US2062818A - Molding sand - Google Patents

Description

Patented Dec. 1, 1936 UNITED STATES MOLDING sANn Roy E Paine, Cleveland, Ohio, assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application November 20, 1934,

Serial No. 753,900.

I 'l Claiins. This invention relates to providing a molding sand composition which is adapted to the production of green sand molds wherein aluminum-magnesium alloys may be cast'without any unfavorable reaction between the metal and the atmosphere or the components of the molding composition.

In the art of casting easily oxidizable metals in green sand molds, which are herein defined to be molds made from water tempered sand, considerable difl'iculty has been encountered because of the reaction between the metal and the water in the molding sand. This reaction results in. the formation of a heavy oxide skin or dross on the surface of the metal which may interfere with the flow of 'metal in the mold. The dross is also likely to adhere to the casting and produce a rough unsatisfactory surface. In the case of certain alloys the reaction may occur below the surface of the casting the result of which cannot be seen by ordinary inspection. Various expedients have been tried in an effort to eliminate water from themolcl, for example, such practices .as drying the mold before introducing the metal, or using a non-aqueous binder have been employed, but they either add to the cost of producing. castings or impair the'molding characteristics of the sand. An extensive search has therefore been made to find more practicable means of overcoming the ill effect of water upon the metal poured in a green sand mold.

The casting of aluminum-magnesium alloys in molds made from water tempered sand presents a peculiarly difflcult problem because of the formation of a porous black ring or frame immediately below the surface of the casting. The occurrence of frame is definitely associated with the presence of water since it does not appear in castings produced in either dry or non-aqueous tempered sand molds; The occurrence of frame is undesirable for the reason that it creates a porouszone below ness is likely to result in an early failure in service.

Oxidation inhibitors have been employed in the production of sand molds for casting magnesium 5 not borne out such an assumption. The differences between magnesium base, and aluminum base-magnesium alloys are well defined. For example, magnesium'base alloys are only attacked on the surface when-cast in green sand molds whereas aluminum base-magnesium alloys develop a sub-surface oxidized zone which cannot be seen by ordinary visual inspection. In commercial practice different types of sand are used in making sand molds for the'casting of magnesium and aluminum base alloys, a coarse porous sand being employed for the former kind of alloys and a fine grained sand for the latter variety of alloy. This difference in the sand used necessitates different molding practices, and-what might be effective in treating one type of sand would not be adapted to use in the other type. A furtherdistinction between the two kinds of base alloys lies in the difference in response to substances intended to inhibit oxidation. Sulphur, for example, which'has been commonly used in sand molds for the production of magnesium base alloy castings is entirely ineffective in preventing the formation of framein an aluminum basemagnesium alloy casting. Other substances also, such "as urea, which are satisfactory for protecting magnesium base alloys afford. little or no protection against the occurrence of frame in aluminum basemagneslu.m alloys. As a result of my investigation it may be safely said that no prediction can be made as to the behavior of one type of alloy in the presence of a protective substance, from the action of the same substance upon the other type of base alloy.

One of the objects of my invention is to inhibit the formation of frame in aluminum basemagnesium alloys that are cast in green sand molds. Another object is to incorporate a substance in the molding sand which Will not only inhibit the formation of frame but, will] not destroythe molding quality of the sand. Still another object is to employ a substance inthe manufacture of sand molds which is not volatile at ordinary temperatures and does not react with the water used to temper the sand. A further object is to use a substance thatdoes not yield noxious fumes in contact with the molten metal in the'mold.

I have found after an extended investigation that the incorporation of from about 1 to 10 per cent of ammonium fluoride in molding sand tempered with water effectively inhibits the formation of frame in aluminum-magnesium alloys east in molds made from such a mixture. If desired, the 'mold may be first made in the usual manner without mixing any ammonium fluoride with the sand, and, then the surface of the mold easily vaporizable alcohols as methyl alcohol and,

2' cavity may be treated by spraying with a solution of the salt or simply dusting thewalls with the powdered salt prior to pouring metal therein.

I have found it advantageous to employ an alcoholic solution of ammonium fluoride for spraying purposes and allow the solvent to evaporate be fore introducing the metal into the mold. Such ethyl alcohol are satisfactory solvents of ammonium fluoride. The inhibitive effect of the ammonium fluoride is the same whether it is mixed with the sand or merely sprayed on the surface of the mold. The manner of using the protective agent will in any casedepend upon the nature and number of castings to be made. Where only a few castings are desired, it is obvious that it would be more economical to coat the mold cavity than to incorporate the salt with the sand mixture.

The sands used in making green .sand molds, where my invention is practiced, are tempered with about 4 to 9 per cent of Water; that is, just enough water is added to the molding sand mixture to make it cohesive when molded. The other components of the mixture need not be .varied on account of' the use of ammonium fluoride, since which was not visible from the surface by ordinary inspection. Another portion of the same alloy was cast in the same kind of mold which had been sprayed with a 4.2 per cent methyl alcohol solution of ammonium fluoride and the alcohol allowed to evaporate before the metal was intro- .duced. No black ring appeared onthe latter cast:

ing except in a portion of the gate. The useful part of the casting, however, was clean and bright. The deleterious efiect of such a ring upon the physical properties of a cast article is shown by a comparison of the properties of the aforementioned castings. The castings made in the untreatedmold had an average tensile strength of 23,400 pounds per square inch, a yield strength of 14,000 pounds per square inch, and an elongation of 6.3 per cent in two inches. Castings made in the second, or treated mold, in marked con trast, had an average tensile strength of 27,800

pounds per square inch, a yield strength of 15,400 pounds per square inch, and an elongation of 8.4 per cent in two inches.

. In preparing a mixture of molding sand and ammonium fluoride, I prefer to use from about 2 to 6 per cent of the salt and temper the mass with 5 to '7 per cent of water. In general, when the water content of the sand is near the maximum allowable amount, a greater quantity of ammonium fluoride should be used as a precaution against the reaction with the water. If the. salt is sprayed'on the surface of, the mold cavity either an aqueous or an alcoholic solution may be,

employed, but in'practice I prefer the alcoholic solvent because of its greater volatility. The use of such a solvent avoids the introduction of more water into the mold, and upon evaporation it leaves a layer of ammonium fluoride on the walls of the mold cavity. Under some conditions a simple dusting of the mold walls with the powdered salt may provide suflicient protection against formation of the black ring. The essential feature of my invention. in any case, is that ammonium fluoride shall be-brought i'nto contact with the molten aluminum-magnesium alloy.

While my invention is applicable to aluminum" base alloys containing -more than 50 per cent aluminum, it is especially effective in alloys containing between about 4 and 15 per cent magnesium, the range usually employed in the manufacture of aluminum-magnesium alloy castings.

Alloys containing this amount of magnesium re-.

bringing ammonium fluoride in contact with the surface of the molten metal next to the mold walls.

2. A method of making castings of aluminum base alloys containing from about 4 to 15 per cent magnesium in green sand molds, comprising tempering the molding mixture with from about 5 to '7 per cent Water, forming the mold,

treating the walls of the mold cavity with ammonium fluoride, and casting the metal' in said mold. v

3. A method of making castings of aluminum base alloys containing from about 4 to 15 per "cent magnesium in green sand molds, comprising adding from about 1 to 10 per cent of ammonium fluoride to the molding mixture,"tempering the mixture with from about 5 to 7 per cent water, forming the mold, and casting the metal therein.

4. A green sand mold for casting aluminum' base alloys .containing from about 4 to 15 per cent magnesium, said mold being made from a molding mixture tempered with about 5 to 7 per cent water, and the walls of said mold being treated with ammonium fluoride prior to the introduction of the metal.

5. A green sand mold for casting aluminum base alloyscontaining from about 4 to 15 per cent magnesium, said mold being made from a moldi'ng mixturecontaining from about 1 'to 10 per cent ammonium fluoride, and tempered with 5 to 7 p'eTcent water.

6. A method of'makingcastings of aluminum basealloys containing from about 4 to 15 per cent magnesium in green sand molds, comprising adding from about 1 to 10 per cent'of'ammonium fluoride to the molding mixture, tempering said niixture with from about 4 to 9 per cent of water, forming the mold, and casting the metal in said mold.

7. A green sand mold, for casting aluminum basealloys containing ,from about 4 t .15.per cent magnesium, said moldbeing made from a molding mixture containing from about 1 to 10 per cent ammonium fluoride, and tempered with 4to 9 per cent water. v

- ROY E. PAINE.