Note: Descriptions are shown in the official language in which they were submitted.
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STABILIZED GRAIN SIZE REFRACTORY
METAL P0111~DER METALLURGY MILL PRODUCTS
BACKGROUND OF THE INVENTION
The invention relates generally to metal mill products (and
fabricated parts) made from powders of refractory metals including the
elemental metals and their alloys and, more particularly to the use of oxide
dopants for grain size stabilization in mils products and fabricated parts to
be subjected to high temperature application usage and/or high tempera-
ture fabrication processes.
Users of refractory metals have had a long-standing interest in
replacing tantalum with niobium. One driving force for such replacement of
tantalum is price as well as the limited availability of tantalum. Many mill
products involve hir,~, h temperature exposure in fabrication and/or use. The
high temperatures can cause grain growth. In various applications large
grains, as a consequence of such grain growth, are detrimental to the
performance of the material. This has been a limitation of niobium
substitution for tantalum. Other limitations include lesser strength and
hardness as-fabricated niobium and its alloys.
Currently, areas of interest include furnace parts, sintering trays
and deep drawn cups as used for manufacturing synthetic diamonds.
These products require material with small grain size. Furnace parts
particularly require the material to have slow grain growth during service in
order to prevent premature deterioration of the mechanical properties.
Currently tantalum material with stabilized grain size, due to alloying
additions or other artifacts, is used for wire or sheet. In one embodiment or
state of interaction, Si02 is used as a grain stabilizer. The disadvantage of
such a manufacturing method (resistance-sintering) for grain size
stabilized tantalum powder metallurgy (P/M) material is that it
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is limited to a lot size of 30 pounds for tantalum and approximately 15
pounds for niobium. It is desirable to make lot sizes of up to 1000 pounds
of tantalum and 500 pounds of niobium respectively.
Current manufacturing methods for large P/M sheet sizes/strip
length are not capable of providing large pieces of sheet or long coils of
sheet with the same low level of oxygen content and good mechanical
properties
It is an object of this invention to provide a powder metallurgy (P/M)
route to fabrication of refractory metals in large lots with low oxygen
content and to provide resultant mill products with low oxygen content.
It is a further object of this invention to provide a P/M source for mill
products and eventual mill products with a finer grain and a decreased
grain growth than are achieved with ingot source materials.
These objects are applicable to refractory metals generally
~ 5 and more particularly to niobium and its alloys.
The objects set forth above as well as further and other objects and
advantages of the present invention are achieved by the invention as
described hereinbelow
0 SUMMARY OF THE INVENTION
The invention relates to a process for making a metal mill product
from a refractory metal powder comprising (a) providing a low oxygen
refractory metal powder; (b) adding to the powder a grain growth inhibitor
to the low oxygen refractory metal powder before consolidating the
25 powder, (c) consolidating the powder by either hot isostatic pressing,
extrusion or another thermomechanical working process; and
(d) subjecting the consolidated powder to subsequent thermomechanical
processing, and thereby forming the mill product. The invention also
relates to products made from such a process.
30 Grain growth inhibitors are added to niobium powder by blending
inhibitors such as Si02 and Y203 prior to consolidation or as a residue of a
de-oxidation process where magnesium is added to capture the oxygen
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from the niobium powder and form magnesium oxide during the de-
oxidation process.
The powder is consolidated either by hot isostatic pressing
(HIPing), extrusion or other thermomechanical working. Such methods of
consolidation are capable of providing suitable P/M sheet bars with a
weight of up to several hundred pounds, e.g., five hundred pounds, one
thousand pounds or more. Subsequent thermomechanical processing of
the P/M sheet bar is applied similarly to then P/M derived refractory metals
as to metals from ingot sources.
The present invention inhibits grain growth in niobium P/M sheets
during high temperature exposure. A low oxygen niobium powder
(< about 400 ppm, preferably < about 200 ppm) is needed as a starting
material. Powders with a higher content in oxygen cannot be consolidated
to full density and/or will not yield good mechanical properties.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a flow chart showing a process of the present invention to
create stabilized grain size powder; and
Figs. 2-4 are flow charts showing examples of consolidating steps
to create products made of stabilized grain size powder.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The invention relates to a process for making a metal mill product
from a refractory metal powder comprising (a) providing a low oxygen
refractory metal powder; (b) adding to the powder a grain growth inhibitor
to the low oxygen refractory metal powder before consolidating the
powder, (c) consolidating the powder by either hot isostatic pressing,
extrusion or another thermomechanical working process; and
(d) subjecting the consolidated powder to subsequent thermomechanical
processing, and thereby forming the mill product. The invention also
relates to products made from such a process.
The low oxygen niobium powder can be any powder, which when
used in accordance to the invention, enables user to meet an object of the
invention. The metal powders with stabilized grain size of the present
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invention are preferably produced via the following procedure as
discussed in U.S. Patent 6,261,337, incorporated herein in its entirety.
Niobium alloys can also be used.
In other embodiments, instead of using niobium powders, powders
made from a refractory metal selected from hafnium, molybdenum,
niobium, rhenium, tantalum, tungsten, vanadium, and zirconium metals
can be used. Also, alloys of these metals can also be used.
As illustrated in Fig. 1, low oxygen niobium and grain growth
inhibitor powders (for example Si02 or Y203) are blended to form low
oxygen powder with grain size inhibitors. Figs. 2-4 illustrate the
consolidation steps with the master blend. The physical processes of
blending and consolidating achieve a uniform distribution of grain growth
inhibiting particles in the powder metal sheet bars. The powders
are made by the process described in US 6,261,337 and as described
herein.
These powders are blended to produce the desired alloy compo-
sition. The powders are then sealed in an evacuated can, heated to a
desired temperature, and extruded such that the extrusion ratio is at least
8:1. This is done to completely consolidate the niobium powders and the
included inhibitors. The can may be removed either just before or just after
the rolling operation.
The above process can afford advantages of more stable grain size
in the final material, more uniform material properties (such as ultimate
tensile strength and hardness), lower manufacturing costs, better control
of fiber size, and greater flexibility for alloy modifications and control of
properties.
Niobium sheets produced from powder blends of niobium and
grain inhibitors, for example silicon, were tested for grain growth, ultimate
tensile strength, and hardness. The test results are presented in Table 1
below.
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Table 1
1150C Ultimate
Silicon 1065C @180 1300C Tensile Hardness
@90 min min @180 min Strength
m ASTM ASTM ASTM KSI VICKERS
0 9.5 9.5 7.5 49.3 114
150 9.5 9.0 8.0 50.3 117
300 9.5 9.5 8.5 49.5 125
Nb I/M 5.5 <1 <1 32 72
P/M sheet with grain growth inhibitors, preferably silicon, of 0, 150,
and 300 ppm were thermomechanical processed to a thickness of 0.015
inches and annealed at 1065°C for 90 minutes to produce grain sizes of
approximately ASTM 9.5. Niobium sheet produced from ingot metallurgy
(I/M) a grain size of approximately ASTM 5.5 under the same anneal heat
treat conditions. The P/M and I/M test samples were subjected to addi-
tional annealing heat treatments at 1150°C for 180 minutes and 1300
°C
for 180 minutes. The P/M test samples yielded grain sizes greater than
ASTM 7.0 compared to I/M test samples that yielded grain sizes coarser
than ASTM 1.
Additionally, the higher P/M Ultimate Tensile Strength of 49.3 KSI,
50.3 KSI, and 49.5 KSI and hardness of 114 VHN, 117 VHN, and 125
VHN are significant improvements over typical I/M material of Ultimate
Tensile Strength of 32 KSI and hardness of 72 VHN. The fine grain sizes
and improved tensile strength and hardness after heat treatment of the
P/M material is a significant advantage, compared to I/M material, in
applications where large amounts of deformation are required during
fabrication, such as deep drawn diamond cups, or capacitor cans.
Alternatively, the blended powders may be isostatically pressed into
a bar prior to canning and extrusion, as illustrated in Fig. 2. The advan-
s
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tage of this method would be to put a higher weight into the compact prior
to extrusion to aid in consolidation and increase yield per extrusion.
Now returning to Fig. 1, niobium hydride powder is placed into a
vacuum chamber, which also contains a metal having a higher affinity for
oxygen, such as calcium or magnesium, preferably the latter. Preferably,
the starting hydride powder has oxygen content less than about 1000
ppm. The chamber is heated to the dehydration temperature to remove
the hydrogen, then heated to the deoxidation temperature to produce a
powder of niobium or alloy of niobium having a target reduced oxygen
content of less than about 400 ppm preferably below 200 ppm and more
preferably below 100 ppm. The magnesium, containing the oxygen, is
then removed from the metal powder by evaporation and subsequently by
selective chemical leaching or dissolution of the powder.
For example, a niobium powder with less than 400 ppm oxygen can
be produced by the deoxidization of niobium hydride under partial
pressure of argon. Niobium hydride powder would be blended with 0.3
' wt.-% magnesium and placed in a vacuum furnace retort, which is
evacuated, and backfilled with argon. The pressure in the furnace would
be set at about 100 microns with Argon flowing and the vacuum pump
running.
The furnace temperature would be ramped to about 650°C in
approximately 50°C increments, held until temperature equalized, then
ramped up to 950°C in approximately 50°C increments. When the
temperature equalized at 950°C it would be held for about two hours.
After
such hold, the furnace is shut down. Once the furnace cools its powder
content is removed from the retort.
The magnesium, containing the oxygen, would then be removed
from the metal powder by acid leaching to produce the resulting niobium
powder having an oxygen content of less than 300 ppm.
As described above, in the process for producing formed powder
metal products of niobium, the metal hydride powder is deoxidized to an
oxygen content of less than about 400 ppm. The powder is consolidated to
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form a niobium or alloy product, having an oxygen content below about
about 400 ppm, or below about 300 ppm or below about 200 ppm or
below about 100 ppm, but for many powder metallurgy purposes between
about 100 ppm and 150 ppm. According to the present invention, a
formed refractory metal product (niobium product), having a stabilized
grain size, may be produced from metal hydride powder, as treated as
described above, by any known powder metallurgy techniques.
Exemplary of these powder metallurgy techniques used for forming
the products are the following, in which the steps are listed in order of
performance. Any of the following single techniques or sequences of
techniques may be utilized in the present invention: cold isostatic pressing,
sintering, encapsulating, hot isostatic pressing and thermomechanicai
processing; cold isostatic pressing, sintering, hot isostatic pressing
thermomechanical processing; cold isostatic pressing, encapsulating, hot
isostatic pressing and thermomechanical processing; cold isostatic
pressing, encapsulating and hot isostatic pressing; encapsulating and hot
isostatic pressing; cold isostatic pressing, sintering, encapsulating,
extruding and thermomechanical processing; cold isostatic pressing,
sintering, extruding, and thermomechanical processing; cold isostatic
pressing, sintering, and extruding; cold isostatic pressing, encapsulating,
extruding and thermomechanical processing; cold isostatic pressing,
encapsulating and extruding; encapsulating and extruding;
mechanical pressing, sintering and extruding; cold isostatic pressing,
sintering, encapsulating, forging and thermomechanical processing;
cold isostatic pressing, encapsulating, forging and thermomechanical
processing; cold isostatic pressing, encapsulating and forging;
cold isostatic pressing, sintering, and forging; cold isostatic pressing,
sintering and rolling; encapsulating and forging; encapsulating and rolling.
cold isostatic pressing, sintering and thermomechanical processing;
mechanical pressing and sintering; and mechanical pressing, sintering,
repressing and resintering; other combinations of consolidating, heating
and deforming may also be utilized.
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The production of a formed niobium product having a stabilized
grain size can be achieved by cold isostatic pressing of various kinds of
known niobium powders to form a compact, followed by a hot isostatic
pressing (HIPing) step to densify the compact and then thermomechanical
processing of the powder compact for further densification and completion
of the bonding, as illustrated in Fig. 3. Preferably, niobium powder with
grain size inhibitors would be cold isostatically pressed at 60,000
pounds/sq. in. and room temperature, into a compact with rectangular or,
preferably, round cross section, then hermetically encapsulated and hot
isostatically pressed (HPed) at 40,000 Ibs.1 sq. in. and 1300°C for
four
hours. The HIPed compact would be unencapsulated and converted to
sheet or foil by thermomechanical processing steps.
A similar process, as illustrated in Fig. 4, of just cold isostatic
pressing, sintering and thermomechanical processing using niobium
powder having an oxygen content of less than 300 ppm can be conducted
by cold isostatically pressing at 60,000 Ibs./sq. in. into a bar shape
preform. This preform would be sintered at 1500°C for two hours in a
vacuum of less than about 0.001 Torr to yield a preform having a density
of about 95% theoretical density (Th) and less than 400 ppm oxygen. The
sintered preform would be converted into sheet and foil by thermomecha-
nical processing steps.
Production of a formed niobium sheet or foil having a stable grain
size by hot extrusion and thermomechanical processing can be made,
using niobium powder having an oxygen content of less than 400 ppm as
the starting powder. This powder can be hermetically encapsulated then
extruded through a rectangular or, preferably, round die at 1000°C to
produce an extruded product having oxygen content of less than 400 ppm.
The extruded product can be converted to sheet or foil by the thermo-
mechanical processing.
Niobium sheet or foil with oxygen content of less than 400 ppm can
be produced by cold isostatic pressing, hot extrusion and thermomecha-
nical processing. This compact made by cold isostatically pressing could
s
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be hermetically encapsulated then extruded at 1000°C to produce an
extruded product with an oxygen content of about 300 ppm which can be
converted into sheet and foil by thermomechanical processing steps.
Niobium products having stable grain size can be prepared by
mechanical pressing, sintering, repressing and resintering.
Niobium powder blend having oxygen content of less than 400 ppm
can be utilized as the starting powder. It is placed in a die and mecha-
nically pressed, using uniaxial pressure. The pressed tablet should be
then sintered at 1500°C for two hours in a vacuum evacuated to less
than
about 0.001 Torr. The sintered tablet would then be repressed and
resintered at 1500°C for two hours in a vacuum evacuated to less than
about 0.001 Torr.
The resintered tablet will have oxygen content of less than about
400 ppm and be suitable for thermomechanical processing to produce a
formed niobium product.
In one embodiment, a copper or steel container is filled with
niobium powder, evacuated, hermetically sealed, and extruded through a
die to give a 10:1 extrusion ratio. The copper container is removed by acid
treatment and the extruded bar is thermo-mechanically processed into a
sheet form flat. In another embodiment, a steel container is filled with the
niobium powder, evacuated, hermetically sealed and HIPed. The steel
container is removed by machining and the HIPed piece is thermo
mechanically processed into a sheet form flat.
Anneals may be used to improve workability of the material in
between two deformation steps or to adjust grain size and texture through
recrystallization although a final anneal may not be necessary. When the
powder is canned during the consolidation (usually to protect it from the
environment at high temperature), the can will bond to the niobium.
In another embodiment, the process provides P/M sheets of large
size (>100 pounds) having good mechanical properties and small stable
grain size, capable of a higher yield than conventional P/M processes for
sheet manufacture, typically 50 pounds or less. Low oxygen niobium
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powder of less than 400 ppm, preferably less than 150 ppm, of
non-spherical particles and sizing less than 250 microns FAPD (Fisher
Average Particle Diameter), is provided per processes described herein.
Powders with a higher content in oxygen cannot be consolidated to full
density and/or will not yield good mechanical properties. The powder is
consolidated to full density either by HIPing (hot isostatic pressing) or by
extrusion. Both methods of consolidation are capable of providing suitable
P/M sheet bars with a weight of up to several hundred pounds.
Thermomechancial processing of the P/M sheet bar is similar to
standard processes.
Numerous variations and modifications may obviously be made
without departing from the present invention. Accordingly, it should be
clearly understood that the forms of the present invention herein described
are illustrative only and are not intended to limit the scope of the
invention.
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