US2056233A - Degassing molten aluminum and its alloys - Google Patents

Description

Patented Oct. 6, 1936 UNITED STATES PATENT OFFICE DEGASSING MOLTEN ALUMINUM AND ITS ALLOYS No Drawing.

Claims.

This invention relates to removing the deleterious eifects of gas and insoluble oxides in aluminum and its alloys. One of the principal v objects of the invention is the provision of means 5 for degassing and purifying large masses of molten metal in an economical yet efficacious manner. Another object is to employ agents for this purpose which leave substantially no residue or undesirable constituent in the metal charge after the treatment. A further object is to degas the metal in such a way as not to impair its casting qualities or its physical properties in the solid state. Still another object is to utilize degassing agents which do not attack the foundry or metal handling equipment.

Molten aluminum and aluminum base alloys are known to sorb a considerable quantity of gas and the amount found in a particular melt depends upon the character of metal employed, Whether it is of virgin or secondary origin, and the conditions under which the melting is done. The eifect of gas in the metal is manifested in the cast product through the occurrence of blowholes and so-called pinhole porosity, and by slivers or blisters on the surface of wrought articles. When in the molten state, the metal is capable of sorbing some gas but as the liquid metal cools and solidifies in the mold, the sorption of the gas decreases with the result that free gas is released. The released gas immediately accumulates and forms bubbles, some of which become trapped in the freezing metal with the resultant formation of a cavity or discontinuity in.

the metallic structure which is a source of weakness in the casting.

Various expedients have heretofore been employed in an effort to'rid the molten aluminum of its gas content and thus overcome the adverse eifects of gas. Melting the charge under a vacuum has been tried and while successful on a laboratory scale in extracting gas from the metal, special equipment is required which has an extremely limited capacity for handling large molten charges. Fused salts on the surface of the molten bath have been tried both as a means of absorbing oxide particles as well as excluding air from the metal. Such fused salts must be carefully excluded from the stream of metal entering the mold, and they are also likely to form a crust around the walls of the metal container which must be removed from time to time. The passage of an inert or active gas through the liquid metal has also been proposed and tried. 5 Although this practice has been partially success- Application January 10, 1935, Serial No. 1,199

ful under some conditions, its limitations have precluded wide adoption.

I have found that the foregoing difficulties may be obviated and the metal effectively degassed through use of a duplex treatment which consists of adding sodium and aluminum chloride to the molten charge. The sequence of making the additions is important in that a treatment with aluminum chloride should follow the introduction of sodium. The sodium may be first added to the melt which has received no preliminary cleansing treatment, and the sodium allowed to difiuse throughout the molten charge before the aluminum chloride is added. My preferred practice, however, is to first treat the molten bath with aluminum chloride, then add the sodium, allow it to difiuse, and finally introduce another charge of aluminum chloride. I have discovered that an initial treatment with aluminum chloride not only clears the melt of suspended dirt particles, but that it accelerates and renders the action of the sodium more effective. The final addition of aluminum chloride reacts to dislodge the gas and virtually clear the bath of any traces of sodium. The sodium and aluminum chloride cooperate to degas the metal to an extent that either substance used alone is unable toaccomplish. I designate my treatment as a duplex process because two substances are employed to degas the metal rather than because of the number of steps involved in the use of these substances.

Although molten aluminum and aluminum base alloys generally sorb gas, this problem is particularly acute in alloys containing a substantial amount of magnesium, that is, more than about 0.5 per cent. The presence of magnesium appears to promote the sorption of gas and the condition is especially apparent where a large part or all of the stock charged to the furnace is of secondary origin. Methods heretofore used in treating aluminum and its alloys have either failed or proved to be only partially successful in degassing alloys containing more than 0.5 per cent magnesium. Through the application of my duplex treatment, however, virtually all traces of gas are removed.

In practicing my invention, the sodium is preferably added to the molten charge as soon as practicable after the charge has melted. In this way an economy of time is effected since the sodium has opportunity to diffuse throughout the charge while the metal bath is being brought up to the desired pouring temperature. A period of from about 15 minutes to 10 hours must in any case be allowed for the charge to stand after the sodium has been added in order to secure the proper diffusion. The sodium also escapes more slowly at lower temperatures and thus a smaller quantity will accomplish the same result as a larger amount added at a much higher temperature. This element may be added in metallic form or as a constituent of a rich alloy used to provide the desired ingredients of the final product. When added in metallic form the sodium is preferably introduced in small pieces of a pound or two in weight at intervals of several minutes rather than making the addition of the total amount at one time. When added as a component of a rich alloy the foregoing precautions are unnecessary and hence in many cases the introduction of sodium in this manner is most convenient and effective. If the sodium is introduced in metallic form, I have found that a special sodiumizer having only a few holes is a very satisfactory means of holding the solid sodium in the bottom of the charge until it has completely melted and diffused into the metal bath.

Whether the sodium is added in the elemental form or as a component of a rich alloy, the melt should be allowed to stand for some time after the sodium addition to secure complete diffusion of the element throughout the bath. The time required to accomplish this purpose varies with the size of the charge and the temperature. For charges of a few hundred pounds a holding period of from 15 minutes up to an hour is generally sufficient. Where several thousand pounds of metal are to be treated, a holding period of from 2 to 10 hours is usually required. During this time the temperature of the melt should not exceed about 1475 F., the preferable range being 1325 to 1425 F. Within this temperature range the action of the sodium is hastened but it is not too rapid, nor is the temperature high enough to cause excessive Ioss through burning.

The amount of sodium to be added depends upon the quantity of gas in the melt; for example, a charge of scrap metal or a magnesium-bearing alloy is likely to contain more gas than a charge of virgin stock. For most purposes from about 0.01 to 0.1 per cent of sodium in relation to the weight of the whole charge is sufficient. For an excessively gassed charge, however, as much as 1 per cent may be added. The amount of sodium remaining in the final product is very small, however, and should not exceed about 0.005 per cent.

Although the sodium may be introduced into the molten charge without any preliminary cleansing of the charge, I have found that an initial treatment with aluminum chloride removes most of the dirt admixed with the melt, and that the action of the sodium is thereby facilitated. The salt not only clears the metal of suspended dross particles but it creates a favorable condition for the action of sodium. When exposed to the air metallic sodium becomes coated with a film which sorbs moisture with the result that some gas is introduced along with the sodium when it is added to the molten charge. The presence of a minute amount of aluminum chloride, however, appears to react with this film and forms a product which does not gas the metal. If, on the other hand, there is no preliminary treatment with aluminum chloride, a longer holding period is required subsequent to the addition of sodium to promote the degassing process. The action of the two substances-does not in any case appear to be restricted to a more mechanical agitation of the liquid metal. The quantity of anhydrous aluminum chloride needed for the preliminary treatment varies with the size and character of the charge, from about to 4 ounces of the salt per hundred pounds of charge being sufficient. The salt may be conveniently added to the melt by placing it in aluminum capsules holding as much as one pound and submerging these in the bath by means of a sodiumizer.

Whether or not aluminum chloride is initially used to treat the molten charge, it is essential that it be introduced after the sodium has diffused throughout the melt. When this has occurred the anhydrous salt is then added to the molten charge while it is in the furnace or in the pouring ladle, or some is added both in the furnace and the ladle. The salt serves both to complete the degassing and to reduce the sodium content of the metal. If the last treatment with aluminum chloride is omitted, the metal will be only partially degassed.

The amount of aluminum chloride employed for the treatment after the sodium addition may vary between about and 4 ounces per hundred pounds of metal treated. When a portion is added in the furnace and the balance in the ladle, I prefer to add the larger portion in the furnace, so as to minimize any cooling of the metal in the ladle, which would occur if a great number of capsules were added. In adding the salt to the furnace, capsules or packets containing as much as one pound may be used, but in adding it to the ladies only an ounce or two should be placed in each capsule and the capsules added one after the other, rather than attempting to introduce the entire amount at one time either in a single container or in several capsules.

I am aware that both sodium and aluminum chloride have been separately employed in the treatment of aluminum and certain aluminum base alloys. Sodium has been extensively used to modify the structure of aluminum-silicon alloys, and when added for this purpose no substantial time interval is allowed to elapse between the addition of this element and the pouring oi the alloy into the mold. To permit a marked lapse of time to occur defeats the purpose for which the element is added and the alloy becomes under-modifled. In some other cases, however, sodium is retained as a recognized constituent of the alloy. In contrast to such uses of this element, I introduce it for the purpose of assisting in removing the effect of any sorbed gas, and after this has been accomplished there is nothing to be gained by retaining it in the alloy. Only a slight trace, if any, remains in the final product. I have furthermore ascertained that the use of sodium alone does not effectively degas aluminum and aluminum base alloys on a commercial scale; that is, some gas remains in the metal which is released upon solidification. It has only been through the combined use of sodium and another substance that I have been able to rid the metal of the deleterious effects of gas and finely dispersed oxide particles.

The use of aluminum chloride alone has been proposed as a suitable fiuxing agent for aluminum base alloys but its utility has been confined to the removal of dirt. According to tests which I have made, treatment with this substance alone fails by a large margin to entirely eliminate the effects of gas, although it partially accomplishes this result by the use of large amounts on small melts. The duplex treatment herein described and in co-pending application Serial No. 1,200 has been the only effective means of completely eradieating the undesirable effects of gas on a commercial scale. While I oiler no explanation of the reaction which occurs in the metal bath. it nevertheless appears that the sodium and aluminum chloride mutually act upon the sorbed gas to disengage it more eifectively than when either substance is used alone.

An illustration of the efiectiveness of the duplex treatment described hereinabove is'shown by the following example. An alloy composed of aluminum, 4.2 per cent copper, 0.5 per cent manganese and 1.5 per cent magnesium is prone to develop blisters on heat treated sheet, particularly if scrap metal alone is used in making up the charge. Under commercial operating conditions a large rejection of sheet is frequently encountered because of the presence of numerous blisters attributable to the release of gas. A 17,000 pound charge of the above alloy in the form of scrap metal was melted under the usual conditions. Approximately i pounds of sodium were added in parcels of' about 1 to 2 pounds each shortly after the charge became molten. The melt was then allowed'to stand for 2 hours at a temperature between 1350 and 1425 F. At the end of this time pounds of anhydrous aluminum chloride in 1 pound capsules were introduced into the metal. The charge was permitted tostand for 4 hours longer within the same temperature range before pouring it into ingot. From about 1 to 3 ounces of aluminum chloride were added to the metal in the ladle from which each 250 pound ingot was poured. Sheet made from these ingots and heat treated failed to show any blisters.

I have also found that the treatment is effective in eliminating gas blisters in sheet made from remelted foil scrap. Foil scrap is especially difficult to remelt because of the large area of metal exposed to the atmosphere. In remelting such stock oxide particles are introduced in the melt. and these particles appear to have an aillnity for gas. Other means of treating the melt than the one outlined above have failed to produce blisterfree sheets. The duplex treatment is especially useful in reclaiming metal and alloys of secondary origin.

I claim:

1. A method of degassing molten aluminum and aluminum base alloys comprising adding thereto from about 0.01 to 1 per cent of sodium, maintaining said molten charge at a temperature below about 1475 F. from 4; to 10 hours, and introducing from about /2 to 4 ounces of anhydrous aluminum chloride per hundred pounds of melt.

2. A method of degassing molten aluminum and aluminum base alloys comprising adding sodium in the molten charge, allowing the sodium to diffuse throughout the melt, and thereafter introducing anhydrous aluminum chloride into said melt.

3. A method of degassing molten aluminum and aluminum base alloys with sodium and aluminum chloride comprising initially treating the melt with anhydrous aluminum chloride, adding less than 1 per cent of sodium, maintaining said melt at a temperature below about 1475" F. for to 10 hours, and finally treating the melt with anhydrous aluminum chloride.

4. A method of degassing molten aluminum and aluminum base alloys with sodium and aluminum chloride consisting of adding to the molten charge from about to 4 ounces of anhydrous aluminum chloride per hundred pounds of metal treated, thereafter adding from about 0.01 to 1 per cent of sodium and maintaining the melt at a temperature below about 1475 F. for a period of A to 10 hours, and finally repeating the treatment with anhydrous aluminum chloride.

5. A method of degassing molten aluminum base alloys containing more than 0.5 per cent magnesium comprising adding from about 0.01 to 1 per cent sodium to the melt, allowing the molten charge to stand for to 10 hours at a temperature between about 1325 and 1425 F. and thereafter introducing from about /2 to 4 ounces of anhydrous aluminum chloride per hundred pounds of metal treated.

6. A method of degassing molten aluminum and aluminum base'alloys, comprising adding thereto from about 0.01 to 0.1 per cent of metallic sodium, allowing the melt to stand until the added sodium has diffused throughout the mass, and thereafter introducing from about to 4 ounces of anhydrous aluminum chloride per hundred pounds of metallic charge treated.

7. In a method of degassing molten aluminum and aluminum base alloys with sodium and aluminum chloride, the step of initially treating the melt with from about to 4 ounces of anhydrous aluminum chloride per hundred pounds of metal treated.

8. In a method of degassing molten aluminum and aluminum base alloys with sodium and alu minum chloride, the steps of initially treating the melt with anhydrous aluminum chloride, adding from about 0.01 to 1 per cent of sodium and repeating the treatment with aluminum chloride.

9. In a method of degassing molten aluminum and aluminum base alloys with sodium and aluminum chloride, the step of adding from about 0.01 to l per cent sodium to the melt and maintaining said melt at a temperature below about 1475 F. for a period of to 10 hours before the final treatment with aluminum chloride.

10. A method of degassing molten aluminum base alloys containing more than 0.5 per cent magnesium with sodium and aluminum chloride comprising introducing from to 4 ounces of anhydrous aluminum chloride per hundred pounds of molten charge, thereafter adding from about 0.01 to 0.1 per cent sodium, maintaining the melt below about 1475" F. for a period of A; to 10 hours, and finally repeating the treatment with aluminum chloride.

PHILIP T. STROUP.

CERTIFICATE OF CORRECTION.

Patent No. 2,056,233. October 6, 1936.

PHILIP T. STROUP.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 2., first column, line '74, for the word "more" read mere; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 8th day 'of December, A. D. 1956.

Henry Van Aradale (Seal) Acting Commissioner of Patents.