Note: Descriptions are shown in the official language in which they were submitted.
; by 3 3 (3 J
13DV 7960
-- 1 --
METHOD AND APPARATUS FOX MAKING ALLOY POWDER
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the manufacture of
alloy powder, and, more particularly, to the manufacture
of a superalloy powder characterized by reduced amounts
of impurities.
2. Description of the Prior Art
A wide variety of alloy powder manufacturing
methods and apparatus are well known in the metallurgical
art. As such manufacture relates to high temperature alloys
and superalloy, for example the type based on Fe, Co, Nix
To or their combinations, current powder production methods
include first melting the alloy elements in a high vacuum
furnace chamber through the use of vacuum electron beam,
vacuum arc, vacuum induction or vacuum plasma melting to
produce an ingot. After production of the alloy ingot,
current powder production converts the alloy ingot into
powder by such methods as gas atomization, rotary atomization
and vacuum atomization utilizing ceramic hearth primary
melting in conjunction with a ceramic tundish and nozzle
for producing a liquid metal stream needed to produce
powder.
Certain high temperature operating and highly
stressed components of gas turbine engines, for example,
turbine disks, use powder metal in their manufacture.
By producing a powder metal preform nearly to the final
shape of the component, manufacturing costs can be
~,~333~1t7
13DV 7960
-- 2 --
reduced. However, it has been recognized that inadequate
powder cleanliness, particularly from ceramic particles
introduced in currently used powder manufacturing
processes, can result in a significant reduction in such
mechanical properties as low cycle fatigue in the finished
component. This reduction is due to the presence in
the consolidated powder metal disks of defects which act as
initiation sites for low cycle fatigue failures. Nearly
all superalloy powder metal for such applications
currently are produced by first providing an ingot, melting
the ingot and then making powder by gas atomization
processes. Such atomization processes utilize ceramic
melting and pouring devices and it has been found
that these devices introduce a significant proportion of
the undesirable ceramic inclusions. It should be recognized
that the present invention can be particularly useful
when the starting materials are relatively free of such
ceramic inclusions.
SUMMARY OF THY INVENTION
It is a principal object of the present
invention to provide an improved method for making an
alloy powder in which the melting is conducted without
contact with ceramic members and powder is made directly
from the molten alloy.
Another object is to provide apparatus for
producing an alloy powder, improved through a means to
melt the metallic materials of the alloy out of contact
with ceramic members.
These and other objects and advantages will be
Gore clearly understood from the following detailed
description of the preferred embodiments and the
drawing all of which are intended to be typical of rather
than in any way limiting on the scope of the present
invention.
Briefly, the method of the present invention,
in one form, provides a melting hearth having fluid-cooled
13DV 7960
-- 3
walls and in which is disposed the metallic material
which define an alloy composition. The metallic material
is then melted in the hearth. In one specific embodiment,
a plasma heat source is directed a-t and may be swept over
the metallic material and the hearth -to provide substantially
uniform heat to the metallic material to initiate and
conduct melting of the metallic material. While melting is
conducted, a cooling fluid is provided in the walls of the
hearth sufficient to resolidify melted metallic material
adjacent to the cooled hearth walls. This forms a skull
of metallic material within the hearth a-t the cooled walls
while maintaining additional molten alloy as a molten
metal reservoir within the skull. Then the additional
molten alloy is introduced from the hearth into a powder
metal producer
One form of the apparatus of the present invention
provides, in combination, means to melt the metallic
material comprising a fluid-cooled hearth for receiving
metallic material, a plasma heat source to melt -the
metallic material in the hearth and to provide a molten
metal reservoir, a powder metal producer, and means to
introduce the molten metal prom the reservoir into the
powder metal producer. In one form, the means to melt
the metallic material is a movable plasma heat source
directed toward the hearth and adapted, during operation,
to sweep a surface of metallic material in the hearth to
provide substantially uniform heat to the metallic material.
BRIEF DESCRIPTION OF THE DRAWING
The drawing is a partially sectional, diagrammatic
view of one form of the present invention including an
improved melt chamber and a metallic powder producer.
DESCRIPTION Ox TIE PREFERRED EMBODIMENTS
_ _
The development of modern aircraft gas turbine
engines has defined requirements for higher temperature
operating materials capable of withstanding high stresses.
The complexity of component design and the advances in
33~'~
13DV 7960
- 4 -
powder metallurgy processing and alloy definition have
made the use of powder metal attractive from an economic
manufacturing viewpoint. In addition, powder alloy use
has the capability of achieving desirable properties such
as low cycle fatigue resistance along with high temperature
operating capability.
Typical of such a component requiring very high
strength, high temperature materials are rotating disks
used in the turbine section of modern gas turbine engines.
Other engine components, such as of Tubs alloys,
sometimes are used in the compressor section. However,
in order to achieve desirable low cycle fatigue capability,
it has been recognized that certain types of impurities
must be eliminated from the powder alloy used in such
processing.
It has been observed that a major impurity which
results in defects in such disks is ceramic in nature and
can be traced to initial starting material or the
subsequent processing required to produce powder from
the alloy. The presence of such defects can reduce the
low cycle fatigue capability of such disks below that
required under high temperature and high stress conditions.
For the production of powder metal from
superalloy, for example of the type based on Fe, Co, No
I or their combinations, gas atomization processes are
used with ceramic melting and pouring devices. Such
ceramic structures introduce a significant portion of
the ceramic impurity material which constitutes defects
serving as low cycle fatigue fracture initiation sites
in the finished component manilfactured by powder
metallurgy techniques.
The present invention avoids contact between
ceramic members and the alloy from which the powder is
manufactured by melting metallic material, out of contact
with ceramic members and introducing that molten alloy
into a powder metal producer. In one form, this is
I 37
13DV 7960
5 --
accomplished by the combination of the use of a fluid-
cooled melting hearth and a plasma heat source which may
be movable, in the melt chamber or melting apparatus in
which the materials of the alloy are melted prior to
introduction into a powder metal producer. The fluid-
cooled hearth causes resolidification of molten material in
the hearth about the walls of the hearth. This forms a
hearth skull of metallic materials as a barrier between
material of the hearth and the molten alloy remaining in
the hearth skull.
Use of a movable plasma heat source, such as one
or more movable plasma torches which together define the
plasma heat source, provides rapid and uniform heating
and melting of the materials defining the composition
of the alloy to be made into powder. In addition,
superheating of the molten material to a temperature
sufficient and practical for introduction into a metal
powder producer can be assisted through the use of such
movable, primary plasma heat source which is adapted to
sweep over a surface of the metallic material in the hearth.
One form of the apparatus of the present invention
it shown in the drawing. The improved means to melt the
metallic material in melting chamber 10 includes a fluid-
cooled hearth 12 including walls 13 having fluid-cooling
passages 14 therein connected with a source of cooling
fluid such as water (not shown. As used herein, the
term "wall" or "walls" may include the base or floor as
well as the side walls, as desired, of the member being
described. Melting chamber I can be adapted to enclose
a desired atmosphere or pressure condition for
example by introducing an inert gas such as argon into
inlet 16, to be evacuated through gas outlet 18.
Appropriate other means to control the atmosphere within
melt chamber 10 will be recognized by those skilled in the
art, according to a variety of methods currently used.
Disposed above hearth 12 is a plasma heat source 20 shown
~;333~'~
13DV 7960
in the drawing as a plurality of plasma torches, which
may be movable, directed toward hearth 12. With metallic
material 22 introduced in the hearth 12, plasma heat
source 20 is adapted to initiate and further -the melting
of such materials. When movable, plasma heat source 20 is
adapted to sweep over a surface of the metallic material
and to provide substantially uniform heat to such material.
During the operation of the above-dessribed
improved melting means, metallic material 22, which defines
an alloy composition, is disposed in hearth 12. Such
introduction can be in a batch-type process or can be in a
continuous or semi-continuous process employing a
supplementary metal feed system of a type well known in the
art. For example, a chute and feed mechanism of the
type shown in US. Patent 3,744,943 - Bomberger, Jr. et at
issued July 10, 1973 can be used.
I1~ith cooling fluid such as water circulating
within cooling passages 14, plasma heat source 20 such as
a battery of movable plasma heat torches are placed
in operation. In this embodiment, the torches are moved to
sweep a surface o-E the material 22 in hearth 12 to melt
such material. As molten material contacts the cooled
inner wall of hearth 12, such material resolidifies into
a hearth skull 24 which acts as a barrier or buffer
between the hearth walls and other melted material and
alloy in the hearth. In this way, hearth material is
prohibited from being introduced into the molten alloy
within the hearth and a reservoir of molten alloy is
provided substantially free of foreign materials.
After a desirable level of melting and superheat
is achieved, the hearth is tipped such as about pivot 26
using a tipping means or mechanism represented by arrow 28.
Molten alloy in the hearth, remaining from that material
which is resolidified to form skull 24, is discharged or
poured from the hearth, conveniently from a hearth lip 30
to provide a molten metal stream 32. In the drawing,
~3~33~79
7 - 13DV 7960
according to one form of the present invention, molten
metal stream 32 is poured into a stream control device in
the form of a fluid-cooled trough 34 for supplemental
handling. however, it should be understood that molten
metal stream 32 can be introduced into any of several other
stream control devices of a type apparent to those skilled
in the art or directly into a powder metal producer.
In the form of the invention shown in the
drawing, molten metal stream 32 is introduced into a
stream control device comprising fluid-cooled trough 34
which includes fluid-cooling passages 36 supplied from a
cooling fluid source such as water (snot shown) in a manner
well known in the art. Similar to -the hearth 12, trough
34 can include a lip 38 to assist flow of molten metal
from trough 34.
In operation, trough 34 receives molten alloy in
stream 32 from hearth 12 while cooling fluid is circulated
through cooling passages 36. As the molten metal contacts
the fooled walls of the trough, a portion of the molten
metal solidifies forming a trough skull 40 similar to
hearth skull 24. Skull 40 -functions, in the same manner,
as a barrier or buffer between walls of the trough and
molten alloy maintained in the trough after solidification
of the trough skull. To maintain such additional alloy in
the trough in the molten state, a secondary plasma heat
source such as shown in the drawing as a plasma heat
torch 42 may be desired or required. During operation,
secondary plasma heat source 42 is directed at the additional
molten alloy in the trough remaining from that which has
resolidified as trough skull 40. A stream 44 of molten
alloy flows from trough 34 into a powder metal producer
shown generally at 46 in the drawing. the stream of molten
alloy is converted from the liquid phase to a powder in
the powder metal producer 46.
Such a metal powder producer can be of a variety
3~t7
13DV 7960
-- 8
of types well known in the art, for example atomization
or other disintegration type devices which produce metal
powders. The drawing shows diagrammatically one of -the
gas atomization type which includes a cooling tower I
having a molten metal inlet 50 about which is disposed
an atomizing gas spray means 52 to inject atomizing gas
such as argon, nitrogen, helium, etc., into molten metal
stream 44 entering cooling tower 48 through inlet 50.
Such an atomizing gas is fed through conduit 54 from a
pressurized gas source (not shown). The atomizing
gas thus introduced into the molten alloy stream causes
the stream to disperse into small particles which solidify
and fall to the bottom of cooling tower 48 to be collected
in metal powder collector 56. As shown in the drawing, it
is convenient to include with such a powder metal producer
an exhaust system shown at 58. Conrail the exhaust
system includes a fines or dust collector 60, for example
of the cyclone collector type well known in the art.
If desired, supplemental heat sources can be
used in melting chamber 10, for example directed at hearth
lip 30 or at trough lip 38, or boil. This can assist
molten alloy streams such as 32 and 44 to pour in a
desired molten condition or superheat.
In one example of the evaluation of the
improved melt chamber or means to melt the metallic
material of the present invention, a nickel-base superalloy
by commercially available as Rune 95 alloy and having a
nominal composition, by weight, of Q.06% C, 13% Or, 8% Co,
3.5% Mow 3.5% Cub, 0.05% Or, 2.5% Tip 3.5% Al, 0.01%B,
3.5~ W with the balance No and incidental impurities was
used. In this evaluation, three plasma heat torches as
the primary heat source 20 were focused on a water-cooled
copper melting hearth 12. An additional plasma heat
torch as a secondary plasma heat source 42 can be focused
on a water-cooled copper pouring trough 34, as shown in
the drawing. In other evaluations of melting in hearth 12,
t'
13DV 7960
_ g _
fewer than three torches were used. The hearth heating
torches, as the primary plasma heat source, were movable
in three orthogonal directions; the pouring trough heating
torch or secondary plasma heat source was movable in the
vertical and one horizontal direction. The sides of the
apparatus and the supports for the plasma torches were
protected by heat shields. As a result of several
trial evaluations, it was found that the combination of
a fluid-cooled hearth and a plasma heat source, which
may be movable, alone or in combination with a pouring
trough as a stream control device, can provide an
improved means to melt a metallic material for the
purpose of producing a powder metal and without a
substantial increase of ceramic impurities which can
act as defect sites.
Through -the use of the apparatus of the
present invention, there is provided an improved method
for making an alloy powder, especially one of a hick
temperature alloy or superalloy such as based on Fe, Jo,
Nix To or their mixtures, the method being characterized
by the substantial avoidance of addition of defect-forming
ceramic materials.
This invention has been described in connection
with specific embodiments and examples. However, it will
be readily recognized by those skilled in the art the
various modifications and variations of which the present
invention is capable without departing from its scope
as represented by the appended claims.
