US2290024A - Aluminum alloy - Google Patents

Description

Patented, July 14, 1942 Walter Bonsack, South Euclid, Ohio, assignor to ,The National smelting Company, Cleveland, Ohio, a corporation of Ohio N Drawing.

Application February 20, 1942, Serial No. 431,681

8 Claims. (Cl. 75-146) This application: is acontinuationin-part of my copending application Serial No. 389,020,

filed April 1-7, 1941; Theinvention relates to alloys, and particularly to aluminum base alloys suitable for casting and; working and having high strengt-hat ordinary and elevated temperatures;

It islan object of this inventionto procluce alloys having relatively high elongation and relatively high tensile strength.

' It is a further objectof thisinvention to pro vide a relatively light alloy which maybe easily cast and machined, which may be used at ele:

vated temperatures without arapid deterioration of desirable properties,and which may be readily treated with anodic treatment to give excellent lustre and finish. s l It is a still furtherobje it ofthis invention to providean alloy having arelatively high proportional limit and ,relatively high fatigue strength, and in which these properties may be obtained without heat treatment. Q

When magnesium and zinc arejadded to aluminumin the proper proportions, a ternary compound of aluminum,"'magnesium jand zinc; is

form aternary compound according to the above formula having the greatest proportion of zinc, increases the brittleness and decreases the duetility of the alloy. For this reason it is undesirable that zinc be present in quantities substan{ tially greater than theamount to react to form such a ternary compound with .magnesium and aluminum. The most desirable properties are obtained when themagnesium 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 AlzM'gaZna, or somewhat larger asrepresented by the formula A13 Mg7Zns; :A small amount of magnesium may be providedto replenish losses that may occur when the alloy metal is remelted. Magnesiumadd s to the hardness and machine ing qualities of the alloy and, as above stated, shouldbepresentinan amount sufiicient to combine with the zinc and aluminumpresent; In greater quantities, magnesium tends to make the alloy sluggish, decreasing castab ility.

Theimproved aluminum alloys may have the ternarycompound of aluminum,-zinc and magformed, which compoundis soluble'in solid solu'- tion in the aluminum. N The presence of this compound in a relatively small amount greatly, improves the characteristics of aluminum and pro,- duces an alloyhaving high strength combined with high ductility, good casting, rolling, ex-

truding and forging properties, good colorand excellent'corrosion resistance. In calculating the amount of magnesium andzinc that should be present in thealuminum alloy to form the desired percentage of ternary compound, only magnesium which is not combined with silicon is to be calculated, as his only such magnesium that is availableto combine with zinc and aluminum to form the ternary compound. i

The ternary compound is said by some investigators to have a composition having substantially the formula AlaMgvZns, and other investigators have considered the formula for the ternary compound as being'AlzMgaZm. It will be seenthat the amountsof magnesium andzinc relative to each other are quite similar in both formulae. The magnesium and zinc should be present in about the proportion necessary to formthe ternary compoundof either formula or,preferably, the ratio of the magnesium-to zinc in the alloy, shouldbebetween the ratios in the formulae.

nesiu m present in: an amount ranging from about 2% to 20%, the preferred range beingbetween about 3% and 15%; At room tempera ture the ternary compound goes into solid solu tion inaluminum alloys in an amount of about An excess of zinc,over and above that which i cooperates with magnesium and aluminum to 2%. The percentage in solidsolution 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 improvetheir properties.

A' small amount of silicon is usually present in aluminum alloys and up to 1.5%, silicon may be present in the alloysofithe present invention,

If the alloys are to be rolled or worked the silicon should be low, from about the minimum obtainable quantity of 414% or, .05% to about ,7%. More than 17% 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 parts, ,by weight, of magnesium to form MgzSi. At least suflicient magnesium is therefore added to the alloy to combine with thefsilicon uncombined with any calcium to form Mg Si, and in addition to combine with all the zinc andform the ternary compound according to, the formula AlzMgsZIls.

Mg Si is more stable than the ternary compound above mentionedand may be maintained in solid solution 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, make as eflicient 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.

It has now been found that aluminum alloys containing magnesium (over that necessary to combine with silicon) and zinc in the proportions to form a ternary compound are greatly improved by the addition of about .05% to 1.5% manganese, about .05% to 1.5% chromium and about .1% to 1.5% nickel, with one or more of the grain refining elements, such as those selected from the group consisting of titanium, columbiurn, zirconium, boron, tungsten, molybdenum, tantalum and vanadium in a total amount of .005% to .5%. Iron may be present in an amount from about .1% to 1.5%.. {Although the metals manganese, chromium and nickel each increase the hardness of the alloy, a=given percentage. of each of these elements improves certain of the properties more thanit does others. Nickel increases the tensile strength, proportional limit and yield strength of the alloy without decreasing its elongation to any appreciable degree. In fact, with certain amounts of nickel the elongation is increased,

so that an alloy-having exceedingly desirable and exceptional properties may be obtained.

Alloys containing nickel may be readily heat treated or age hardened to give somewhat superior properties, but very desirable properties which are almost equivalent to the heat treated alloys are alsoobtained when castings are simply aged at room temperature, with or without quenching from the'mold.

Nickel isquite an effective element in the alloy and appreciable improvements in properties of the alloy are noted when it\ ,is present in an amountof about .l%, or more. The preferred properties are obtained with about .3% to about .8% or-1% nickel, andin some cases it is desirable to have the nickel present in amounts as great. as 1.5%.

Manganese, although it decreasesthe tensile.

are obtainable when castings are simplyaged at room temperature, 01' when quenched from themol'd and aged.

Manganese is a very effective element in the alloy and desirable improvements are noted when about .1% or even a little less, such as .05%, is

present in the alloy. The preferred properties are obtained with about .2% to about .5% 'or .8% manganese, and in some cases it is desirable to 'have the manganese present in amounts as greatas about 1%, or even 1.5%.

Chromium is a particularly effective alloying element in the alloy of the present invention.

Although it does not appear to improve the proportional limit and yield strength of the alloy quite as much'as does manganese, it increases the elongation and further increases corrosion resistance. It 'is,. therefore, particularly advantageous that both chromium and manganese be present. As little as .05% or .1% chromium,

and particularly with manganese, is effective in improving the properties of the alloy, but .2% or .3% to about 1%, or even 1.5%, is desirable. When manganese is also present, the total of manganese and chrominum should preferably be between about .3% and 2.5% .of the alloy.

The quantity of each of thehardening metals desired in a given alloy also depends somewhat upon the quantity of other hardening ingredients present and upon the amount of ternary compound, a given hardness and tensile strength often being obtainable either with a relatively large amount of strength-improving hardening metal and a relatively small amount of ternary compound, or with a relatively small amount of such metal and a relatively large amount of magnesium and zinc in the proportions of a ternary compound.

An alloy containing 2%, or even as little as 1%, 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 larger percentage of the ternary compound present, such as 4% to 8% or 9% for casting purposes, 9% ternary compound containing about 5.4% zinc. When the casting is more or less intricately shaped, still greater percentages, such as 10% to 12% or 15%, 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 15% or so, whereas less of the ternary compound, such as 4% to 6%, is preferred in alloys which are quenched upon removal 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 magnesium, zinc and silicon proportioned as herein described, and also containing manganese, chromium, nickel and a grain refining metal such as is set forth in the above group, is improved by the presence of iron in suitable proportion.

Iron in suitable amounts further increases the hardness and tensile strength of 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. Iron also improves castability. These alloys containing iron may be readily heat treated or age hardened to give somewhat superior properties, but the iron in. combination with manganese, chromium and the ternary elements in the above proportion is also outstanding in that almost as desirable properties are obtained when castings 2,290,024 are aged at roomtemperature without a heat treatment or quenching.

Iron has generallybeen considered to crystal lize in large platelike crystals, whichweaken the alloy;, Iron inthe presence oi the ternary compound appears-to crystallizein finely dispersed form and the ternary compound also seems to be dispersed, thus producing a highly desirable e1 y- The quantity ,of iron 'desired in the alloy de pends uponthe quantity of other hardeningin gredients present, upon the'amount of ternary about 0.1% and about 1.5%. With the larger percentage of iron in the alloys, the percentages 'of manganese and chromium may be reduced considerably. If bothmanganese and chromium are present in substantial amounts and if workability, ductility andcorrosionresistanceare not to be impaired, the iron shouldbe low, 1. e.,

lected from the group consisting of titanium,

tungsten, molybdenum, zirconium and vanadium, andespecially tungsten, and molybdenum, im'- prove, both the strength and the elongation of the castings.

Titanium being readily available is frequently used,,,but when special properties of the fabricatedarti'cle are important it is desirable to select the grain refiner that is most suitablefor such properties. The grain refiners boron,

colurnbium and, tantalum may advantageously present, and upon what properties ar most in'iabout 0.1% or less to about 5% or.7% of the alloy. If yield strength and high tensile strength are most important and ductility less important, the amount of iron may be greater.

The grain refining elements are particularly desirable in an" aluminum alloy containing iron,

manganese, chromium and the, ternary compound. Although the iron itself improves the properties of the alloy, the manganesachromium and grain refining elements exert a still furthe improvement independently of iron. f w

The aluminum alloys'of the present invention containing magnesium, uncombined with silicon,

and zinc in the proportion of a ternary compound, whencast in molds of adesign such that chilling takes place substantially simultaneously in the various portions of the casting, solidify without the use of grain refining agents to form good castings. However, it hasfbeen found that certain grain refiningelements substantially improve the propertieso'f the aluminum alloy con-,-

taining the ternary compound, iron, manganese,

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

amount of 01%, to .5%, molybdenum in the amount of .0l% to .5%, vanadium in the amount of 01% to .5%, titanium in the amount of .05% l to .5 columbium in the amount of 01% to 5% and tantalum, in the amount, of .05% to" .5%.

These grainrefining elements should preferably be present in a total amountof 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 desirable in the alloys of the presentinvention, 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 upon the particular condition which must be satisfied. The grain refiners Se- .be used where, appearance, finish and corrosion resistance are important.

The above described hardening elements, manganese and chromium, substantially decrease the hot shortness, improve the properties of the ai by 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 oi zirconium, tungsten, molybdenum,,vanadium and titanium,

alsohave thisproperty, particularly. when present in larger amounts, such as 2% or .3 orso.

It may, therefore, be desirable to have up to,.5,%

or so of these latter elements present.v 1

, The following example illustrates the alloys of the present invention.

An aluminumbase alloy containing ma g nesi-t um, zinc and silicon in proportion to form 6% of a ternary compound based on the formula AhMgvZns, about .3% manganese, about;.25% chromium,,about .2 %,.titanium, about .6 iron and about 22% ni,ckel,with the balance substantially all aluminum and minor impurities, was prepared and chill cast into test bars, The test bars, air cooled after removal fromthemold and aged seven days at room temperature, had

atensile strength of 40,700 lbs/sq. in., a proportional limit of 18,400 lbs/sq. in., a yieldstrength of 25,600 lbs/ sq. in., a hardness of Rockwell E 81.5 and anelongation of 7.2%.

Copper is frequently an impurity in aluminum alloys and, unlike the hardeners present in alloys of the present invention, copper is a precipitation hardening ingredient whichis much more soluble at high temperatures than at low temperatures. Substantial amounts of copper are desirable when the alloy is to be. solution heat treated. Alloys of the present type containing copper in an amount of more thana1% or so form the subject matter of copending applications.

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 inaddition to high strength the alloys are light in weight and are, therefore, especially adapte'd to aircraft construction and the like. This is particularly true when the quantity of ternary compound is sufiiciently low so that the alloy may be drawn or rolled into structural members. v

i If the alloy contains uncombined silicon, about 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 AlzMgaZm 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 .5%, 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 20% AlaMg1Zns would be about 7% and 12%, respectively. The alloys described herein include aluminum, magnesium and zinc, the magnesium, uncombined with silicon, being proportioned to the zinc in 'the ranges of the formulae given for the ternary compound. The proportions for the formation of the ternary compound in the alloy exist when 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 thisrange, or about 35% to 40% of the zinc.

' In the above example 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 upto approximately 50% of its initial value by aging at room temperature for relatively long periods of The same improvetime, such as a few months. ment in tensile strength can, of course, be obtained relatively quickly by aging at temperatures above room temperature. I

To obtain properties even of the same order of magnitude 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 improvement in properties.

The alloys of the present invention have good fatigue and tensile strength and a relatively high proportional limit, even at relatively high temperatures; they have to be heat treated if it is desired to improved and modify their properties, although such heat treatment is not required; and they have sufficient ductility and hardness so that they can be rolled or formed into sheets, rods, wire, 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, 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.

As pointed out above, grain refiners are usually desirable in alloys of the present invention. However, it has been found that the alloys of the present invention have relatively high strength, high proportional limit and ductility without solution heat treatment, even in the absence of the grain refiners. The alloys may be cast, rolled, forged or otherwise shaped with or without grain refiners.

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 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 presented for purposes of explanation and illustration, and that various equivalents can be used, and modifications of said procedure can be made without departing from my invention as defined in the appended claims.

What I claim is:

1. An aluminum base alloy containing magnesium, zinc, about -.1% to about 1.5% iron, about .05% to 1.5% manganese, about .05% to about 1.5% chromium, about .1% to 1.5% nickel, silicon in an amount up to 1.5%, and one or more grain refining metals, with the balance substantially all aluminum and minor impurities, the amount of zinc in the alloy being about .6% to about 9%, and the amount of magnesium in the alloy uncombined with silicon being about 35% to about 45% of the zinc content, the total magnesium being within the range of about .5% to about 7%.

2. An aluminum base alloy containing magnesium, zinc, about .l% to about 1.5% iron, about .05% to 1.5% manganese, about .05% to about 1.5% chromium, about .1% to 1.5% nickel, silicon in an amount up to 1.5%, and one or more grain refining metals in a total amount of about .005% to about .5%, with the balance substantially all aluminum and minor impurities, the amount of zinc in the alloy being about .6% to about 9%, and the amount of magnesium in the alloy uncombined with silicon being about 35% to about 45% of the zinc content, the total magnesium being within the range of about .5% to about 7%.

3. The alloy set forth in claim 1 in which the zinc content is about 1.2% to 6% and the magnesium, content is within the range of about .5% to 6%.

-4. The alloy set forth in claim 1 in which the zinc content is about 1.2% to 4.8%, and the magnesium content is within the range of about .5% to 5%.

5. The alloy set forth in claim 2 in which titanium is present in the amount of about .05% to .5%.

' 6. The alloy set forth in claim 2 in which'molybdenum is present in the amount of about .01% to .5%.

7. The alloy set forth in claim 2 in which zirconium is present in the amount of about .05% to .5%.

8. An aluminum base alloy containing magiron, 1 about .05% to 1.5% manganese, about .05% t0= WALTER BONSACK.