U.S. patent application number 11/945341 was filed with the patent office on 2011-04-28 for methods for centrifugally casting highly reactive titanium metals.
This patent application is currently assigned to GENERAL ELECTRIC COMPANY. Invention is credited to THOMAS JOSEPH KELLY, MICHAEL JAMES WEIMER.
Application Number | 20110094705 11/945341 |
Document ID | / |
Family ID | 40291097 |
Filed Date | 2011-04-28 |
United States Patent
Application |
20110094705 |
Kind Code |
A1 |
KELLY; THOMAS JOSEPH ; et
al. |
April 28, 2011 |
METHODS FOR CENTRIFUGALLY CASTING HIGHLY REACTIVE TITANIUM
METALS
Abstract
Methods for centrifugally casting a highly reactive titanium
metal involving providing a cold wall induction crucible having a
plurality of induction coils and a removable bottom plate, using a
power source to heat a titanium metal charge in the induction
crucible to obtain a molten metal, preheating a secondary crucible
and placing the preheated secondary crucible into a centrifugal
casting machine, positioning the centrifugal casting machine having
the secondary crucible beneath the induction crucible, withdrawing
the bottom plate of the induction crucible and turning off the
power source to the induction crucible to allow the molten metal to
fall from the induction crucible into the secondary crucible, and
accelerating the secondary crucible to centrifugally force the
molten metal into a casting mold to produce a cast component.
Inventors: |
KELLY; THOMAS JOSEPH;
(Cincinnati, OH) ; WEIMER; MICHAEL JAMES;
(Loveland, OH) |
Assignee: |
GENERAL ELECTRIC COMPANY
|
Family ID: |
40291097 |
Appl. No.: |
11/945341 |
Filed: |
November 27, 2007 |
Current U.S.
Class: |
164/493 |
Current CPC
Class: |
B22D 13/00 20130101;
B22D 13/06 20130101 |
Class at
Publication: |
164/493 |
International
Class: |
B22D 13/00 20060101
B22D013/00; B22D 27/02 20060101 B22D027/02 |
Claims
1. A method for centrifugally casting a highly reactive titanium
metal comprising: providing a cold wall induction crucible having a
plurality of induction coils and a removable bottom plate; using a
power source to heat a titanium metal charge in the induction
crucible to obtain a molten metal; preheating a secondary crucible
and placing the preheated secondary crucible into a centrifugal
casting machine; positioning the centrifugal casting machine having
the secondary crucible beneath the induction crucible; withdrawing
the bottom plate of the induction crucible and turning off the
power source to the induction crucible to allow the molten metal to
fall from the induction crucible into the secondary crucible; and
accelerating the secondary crucible to centrifugally force the
molten metal into a casting mold having a facecoat comprising an
oxide selected from the group consisting of scandium oxide, yttrium
oxide, hafnium oxide, a lanthanide series oxide, and combinations
thereof to produce a cast component wherein the secondary crucible
comprises ceramic or a niobium liner.
2. The method of claim 1 wherein the titanium metal charge
comprises a titanium aluminide alloy.
3. The method of claim 1 comprising withdrawing the bottom plate of
the induction crucible using a method selected from the group
consisting of sliding, rotating and dropping.
4. The method of claim 1 wherein the cast component comprises a low
pressure turbine blade.
5. The method of claim 2 comprising using the induction coils of
the induction crucible to heat the metal charge to a temperature of
from about 1480.degree. C. to about 1557.degree. C. to obtain the
molten metal.
6. The method of claim 1 wherein the molten metal becomes suspended
within the induction crucible.
7. The method of claim 2 comprising preheating the secondary
crucible to a temperature of at least about 1000.degree. C. when
the secondary crucible comprises niobium and at least about
1082.degree. C. when the secondary crucible comprises ceramic.
8. The method of claim 1 comprising: keeping the secondary crucible
stationary for from about 0.5 to about 2 seconds after the molten
metal falls into the secondary crucible; and thereafter
accelerating the secondary crucible to from about 100 rpm to about
600 rpm within from about 1 second to about 2 seconds to
centrifugally force the molten metal into the casting mold.
9. (canceled)
10. A method for centrifugally casting a highly reactive titanium
metal comprising: providing a cold wall induction crucible having a
plurality of induction coils and a removable bottom plate; using a
power source to heat a titanium metal charge in the induction
crucible to obtain a molten metal; preheating a secondary crucible
and placing the preheated secondary crucible into a centrifugal
casting machine; positioning a funnel beneath the induction
crucible; positioning the centrifugal casting machine having the
secondary crucible beneath the funnel; withdrawing the bottom plate
of the induction crucible and turning off the power source to the
induction crucible to allow the molten metal to fall from the
induction crucible through the funnel and into the secondary
crucible; and accelerating the secondary crucible to centrifugally
force the molten metal into a casting mold having a facecoat
comprising an oxide selected from the group consisting of scandium
oxide, yttrium oxide, hafnium oxide, a lanthanide series oxide, and
combinations thereof to produce a cast component wherein the
secondary crucible comprises ceramic or a niobium liner.
11. The method of claim 10 wherein the titanium metal charge
comprises a titanium aluminide alloy.
12. The method of claim 10 comprising withdrawing the bottom plate
of the induction crucible using a method selected from the group
consisting of sliding, rotating and dropping.
13. The method of claim 10 wherein the cast component comprises a
low pressure turbine blade.
14. The method of claim 11 comprising using the induction coils of
the induction crucible to heat the metal charge to a temperature of
from about 1480.degree. C. to about 1557.degree. C. to obtain the
molten metal.
15. The method of claim 10 wherein the molten metal becomes
suspended within the induction crucible.
16. The method of claim 11 comprising preheating the secondary
crucible to a temperature of at least about 1000.degree. C. when
the secondary crucible comprises niobium and at least about
1082.degree. C. when the secondary crucible comprises ceramic.
17. The method of claim 10 comprising: keeping the secondary
crucible stationary for from about 0.5 to about 2 seconds after the
molten metal falls into secondary crucible; and thereafter
accelerating the secondary crucible to from about 100 rpm to about
600 rpm within from about 1 second to about 2 seconds to
centrifugally force the molten metal into the casting mold.
18. A method for centrifugally casting a highly reactive titanium
aluminide comprising: providing a cold wall induction crucible
having a plurality of induction coils and a slidably removable
bottom plate; using a power source to heat a titanium aluminide
charge in the induction crucible to obtain a molten titanium
aluminide; preheating a secondary crucible and placing the
preheated secondary crucible into a centrifugal casting machine;
positioning a niobium funnel beneath the induction crucible;
positioning the centrifugal casting machine having the secondary
crucible beneath the niobium funnel; slidably removing the bottom
plate of the induction crucible and turning off the power source to
the induction crucible to allow the molten titanium aluminide to
fall from the induction crucible through the niobium funnel and
into the secondary crucible; keeping the secondary crucible
stationary for from about 0.5 to about 2 seconds after the molten
titanium aluminide falls into secondary crucible; and accelerating
the secondary crucible to from about 100 rpm to about 600 rpm
within from about 1 second to about 2 seconds thereafter to
centrifugally force the molten titanium aluminide into a casting
mold having a facecoat comprising an oxide selected from the group
consisting of scandium oxide, yttrium oxide, hafnium oxide, a
lanthanide series oxide, and combinations thereof to produce a cast
low pressure turbine blade wherein the secondary crucible comprise
ceramic or a niobium liner.
19. The method of claim 18 comprising using the induction coils of
the induction crucible to heat the metal charge to a temperature of
from about 1480.degree. C. to about 1557.degree. C. to obtain the
molten metal.
20. The method of claim 19 comprising preheating the secondary
crucible to a temperature of at least about 1000.degree. C. when
the secondary crucible comprises niobium and at least about
1082.degree. C. when the secondary crucible comprises ceramic.
Description
TECHNICAL FIELD
[0001] Embodiments described herein generally relate to methods for
centrifugally casting highly reactive metals. More particularly,
embodiments herein generally describe methods for centrifugally
casting highly reactive titanium alloys, and in particular,
titanium aluminide alloys.
BACKGROUND OF THE INVENTION
[0002] Turbine engine designers are continuously looking for new
materials with improved properties for reducing engine weight and
obtaining higher engine operating temperatures. Titanium alloys (Ti
alloys), and in particular, titanium aluminide based alloys (TiAl
alloys), possess a promising combination of low-temperature
mechanical properties, such as room temperature ductility and
toughness, as well as high intermediate temperature strength and
creep resistance. For these reasons, TiAl alloys have the potential
to replace nickel-based superalloys, which are currently used to
make numerous turbine engine components.
[0003] Vacuum Arc Re-melting (VAR) is one technique commonly used
to melt Ti alloys. VAR generally involves striking an arc between a
titanium alloy electrode and pieces of the same alloy (electrode
ends, for example) placed in a water-cooled copper crucible. A
molten pool is established and the electrode progressively melts.
When sufficient molten metal is available, the electrode can be
withdrawn and the crucible tilted to pour the metal into a mold for
casting components.
[0004] The VAR technique can have several drawbacks. Titanium
electrodes used in the VAR process can be expensive because of the
high cost of titanium billets/forgings, and the high cost of labor
involved in creating an electrode from certified scrap or revert
material. Also, the requirement for a pre-alloyed electrode can
make it difficult and expensive to produce non-standard alloys.
Furthermore, the need to use a water-cooled crucible can limit the
degree of superheat achievable in the metal, which in turn can
affect fluidity, leading to difficulty in filling thin-wall
castings. Moreover, the highest temperature exists where the arc
strikes the metal, and high temperature gradients exist in the
molten metal. This can also affect the filling of molds and sets up
poor temperature gradients in the solidifying casting.
[0005] In view of the previously described issues with VAR
techniques, another method that can be employed when melting Ti
alloys is Vacuum Induction Melting (VIM). VIM was developed for
processing specialized and exotic alloys that contain reactive
elements, such as titanium and aluminum, which cannot be melted and
cast in air. As the use of such alloys continues to increase, VIM
is consequently becoming more commonplace.
[0006] Vacuum induction melting generally involves heating a metal
in a crucible made from a non-conductive refractory alloy oxide
until the charge of metal within the crucible is melted down to
liquid form. In this technique, pieces of solid titanium alloy are
placed in a cooled metal hearth, usually made of copper, and melted
in an inert atmosphere using a very intense heat source, such as an
arc or plasma. A molten pool will form initially on the interior
and top surface of the charge of titanium while the titanium
adjacent to the confining wall of the copper hearth remains solid.
This "skull" of solid titanium that develops contains the liquid
titanium metal free of contamination. See U.S. Pat. No. 4,654,858,
issued to Rowe for a general discussion of cold wall induction
melting.
[0007] As referenced previously, copper crucibles are most often
employed in cold wall induction melting of highly reactive alloys
for a number of reasons. For example, melting and casting from
ceramic crucibles can introduce significant thermal stress on the
crucible, which can result in the crucible cracking. Such cracking
can reduce crucible life and cause inclusions in the component
being cast. Moreover, the highly reactive TiAl alloys can break
down the ceramic crucible and contaminate the titanium alloy with
both oxygen and the refractory alloy from the oxide. Similarly, if
graphite crucibles are employed, the titanium aluminide can
dissolve large quantities of carbon from the crucible into the
titanium alloy, thereby resulting in contamination. Such
contamination can result in a loss of mechanical properties of the
titanium alloy. Copper is less likely to exhibit the previously
described problems associated with ceramic and graphite crucibles,
which is why copper crucibles are typically employed when using
cold wall induction melting to melt highly reactive metal
alloys.
[0008] However, while cold crucible melting in copper crucibles can
offer metallurgical advantages for the processing of the highly
reactive alloys described previously, it can also have a number of
technical and economic limitations including low superheat, yield
losses due to skull formation and high power requirements. In
particular, the cold wall induction crucible suffers heat loss when
the power to the crucible is terminated and the metal is allowed to
slump against the water-cooled copper sides of the mold.
[0009] One development that has been employed to address the
previously described issues with vacuum induction melting is bottom
pouring from a cold hearth melting system through a nozzle. See
U.S. Pat. Nos. 4,546,858 issued to Rowe and 5,164,097 issued to
Wang et al. The nozzle material typically employed has been copper
or brass, which are considered good thermal conducting materials.
Graphite and thermally insulating materials have also been
mentioned for use as nozzle material.
[0010] While the use of nozzles can provide many benefits over
other common practices, the use of nozzles is not entirely without
the potential for complications. For example, cold hearth melting
and bottom pouring of reactive metals like titanium can result in
undesirable melt freeze-off in the nozzle. In addition, many
crucible/nozzle systems can struggle to provide the requisite
control of liquid flow rate, minimize erosion of the nozzle, and
minimize melt contamination.
[0011] Another development that has been employed to address the
previously described issues with vacuum induction melting is
levitation melting, which generally involves using energy from
induction coils to electromagnetically suspend the metal being
melted. See U.S. Pat. No. 5,275,229, issued to Fishman et al. for a
general discussion of levitation melting. However, while the
magnetic induction field can both heat the metal and hold the
molten metal suspended in space within the crucible, once the power
source for the system is turned off, the metal can slip back into
the water-cooled crucible and chill again before it can be poured.
This can result in incomplete filling of the mold.
[0012] Therefore, in spite of such advances, there remains a need
for improved methods for melting highly reactive metal alloys, such
as TiAl, that allow the alloy to remain molten during pouring, yet
reduces the occurrence of the issues associated with conventional
melting processes.
BRIEF DESCRIPTION OF THE INVENTION
[0013] Embodiments herein generally relate to methods for
centrifugally casting a highly reactive titanium metal comprising
providing a cold wall induction crucible having a plurality of
induction coils and a removable bottom plate, using a power source
to heat a titanium metal charge in the induction crucible to obtain
a molten metal, preheating a secondary crucible and placing the
preheated secondary crucible into a centrifugal casting machine,
positioning the centrifugal casting machine having the secondary
crucible beneath the induction crucible, withdrawing the bottom
plate of the induction crucible and turning off the power source to
the induction crucible to allow the molten metal to fall from the
induction crucible into the secondary crucible, and accelerating
the secondary crucible to centrifugally force the molten metal into
a casting mold to produce a cast component.
[0014] Embodiments herein also generally relate to methods for
centrifugally casting a highly reactive titanium metal comprising
providing a cold wall induction crucible having a plurality of
induction coils and a removable bottom plate, using a power source
to heat a titanium metal charge in the induction crucible to obtain
a molten metal, preheating a secondary crucible and placing the
preheated secondary crucible into a centrifugal casting machine,
positioning a funnel beneath the induction crucible, positioning
the centrifugal casting machine having the secondary crucible
beneath the funnel, withdrawing the bottom plate of the induction
crucible and turning off the power source to the induction crucible
to allow the molten metal to fall from the induction crucible
through the funnel and into the secondary crucible, and
accelerating the secondary crucible to centrifugally force the
molten metal into a casting mold to produce a cast component.
[0015] Embodiments also generally relate to methods for
centrifugally casting a highly reactive titanium aluminide
comprising providing a cold wall induction crucible having a
plurality of induction coils and a slidably removable bottom plate,
using a power source to heat a titanium aluminide charge in the
induction crucible to obtain a molten titanium aluminide,
preheating a secondary crucible and placing the preheated secondary
crucible into a centrifugal casting machine, positioning a niobium
funnel beneath the induction crucible, positioning the centrifugal
casting machine having the secondary crucible beneath the niobium
funnel, slidably removing the bottom plate of the induction
crucible and turning off the power source to the induction crucible
to allow the molten titanium aluminide to fall from the induction
crucible through the niobium funnel and into the secondary
crucible, keeping the secondary crucible stationary for from about
0.5 to about 2 seconds after the molten titanium aluminide falls
into secondary crucible, and accelerating the secondary crucible to
from about 100 rpm to about 600 rpm within from about 1 second to
about 2 seconds thereafter to centrifugally force the molten
titanium aluminide into a casting mold to produce a cast low
pressure turbine blade.
[0016] These and other features, aspects and advantages will become
evident to those skilled in the art from the following
disclosure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a schematic cross-sectional view of one embodiment
of a cold wall induction crucible having a metal charge placed
therein in accordance with the description herein;
[0018] FIG. 2. is a schematic cross-sectional view of one
embodiment of a cold wall induction crucible having the bottom
plate removed and the molten metal suspended therein in accordance
with the description herein;
[0019] FIG. 3. is a schematic cross-sectional view of one
embodiment of a centrifugal casting system in accordance with the
description herein; and
[0020] FIG. 4 is a schematic perspective view of one embodiment of
a component, a low pressure turbine blade, which can be cast in
accordance with the description herein.
DETAILED DESCRIPTION OF THE INVENTION
[0021] Embodiments described herein generally relate to methods for
centrifugally casting highly reactive metals, and in particular
titanium alloys and titanium aluminide alloys, into net shape
components, though the description that follows should not be
limited to such.
[0022] In accordance with the description herein below, a cold wall
induction crucible 10 having a body 12, can be provided, as shown
in FIG. 1. Body 12 can be made from any metal having good thermal
and electrical conductivity, such as, for example copper. Body 12
may be water-cooled to prevent the copper from melting during the
heating of the crucible. More particularly, copper generally melts
at about 1900.degree. F. (about 1038.degree. C.) and TiAl melts at
about 2600.degree. F. (about 1427.degree. C.), and the copper in
the crucible can form a low melting eutectic with the titanium.
Water-cooling the crucible can prevent this from occurring.
Water-cooling inlet 24 and outlet 26 may be used to circulate the
cooling water through a plurality of channels 28 positioned about
body 12. While body 12 may have any shape desired and acceptable
for use in induction melting, in one embodiment, body 12 may be
generally shaped as a hollow cylinder. Body 12 may have a plurality
of induction coils 14 positioned thereabout, which can be heated
using a power source 21. Coils 14 can serve as a heat source to
melt a metal charge placed within the crucible and maintain its
molten state, as described herein below.
[0023] Crucible 10 may also have a removable bottom plate 16 as
shown in FIG. 1. Like crucible 10, bottom plate 16 may comprise any
metal having good thermal and electrical conductivity, and in one
embodiment, may comprise copper. Bottom plate 16 may also be
water-cooled and have a plurality of induction coils 14 positioned
thereunder, again, to help melt the metal charge placed with
crucible 10 and maintain its molten state. Additionally, an
electric insulator plate 19 may circumscribe bottom plate 16 to
help maintain heat in the bottom of crucible 10. As described
herein below, bottom plate 16 may be removed from body 12 in a
variety of manners, including, but not limited to, sliding (as
shown in FIGS. 2 and 3), rotating, dropping and the like.
[0024] In use, a metal charge 18 comprising a highly reactive alloy
may be placed inside body 12 of crucible 10 as shown in FIG. 1. In
one embodiment, metal charge 18 may comprise a titanium alloy, and
more specifically a titanium aluminide alloy, and may take any
acceptable form, which may include, but should not be limited to,
lumps, ingots, granules, plates, powders, and mixtures thereof.
Those skilled in the art will understand that the amount of metal
charge 18 placed into crucible 10 can vary depending on intended
use, however, in one embodiment, from about 1 pound (about 454
grams) to about 3.5 pounds (about 1588 grams), and in another
embodiment from about 1.25 pounds (about 567 grams) to about 3.3
pounds (about 1497 grams) of metal charge 18 can be used to make
net shaped low-pressure turbine blades as described herein
below.
[0025] Once metal charge 18 is placed inside crucible 10, a cover
20, which in one embodiment, may be made from the same material as
crucible 10, may be positioned on top of body 12 and held in place
with a cover ring 22, to ensure crucible 10 is sealed. Power source
21 may be turned on and metal charge 18 can melt when the
appropriate temperature is attained, which in one embodiment may be
from about 2700.degree. F. to about 2835.degree. F. (about
1480.degree. C. to about 1557.degree. C.). Those skilled in the art
will understand that the electromagnetic field generated by the
induction coils causes the metal charge to heat itself internally
due to resistance heating caused by current flow within the metal
charge. As metal charge 18 begins to melt, the resulting molten
metal 30 may become suspended within body 12 of crucible 10 such
that molten metal 30 does not come into contact with the inside of
body 12 as long as the power is being supplied to crucible 10. This
suspension of molten metal 30 can prevent the formation of a
skull.
[0026] Concurrent with melting the metal charge in induction
crucible 10, a secondary crucible 32, or other like holding device,
may be preheated using any acceptable means, such as, but not
limited to, microwave or radiant energy. Secondary crucible may be
made from graphite or ceramic, and may optionally have a metal
liner, such as for example, niobium. Secondary crucible 32 can aid
in the transfer of the molten metal to a casting mold without
losing any of the superheat in the molten metal generated during
induction melting in the induction crucible 10. More specifically,
secondary crucible 32 can be preheated to at least about
1832.degree. F. (about 1000.degree. C.), and in one embodiment from
about 1832.degree. F. to about 2200.degree. F. (1000.degree. C. to
about 1200.degree. C.), when secondary crucible 32 comprises
niobium, and to at least about 1980.degree. F. (about 1082.degree.
C.), and in one embodiment from about 1980.degree. F. to about
2400.degree. F. (1082.degree. C. to about 1316.degree. C.), when
secondary crucible comprises ceramic. Preheating can help prevent
thermal shock and cracking of secondary crucible 32, which would
allow for reuse thereof. Preheated secondary crucible 32 may then
be placed in the rotating arm 34 of a centrifugal casting machine
36 and positioned below induction crucible 10, as shown generally
in FIG. 3. Any conventional centrifugal casting machine is
acceptable for use herein, such as for example, the Linn High-Therm
Titancast 700 (Germany) or the SEIT Supercast (Italy).
[0027] Removable bottom plate 16 may then be withdrawn from body 12
of crucible 10, as previously described. In the embodiments shown
in FIGS. 2 and 3, bottom plate 16 may be slidably removed from
crucible 10 using any acceptable mechanism, such as, but not
limited to, tracks or guides. Although bottom plate 16 is removed,
the electromagnetic field generated by induction coils 14 can
maintain molten metal 30 in a suspended state within body 12 of
crucible 10 as shown in FIG. 2, until further processing.
[0028] When power source 21 is turned off, molten metal 30 is
allowed to fall from induction crucible 10 through a niobium funnel
33 and into preheated secondary crucible 32, which can remain
stationary within casting machine 36 just long enough for molten
metal 30 to complete its transfer into secondary crucible 32, which
in one embodiment may be from about 0.5 to about 2 seconds. Once
the transfer of molten metal 30 is complete, secondary crucible 32
can be rapidly (about 1 to about 2 seconds) accelerated to full
speed, which may be from about 100 rpm to about 600 rpm. Casting
machine 36 can centrifugally force molten metal 30 out of secondary
crucible 32 and into casting mold 38 through port 40, which may
comprise at least one of a slit, hole, tube, or combination
thereof. This quick transfer from secondary crucible 32 to casting
mold 38 results in a contact time between the two of less than
about 5 seconds. This brief contact time not only significantly
reduces heat loss, but also helps ensure that there is no
undesirable reaction between the molten metal and the graphite or
ceramic used to construct secondary crucible 32.
[0029] Casting mold 38 may comprise any ceramic investment casting
system that provides an inert face coat and thermal insulating
backing materials. As an example, in one embodiment, casting mold
38 may comprise a face coat including an oxide. As used herein,
"oxide" refers to a composition selected from the group consisting
of scandium oxide, yttrium oxide, hafnium oxide, a lanthanide
series oxide, and combinations thereof. Furthermore, the lanthanide
series oxide (also known as "rare earth" compositions) may comprise
an oxide selected from the group consisting of lanthanum oxide,
cerium oxide, praseodymium oxide, neodymium oxide, promethium
oxide, samarium oxide, europium oxide, gadolinium oxide, terbium
oxide, dysprosium oxide holmium oxide, erbium oxide, ytterbium
oxide, lutetium oxide, and combinations thereof. Casting mold 38
may comprise a backing including a refractory material selected
from the group consisting of aluminum oxide, zirconium silicate,
silicon dioxide, and combinations thereof, in a colloidal silica
suspension.
[0030] Once the molten metal has been substantially transferred
into casting mold 38, centrifugal casting machine 36 can be turned
off. The resulting component, which in one embodiment may be a low
pressure turbine blade 42, as shown in FIG. 4, can be removed from
casting mold 38 using conventional practices. Because of the use of
centrifugal casting, blade 42 needs little post-cast processing.
The centrifugal forces generated by casting machine 36 provides for
the optimized filling of casting mold 38 by improving the filling
of thin sections of the mold, thereby providing a net shape
component.
[0031] Moreover, because cold wall crucibles are used to melt the
metal charge, there is less thermal stress on the crucible, and
therefore, less crucible cracking. This can allow for both reuse of
the crucible and fewer inclusions in the cast component.
Additionally, since contact between the molten metal and secondary
crucible is limited, there is a reduced likelihood of contamination
of the molten metal from breakdown of the crucible. Less
contamination can result in improved mechanical properties of the
titanium alloy.
[0032] This written description uses examples to disclose the
invention, including the best mode, and also to enable any person
skilled in the art to make and use the invention. The patentable
scope of the invention is defined by the claims, and may include
other examples that occur to those skilled in the art. Such other
examples are intended to be within the scope of the claims if they
have structural elements that do not differ from the literal
language of the claims, or if they include equivalent structural
elements with insubstantial differences from the literal language
of the claims.
* * * * *