US2160812A

US2160812A - Making castings of aluminum and aluminum-base alloy

Info

Publication number: US2160812A
Application number: US194766A
Authority: US
Inventors: Alden John Herbert; Stroup Philip Trimble; Budge Pasqual Michael
Original assignee: Aluminum Company of America
Current assignee: Howmet Aerospace Inc
Priority date: 1938-03-09
Filing date: 1938-03-09
Publication date: 1939-06-06
Anticipated expiration: 1956-06-06

Description

Patented June 6, 1939 UNITED STATES PATENT OFFICE MAKING CASTINGS ALUMHVUM- John Herbert Alden New Kensington,

OF ALUMINUM AND BASE ALLOY .1 Philip Trimble Stroup, and Paaqual Michael Budge, Fairfield, Conn, assignors to Aluminum Company of America, ration of Pennsylvanl Pittsburgh, Pa., a corpoa No Drawing. Application March 9. 1938.

Serial No. 194,766

13 Claims.

This invention relates to the casting of aluminum and aluminum-base alloys, that is, alloys in which aluminum is not less than about 50 per cent of the whole, particularly aluminum alloys containing copper as the principal alloying element or one of such elements. More specifically the invention relates to the prevention of the defect commonly known as "pinhole porosity" in castings, especially castings of intricate shape.

This defect has long been known in the art, and

various methods of combating it have been suggested. based 'on the theory that it is caused by the presence of gas in the melt and the release of the gas as the metal solidifies. One method proposed for overcoming the evil consists in mechanically agitating the molten metal in an atmosphere of nitrogen, carbon monoxide, carbon dioxide, or other hydrogen-free gas which is also free from water vapor, while the surface of the melt is covered with a flux capable of dissolving oxides present or formed during the agitation. This process is based on the theory that the most important factor is the taking up of hydrogen by the melt which frequently comes from the reduction of water by the hot metal in suflicient amount to impair to a serious extent the de-gassing effected by the mechanical agitation. Mechanical agitation, however, is inadequate in some cases, as in casting finned cylinder heads for aircraft engines. In such cases our experience shows that to insure substantially complete freedom from pinhole porosity the agitation should be eifected by bubbling a gas through the molten metal while protecting the metal from contact with excessive moisture. We have further found that the use of an atmosphere of nitrogen or an oxide of carbon is unnecessary, and that an atmosphere of dried air, otherwise of normal constitution, is equally if not more effective. The essential feature of our process accordingly consists in bubbling a gas through the molten metal while the metal is in an atmosphere of dry air. In general the moisture-content of the atmosphere should not exceed 1 grain per cubic foot. and may with ad- 46 vantage be not more than .5 grain per cubic foot,

especially when the alloy contains copper in substantial amount, say about 2 per cent or more. The permissible moisture content of the air depends chiefly upon the pouring temperature and the qualitative and quantitative composition of the metal, the pouring temperature itself depending largely upon the intricacy of the casting. These factors are widely variable, and it will therefore be recognized that an upper limit of as 1 grain per cubic foot is to a certain extent arbitrary. That is, it is not critical as meaning that more moisture is always fatal and a little less always effective, but experience indicates that in general more than the amountjust stated is apt to be ineffective, and that for castings having many thin sections and thus requiring a relatively high pouring temperature the moisture content should not be more than .5 grain per cubic foot. The bubbling oi the gas through the molten metal has a fluxing effect in that dross inclusions and the like are brought to the surface, and if the passing of the gas through the melt is effected in an atmosphere of dried air and continued in such atmosphere until the metal is ade-' quately clean it will usually be found that enough gas has been removed to permit castings to be made which are substantially free from the defects sought to be eliminated, as can be determined by trial.

I'he de-gassing of the molten alloy can be eifected to any extent necessary or desirable by means of any hydrogen-free non-oxide-forming gas, preferably dry chlorine or other halogen. By hydrogen-free gas we mean one which does not contain or release hydrogen at the temperature of the de-gassing operation. The gas can be introduced as such through the bottom or sides of the vessel containing the melt, or through one or more pipes extending into it, or by the thermal decomposition or volatilization of one or more compounds, by preference a halogen-containing compound, submerged in the molten metal. Aluminum or zinc chloride can be used advantageously, or a mixture of the two. Other conditions being the same, the extent of de-gassing necessary in our process for the production of castings substantially free from pinhole porosity depends largely upon the qualitative and quantitative composition of the metal. For an alloy composed of copper 10 per cent, iron 1 per cent, magnesium .25 per cent, and the rest aluminum with the usual impurities, an effective practice is to pass through the melt such quantity of chlorine as would amount (at atmospheric pressure) to 0.03 to 0.04 cubic foot per pound of metal. A convenient method for the purpose is to draw oil from a cylinder of the gas at high pressure an amount of gas which would, when expanded to a convenient lower pressure, afford the above quantity per pound of metal. For example, for a melt of 2'75 pounds of the above alloy having an exposed surface area of about 300 square inches and a depth of about 13 inches, we are accustomed to take about 3.15 cubic feet of chlorine at 60 pounds pressure per square inch, discharge the gas from perforations around the lower portion of a pipe extending into the melt from the top down to the bottom of the vessel, and continue the discharge until the pressure (as shown by the gauge on the cylinder containing the gas) has fallen to 10 pounds. This usually takes about 12 minutes. We have also found that the amount stated is suitable for other compositions, for example an alloy composed of copper 4 per cent, nickel 2 per cent, magnesium 1.5 per cent, and the rest aluminum with the usual iron and silicon impurities. In any case an advantageous amount can be determined by trial with a few castings. We have observed further that with different batches of alloy of supposedly the same composition it is sometimes necessary to increase the amount of chlorine, and that on the other hand the amount may sometimes be decreased. Alloys containing still less copper, and even none at all, are in some cases liable to give castings having pinhole porosity, and where this type of porosity is found to occur with such alloys they will in general be benefited by the treatment described above, and in some cases, easily determined by trial, the amount of chlorine can be decreased.

The following is an example of practicing our invention in an efficient manner. The alloy, previously compounded to a desired composition, is melted and skimmed in the ordinary way, usually without fiuxing, and is transferred to a holding furnace through which a current of dried air is passed. The drying of the air may be efi'ected in any convenient way, preferably by! means of activated alumina or other eiilcient adsorbent of moisture. The holding furnace need not be closed if the current of dried air is of sufficient volume to insure protection of the metal from outside air which contains more than the minimum amount of moisture as determined by experience with the particular alloy and other conditions obtaining. While in the atmosphere of dehydrated air the melt is de-gassed by bubbling chlorine through it. and the metal is raised to the desired pouring temperature. In general we have found it advantageous to maintain a dry air atmosphere over the melt until the metal is ladled out for pouring into the molds. Notwithstanding that the dry air contains about 20 per cent of oxygen, and that the hot, agitated, readily oxidizable metal is therefore constantly bathed or in contact with a large volume of free oxygen, we have found that the resulting oxidation is inconsequential. This we have observed to be true in the difficult operation of casting airplane engine cylinder heads, elaborately finned for cooling purposes, from an aluminum alloy containing copper 10 per cent, iron 1 per cent, and a small amount of magnesium (less than 1 per cent. say .25 per cent).

In this case the melt was raised, in the holding furnace, from a temperature between 1200" and 1250 F. to a pouring temperature between 1400 and 1500 F., with the current of dry air (containing a residual moisture content of about .5 grain per cubic foot) maintained at full volume, which for about 2'75 pounds of metal, having an exposed area of about 300 square inches and a depth of approximately 13 inches, is preferably 135 to 140 cubic feet per minute; though for purposes of economy about a quarter of the volume stated is employed in completing the heating of the melt to the pouring temperature if the desired temperature has not been attained during the degassing operation.

In another emcient practice of our method,

with the 4 per cent copper alloy given above, the melting, de-gassing, and raising to the pouring temperature were effected in the same furnace. With 300 pounds of the alloy in as oylindrical vessel 1'! inches, approximately, in diameter inside, and having a hemispherical bottom, the exposed surface area of the melt is about 227 square inches and the greatest depth is about 20 inches. With this charge the dried air was turned on when the metal began to melt, at which time the temperature is about 1100 F. Maintaining the fire under the pot the temperature is brought up to 1400" F. The passing of chlorine through the melt is then begun, approximately 16 or 1'? cubic feet of the gas being passed through in about 18 minutes. During this operation the temperature of the melt may fall somewhat, say to 1350 F. A temperature of 1380 F. is suitable for this alloy but to allow for incidental cooling between the furnace and the mold the melt is brought to 1400" F. or somewhat higher, the flow of dried air over the metal being continued during this reheating. In the degassing operation the chlorine used was taken from a tank containing approximately 3.5 cubic feet of the gas at 60 pounds pressure per square inch above atmospheric, and the gas was bubbled through the molten metal until the pressure had fallen almost to atmospheric.

It is to be understood that the invention is not limited to the details herein specifically described but can be carried out in other ways without departing from its spirit as defined by the subjoined claims.

We claim-- 1. In a method of making castings oi aluminum and aluminum-base alloys free from pinhole porosity, the step comprising cle-gassing the molten metal by bubbling a non-oxidizing hydrogen-free gas through the metal while in an atmosphere of air in which any content of moisture present has been reduced to not more than 1 grain per cubic foot.

2. In a method of making castings of aluminum and aluminum-base alloys free from pinhole porosity, melting the alloy, protecting the molten alloy from contact with excess moisture by means of an atmosphere of air in which the moisture content is not more than 1 grain per cubic foot, and while the alloy is so protected de-gassing it by bubbling chlorine through the molten metal.

3. In a method of making castings of aluminum and aluminum-base alloys free from pinhole porosity, comprising melting the alloy. Drotecting the molten alloy from contact with excessive moisture by means of an atmosphere of air in which the moisture content is not more than .5 grain per cubic foot, and while the alloy is so protected, de-gassing it by bubbling a nonoxide-forming gas through the molten metal, and heating the metal to the pouring temperature.

4. In a method of making castings of aluminum and aluminum-base alloys free from pinhole porosity, melting the alloy, transferring the alloy to a holding chamber, drying air to a moisture content not more than '1 grain per cubic foot, delivering such dried air to the holding chamber to provide a protective atmosphere therein, and while the alloy is in such atmosphere de-gassing it by bubbling a non-oxidizing hydrogen-free gas through the molten metal.

5. In a method of making castings of aluminum and aluminum-base alloys free from pin- 15 hole porosity, melting the alloy, transferring the alloy to a. holding chamber, providing therein an atmosphere of air in which the moisture content is not more than .5 grain per cubic foot, and while the alloy is in such atmosphere de-gassing it by bubbling a non-oxidizing hydrogen-free gas through the molten metal.

6. In a method oi making castings of aluminum and aluminum-base alloys free from pinhole porosity, melting the alloy, transferring the alloy to a holding chamber, providing therein an atmosphere in which the moisture content is not more than .5 grain per cubic foot, and while the alloy is in such atmosphere de-gassing it by bubbling chlorine through the molten metal.

'7. In a method 01' making castings o1 aluminum-copper alloy free from pinhole porosity, the step comprising de-gassing the molten metal by bubbling a non-oxidizing hydrogen-free gas through the metal while in an atmosphere of air from which any content of moisture present has been reduced to not more than .5 grain per cubic i'oot.

8. In a, method of making castings oi aluminum and aluminum-base alloys free from pinhole porosity, melting the alloy in an atmosphere oi! air in which any content of hoisture has been reduced to not more than 1 grain per cubic foot, and while in such atmosphere de-gassing the alloy by bubbling chlorine through it.

9. In a method of maldng castings of aluminum and aluminum-base alloys free from pinhole porosity, melting the allow in a current of air having a moisture content not exceeding .5 grain per cubic foot, de-gassing the alloy by bubbling through it a non-oxidizing hydrogenfree gas while the alloy is protected by such current, and thereafter decreasing the volume of the current and while the alloy is protected by the current of decreased volume raising the alloy to a desired pouring temperature.

10. In a method or making castings of aluminum and aluminum-base alloys tree from pinhole porosity, de-gassing the molten alloy in a current of air having a moisture content not exceeding .5 grain per cubic foot.

11. In a method of making castings of aluminum and aluminum-base alloys free from pinhole porosity, melting the alloy, transferring the alloy to a holding chamber, passing through the chamber a current of air having a moisture content not exceeding .5 grain per cubic foot, degassing the alloy by bubbling through it a nonoxidizing hydrogen-free gas while the alloy is protected by such current, and thereafter decreasing the volume of the current and while the alloy is protected by the current of decreased volume raising the alloy to a desired pouring temperature.

12. In a method of making castings oi aluminum and aluminum-base alloys free from pinhole porosity, melting the alloy, transferring the alloy to a holding chamber, passing through the chamber a current of air having a moisture content not exceeding .5 grain per cubic foot, degassing the alloy by bubbling chlorine through it while it is protected by such current, and thereafter decreasing the volume of the current and while the alloy is protected by the current of decreased volume raising the alloy to a desired pouring temperature.

13. In a method of making castings of aluminum and aluminum-base alloys free from pinhole porosity, de-gassing the molten alloy in a current or air having a moisture content not exceeding .5 grain per cubic foot by bubbling chlorine through the alloy, and thereafter decreasing the volume of the current and while the alloy is protected by the current of decreased volume raising the alloy to a desired pouring temperature.

JOHN HERBERT ALDEN. PHILIP 'I'RIMBLE STROUP. PASQUAL MICHAEL BUDGE.

CERTIFICATE OF CORRECTION.

Patent no. 2,160,812.

June a 9 JOHN HERBERT ALDEN, ET AL.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 3, first column, line 27, claim 8, for "hoisture' read moisture; line 35, claim 9, for the word "allow" read alloy; and that the said Letters Patent should be read of the case in the Patent Office.

with this correction thereinthat the some may conformto the record Signed and sealed thiellth day of July, A. D. 1939.

(Seall Henry Van Arsdale Acting Commissioner oi Patmts.

hole porosity, melting the alloy, transferring the alloy to a. holding chamber, providing therein an atmosphere of air in which the moisture content is not more than .5 grain per cubic foot, and while the alloy is in such atmosphere de-gassing it by bubbling a non-oxidizing hydrogen-free gas through the molten metal.

JOHN HERBERT ALDEN. PHILIP 'I'RIMBLE STROUP. PASQUAL MICHAEL BUDGE.

CERTIFICATE OF CORRECTION.

Patent no. 2,160,812.

June a 9 JOHN :m' :m- ALDEN, ET AL.

(Seall Henry Van Arsdale Acting Commissioner oi Patmts.