US2056234A - Degassing molten aluminum and its alloys - Google Patents

Description

Patented Get. 6, i936 IDEGAS SING MOLTEN AL 1 II'llS ALLOYS No lllira in g.

Application January 10, 1935,

Serial No. 1,200

8 Claims.

This invention relates to removing the deleterious effects of gas and insoluble oxides in aluminum and its alloys. One of the principal objects of the invention is the provision of means 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 physical properties in the solid state. Another object is to eliminate void spaces in castings which result from the release of gas. Still another object is to utilize agents which do not attack 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 the metal employed, whether it is of virgin or secondary origin, the composition of the alloy, and the conditions under which the melting is done. The effect of the 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. Such cavities, particularly those of the pinhole type may contribute to leakage of gas or liquid if the finished product is used as a container for a liquid or gas. When in the molten state, aluminum and its alloys are capable of sorbing 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 liberated. The released gas immediately accumulates and forms bubbles, some of which become trapped in the freezing mass with the resultant formation of a cavity or discontinuity in the metallic structure that is a source of weakness in the casting.

Various expedients have been heretofore tried in an effort to rid molten aluminum of any sorbed gas and, overcome its adverse effects. 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 a limited capacity for handling large quantities of liquid metal. Fused salts have been used on top of the molten charge to exclude air or furnace gases, but such fused salts do not penetrate the body of the melt. Such fused salts must be carefully excluded from the metal entering the mold and they are also likely to form a crust on the walls of the metal container which must be chipped off from time to time. In some instances the salts may attack the container, thereby weakening it and possibly introducing impurities into the metal bath. The passage of an inert or an active gas through the molten metal has also been proposed and tried. Although this practice has been partially successful under certain conditions, its limitations have precluded wide adoption.

I have found that the foregoing difilculties 10 may be obviated and the metal effectively degassed through a duplex treatment which con sists of adding sodium to the molten charge, permitting the sodium to permeate the bath, and then finally passing a gaseous fluorinated hydrocarbon through the liquid metal. Only a minute trace of sodium remains in the final product and no residues of the degassing agents are left in the metal to impair its quality. Furthermore, these substances do not attack the foundry equipment or damage the metal holding 20 containers.

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 25 than about 0.5 percent. The presence of magnesium appears to promote the sorption of gas, and this condition is especially apparent where a large part or all of the stock charged to the melting 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 as soon as the charge has melted, the temperature generally being below 40 about 1350 F. By adding the sodium at such a temperature, an economy of time is effected I in that the sodium has an opportunity to diffuse throughout the bath while the temperature is being raised to the proper point for a pouring of the metal. 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 escapes more slowly at a low than at a 50 high temperature and hence a smaller quantity of the element need be used when it is added at, say, 1250 F. than at 1450 F. This element may be added in metallic form or as a constituent of a rich alloy used to provide the de 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 elemental form or as a component of a rich alloy, the melt should be allowed to stand for some time 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 charge and the temperature. For charges of a few hundred pounds, a holding period of from 15 minutes up to an hour is gen erally sufiicient. Where several thousand pounds of metal are to be treated, a holding period of 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 loss through burning. Afterthe sodium has been added and allowed to permeate the metal charge, a gaseous fluorinated hydrocarbon is introduced which reacts with the sodium-treated metal to remove the gas. The gaseous substance may be introduced in any convenient-manner such as thrusting the open end of a refractory tube below the surface of the molten bath and passing gas through the tube. The gas should be bubbled through the metal at such a rate as to avoid dangerous spattering. The passage of the gas through the molten charge usually provides enough agitation to thoroughly mix the metal and obviate the necessity of stirring it by mechanical means. The amount of fluorinated hydrocarbon required depends upon the gas content and size of the charge. In general, about 1 to 4 ounces of gas per hundred pounds of metal treated is required and the time used to introduce the gas may vary between 1 and 15 minutes. I prefer to release the fluorine-bearing gas in the metal at a moderate rate of about 0.05 to 0.15 cubic foot per minute.

The molten metal may be treated either in the melting furnace or in the pouring ladle. I have found it to be most convenient to introduce the gaseous fiuorinated hydrocarbon when the metal is in the pouring ladle, for then there is a minimum exposure to air before filling the molds.

The class of substances known as the gaseous fiuorinated hydrocarbons referred to hereinabove includes substances of the type represented by difluorodichloromethane, CFzClz. Other members of this group having the following formulas, CFCla, CF3C1, C2F4Clz, and C2F3C13, exhibit the same characteristics but with varying degrees of effectiveness. The compound must be a hydrocarbon derivative, and contain fluorine, and preferably contain chlorine also as in the substances named above. Compounds of this class have a distinct effect in degassing aluminum previously treated with sodium.

When used alone, however, these substances do 'action differs from that where fused fluoride salts are brought in contact with the metal. I offer no explanation of the mechanics of the action by the sodium and fiuorinated hydrocarbon since other gaseous substances have been tried which would be expected to diminish the eifects of sorbed gas, and these failed to accomplish the desired result. The elimination of porosity in cast metal attributable to the release of gas appears to be the peculiar result of combining certain added substances within the molten metal, and is not a matter susceptible of prediction from the behavior of these substances in other environment. Sodium and difluorodichloromethane, for example, in combination achieve a result of which either substance alone is incapable and which cannot be foreseen from the properties of either material.

When the aluminum or aluminum base alloy is badly gassed, I have found it to be advantageous to first treat the molten charge with anhydrous aluminum chloride. This salt not only clears the metal of suspended dirt particles, but

it appears to aid the action of the sodium. From about to 4 ounces of the anhydrous salt are required per hundred pounds of metal treated. The aluminum chloride is preferably added in ,capsules containing from a few ounces up to a base alloys containing more than 3 per cent silicon have been so treated, but in these cases the alloy is cast soon after the sodium has been added because any considerable lapse of time after the addition defeats the purpose for which the element is added and the alloy becomes under-modified. In some other instances sodium has been retained as an intentional alloying constituent, but in the present case only a trace of the element remains in the final product. Af ter the sodium has accomplished its purpose in the molten alloy, there is nothing to be gained by retaining it in the solid alloy. I have furthermore ascertained that sodium alone does not completely degas aluminum and aluminum base alloys on a commercial scale, but that it must be employed in combination with some other substance to achieve the desired result.

My invention is particularly adapted to the treatment of secondary metal. The remelting of foil scrap has presented an especially difficult problem because of the extreme thinness of the metal sheet compared to area, and the resultant ease of oxidation in the melting furnace. The melt obtained from reclaiming foil scrap therefore contains a large amount of oxide particles and occluded gas. To render the metal fit for subsequent use, it must be treated to remove the effect of the'gas in creating blowholes or other void spaces in the casting. As an example of the eflicacy of my duplex treatment in reclaiming foil scrap, one test may be cited where a 17,000

nealed. The, balance of the charge in the fur- 'nace was then treated with about 0.1 per cent sodium and allowed to stand for about 6 hours at a temperature between 1350 and 1400 F. A portion of this metal was tapped from the furnace. poured into ingot molds, and rolled to sheet. When annealed, the sheet revealed some blisters as did the metal treated with difluorodichloromethane. The remainder of the charge in the furnace was transferred to the pouring ladle and difluorodichloromethane passed through each ladle full of metal for a period of 2 minutes.

Sheet obtained from this part of the original charge revealed no, blisters indicating that the effects of liberated gas had been efiectively overcome.

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 the melt at a temperature below about 1475" F. fora period of about 1A to 10 hours and thereafter passing'a gaseous fluorinated hydrocarbon through the melt for a period of about 1 to 15--minutes.

2. A method of "degassing molten aluminum and aluminum base alloys comprising adding thereto about 0.01 to 1 per cent sodium, allowing the sodium to difiuse throughout the melt, and thereafter passing a gaseous fluorinated hydrocarbon through said molten metal.

3. A method of degassing molten aluminum'and aluminum base alloys comprising adding thereto from about 0.01 to 0.1 per cent sodium, maintaining the melt within the temperature range of 1325' to 1425 F. for a period or it, to 10 hours and thereafter passing gaseous difluorodichlorm the melt below about1475 methane through said meltror 1 to 15 minutes.

4. 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 said alloy, maintaining the melt at a temperature below 1475v F. for a period of about A; to 10 hours, and thereafter passing a gaseous fluorinated hydrocarbon through said melt for a period of about 1 to 15 minutes.

5. A method of degassing molten aluminum and aluminum base alloys comprising adding thereto from about 0.01 to 0.1 per cent sodium, maintaining the melt at a temperature below about 1475 F. for a period of $4 to 10 hours, and

-thereafter passing a gaseous fluorinated hydrocarbon through said melt at the rate of about 0.05 to 0.1 cubic foot per minute for aperiod of about 1 to 15 minutes.-

6. In a method of degassing molten aluminum and aluminum base alloys with sodium and a gaseous fluorinated hydrocarbon, the steps of adding from about 0.01 to 1 per .centsodium to the molten charge and allowing said charge toremain at a temperature below about 1475 F. for a period of A to 10 hours before introducing the gaseous fluorinated hydrocarbon.

'7. A method of degassing molten aluminum and aluminum base alloys comprising preliminarily introducing from about to 4 ounces of anhydrous aluminum chloride per hundred pounds of charge treated, adding from about 0.01 to 1 per cent sodium to said to 10 hours, and finally passing a gaseous fiuori nated hydrocarbon through the melt for a period of about 1 to 15 minutes.

8. A method of degassing molten aluminum and aluminum base alloys. comprising preliminarily treating the molten charge with anhydrous aluminum-chloride, adding sodium in quantities suflicient to serve as a degassifier and allowing the sodium to diffuse throughout the metal, and

thereafter passing a gaseous fluorinated hydrocarbon through said molten metal.

PHILIP T. STROUP.

charge, maintainin F F. for a period of A