US2274657A - Aluminum alloy - Google Patents

Description

Patented Mar. 3, 1942 ALUMINUM ALLOY Walter Bonsack, South Euclid, Ohio, assignor to The National Smelting Company, Cleveland, Ohio, a corporation of Ohio No Drawing. Application April 17, 1941,

Serial No. 389,019

2 Claims.

This invention relates to alloys, and particularly to aluminum base alloys having high strength at ordinary and elevatedtemperatures and being suitable for casting, working and heat treating.

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

It is 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 in the desirable properties, and which may be readily treated with anodic treatment to give excellent lustre and finish.

It is a still further object of this invention to provide an alloy having a relatively high proportional limit and a relatively high fatigue strength, and in which these properties may be further enhanced by heat treatment.

' In my copending application Serial No. 284,527,

filed July 14, 1939, of which this application is a continuation-in-part, I have described an aluminum alloy containing magnesium and zinc in the proportions to form, with part of the aluminum, a ternary compound of aluminum, magnesium and zinc. When magnesium and zinc are added to aluminum in the proper proportions, a ternary compound of aluminum, magnesium and zinc is formed, which compound may be in solid solution in the aluminum. The ternary compound improves the. characteristics of aluminum, giving the alloy high strength combined with relatively high ductibility, good color and excellent corrosion resistance. The casting properties of such an alloy may be greatly improved, however, by the presence of silicon in an amount more than an impurity. In an alloy containing sufiicient magnesium to combine with all the silicon, silicon does not exist as free silicon, but avidly combines with the magnesium to form magnesium silici-de (MgzSi), which compound improves the casting properties. a

In calculating the amount of magnesium and zinc that should be present in the aluminum alloy to form the desired percentage of ternary compound, the magnesium which is not used up in combining with the silicon is to be considered as that available for combination with the zinc and a portion of the aluminum to form the ternary compound. Therefore, in making the alloy one should first determine th amount of magnesium silicide desired before calculating the total amount of magnesium that should be present in the alloy to form the desired amount of ternary compound.

The ternary compound is said by some investigators to have a composition having substantially the formula Al3Mg7Zn6, and other investigators have considered the formula for the ternary compound as being AlzMgsZns. It will be seen that the amounts of magnesium and zinc relativ 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 the proportion necessary to form the ternary compound of either formula.

An excess of zinc, over and above that which cooperates with magnesium and aluminum to form a ternary compound according'to the above formula having the greatest proportion of zinc,

increases the brittleness and decreases the ductility of the alloy. Also, it tends to cause a decline in the ultimate strength and proportional limit. For this reason it is undesirable that zinc be present in quantities substantially greater than the amount sufficient'to react to form such a ternary compound with magnesium and aluminum. The most desirable properties are ob tained when the magnesium and zinc are proportioned so that the ratio of magnesium (uncombined With any silicon) to zincis about equal to the ratio represented by the formula AlzMgsZns, or somewhat larger, as represented by the formula AlsMgvzns- It is usually desirable to have the magnesium present on the rich side to prevent the silicon from-being in excess and taking magnesium away from the ternary compound.

Magnesium adds to the hardness and machining qualities of the alloy and, as above stated, should be present in an amount suflicient to combine with the available silicon to form the ternary compound with the zinc and aluminum present. Asmall amount of magnesium may be provided to replenish losses that may occur when the alloy metal is remelted. In greater quantities magnesium tends to make the alloy sluggish, de-

' tion may have the ternary compound of aluminum, zinc and magnesium present in an amount ranging from about 1% to about 8%. But. with the preferred alloys, the zinc and the magnesium uncombined with the silicon are proportioned to form about 2% to about 6% of the ternary compound, with possibly a small amount of magguesium uncombined with the silicon and the ter- 7 prove their properties.

Magnesium silicide small proportions may be present in aluminum alloys containing sufiicient magnesium to react with the silicon present as an impurity. Whilethe magnesium-silicide equivalent to such a quantity of silicon may slightly improve the casting properties of alloys containing the ternary compound of aluminum,

magnesium and zinc, it has been found that such ternary alloys having more magnesium silicide, preferably between about 3% and about have their casting properties considerably improved. As' much as 8% magnesium silicide may be present in the. alloys, however. Calcium combines even more avidly with silicon than magnesium, and its silicide, in small amounts, may besubstituted for part of the magnesium silicide. Calcium silicide supplements the magnesium compound without materially affecting the properties of the alloy. When calcium silicide is also present, the totalsilicide should preferably be between about 3% and 3%.

In forming the alloys of the present invention the magnesium'ishould, therefore, be present in suficient amount to combine with the free silicon, or-that uncombined with any calcium, to

form Mgzsi and in addition to combine with the zinc to form the'ternary compound according to the formula AlzMgaZns, the formula requiring the least magnesium relative to the zinc.

2% or 3% of the ternary compound of alumi-- num, magnesium and zinc improves the properties of aluminum or aluminum alloys, and such alloys may be cast. The casting properties, however, are. usually also-improved when the alloy contains more of the ternary compound than an increase in strength and a demagnesium silicide (MgzSi) before any ternary compound will be formed. For example, if 1% of the ternary compound on the basis of AlzMgaZm be desired in an alloy having 1%. silicon, the amount of magnesium to be added to form the ternary compound will be about 22%, and the magnesium to combine with 1% silicon will be about 1. 75%, making a total of about 2%.

3.6%. The magnesium should be-present within the limits of about 2% and about 8%, and preferably between about 2% and 5.5% or 6%, depending on the; amount of zinc and silicon in the alloy. Magnesium silicide should be present in amounts of 3% to 8%, and preferably between 3% and about 5%. The amount of magnesium uncombined with silicon should be proportioned to the zinc to form with the zinc and crease in elongation are ordinarily obtained with increased quantities of the ternary compound.

The presence of the ternary compound of aluminum, magnesium and zinc in aluminum base alloys is quite advantageous in producing alloys having relatively high tensile strength,

relatively high proportional limit, relatively high' somewhat elevated temperatures, and which alloys may have their properties further improved by heat treatment. As before explained, the ternary compound is particularly of advantage in aluminum base alloys having a relatively low silicon content, such as less than 1.5%.

The percentages of zinc and magnesium in the ternary compound are almost the same, regardless of which of the above mentioned formulae are considered the basis for the computation. Thus, in' each percentage of the ternary, on the basis of the formula AhMg'zZnc there is about 26% magnesium and about 5% zinc, and on the basis of the formula AlzMgaZna there is about 22% magnesium and about 5% zinc.

About 1.75% magnesium is required to combine with each percent of uncoznbined silicon to form part of the aluminum a ternary compound of aluminum, magnesium and zinc. magnesium should also be present to combine with the silicon to form magnesium silicide, and to provide an excess, such as 5%. That is, for the formation of the ternary compounds and the improved alloys of the present invention, the magnesium is related to the zinc and silicon and is about 35% to 45% of the zinc plus 1.75% of the silicon content.

The aluminum alloys of the present invention containing magnesium, uncombined with silicon, and zinc in the proportion of a ternary compound, when cast in molds of a design such that chilling takes place substantially simultaneously in the various portions of the casting, form good castings without the use of grain refining agents. However, it has been found that certain grain refining elements substantially improve the properties of the aluminum alloy containing the ternary compound.

The grain refiners which I have found improve the properties of the alloy are members of the group consisting of boron in the amount of .005% to .1%, zirconium in the amount of .01% to .5%,

tungsten in the amount of .01% to 5%, molyb-.

from .005% to 5%, and it is frequently desirable to have more than one of these elements present in a given alloy.'

While the grain refiners in the above group are disirable in the alloys of the present invention, not all of the grain refiners affect the properties in the same way. The particular refiner or group of refiners selected in any given instance depends on the particular condition which must be satisfied. The grain refiners selected from the group consisting of titanium, tungsten, molybdenum, zirconium and vanadium, andespecially tungsten and molybdenum, improve the strength and the elongation of the castings.- Of these, titanium, being less expensive, is usually used Additional for ordinary castings, but in cases where the highest elongation, together with strength, is necessary, it is preferred to use tungsten or molybdenum. The grain refiners selected from the group consisting of boron, columbium and tantalum do not appear to appreciably increase the strength or elongation of the alloy and are usually used where appearance, finish and corrosion resistance are most important and where strength is of less consequence. Certain grain refiners, such as cerium, were found to berelatively undesirable in the alloy.

The above described hardening elements, manganese, chromium, nickel and copper, substantially decrease the hot shortness, improve the properties of the alloy, and assist in maintaining the improved properties at high temperatures, such as are encountered in internal combustion engines. The above grain refining elements, particularly members of the group consisting of tungsten, molybdenum, vanadium and titanium, also have this property when present in substantial amounts, such as .2% or .3%, or so. It is,

therefore, especially desirable to have up to .5% or so of these latter elements present when the hardening ingredients are absent.

An aluminum base alloy containing about 3% magnesium silicide (MgzSi) magnesium and zinc in the proportion to form with the aluminum about 6% of the ternary compound AlsMgrZne, and .2% titanium was chill cast into test bars.

The best bars were quenched from the mold and aged for seven days at room temperature, and, when tested, showed a tensile strength of 38,000

lbs/sq. in., a yield strength of 28,300 lbs/sq. in.,

. nesium silicidewas present, was chill cast into test bars which were quenched and aged as above. While the test bars thus prepared had a tensile and tensile strengths, and a relatively high proportional limit, even at relatively high temperatures; they-may be heat treated to improve and modify their properties; and they have sufiicient ductility and hardness so that they'can be used as structural shapes, castings, machine parts, etc. These alloys have a desirable color, high corrosion resistance, and may be anodically finished or highly polished With excellent results.

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 departingfrom my invention as defined in the appended claims.

What I claim is:

strength and ductility, comprising magnesium, zinc, magnesium silicide, one or more metals for hardening or grain refining, and the balance substantially all aluminum and minor impurities, the zinc being present in the amount of about .6%' to 4.8%, the magnesium silicide being present in the amount of about 3% to 8%, the amount of magnesium in the alloy uncombined with silicon being about 35% to 45% of the zinc content, the total magnesium being within the range of about 2%-to 8%, and the total silicon being within the range of about 1% to 3%.

2. An aluminum base alloy of relatively high strength and ductility, comprising magnesium, zinc, magnesium silicide, one or more metals for hardening or grain refining, and the balance substantially all aluminum, and minor impurities, the zinc being present in the amount of about 1.2%

to 3.6%, the-magnesium silicide being present in.

the amount of about 3% to 5%, the amount of magnesium in the alloy uncombined with silicon being about 35% to 45% of the zinc content, the total magnesium being within the range of about 2% to 5.5%, and the total silicon being within the range of about 1% to 2%.

WALTER BONSACK.