U.S. patent application number 09/962597 was filed with the patent office on 2002-04-18 for silver and silver alloy articles.
Invention is credited to Seuntjens, Jeffrey M..
Application Number | 20020043392 09/962597 |
Document ID | / |
Family ID | 25259212 |
Filed Date | 2002-04-18 |
United States Patent
Application |
20020043392 |
Kind Code |
A1 |
Seuntjens, Jeffrey M. |
April 18, 2002 |
Silver and silver alloy articles
Abstract
A new method for fabricating silver or silver alloy tube stock
is described. The method provides silver or silver alloy tube stock
with a structure that is substantially free of defects, has a fine
grain size, and is amenable to uniform deformation. The silver or
silver alloy tube stock is used to make silver-superconductor
monofilament or multifilament precursor articles and
composites.
Inventors: |
Seuntjens, Jeffrey M.;
(Spencer, MA) |
Correspondence
Address: |
FRANK R. OCCHIUTI
Fish & Richardson P.C.
225 Franklin Street
Boston
MA
02110-2804
US
|
Family ID: |
25259212 |
Appl. No.: |
09/962597 |
Filed: |
September 25, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09962597 |
Sep 25, 2001 |
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08831504 |
Mar 31, 1997 |
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Current U.S.
Class: |
174/126.1 ;
174/125.1; 29/419.1; 29/599 |
Current CPC
Class: |
Y10T 29/49014 20150115;
H01L 39/248 20130101; Y10T 29/49801 20150115; B21C 23/085
20130101 |
Class at
Publication: |
174/126.1 ;
174/125.1; 29/599; 29/419.1 |
International
Class: |
H01B 012/00; H01B
005/00 |
Claims
What is claimed is:
1. A method of manufacturing a seamless metal tube comprising a
fine-grained silver or silver alloy and having a desired inner
diameter, a desired outer diameter, and a desired cross-sectional
area determined by the desired outer diameter and the desired inner
diameter, comprising the steps of: providing an ingot of silver or
a silver alloy having a long axis and a hole parallel to the long
axis and having a diameter approximately equal to the desired inner
diameter of the seamless metal tube, and an initial cross-sectional
area at least 8 times greater than the desired cross-sectional
area; inserting a mandrel having a diameter approximately equal to
the desired inner diameter of the seamless metal tube into the hole
of the ingot prior to the pressing step; and extruding the ingot
through a die having a diameter approximately equal to the desired
outer diameter of the seamless metal tube to produce the seamless
metal tube and reduce the average and maximum grain size of the
silver or silver alloy.
2. The method of claim 1, wherein the initial cross-sectional area
is at least 20 times greater than the desired cross-sectional
area.
3. The method of claim 1, further comprising the steps of: heating
the ingot to a temperature between about 200 to 450.degree. C.
prior to the extruding step; and cooling the seamless metal tube to
below nominally 200.degree. C. within 5 minutes of the extruding
step.
4. The method of claim 1, wherein the average diameter of the ingot
is greater than 100 millimeters.
5. The method of claim 1, wherein the desired outer diameter of the
seamless metal tube is between 25 and 100 millimeters.
6. The method of claim 1, wherein the seamless metal tube is
substantially free of major defects.
7. The method of claim 1, wherein the average grain size of the
fine-grained silver or silver alloy is less than 50
micrometers.
8. The method of claim 7, wherein the average grain size is less
than 20 micrometers.
9. The method of claim 1, wherein the maximum grain size of the
fine-grained silver or silver alloy is about 100 micrometers.
10. The method of claim 9, wherein the maximum grain size is about
50 micrometers.
11. The method of claim 1, wherein the step of providing the ingot
comprises the steps of providing a silver or silver alloy casting
having a diameter smaller than the average diameter of the ingot
and upsetting the casting one or more time, in a press to produce
the ingot at a desired diameter larger than the diameter of the
casting.
12. The method of claim 11, wherein the average diameter of the
ingot is greater than 150 millimeters.
13. A method of reducing the grain size of a silver or silver alloy
article having a desired outer diameter and a desired
cross-sectional area comprising the steps of: providing an ingot of
silver or a silver alloy having a long axis and an initial
cross-sectional area at least 8 times greater than the desired
cross-sectional area; and extruding the ingot through a die having
a diameter approximately equal to the desired outer diameter of the
article to produce the article having a fine grain size.
14. The method of claim 13, further comprising the steps of:
heating the ingot to a temperature between about 200 to 450.degree.
C. prior to the extruding step; and cooling the article to below
nominally 200.degree. C. within 5 minutes of the extruding
step.
15. The method of claim 14, wherein the ingot further includes a
hole parallel to the long axis and having a diameter approximately
equal to a desired inner diameter of the article and the method
further includes the step of inserting a mandrel having a diameter
approximately equal to the desired inner diameter of the
article.
16. The method of claim 15, wherein the initial cross-sectional
area is at least 20 times greater than the desired cross-sectional
area.
17. The method of claim 13, wherein the ingot has a first average
grain size and the article has a second average grain size that is
a factor of 100 smaller than the first grain size.
18. The method of claim 13, wherein the silver or silver alloy of
the article has a maximum grain size of about 100 micrometers.
19. The method of claim 18, wherein the maximum grain size of about
50 micrometers.
20. The method of claim 13, wherein the silver or silver alloy has
an average grain size less than 50 micrometers.
21. The method of claim 20, wherein the average grain size is less
than 20 micrometers.
22. A thick-walled, seamless metal tube comprising silver or a
silver alloy having an average grain size that is less than 200
micrometers and a maximum grain size less than about 300
micrometers.
23. The seamless metal tube of claim 22, wherein the tube has an
outer diameter greater than 25 millimeters.
24. The seamless metal tube of claim 22, wherein the seamless metal
tube is substantially free of major defects.
25. The seamless metal tube of claim 24, wherein the average grain
size is less than 100 micrometers and the maximum grain size is
less than about 200 micrometers.
26. The seamless metal tube of claim 25, wherein the average grain
size is less than 20 micrometers, and the maximum grain size is
less than about 50 micrometers.
27. A superconducting article precursor comprising a superconductor
or superconductor precursor contained in a thick-walled, seamless
metal tube comprising silver or a silver alloy.
28. The superconducting article precursor of claim 27, wherein the
thick-walled, seamless metal tube has an outer diameter greater
than 25 millimeters.
29. The superconducting article precursor of claim 27, wherein the
seamless metal tube is substantially free of major defects.
30. A method of manufacturing a composite superconducting article,
comprising the steps of: providing a precursor article including a
superconductor or superconductor precursor contained in a seamless
metal tube comprising fine-grained silver or silver alloy, the
seamless metal tube having an outer diameter greater than 25
millimeters; and treating the precursor article to obtain the
superconducting article.
31. A method of manufacturing a fine-grained silver or silver alloy
article independent of initial grain size of the silver or silver
alloy, the article having a desired outer diameter and a desired
cross-sectional area, the method comprising the steps of: providing
an ingot of silver or a silver alloy having a long axis and an
initial cross-sectional area at least 8 times greater than the
desired cross-sectional area; and extruding the ingot through a die
having a diameter approximately equal to the desired outer diameter
of the article to produce the article.
32. The method of claim 31, further comprising the steps of:
heating the ingot to a temperature between about 200 to 450.degree.
C. prior to the extruding step; and cooling the article to below
nominally 200.degree. C. within 5 minutes of the extruding
step.
33. The method of claim 31, wherein the initial cross-sectional
area is at least 20 times greater than the desired cross-sectional
area.
34. The method of claim 31, wherein the article is substantially
free of major defects.
35. The method of claim 31, wherein the article has an average
grain size is less than 100 micrometers.
36. The method of claim 35, wherein the average grain size is less
than 20 micrometers.
37. The method of claim 36, wherein the article has a maximum grain
size of about 100 micrometers.
38. The method of claim 37, wherein the maximum grain size is about
50 micrometers.
39. A metal article comprising fine-grained silver or silver alloy,
wherein the metal article has a minimum cross-sectional area
greater than about 100 square millimeters and is substantially free
of major defects.
Description
[0001] The invention relates to tubes of silver and silver
articles.
BACKGROUND OF THE INVENTION
[0002] Oxide superconductors exhibit superconductivity at
relatively high temperatures in comparison to their traditional
metallic counterparts. However, the oxide superconductors, being
ceramics, are generally brittle and are difficult to process and
manipulate. In contrast, composites of oxide superconductors
supported by metal matrices, such as metal sheaths, have mechanical
and electrical properties that are improved relative to the oxide
superconductors alone. The metal sheath, matrix, or container,
which holds the oxide superconductor powder prior to and during
processing is typically composed of a noble metal such as silver or
a silver alloy.
[0003] Oxide dispersion strengthened (ODS) silver alloys have been
used as matrix materials, where the oxide dispersion increases the
strength and hardness of the matrix. The use of silver and silver
alloys in the production of silver-sheathed superconductor
composites (silver-superconductor composites) is described, for
example, in Lay, U.S. Pat. No. 5,384,307, Yamamoto, et al., U.S.
Pat. No. 5,232,906, Flukiger, U.S. Pat. No. 5,232,906, and U.S.
Ser. No. 08/626,130 filed Apr. 5, 1996 and entitled "Oxygen
Dispersion Hardened Silver Sheathed Superconductor Composites,"
each of which is incorporated herein by reference.
[0004] Silver-sheathed superconductor composites require the
fabrication of thick walled and large diameter silver tube stock
(used as the metal sheath) for processing, however, available
silver tube stock has primarily been prepared to the specifications
of the jewelry industry and has relatively thin walls and small
diameters. Larger articles of silver are available as cast ingots
having diameters up to about 100 millimeters. The properties of the
silver-superconductor composites are influenced by the starting
grain size of the silver.
[0005] Cast silver is not well-suited for use in
silver-superconductor composites due to the large average grain
size and possible high level of porosity in the silver or silver
alloy castings. In making these castings, only small temperature
gradients exist in the solidifying metal due to the high thermal
conductivity of the silver, which results in very large grains in
the castings. The grain sizes typically can be on the order of 300
micrometers to a few millimeters in diameter. In addition, cast
silver can have porosity due to bubble formation during cooling of
the silver when the melt is not properly degassed. The high level
of porosity occurs because the solubility of gases in solid silver
is much lower than the solubility of gases in liquid silver. The
large grain sizes and porosity lead to difficulties in
silver-superconductor composite processing steps.
[0006] Presently, the process for preparing silver tube stock
suitable for use in silver-superconductor composites is to cast
molten silver into a tube up to about 50-75 millimeters in
diameter. The cast silver tube is then drawn to the desired inner
diameter (ID) or outer diameter (OD). Drawing of the cast silver
tube to reduce its OD by a nominal factor of about 2 (an OD on the
order of 25-35 millimeters) produces a number of flaws, such as
folds, in the product that relate to the large initial grain size
and high porosity of the cast silver tube. First, the large grains
can exaggerate the formation of steps and folds on the inner
surfaces of the drawn silver or silver alloy tube. These may extend
up to 1 millimeter below the surfaces of the drawn tube. The steps
and folds are typically formed on the inner surface due to the
unsupported collapse of the inner diameter of the tube during
drawing. Drawing a tube with insufficient mandrel support on inside
of tube can may cause folds in fine grains as well. Once created,
the folds can only be removed by machining the effected portion of
the surface, resulting in a yield loss of material. Second, the
large grains can result in non-uniform deformation and formation of
localized cracks or tears on the surface of the tube during
processing. Cracks or tears can form either on the inner or outer
surface of the tube during drawing. During drawing, junctions where
three grain boundaries meet (grain boundary triple points) can open
in early stages of drawing. These openings becomes lenticular
during further drawing steps and eventually becomes an axial
"split" in the silver tube of the monofilament. The split remains
in the monofilament, even though the drawing work eventually
refines the silver grains to smaller grain sizes. The split can
result in bridging between filaments in a multifilamentary
composite configuration. Preferably, filaments are unbridged.
Third, porosity in the cast silver material is not healed in the
processing since the drawing forces are primarily tensile rather
than compressive and are applied at or near room temperature.
[0007] The tube casting and tube drawing approaches to the
production of silver or silver alloy tube stock needed to make
silver-sheathed superconductor composites can be expensive. Tube
casting is more expensive than production cylindrical billet
casting. The tube drawing process has yield loss on the ends of
each drawn length. Tube drawn product must be post-machined to
remove sizable surface defects such as the folds that can be up to
1 millimeter deep.
[0008] The requirements for cast and drawn silver or silver alloy
monofilament processability, final tape performance, and processing
costs indicate that alternative routes to silver or silver alloy
tube stock are desirable. Moreover, defects and failures
encountered in processed silver-superconductor composites suggest
that silver or silver alloy tube quality needs to be improved. In
addition, larger diameter, thicker wall silver or silver alloy tube
stock than that typically used in the jewelry industry that is
suitable for use in composite processing is needed.
[0009] One family of oxide superconductors includes
bismuth-strontium-calcium-copper-oxide (BSCCO) compositions such as
Bi.sub.2Sr.sub.2CaCu.sub.2O.sub.8 (BSCCO-2212) and
Bi.sub.2Sr.sub.2Ca.sub.2Cu.sub.3O.sub.10 (BSCCO-2223). The BSCCO
superconductors include compositions where bismuth is partially
substituted by dopants such as lead (that is, (Bi,Pb)SCCO). Other
families of oxide superconductors include
yttrium-barium-copper-oxide (YBCO) compositions, such as
YBa.sub.2Cu.sub.3O.sub.7 (YBCO-123), and
thallium-barium-calcium-copper-oxide compositions.
[0010] The oxide superconductor-silver composites can be prepared
in elongated forms such as wires and tapes by processes, such as
the well-known powder-in-tube (PIT) process, that typically include
a number of stages. In the PIT process, first, a powder of a
precursor to a superconductor is prepared. The precursor can be a
single material or a mixture of materials. Second, a silver or
silver alloy container (for example, a tube, billet or grooved
sheet) is filled with the precursor powder. The silver container
serves as a matrix for constraining the superconductor. Third, the
filled container is deformed in one or more iterations to reduce
the cross-sectional area of the container in a draft reduction
step. A number of filled containers (filaments) can be combined and
surrounded by another silver or silver alloy matrix to form a
multifilament article. Finally, the material is subjected to one or
more deformation and annealing cycles which together form and
sinter the oxide superconductor. This thermomechanical processing
helps precursor grains align and grow to form a textured
superconductor article, which is predominantly composed of one
phase and has a high critical current density (J.sub.c).
[0011] If the precursor powder is composed of one or more oxides,
the process is known more specifically as oxide-powder-in-tube
(OPIT) processing. If the precursor powder is composed of elemental
metal alloys, the process is known more specifically as
metal-powder-in-tube (MPIT) processing. See, for example, Yamamoto,
et al., U.S. Pat. No. 5,232,906, Otto, et al., "Progress toward a
long length metallic precursor process for multifilament Bi-2223
composite superconductors", IEEE Transactions on Appl. Supercon.,
Vol. 5, No. 2 (Jan. 1995), pp. 1154-1157, Yurek, et al., U.S. Pat.
No. 4,826,808, Yurek, et al., U.S. Pat. No. 5,189,009, Gao et al.,
Superconducting Science and Technology, Vol. 5, 1992, pp. 318-326,
and Sandhage, et al., "The oxide-powder-in-tube method for
producing high current density BSCCO superconductors", Journal of
Metals, Vol. 43, No. 3, 1991, pp. 21-25, each of which is
incorporated herein by reference.
SUMMARY OF THE INVENTION
[0012] The invention relates to a new method for fabricating silver
or silver alloy articles and tube stock. It is particularly useful
for manufacturing thick-walled tube stock and articles having a
minimum cross-sectional area greater than about 100 square
millimeters. The invention features silver or silver alloy articles
or tube stock with a structure that is substantially free of
defects, has a fine grain size, and is amenable to uniform
deformation. The silver or silver alloy tube stock can be used to
make silver-sheathed superconductor monofilament or multifilament
precursor articles and composites.
[0013] By "fine-grained silver or silver alloy" is meant silver or
silver alloy which has an average grain size that is typically less
than 300 micrometers, preferably less than 200 micrometers, more
preferably less than 100 micrometers, even more preferably less
than 50 micrometers, or, most preferably less than 20 micrometers.
The maximum grain size is typically less than about 300
micrometers, preferably less than about 200 micrometers, more
preferably less than about 100 micrometers, and most preferably
less than about 50 micrometers. Articles with average grain sizes
of 10-20 micrometers and maximum grain sizes typically less than
about 50 micrometers may be obtained. The average and maximum grain
sizes can be determined, for example, by metallography. Preferably,
the metal tube is substantially free of porosity and has a surface
that is substantially smooth and defect-free. A 3:1 ratio of
maximum-to-average grain size is typical of silver or silver alloys
produced by this process.
[0014] By "thick-walled" is meant a tube having a ratio of inner
diameter to outer diameter in the range of about 0.60-0.85.
[0015] In one aspect, the invention relates to a method of
manufacturing a fine-grained silver or silver alloy article
independent of initial grain size of the silver or silver alloy.
The article has a desired outer diameter and a desired
cross-sectional area. First, the method features the step of
providing an ingot of silver or a silver alloy having a long axis
and an initial cross-sectional area at least 8 times greater than
the desired cross-sectional area. Next, the method features the
step of extruding the ingot through a die having a diameter
approximately equal to the desired outer diameter of the article to
produce the article.
[0016] In another aspect, the invention relates to a method of
manufacturing a seamless metal tube comprising a fine-grained
silver or silver alloy and having a desired inner diameter, a
desired outer diameter, and a desired cross-sectional area
determined by the desired outer diameter and the desired inner
diameter. Preferably, the ratio of the desired inner diameter to
the desired outer diameter is about 0.60 to 0.85. First, the method
features the step of providing an ingot of silver or a silver alloy
having a long axis and a hole parallel to the long axis and having
a diameter approximately equal to the desired inner diameter of the
seamless metal tube. The ingot has an initial cross-sectional area
at least 8 times greater than the desired cross-sectional area.
Second, the method features the step of inserting a mandrel having
a diameter approximately equal to the desired inner diameter of the
seamless metal tube into the hole of the ingot prior to the
pressing step. Next, the method features the step of extruding the
ingot through a die having a diameter approximately equal to the
desired outer diameter of the seamless metal tube to produce the
seamless metal tube and reduce the average and maximum grain size
of the silver or silver alloy.
[0017] In another aspect, the invention features a method of
reducing the grain size of a silver or silver alloy article having
a desired outer diameter and a desired cross-sectional area. First,
the method features the step of providing an ingot of silver or a
silver alloy having a long axis and an initial cross-sectional area
at least 8 times greater than the desired cross-sectional area.
Next, the method features the step of extruding the ingot through a
die having a diameter approximately equal to the desired outer
diameter of the article to produce the article having a fine grain
size.
[0018] The ingot can have a first average grain size and the
article can have a second average grain size that is a factor of 50
to 200 times smaller than the first grain size. In other words, the
average grain size is typically reduced by a factor of about 50 to
200. Fine-grained articles may be obtained independent of the ingot
grain size.
[0019] The methods can include a heating step before extrusion and
a cooling step after extrusion. Typical extrusion conditions
suitable for silver or silver alloy tube extrusion include a 200 to
450.degree. C. pre-heat of the ingot, preferably 260 to 340.degree.
C. After extrusion, the article is cooled. The preferred cooling
rate is to cool the article to below nominally 200.degree. C. and,
preferably, room temperature within 5 minutes of the article
exiting the extruder.
[0020] In preferred embodiments, the method features the steps of
drilling a hole in the ingot and inserting a mandrel having a
diameter approximately equal to the inner diameter of the tube into
the hole of the ingot prior to the pressing step.
[0021] In other preferred embodiments, the method features the step
of upsetting a silver or silver alloy casting having a diameter
smaller than the average diameter of the extrusion ingot in a press
to produce an upset ingot having a diameter larger than the
diameter of the casting. The upset ingot preferably has an the
average diameter of between 100 and 300 millimeters. The upsetting
step can be repeated on the upset ingot to increase the average
diameter of the upset ingot to between 150 and 300 millimeters.
[0022] In another aspect, the invention features a metal article
with a minimum cross-sectional area greater than about 100 square
millimeters including fine-grained silver or a silver alloy which
is substantially free of major defects. The article may be machined
to produce a surface that is substantially smooth and
defect-free.
[0023] In another aspect, the invention features a thick-walled,
seamless metal tube comprising silver or a silver alloy having an
average grain size that is less than 200 micrometers and a maximum
grain size less than 300 micrometers.
[0024] In another aspect, the invention features a superconducting
article precursor including a superconductor or superconductor
precursor contained in a thick-walled seamless metal tube composed
of fine-grained silver or a silver alloy. The seamless metal tube
can have an outer diameter greater than 25 millimeters. The
seamless metal tube can be substantially free of major defects. In
another aspect, the invention features a method of manufacturing a
composite superconducting article by treating the precursor article
to obtain the superconducting article. The composite can be
multifilamentary, the seamless metal tube can be substantially
smooth-walled, and the superconducting article can be
unbridged.
[0025] In other preferred embodiments, the average diameter of the
ingot is between 100 and 300 millimeters and the outer diameter of
the tube is between 25 and 100 millimeters.
[0026] The invention may provide one or more of the following
advantages. Cast and drawn silver or silver alloy tube stock has
large grain size, porosity, folds, cracks and splits in the tube
which can lead to breakage during processing of the
silver-superconductor composite, particularly in wire drawing where
the defects act as stress risers under tensile load. Typically, the
larger the casting diameter, the larger the grain size of the
silver or silver alloy. In the case of processing multifilament
composites, the filaments can be sausaged or otherwise damaged
without complete composite failure. The resulting defective
multifilament composite can lead to forced current transfer to
other filaments, degraded overall composite performance. The cast
and drawn silver tube stock is not amenable to scale-up by, for
example, upsetting, since larger tube sizes will not encounter the
deformation needed to refine grain size during processing,
resulting in even lower quality tube products. Moreover, larger
cast silver or silver alloy tubes are not commercially
available.
[0027] The extruded silver or silver alloy article is substantially
free of major defects. In particular, the extruded tube stock is
free of porosity, has a fine grain size and is free of surface
folds and cracks. As a result, the extruded silver or silver alloy
tube stock performs better in silver-superconductor composite
processing due to improved tube quality. The extrusion pressure
(typically over 700 MPa) closes casting porosity and routinely
provides a fine-grained tube with preferred texture for uniform
flow in subsequent silver-superconductor drawing processes. The
lack of porosity and other major defects in the fine-grained silver
or silver alloy contributes to smooth-walled, uniformly dense
superconducting filaments. The improved properties of the silver or
silver alloy tube stock can help eliminate bridging of filaments,
which can be correlated with high J.sub.c. The fine grain size
helps eliminate folds and cracks on the surfaces of the tube. Due
to the flexibility of processing conditions, the extruded tube
stock has more consistent and predictable material properties than
the cast and drawn material. In addition, extruded silver or silver
alloy tube stock can be made with larger diameters (up to 75
millimeters in diameter) and with a range of wall thicknesses,
including thick-walled tube stock.
[0028] The extrusion of the silver or silver alloy tube stock also
eliminates processing steps and material loss (particularly
processed material loss) in comparison to the cast and drawn
processes of forming silver tubes. For example, uniform tube
drawing processes can become difficult and unreliable when the
superconductor powder fraction is greater than approximately 50
percent. The fine grain size of the tube can facilitate
superconductor monofilament fabrication with high fill factors even
in excess of 55 percent. Fill factor is the measured percentage of
oxide superconductor in a cross section of the conductor, and is an
important determinant of the overall current carrying capacity of
the conductor. Moreover, the silver scrap from tube extrusion is
recoverable at about 80 percent of the ingot purchase price.
[0029] A silver alloy is a mixture of one or more metals with
silver, where silver is the dominant amount of material in the
alloy. The metal dissolved in the silver can include gold,
platinum, palladium, aluminum, magnesium, copper, lithium, sodium,
potassium, calcium, beryllium, strontium, barium, yttrium,
scandium, lanthanum, cerium, praseodymium, neodymium, promethium,
samarium, europium, gadolinium, terbium, dysprosium, holmium,
erbium, thulium, ytterbium, lutetium, titanium, zirconium, hafnium,
vanadium, niobium, tantalum, silicon, germanium, antimony, tin,
lead, gallium, indium, thallium, zinc, bismuth, or mercury. The
preferred metals included in a silver alloy are gold, platinum,
palladium, aluminum, magnesium, antimony, or copper. As used
herein, "silver alloy" also encompasses silver containing a
dispersion of oxide particles. Oxide dispersion strengthened (ODS)
is described, for example, in Lay, U.S. Pat. No. 5,384,307,
Yamamoto, et al., U.S. Pat. No. 5,232,906, Flukiger, U.S. Pat. No.
5,232,906, and U.S. Ser. No. 08/626,130 filed Apr. 5, 1996 and
entitled "Oxygen Dispersion Hardened Silver Sheathed Superconductor
Composites."
[0030] "Article," as used herein, means a wire, tape, cable, or
other extruded shape. A superconducting article is an article that
includes an oxide superconductor component, such as a tape, wire,
or cable, and can be made up of a single oxide superconductor
filament (monofilament) or a plurality of oxide superconductor
filaments (multifilament).
[0031] "Extrusion ingot-to-article area reduction ratio" or "R," as
used herein, means the ratio of the initial cross-sectional area of
the upset silver ingot (A.sub.i) to the final cross-sectional area
of the extruded article (A.sub.f). The cross-sectional area an
object such as the article or ingot is determined by the outer
diameter and, if present, the inner diameter of the object. The
cross-sectional area of the object does not include any hole in the
object; or any mandrel or liner dimension. For example, when the
object is a tube, the cross-sectional area of the tube is equal to
.pi.(outer radius).sup.2-.pi.(inner radius).sup.2. The extrusion
pressure, P, is related to R by the equation P=k ln(R), where k is
a materials property. When R is greater than about 8
(A.sub.i.gtoreq.A.sub.f), the extrusion pressure and deformation at
the appropriate temperature can be sufficient to close any residual
porosity in the silver or silver alloy due to casting and to refine
the silver or silver alloy grains. More preferably, R is greater
than about 20.
[0032] "Extruding" or "extrusion," as used herein, means forcing or
pressing a material through a die to give a shaped article.
"Ingot," as used herein, means a piece of metal that is the source
of material for extrusion or pressing. The ingot can be
cylindrical. "Casting," as used herein, means a metal article that
is prepared from molten metal that is solidified into the shape of
the article, such as a circular cylindrical shape.
[0033] "Included angle," as used herein, means the full angle
formed between the faces of the extrusion die as measured in a
cross section of the die through the center of the die opening.
[0034] "Aspect ratio," as used herein, means the ratio of a first
dimension of an article to a second dimension of the article. In
the case of a cylinder, such as a silver casting, the aspect ratio
is the ratio of the length to the diameter.
[0035] "Upset," as used herein, means to deform an article by
pressing to increase one dimension and decrease another dimension,
thereby decreasing the aspect ratio of the article. For example, a
silver ingot having a length and diameter can be upset to give an
upset ingot having a smaller length and a larger diameter.
[0036] Other features and advantages of the invention will be
apparent from the description of the preferred embodiment thereof,
and from the claims.
BRIEF DESCRIPTION OF THE DRAWING
[0037] FIG. 1 is a perspective view of an ingot.
[0038] FIG. 2 is a cross-sectional view of an extruder.
[0039] FIG. 3 is a perspective view of an extruded tube.
[0040] FIG. 4 is a metallograph of a cross section of the metal
tube transverse to the direction of extrusion.
[0041] FIG. 5 is a metallograph of a cross section of the metal
tube parallel with the direction of extrusion.
DETAILED DESCRIPTION
[0042] According to the invention, a silver tube is extruded from a
cylindrical casting of silver (i.e., an ingot), such as the
castings available from Handy & Harman, Fairfield, Conn. or
A.T. Wall Company, Warwick, Rhode I. The largest suitable silver
casting currently known by the inventors to be available has a
diameter of approximately 114 millimeters, however, larger diameter
silver starting material, required for larger diameter tube demands
in order to achieve a suitable area reduction ratio, can be
fabricated by upsetting the casting. The 114 millimeter diameter
cast silver ingot can be upset in an extrusion press with a 150
millimeter diameter liner to yield an upset ingot with a diameter
on the order of 150 millimeters. "Liner," as used herein, means the
wall of material located on the inside of the extrusion press
between the ram and the die that constrains the metal during
pressing so that it flows through the die. Larger diameter silver
starting materials can be fabricated by upsetting 150 millimeter
diameter ingot in a larger diameter liner. Larger diameter
materials are prepared by sequential upset of the ingots to avoid
buckling.
[0043] Referring to FIG. 1, the silver ingot with an appropriate
radius is an ingot 14. A hole 20 having a diameter approximately
equal to, but slightly larger than, the desired inner diameter of
the tube is bored into ingot 14 along the cylindrical axis of the
ingot 14. The hole 20, can be any diameter consistent with the area
reduction ratio requirements for the tube extrusion. The ingot can
be 30 inches or greater in length.
[0044] The ingot of silver or silver alloy having the hole is tube
extruded in an extrusion press. The ingot is coated with a
lubricant prior to placing the ingot in the extruder. The lubricant
is preferably applied to the ingot prior to inserting the mandrel
into the ingot or prior to inserting the ingot into the extrusion
press. The lubricant can be sprayed or painted on the ingot. The
lubricant preferably includes graphite, grease, or a combination
thereof that does not substantially decompose during the extrusion
process. It is preferred that the lubricant be removed from the
tube after the pressing step.
[0045] Referring to FIG. 2, an extruder 12 includes silver ingot 14
within a liner 16 which contains the ingot during pressing. A
mandrel is inserted through the ingot to produce a tube. Ingot 14
has a hole 20 along the long axis of the ingot. A mandrel 22 is
inserted into the hole 20. Prior to extrusion, ingot 14 is warmed
in a pre-heat step to facilitate extrusion. A ram 24 enters liner
16 and pushes ingot 14 through the extruder. (FIG. 2 shows the
ingot already partly extruded.) Extrusion of ingot 14 into an
extruded tube 31 takes place by forcing or pressing the
ingot/mandrel assembly through an opening 32 in a die 34. The
included angle of die 34 is the angle between a surface 40 and a
surface 41 of the die, which is typically around 45-120 degrees. A
layer of lubricant 30 between mandrel 22 and the surface of hole 20
and between liner 16 and the outer surface of ingot 14 reduces
friction and helps keep extrusion pressure low during
processing.
[0046] Referring to FIG. 3, tube 31 is extruded having hole 20 and
mandrel 22 which defines the inner diameter of the tube. The
mandrel 22, a silver or silver alloy article is obtained without
hole 20.
[0047] Referring to FIG. 2, the diameter of opening 32 determines
the final cross-sectional area and outer diameter of the extruded
tube 31 and the diameter of liner 16 determines the initial
cross-sectional area of the extrusion of ingot 14, the ratio of
which is the area reduction ratio for the extrusion. As mentioned
above, the area reduction ratio relates to the pressure, P, and the
"k" for the process. The difference in the mandrel diameter and the
extrusion die opening diameter corresponds to the tube wall
thickness.
[0048] A follower block 36 is placed at the tail end of ingot 14,
and, in response to ram 24, pushes all of the silver ingot through
the die. The follower block is a relatively inexpensive metal, such
as copper, that increases the yield of silver or silver alloy tube
from the extrusion.
[0049] Typical extrusion conditions suitable for silver or silver
alloy tube extrusion include a 4-9 millimeter per second ram speed,
200 to 450.degree. C. pre-heat of the ingot, preferably 260 to
340.degree. C. It is desirable to rapidly cool the extrudate in
order to maintain the fine grain sizes during the extrusion. The
cooling rate depends on the extrusion temperature and the
cross-sectional area of the extruded article. Cooling rate is
optimized to minimize grain growth. The preferred cooling rate is
to cool the extrudate to below nominally 200.degree. C. and
preferably room temperature within 5 minutes of exiting the
extruder.
[0050] The mandrel is a long rod having a diameter approximately
equal to the inner diameter of the extruded tube. The diameter of
the mandrel is small enough to slide through the hole in the ingot,
but is close enough in diameter to the hole to avoid buckling of
the mandrel during the upset immediately prior to extrusion of the
silver or silver alloy through the die. The difference between the
diameter of the mandrel and the diameter of the hole is between
about 2% and 8% of the diameter of the hole, preferably about 5% of
the diameter of the hole. The mandrel is made of a material that is
hard enough to avoid deformation during extrusion, but is not
brittle enough to break during the process. Examples of suitable
mandrels are hardened steel having a Rockwell C hardness of about
50. For silver or silver alloy tube extrusion, the mandrel is much
harder and much stronger than the silver or silver alloy under the
extrusion conditions.
[0051] As noted above, the diameter of the ingot is approximately
equal to the diameter of the liner, but smaller by about 1% to
about 5%, and preferably about 2.5% of the diameter of the liner.
The diameter of the ingot is small enough to slide into the liner.
If the diameter of the ingot is significantly smaller than the
diameter of the liner, the ingot is inserted into a fitting sleeve
to reduce the difference in diameter. The fitting sleeve is welded
to the ingot prior to extrusion by, for example, tungsten inert gas
welding. The fitting sleeve is usually copper. Under optimized
extrusion conditions, it may not be necessary to use the sleeve to
modify the ingot diameter.
[0052] After extrusion, the silver or silver alloy tube is cooled.
If a fitting sleeve was used, it can be peeled away from the tube
after extrusion. A short section of the tube obtained can be
contaminated with the follower block metal which co-extrudes with
the silver or silver alloy at the end of the extrusion process. The
contaminated region is cut from the end of the silver or silver
alloy tube. A small portion of the inner and outer surfaces of the
tube are machined to ensure removal of the lubricant. Between 0.05
and 0.20 millimeters of material can be removed from each surface
to remove the lubricant, and up to 5 millimeters may be removed to
ensure concentricity of the tube. Less machining is required in
comparison to the tube casting and drawing process since extruded
material does not contain the up to 1 millimeter deep surface
defects described above. Extruded tube 31 can be sectioned to
desired lengths.
[0053] Any diameter die openings can be used for silver or silver
alloy tube extrusion to obtain tubes for other uses so long as the
diameters of the liner and die are consistent with the area
reduction ratio requirements for the extrusion process.
[0054] Extruded silver tube in accordance with the invention
typically has an average grain size of less than 50 micrometers and
is free of surface defects and casting porosity. The average grain
size is typically about 50 to 200 times smaller than the average
grain size of the initial silver casting, which typically has a
minimum grain size of about 300 micrometers and average grain size
of about 1 millimeter.
[0055] The uniform small grain size of the extruded silver tube
allows subsequent silver-superconductor composite processing to
rely on intrinsic silver properties of the tube. Previously,
composite processing relied on the conditions required to process
the defect-laden silver tube stock. The substantially defect-free,
fine grain size, extruded silver tube stock can be used in uniform
drawing processes in high fill factor composites even those with
fill factors over 50% where it will overcome the material flaws in
the drawn, thin-walled silver tube stock.
[0056] Upset of the casting to form larger diameter silver articles
for extrusion is achieved in a press similar to that shown in FIG.
2, where the die is replaced with a solid block. A liner having the
desired diameter is used and the ram press upsets the silver
casting to the diameter of the liner.
[0057] A composite superconducting article can be prepared by, for
example, the well-known powder-in-tube (PIT) process, that
typically include a number of stages. In the PIT process, first, a
powder of a precursor to a superconductor is prepared. The
precursor can be a single material or a mixture of materials.
Second, a metal container (for example, a tube, billet or grooved
sheet) is filled with the precursor powder. The metal container
serves as a matrix, constraining the superconductor. Third, the
filled container is deformed in one or more iterations (with
optional intermediate annealing steps) to reduce the cross
sectional area of the container in a draft reduction step. A number
of filled containers (filaments) can be combined and surrounded by
another metal matrix to form a multifilament article. Finally, the
material is subjected to one or more deformation and phase
conversion heat treatment cycles which together form the desired
oxide superconductor from the precursor and helps the oxide
superconductor grains align and grow to form the textured
superconductor article.
[0058] If the precursor powder is composed of one or more oxides,
the process is known more specifically as oxide-powder-in-tube
(OPIT) processing. See, for example, Rosner, et al., "Status of
superconducting superconductors: Progress in improving transport
critical current densities in superconducting Bi-2223 tapes and
coils" (presented at the conference `Critical Currents in High Tc
Superconductors`, Vienna, Austria, April, 1992), and Sandhage, et
al., "The oxide-powder-in-tube method for producing high current
density BSCCO superconductors", Journal of Metals, Vol. 43, No. 3,
1991, pp. 21-25, all of which are incorporated herein by
reference.
[0059] Methods of preparing BSCCO superconducting materials,
particularly lead-doped BSCCO, are described in U.S. Ser. No.
08/467,033 filed Jun. 6, 1995 and entitled "Processing of (Bi,Pb)
SCCO Superconductor in Wires and Tapes," and U.S. Ser. No.
08/331,184 filed Oct. 28, 1994 and entitled "Production and
Processing of (Bi,Pb) SCCO Superconductors," both of which are
incorporated herein by reference. The composition of the precursor
powder is controlled to improve processing of the precursor
powder.
EXAMPLE
[0060] A length of a continuously-cast silver ingot was obtained
from Handy and Harman. The product is nominally 114 millimeters in
diameter, however, the diameter of the casting actually measured
108 millimeters with an ovality of .+-.0.50 millimeters. The
casting was 293 millimeters long. The ovality was caused by the
"caterpillar track" extraction of the ingot from the continuous
caster process employed in its manufacture. Although metallography
was not performed on the casting, the grains were visible to the
unaided eye on the outer surface and were estimated to have an
average diameter of 2 to 3 millimeters.
[0061] A machining fixture was built to hold the non-round casting
in a lathe chuck so it could be rotated about its centroid for
drilling the tube extrusion hole. A steady rest was used to hold
the other end of the casting. A 20 millimeter diameter hole was
drilled parallel to the long axis of the casting, just over half
way through the casting. The casting was turned around and the
matching hole was drilled from the other side of the casting. The
two holes met within an estimated 0.50 millimeter. A 20 millimeter
ream was used to blend the axial holes, removing most of the "step"
between the two drillings, as well as most of the drill bit scars,
such as scratches and small folds, along the length of the hole.
This hole represents a 3% yield loss of material from the
as-received casting.
[0062] The tube was extruded on a 1300 metric ton press using a
liner having a diameter of 115 millimeters. A rod-shaped steel
mandrel having a diameter of 19 millimeters was used for the tube
extrusion. The extrusion die opening had a diameter of 33
millimeters and a 90 degree included angle. The diameter of the
liner and die opening represent extrusion area reduction ratio of
18.3. The recommended billet (ingot) OD was 111 millimeters, and
because the ingot was under-sized, a copper sleeve was added to the
casting. The casting was machined to a diameter of 109 millimeters
and slid into a copper fitting sleeve with a 109 millimeter ID and
111 millimeter OD. The can was tungsten inert gas (TIG) welded to
the silver without evacuation.
[0063] The billet was pre-heated to 427.degree. C. for 2-3 hours to
ensure uniform heating of the silver. A conservatively high
temperature was used to ensure that the billet would not stall if
the unknown mandrel friction was very high, but lower temperatures
(260-370.degree. C.) could be used. A copper follower was placed
behind the billet and ahead of the mandrel to ensure extrusion of
all of the silver off of the mandrel (the mandrel tip was about 300
millimeters past the die at the end of the extrusion). The ram
speed was 4.2 millimeters per second for this extrusion. The
extrudate was hand-placed into a water quench tank within 30
seconds after the extrusion.
[0064] The extrusion break-through force was 570 metric tons and
the run force was 380 metric tons. The k factor was calculated to
be 190 MPa at the break-through pressure and 130 MPa at the running
flow. The extruded length was about 5.4 meters. The extrudate was
cut in half for handling. The copper fitting sleeve was peeled off
with a pliers. The billet yield of silver tube was high, well over
90%.
[0065] Metallography of the middle section was taken as a
representative sample. Referring to FIGS. 4 and 5, transverse and
longitudinal cross sections have indistinguishable microstructures
under these extrusion conditions. The average grain size was
estimated to be about 15 micrometers, with the largest grains
having diameters of about 50 micrometers. The observed grains
represented a 100-fold reduction in grain size from the initial
silver casting and a 10-fold reduction in grain size over cast and
drawn tube stock. A few large grains (up to 100 micrometers in
diameter) remained immediately at the inner and outer surface.
These are easily and ordinarily removed by machining or etching,
since the extruded product must have 0.05 to 0.2 millimeters of
material removed from inside and outside surfaces to ensure removal
of the lubricant. There was no evidence of porosity or folds in the
middle section of the extruded tube. In addition, the inner surface
of the silver tube appears to be concentric with the outer surface
of the tube; measurements of the wall thickness on the middle
metallography section do not vary by more than 0.25
millimeters.
[0066] The equiaxed longitudinal microstructure indicates that the
larger grains in the bulk of the material can be due in part to the
higher than necessary extrusion temperature and delay in quenching
of the extrudate used in this example. Smaller grain size may be
achieved upon optimization of extrusion conditions by decreasing
the temperature until the break-through pressure reaches about 80
percent of the press capacity, or by increasing the cooling rate
after extrusion. However, the extrusion temperature must not be too
cold, otherwise dynamic reactions can occur. For example,
extrusions that are too cold may result in larger, elongated grains
(unrefined grains) in the extruded product.
[0067] Other embodiments are within the claims.
* * * * *