CA1248761A - Alloying system - Google Patents

Abstract

Abstract of the Disclosure A method for adding alloying material to a molten metal media, such as molten aluminum, is disclosed. The method includes converting the alloying material into a spray of superheated alloy material and directing the spray into the molten metal media at a predetermined depth below the media's surface, the depth having been determined beforehand to enhance dissolution and dispersion of the alloying material in the molten media.

Description

The invention relates generally to a method and system for adding alloying elements to molten metals. More partic~-larly, however, it relates to the addition of elemen-ts which normally dissolve slowly and with difficulty in molten metals, particularly aluminum.
Many different methods have been employed to add alloying elements to molten metals. Conventional methods typically add the elements directly to the melt in the form of a lump, a bar or the like. In some cases, they are added directly to molten metal being tapped into a ladle, and in other cases, they may be placed in the ladle prior to tapping.
Another method for adding alloying elements to molten metals, particularly molten steel, is disclosed in U.S. Patent 3,768,g99 to Ohkubo et al. In Ohkubo, alloying is accomplished by feeding a wire rod into the molten metal. The rod is coat~d with additives for the molten metal and an organic binder which decomposes into gaseous products in the molten metal. The generated gas stirs the molten metal and thus uniformly incorporates the added components throughout the molten metal.
U.S. Patent 3,7~9,309 to ~awawa also discloses a method for adding alloying elements in the form of a wire rod to molten metals. The rod has a controlled size and is added to a molten metal bath by inserting it at a controlLed speed~ so as to produce a refined and purified metal alloy.
The above methods of adding alloying elements to molten metal work fairly well with alloying elements which dissolve and disperse easily in the molten metal. ~owever, such methods do not work so well with elements having limited liquid '~' 1 ~

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solubility such as Pb, Bi and Sn and high o~idizing potential such as Mg and Zn.
U.S. Patent 3,9~7,265 to Guzows~i et al proposes a solution to the problem of adding such "hard-to-alloy" materials to molten metal. The process employs a high current arc which is formed between the molten base metal and the alloying addition. The alloying addition is passed through the arc where it is melted and converted into a spray of finely divided superheated molten particles. In such a condition, the particles are able to rapidly dissolve in the molten metal upon contact therewith. While the Guzowski concep~ of alloying is certainly an interesting one, a need still exists for a process capable of providing improved results.
Accordingly, an object of the present inveniion is to provide an improved process for adding "hard-to-alloy" alloying materials to molten metals.
Another object of the present invention is to provide an alloying process having high dissolution rates.
Another object of the present invention is to provide an alloying process which is amenable ~o continuous casting processes.
Another object of the present invention is to provide a lead alloyed, aluminum based metal article having high machinability.
Another object of the present invention is to provide a process for adding alloying material to a molten media that additionally adds heat to the molten media.

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Addi~ional objects and advan~ages of the present invention will become apparent to persons skilled in the art from the following specification and drawings.
In accordance with the present invention, a me-thod is provided for adding alloying material to a mol~en metal media, such as molten aluminum. The method includes the step of converting the alloying material into a spray of superheated alloy material and directing the spray into the molten metal media at a predetermined dep-th below the media's surface, the depth having been determined beforehand to enhance dissolution and dispersion of the alloying material into the molten media.
In a preferred embodiment, the alloying material is converted into the spray of superheated alloy material in a chamber means, i.e. spark cup means which is at least partially immersed in the molten media body. The al]oying material is for example lead, bismuth, antimony, magnesium, zinc and copper.
Among them, lead is particularly preferred in this process. The spark cup has a lower open end which is exposed to the molten media and an upper inlet, at least a portion of which is located above the exposed or exterior surface of the molten media. The lower open end of the spark cup is maintained or immersed a predetermined depth ~elow the surface of the molten media. The alloying material, preferably in the form of an elongated element having a free end, is continually fed into the spark cup through its upper inlet, and an electrical arc discharge between the submerged molten metal surface and the alloying element in the spark cup is maintained with a current that exceeds the globular/
spray transltion current density of the alloying material. At such ~ - 3 ~
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a current, the free or exposed end of the alloying element is converted into a spray of superheated material. Arc shielding gas, preferably an ionizable gas and a non-reactive gas, is continually supplied to the spark cup through its upper inlet ~ 3a-also. In addition to shielding the arc clischarge, the gas slightly pressurizes the spark cup and thereby prevents molten media from entering its open end. As such 7 a submerged interior surface of molten metal media is created in the spark cup's open end at the aforementioned predetermined depth. The shielding gas also carries or projects the superheated spray of alloy material into the molten media through the submerged molten metal surface so as ~o permit dissolution and dispersion of the alloy material in the media. The predetermined depth of immersion has been found to significantly enhance dispersion and dissolution of the alloying material into the media.
The present invention also provides a lead alloyed, aluminum based article having high machinability. The article is produced by converting lead alloy material into a spray of superheated alloy material which is injected into a bath o molten aluminum at a predetermined depth below the molten bath's surface. The spray i5 formed by establishing an electrical arc discharge between a submerged surface of the molten media and the alloying material. The discharge is maintained with a current that exceeds the globular/spray transition current density of the alloying material. The spray of superheated alloying material is directed onto the submerged interior surface of the media where dissolution and dispersion of the alloy material into the media take place. The submerged surface is maintained at the predetermined depth below the bath's surface having been found to enhance said dissolution and dispersion of the lead into molten aluminum bath. The article so produced has acicular shaped particles of lead which are smaller and more uniformly sized and dispersed than those which .

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are made by adding lead at the surface of the molten aluminum or at a depth above the aforesaid predetermined depth.
Figure 1 illustrates an embodiment of the present invention.
FigurQ 2 illustrates the spark cup depicted in Figure 1.
Figure 3 is a graph plotting alloy dissolution rate in pounds per minute versus spark cup immersion depth.
Figure 4 is a graph illustrating the relationship of actual recovery in percentages versus immersion depth in inches.
Figure 1 illustrates the addition of a wire 10 of alloying material into a bath or melt 12 of molten media in a flow through furnace 14. The surface of melt 12 is referred to herein as exposed or exterior surface 16. Wire 10 is being fed by a feeder 18 which passes it through a triplex ~eed cable 20 into a spark cup 22, the spark cup being partially immersed in melt 12. In spark cup 22, alloy wire lO is converted into a ~ spray 24 of superheated alloy material by passing it through a ; plasma arc discharge (not numbered). The plasma arc discharge is established between a submerged surface 26 of the molten metal which is maintained within an open end 28 of spark cup 22 and a free end 30 of alloy wire 10. The arc discharge is shielded with a shiel~ing gas 32, preferably argon, which is provided via feed cable 20 by an arc shielding gas source 34.
In addition to providing a shielding atmosphere for ~he arc in the spark cup, the shielding gas source 34 pressurizes the spark cup at a pressure which is suffic~ent to prevent molten rnetal from entering open end 28 of the spark cup. Such pressurization also facilitates maintenance of the aforementioned submerged ~:- 5 ..

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surface at a certain predetermined depth below exposed surface 16 (more on this, infra). Returning to Figure 1, it will be seen that the arc dlscharge is powered by a constant current power supply source 36 (more on this, infra). Melt 12 serves as an anode with wire 10 serving as a consumable electrode. The electrical circuit leading back to current source 36 is completed by a return wire 38 which is attached to a rod 40 immersed in melt 12. The superhea~ed spray produced by the arc discharge is directed or projected by the supply of shielding gas onto submerged surface 26 where the alloy material rapidly dissolves and disperses in melt 12. The gas is preferabl~
supplied at a flow rate that maximizes the projection of the spray into the melt. An impeller 42 or agitating means is also provided to further enhance dispersion of the alloy material throughout the melt. Spray 24 can be maintained as long as is desired by continually advancing or feeding the alloying wire into the spark cup. Feeder 18 can also be controlled to maintain or vary the rate at which wire 10 is fed into the spark cup .
The alloying material can be provided in wire form, as described above, or in the form of rod, tube, strip or in powdered form wherein the powders are encased in a hollow tu~e made from a suitable metal which has been swaged or otherwise worked to reduce its diameter and compact the powdered material in the tube. The only real limitation on the form of the alloying is that it should have a form which permits it to be fed into the feed cable in a seal-tight fashion, thereby enabling the pressurized atmosphere in the spark cup to be maintained. If the pressurized a~mosphere in the spark cup is 7~:L

not maintained, molten metal will, quite obviously, enter Lhe spark cup through its open end 28, thereby raising submerged surface 26 to a depth above its predetermined depth. Such raising of submerged surface 26 will result in lower dissolution and dispersion rates. (The importance of maintaining submerged surface 26 at its predetermined depth will be discussed in more detail, infra.) While no means for sealing the wire is depicted in Figure 1~ those skilled in the art will be aware of num~rous means having the capability of providing an efective seal.
lQ Such means could include elastomer and pneumatic seals. In addition, feeder 18 is preferably a consistent feed rate tractor drive.
Constant current source 36 is preferably of the type which maintains a relatively constant current regardless of voltage ~luctuations. The arc produced thereby has self~stabilizing characteristics and is relatively insensitive to changes in arc length which might be caused by fluctua~ions in the submerged molten metal depth. It may also be desirable in certain situations to further enhance arc stability by

2~ seeding the plasma discharge with certain additives, such as alkali metals which are known to promote arc stability. Arc stability can also be enhanced by using various fluxes known to those skilled in the relevant art.
As mentioned in U.S. Patent 3,947,265 to Guzowski, it may be desirable to add a high fre~uency, high voltage component to ~he arc which is particularly useful if AC current is used.
This apparently reduces the tendency of the arc to extinguish every time the voltage passes through zero, increases the 7~;~

stability of the arc and makes initiation of the arc less dif~icult.
An important aspect of the present invention requires that the current supplied by power source 36 exceed the globular/spray transition current density of the alloyed material. As used herein, the globular/spray transition curren~
density defines the boundary line separating the two di~ferent types of metal transfer that are capable of occurring in the plasma arc discharge. (As pointed out by Guzowski in U.S.
Patent 3,947,265, this transition point can vary with such factors as alloy type, wire size and wire speed~) In cases with current densities below the transition point, alloy material being trans~erred through the arc detaches into large drops which dissolve and disperse slowly in the molten metal media.
At current densities above the transition point, the trans~er mechanism changes causing the alloy material to convert a ~ine spray of superheated alloy material. In this condition, the alloy material rapidly dissolves and disperses in the molten media upon contact with submerged surface 26.
Shielding gas 32 carrying or projecting spray 2~ into the melt also ~ypically enters the melt. This, however, should not introduce or cause any melt contamination since such gas simply escapes from the melt by ~ubbling through the melt to exterior surface 16. As previously mentioned, the preferred shielding gas is argon; however, other shielding gases, such as helium, carbon monoxide and carbon dioxide, may also be used in appropriate si.tuations.
The spark cup is preferably cylindrically shaped.
Such a shape provides a relatively high spark cup sur~ace area ~24~

to volume ratio which facilitates conductive heat transfer from the spark cup to the melt. It is important to facilitate such heat transfer to prevent the spark cup from overheating.
Moreover, those skilled in the relevant art will appreciate that suc~ heat transfer to the melt is advantageous in that it provides a convenient way of adding heat to the melt, thereby reducing furnace fuel needs. Conventional alloy adding processes such as that disclosed in Guzowski et al U.S. Paten~
No. 3,947,265 do not add much, if any, heat to their respective melts. For example, most of the heat generated during melting of the alloy material in Guzowski et al is lost to the atmosphere since the superheated spray is formed entirely above the melt surface.
The spark cup's cylindrical shape also enhances projection of the shielding gas carrying the superheated spray into the melt. Such pr~jection is important in that it enhances dissolution and dispersion of the alloying material into the melt. While a cylindrical shape is preferred, other shapes, such as an inverted frustoconical shape, which provide enhanced projection and heat transfer are considered to be within the purview of the present invention.
The spark cup's composition is another important aspect of the present invention. Preferably, i~ is made from material having the following characteristics:
1. High radiation heat transfer so as to maximize the transfer of radiation heat from the arc discharge to the melt, thereby reducing the possibility of overheating in the spark cup.

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2. ~igh resistance to thermal and mechanical shock.

3. High thermal and chemical stability in the melt.
Borosilicate, alumina, mullite and silica are some materials known to possess the desired characteristics.
Another briefly alluded to but important aspect of the present process invention is directed to immersing the spark cup and maintaining submerged surface 26 in the open end of the spark cup at its predetermined depth below exposed surface 16.
Such depth will be referred to hereinafter as the predetermined 10 immersion depth. It has been found that a difference of one or two inches in the immersion depth can have a signlficant impact upon the rate at which alloying material dissolves and disperses in the molten media. Figures 3 and 4 set forth test data from experiments conducted to determine the effects of immersion depth upon dissolution and dispersion. Figure 3 sets forth data respecting dissolution rate in pounds per minute versus immersion depth, and Figure 4 shows actual recovery in percentages versus immersion depth. The goal of the experiments was to add 0.5% lead to a substantially lead-free body of molten 20 aluminum. The experiments were conducted with a setup similar to that disclosed in Figure 1 except that a constant voltage supply source was used instead of the preferred constant current supply source. The flow-through furnace used in the experiments contained approximately 1000 pounds of aluminum. The bath of mol~en aluminum in the furnace had a depth of approximately 30 inches with a diameter of approximately 23 inches. One-eighth inch diameter lead wire was fed into a borosilicate spar~ cup at a feed rate of about 30 inches per minute via a triplex ~eed r cable. The spark cup was cylindrically shaped and had a lower . :

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opening similar to that described in Figure 1 with a diameter of approximately five centimeters. The spark cup's length to diameter ratio was approximately 6 to 1. Argon shielding gas was fed into the spark cup via the feed cable at a flow rate of about 10 standard ft3/hr. A plasma arc discharge was established in the spark cup between the free end of the lead wire and the submerged molten metal surface at a voltage of about 35 volts and a current of about 125 amperes, which translates into a current density of about 10,000 amp/in2. As such, the free end of the wire melted and converted into an axial spray of superheated alloy material upon entering the arc discharge. The spray wa~ directed onto the submerged melt surface by the shielding gas. After adding an appropriate amount of lead wire to the bath of molten aluminum, the alloyed molten aluminum was continuously cast into several ingots having dimensions of 6 in. x 6 in. ~ 36 in.
From Figure 2, those skilled in the art will appreciate that a dramatic increase in lead's dissolution rate (that is, the rate at which lead dissolved into the molten media) resulted when the spark cup immersion depth was increased from five to six inches. It will be noted that further increases in immersion depth did not seem to have much of an effect upon the dissolution rate. Similarly, in Figure 3, it can be seen that actual recovery in percentages increased dramatically when the immersion depth was increased from four to six inches. Moreover, further increases in the immersion depth showed further increases in actual reco~ery; howe~er, not nearly - as dramatic as those that occurred from four to si~ inches.
Actual recovery was measured by optical emission spectroscopy.

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Metallographic examination revealed that the particles of lead in the cast ingot were smaller, more acicular shaped and more uniformly sized and dispersed than those added by conventional methods. Moreover, it is believed that such ingot provided by the present invention has improved machinability.
While the immersion depth providing enhanced dissolution and dispersion in accordance with the present invention will vary with the material being added, bath size, bath 10w rate, alloy feed rate and size, inter alia, and will have to be determined for each setupi those skilled in the relevant art will appreciate that the method and apparatus of the present invention can result in greatly increased dissolution rates, particularly for alloy material with limited solubility, such as lead, bismuth and tin and for high oxidiæable materials such as magnesium and zinc.
Those skilled in the art will also appreciate that the present inven~ion is amenable to continu~us casting processes.
Continuous casting processes are those that permit the continual flow of metal from a melting furnace into a casting mold. Since continuous casting usually proceeds at a uniform rate, it will be easy ~o calculate the desired alloy ~eed rate with the method of the presen~ invention. The in~ention, however, is particularly amenable to continuous casting processes wherein the casting rate varies. Suitable instrumentation can be installed on ~he casting line to detect any changes in the casting rate ~hich can then be used to make adjustments in the alloy feed rate.
Various modifications may be made in the in~ention without departing from the spirit thereof, or the scope of the .

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claims, and therefore, the exact form shown is to be ~aken as illustrative only and not in a limiting sense, and it is desired that only such limitations shall be placed thereon as are imposed by the prior art, or are specifically set forth in the appended claims.

Claims (23)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for adding alloying material to a molten aluminum, said process being capable of adding said material in substantial quantity, comprising the steps of:
(a) providing a body of molten aluminum having an exterior surface and an interior surface below said exterior surface, said interior surface being provided by:
(i) immersing elongated chamber means into the molten aluminum body, said chamber means having a lower open end which is exposed to the molten aluminum and an upper inlet, at least a portion of which is located above the exterior surface of the molten aluminum, and (ii) continuously supplying the chamber means with a gas comprising an ionizable gas under sufficient pressure to maintain the molten aluminum's interior surface at the chambers's open end;
(b) feeding the material into the chamber means through the inlet;
(c) converting the material into a superheated spray of material in the chamber means by establishing an electrical arc discharge between the interior surface of the molten aluminum and the material being fed into the chamber means, said discharge being maintained with a current sufficient to convert said material into said spray; and (d) maintaining a plasma within said chamber at least between the material and the molten aluminum interior surface; wherein said gas is supplied at a sufficient rate to project said material into the interior surface of said aluminum to enhance entry there-into; and said interior surface is maintained at a depth sufficient-ly below the exterior surface such that at least 50% of material being added to the molten aluminum is recoverable in the molten aluminum.

2. A process for adding an alloying material to a molten metal media, said process being capable of adding said material in substantial quantity, comprising the steps of:
(a) providing a body of molten metal media, (b) providing a chamber means having an open discharge end positioned within said media body;
(c) introducing into said chamber means a gas comprising an ionizable gas under sufficient pressure to maintain the interior molten media surface substantially at said chamber's discharge end region;
(d) conducting an electric arc and maintaining a plasma within said chamber, said plasma substantially extending at least from said interior molten media surface to a site within said chamber and spaced from said interior media surface; and (e) supplying said material to said site within said chamber and converting said material into a superheated spray substantially within said plasma and carried toward said interior molten media surface.

3. A process as recited in claim 2 wherein said gas is substantially continuously introduced into the chamber means at a rate sufficient to project said material into said molten media and enhance mixing therewith.

4. A process as recited in claim 2 wherein said arc is maintained at a current sufficient to convert said material into a spray.

5. A process as recited in claim 2 wherein gas is sub-stantially continuously introduced at a rate and the chamber means is sized and shaped so as to provide projection of said material into said molten media.

6. A process as recited in claim 2, 3, or 4, wherein the added material is supplied to the chamber means in the form of solid wire or rod.

7. A process as recited in claim 2 wherein the molten media comprises aluminum.

8. A process as recited in claim 2 wherein the gas compri-ses a nonreactive gas.

9. A process as recited in claim 8 wherein said gas is seeded with at least one additive to promote arc stability or oxygen scavenging.

10. A process as recited in claim 2, 3, or 7, wherein said electric arc is formed by a direct current.

11. A process as recited in claim 2, 3, or 7 wherein the length of the chamber along the direction of projection into the molten media exceeds the transverse dimension of the chamber outlet.

12. A process as recited in claim 2, 3, or 7, wherein the material added comprises one or more of the group consisting of lead, bismuth, antimony, magnesium, zinc and copper.

13. A process as recited in claim 2, 3, or 7, wherein said arc discharge is established with a constant current power supply.

14. A process as recited in claim 2, 3, or 7, wherein the chamber means is made from alumina, borosilicate, mullite or silica.

15. A process as recited in claim 2, 3, or 7, wherein said molten media is moving past the discharge end of said chamber.

16. A process as recited in claim 2, 3, or 7, wherein the media is agitated to further enhance dispersion of said material within said media.

17. A process for adding an alloying, material to a molten metal media, said process being capable of adding said material in substantial quantity, comprising the steps of:
(a) providing a body of molten metal media;
(b) providing a chamber means having an open discharge end positioned a substantial depth within said media body, said chamber extending upwardly above an upper surface of said media;
(c) supplying said chamber means with a gas comprising an ionizable gas under sufficient pressure to maintain an interior molten media surface substantially at said chamber's discharge end region;
(d) supplying said material to a site within said chamber above said interior molten media surface;

(e) conducting an electric arc between said material at said site and said interior molten media surface;
(f) maintaining a plasma within said chamber substantially extending from said site within said chamber to said interior molten media surface; and (g) maintaining said arc at a sufficient current to convert said material into a superheated spray substantially within said plasma projected toward said interior molten media surface; wherein said supplying of said gas to said chamber is of sufficient rate to project said material into said media so as to enhance entry of said material into said media.

18. A process as recited in claim 17 wherein the chamber means is sufficiently immersed in the body of molten media such that the interior surface is maintained at a depth below the exter-ior surface which is sufficient to enable at least 50% of the alloying material to be recovered in the molten media.

19. A process as recited in claim 17 or 18 wherein the molten media is maintained in motion relative to the chamber discharge.

20. The process according to claim 2, 3, or 7, wherein substantial heat is transmitted to the molten media through the chamber portion submerged in the media.

21. The process according to claim 2, 3, or 7, wherein said chamber is immersed a substantial distance into said molten media such that said added material is introduced into said molten body at a substantial distance within said body.

22. The process of claim 1, 2, or 7, wherein the alloying material is lead.

23. A lead-alloyed, aluminum based metal article pro-duced according to the steps of: providing a body of molten aluminum media having an exterior surface and a submerged interior surface; converting lead alloying material into a spray of super-heated alloy material by establishing an electrical arc discharge between the submerged surface of the molten media and the alloying material, said discharge being maintained with a current that ex-ceeds the globular/spray transition current density of the lead alloying material; and directing the spray of superheated alloying material onto a submerged interior surface of the media so as to enable dissolution and dispersion of the lead alloying material into the media, said submerged surface being maintained at a predetermined depth below the exterior surface which is sufficient to enhance said dissolution and dispersion; said article having smaller, more acicular shaped and more uniformly sized and disper-sed particles of lead than that which is made by adding said lead at the surface or at a depth above said predetermined depth.