoreq.32% Al, .ltoreq.2% Mo,
.ltoreq.1% Zr, .ltoreq.2% Si, .ltoreq.30% Ni, .ltoreq.10% Cr,
.ltoreq.0.3% C, .ltoreq.0.5% Y, .ltoreq.0.1% B, .ltoreq.1% Nb and
.ltoreq.1% Ta.
24. The method of claim 1, wherein the aluminide comprises an iron
aluminide alloy which includes, in weight %, 20-32% Al, 0.3-0.5%
Mo, 0.05-0.3% Zr, 0.01-0.5% C, .ltoreq.0.1% B, .ltoreq.1% oxide
particles, balance including Fe.
25. The method of claim 1, wherein the sintering step provides a
sintered product having an average grain size less than 40
.mu.m.
26. The method of claim 1, wherein the powder mixture consists
essentially of Fe.sub.2Al.sub.5 and pure Fe.
27. The method of claim 1, wherein the aluminide comprises an alloy
of Ni.sub.3Al containing, in weight %, 0.005 to 0.05% B, 6 to 12%
Al, 4 to 8% Mo, 2 to 4% Ti, balance Ni.
28. The method of claim 1, wherein the aluminide comprises an alloy
of Ti.sub.3Al containing, in weight %, 2 to 20% Nb, 0.5 to 10% W,
0.5 to 10% Ta, 0.1 to 0.5% B, and/or up to 10% Mo.
29. The method of claim 1, wherein the aluminide comprises an alloy
of TiAl containing, in weight %, 2 to 20% Nb, 0.5 to 10% W, 0.5 to
10% Ta, 0.1 to 0.5% B, and/or up to 10% Mo.
30. The method of claim 1, wherein the aluminide compact is forged
into an automotive valve.
31. The method of claim 1, wherein the aluminide compact is formed
into a fuel injection nozzle for direct pressure fuel injection
systems.
32. The method of claim 1, wherein the aluminide compact is formed
into a fuel injection nozzle for automobile, diesel or marine
engines.
33. The method of claim 1, wherein the aluminide compact includes
sufficient tungsten carbide and/or oxide particles to provide
improved wear resistance.
34. The method of claim 1, wherein the first and second powders
have a mean particle size of 2 to 20 .mu.m.
Description
FIELD OF THE INVENTION
[0001] The invention relates to improvements in powder processing
of aluminides.
BACKGROUND OF THE INVENTION
[0002] Iron base alloys containing aluminum can have ordered and
disordered body centered crystal structures. For instance, iron
aluminide alloys having intermetallic alloy compositions contain
iron and aluminum in various atomic proportions such as Fe.sub.3Al,
FeAl, FeAl.sub.2, FeAl.sub.3, and Fe.sub.2Al.sub.5. Fe.sub.3Al
intermetallic iron aluminides having a body centered cubic ordered
crystal structure are disclosed in U.S. Pat. Nos. 5,320,802;
5,158,744; 5,024,109; and 4,961,903. Various articles on iron
aluminides are incorporated in a special issue of Materials Science
and Engineering A, Vol. A258 (1998) edited by Deevi et al. which
includes papers presented at a conference on Iron Aluminides: Alloy
Design, Processing, Properties and Applications held Feb. 15-19,
1998 in San Antonio, Tex.
[0003] A 1990 publication in Advances in Powder Metallurgy, Vol. 2,
by J. R. Knibloe et al., entitled "Microstructure And Mechanical
Properties of P/M Fe.sub.3Al Alloys", pp. 219-231, discloses a
powder metallurgical process for preparing Fe.sub.3Al containing 2
and 5% Cr by using an inert gas atomizer. This publication explains
that Fe.sub.3Al alloys have a DO.sub.3 structure at low
temperatures and transform to a B2 structure above about
550.degree. C. To make sheet, the powders were canned in mild
steel, evacuated and hot extruded at 1000.degree. C. to an area
reduction ratio of 9:1.
[0004] A 1991 publication in Mat. Res. Soc. Symp. Proc., Vol. 213,
by V. K. Sikka entitled "Powder Processing of Fe.sub.3Al-Based
Iron-Aluminide Alloys," pp. 901-906, discloses a process of
preparing 2 and 5% Cr containing Fe.sub.3Al-based iron-aluminide
powders fabricated into sheet. This publication states that the
powders were prepared by nitrogen-gas atomization and argon-gas
atomization. The nitrogen-gas atomized powders had low levels of
oxygen (130 ppm) and nitrogen (30 ppm). To make sheet, the powders
were canned in mild steel and hot extruded at 1000.degree. C. to an
area reduction ratio of 9:1.
[0005] A paper by V. K. Sikka et al., entitled "Powder Production,
Processing, and Properties of Fe.sub.3Al", pp. 1-11, presented at
the 1990 Powder Metallurgy Conference Exhibition in Pittsburgh,
Pa., discloses a process of preparing Fe.sub.3Al powder by melting
constituent metals under a protective atmosphere, passing the metal
through a metering nozzle and disintegrating the melt by
impingement of the melt stream with nitrogen atomizing gas. An
extruded bar was produced by filling a 76 mm mild steel can with
the powder, evacuating the can, heating 11/2 hour at 1000.degree.
C. and extruding the can through a 25 mm die for a 9:1 reduction. A
sheet 0.76 mm thick was produced by removing the can, forging 50%
at 1000.degree. C., rolling 50% at 850.degree. C. and finish
rolling 50% at 650.degree. C.
[0006] A publication by D. J. Gaydosh et al., entitled
"Microstructure and Tensile Properties of Fe-40 At.Pct. Al Alloys
with C, Zr, Hf and B Additions" in the September 1989 Met. Trans A,
Vol. 20A, pp. 1701-1714, discloses hot extrusion of gas-atomized
powder wherein the powder either includes C, Zr and Hf as
prealloyed additions or B is added to a previously prepared
iron-aluminum powder.
[0007] A publication by C. G. McKamey et al., entitled "A review of
recent developments in Fe.sub.3Al-based Alloys" in the August 1991
J. of Mater. Res., Vol. 6, No. 8, pp. 1779-1805, discloses
techniques for obtaining iron-aluminide powders by inert gas
atomization and preparing ternary alloy powders based on Fe.sub.3Al
by mixing alloy powders to produce the desired alloy composition
and consolidating by hot extrusion, i.e., preparation of
Fe.sub.3Al-based powders by nitrogen- or argon-gas atomization and
consolidation to full density by extruding at 1000.degree. C. to an
area reduction of <9:1.
[0008] U.S. Pat. Nos. 4,917,858; 5,269,830; and 5,455,001 disclose
powder metallurgical techniques for preparation of intermetallic
compositions by (1) rolling blended powder into green foil,
sintering and pressing the foil to full density, (2) reactive
sintering of Fe and Al powders to form iron aluminide or by
preparing Ni-B-Al and Ni-B-Ni composite powders by electroless
plating, canning the powder in a tube, heat treating the canned
powder, cold rolling the tube-canned powder and heat treating the
cold rolled powder to obtain an intermetallic compound.
[0009] U.S. Pat. No. 5,484,568 discloses a powder metallurgical
technique for preparing heating elements by micropyretic synthesis
wherein a combustion wave converts reactants to a desired product.
In this process, a filler material, a reactive system and a
plasticizer are formed into a slurry and shaped by plastic
extrusion, slip casting or coating followed by combusting the shape
by ignition.
[0010] U.S. Pat. Nos. 5,098,469 and 5,269,830 disclose techniques
for preparing intermetallic alloy compositions by powder
metallurgical techniques which include pressureless sintering. The
'469 patent discloses a four step pressureless sintering process
for producing Ni-Al-Ti intermetallic aluminide alloys wherein a
compact of nickel powder and prealloyed aluminide powder is heated
without cool down steps and with a heating rate of 10.degree. C.
per minute between the processing steps. The '830 patent discloses
a pressureless sintering process for producing Fe.sub.3Al and FeAl
compounds wherein elemental powders of iron and aluminum are heated
under conditions of temperature and pressure to produce an
exothermic reaction and densification is achieved by sintering in
vacuum or by pressure assisted densification by heating during
compression. According to the '830 patent, pressureless sintering
achieves near 75% of full density.
[0011] U.S. Pat. No. 5,768,679 discloses a powder metallurgical
technique for making a TiAl intermetallic compound by preparing a
mixture of materials selected from Ti, Ti alloys, Al, Al alloys and
TiAl compounds, and sintering the mixture.
[0012] U.S. Pat. No. 5,950,063 discloses a method of making powder
injection molded parts by sintering a mixture of a powder and
binder, wherein the powder can be ceramic, metallic and/or
intermetallic and the metallic powder can be a mixture of
prealloyed powder and an elemental/semi-elemental powder or a
prealloyed powder and an elemental/master-alloy powder.
[0013] U.S. Pat. No. 4,762,558 discloses a reactive sintering
process for producing Ni.sub.3Al by compacting elemental Ni and Al
powders and vacuum sintering the powders through an exothermic
reaction. U.S. Pat. No. 2,755,184 also discloses a technique for
making Ni.sub.3Al but by using a precursor compound NiAl and
elemental Ni powders. Likewise, U.S. Pat. No. 5,905,937 discloses a
powder technique for making Ni.sub.3Al by sintering a mixture of a
brittle nickel aluminide powder with elemental Ni.
[0014] A porous aluminide structure is disclosed in U.S. Pat. No.
4,990,181. The '181 patent discloses a porous sintered aluminide
made by mixing the aluminide with a metal or aluminide as a
prealloyed powder with an organic binder and solvent, burning out
carbon in the mixture and sintering to form the porous sintered
product.
[0015] Sintered articles made from powder mixtures including
ferrous metal compositions including FeAl powder (50 wt. % Al and
50 wt. % Fe) are disclosed in U.S. Pat. Nos. 4,758,272; 4,992,233;
and 5,864,071. Of these, the '272 and '233 patents seek to produce
a porous body having 5-50 wt. % Al by sintering binder, FeAl powder
and Fe powder and the '071 patent seeks to produce a ferrous metal
composition having 0.5-5 wt. % Al by sintering FeAl and low alloy
or stainless steel compositions.
[0016] Based on the foregoing, there is a need in the art for an
economical technique for preparing intermetallic compositions such
as iron, titanium or nickel aluminides. For instance, conventional
powder metallurgical techniques of preparing iron-aluminides
include melting iron and aluminum and inert gas atomizing the melt
to form an iron-aluminide powder, canning the powder and working
the canned material at elevated temperatures or reaction synthesis
can be used to react elemental powders of iron and aluminum. It
would be desirable if iron-aluminide could be prepared by a powder
metallurgical technique wherein it is not necessary to can the
powder and wherein it is not necessary to subject the iron and
aluminum to any hot working steps in order to form an
iron-aluminide product such as a sheet. It would be further
desirable to increase the sintered density of aluminide
compacts.
SUMMARY OF THE INVENTION
[0017] The invention provides a method of manufacturing a sintered
aluminide compact by a powder metallurgical technique, comprising
steps of forming a powder mixture comprising a first powder of
Me.sub.xAl.sub.y or alloy thereof wherein M is Fe, Ti or Ni,
x.gtoreq.1, y.gtoreq.1, x>y or y>x and a second powder
comprising M or alloy thereof, and heating the powder mixture so as
to react the first powder with the second powder to form a sintered
aluminide compact.
[0018] In preparing an FeAl or Fe.sub.3Al iron aluminide compact,
the first powder can comprise one or more materials selected from
Fe.sub.2Al.sub.5, FeAl.sub.3, FeAl.sub.2, Fe.sub.3Al or alloys
thereof and the second powder can comprise one or more materials
selected from FeAl, Fe.sub.2Al.sub.5, FeAl.sub.3, FeAl.sub.2,
Fe.sub.3Al or alloys thereof and/or Fe or an iron base alloy
powder. For example, during the heating step, Fe.sub.2Al.sub.5 can
be reacted with Fe to form FeAl or Fe.sub.3Al. By adjusting the
contents of aluminum, iron and optional alloying additions in the
powder mixture, it is possible to form a sintered compact
consisting of FeAl or Fe.sub.3Al or alloys thereof. The heating
step is preferably carried out in a vacuum or inert gas (e.g., Ar,
He, N.sub.2, etc.) environment such that expansion of the sintered
compact due to volume change during formation of the FeAl or
Fe.sub.3Al is less than 10% and/or the FeAl or Fe.sub.3Al initially
forms as a layer between the iron or iron base alloy powder and the
Fe.sub.2Al.sub.5. In a preferred process, the powder mixture is
heated at a heating rate of less than 15.degree. C./min and/or the
sintered compact is heated sufficiently to increase the density of
the sintered compact to at least 98% of the theoretical density.
The process can include a step of pressing the powder mixture into
a shaped article such as a molded part or a worked article such as
a sheet. According to the process, reactions which can sequentially
occur during the heating steps include the initial formation of
FeAl or Fe.sub.3Al by an interfacial reaction between pure Fe or an
iron base alloy powder and Fe.sub.xAl.sub.y, and the balance of the
FeAl or Fe.sub.3Al is formed by solid state diffusion.
[0019] In preparing an NiAl or Ni.sub.3Al iron aluminide compact,
the first powder can comprise one or more materials selected from
Ni.sub.2Al.sub.3, Ni.sub.3Al.sub.2, Ni.sub.5Al.sub.3, NiAl.sub.3 or
alloys thereof and the second powder can comprise one or more
materials selected from Ni.sub.2Al.sub.3, Ni.sub.3Al.sub.2,
Ni.sub.5Al.sub.3, NiAl.sub.3 or alloys thereof and/or Ni or Ni base
alloy powder. For example, during the heating step, NiAl.sub.3 can
be reacted with Ni to form NiAl or Ni.sub.3Al. By adjusting the
contents of aluminum, Ni and optional alloying additions in the
powder mixture, it is possible to form a sintered compact
consisting of NiAl or Ni.sub.3Al or alloys thereof. The heating
step is preferably carried out in a vacuum or inert gas (e.g., Ar,
He, N.sub.2, etc.) environment such that expansion of the sintered
compact due to volume change during formation of the NiAl or
Ni.sub.3Al is less than 10% and/or the NiAl or Ni.sub.3Al initially
forms as a layer between the Ni or Ni base alloy powder and the
Ni.sub.xAl.sub.y. In a preferred process, the powder mixture is
heated at a heating rate of less than 15.degree. C./min and/or the
sintered compact is heated sufficiently to increase the density of
the sintered compact to at least 98% of the theoretical density.
The process can include a step of pressing the powder mixture into
a shaped article such as a molded part or a worked article such as
a sheet.
[0020] In preparing an TiAl or Ti.sub.3Al iron aluminide compact,
the first powder can comprise one or more materials selected from
TiAl.sub.2, TiAl.sub.3 or alloys thereof and the second powder can
comprise one or more materials selected from TiAl.sub.2, TiAl.sub.3
or alloys thereof and/or Ti or Ti base alloy powder. For example,
during the heating step, TiAl.sub.3 can be reacted with Ti to form
TiAl or Ti.sub.3Al. By adjusting the contents of aluminum, Ti and
optional alloying additions in the powder mixture, it is possible
to form a sintered compact consisting of TiAl or Ti.sub.3Al or
alloys thereof. The heating step is preferably carried out in a
vacuum or inert gas (e.g., Ar, He, N.sub.2, etc.) environment such
that expansion of the sintered compact due to volume change during
formation of the TiAl or Ti.sub.3Al is less than 10% and/or the
TiAl or Ti.sub.3Al initially forms as a layer between the Ti or Ti
base alloy powder and the Ti.sub.xAl.sub.y. In a preferred process,
the powder mixture is heated at a heating rate of less than
15.degree. C./min and/or the sintered compact is heated
sufficiently to increase the density of the sintered compact to at
least 98% of the theoretical density. The process can include a
step of pressing the powder mixture into a shaped article such as a
molded part or a worked article such as a sheet.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0021] The invention provides a powder metallurgical technique for
making high strength, high density powder products of aluminides
which can be designed to have excellent strength and ductility at
room temperature as well as high tensile and creep strengths at
elevated temperatures. The sintering process according to the
invention is an improvement over conventional processes wherein
water or gas atomized prealloyed powders are combined with a binder
and the sintered product includes porosity due to binder burnout
with the result that the product attains a maximum density of
around 90% of theoretical density. According to the invention it is
possible to obtain densities of at least 95%, preferably at least
98% or higher.
[0022] According to the invention, powder products are manufactured
by a powder metallurgical technique which provides a highly dense
aluminide. In the process, a powder mixture of the aluminide can be
formed into continuous shapes such as sheet, tube, rod, wire or
extruded shapes. For instance, the powder mixture can be formed
into continuous sheet by tape casting or roll compaction.
Alternatively, the powder mixture can be formed into desired shapes
by die compaction or injection molding. The powder mixture
preferably includes powder in the form of an intermediate
aluminum-containing intermetallic compound to avoid thermal
expansion associated with the formation of intermediate
aluminum-containing intermetallic compounds during sintering of the
powder mixture. Such thermal expansion can produce Kirkendall
porosity which results in lowered density during the sintering
process. Thus, because the powder mixture starts off with an
intermediate aluminum-containing intermetallic compound, the powder
mixture can be sintered to higher density than in the case of using
elemental Fe, Ti or Ni and aluminum powders.
[0023] According to one embodiment of the invention wherein an iron
aluminide is produced, it is possible to include alloying additions
such as pure iron powder and/or prealloyed iron or aluminum base
powders to achieve a desired iron aluminide composition such as
Fe.sub.3Al or FeAl (36 to 50 at % Al) or alloys thereof. The
powders can optionally include conventional binder materials to aid
in molding of the powder mixture. Alternatively, the powder mixture
can be binder-free to reduce the porosity otherwise associated with
porosity formed during binder burnout. Also, the powder mixture can
be free of elemental aluminum to avoid the volume expansion which
otherwise may occur during formation of intermediate iron-aluminum
intermetallic compounds during the sintering step. In the case of a
binder-free composition, it is advantageous to include elemental
iron powder to improve compaction of the powder mixture.
[0024] Of the iron aluminides, FeAl has a B2 structure and exists
over a wide range of Al concentrations at room temperature (36
to.about.50 atomic %). Iron aluminides based on FeAl exhibit better
oxidation, carburization and sulfidation resistance than Fe.sub.3Al
alloys and have lower densities compared to the steels and
commercial iron based alloys, offering better strength-to weight
ratio. In addition, FeAl exhibits high electrical resistivity in
the range of 130 to 170 .mu..OMEGA.-cm as compared to many of the
commercial metallic heating elements. These properties allow them
to be considered as high temperature structural materials, gas
filters, heating elements, and as fasteners.
[0025] According to one embodiment of the present invention,
M.sub.xAl.sub.y or alloy thereof is prepared by a sintering
process. Sintering is useful for forming precision,
high-performance products operating in demanding applications such
as automotive engines, aerospace hardware, manufacturing tools and
electronic components. Sintering delivers net shape processing,
uses limited material, and eliminates deformation processing and
machining of the components. It also allows microstructural control
of the product. After shaping the powder into compacts, the
compacts are heated to elevated temperatures (approximately
one-half of the absolute melting temperature) to bond the particles
and increase the strength.
[0026] In forming binary iron aluminide or iron aluminide alloy
compositions, various iron containing powders can be mixed together
and reacted to form the iron aluminide. For example, Fe or its
alloy can be reacted to form FeAl, Fe.sub.3Al can be reacted with
FeAl.sub.2 to form FeAl, FeAl.sub.2 can be reacted with Fe to form
FeAl or Fe.sub.3Al, Fe.sub.2Al.sub.5 can be reacted with Fe to form
FeAl or Fe.sub.3Al, and FeAl.sub.3 can be reacted with Fe to form
FeAl or Fe.sub.3Al. These are just a few examples of using high
aluminum intermetallics with low melting points to form a valuable
high melting point intermetallic.
[0027] In forming binary Ni aluminide or Ni aluminide alloy
compositions, various Ni containing powders can be mixed together
and reacted to form the nickel aluminide. For example,
Ni.sub.2Al.sub.3 can be reacted with Ni.sub.3Al.sub.2,
Ni.sub.5Al.sub.3 or NiAl.sub.3 to form NiAl or Ni.sub.3Al,
Ni.sub.3Al.sub.2 can be reacted with Ni.sub.5Al.sub.3 or NiAl.sub.3
to form NiAl, Ni.sub.5Al.sub.3 can be reacted with NiAl.sub.3 to
form NiAl or Ni.sub.3Al, etc.
[0028] In forming binary Ti aluminide or Ti aluminide alloy
compositions, various Ti containing powders can be mixed together
and reacted to form the Ti aluminide. For example, Ti can be
reacted with TiAl.sub.2 or TiAl.sub.3 to form TiAl or Ti.sub.3Al,
TiAl.sub.2 can be reacted with TiAl.sub.3 to form TiAl, etc.
[0029] The bonds between the particles grow by various mechanisms,
which occur at the atomic level. Common mechanisms for metal
bonding are solid-state diffusion and liquid state sintering (with
liquid phase present during the process). Classical sintering
processes include several stages: contact formation, neck growth,
pore rounding and pore closure, and final densification of the
product.
[0030] Many powder metallurgical processes use prealloyed powders
as a starting mixture for dense intermetallic production, which can
be obtained by atomization or mechanical alloying. Further
consolidation (sintering) involves the use of complex and costly
processes based on hot isostatic pressing or hot extrusion.
Therefore, it would be desirable to develop low cost processing
methods for intermetallic products. The invention provides a novel
processing technique which involves use of an intermediate
iron-aluminum intermetallic compound such as Fe.sub.2Al.sub.5, an
aluminum-containing powder which undergoes reaction with iron to
produce FeAl during sintering. By varying the iron/aluminum ratio
in the composition to be sintered, the same technique can be used
to form Fe.sub.3Al compacts.
[0031] The process according to one embodiment of the invention is
an improvement over the conventional reaction synthesis processes
wherein heating of elemental iron and aluminum powders is
accompanied by heat generation due to an exothermic reaction
between the mixed powders. It has been reported that the main
disadvantage of this process is the large porosity of the final
products. To eliminate porosity, some researchers have reported the
use of the application of pressure during the combustion or
sintering, which increases the complexity of the process. Such
large porosity can be avoided according to the process of the
invention wherein an intermediate intermetallic composition is used
to form value added FeAl or Fe.sub.3Al intermetallic alloys.
[0032] According to one aspect of the present invention,
pressureless sintering can be used to from the sintered aluminide
products such as FeAl, Fe.sub.3Al, NiAl, Ni.sub.3Al, TiAl,
Ti.sub.3Al or alloys thereof. Pressureless sintering is based on
the thermal bonding of the particles into the solid structure
without the assistance of the pressure and is widely used by
automotive industry.
[0033] According to the invention it is possible to prepare
aluminide composites with minimum or no melting of aluminum. In
particular, by providing the aluminum content of the final compact
in the form of an intermediate compound M.sub.xAl.sub.y such as
Fe.sub.2Al.sub.5 or alloy thereof it is possible to form a sintered
aluminide compact by a solid state process wherein the components
of the aluminide undergo an interfacial reaction, e.g., an
interfacial reaction between Fe and Fe.sub.2Al.sub.5 or alloys
thereof to form FeAl or Fe.sub.3Al. This reaction may be
accompanied by the formation of voids which provide an escape path
for volatile impurities present in the original powder. After
formation of a small amount of the FeAl or Fe.sub.3Al phase, the
dominant process is solid state diffusion. At 1000.degree. C.,
formation of the desired FeAl or Fe.sub.3Al phase can be completed.
However, a controlled thermal expansion or swelling of the compact
may occur up to 1150.degree. C. followed by the sintering.
[0034] After synthesizing of 100% FeAl, Fe.sub.3Al or alloy
thereof, the next step is the densification of the compacts
involving diffusion, which is driven by a reduction of the surface
area. Final densification of the samples depends on the expansion,
which occurs prior to the complete formation of FeAl or Fe.sub.3Al.
Densification can be expected to start at 1150.degree. C. and
rapidly increase from 1200.degree. C.
[0035] The sintering limit can be achieved faster at higher (e.g.,
1350.degree. C.) temperatures. The same densification can be
obtained at the lower temperatures (e.g., 1200.degree. C.), but
with longer heating times. Accordingly, high temperatures on the
order of 1200-1350.degree. C. can be used to achieve densities of
.about.98% and above.
[0036] One of the challenges in Fe-Al sintering is to reduce the
large pores left by diffusion of aluminum. According to sintering
theory, early in the sintering process the pores remain attached to
the grain boundaries and as the temperature increases the rate of
grain boundary motion increases. After isolation of the pores from
each other and further shrinkage, the grain boundaries break away
from the pores leaving them trapped in the interior of the grains.
Generally, pores at the grain interior shrink much slower than
pores on the grain boundary. Separation of the pores from the
boundaries thus limits the final density. As such, it is desirable
to avoid the formation of pores inside the grains. According to the
present invention, pore formation and location can be minimized by
using an intermediate intermetallic compound as a precursor powder
in the sintering process.
[0037] In preparing powders for use in the process according to the
present invention, fine powders on the order of 0.1 to 40 .mu.m,
preferably 2 to 20 .mu.m can be prepared by an atomization
technique wherein the nozzle configuration and/or pressure of
atomization is modified to achieve the fine powder size. Also,
nanosized powders can be incorporated in the powder mixture to
improve compaction of the powder mixture. Conventional powder
metallurgical techniques use powder mixtures wherein the powders
are 40 to 150 .mu.m in size. By using finer sized powders in the
process according to the invention, it is possible to achieve a
smoother surface in the final sintered product and thereby reduce
production cost by obviating the need for additional machining of
the sintered part.
[0038] According to a second embodiment of the invention, an
intermetallic alloy composition is formed into a worked product
such as a sheet by consolidating a mixture of M.sub.xAl.sub.y or
alloy thereof with M or M alloy powder, rolling and heat treating
the rolled sheet. The invention overcomes porosity problems
associated with working intermetallic alloys made by tape casting
or roll compacting prealloyed powders.
[0039] According to this embodiment, a sheet having an
intermetallic alloy composition is prepared by a powder
metallurgical technique wherein a non-densified metal sheet is
formed by consolidating a powder mixture having elements in amounts
which form an intermetallic alloy composition upon sintering
thereof. The non-densified sheet can be debindered (if the powder
mixture contains binders), react the M.sub.xAl with the M or M
alloy powder to form the desired aluminide or aluminide alloy and
sinter the powders and thus increase the density of the sheet prior
to or after rolling the non-densified sheet. A rolled sheet is
formed by hot and/or cold rolling the densified or non-densified
metal sheet so as to reduce the thickness thereof, and the rolled
sheet is heat treated to further sinter, anneal and stress relieve
the sheet. For example, a sheet or strip can be formed by hot
rolling or cold rolling the sintered powders in one or more passes
to a final desired thickness with at least one heat treating step
such as a sintering, annealing or stress relief heat treatment.
This technique can be employed to manufacture a sheet of Fe.sub.3Al
or an FeAl alloy with 6-32 atomic % Al, preferably 6-26 at. % Al.
Nickel aluminide and titanium aluminide intermetallic alloys can be
made in like manner. If desired, the heat treatment can be carried
out by passing the sheet or strip through a furnace arrangement
which heats the strip to a desired temperature such as up to the
melting point of one or more constituents of the aluminide, e.g.,
flash annealing as described in commonly owned U.S. Pat. No.
6,143,241, the disclosure of which is hereby incorporated by
reference.
[0040] The foregoing process provides a simple and economic
manufacturing technique for preparing intermetallic alloy materials
such as iron, nickel or titanium aluminides which are known to have
poor ductility and high work hardening potential at room
temperature.
[0041] In roll compacting the powder mixture, water or polymer
atomized powder is preferred over gas atomized powder for
subsequent roll compaction since the irregularly shaped surfaces of
the water atomized powder provide better mechanical interlocking
than the spherical powder obtained from gas atomization. Details of
the roll compaction process can be found in commonly owned U.S.
Pat. No. 6,030,472, the disclosure of which is hereby incorporated
by reference.
[0042] In the roll compaction process, the powder is sieved to a
desired particle size range, blended with an organic binder, mixed
with an optional solvent and blended together to form a blended
powder. The sieving step preferably provides a powder having a
particle size within the range of -100 to +325 mesh which
corresponds to a particle size of 43 to 150 .mu.m. In order to
improve the flow properties of the powder, less than 5%, preferably
3-5% of the powder has a particle size of less than 43 .mu.m. The
binder is preferably an organic binder which will decompose in a
controlled manner such as polyvinyl alcohol and methyl cellulose,
etc. and is blended with the powder in an amount such as up to
about 5 wt %. The cellulose based binder can be methylcellulose
(MS), carboxymethylcellulose (CMS) or any other suitable binder
such as polyvinylalcohol (PVA). The surface of the powder is
preferably contacted with enough binder to cause mechanical bonding
of the powder (i.e., the powder particles stick to each other when
pressed together). The solvent can be a liquid such as purified
water in any suitable amount such as up to about 5 wt %. The
mixture of the binder-adhered powder and solvent provides a "dry"
blend which is free flowing while providing mechanical interlocking
of the powders when roll compacted together.
[0043] Green strips are prepared by roll compaction wherein the
blended powder is fed from a hopper through a slot into a space
between two compaction rolls. In a preferred embodiment, the roll
compaction produces a green strip of aluminide having a thickness
of about 0.026 inch and the green strip can be cut into strips
having dimensions such as 36 inches by 4 inches. The green strips
are subjected to a heat treatment step to remove volatile
components such as the binder and any organic solvents. The binder
burn out can be carried out in a furnace at atmospheric or reduced
pressure in a continuous or batch manner. For instance, a batch of
iron aluminide strips can be heated in a furnace set at a suitable
temperature such as 700-900.degree. F. (371-482.degree. C.) for a
suitable amount of time such as 6-8 hours or for shorter times at a
higher temperature such as 950.degree. F. (510.degree. C.). During
this step, the furnace can be at 1 atmosphere pressure with
nitrogen gas flowing therethrough so as to remove most of the
binder, e.g., at least 99% binder removal. This binder removal step
results in very fragile green strips which are then subjected to
primary sintering in a vacuum furnace.
[0044] In the primary sintering step, the porous brittle
de-bindened strips are preferably heated under conditions suitable
for effecting reaction of the Fe.sub.xAl.sub.y powders with the Fe
and/or Fe alloy powders so as to form FeAl or Fe.sub.3Al as well as
sintering and densification of the powder. This sintering step can
be carried out in a furnace at reduced pressure in a continuous or
batch manner. For instance, a batch of the de-bindened iron
aluminide strips can be heated in a vacuum furnace at a suitable
temperature such as 1000 to 1260.degree. C. for a suitable time
such as one hour. The vacuum furnace can be maintained at any
suitable vacuum pressure such as 10.sup.-4 to 10.sup.-5 Torr. In
order to prevent loss of aluminum from the strips during sintering,
it is preferable to maintain the sintering temperature low enough
to avoid vaporizing aluminum yet provide enough metallurgical
bonding to allow subsequent rolling. Further, vacuum sintering is
preferred to avoid oxidation of the non-densified strips. However,
protective atmospheres such as hydrogen, argon and/or nitrogen with
proper dew points such as -50.degree. F. or less thereof could be
used in place of the vacuum.
[0045] In the next step, the presintered strips are preferably
subjected to hot or cold rolling in air to a final or intermediate
thickness. Due to the hardness of the intermetallic alloy, it is
advantageous to use a 4-high rolling mill wherein the rollers in
contact with the intermetallic alloy strip preferably have carbide
rolling surfaces. However, any suitable roller construction can be
used such as stainless steel rolls. If steel rollers are used, the
amount of reduction is preferably limited such that the rolled
material does not deform the rollers as a result of work hardening
of the intermetallic alloy. The hot/cold rolling step is preferably
carried out to reduce the strip thickness by at least 30%,
preferably at least about 50%. For instance, the 0.026 inch thick
presintered iron aluminide strips can be cold rolled to 0.013 inch
thickness in a single cold rolling step with single or multiple
passes.
[0046] After the hot/cold rolling, the hot/cold rolled strips are
subjected to heat treating to anneal the strips. This primary
annealing step can be carried out in a vacuum furnace in a batch
manner or in a furnace with gases like H.sub.2, N.sub.2 and/or Ar
in a continuous manner and at a suitable temperature to relieve
stress and/or effect further densification of the powder. In the
case of iron aluminide, the primary annealing can be carried at any
suitable temperature such as 1652-2372.degree. F. (900 to
1300.degree. C.), preferably 1742-2102.degree. F. (950 to
1150.degree. C.) for one or more hours in a vacuum furnace. For
example, the hot/cold rolled iron aluminide strip can be annealed
for one hour at 2012.degree. F. (11000.degree. C.) but surface
quality of the sheet can be improved in the same or different
heating step by annealing at higher temperatures such as
2300.degree. F. (1260.degree. C.) for one hour.
[0047] After the primary annealing step, the strips can be
optionally trimmed to desirable sizes. For instance, the strip can
be cut in half and subjected to further rolling and heat treating
steps.
[0048] In the next step, the rolled strips can be further rolled to
reduce the thickness thereof. For instance, the iron aluminide
strips can be rolled in a 4-high rolling mill so as to reduce the
thickness thereof from 0.013 inch to 0.010 inch. This step achieves
a reduction of at least 15%, preferably about 25%. However, if
desired, one or more annealing steps can be eliminated, e.g., a
0.024 inch strip can be rolled directly to 0.010 inch.
Subsequently, the rolled strips are subjected to secondary
sintering and annealing. In the secondary sintering and annealing
step, the strips can be heated in a vacuum furnace in a batch
manner or in a furnace with gases like H.sub.2, N.sub.2 and/or Ar
in a continuous manner to achieve full density. For example, a
batch of the iron aluminide strips can be heated in a vacuum
furnace to a temperature of 2300.degree. F. (1260.degree. C.) for
one hour.
[0049] After the secondary sintering and annealing step, the strips
can optionally be subjected to secondary trimming to shear off ends
and edges as needed such as in the case of edge cracking. Then, the
strips can optionally be subjected to further rolling wherein the
thickness of the strips is further reduced such as by 15% or more.
For example, the strips can be cold rolled to a final desired
thickness such as from 0.010 inch to 0.008 inch.
[0050] After the final rolling step, the strips can be subjected to
a final annealing step in a continuous or batch manner at a
temperature above the recrystallization temperature. For instance,
in the final annealing step, a batch of the iron aluminide strips
can be heated in a vacuum furnace to a suitable temperature such as
2012.degree. F. (1100.degree. C.) for about one hour. During the
final annealing the rolled sheet is preferably recrystallized to a
desired average grain size such as about 10 to 30 .mu.m, preferably
around 20 .mu.m. Then, the strips can optionally be subjected to a
final trimming step wherein the ends and edges are trimmed and the
strip is slit into narrow strips having the desired dimensions for
further processing such as into tubular heating elements or other
desired product.
[0051] The trimmed strips can be subjected to a stress relieving
heat treatment to remove thermal vacancies created during the
previous processing steps. The stress relief treatment increases
ductility of the strip material (e.g., the room temperature
ductility can be raised from around 1% to around 3-4%). In the
stress relief heat treatment, a batch of the strips can be heated
in a furnace at atmospheric pressure or in a vacuum furnace. For
instance, the iron aluminide strips can be heated to around
1292.degree. F. (700.degree. C.) for two hours and cooled by slow
cooling in the furnace (e.g., at .ltoreq.2-5.degree. F./min) to a
suitable temperature such as around 662.degree. F. (350.degree. C.)
followed by quenching. During stress relief annealing it is
preferable to maintain the iron aluminide strip material in a
temperature range wherein the iron aluminide is in the B2 ordered
phase.
[0052] The stress relieved strips can be processed as tubular
heating elements, honeycomb structures, thermal protection
structures or other products by any suitable technique. Iron
aluminide, nickel aluminide and titanium aluminide strips can be
made by any combination of the foregoing steps or modification of
such steps to achieve desired dimensions and/or properties. For
heater elements, the strips can be laser cut, mechanically stamped
or chemical photoetched to provide a desired pattern of individual
heating blades. For instance, the cut pattern can provide a series
of hairpin shaped blades extending from a rectangular base portion
which when rolled into a tubular shape and joined provides a
tubular heating element with a cylindrical base and a series of
axially extending and circumferentially spaced apart heating
blades. Alternatively, an uncut strip could be formed into a
tubular shape and the desired pattern cut into the tubular shape to
provide a heating element having the desired configuration.
[0053] In general, rolled FeAl material having a composition, in
weight %, of 23% Al, 0.005% B, 0.42% Mo, 0.1% Zr, 0.2% Y, 0.03% C,
balance Fe can exhibit room temperature yield strength of 55-70
ksi, ultimate tensile strength of 65-75 ksi, total elongation of
1-6%, reduction of area of 7-12% and electrical resistivity of
about 150-160 .mu..OMEGA..multidot.cm whereas the elevated
temperature strength properties at 750.degree. C. include yield
strength of 36-43 ksi, ultimate tensile strength of 42-49 ksi,
total elongation of 22-48% and reduction of area of 26-41%
elongation values.
[0054] Iron aluminide, nickel aluminide and titanium aluminide
sheet products according to the invention can be made by roll
compaction such as by the various steps described above or other
technique such as tape casting. Such techniques, however, can be
modified to form other shaped products as will be apparent to those
skilled in the art.
[0055] In the tape casting process, a powder mixture is processed
by substituting tape casting for the roll compaction step in the
foregoing roll compaction embodiment. However, whereas an
irregularly shaped powder is preferred for the roll compaction
process, gas atomized powder is preferred for tape casting due to
its spherical shape and low oxide contents. The gas atomized powder
is sieved as in the roll compaction process and the sieved powder
is blended with organic binder and solvent so as to produce a slip,
the slip is tape cast into a thin sheet and the tape cast sheet is
hot/cold rolled and heat treated as set forth in the roll
compaction embodiment. Details of the tape casting process can be
found in commonly owned U.S. Pat. No. 6,030,472, the disclosure of
which is hereby incorporated by reference
[0056] The foregoing has described the principles, preferred
embodiments and modes of operation of the present invention.
However, the invention should not be construed as being limited to
the particular embodiments discussed. Thus, the above-described
embodiments should be regarded as illustrative rather than
restrictive, and it should be appreciated that variations may be
made in those -embodiments by workers skilled in the art without
departing from the scope of the present invention as defined by the
following claims.
* * * * *