US2436520A - Grain refining magnesium alloys - Google Patents

Description

Patented Feb. 24, 1948 UNITED S EN T fliF F l-EL GRAIN REFINING MAGNESIUM' ALLOYS Charles H. Mahoney, Harley C. Lee, Allan L. Tarr,

and Pierre E. Le :Grand, BoulderCity, Nev.., as'-' signors, .by, memo assignments, to Reconstruction Finance Corporation, a corporation of the United States (Cl. BIS- 67) '7 Claims. 1

This invention relates :tomagnesiumalloys and more particularly to magnesium alloys containingaluminum, and. has. iorits object the provision of an improved methodofuproducing such alloys of fin'elgrain structure.

Magnesium alloys, particularly those containingaluminum' with or Without zinc, when made by customary methods: of alloying and refining, inherently solidify in a coarse grain structure. It .nowa matter of common knowledge that such magnesium alloys Whenof; fine grain structu-repossess superior mechanicalproperties, enhancedamena'bility to solution treatment, shorter heat-treatingrequirements, and improved :ma-

chinability. It is also nowa matter of common knowledge that this desirable fine-grainstructure canzbeuobtained-by superheating' the molten alloy to temperatures far above the melting temperatune-of thealloy, as, for example, by superheating; at aztemperature between 900 and 950? C. The alloy is heated to the superheatingtemperature, and then cooled to pouring or casting temperature generalIy in the neighborhood of650 to 800 C. This super-heating, as it is called, imparts to. the solidified 21103 13,- relatively fine grainstructure with the-attendant superior properties hereinbefore mentioned. The fine grain structure tends to persist through subsequent remeltings of the alloy, so that castings subsequently produced from superheated alloy possess a fine grain structure.

Superheating of the magnesium alloy is expensive, both on account of the labor and equipment required, and is moreover characterized by uncertainty of duplication and difiiculty of control. The temperaturerequired for superheating varies with alloy composition and conditions, and sometimes a. superheating temperature giving satisfactory grain refinement with a particular melt may actually give unsatisfactory results when applied to another melt.

We have discovered that carbon-containing substances when introduced into a molten ma nesium base alloy at a temperature only slightly above its melting temperature impart to the solidified or subsequently cast alloy a fine-grained structure. Based on that discovery, the present invention involves treating a molten magnesium base alloy with a carbonaceous substance (grain refining. agent) which acts on the molten alloy at the temperature of treatment to give the solidified alloy a fine grain. structure. The treatment temperature may be only sufiicient to maintain.

the alloy in a moltenstate, but in practice is preferably somewhat higher, although no advantage has been observed with treatment temperae' tureszin excesswotabout 800 C. The reaction oi.

the carbonaoeousmaterial with the molten alloy.-

takes @placevpromptly, and treatment periodsv of a few minutes, for example 1 to 1 5 minutes, are

satisfactory.

It is our present belief that any organic .or carbonaceous. material in which carbon is released or madeavaiiableinan active form at the temperature of treatment brings about a grainrefinementin the solidified alloy. .It seems li-lrely that-the carbon reacts with .one of the metalsoi-the alloy, probablyaluminum in alloys containing that metal, to form a metal carbide which increases the number of grains. in the solidified structure in contrast with the natural coarsegrainedv structure of castings. ad f the untreated (and non-superheated) alloy.

The .carbonaceous'grain-refining agent maybe introduced into the molten alloy-in- .gaseous or solid, or even liquid,.state. Among thecarbonaceous substances which may be successfully. used in practicing the inventionare hydrocarbon gases such as methane, propane and; the like, fiuegases, coal, coke, peat, carbonmonoxide, carbondioxide, magnesium carbonate and other carbonates which. will react with molten. magnesium. base a1- loys to give reactive carbon. Air, which contains some carboiroioxide even efiects some grain refinement, but the action is ineliective commercially practical time periods of treatment; Similarly, thecarbon in graphite and silicon carbide. is relatively so. inactive that unduly. longv periodslof treatment arenecessary to bringabout any noteworthy grain refinement. on the other hand, propane and other carbonaceous. substanceswhioh are eitherin whole or in part vaporized to give carbon-containing gasesat the temperature of treatment, and which lcanibe introduceddnto the molten alloy at the temperature of treatment to give. reactive carbon, as for example by cracking of the carbonaceous substancernay be-eflectivelyusedin the practice of the invention. Mixtures of carbonaceous. gases, such. as propane and carbon dioxide, give particularlygoodresults.

Most organic substances,.including petroleum products, coal derivatives, sugars, synthetic plastics and plant fibers and their derivatives, are suitable grain-refiningagents of the invention; In choosing organic substances for the purpose of. the invention, those yielding highpercentages of reactive earbonon contacting moltenmagnesium base alloys .are most descrable, while those .con-. taming. appreciable percentages of chlorine, ni-

trogen, oxygen, sulphur and phosphorous are less desirable. The choice of metallo-organic substances and of carbonates will be governed by economy and by the desire to incorporate or exelude alloying elements which may be natural to the substance. For instance, the use of nickel carbonate would result in contamination of the alloy with metallic nickel which is generally recognized as undesirable because of lessened corrosion resistance of the resulting nickel containing alloy.

Carbon monoxide and carbon dioxide introduced into the molten alloy in gaseous form, or in solid substances which evolve carbon dioxide at the temperature of treatment, are highly efiective in achieving grain refinement, but their use is accompanied by the formation of considerable magnesium oxide with incidental metal loss and the necessity of careful subsequent refining of the metal to remove the metal oxide. Treatment with carbon monoxide is preferred to treatment with carbon dioxide because of the lesser amount of oxide formation incident to the use of carbon monoxide. Pulverized coke can be successfully used, but is slower in its action and hence not as desirable as some other sources of carbon. Air (containing carbon dioxide) and flue gas (containing carbon dioxide or carbon monoxide or both and perhaps other carbonaceous gases or substances) require a longer time of treatment to achieve efiective grain refinement than does carbon dioxide alone, and moreover result in objectionable nitride and oxide forma-' tion, and hence are less desirable grain refining agents of the invention.

The gaseous grain-refining agents of the invention, such for example as propane, may be used in compressed form and bubbled through the molten alloy, in a melting crucible or the like, under sufiicient pressure to overcome the molten metal head. Solid agents, such for example as coal or magnesite, are ground or pulverized and added to the molten metal. Where these agents are heavier than the molten magnesium alloy, they will travel downwardly through the molten alloy in a finely disseminated or dispersed condition and thereby efiect the necessary contact with the molten alloy. Alternatively, the solid agent may be intimately mixed with the molten alloy by agitation, such as stirring or plunging. In all cases, the surface of the molten alloy may be provided with the usual protective covering, such as a suitable flux or inert gas, to prevent excessive oxidation of the metal. Following the grain refining treatment, it is usually desirable to refine the melt by stirring in a refining flux, as is customary in magnesium refining practice, and separate the small proportion of oxide and other solid and gaseous by-products of the reaction from the molten metal. This cleansing may be aided by allowing the molten metal to remain quiescent for a few minutes before casting. For example, effective elimination of contaminants introduced by the treatment has been achieved in 150 lb. alloy melts by allowing the melt to remain quiescent for 15 minutes at 690 C. prior to casting into ingots.

A surprisingly small quantity of the grain-refining agent is adequate for the purposes of the invention. In practice the amount of carbonaceous material required will in general depend upon the reactivity of the carbonaceous material and the degree of dispersion achieved throughout the molten metal. Thus, in the case of solid agents like coal or magnesite, a quantity of the agent capable of furnishing one percent or less of carbon based on the weight of the magnesium alloy is adequate, and equivalent quantities of gaseous agents are generally adequate. For example, 75 lbs. of propane were sufficient'to adequately treat 56,000 lbs. of magnesium alloy, and in another example 10 grams of solid carbonaceous material in the form of pulverized coal were sufiicient to satisfactorily treat lbs. of magnesium alloy.

Practice of the invention consistently produces exceptionally fine-grained magnesium alloys which are characterized by initially good me-' chanical properties which are carried over on subsequent remelting cycles, even though neither re-treatment in accordance with the invention nor superheating is resorted to in such subsequent remelting cycles. However, fine-grained magnesium alloys producedin accordance with the invention can be superheated or retreated in accordance with the invention on the remelt fioor without injury. In fact, brief re-treatment in accordance with the invention on the remelt floor tends to quickly establish an optimum fine grain structure so that. the quantity of recycled scrap metal on the melting floor becomes unimportant whereas in present practice it is generally con sidered desirable to add at least 30% of risers, runners and similar scrap to each heat.

The treatment temperature for optimum efiectiveness in grain refining is largely dependent on the carbonaceous agent employed. For example in treating a magnesium base alloy with propane at a temperature of 760 C. or slightly lower, maximum grain refinement has been realized in treatment intervals of 2 to 4 minutes, and average grain intercept values of less than 0.0025 inch were obtained in cast ingots. For economy it is desirable to hold the treatment temperature below about 800 C. Heating of the molten alloy to temperatures in excess of that required for achieving maximum grain refinement in accordance with the invention does not impair the elfectiveness of the method of the invention, and has no injurious or objectionable efiect other than the resulting unnecessary oxidation losses and increased cost.

The invention provides a simple and rapid method of grain refinement permitting the production of fine-grained magnesium alloy structures possessing the properties and characteristics of the fine-grained structures produced by the heretofore customary superheating practices. The grain refinement efiected by the invention can be consistently duplicated in successive heats of the same alloy with high regularity in grain size. Thus, the invention enables the consistent production of cast alloys possessing grain sizes smaller than 0.005 inch average intercept value, and, with careful practice, of cast alloys having grain sizes as small as 0.001 inch average intercept value. Like grain refinement by superheating, the invention is particularly applicable to magnesium base alloys containing aluminum, say 2.5 to 10% aluminum, with or without zinc, say 0.2 to 3% zinc. The invention is, however, applicable to all magnesium base alloys capable of responding to present superheating practices to bring about grain refinement, and the invention creates or sets up in the molten alloy a condition that results in the production in the subsequently cast alloy of a very fine-grained structure possessing the properties and characteristics of the fine-grained structure heretofore produced by superheating.

We claim: 1. The method of producing magnesium base alloys of fine grain structure which comprises melting a magnesium base alloy and then treating the molten alloy with a hydrocarbon in which carbon is released in active form at the temperature o treatment.

2. The method of producing magnesium base alloys of fine grain structure which comprises melting a magnesium base alloy and then treatin the molten alloy with a hydrocarbon gas.

3. The method of producing magnesium base alloys of fine grain structure which comprises melt ng a magnesium base alloy and then treating the molten alloy with propane.

4. The method of producing magnesium base alloys of fine grain structure which comprises melting a magnesium base alloy and then treating the molten alloy at a temperature not exceeding about 800 C. with a hydrocarbon in which carbon is released in a form acting on the molten alloy at the temperature of treatment to give a fine grain structure in the subsequent cast alloy.

5. The method of producing magnesium base alloys of fine grain structure which comprises melting a magnesium base alloy and then treating the molten alloy at a temperature not exceeding about 800 C. with a hydrocarbon gas.

6. The method of producing magnesium base alloys of fine grain structure which comprises melting a magnesium base alloy and then treating the molten alloy for a few minutes with a hydrocarbon gas.

7. The method of producing magnesium base alloys of fine grain structure which comprises melting a magnesium base alloy and then treating the molten alloy with an amount of a gaseous hydrocarbon capable of furnishing upon decomposition in the molten metal up to about 1% of carbon by weight of the molten metal.

CHARLES H. MAI-IONEY.

HARLEY C. LEE.

ALLAN L. TARR.

PIERRE E. LE GRAND.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,854,694 Wood Feb. 16, 1932 1,856,293 Reitmeister May 3, 1932 2,126,786 Lasch Aug. 16, 1938 2,151,779 Lasch Mar. 28, 1939 2,380,863 Nelson July 31, 1945 FOREIGN PATENTS Number Country Date 232,581 Germany June 7, 1910 OTHER REFERENCES Battelle Memorial Institute, Columbus, Ohio. Reports on Investigation of Cast Magnesium Alloys and of the Existing Foundry Techniques and Practices; Report W-133, published by O. T. 8., Apr. 1946; page 43 and pages 1-4 of Preface.

Ser. No. 387,769, Lepp (A. P. C.) published May 4, 1943.

Dow, Metals Handbook, 1939 edition, page 1054; published by American Society for Metals.

Beck, The Technology of Magnesium and its Alloys, published in 1940 by F. A. Hughes and Co., Ltd., Abbey House, N. W. 1., London. Page 314.