US2290020A - Aluminum alloy - Google Patents

Description

Patented July 14, 1942 I I ALUMINUM ALLOY Walter Bonsack, South Euclid, Ohio, assignor to The National Snielting Company, Cleveland, Ohio, a corporation of Ohio No Drawing. Application August 7, 1941, Serial No. 405,884

1 the proportion necessary to form the ternary V 4 Claims. This application isa continuation-in-part of my copending application Serial No. 389,020, filed,

April 17, 1941. The invention relates to alloys, and particularly to aluminum base alloys suitable for casting and working and having high strength at ordinary and elevated temperatures.

Itfis an object of this invention to produce alloys having relatively high elongation and relatively-high tensile strength.

It is a further object of this invention to provide a relatively light alloy which may be easily cast and machined, which may be used at elevated temperatures without a rapid deterioration of, desirable propertiesand which may be readily treated with anodic treatment to give excellent lustre and' finish.

It is a still further object of ths invention to provide an alloy having a relatively high proporv tional limit and relatively high fatigue strength, and inv which these properties may be obtained without heat treatment.

It has been'found that an aluminum alloy containing iron, and having zinc and magnesium present in proper proportions, Will produce an alloy that may be readily cast and have improved physical properties for use both at ordinary and elevated temperaturea'and which may have these properties improved by heat treatment.

When magnesium and zinc are added to aluminum in the proper proportions, a ternary compound of aluminum, magnesium and zinc is formed, which compound is' soluble in solid socompoundof either formula.

An excess of zinc, over and above that which cooperates with magnesium and aluminum to form a ternarycompound according to the above formula having the greatest proportion of zinc, increases the brittleness and decreases the ductility of the alloy. For this reason it isundesirable that zinc be present in quantities substanlution in the aluminum. The presence of this compound in relatively small amounts greatly improves the characteristics of aluminum and produces an alloy having high strength combined with high ductility, good casting and forging properties, good color and excellent corrosion resistance. In calculating the amount of magnesium and zinc that should be present in the aluminum alloy to form the desired percentage of ternary compound, only magnesium which is not combined with silicon is .to be calculated, as it is only such magnesium that is available to combine with zinc and aluminum to form the ternary compound.

The ternary compound is said by some investigators to have a composition having substantially the formulaAlaMgvZne, and other investigators have considered the formula for the ternary compound as being AI MgaZns. It will be seen that the amounts of magnesium and zinc relative to each other are quite similar in both formulas and, for the purposes of the improved alloy, the

magnesium and zinc should be present in about tially greater than the amount to react to form such a ternary compound with magnesium and aluminum. The most desirableproperties are obtained when the magnesium and the zinc are proportioned so that the ratio of magnesium (uncombined with any silicon) to zinc is about equal to the ratio represented by the formula AlzMgsZns,

or somewhat larger as represented by the formula AlgMgvZns. -A small amount of magnesium may be provided to replenish losses that may occur when the alloy metal is remelted.

Magnesium adds to the hardness and machining qualities of the alloy and, as above stated, should be present in an amount sufficient to combine with the zinc and aluminum present. In

greater quantities, magnesium tends to make the alloy sluggish, decreasing castability.

Magnesium and zinc have heretofore been added to aluminum in the-proportion represented by the formula MgZn-z. It has been found, however, that a given percentage of ternary compound is more effective in producing desirable properties, and because the zinc content of the ternary alloys is less they have a lower density.

The improved aluminum alloys may have the ternary compound of aluminum, zinc and mag nesium present in an amount ranging from about 2% to 20%, the preferred range being between about 3% and 15%. At room temperature the V ternary compound goes into solid solution in alu-' minum alloys in an amount of about 2%. The percentage in solidsolution increases at high temperatures and decreases upon cooling, the

excess precipitating out.

A small amount of silicon is usually present in aluminum alloys and from .15% to about';7%

is desirable in alloys of the present invention which are tobe forged or drawn; more than 37% is frequently desirable in casting alloys. Silicon combines with magnesium in preference to mostelements, each part by weight of silicon combining with about 1.75%, by weight, of magnesium to form MgzSi. At least sufiicient magnesium is therefore added to the alloy to combine with the silicon uncombined with any calcium to form MgzSi, and in addition to combine with all the zinc and form the ternary compound -tion in aluminum alloys in an amount up to about 1.85%, which is the quantity of MgzSi present if the silicon is present in the alloy, and acts as a hardener which is sometimes desirable in conjunction, with the ternary compound. MgzSi does not, however, mak as efficient use'of the magnesium as does the above mentioned ternary compound. Therefore, it is desirable to have the magnesium present on the rich side to prevent the silicon from being present in excess and taking magnesium away from the ternary compound.

If the alloy is desired particularly for casting purposes, more silicon, such as up to about 1.5%, may be present. If however a somewhat larger amount of silicon is present in the alloy than is desirable for the purpose for which the alloy is intended, and such amount of silicon is not too excessive, then a small amount of calcium may be added. Calcium has an even stronger affinity for silicon than has magnesium and, therefore, it can be used to reduce the amount of silicon available for combination with magnesium. The amount .of the relatively expensive magnesium available for the formation of the ternary compound may thus be increased. Although much more'than 1.85% silicide acts as a supplemental hardener and more than about 3% or so makes the alloy more sluggish and adversely affects the castability of the alloy, up to 3% is desidable and more than 3% may in some cases be desirable in the production of hard castings having less intricate shapes, particularly when a large amount of the ternary compound 'is present in the alloy. Usually, however, the

amount of silicon should be between .5% and 1.5%, especially in castings not heat treated. This is true even when calcium is present, although with the latter element more magnesium is available for formation of the ternary compound.

While 2% -or 3% of the ternary compound of alumninum, magnesium'and zinc improves the properties of aluminum or aluminum alloys having low silicon content, alloys containing such low percentages of a ternary compound are relatively diilicult to cast. I I

An alloy containing 2% of the ternary compound may be used for casting purposes. The castability, however, is improved with an increase in the amount of ternary compound and it is, therefore, preferred to have a largerpercentage of the ternary compound present, such as 4% to 8% for casting purposes. When the casting is more or less intricately shaped, still greater percentages, such as 10% to or of the ternary compound may be present. For alloys to be forged or shaped after casting, the ternary compound should be present in the lower ranges, such as 2% to 8% or so, as the metal is less hard with the lower percentages of the ternary compound.

A larger proportion of the ternary compound may be present in alloys which are to be given a so-called solution treatment than in alloys to be given only an aging treatment, or those to be quenched from the casting mold and aged at relatively low temperatures. Thus, the desirable properties of the solution heat treated alloys may be obtained'when they contain the ternary compound in amounts up to 20% or so, whereas less of the ternary compound, such as 4% to 15%,

is preferred in alloys which are quenched upon rcmoval from the mold and heat treated at a low temperature, or aged at room temperature.

It has generally been considered that aluminum alloys of magnesium containing iron much above the impurity value in commercial aluminum are of little commercial value; but it has also now been found that an alloy containing the above described ternary compound is improved by the presence of iron in suitable proportion. 1

Copper present in suitable amounts makes the alloy more responsive to heat treatment. When heat treatable alloy is desired, it is preferable (that from 2% to about 1% or even up to about 1.5%, copper be present in the alloy. Copper is a precipitation hardening ingredient, and the benefits of this element are obtainable when the alloy is subjected to conditions of heat treatment which precipitate the element from the solid solution. The presence of from .5% to 1.5% copper permits a reduction in'the amount of the ternary compound, which may be as low as 1%, and preferably should not exceed about 10% or 12%. 1

Example 1 An alloy containing about 1% copper, 2% silicon and 6% of the ternary compound of aluminum, magnesium and zinc was chill cast, quenched'and aged three hours at C. Upon being tested it was found to have a tensile strength of 42,000 lbs/sq. in., an' elongation of 4.7%, a Brinell hardness of 86, a proportional limit of 20,000 lbs/sq. in., and a yield strength of 28,400 lbs/sq. in. The same alloy when aged seven days at room temperature had a tensile strength of 40,300 lbs/sq. in., an elongation of 6.5%, a Brinell hardness of 82, a yield strength of 23,900 lbs/sq. in., and a proportional limit of 17,500 lbs/sq. in. When the. above castings were simply air cooled and aged seven days at room temperature the tensile strength was about 39,000 lbs/sq. in.

Example 2 Example 3 When the aluminum base alloy of Example 2 contained about 50% copper instead of 25% copper, the tensile strength was about 39,600 lbs/sq. in., and a yield strength of about 25,500 lbs/sq. in., a proportional limit of about 18,600 lbs/ in., a hardness of about 84 Rockwell E, and an elongation of abaut 5.4% were obtained.

Since the molecular proportion of zinc is never more than the molecular proportion of the relatively light magnesium in the ternary compound, it is seen that in addition to high strength the alloys are light in Weight and are, therefore, especially adapted to aircraft construction and the like. This is particularly true when the quantity of ternary compound is sufficiently low so that the alloy may be drawn or rolled into structural members.

If the alloy contains uncombined silicon, about a 1.75% magnesium is required to combine with each percent of uncombined silicon to form magnesium 'silicide (MgzSi) before any ternary compound-will be formed. For example, if 2% of the ternary compound on the basis of AhMgaZna be desired in an alloy having .3% silicon, the amount of magnesium to be added to form the ternary compound will be .45%, or about and the magnesium to combine with 3% silicon will be about 5%, making a total of about 1%.

The magnesium and zinc in an alloy containing .7% free silicon and AlaMgwZIls would be about 7% and 12%respectively. The alloys described herein include aluminum, magnesium and zinc, the zinc being present in the amounts of 1.2% to 12% or the alloy, and the magnesium,

uncombined with silicon, being proportioned to the zinc in the ranges of the formulas given for the ternary compound. The proportions for the formation of the ternary compound in the alloy exist when themagnesium is about to 45% of the zinc content plus 175% of the silicon con tent. Most desirable properties may be obtained when the magnesium (uncombined with silicon) is in the lower portion of this range, or about 35% of the zinc.

In the above examplesof alloys of the present invention it is to be noted that excellent tensile strength and hardness are obtainable in a relatively short time by agingat room temperature. A-very astounding fact has been discovered, however, in connection with these alloys, namely, that the tensile strength may increase up to approximately 50% of its initial value by aging at room temperature for relatively long periods of time, such as a few months. The same improvement in tensile strength can, of course, be obtained relatively quickly by aging at temperatures above room temperature.

The alloys of the present invention have good fatigue and tensile strength and a relatively high proportional limit, even at relatively high tem-- peratures; they may be heat treated to improve and modify their properties; and they have sufficient ductility and hardness so that they can be used as sheets, rods, wire, structural shapes, castings, machine parts, etc. a desirable color, high corrosion resistance, and maybe anodically finished or, highly polished with excellent results, and are suitable for many uses, among them being the production of castings which are shaped or formed to some extent after casting. The alloys having the lower percentages of ternary compound may even be forged at room temperature and are thus useful for many special purposes.

These alloys have It is to be understood that in considering the amount of zinc and magnesium to add to aluminum alloys to form the ternary compound of aluminum, magnesium and zinc in the alloy, such magnesium as is necessary to combine with the uncombined silicon is not to be considered as part of the ternary compound.

It is to be understood that the particular compounds disclosed and the procedure set forth are presented for purposes of explanation and illustration, and that various equivalents can be used, and modifications of said procedure can bemade without departing from my invention as defined in-the appended claims.

What I claim is:

1. An aluminum base alloy containing magnesium, zinc, about .2% to 1.5% copper, and silicon in an amount up to' 1.5%,. with the balance substantially all aluminum and minor impurities, the percentage of zinc in the alloy being about .6% to 12%, the amount: of magnesium in the alloy uncombined with the silicon being about 35% to about of the zinc content, the total amount of magnesium being within the range of about .5% to 7%.

2. An aluminum base alloy containing magnesium, zinc, about to 1.5% copper, and silicon in an amount up to 1.5%, with the balance substantially all aluminum and minor impurities, the

percentage of zinc in the alloy being about 2.4% s

to 7.2%, the amount of magnesium in the alloy uncombined with the silicon being about 35% to about45% of the zinc content, the total, amount of magnesium being within the range of about .5% to 7%.

3. An aluminum base alloy containing mag.-.

nesium, zinc, about .2% to 1% copper, and silicon in an amount up to 1.5 with the balance substantially all aluminum and minor impurities, the percentage of zinc in the alloy being about .6% to 6%, the amount of magnesium in the alloy uncombined with the silicon being about 35% to about 45% of the zinc content, the total percentage of zinc in the alloy being about .6%

to 4.8%,. the amount of magnesium in the alloy uncombined with the silicon being about 35% to about 45% of the zinc content, the total amount of magnesium being within the range of about 5% to 5%.

\ WALTER BONSACK.