US2290018A - Aluminum alloy - Google Patents

Description

' for Aluminum alloys.

Pat nted July 14, 1942 UNIT-Eu STAT azsaoia ALUmN'r'J'u Arno! Walter Bonsack, stat-mus, M0, assignor to The National smelting Company, Cleveland,

Ohio, a corporation of Ohio application April O i i al 8e No. 389,020. Divided and this applies...

tion June 16, 1941, Serial No. 398,315

This invention relates to alloys, and particularly to aluminum base alloys suitable for castin and working, and having high strength at ordi'- nary-and elevated temperatures.

This application is a division. of my copending April 17, 1941,.

application Serial No. 389,020, filed It is 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'pro- Q vide a relatively, light alloy which may be easily cast and machined, which may be used at elevated temperatures without, a rapid deterioration ofdesirable properties, and which may be readily treated with anodic treatment excellent lustre and finish.

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

Ithas been found that an aluminum alloy containing iron, and having zinc .and magnesium" present in proper proportions, will produce an alloy thatmay beeiie dily cast and have improved physical pr opertiesrfor use both at ordinary and elevated temperatures, and which may have these v properties improved by heat treatment.

Whenmag'nesium and zinc are added to aluminum in the proper proportiona a' ternary compound 'of aluminum, magnesium and zinc is 'formed, which compound is solubledr' 'gsolid solution in the aluminum. The presence of this compound in relatively, small amounts greatly to give form a ternary increases the brittleness and decreases the ductility of the alloy; For this reason it is undesirable that zinc be present in quantities substantially greater than the amount to react to form such a ternary compound with magnesium and aluminum. The most desirable properties 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 theratio represented by the formula AlzMgaZna, or somewhat larger. as represented by the formula AIJMC'IZDI- 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 sumcient 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:.- It has been found, however, that a. given. percentage of ternary comproperties and, because the zinc content of the 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 magnesiumwhich 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.

f The temary compound is said by gators to have a composition having substantially the formula AbMgrZm, and other investigators have considered the formula for .theternary compourid as beingAlaMgsZna. It will be seen that the amounts of magnesium and zinc'relative toeach other are quite similar in both formulas and, for the purposes of the improved alloy, the magnesium and zinc shouldbe. present in about the proportion necessary to form the ternary compound of either formula.

Anexcess of zinc, over andabove that which some invesu- V pound is more effective in producing desirable ternary alloys is less, they have a lower density.

The improved aluminum alloys may have. the ternary compound of aluminum, zinc and magnesium present in an amount ranging from about 2% to 20%, the preferred range being between about 3% and 15%. At room temperature the ternary compound goes into solid solution in aluminum alloys in an amount of about 2%. The percentage in solid solution. increases at high temperatures and decreases upon cooling, the excess precipitating out. Aluminum alloys containing the ternary compound may, therefore, be

advantageously heat treated to improve their properties.

A small amount of silicon is usually present in aluminum alloys and from .15% to about 17% is desirable in alloys of the present invention which are to be forged or drawn; more than .7% is frequently desirable in casting alloys. Silicon combines with magnesium in preference to most elements, each part by weight of silicon: combining with about 1.75%, by weight, of mag- At least sufficient magnesium to form MgaSi.

nesium is therefore added to the alloy to combine with the silicon uncombined with any calcium to form MgaSi, and, in addition, to combine with all the 'zinc and form the ternary com.-

pound according to the formula AlzMgaZm.

MgzSi is more stable than the ternary compound above mentioned and may be maintained in solid solution in aluminum alloys in an amount up to cooperates with magnesium and aluminum to so about 1.85%, which is the quantity of M5251 compound according .to' the above formula having the greatest proportion of zinc,

present if the silicon is present in the alloy, and acts as a hardener, which is sometimes desirable in conjunction with the ternary compound. MgaSi does not, however, make as eflicient use of the magnesium as does the-above mentioned above the impurity value in commercial alumibe added. Calcium has an even stronger aillnity 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 ofthe ternary compound may thus be increased. Although much more than 1.85% siiicide acts as a supplemental hardener and more than about 3% or. so makes the alloy more sluggish and adversely afiects the castability of the alloy, up to 3% is'desirable, 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 aluminum, magnesium and zinc improves the properties of aluminum or aluminum alloys hav ing low silicon content, alloys containing such low percentages ofa ternary compound are relatively difllcult to cast.

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, audit is,

' therefore, preferred to have a larger percentage of the ternary compound present, such'as 4% to 8%, for casting purposes. When the casting is more or less intricately shaped, still greater per-' centages, such as 10% to 15% or 20%. 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 I and metals of the above hardening group may be present in alloys which are to be given a socalled solution treatment'than in alloys to be num 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.

Iron in suitable amounts further increases the hardness and tensile strengthof the alloy without decreasing its ductility a substantial amount. 'A small amount of iron thus permits one to obtain the properties desired with a smaller amount of magnesium and zinc. These alloys containing iron may be readily heattreated or age hardened to give somewhat superior properties, but the iron in combination with the ternary elements in the above proportion is also outstanding, in that almost as desirable properties are obtained when castings are aged at room temperature without a heat treatment or quenching.

Iron has generally been considered to crystallize in large platelike crystals, which weaken the alloy. Iron in the presence of the ternary, compound appears to crystallize in finely dispersed form,. and the ternary compound also seems to be dispersed, thus producing a highly desirable alloy.

Iron in the amount of .4% or more in the alloys of the present invention gives noticeable eiiects in improving the properties of the alloy, and as much as 2% has-been found to be desirable for some purposes. For most castings it is desirable that the alloy have .6% or .7% to 1.5% of iron, although about 1% is usually preferred. The

quantity of iron desired in the alloy depends also upon thequantity of other hardening ingredients and upon the amount of ternary present, a given hardness and tensile strength often being 0btainable with a relatively larger amount of iron and a relatively smaller amount of ternary com- Pound, or a relatively'smaller amount of iron and a relatively larger amount of ternary.

An'aluminum base alloy containing .2% silicon and magnesium, (uncombined with silicon) and zinc in the proportions represented by the formula AlaMgrZnc, and in sumcient amounts to produce 6% of this ternary compound in the alloy, was prepared. From this base alloy three diil'erent alloys were prepared by incorporating the proportions of iron indicated in the follow ing Table 1, and chill cast in standard test bar molds. Several bars of ea'ch alloy were given the indicated heat treaments, that is, some of the bars of each alloy were removed from the mold while hot andallowed to .cool in air, and then aged seven days at room temperature;

another set of bars was removed from the mold before the bars had cooled sufliciently to precipitate the hardening ingredients, then quenched in water and allowed to age at room temperature for seven days.

Tsar: 1

m... Yield Tensile, namu Alloy Fe tion m strength strength hardness 'Pa m4 Per cent Lbs at. LbL/hl. Lbx. 111. k Q 1-, -.81 02 10,100 20,000 30,4000 10 2a.... 1.04 4.4 10,200 2:0400 30,500 as s 1.87 a4 10.500 25,300 00,000 04 1'2... .81 1.0 10,400 2;,100 30.400 70 z n-.- 1.04 so 10,400 20,000 41,100 as 0'2... 1. s7 5.8 11,400 25,000 02' Alloy air-cooled; aged at room temperature seven da "Alloy quenched; aged at room temperature seven 00" It is seen from the test results of the above table that, although the tensile strength may be ment, almost equal results are obtained by simply air-cooling the casting and aging it at room 2,2 oo,o1e increased to some extent by a quenching treattemperature. Before the quenched casting is aged a tensile strengh of over 30,000 lbs/sq. in. is obtained, while at the same time the casting has an elongation of 12%. a tensile strength of even greater ian 40,000

. lbs/sq. in, together with an elongaticl of almost 7 is obtained. In an aged casting maximum heat treatment as quenching from the mold, the

alloy is especially usefulformany purposes, such as large castings or iorgings, wherein it is diflicult to heat treat or quench.

Copper makes the alloy more responsive to heat treatment, and from .2% to about 1%, or even up to about 1.5%, copper may advantageously be present. Copper, is a precipitation hardening ingredient, and the benefits of this element are obtainable when the alloy is subjected to heat treatment, which precipitates the copper from the solid solution. -The presence of from .5% to 1.5% copper permits a reduction in the amount of ternary compound, which may be as low as 1%, and preferably should not exceed about or 12%.

' Example 1 An aluminum base alloy containing 6% of the above described ternary compound based on the formula AlaMg-zZnu, about 1% iron, about .3% silicon, and about-.5% copper, was chill cast into test'bars, quenched and aged seven days at room temperature. When tested, these test bars lbs/sq. in., a hardness of. about 81 Rockwell E,

and an elongation of about 7.8%.

Example 2 When the aluminum base alloy ofExample 1 contained only about copper instead of .5% copper, the tensile strength was about 39,000 lbs/sq. in., and a yield strength of about 22,700 lbs/sq. in., a proportional limit of about 16,800 lbs/sq. in., a hardness of about 83 Rockwell E. and an elongation of about 7% 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 sufllciently low so that the alloy may be drawn or rolled into structural members.

If the alloy contains uncombined silicon, about "1.75% magnesium is required to combine. with each percent of uncombined silicon to formmagnesium silicide '(MgzSi) before any ternary compound will be formed. For example, if 2% of the ternary compound on the basis of AhMgaZn: be

In an aged casting;

desired in an alloy having .3% silicon, the

amount of magnesium to be added to form the ternary compound will be .45%, or about .5%, and the magnesium to combine with .3% silicon will be about .5%. makin a total of about 1%.

The magnesium and zinc in an alloy containing .7% free silicon and 20% AlsMg-zZns 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% of 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 existwhen the magnesium is about 35% to 45% of the zinc content plus 175% of the silicon content. Most desirable properties may be obtained when the magnesium (uncombined with silicon) is in the lower portion of this range, or about to of the zinc.

In the above examples of alloys of the present invention it is to be noted that excellent tensile strength and hardness are obtainable in a relatively short time by aging at 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 iorrelatively long periods of I 1 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 improvement of properties isillustrated by the following table showing the improvement in an alloy containing a small percentage of silicon, about 6% ternary compound, about 1% iron,

and about .2% titanium. The test bars were showed a tensile strength of about 41,200

chill cast, quenched from the mold, and tested after aging at room temperature for the period indicated.

i TABLE 2 Elonga- Tensile Hardness m time tion strength Rockwell E Percent l2 v31,200 56 3 7.9 36, 700 76 l 7.3 38,300 80 2 6.6 39,700 82 3 6.0 40,000 84 4 6.3 40,300 83 s 0.0 40,900 85 2 z 0.0 41,350. 85.1 a 4.8- 41, 600, 88.5 4 4.8 42,500 87.6 5 5.2 43,400 89.1 6 5.1 43,700 89.1

To obtain these exceptional properties in aluminum base alloys commonly in use one has to resort to a solution and aging heat treatment, whereas in alloys of the present invention it is not necessary to solution heat treat for improvementin properties.

flcient ductility and hardness so that they can be used as sheets, rods, wire, structural shapes, castings, machine parts, etc. These alloys have a desirable color, high corrosion resistance, and

may be anodically finished or polishedwith excellent results, and are suitable for many uses, among them being the production of castforged at room temperature, and are thus useful for many special purposes.

, It is to be understood that, inconsidering the amount of zinc and magnesium to add to aluminum alloys to form the ternary compound or aluminum, magnesium and zinc .in the ailoiilsuch the magnesium as is necessary to combine wi uncombin'ed silicon is not to be considered as part oi the magnesium necessary to form the specified amount of ternary compound.

It is to be understood that the particular compounds disclosed and the procedure set forth are preaented'for purposes of explanation and illustration, and that various equivalents can be used and modifications 01 said procedure can be made without departing from my invention as defined in the appended claims. I

What I claim is: 4

1. An aluminum base alloy containing magnesium, zinc, about .15% to 1.5% silicon, about 2% to 1.5% copper, and about 4% to 2% iron, with the balance substantially all aluminum and minor impurities, the percentage of zinc in the alloy being about 1.2% to 7.2%, and'the amount of magnesium uncomblned with the silicon being about 35% to about 45% of the zinc content, the

neslum,-zinc,qabout .2% to 1.5%. Qi per, silicon aluminum and minor impurities, the percentage within'the range of about .5% to 7%.

3. An aluminum base alloy containing magneslum, zinc, about .2% to 1.5% copper, silicon in an amount up to about 1.5%, and about .4% to 2% iron, with the balance substantially all aluminum and minor impurities, the percentage of zinc in the alloy beingJrom about .6% up to about 6%, and the amount 0! magnesium uncombined with the silicon being about to about ot the zinc content. the total amount oi magnesium being within the range of about .5% to 6%.

4. An aluminum base alloy containing magnesium, zinc, about .2% o 1.5% copper, silicon in an amount up to about 1.5%, and about .4% to 2% iron, with the balance substantially all aluminum and minor impurities, the percentage of zinc in the alloy being from about .6% up to about 4.8%, and the amount of magnesium un-' combined with the silicon being about 35% to about 45% of the zinc content, the total amount total magnesium being within the range of about 30 otvmagnesium being within the range of about 5% to 5%. A

- wan'rna nousacx,