System and method of forming a solid casting

A method and a system for forming a solid casting. A material is fed into a mold having a retractable bottom. A first portion of the material at a first, lower position within the mold is allowed to solidify to thereby form a portion of the casting. The retractable bottom is withdrawn downwards at a withdrawal rate. A second portion of the material at a second, upper position within the mold is maintained in a liquid state by application of heat thereto, using a plasma arc generated by a plasma arc torch. A voltage of the plasma arc is measured, and the withdrawal rate of the retractable bottom is controlled based on the voltage of the plasma arc.

BACKGROUND OF THE INVENTION

This invention relates generally to a system and method of forming a solid casting, and more particularly to forming an ingot while controlling the withdrawal rate of the ingot while the ingot is solidifying within a mold.

To form a metal ingot, molten metal is poured into a mold, where it subsequently freezes. One example of such a mold is a withdrawal crucible. In a withdrawal crucible, a puller forms the bottom of the mold at the start of the casting process. The puller is moved down within the mold as the metal is poured in the top.

In some withdrawal crucibles, the top portion of the metal is maintained in a molten state with a separate heater such as a plasma arc torch.

Such systems are generally described in Applicant's copending application Ser. No. 14/031,008, the disclosure of which is hereby incorporated by reference, and on Applicant's website at http://www.retechsystemsllc.com.

BRIEF SUMMARY OF THE DISCLOSURE

A method and a system are provided, for forming a solid casting. A material is fed into a mold having a retractable bottom. A first portion of the material at a first, lower position within the mold is allowed to solidify to thereby form a portion of the casting. The retractable bottom is withdrawn downwards at a withdrawal rate. A second portion of the material at a second, upper position within the mold is maintained in a liquid state by application of heat thereto, using a plasma arc generated by a plasma arc torch. A voltage of the plasma arc is measured, and the withdrawal rate of the retractable bottom is controlled based on the voltage of the plasma arc.

The measured voltage may be a voltage between a power supply of the plasma arc torch and a ground. The ground may be measured at the casting.

The voltage may be indicative of a distance between the plasma arc torch and a top surface of the second portion of the material, such as by being directly proportional to the distance.

The control of the withdrawal rate may include filtering and/or processing a signal of the voltage, proportional control based on the voltage, integral control based on the voltage, derivative control based on the voltage, or combinations thereof.

The mold may be a crucible and the casting may be an ingot.

For a more complete understanding of the nature and advantages of the present invention, reference should be made to the ensuing detailed description and accompanying drawings.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Throughout this description for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the many embodiments disclosed herein. It will be apparent, however, to one skilled in the art that the many embodiments may be practiced without some of these specific details. In other instances, well-known structures and devices are shown in diagram or schematic form to avoid obscuring the underlying principles of the described embodiments.

In one exemplary method of forming a metal ingot, molten metal is poured into a mold with a movable plug or withdrawal ram disposed within it. At the beginning of the casting process, the molten metal impacts the ram and subsequently freezes to form the bottom of the ingot. As additional molten metal is poured into the top of the mold, the ram is withdrawn downwards.

For various reasons, it is often desirable to maintain a top portion of the metal within the mold in a molten state by using a heater, such as a plasma arc torch disposed above the top of the molten pool.

For various reasons, it is desirable to maintain the distance between the plasma arc torch and the top of the molten pool constant, which has heretofore been extremely difficult.

This distance has a rather linear correlation with the voltage of the plasma arc, i.e. the voltage drop between the power supply of the plasma arc torch and the ground. Thus, in the embodiments described herein, the withdrawal rate of the withdrawal ram is controlled based on the voltage of the plasma arc to maintain the distance between the plasma arc and the top of the molten pool constant. In other words, exemplary embodiments of the presently claimed invention use plasma arc voltage feedback to control a constant pool level in a withdrawal crucible while incoming molten material is filling the crucible and forming an ingot.

In more detail, turning toFIG. 1A, in some embodiments, raw material12is fed to a melting hearth20where it is melted by a first heat source30, which may be a plasma arc torch or any other appropriate heat source. The melted material14is then poured into a withdrawal crucible50where it is acted upon by a second heat source: a plasma arc torch60. The material16ain the lower portion of the crucible50solidifies to become a portion of what will later be the finished ingot, while the material16bin the upper portion of the crucible50is maintained in the molten state by the plasma arc torch60. The delineation between the solid16aand molten16bmetal within the crucible50is not illustrated.

In exemplary embodiments, the mold50has a retractable bottom52which is moved downwards as the material16fills the mold to maintain the surface level substantially constant. The bottom52may be, for example, a near net fit dovetail joint or puller that occupies the crucible and forms the bottom at the start of the casting process. Molten metal16pours into the dovetail joint and freezes. As the level begins to fill in the crucible, the material16ain contact with the dovetail puller52is allowed to solidify through, for example, a water cooling system integrated into the crucible. As the material16is fed into the crucible50, the withdrawal position of the bottom52moves down in order to maintain a constant molten pool level position in the crucible.

In more detail, as the molten metal14begins to flow into the mold50, the molten metal flows into an undercut region that forms the part of the ingot that is gripped by the puller52to pull the ingot vertically downwards. There is either a way to separate two pieces of the puller52, or there is a relief on one side allowing horizontal removal. In more detail yet, and referring toFIGS. 6A and 6B, the bottom52of the mold50may be a dovetail puller, such as a two-piece dovetail puller as shown inFIG. 6A, or alternatively, a one-piece dovetail puller as shown inFIG. 6B. The two halves of the two-piece puller shown inFIG. 6Amay be bolted together and/or may be attached to one another with a hinge, so that they can be separated from one another to remove the finished ingot. The one-piece puller shown inFIG. 6Bmay have an open edge, as shown, to allow the finished ingot to be slid laterally out of the puller. The bottom52may include a base, a chill plate, and a dovetail plate. The puller base may be mounted to and receive cooling water from the water-cooled housing of the mold50. The chill plate is mounted to the top of the puller base, while the copper dovetail plate52is mounted to the top of the chill plate. The dovetail plate52is undercut on its inside diameter to provide a relief for the ingot dovetail as liquid metal begins flowing into the mold50.

In an ideal world, if equipment and operators were perfect, the feed rate of the material12into the hearth20, the rate at which the raw material12is melted within the hearth20to form the melted material14, the rate at which the melted material14is poured from the hearth20into the crucible50to form the ingot16, and the rate at which the ingot material16is withdrawn downwards within the crucible50would all be equal to one another. In other words, the liquid pour rate into the crucible would be smooth, steady, and continuous. The withdrawal rate would be identical to the pour rate, and the liquid level within the crucible50would be exactly constant over time.

However, turning toFIG. 1B, the liquid level within the crucible50has previously been difficult to measure because of heat, bright light, and dust. Therefore, in the typical prior art, the liquid level often varies. In the example ofFIG. 1B, the withdrawal of the ingot16has outpaced the pouring of the material14into the crucible50, and the liquid level of the top of the molten portion16bis lower than ideal. Known systems have attempted to use non-contact devices such as lasers, ultrasound, and optical vision equipment, with mixed results. Even with viewport purges and brushes, the “view” for such devices can be disturbed by dust, pitting of glass, etc. Generally, the effectiveness of these systems has been poor compared to the cost of equipment, maintenance, and implementation. Traditionally, operators have been required to monitor liquid levels through video camera views—at least as a backup to automation attempts.

Turning toFIG. 2, there is a rather linear correlation between the torch standoff (Z-height) and the torch voltage when as plasma arc torch is used as the heat source60. This correlation is generally discussed in Applicant's U.S. Pat. No. 5,239,162, the disclosure of which is hereby incorporated by reference. This correlation means that the plasma arc torch voltage could be used as a way to measure the distance from the torch to the liquid surface in the crucible. Once the torch height or torch pattern height has been set, an algorithm is used to process the signal and thereby smooth the signal for control purposes. The withdrawal rate is controlled based on the smoothened signal, thereby maintaining the liquid level constant.

The plasma arc voltage is measured in the electrical connection between the power supply and the ingot ground. The voltage of the arc is proportional to the distance from the start of the arc to the top molten surface of the solidifying ingot, and therefore can be used to measure the height of the top of the molten pool16bin real time. This voltage is used in a closed loop feedback control system to adjust the ingot withdrawal rate and control the molten pool level in the crucible by maintaining a target voltage.

In other words, referring back toFIGS. 1A and 1B, and also toFIGS. 3A and 3B, if the withdrawal rate starts to outpace the melting and pouring rates as shown inFIGS. 1B and 3B, the control system notes the corresponding change in voltage and responds by slowing the withdrawal rate. Conversely, if the melting and pouring rates start to outpace the withdrawal rate, the control system notes the corresponding change in voltage and responds by speeding the withdrawal rate.

In a presently preferred embodiment, the control system processes the voltage signal and subsequently uses proportional-integral-derivative (PID) control, but any appropriate control system may be used. The signal processing may include filtering, such as with a linear filter, a non-linear filter, a time-variant filter, a time-invariant filter, a causal filter, a non-causal filter, an analog filter, a digital filter, a discrete-time filter, a continuous-time filter, a passive type of continuous-time filter, an active type of continuous-time filter, an infinite impulse response type of filter, or a finite impulse response type of filter.

FIG. 4is a photograph showing an exemplary embodiment of the system during use. In this embodiment, the crucible50is water-cooled to solidify the bottom portion of the ingot16a. The molten pool16bcan be seen as a bright spot at the top of the solidified ingot16a.

Still more detail of an exemplary method is shown inFIG. 5. In its broadest form, the exemplary method includes three steps: step100: forming the ingot; step200: measuring the voltage of the plasma arc torch; and step300: controlling at least one aspect of the forming step100based on the voltage measured in step200.

In more detail, step100of forming the ingot includes step110of introducing the material12,14into the crucible50; step122of applying heat to the top portion16bof the material using the plasma arc torch60; step124of allowing the bottom portion16aof the material to solidify; and step126of withdrawing the material downwards within the crucible50. Step110can be further subdivided into step112of feeding the raw material12into the hearth20; step114of melting the fed raw material12with the first heat source30within the hearth20to form the molten material14; and step116of pouring the molten material14from the hearth20into the crucible50.

It will be appreciated that the system and method heretofore described provide at least the following benefits: No extra hardware is required, other than changes to the control system to adjust the withdrawal rate in response to changes in voltage, which are indicative of changes in liquid level. In other words, an existing system can be retrofitted to implement the exemplary method, simply by updating the control system. The feed mechanism, melting hearth, plasma arc torches, and mold with associated movable bottom need not be changed. Runout and overflow are avoided, leading to higher quality finished products and less waste. The surface finish may be of higher quality. Furthermore, the complete automation of the withdrawal rate frees up operators to concentrate on the feeding, melting, and refining steps, which will result in much less human error.

In simulations, even using arc voltage noises of an atypical +−50 volts, the Applicant predicts control of the liquid level position to within 2 mm of target or better.

Velocity corrections are made automatically by the controller to adapt to varying melt rate conditions. In other words, when the pour rate starts to outpace the withdrawal rate, the withdrawal rate is sped up, and when the withdrawal rate starts to outpace the pour rate, the withdrawal rate is slowed down. Because the pour rate depends on the earlier steps, this method compensates not only for variations in the pour rate, but indirectly compensates for any variation upstream in the process, for example, the feed rate of the raw material12, and the melt rate of the raw material12into the molten material14; as well as directly compensating for variation in the pour rate of the molten material14into the mold50. This exemplary withdrawal system has the ability to facilitate fully automatic withdrawal positioning without other external sensors or human intervention to monitor for overflow or low level conditions.

A system of this nature can be used to control the liquid metal level not only in continuous hearth melting systems, but also any system where liquid metal is fed into a container that is heated by a plasma torch. For example, another embodiment of the invention provides a semi continuous casting cold wall induction system where material is melted and mixed in a water cooled copper hearth, then the hearth is tilted to pour metal into the cold wall induction crucible for casting. Yet another embodiment provides a system where material is melted and mixed in a water cooled copper hearth, then the hearth is tilted to pour metal into a plasma heated tundish.

The above description is illustrative and is not restrictive, and as it will become apparent to those skilled in the art upon review of the disclosure, that the present invention may be embodied in other specific forms without departing from the essential characteristics thereof For example, any of the aspects described above may be combined into one or several different configurations, each having a subset of aspects. These other embodiments are intended to be included within the spirit and scope of the present invention. The scope of the invention should, therefore, be determined not with reference to the above description, but instead should be determined with reference to the following and pending claims along with their full scope of equivalents.