Patent Application
20080298425
1. Technical Field
The present invention relates generally to a furnace and method for melting metals. More particularly, the present invention relates to the melting of metals using a DC power source and an AC power source. Specifically, the invention relates to a furnace and method which utilizes a DC direct arc electrode or plasma torch and an induction coil powered by an AC power source.
2. Background Information
While induction melting of metals is well known in the art, it is most restricted in the lower temperature regimes before the metal or alloy reaches its melting point. Typically, the electromagnetic coupling becomes more efficient once the charge material becomes molten. Thus, any process which aids in the initial low temperature regimes to establish a molten pool of metal helps increase the efficiency of the electromagnetic coupling of the induction field to the metal charge.
Another weakness of the induction melting of metal relates to the fact that the majority of the heating is done at the crucible wall with which the molten metal is in direct contact. This is particularly true in the case of high frequency induction melting. The higher temperatures at the crucible-metal interface leads to a high potential for reactions between the molten metal and the material forming the crucible. The products of these reactions include microscopic “dirt” in the form of oxides (often called low density inclusions), oxygen, carbon, hydrogen and other reaction products. The formation of such oxides occurs because the oxides are very stable while the metals are very unstable. Thus, these compounds are often referred to as oxygen scavengers or “oxygen getters”. For this reason, significant effort and cost has gone into the development of refractory ceramics which lessen the likelihood of metal-ceramic reactions. However, it is very difficult to alleviate all likelihood of such reactions, and with regard to certain metals like titanium, zirconium and their alloys, it is virtually impossible.
In addition, extremely complex and expensive systems have been designed to lessen or alleviate the problem with crucible-metal interface reactions while substituting a copper crucible for a refractory crucible and cooling the copper typically with water in an effort to prevent the melting of the copper due to the high melt temperature of molten metals such as titanium, which is substantially higher than the melting point of copper. Induction melting systems which incorporate this feature are typically referred to as I.S.M. or induction skull melters. This feature is also used with DC ingot and casting systems and is referred to by a number of names, including V.A.R. or vacuum arc remelting, or vacuum arc casting.
Each of these systems has drawbacks. They are expensive to build and operate because the machines have fabricated copper components which are expensive to manufacture. They also require expensive cooling by expensive water systems. Thus, they are not practical to use except in very large size and volume applications.
They can also be dangerous to operate. If the cooling system fails to work properly, the copper shell will melt, allowing the molten charge such as molten titanium to breach the system and combine with the water remaining in the crucible. This typically leads to a steam explosion at the least, and if not arrested, can quickly lead to a much larger and more devastating hydrogen explosion. Unfortunately, there have been some deadly accidents of this sort in the past.
The furnace and the method of the present invention address these and other problems in the art.
The present invention provides a method comprising the steps of melting with a DC arc source a metal within a melting cavity of a crucible bounded by a crucible wall to form molten metal therewithin; and maintaining a skull along the crucible wall with an induction coil in electrical communication with an AC power source to protect against contact between the molten metal and crucible wall.
The present invention also provides a furnace for melting a metal charge, the furnace comprising a melting crucible defining a melting cavity adapted to receive the metal charge; an electrode adjacent the melting cavity adapted to melt the metal charge; a DC power source in electrical communication with the electrode; an induction coil adjacent the crucible; and an AC power source in electrical communication with the induction coil.
Similar numbers refer to similar parts throughout the drawings.
A first embodiment of the furnace of the present invention is indicated generally at 10 in
Referring to
Crucible 12 has a substantially flat bottom wall 24 and a vertical side wall 26 which is typically cylindrical and extends upwardly therefrom to define therewithin a cylindrical melting cavity 28 for receiving a metal charge 30. Crucible 14 is preferably formed of a carbon-graphite material or a ceramic material such as zirconium oxide or yitria oxide. An electrode 32 which may be either an anode or cathode is mounted on bottom wall 24 and extends through a hole formed therein to communicate with melting cavity 28 so that it is in electrical communication with charge 30. Electrode 32 is in electrical communication with a DC power source 34 which involves a power converter for converting AC to DC power. DC power source 34 is likewise in electrical communication with heat source 16 via clamp 18. Heat source 16 may be a direct arc electrode or a plasma torch. Induction coil 14 is in electrical communication with an AC power source 36. AC power source 36 has controls for regulating the amount of power (in kilowatts) and the frequency. Coil 14 is tubular and thus forms a passage so that water from water source 38 may be pumped by pump 40 through the passage to cool coil 14.
The operation of furnace 10 is now described with reference to
Initially, the power to induction coil 14 from AC power source 36 is in an off condition and water is being circulated via water source 38 and pump 40 through coil 14 in order to actively cool crucible 12 primarily along side wall 26. DC electrode 16 is then powered by DC power source 34 in order to strike an arc to charge 30. The position of electrode 16 is controlled by a three axis positioning system (not shown) which is able to move electrode 16 along axes X, Y and Z and is typically operated via a voltage or current driven feedback loop system. The heat produced by the arc between electrode 16 and charge 30 will melt the metal to create a molten metal bath 42 which has a generally cylindrical shape. The heat produced by the arc of electrode 16 will be generally hottest at the contact with molten bath 42 with the temperatures gradually decreasing radially outward toward side wall 26. In order to prevent molten bath 42 from contacting crucible 12, the cooling provided by the water passing through induction coil 14 cools crucible 12 and in turn cools the metal along the walls of the crucible in order to maintain a solid or semi-solid boundary layer or skull 44 along side wall 26 and bottom wall 24. Thus, the reactive molten metal of bath 42 either does not contact crucible 12 or only does so minimally. Thus, the problem of contamination which would otherwise occur due to the molten metal-crucible interface is eliminated or substantially avoided.
During the melting process, induction coil 14 may be powered by AC power source 36 in order to provide electromagnetic heating and stirring of the molten bath 42. A particular advantage of heating with coil 14 is the ability to maintain a uniformly thick skull 44 along crucible side wall 26. More particularly, skull 44 includes a flat circular bottom wall portion and a cylindrical side wall portion extending vertically upwardly therefrom. In addition, coil 14 may be used to melt the material of skull 44 to allow for its easy removal in the event that the crucible is to be used for another alloy having a different chemistry. Typically, molten material 42 will be poured or otherwise transferred out of crucible 12 and used in the molding of various objects. During pouring, molten material 42 will contact crucible 12 for a relatively brief period so that contamination therebetween is minimal. Once molten material 42 is transferred out of crucible 12, coil 14 is powered to completely melt skull 44 to form additional molten material which may be poured or otherwise transferred from crucible 12 and may be maintained separate from the original molten material 42 to prevent contamination therebetween.
Furnace 100 is now described with reference to
Furnace 100 is operated in essentially the same manner as furnace 10 except that the metal charge is melted to form a molten bath 60 which is generally conical in shape and a skull 62 which forms along side wall 56 and bottom wall 54 which is frustoconical and thus has a generally V-shaped cross-section. Thus, the molten bath 60 is wider at its upper surface than at its bottom. Since melting cavity 51 has a diameter which increases from the bottom upward, it provides a greater diameter where the DC arc contacts the surface of molten bath 60 where the greatest amount of heat is produced. This configuration helps to insure that skull 62 has a substantially uniform thickness and also adds to the volume of the crucible without creating hot zones in the crucible. Skull 62 has a conical or frustoconical shape.
In addition to the various advantages noted above, furnaces 10 and 100 may be used in a more standard fashion. For instance, if the metal or metal alloy to be melted is relatively non-reactive with the material of the crucible, such as a copper or stainless steel charge, it may be preferred to use the furnace as a traditional induction furnace. In addition, if it is desired to provide vigorous stirring of the molten metal in order to homogenize the bath, the use of the AC induction coil may also be preferred.
On the other hand, the combined use of the DC arc source and the AC induction coil may be preferred in order to rapidly melt a given charge. For instance, if an alloy contains constituents with extremely low vapor pressure points, it may be desirable to reduce the residence time of the constituents in the molten bath in order to reduce the chances of vaporizing or oxidizing the constituents and altering the bath chemistry. Thus, furnaces 10 and 100 provide new advantages as well as versatility.
In the foregoing description, certain terms have been used for brevity, clearness, and understanding. No unnecessary limitations are to be implied therefrom beyond the requirement of the prior art because such terms are used for descriptive purposes and are intended to be broadly construed. Moreover, the description and illustration of the invention is an example and the invention is not limited to the exact details shown or described.
