U.S. patent application number 13/266918 was filed with the patent office on 2012-02-16 for method and apparatus for manufacturing titanium alloys.
Invention is credited to Michael K. Popper.
Application Number | 20120037330 13/266918 |
Document ID | / |
Family ID | 42236283 |
Filed Date | 2012-02-16 |
United States Patent
Application |
20120037330 |
Kind Code |
A1 |
Popper; Michael K. |
February 16, 2012 |
Method and Apparatus for Manufacturing Titanium Alloys
Abstract
A system and method for producing a metallic ingot using, for
example, a VAR furnace, includes a primary crucible receiving a
melted metal from a source of metal and collecting the melted metal
to for a pool of melted metal, the primary crucible including an
overflow lip, a secondary crucible receiving the melted metal from
the overflow lip of the primary crucible, the secondary crucible
being smaller than and electrically isolated from the primary
crucible, and a withdrawal device withdrawing the molten metal,
solidified by cooling, from the secondary crucible in the form of
solidified ingots, wherein the solidified ingots have a smaller
diameter than a diameter of the source of metal. A cutting device
periodically cuts the withdrawn solidified ingots as they are
withdrawn from the secondary crucible.
Inventors: |
Popper; Michael K.;
(Sewickley, PA) |
Family ID: |
42236283 |
Appl. No.: |
13/266918 |
Filed: |
May 7, 2010 |
PCT Filed: |
May 7, 2010 |
PCT NO: |
PCT/US10/34039 |
371 Date: |
October 28, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61176340 |
May 7, 2009 |
|
|
|
Current U.S.
Class: |
164/460 ;
164/250.1; 164/437; 164/459; 164/469; 164/514 |
Current CPC
Class: |
C22B 9/20 20130101; B22D
11/126 20130101; B22D 11/001 20130101; C22C 14/00 20130101; B22D
21/005 20130101; B22D 11/04 20130101; B22D 11/141 20130101; C22C
1/02 20130101 |
Class at
Publication: |
164/460 ;
164/437; 164/250.1; 164/514; 164/459; 164/469 |
International
Class: |
B22D 11/126 20060101
B22D011/126; B22D 27/02 20060101 B22D027/02; B22D 11/10 20060101
B22D011/10 |
Claims
1. A system for producing a metallic ingot comprising: a primary
crucible receiving a melted metal from a source of metal and
collecting the melted metal to for a pool of melted metal, the
primary crucible including an overflow lip; a secondary crucible
receiving the melted metal from the overflow lip of the primary
crucible, the secondary crucible having heated side walls, being
smaller than and electrically isolated from the primary crucible
and having an opening through which the molten metal may pass as
the metal solidifies into an ingot, the opening having a smaller
diameter than a diameter of the source of metal; and a withdrawal
device withdrawing the melted metal, solidified by cooling, from
the secondary crucible in the form of a solidified ingot
2. The system of claim 1, further comprising a cutting device
periodically cutting the withdrawn solidified ingot as the ingot is
withdrawn from the secondary crucible.
3. The system of claim 1, further comprising a heat source provided
above the secondary crucible for keeping the melted metal at the
top molten.
4. The system of claim 3, wherein the heat source comprises a
non-consumable electrode.
5. The system of claim 1, wherein the source of metal comprises
titanium or a titanium alloy.
6. The system of claim 1, wherein the source of metal comprises a
consumable electrode of the metal.
7. The system of claim 1, wherein the source of metal is melted
using a VAR furnace.
8. A method of manufacturing a metal comprising: melting a source
of metal to form a pool of melted metal in a primary crucible;
transferring the melted metal from the primary crucible to a
secondary crucible, the secondary crucible having sidewalls and
being smaller than and electrically isolated from the primary
crucible; heating the sidewalls of the secondary crucible; allowing
a portion of the melted metal to cool and solidify within the
secondary crucible; and continuously withdrawing the cooled and
solidified metal from the secondary crucible.
9. The method of claim 8, further comprising periodically cutting
the withdrawn cooled and solidified metal as it is withdrawn from
the secondary crucible to form solidified ingots.
10. The method of claim 8, wherein the transferring step comprises
directing the melted metal from the primary crucible via an
overflow lip which allows the melted metal to flow from the primary
crucible to the secondary crucible.
11. The method of claim 8, further comprising heating the secondary
crucible to keep the melted metal at the top molten.
12. The method of claim 8, wherein the source of metal comprises
titanium or a titanium alloy.
13. The method of claim 8, wherein the source of metal comprises a
consumable electrode of the metal.
14. The method of claim 8, wherein the source of metal is melted
using a VAR furnace.
15.-19. (canceled)
20. A method of manufacturing a metal comprising: melting a source
of metal to form a pool of melted metal in a crucible, wherein the
crucible includes a hole formed at a bottom thereof, the hole
defined by sidewalls extended downward from the crucible bottom;
providing a starter piece of metal in the crucible hole, wherein
the starter piece of metal is compositionally the same as the
source of metal; allowing the melted metal to pool in the crucible;
and upon the pooled melted metal reaching approximately 1-2'' in
height in the crucible, withdrawing the starter piece of metal from
the crucible hole, wherein the starter piece of metal has cooled
and solidified metal attached to it.
21. The method of claim 20, further comprising periodically cutting
the withdrawn cooled and solidified metal as it is withdrawn from
the crucible hole to form solidified ingots.
22. The method of claim 20, wherein the source of metal comprises
titanium or a titanium alloy.
23. The method of claim 20, wherein the source of metal comprises a
consumable electrode of the metal.
24. The method of claim 20, wherein the source of metal is melted
using a VAR furnace.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of co-pending
Provisional Patent Application Ser. No. 61/176,340 entitled "Method
and Apparatus for Manufacturing Titanium Alloys", filed on May 7,
2009, the entire disclosure of which is incorporated by reference
herein.
FIELD OF THE INVENTION
[0002] The present invention is directed toward the processing and
manufacture of metals and, more particularly, toward the processing
and manufacture of titanium and titanium alloys.
BACKGROUND OF THE INVENTION
[0003] For over sixty (60) years, titanium and titanium alloys have
been found to be of great use in industry. Initially, such use was
found in aerospace applications, and later in industrial
applications such as the chemical process industry, oil and gas
industry, medical industry, etc. The unique combination of
titanium's strength-to-weight (useful in aerospace applications)
and corrosion resistance (useful in chemical process, oil and gas,
and medical applications) have created a large demand for its use.
As a result, the production of titanium has grown consistently over
the years. Due to advances in the aerospace industry and aircraft
manufacturing techniques over the past ten to fifteen (10-15)
years, the use of titanium products in aerospace applications in
the near future is expected to grow much more rapidly than at any
period in its history.
[0004] Manufacturing methods for making various small parts from
titanium and titanium based alloys are well established, from the
initial melting of the alloy through the final small part
fabrication. Such small parts may include, for example, fasteners,
fittings, and small machined, forged or formed parts. Titanium has
a very diverse use, which varies from medical devices, such as hip
implants, to oil well logging tools. Typically, the titanium
products are produced first by normal ingot forging and other hot
working operations, and then by cold drawing and machining from
titanium mill products, such as billets or bars, or even smaller
mill products, such as hot rolled coil. Hot rolled coils of
titanium are spooled coils of metal where the diameter of the metal
in the coil typically may be from 0.200'' (5.08 mm) up to 0.875''
(22.225 mm) in diameter. This latter category of coil (hot rolled
coil) has become very significant over the past few years as new
generations of commercial and military aircraft employ more
composite materials in their (the aircraft's) fabrication.
[0005] Previously used aluminum fasteners are not compatible with
the increased use of composite materials due to the possibility of
increased corrosion between aluminum and composite (graphite)
materials. This has dramatically increased the already significant
use of titanium fasteners in the material systems for the new
aircrafts. As an example, previous generations of commercial
aircraft, such as the Boeing 737, 747 and 767, as well as other
commercial aircraft, variably used between 20,000 and 40,000 pounds
(9,080 and 18,160 kg) of titanium alloys in their airframes,
including the fasteners. The newer generation aircrafts, such as
the Boeing 787, will use in excess of 200,000 pounds (90,800 kg) of
titanium in their airframes, and this is excluding the engine.
Newer generations of commercial aircraft produced by, for example,
Airbus Industries, also will see a similarly significant increase
in the use of composites and, as a result, will thereby require
large quantities of titanium fasteners. Most of the larger use of
titanium is comprised of various titanium alloy fastener systems to
secure the composite skin of the aircraft to the airframe.
[0006] Current methods for making small diameter titanium and
titanium alloy (e.g., NiTi alloy) parts, as well as some steel,
cobalt or nickel based alloy parts, or even other reactive metal
parts (e.g., zirconium, vanadium, titanium aluminum, etc.) are
generally as described below
[0007] Titanium and titanium alloys are melted using either virgin
titanium sponge and other alloying metals and/or master alloys, or
by using scrap, or by using some combination of virgin materials
and scrap depending upon the demands of the particular application.
There are a number of melting practices that can produce a useable
titanium or reactive metals ingot that can then be processed into a
mill product configuration suitable for the kind of applications
generally discussed above. These melting practices currently
include, for example, Vacuum Arc Melting, Plasma Arc Furnaces or
Electron Beam Furnaces, or Cold Walled Induction Skull Melting
Furnaces.
[0008] Vacuum Arc Melting of elemental titanium sponge and alloying
elements starts with compacted forms of sponge and scrap that are
welded together or otherwise attached and then suspended in a
vacuum chamber and remelted. A typical configuration of a Vacuum
Arc Remelt ("VAR") furnace can be seen in the upper portion of the
attached FIGS. 1-2. The melting occurs when the remelt electrode,
which is composed of virgin titanium or other reactive metals or
scrap or some combination of virgin metal and scrap, is placed
inside a vacuum chamber which also contains a copper crucible as
part of the vacuum chamber. The electrode is lowered into the
copper crucible until it strikes the bottom of the crucible which
is at the opposite electrical potential. This causes an arc between
the crucible bottom, or anode, and the remelt electrode, which is
the cathode in the electrical circuit, which generates sufficient
heat to cause the electrode, which becomes sacrificial, to melt and
to drip molten titanium metal onto the bottom of the crucible, thus
forming a new ingot which is more homogenous than the remelt
electrode from which it derived its raw material. Until this point,
the new "VAR" ingot that is crystallized inside the copper crucible
has always been larger in diameter than the remelt electrode from
which it draws its source metal prior to striking an arc. For
example, see U.S. Publication No. US 2006/0230876 and Zanner,
"Vacuum Arc Remelting--An Overview". This is always true because
the Vacuum Arc Remelting process requires a space, or annulus,
between the wall of the copper crucible and the electrode to be
remelted in order to prevent the to be remelted electrode from
coming into contact with the copper crucible side wall and
injuriously and prematurely arcing there. This may cause the melt
to progress improperly or otherwise operate in a non-controllable
fashion and prematurely end the melt and possibly damage the copper
crucible or even the VAR melt equipment itself. The only proper
location for the arc in the Vacuum Arc Remelt furnace system is off
the bottom of the remelt electrode, just above the crucible bottom
and the pool of the newly melted and crystallized ingot. Thus, the
newly melted ingot may be, for instance, thirty inches (30'') (76.2
cm) in diameter, whereas the source remelt electrode from which it
was formed may only be, for instance, twenty-four inches (24'')
(60.96 cm) or twenty-six inches (26'') (66.04 cm) in diameter.
[0009] This Vacuum Arc Remelting process may be performed several
times in order to progressively cause the new ingot to become more
homogenous and to contain fewer defects, and, in the case of
forging and hot working efficiency, to provide a larger single
ingot of metal. With each sequential remelt in the Vacuum Arc
Remelt furnace, the crystallized ingot that is formed in the copper
crucible becomes larger in diameter for the reason mentioned in the
previous paragraph.
[0010] In addition to Vacuum Arc Remelting, other melt techniques
for producing titanium ingots are to use Plasma Arc furnaces or
Electron Beam Furnaces, or Cold Walled Induction Skull Melting
Furnaces. Generally, although not in all cases, these furnaces are
used to consolidate various raw material forms, such as titanium
sponge, master alloy or different geometries of scrap, so as to
easily produce a remelt electrode that can be subsequently remelted
in a VAR furnace. In some cases for the Plasma and Electron Beam
Furnaces, certain customer and industry specifications allow for
the direct use of as-melted Plasma Arc melted ingots, or Electron
Beam melted ingots. This is typically not now the case, however,
where the final titanium products are used in critical aerospace
applications, such as airframes and fasteners for airframes, and in
the medical products industry. The appropriate industry
applications (specifications) call for multiple vacuum melting
cycles, the last of which must be Vacuum Arc Remelting.
[0011] As a result of the above, titanium, reactive metals and
other industries require VAR melting as the final of multiple melts
in an ingot melting cycle. Progress in the melting of these
products has focused around producing larger VAR ingots, thus
giving the ingot producer and processor more efficient quantities
of metal in one heat (the product of one ingot melted at one time)
for the subsequent production of smaller diameter round products to
be used in products, such as fasteners or small round parts, for
use in, for example, the medical or chemical or oil and gas
industries. Progress has also focused on automating VAR melting,
introducing the use of computers, melt profiles, start up and melt
completion procedures, and related measures to assist with the
production of larger ingots that melt both larger amounts of metal
at one time, and also more complex alloys.
[0012] Complex titanium and other reactive metal alloys exhibit
tendencies to not remain completely homogenous during the melt
cycle, especially as the ingots melted become larger. Thus, the
progress in melt management and scale is partially offset by the
production of a not immediately useful titanium or reactive metal
ingot due to ingot homogeneity problems caused by large melt pools
that have a tendency to see various alloy components segregate upon
the slow cooling that takes place in the center of the larger
ingot. This problem may be overcome by extended solid state heating
and homogenizing of the complex ingot, but it is costly and
requires a significant amount of energy, in the form of natural gas
or electricity, for the heating and homogenizing of the ingot.
[0013] All large titanium and reactive metal ingots experience
multiple hot working and deformation cycles to reduce their
diameter from the as-melted ingot diameter at, for instance, thirty
inches (30'') (76.2 cm) in diameter to the diameter of a hot rolled
coil at, for instance, approximately 0.450'' (11.43 mm) in
diameter, from which an aerospace fastener might be fashioned.
Specifically, an ingot that is thirty inches (30'') (76.2 cm) in
diameter that has been final melted in a VAR will undergo
processing very similar to that described below
[0014] First, the ingot is conditioned by grinding to remove
artifacts of the melt on the surface of the ingot. There is very
significant capital equipment and yield loss involved here. Yield
loss may typically vary from approximately 2% to 5% of the ingot
weight.
[0015] Second, the ingot is heated to a very high temperature, on
the order of approximately 50-70% of the melt point of the alloy,
roughly 1,800-2,200.degree. F. (982.22-1,204.44.degree. C.), and
forged from an as-melted ingot to an intermediate form. The
intermediate form is generally around eighteen inches (18'') (45.72
cm) round, octagonal or square by about four times the length of
the original thirty inch (30'') (76.2 cm) ingot. Significant
capital equipment in the form of very large forging presses or
rolling and cogging mills, and large and multiple heating furnaces
are involved for this step in order to have a product that is ready
for the coil mill rolling operation.
[0016] Third, the intermediate eighteen inch (18'') (45.72 cm)
billet from the parent ingot is conditioned by surface grinding to
remove defects which could cause problems in the subsequent hot
working steps.
[0017] Fourth, the eighteen inch (18'') (45.72 cm) billet is heated
to a high temperature, or red heat, and forged again to about nine
inches (9'') (22.86 cm) round or square, again about a 4:1
reduction of cross-sectional area.
[0018] Fifth, the nine inch (9'') (22.86 cm) billet is conditioned
by grinding to remove deleterious surface imperfections prior to
the next processing step. There is additional yield loss at this
step.
[0019] The sixth operation involves the heating and forging or
rolling of the nine inch (9'') (22.86 cm) billet to a billet
approximately four inches (4'') (10.16 cm) round. This again is a
4:1 reduction of the cross-section of the nine inch (9'') (22.86
cm) billet. Again, heating and forging or roll cogging operations
here are costly and repetitive.
[0020] The seventh operation is the surface conditioning of the
approximately four inch (4'') (10.16 cm) round bar or billet in
order to have its surface in smooth condition as it is processed
into a coil for further subsequent processing into small parts,
such as fasteners.
[0021] The eighth and generally the final hot working operation is
the rolling of the nominally four inch (4'') (10.16 cm) round bar
or billet to a coil that can then be processed by methods other
than hot working methods (e.g., cold working, drawing, and
machining) to produce products such as fasteners and other small
round components.
[0022] It can clearly be seen that each time the ingot is heated,
significant energy is expended. Also, as a result of the various
grinding steps and related conditioning of the ingot and
intermediate forged billets, a large portion of the starting ingot
is lost, or yielded, due to the repetitive oxidizing of the ingot
or billet surface at high temperatures and the necessity of
removing this layer along with any cracking or defects induced in
the multiple of hot working steps. It is generally accepted in the
industry that the loss from an as-melted VAR titanium ingot to the
point at which a four inch (4'') (10.16 cm) round billet is formed
is on the order of 15% of the starting ingot weight. This means
that only 85% of the starting ingot is available to commence the
final hot working cycle, whereby the four inch (4'') (10.16 cm) bar
or billet is converted to a small diameter round or hot rolled
coil.
[0023] The present invention is directed at overcoming one or more
of the above-mentioned problems, and reducing the economic loss
from multiple heating, forging and grinding operations.
SUMMARY OF THE INVENTION
[0024] In one embodiment, a system for producing a metallic ingot
is provided which includes a primary crucible receiving a melted
metal from a source of metal and collecting the melted metal to
form a pool of melted metal, the primary crucible including an
overflow lip, a secondary crucible receiving the melted metal from
the overflow lip of the primary crucible, the secondary crucible
being smaller than and electrically isolated from the primary
crucible, and a withdrawal device withdrawing the melted metal,
solidified by cooling, from the secondary crucible in the form of
solidified ingots, wherein the solidified ingots have a smaller
diameter than a diameter of the source of metal. A cutting device
periodically cuts the withdrawn solidified ingots as they are
withdrawn from the secondary crucible.
[0025] The system may include a heat source provided above the
secondary crucible for keeping the melted metal at the top molten.
In one form this heat source includes a non-consumable
electrode.
[0026] The source of metal may include titanium or a titanium alloy
or other reactive metal. In one form, the source of metal includes
a consumable electrode of the metal which is melted using a VAR
furnace.
[0027] In another embodiment, a method of manufacturing a metal is
provided, which includes the steps of melting a source of metal to
form a pool of melted metal in a primary crucible, transferring the
melted metal from the primary crucible to a secondary crucible, the
secondary crucible being smaller than and electrically isolated
from the primary crucible, cooling and solidifying the melted
metal, and withdrawing the cooled and solidified metal from the
secondary crucible. The withdrawn cooled and solidified metal is
then periodically cut as it is withdrawn from the secondary
crucible to form solidified ingots.
[0028] The transferring step may include directing the melted metal
from the primary crucible via an overflow lip formed in the primary
crucible. As the primary crucible is filled with the melted metal,
the overflow lip directs the flow from the primary crucible to the
secondary crucible. Preferably, the secondary crucible is heated to
keep the melted metal at the top molten. In one form, this heating
is accomplished using a non-consumable electrode.
[0029] The source of metal may include titanium or a titanium alloy
or other reactive metal. In one form, the source of metal includes
a consumable electrode of the metal which is melted using a VAR
furnace.
[0030] In a further embodiment, a system for producing a metallic
ingot is provided including a crucible receiving a melted metal
from a source of metal and collecting the melted metal to form a
pool of melted metal, wherein the crucible includes a hole formed
the bottom thereof, the hole defined by sidewalls extended downward
from the crucible bottom, and a withdrawal device withdrawing the
melted metal, solidified by cooling, from the crucible hole in the
form of solidified ingots, wherein the solidified ingots have a
smaller diameter than a diameter of the source of metal. A cutting
device is provided which periodically cuts the withdrawn solidified
ingots as they are withdrawn from the crucible hole.
[0031] In yet a further embodiment, a method of manufacturing a
metal is provided including the steps of melting a source of metal
to form a pool of melted metal in a crucible, wherein the crucible
includes a hole formed the bottom thereof, the hole defined by
sidewalls extended downward from the crucible bottom, providing a
starter piece of metal in the crucible hole, wherein the starter
piece of metal is compositionally the same as the source of metal,
allowing the melted metal to pool in the crucible, and upon the
pooled melted metal reaching approximately one to four inches
(1-4'') (2.54-10.16 cm) in height in the crucible, withdrawing the
starter piece of metal from the crucible hole, wherein the starter
piece of metal has cooled and solidified metal attached to it. The
withdrawn cooled and solidified metal is periodically cut as it is
withdrawn from the crucible hole to form solidified ingots.
[0032] It is an object of the present invention to continuously
produce metal ingots having a diameter suitable for coil, bar or
rod rolling while allowing the melt to continue uninterrupted.
[0033] It is a further object of the present invention to minimize
the number of melts required to produce metal ingots having a
diameter suitable for coil, bar or rod rolling while still honoring
industry specifications calling for VAR melting as the final melt
in a multiple melt cycle.
[0034] Other objects, aspects and advantages of the present
invention can be obtained from a study of the specification, the
drawings, and the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] FIG. 1 illustrates a first embodiment of the present
invention incorporated into a VAR furnace; and
[0036] FIG. 2 illustrates a second embodiment of the present
invention incorporated into a VAR furnace.
DETAILED DESCRIPTION OF THE INVENTION
[0037] The present invention relates to methods and apparatus for
producing smaller cross-sections (diameters) of as-melted ingots
from a larger input remelt electrode by, for example, VAR melting
an ingot that can subsequently be processed directly, or with a
maximum of one additional hot working cycle, into smaller mill
products of titanium and other metal products. These smaller
cross-sectional or smaller round ingots will be continuously
withdrawn from a crystallization crucible or mold and continuously
removed in order to allow the melt, which is being fed by a larger
cross-section remelt electrode or ingot, to continue to progress
uninterrupted. These smaller ingots are melted from larger remelt
electrodes in, for example, a conventional VAR furnace and
progressively withdrawn from the copper crucible, or a companion
copper crucible, as the ingot crystallizes. This allows for the
continued use of larger lot sizes (melt sizes) while producing a
final melt ingot that is closer to the input diameter for coil
rolling or bar or rod rolling mills. This also completely honors
all industry standards and specifications calling for VAR melting
as the "final" melt in a multiple melt cycle to produce a final
ingot. The inventive process produces a very large energy savings,
capital equipment savings, and yield savings of the titanium (or
other alloy ingots) processed in this manner.
[0038] The present invention allows for the production by VAR
melting and continuous withdrawal of an ingot smaller in diameter
than the input remelt electrode/ingot from which it was melted.
[0039] The present method and apparatus can produce, for example, a
four inch (4'') (10.16 cm) round (or similar diameter or size bar
or billet) that is ready to be coiled directly. The present method
and apparatus utilizes less than 30% of the energy that
conventional VAR ingots normally utilize in order to be processed
from a larger ingot into a shape that is ready for coiling. The
present invention also provides for these smaller diameter ingots
to be produced by continuously withdrawing the ingot from the
crystallization chamber by unique means as described below.
[0040] In the patent literature, the term "continuous casting" was
used to mean that the process of melting itself progressed
continuously from the beginning to the end. This is self
evident.
[0041] In the present inventive method and apparatus "continuous
casting" as discussed herein, means the continuous melting,
casting, and withdrawing of an ingot from its crystallization mold
as the melt progresses. This continual production, withdrawal and
removal of an ingot in the context of VAR melting are completely
new and previously unpracticed. It does not mean or is meant to
mean the continuous casting and filling of a static melt
crystallization crucible (or other cooling chamber or receptacle)
as is now the established practice in VAR melting. In U.S. Pat. No.
5,103,458, the terms "withdrawal of the ingot" and "withdrawing the
ingot from the mold" are used. To be clear, this is a reference to
the standard technique of completing a VAR melt cycle and
"withdrawing" or removing the ingot from the melt apparatus once
the melt process is complete and an ingot has been formed in the
melt crucible. Conversely, the present invention provides for the
continuous withdrawal and removal of the as-melted smaller diameter
ingot while the melt process is continuing.
[0042] The present invention also provides for an as-melted ingot
to be produced that may be then introduced, with a minimum of
handling or conditioning, and with potentially no additional hot
working cycles, to a continuous coil or bar rolling mill that will
reduce the input approximately four inches (4'') (10.16 cm) in
diameter to a small cross-section hot rolled coil or multiple
straight strands of product.
[0043] The present invention also provides for a removal method
from the bottom of the copper crucible or from the side of the
copper crucible that may be controlled via a logic loop between a
computer and various controllable aspects and parameters of the
ingot melt furnace.
[0044] The present invention also provides for parts of the removal
apparatus that are new and unique to VAR melting and that enable
the small diameter ingot to be easily and continuously removed from
the molten melt pool without undue difficulty, and which produces a
high quality as-melted ingot with minimal surface perturbations and
defect injuries to the subsequent coil or small round rolling
cycle.
[0045] The present invention and description of continuous ingot
removal methods from the liquid melt may be processed in the
substantially vertical plane or the substantially horizontal plane.
These examples discussed herein are in the substantially vertical
plane.
[0046] The present invention provides for the intermittent cutting
and or removal of the as-cast vertical or horizontal continuously
melted and withdrawn ingot under vacuum, and while the melt
continues.
[0047] The present method and apparatus are new and unique in that
prior to the present invention, a VAR melted ingot has never been
removed continuously from the melt crystallizer while the electrode
to be remelted is arcing, dripping molten metal into a crystallizer
mold and thereby remelted under vacuum.
[0048] As can be seen in the upper portion of the attached FIG. 1,
the current state of the art for VAR melting provides for the
production of an as-melted ingot larger than the diameter of the
sacrificial remelt electrode/ingot. Those knowledgeable in the art
will note that virtually as soon as molten titanium comes into
contact with the copper walls of a crystallization chamber, whether
it is a crucible in VAR melting or a withdrawal mold in Plasma
and/or Electron Beam melting, the surface of the molten titanium in
contact with the copper crucible immediately forms a meniscus or
thin layer of solidified titanium metal. The progressive growth of
this meniscus and additional layers of solidified titanium next to
the meniscus at the contact zone of the molten metal with the
crucible wall presents a large impediment to the facile withdrawal
of the ingot from the crystallization container, whether it is a
VAR crucible or a withdrawal mold as practiced in Electron Beam,
Plasma or other forms of reactive metals melting. Solidification
conditions in this case (in the area of the meniscus) are quite
sensitive to the details in the contact zone (see Zanner, "Vacuum
Arc Remelting--An Overview"). The contact zone between the ingot
and copper crystallization mold or crucible then becomes the single
most important focus for a successful withdrawal technique. Failure
to properly address the issue of meniscus formation and
solidification of the reactive (or any metal or alloy) metal will
cause the ingot to become lodged in place or otherwise immoveable.
Alternatively, if the smaller diameter ingot being withdrawn is
moveable, the progressive withdrawal of a useable ingot may be
inhibited by unacceptable tearing of the ingot surface as the
withdrawal puller exerts axial tensile forces on the smaller
diameter ingot to remove it from the melt pool.
[0049] There are then two primary issues to be overcome in order to
successfully withdraw a smaller diameter ingot from a larger
diameter input VAR remelt electrode. The first is to achieve a
steady state of withdrawal that overcomes the issues noted above
regarding the formation of a meniscus and dealing with the issues
that arise as a result of the meniscus.
[0050] The second is to maintain sufficient heat on the surface of
the smaller ingot being withdrawn so that the edges and top of the
smaller ingot closest to the side walls of the withdrawal crucible
do not prematurely solidify and allow for additional hot metal to
flow over this solidified area, thus forming what are known as
"shuts" or "cold shuts". These cold shuts need to be removed after
melting in order to have a useable ingot diameter that will not
crack, split or otherwise fail during subsequent hot rolling to
rod, coil or bar operations. The needed removal of these cold
shuts, however, reduces the useable diameter of the ingot
significantly. For instance, if a cold shut were to migrate 0.25''
(6.35 mm) in from the edge of a four inch (4'') (10.16 cm) round
ingot (i.e., off the radius), then 0.50'' (12.7 mm) of the four
inch (4'') (10.16 cm) diameter ingot would require removal from the
diameter in order to have a smooth defect free bar or billet that
is ready for the hot rod, bar or coiling operation. This would
result in a yield loss of the three and one-half inch (3.5'') (8.89
cm) bar divided by the diameter of the four inch (4'') (10.16 cm)
diameter as-melted ingot, or 9.616 in.sup.2/12.56 in.sup.2 (62.504
cm.sup.2/81.64 cm.sup.2), or a 23.44% yield loss at this stage of
the melting alone. From an economic standpoint, this is
unacceptable. For this reason, if the smaller diameter ingot is
removed as discussed in the method below, the surface of the ingot
in the withdrawal crucible needs to receive additional heating
sufficient to minimize the edge effects of cold shuts.
[0051] Additional heating to the top of the smaller ingot being
withdrawn may come from a number of sources. They potentially are:
[0052] A plasma torch that could heat the top of the smaller
diameter ingot in the withdrawal mold. [0053] An Electron Beam Gun
that could heat the top of the smaller diameter ingot being
withdrawn from the mold. [0054] The cold wall Induction Crucible
could be used as the withdrawal crucible. This cold wall Induction
Crucible would be heated by an induction coil on the outside so as
to cause the smaller diameter ingot inside the crucible to remain
hot and/or liquid until a point where all edge effects or cold
shuts will not possibly form and affect ingot quality. [0055] A
non-consumable copper electrode that could provide electrical
heating via an arc directly applied onto the top of the smaller
diameter ingot being withdrawn, delaying its solidification near
the top and allowing for a more uniform defect free ingot as it is
withdrawn.
[0056] Prior to acceptance or rejection of any of the above
techniques, it is important to understand the basic nature of VAR
melting. VAR melting takes place, in practical operation, when the
remelt electrode, or the ingot to be remelted, is lowered to short
itself (note that the bottom of the copper crucible is the anode in
the circuit and the remelt electrode is the cathode in the circuit
from which the melting arc is created) either against the bottom of
the copper crucible or upon metal that is of a similar composition
to the metal being remelted, be it a "strike plate" of similar
composition, or a small pile or agglomeration of particles of a
like composition metal that has been placed on the bottom of the
copper crucible. Once the aforementioned short takes place, the
remelt electrode is immediately withdrawn a short distance to
create a gap across which the short is replaced by a melting arc
that is generated as long as the gap is not too large. Typically,
the gap will be on the order of 6-12 mm (0.24-0.48''). Ideally, the
melting arc generates a uniform plasma by Joule heating (see
Zanner, "Vacuum Arc Remelting--An Overview"). This arc and/or
plasma is highly susceptible to stray or unintended magnetic fields
that may disrupt, disturb or otherwise create large
non-uniformities of the arc, the metal pool formed under it, and/or
the uniform process of the melt. So care must be exercised when
evaluating a heating technique for the top of the small ingot being
withdrawn so that the technique does not generate strong magnetic
fields or currents that would interfere with the uniformity of the
primary arc melting taking place under the remelt electrode and
between that electrode and the crucible bottom or molten pool that
has formed on the crucible bottom.
[0057] Since meniscus formation and associated crystallization are
completely unavoidable in the copper crucible ingot formation
system, the success of the withdrawal technique becomes dependent
upon incorporating and minimizing those effects, and building a
model and process control capability for ingot withdrawal that
anticipates them. The model should anticipate: [0058] Meniscus
tearing from the axial tensile ingot removal forces. [0059]
Minimizing the amount of the meniscus tearing on each progressive
axial pull. [0060] Heating of the top of the small ingot to be
withdrawn continuously from the melt in a manner sufficient to
maintain the meniscus sufficiently thin to be manipulated as
necessary for the formation and withdrawal of the small ingot.
[0061] This model must also anticipate that the ingot must be
withdrawn incrementally, regularly and progressively in such a
manner that only minimally overcomes the strength of the meniscus
and associated crystallized metal build-up in the copper crucible
contact area. The withdrawing of the ingot should disrupt the
meniscus only enough for the ingot to be pulled down a small
increment, allowing for the formation of a new meniscus immediately
as molten metal from the pool flows to meet the cold copper
crucible wall where the tear occurred, as a result of the tearing
and downward removal of the ingot. The withdrawing needs to be done
in small increments and repeated with a frequency sufficient to
satisfy the model criteria.
[0062] With the foregoing discussion of VAR melting, contact zone
issues, meniscus formation and metal crystallization, and small
ingot heating requirements provided as infrastructure for the
presentation of the embodiments of the present invention, two
specific embodiments are proposed as definitive methods for
implementing the present invention. However, these embodiments are
for illustrative purposes only and are not meant to be limiting.
One skilled in the art will appreciate that other embodiments can
be implemented that fall within the spirit and scope of the present
invention.
[0063] The first embodiment is shown in FIG. 1, in which the upper
portion depicts a conventional VAR furnace having an electrode feed
drive 1, a furnace chamber 2, a melting power supply 3,
bussbars/cables 4, an electrode ram 5, a water jacket 6, a vacuum
suction port 7, an X-Y axis adjustment 8, and a load cell system 9.
The VAR furnace operates in conventional format to melt the remelt
electrode or ingot. The first embodiment contemplates the
withdrawal of an ingot smaller than the to be remelted feed ingot
or electrode by overflowing the VAR copper crucible, shown at 10 in
FIG. 1. The copper crucible 10 is preferably a shallow water cooled
crucible. Metal melts off of the electrode 12 and drips into the
shallow crucible 10. The copper crucible 10 includes a spout 14
that directs the molten metal from the primary crucible 10 to a
secondary, smaller copper crucible 16. Heat from the arc keeps the
metal molten until it fills the crucible 10 and spills out the
spout 14 and is collected into the secondary crucible 16.
Preferably, the secondary crucible 16 is a collar chill crucible
and has a withdrawal mechanism in the bottom of it. The smaller,
secondary crucible 16 needs to have the following
characteristics.
[0064] The secondary crucible 16 should be close enough to the
primary VAR copper crucible 10 so that metal flows or falls easily
into it. The secondary crucible 16 should also be electrically
isolated from the primary copper crucible 10. Previous methods of
withdrawal ingot technology for both Electron Beam and Plasma
Melting do not provide for electrical isolation of any smaller
crucible from which any smaller ingot is withdrawn. In these
previous cases, the withdrawal crucible and feed crucible or hearth
have been made of either a single integral manufacture or, if not,
have been connected with metal connectors in such a fashion as to
provide a single electrical circuit for the complete assembly. In
the case of VAR melting, the consideration of electrical isolation
of a smaller crucible from a larger feed crucible had not been
addressed in any previous literature or embodied in any previous
design. Electrical isolation is critical in VAR melting due to the
requirement for maintaining a completely different arc heating
power level for the smaller withdrawal crucible than the primary
melt crucible. Failure to provide a separate circuit would allow
the smaller crucible to become part of the larger melt crucible
electrical ground, and thus the arc path of the larger VAR remelt
crucible circuit, thereby interfering with the lower power arc
required for the smaller withdrawal crucible.
[0065] As an example, the primary Vacuum Arc Remelt electrode may
be between sixteen inches (16'') (40.64 cm) and twenty-six inches
(26'') (66.04 cm) in diameter, and would then experience a melt
current, I arc, on the order of 14KA to 26KA with a voltage, V, of
approximately 26-40 volts. The secondary, smaller withdrawal
crucible would require a current, I arc, for heating the top of the
ingot of between 4KA and 8 KA, and a voltage, V, of between 24 and
30 volts. It is critical that these different electrical demands
and requirements remain independently controllable and isolated.
Hence the reason for electrical isolation of the primary VAR
crucible 10, and the smaller, withdrawal crucible 16.
[0066] The collection and withdrawal crucible 16 for the smaller
ingot requires an additional source of heating energy to prevent
the undue formation of cold shuts (as discussed above) on the new
smaller ingot as it is being formed and withdrawn. An evaluation of
all of the energy sources available (including Plasma Arc, Electron
Beam, Cold Wall Induction Heating, and Vacuum Arc Heating)
determined that an additional Vacuum Arc source of heat, on a
different electrical circuit so as to be independently controlled
from the primary Vacuum Arc in the larger remelt crucible, would
solve the problem of secondary heating and also meet the following
three primary criteria for secondary heating of a withdrawal
crucible: [0067] 1) That the secondary heating arc generate
minimal, little or no magnetic field interference with the nearby
primary arc used for the melt down of the larger remelt electrode
in the primary Vacuum Arc Remelt crucible. [0068] 2) That the
secondary heating arc provide sufficient heating to the surface of
the smaller, to be withdrawn ingot so as to eliminate premature
cooling and freezing of the surface of the smaller ingot, thus
eliminating or reducing cold shuts or other deleterious
imperfections on the surface of the withdrawn ingot to an
acceptable level. [0069] 3) That the secondary heating arc meet the
letter and spirit of all relevant specifications that demand Vacuum
Arc Remelting as the final melt in a series of melts or melt
cycles.
[0070] These requirements can be met, for example, with a small,
non consumable water cooled copper electrode 18 that arcs in a
fashion essentially similar to the arc from the primary melt down
arc, but is not consumed and operates at a lower power only
sufficient to heat the top of the smaller electrode as discussed.
However, one skilled in the art will appreciate that other heat
sources may be used above the secondary crucible 16 without
departing from the spirit and scope of the present invention.
[0071] Then, with the provisions made for a first embodiment of the
present invention, the following elements are present: [0072] A
primary copper crucible 10 which operates in a conventional VAR
fashion by collecting the remelted metal that drips off of the
remelt electrode 12, thereby forming a pool of melted metal. [0073]
An overflow lip 14 in the primary copper crucible 10 that allows
metal from the pool to drip or flow out of the crucible 10 and into
a smaller collection crucible 16 that is part of a completely
separate electrical circuit (apart from the main crucible). [0074]
A smaller collection crucible 16 that collects the molten metal
flowing from the VAR main crucible 10 and forms it into an ingot of
smaller diameter that is subsequently withdrawn to form a smaller
diameter ingot. [0075] A non-consumable water cooler copper
electrode 18 that arcs to the top of the small ingot being
withdrawn so as to keep the top molten and free of shuts or
problems that may reduce its economical use.
[0076] A programmable logic controller ("PLC") automatic control
loop may be provided that causes the withdrawal rate of the smaller
ingot, the melt rate and power levels of the larger remelt ingot
and the smaller withdrawal ingot, and the power levels of the
auxiliary heating arc on the smaller remelt ingot, all to be
integrated. This is done to ensure that the rate at which the
smaller withdrawal ingot is being removed from the smaller copper
crystallization crucible/mold 16 is equal to the rate at which the
molten metal is dripping off the larger remelt electrode 12, into
the pool of the large copper crucible 10, over the lip 14 and into
the smaller withdrawal crucible 16. This ensure continuous
operation and withdrawal of the smaller diameter ingot.
[0077] A withdrawal device, show at 20, is provided at the bottom
of the smaller crucible 16 and withdraws and removes the ingot from
the small crystallization crucible or mold 16 continually during
the conduct of the melt in the direction of the arrow. The
withdrawal device 20 withdraws the ingot out of the smaller
crucible 16 in a regular progressive fashion. A cutting device 22
is provided which periodically cuts the ingot and moves it aside so
that the entire process can continue uninterrupted.
[0078] A second embodiment of the present invention is shown in
FIG. 2 with like elements from FIG. 1 indicated with the same
reference number and those elements requiring modification
indicated with a prime. The second embodiment approaches the
withdrawal of a smaller ingot from a larger VAR melt crucible from
a more straightforward method. The upper portion of FIG. 2 again
depicts a conventional VAR furnace having an electrode feed drive
1, a furnace chamber 2, a melting power supply 3, bussbars/cables
4, an electrode ram 5, a water jacket 6, a vacuum suction port 7,
an X-Y axis adjustment 8, and a load cell system 9. The VAR furnace
operates in conventional format to melt the remelt electrode or
ingot. In the second embodiment, the withdrawal device 20' is
situated in the bottom of the primary VAR remelt crucible 10'. A
secondary, smaller, electrically isolated crucible as described in
the first embodiment is not required. In the second embodiment, the
withdrawal device 20' for the smaller ingot is built into the
primary VAR crucible 10'. The VAR crucible 10' then acts as its own
tundish, filling the withdrawal ingot from the molten metal
directly above it and directly underneath the melt pool formed by
the dripping metal off the remelt electrode 12.
[0079] As an example, if the bottom of the VAR crucible is round
and is approximately twenty-four inches (24'') (60.96 cm) in
diameter, a four inch (4'') (10.16 cm) hole in the bottom of the
crucible 10' could be fabricated. The four inch (4'') (10.16 cm)
hole would extend downward, out of the bottom of the crucible 10'
and for some distance, as shown at 24. The sides of the withdrawal
hole 24 are preferably copper, as would be expected from a standard
withdrawal crucible.
[0080] At the commencement of a VAR melt cycle, a starter piece of
titanium or metal that is compositionally the same as the to be
remelted ingot would be inserted into the four inch (4'') (10.16
cm) hole and fixed to a pulling device 20' that is capable of
withdrawing the smaller ingot out of the bottom of the VAR crucible
10' in the direction of the arrow as it is filled or built up from
the melt pool directly overhead. The VAR melt is then initiated,
and molten drops of metal will commence accumulating against the
bottom of the crucible 10' and forming the previously discussed
meniscus. The molten drops of the metal would also fall onto the
starter bar in the ingot puller and initially solidify. However, as
is the case with titanium, the heat transfer of titanium is so low
that there would, with the continuation of the arc current from
immediately above the puller, soon be some melting on the face of
the puller bar. The melt would then proceed until approximately one
to four inches (1-4'') (2.54-10.16 cm) of molten metal fills the
bottom of the VAR crucible 10'. At this point, the pulling of the
small diameter ingot would then commence, thus withdrawing the
ingot from the bottom of the crucible 10'. As previously described
with respect to the first embodiment, a cutting device 22 is
provided which periodically cuts the ingot and moves it aside to
that the entire process can continue uninterrupted.
[0081] While the present invention has been described herein with
particular reference to the drawings, it should be understood that
various modifications could be made without departing from the
spirit and scope of the present invention. Those skilled in the art
will appreciate that various other modifications and alterations
could be developed in light of the overall teachings of the
disclosure. The presently preferred embodiments described herein
are meant to be illustrative only and not limiting as to the scope
of the invention, which is to be given the full breadth of the
appended claims and any and all equivalents thereof.
* * * * *