Apparatus and process for controlling the flow of a metal stream

An apparatus that controls the flow of a stream of metal, such as produced from the bottom of a hearth, includes a cylindrical metallic nozzle body having a hollow wall which includes a slit extending substantially parallel to the axis of the cylinder so that there is no electrical continuity around the nozzle wall across the slit. The walls of the cylinder are preferably formed of hollow tubes through which cooling water is passed. A sensor senses a performance characteristic of the apparatus, such as the temperature of the nozzle body. An induction heating coil surrounds the nozzle body, and a controllable induction heating power supply is connected to the induction heating coil to provide power. A controller controls the power provided to the induction heating coil by the induction heating power supply responsive to an output signal of the sensor, so that a selected performance characteristic of the apparatus may be maintained.

BACKGROUND OF THE INVENTION 
This invention relates to metallurgical technology, and, more particularly, 
to controlling the flow of a stream of molten metal. 
Metallic articles can be fabricated in any of several ways, one of which is 
metal powder processing. In this approach, fine powder particles of the 
metallic alloy of interest are first formed. Then the proper quantity of 
the particulate or powdered metal is placed into a mold or container and 
compacted by hot or cold isostatic pressing, extrusion, or other means. 
This powder metallurgical approach has the important advantage that the 
microstructure of the product produced by powder consolidation is 
typically finer and more uniform than that produced by conventional 
techniques. In some instances the final product can be produced to 
virtually its final shape, so that little or no final machining is 
required. Final machining is expensive and wasteful of the alloying 
materials, and therefore the powder approach to article fabrication is 
often less expensive than conventional techniques. 
The prerequisite to the use of powder fabrication technology is the ability 
to produce a "clean" powder of the required alloy composition on a 
commercial scale. (The term "clean" refers to a low level of particles of 
foreign matter in the metal.) Numerous techniques have been devised for 
powder production. In one common approach, a melt of the alloy of interest 
is formed, and a continuous stream of the alloy is produced from the melt. 
The stream is atomized by a gas jet or a spinning disk, producing 
solidified particles that are collected and graded for size. Particles 
that meet the size specifications are retained, and those that do not are 
remelted. The present invention finds application in the formation and 
control of the stream of metal that is drawn from the melt and directed to 
the atomization stage. More generally, it finds application in the 
formation and control of metal streams for use in other clean-metal 
production techniques. 
The alloys of titanium are of particular interest in powder processing of 
aerospace components. These alloys are strong at low and intermediate 
temperatures, and much lighter than cobalt and nickel alloys that are used 
for higher temperature applications. However, molten titanium alloys are 
highly reactive with other materials, and can therefore be easily 
contaminated as they are melted and directed as a stream toward the 
atomization stage unless particular care is taken to avoid contamination. 
Several approaches have been devised for the melting and formation of a 
stream of a reactive alloy such as a titanium alloy. In one such approach, 
the alloy is melted in a cold hearth by induction heating. The alloy 
stream is extracted through the bottom of the hearth and directed toward 
the atomization apparatus. The stream may be directed simply by allowing 
it to free fall under the influence of gravity. To prevent excessive 
cooling of the stream as it falls, electrical resistance heating coils 
have been placed around a ceramic nozzle liner through which the stream 
passes, as described for example in U.S. Pat. No. 3,604,598. Another 
approach is to place an induction coil around the volume through which the 
stream falls, both to heat the stream and to control its diameter, as 
described for example in U.S. Pat. No. 4,762,553. These and similar 
techniques have not proved commercially acceptable for the control of a 
stream of a reactive titanium alloy for a variety of reasons. 
There therefore exists a need for an improved approach to the formation and 
control of a stream of a metal, and particularly for reactive metals such 
as titanium alloys. The present invention fulfills this need, and further 
provides related advantages. 
SUMMARY OF THE INVENTION 
The present invention provides an apparatus for controlling the flow of a 
metal stream, without contaminating the metal by contact with foreign 
substances. The apparatus permits precise control of the metal stream 
based upon a variety of control parameters. 
In accordance with the invention, apparatus for controlling the flow of a 
metal stream comprises a hollow frustoconical metallic nozzle body having 
a hollow wall, the hollow wall having an inner surface and an outer 
surface extending from a first base to a second base for a height h, the 
height h being the perpendicular distance between the first base and the 
second base, the frustoconical nozzle body further having at least one 
slit extending from the first base to the second base so that the wall 
lacks electrical continuity across the slit, and means for cooling the 
nozzle body. An induction heating coil surrounds the nozzle body, and a 
controllable induction heating power supply is connected to the induction 
heating coil. A sensor senses a performance characteristic of the 
apparatus. A controller controls the power provided to the induction 
heating coil by the induction heating power supply responsive to an output 
signal of the sensor, to maintain a selected performance characteristic of 
the apparatus. 
The flow of metal is typically controlled to maintain the nozzle 
temperature within a preselected range, and also to maintain a preselected 
metal stream diameter or flow rate. The metal stream diameter is selected 
to be less than an inside dimension of the nozzle body, so that there is a 
solidified layer of the metal, termed a "skull" in the art, between the 
flowing metal of the stream and the inner surface of the nozzle body. The 
skull prevents contact between the flowing metal and the wall inner 
surface of the nozzle body, ensuring that the material of the wall cannot 
dissolve into the metal stream and contaminate it. Decreasing the power to 
the induction coil or operating at a lower frequency will cause the skull 
to thicken, ultimately becoming so thick that the flow of metal is stopped 
altogether. Thus, the apparatus can act as a valve for the metal stream. 
The required degree of control cannot be achieved in the absence of a 
cooled nozzle body and induction heating of the skull and stream. This 
system establishes a delicate heat balance which can be readily controlled 
to produce the desired results. The cooled nozzle body extracts heat from 
the portion of the skull closest to it. Simultaneously, electromagnetic 
currents induced within the skull by the induction coil limit the amount 
of heat extracted from the flowing metal stream. Although much of the heat 
generated by induced current flows radially outward toward the nozzle wall 
for extraction, sufficient heat is applied to achieve the desired skull 
thickness and stream diameter. Increasing induction power increases the 
total heat input into the system and melts away a portion of the skull 
inner surface, resulting in an increase in stream diameter. Decreasing the 
induction power reduces the heat input and will increase the skull inner 
surface, if desired to the point of freeze off. The feedback control 
system is useful in maintaining preselected values throughout the course 
of extended operation to maintain the required heat balances and achieve 
the desired results. The use of electrical resistance heating in place of 
induction heating is unacceptable, because the heat input rate is too low 
and because the thickness of the skull layer cannot be adequately 
controlled. Unlike induction heating, resistance heating cannot be 
controlled to selectively act to heat the metal skull or stream without 
undesirably and uncontrollably affecting the nozzle body. 
Other features and advantages of the invention will be apparent from the 
following more detailed description of the preferred embodiment, taken in 
conjunction with the accompanying drawings, which illustrate, by way of 
example, the principles of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A preferred application of the apparatus for controlling the flow of a 
metal stream is in a metal powder production facility. The apparatus for 
controlling the flow of a metal stream may be used in other applications, 
such as, for example, a metal ingot production facility. The metal powder 
production facility is the presently preferred application, and is 
described so that the structure and operation of the present invention can 
be fully understood. 
Referring to FIG. 1, a powder production facility 20 includes a crucible 22 
in which metal is melted on a hearth 24. The molten metal flows as a 
stream 26 through an opening in the hearth 24. After leaving the hearth, 
the stream 26 passes through a nozzle region 28 where control of the 
stream is achieved, and which will be discussed in detail subsequently. 
The stream 26 is atomized into fine liquid metal particles by impingement 
of a gas flow from a gas jet 30 onto the stream 26. The atomization gas is 
typically argon or helium in the case where the metal being atomized is a 
titanium alloy. The particles quickly solidify, and fall into a bin 32 for 
collection. (Equivalently, the particles can be formed by directing the 
stream 26 against a spinning disk.) 
In accordance with the invention, apparatus for controlling the flow of a 
metal stream from a water-cooled hearth comprises a frustoconical nozzle 
body made of a conductive metal, such as copper, having a hollow wall, the 
hollow wall having an inner surface and an outer surface extending from a 
first base to a second base for a height h, the height h being the 
perpendicular distance between the first base and the second base, the 
frustoconical nozzle body further having at least one slit extending from 
the first base to the second base so that there is no electrical 
continuity in the nozzle wall, means for cooling the nozzle body, and 
further including a temperature sensor that senses the temperature of the 
nozzle body. The nozzle body, which may include provisions for circulating 
optional cooling fluid, has a flange at one end or base thereof suitable 
for attachment to the fluid-cooled hearth. This base may be electrically 
conductive and have electrical continuity. The preferred fluid is water 
although other fluids such as inert gases, and other liquid or gaseous 
media may be used. An induction heating coil surrounds the nozzle body, 
and a controllable induction heating power supply provides power to the 
induction heating coil. A controller controls the power provided to the 
induction heating coil by the induction heating power supply responsive to 
an output signal of a monitoring sensor, preferably a signal responsive to 
the temperature measured by the temperature sensor. 
Referring to FIGS. 2 and 3, a nozzle body 40 is formed of a plurality of 
hollow tubes 72 positioned around a circumference and extending from a 
first base 89 to a second base 90, each tube spaced from an adjacent tube 
sufficiently so that there is no electrical continuity among the tubes, 
and having the general shape of a right-angle frustocone, and preferably 
is in the form of a substantially right circular hollow cylinder wherein 
the size of the nozzle entrance and nozzle exit, located at the first end 
and the second end respectively, are substantially the same. In the 
general form of a frustocone, the nozzle body is tapered from a first end 
or base 89 to a second end or base 90 so that the geometry of the nozzle 
at the first base 89 or entrance, where metal enters is less restrictive 
than at the second end or base 90 where the metal exits. In this 
configuration, bottom pouring and tapping of the melt as well as steady 
state flow is facilitated by the tapered configuration. In the preferred 
embodiment, steady state flow and operation is achieved by balancing heat 
input and output within and through the nozzle solely by means of the 
controls system. The detailed construction of the walls of the nozzle body 
40 will be discussed in greater detail in relation to FIG. 3. 
The nozzle body 40 is elongated parallel to a cylindrical axis 42. At the 
upper end of the nozzle body 40 is a flange 44, which may be fluid-cooled 
and which may supply cooling fluid to the tubes which form the nozzle. 
This flange 44 permits the nozzle body 40 to be attached to the 
fluid-cooled hearth 24. It is understood that the same fluid cooling 
medium will be used in the nozzle and the hearth when they are integrally 
connected, providing for a more economical arrangement, although each may 
be served by independent cooling systems. The nozzle body 40 is usually 
made of a conductive metal such as copper, or a refractory metal selected 
from the group consisting of tungsten, tantalum and molybdenum. 
An induction heating coil 46 is placed around the nozzle body 40, in the 
shape of the nozzle body exterior. In the general form, this shape is a 
right-angle frustocone, while in the preferred embodiment, this shape is 
substantially a cylinder. The induction heating coil 46 is typically a 
helically wound coil of hollow copper tubing through which cooling fluid, 
preferably water, is passed, and to whose ends a high frequency 
alternating current is applied by a controllable induction heating power 
supply 48. The alternating current is in the range of about 3-450 KHz, 
typically about 10-50 KHz, or higher depending upon the nozzle dimensions 
and the desired metal flow rate. Although induction heating coil 46 in 
FIG. 2 is depicted as having uniform coil spacing, it will be understood 
that coil spacing may be varied to better match heat input to local losses 
to aid in providing a more uniform and controllable skull thickness, 
particularly at the entrance and exit of the nozzle body 40. 
In the view of FIG. 2, the induction heating coil 46 is encased within a 
protective ceramic housing 48, a technique known in the art. 
Alternatively, the induction heating coil may be suspended around the 
nozzle body 40 without any covering, as shown in the embodiment of FIG. 3. 
A sensor to measure a performance characteristic of the apparatus is 
provided. The sensor may be a temperature sensor 52 such as a thermocouple 
contacting, or inserted into, the nozzle body 40 on its side wall or a 
temperature sensor 54 such as a thermocouple contacting, or inserted into, 
the flange 44 portion of the nozzle body 40. Alternatively, the 
performance may be monitored by a temperature sensor positioned in or 
proximate to the skull (not shown) to monitor the skull temperature. Some 
other sensors are depicted in FIG. 1. The sensor may be a diametral sensor 
56 that measures the diameter of the metal stream 26. Such a diametral 
sensor 56 operates by passing a laser or light beam from a source 58 to a 
detector 60, positioned so that the object being measured is between the 
source 58 and the detector 60. The light beam is wider than the expected 
maximum diameter of the object, here the stream 26. The amount of light 
reaching the detector 60 depends upon the diameter of the stream 26, and 
gives a measure of the stream diameter. The diametral sensor can 
alternatively be a position sensor 62, such as a video position analyzer 
with a source described in U.S. Pat. Nos. 4,687,344 and 4,656,331 (whose 
disclosures are incorporated by reference), and a signal analyzer 
available commercially from Colorado Video as the Model 635. 
Alternatively, the weight change of the bin 32 as a function of time 
provides the mass flow of metal. 
The output signal of each of the sensors 52, 54, 56, 60 and 62, or other 
type of sensor that may be used, is provided as the input to a controller 
64. The controller 64 may be a simple bridge type of unit, or, more 
preferably, may be a programmed microcomputer into which various 
combinations of control commands and responses to particular situations 
can be programmed. The output of the controller 64 is a command signal to 
the induction heating power supply 48. The command signal 66 closes a 
feedback control loop to the induction heating coil 46, so that the heat 
input to the nozzle region 28 is responsive to the selected performance 
characteristic of the apparatus. For example, the controller 64 may be 
operated to maintain the diameter of the metal stream 26 within certain 
limits, and also not to permit the temperature measured by the temperature 
sensors 52 and 54 to become too high. The controller varies the command 
signal 66 to achieve this result, and may also be programmed to control 
other portions of the system such as the power to the crucible 22 or the 
water cooling flow to any portion of the system. 
The structure of the nozzle is shown in perspective view in FIG. 3. The 
nozzle body 40 is formed from a plurality of hollow tubes 72 arranged 
around the circumferential surface of a cylinder, on a cylindrical locus, 
with the tubes 72 parallel to the cylindrical axis 42 which is 
perpendicular to the plane formed by the circumference of the cylinder. A 
tubular construction, with each tube representing a finger, is utilized so 
current induced in the nozzle 40 by induction coil 46 will flow around the 
individual tubes 72 and into the nozzle inner diameter. Each tube is 
sufficiently spaced from the other tubes so there is no electrical 
continuity among adjoining tubes, except in the general region of the 
manifold 76, positioned at the first base 89 or upper end of the nozzle. 
This construction forces induced currents in the fingers to travel around 
the outer diameter of the individual tubes creating a magnetic field 
inside the nozzle. This magnetic field in turn penetrates the skull 84 
inducing a current flow at right angles to it in accordance with the right 
hand rule and generating heat within the skull 84. The depth of the 
penetration of this magnetic field is dependent on the frequency of the 
current flow and the conductivity of the skull material. In this way, the 
electromagnetic field generated from the current in the tubes "couples" to 
the skull 84 to provide a method for controlling the metal stream 26. If 
there is electrical continuity in the nozzle, as when there is no 
effective slit or when the tubes are sufficiently close together, the 
nozzle is ineffective. 
To provide structural continuity, an insulating material such as a 
high-temperature cement can be placed into the slits or interstices 75 
between the tubes 72 around the periphery of the nozzle body 40. 
At the upper end or first base 89, the tubes 72 are fixed to a hollow 
cylindrical manifold 76, which in turn is fixed to the flange 44. Within 
each of the tubes 72 is a second set of smaller tubes 73, having a smaller 
diameter than tubes 72 such that an annulus 77 is formed between tubes 72 
and smaller tubes 73, extending from the manifold 76 almost to the lower 
end or second base 90. The cooling fluid, which may be water or a cooling 
gas, is supplied through these smaller tubes 73 and returns in the annulus 
77 between the two tubes 72, 73 making each pair of tubes 72, 73 an 
individual cooling circuit. The manifold 76 is supplied with external 
coolant connectors 80 and 82, respectively, so that a flow of cooling 
water can be passed through the tubes 72, 73. The flange 44 is provided 
with bolt holes or other attachment means to permit it to be attached to 
the underside of the hearth 24. 
The present invention extends to the operation of the apparatus for 
controlling the metal stream. In accordance with this aspect of the 
invention, a process for controlling the flow of a stream of molten metal 
comprises the steps of providing an apparatus comprising a hollow 
frustoconical metallic nozzle body 40 having a hollow wall, the hollow 
wall having an inner surface and an outer surface extending from a first 
base 89 to a second base 90 for a height h, the height h being the 
perpendicular distance between the first base 89 and the second base 90, 
the frustoconical nozzle body 40 further having at least one slit 
extending from the first base 89 to the second base 90 so that there is no 
electrical continuity in the nozzle wall, means for cooling the nozzle 
body, an induction heating coil 46 surrounding the nozzle body 40, a 
sensor that senses a performance characteristic of the apparatus, a 
controllable induction heating power supply connected to the induction 
heating coil, and a controller that controls the power provided to the 
induction heating coil by the induction heating power supply responsive to 
an output signal of the sensor, to maintain a selected performance 
characteristic of the apparatus; and controlling the power provided to the 
induction heating coil 46 to maintain a preselected flow of metal in the 
stream. 
The induction heating coil 46 is positioned on the exterior of the nozzle 
body and may assume the shape of the exterior of the nozzle body. The 
induction coil may have variable spacing of the coils to permit a 
preselected, tailored heating profile along the length of the nozzle. For 
example, the coil may have a concentration of turns at the second base or 
lower end of the nozzle to provide more heat input at this location to 
facilitate melting off of adhering metal at this location. A multi-turned 
coil is preferred. 
Thus, an apparatus such as those described previously is used to attain and 
maintain a preselected set of conditions. In one typical operating 
condition, the alternating current frequency and power applied by the 
power supply 48 to the induction heating coil 46 are selected to maintain 
a solid metal skull 84 between the outer periphery of the metal stream 26 
and the inner wall of the nozzle body 40. That is, radially outward heat 
loss from the stream 26 into the nozzle body 40 is sufficiently fast to 
freeze the outer periphery of the metal stream 26 to the inner wall of the 
nozzle body 40. The unfrozen, flowing metal stream 26 within the nozzle 
body 40 contacts only the frozen metal comprising the skull 84 having its 
own composition, and does not contact any foreign substance used in the 
construction of the wall of the nozzle body. There is no chance of 
contamination of the moving flow of metal by contact with walls of another 
material This feature is highly significant for the control of metal 
streams of reactive metals such as titanium alloys, which readily absorb 
contaminants. Although control of the frequency and the power provides 
maximum flexibility in the system, the same results can be accomplished by 
varying only the power. 
The skull 84 can be made thicker or thinner by selectively controlling the 
power supply 48 and the cooling of the nozzle body 40, with commands from 
the controller 64. Cooling may be accomplished by any one of a variety of 
means, such as by flowing a cooling fluid through the hollow nozzle body 
or through the tubes comprising the nozzle body, or by flowing a stream of 
cooling gas across the exterior of the nozzle body. If the skull 84 is 
made thicker, the diameter of the flowing portion of the metal stream 26 
becomes smaller. If the skull 84 is made thinner, the diameter of the 
metal stream 26 becomes larger. The control of skull thickness is used as 
a valve to decrease or increase the size of the flowing stream 26 and 
thence the volume flow rate of metal By increasing the thickness of the 
skull 84 indefinitely, the flow of metal can be shut off entirely by the 
solid skull that reaches across the full width of the nozzle body 40 The 
flow can be restarted by reversing the process and decreasing the 
thickness of the skull. Since this degree of control may require delicate 
manipulations, it is preferred that the controller 64 be a programmed 
minicomputer. 
Using the approach of the invention, full metal stream flow control is 
achieved reproducibly and neatly without contamination of the metal of the 
metal stream. Although the present invention has been described in 
connection with specific examples and embodiments, it will be understood 
by those skilled in the arts involved, that the present invention is 
capable of modification without departing from its spirit and scope as 
represented by the appended claims.