US2380201A - Manufacture of castings - Google Patents

Description

Patented July 10, 1945- Philip T. Stroup and George F. Sager, New Ken- W V H slngton, Pa., assignors to Aluminum Company of America, Pittsburgh;

Pennsylvania Pa., a corporation of No Drawing. Application July 10, 1942, Serial No. 450,408

5 Claims. (01. 22-200) This invention relates to methods of making sand castings and permanent mold gravity poured castings from magnesium and those alloys of magnesium which contains 50 per cent or more by weight of magnesium and are usually, as they are here, referred to as magnesium base alloys. The general object of the invention is to provide methods of the type indicated which will, at low cost and-without material harm to the physical properties of the final product, overcome, at, least in part, well-known difllculties encountered by the founder in the handling of magnesium and magnesium basealloys and caused by the propensity of such metal to oxidize severely when in the molten state and in contact with air, moisture and other oxidizing media.

The invention is directed to the improvement of two well-known methods of casting which are defined as follows:

Sand casting.-The art of forming'metal articles by pouring molten metal into a cavity whose defining walls are formed in substantial part of foundry sand or'simllarly agglomerated materials, allowing the metal to solidify and removing the solidified metal from the mold, such removal involving destruction of a large part of the walls of said cavity.

Permanent mold gravity pour casting-The art of forming metal articles by pouring molten metal into a cavity whose defining walls are formed in substantial part of a permanent substance, such as metal. possessing continuity of structure or,of a semi-permanent substance, such-as plaster. or a pressure consolidated agglomerate, allowing the metal tov solidify and removing the solidified metal from the mold, such removal involving no destruction of any large part of the cavity-forming surfaces. The product may be a shaped casting or an ingot for working, such as a rolling or extrusion ingot. The mold cavity may be of any shape including those molds without top or bottom, such as are used in continuous casting processes. It is emphasized here that the molten metal proceeds into the cavity only under the force imparted to it by gravity, thus distinguishing from the use of pressure on the flowing molten quality castings therefrom under conditions which would justify the commercial production of sand casting and permanent mold gravity poured castings. Gradual development of methods of preventing oxidation during casting finally led to the commercial production of castings of these types, but the methods necessarily used in their pro-' 'duction add materially to the cost of manufacture and often have detrimental effects upon the properties of the final product. If the molten metal is toibe molded by sand casting, the sand molds are either completely dried (an expensive and unsatisfactory expedient) or there are mixed with the sand, or similar agglomerate,

special inhibitors and tempering agents designed to protect against oxidation of the metal during solidification in the mold cavity. Such inhibitors and agents are well known to the art. Theylnclude such substances andcompounds as sulphur and compounds thereof, fluorides, fiuosilicates and other compounds of fluorine, boric acid, ammonia, glycerine, glycols and many other materials, most of which produce irritating fumes during the casting process and are difficult and expensive to handle. In addition to such costly procedures, calcium is sometimes added to the molten metal since the presence of calcium tends to decrease the propensity of the metal to oxidize. Under'these conditions it is possible to produce satisfactory magnesium and magnesium base alloy sand castings in specially equipped foundries despite the fact that the foundry sand must be continually reconditioned, the operators must contend with the acrid fumes given off by the in:- hibitors and the action of the inhibitors causes the formation of voids and gas pockets in the surfaces of the castings. g a

In the production of permanent mold gravity poured castings the methods used to protect the molten magnesium and magnesium base alloy against oxidation are somewhatdifl'erent. The continuity of structure which characterizes the Walls of the mold cavity in this process prevents incorporation of inhibitors in the cavity surface. Inhibitors, if used, must be ingaseous form, such as, for instance, sulphur vapor or sulphur dioxide. The use of such gases is cumbersome and interferes with the comparative rapidityof production which is a. feature of this type of casting process. Consequently, some founders avoid the use of such inhibitors and instead provide the molten metal with a calcium content and also treat the casting cavity walls with mold washes designed to coat these walls and to thusprevent action between cavity wall and 'molten metal during the casting operation. This involves difficulties. Calcium will itself oxidize or burn while in the molten metal, thus necessitating frequent additions of that element to the molten magnesium or magnesium base alloy which is be- 5 ing processed. As a result, the exact amount 0L- calcium present in the melt at any given time is not known to the operator until a deficiency occurs, in which case the molten metal suffers severe oxidation and interferes with the casting process. The mold wash coating On the cavity walls is not stable and attack thereon by the molten metal necessitates constant renewing of this coating, thus detracting from the emciency of the casting process. Evenunder the best of these conditions the castings produced often have black or dark discolored surfaces and show skin folds and similar surface defects. 7 7

The present invention, which provides methods designed to overcome or diminish the difilculties above mentioned, is predicated upon the presence in the magnesium or magnesium base alloy during sand casting or permanent mold gravity pour casting of certain small quantities of beryllium. This results in advantages hereinafter described. Prior to this invention it was known that beryllium acted as an anti-oxidant in magnesium and magnesium base alloy, but attempts to realize upon the anti-oxidant properties in sand casting or permanent mold gravity pour casting had failed because the resultant castings had poor physical characteristics; We have discovered that the failure of prior workers in this field was ,caused by the use of improper amounts of beryllium (0.005 or more percent by weight). We have further discovered that when beryllium is used in the proper amounts hereinafter set forth, it is possible not only to utilize its anti-oxidation effect in sand casting and in permanent mold gravity pour casting, 4 but to also obtain results and advantages heretofore unknown."

When our, invention is applied to the art of sand casting the magnesium or magnesium base alloy to be cast.is provided with a beryllium 5 content of not more than about 0.002 or less than 0.00005 per cent by weight of the total metal and cast into the mold cavity. The tendency of the molten metal to react with moisture or other oxidizing media present in the molding sand is so substantially reduced that the amount of inhibitor heretofore necessarily present in the sand may be substantially decreased and in some cases, particularly in the case of core sands, eliminated. The necessity for the addition of calcium to the metal is also eliminated. The cost of processing is therefore reduced, the number of castings rejected for surface voids is also reduced. The elimination of a part of the inhibitor previously used also reduces the concentration of acrid 6o fumes therefrom aand betters the working conditions in the foundry. We have consistently obtained these results on a commercial scale without substantial decrease in the properties of the resultant castings, particularly when, as is es preferred, the amount of beryllium present in the metal does not exceed about 0.0015 per cent by weight.

When our invention is applied to the art of permanent mold gravity pour casting the magnesium or magnesium base alloy to be cast is provided with a beryllium content of not more than about 0.002 or less than about 0.00005 per cent by weight of the total metal and cast into the mold cavity. The calcium previouslyv used be positively added thereto.

in such processes can be dispensed with completely or in large part and thus the uncertainties due to the use .of calcium can be eliminated since the beryllium content is stable throughout the casting operation. The presence of beryllium also protects the mold wash coating on the cavity surfaces, and when our invention is practiced it is not necessary to so frequently interrupt the process to renew this coating. The castings produced do not exhibit the dark unsightly surface of castings previously produced and the surface thereof contains less skin folds and other surface defects than the castings of prior processes of this type. The final casting has properties not materially affected, if at all, by the practice of the invention. In fact, the elimination of the necessity for any substantial calcium content makes possible the production of castings which will respond uniformly to thermal treatment and which have, as the result of the thermal treatment, better properties than were obtained in previous castings of equal composition, which contained calcium and no beryllium or beryllium in amount heretofore proposed in the art.

The minute amounts of beryllium used inv the practice of the invention as above described are,

despite the fact that they are of an order usually disregarded as a trace of an element, not present in primary magnesium metal and must Where secondary magnesium metal or alloy is available and contains beryllium, it may, of course, be used and if the beryllium content is not within the range postulated by this invention, adjustment thereof may first be made in any suitable manner.

Where beryllium is to be added to magnesium or magnesium base alloy the method of. addition is not important, the factor of importance being that the proper quantity is achieved in the metal. An intermediate alloy of the beryllium and magnesium or of beryllium and one or more of the other elements of which the alloy is to be composed can be used as a source of beryllium. Another method consists in adding. to the molten magnesium or magnesium base alloy a beryllium salt decomposable at the temperature of the molten metal. Other methods will suggest themselves to those skilled in the art of alloying.

The magnesium base alloys referred to herein and in the appended claims are those alloys containing at least 50 per cent and usually more by weight of magnesium and having as additional constituents, apart from the beryllium, other elements such as manganese, zinc, cadmium, aluminum, silicon, copper, cerium, zirconium, silver and such others as are usable or used to produce useful properties in, or to enhance the properties of, magesium.

We claim: 1. In th sand casting of, magnesium and magnesium base alloy, the step of providing the metal prior to the casting operation with a content of beryllium not greater than about 0.002

and not less than 0.00005 per cent by weight of the total metal.

2. In the sand casting of magnesium and magnesium base alloy, the step of providing the metal prior to the casting operation with a content of beryllium not greater than about 0.0015 and not less than0.00005 per cent by weight of the total metal.

3. In that process of sand casting magnesium and magnesium base alloy which comprises mixgreater than about 0.002 and not less than about 0.00005 per cent by weight of the total metal.

5. In the gravity pour casting of magnesium and magnesium base alloy, the step of providing the metal prior to the casting operation with a content of beryllium not greater than about 0.002 per cent and not less than about 0.00005 per cent by weight of the total metal.

PHILIP T. STROUP. GEORGE F. SAGER.