US2290021A - Aluminum alloy - Google Patents

Description

provide an alloy Patented July 14; 1942 s1 PATENT c cE -f flammNUMAL or b No Drawing; Application l l b Walter Bonsack, South Euclid, Ohio,

Serial No.

v b dClaims. This invention relates to alloysli and particularly to aluminum base alloys suitable for casting having high strength at ordi-' and. working, and

nary and elevated temperatures. c o v o This application is a division of mycopending application SerialNo.389,020,filedApri1 17, 1941,

assignor to Cleveland,

Au u ta, 1941, 405,885

gators the formula AlsMgrZm, and other investigators for Aluminum alloys andofmy copending appli- 398,314 filed Aluminumalloys. I b p o a o of this invention ,to produce alloys having relatively hi h elongation and relatively hlQhItBIlSflBStIBnEth. l It is a further object of vide a relatively light alloywhlch may be easily cast and machined, which may be used atelewithout a rapid deterioration June 16,1941. for

vated temperatures o of desirable properties and which maybe readily this invention to pro treated with anodic treatment to give excellent lustre andfinish. l v i v. It is a still further? object of this invention to having a relatively highfproportional limit and relatively high fatigue stren th, andin which these properties may be obtained without heat treatment. b. I o

o It has beenfound that an aluminum alloy containing iron, and present in proper proportionsgwill produce an alloy that may be readily cast and have improved physical properties for useboth at ordinary and t elevated temperatures; and which may have these "properties improved byheat treatment. l When magnesium and zinc areaddedtoaluminum in the proper proportions, a ternary compound of aluminum, magnesium andzinc is formed, which compound is solublein solid solution in the aluminum. The presence pound in relatively small amounts greatly improves the characteristics or aluminum and produces an alloyyhaving high with high ductility, good. casting and forging properties, good color and excellent corrosion re-" sistance.- In; calculating the amount of magnesium and zinc thatshould be present in the 1 aluminum alloy.

to form the desired percentage of ternary compound,

having zinc and magnesium strength combined of this como o by the formula MgZm. It has temary compound of tive to each other are quite similar in mules and. l for ternary compound goes minumalloys in an amount ofabout 2%. The

compound as being AlzMgaZns. It will be seen that the amounts of magnesium and zinc rela-.

both forthe purposes of the improved alloy. the magnesium and zinc should. be present in about the proportion necessary ternarycompound-of either formula. p b 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,

brittleness and decreases the duos For this reason it is undesirable thatzin'c be present stantially greater than the amount toreact to form such a ternary compound with magnesium and aluminum. The mostdesirable properties are obtained when the magnesium and the zinc are proportioned so thatv the ratio of magnesium (uncombined with any silicon) to zinc is about equalto the ratio represented by the formula or somewhat increases the small amount of to replenish losses alloy metal is remelted. Magnesium adds to the hardness blue with the zinc and greater quantities, alloy sluggish, decreasing castability.

Magnesium and zinc have heretofore been added to aluminum in the proportion represented been found, however, that. a given percentage of ternary compoundFis more eflective 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 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 'temperatur the into solid solution in alupercentage in solid solution increases at high temperatures and decreases upon cooling, the

tohave a'composition having substantially to form the in quantitie sublarger. as represented l and machining qualitiesof the alloy and, as above stated.

should be presentin an amount suflicient to comaluminum present. {ln

magnesium tends to make 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 .7% is desirable in -alloys of the present invention which are to be forged or drawn; 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%, 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 MgaSi, and, in addition, to combine with all the zinc and form the ternary compound according to the formula AlzMgzZm. Mg2Si is more stable than the ternary compound above mentioned and 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. MgaSi does not, however, make 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 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 having low silicon content, alloys containing such low percentages of a ternary compound are relatively diflicult to cast. 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 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 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 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 above described ternary compound is improved by the presence of iron in suitable proportion.

Iron in suitable amounts further increases the hardness and tensile strength of the alloy with- 'cut 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 heat treated 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 effects 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 the quantity of other hardening ingredients and upon the amount of ternary present, a given hardness and tensile strength often being obtainable with a relatively larger amount of iron and a relatively smaller amount of ternary compound, 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 AlaMgvZns, and in suflicient amounts to produce 6% of this ternary compound in the alloy, was prepared. From this base alloy three different alloys were prepared by incorporating the proportions of iron, indicated in the following Table 1, and chill cast in standard test bar molds. Several bars of each alloy were given the indicated heat treatments, that is, some of the bars of each alloy were removed from the mold while seven days at room temperature; another set or bars was removed from the mold before the 1 bars had cooled sufilciently topreeipitate the hardening ingredients, then quenched in water and allowed to, age1at room temperature ior 'seven days. i

Table 1 i Propora Elonga- Yield Tensile Brinell Alloy Fe tion g fi strength strength hardness Per writ Per cent .llbt Jin. Lbs/in. Lo m. KgJmm. 1 '.81 6.2 16,100 23,300 36,900 79 2". 1. 34 4. 4 19, 200 26,4) 39, 500 85 3' 1.87 5.4 18, 500 25,300 39.6) 84 1" ,.,.s1 7.9 16,400 2;,100 39,400 19 2". 1. 34 5. 19,400 41,100 s5 3"- 1. 87 5. 8 17,400 .25, 000 40, ,400 83 Alloy air-cooled; aged at room temperature seven days.

'fAlloy quenched;aged at room temperature seven days. i

It is seen from the test results ofythe above 1.27.15 12% of the alloy, and the magnesium,

uncombined with, silicon, being'proportioned to the zincin theranges of the formulas 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 con-,

tent. Most desirable properties may be obtained when the magnesium (uncombined with silicon) is in the lower portion of this range, or about t 35% to 40% or the zinc.

table .that, although the tensile strengthmay be increased to some extent by a quenching Itreatment," almost equal results are obtained by simv ply air-cooling the casting and aging it at room temperature; Before the quenched casting is aged, a tensile strength of over 30,000 lbs/sq. in.

is obtained, while at the same time the casting has an elongation of 12%. In an aged casting a tensile strength of even greater than 40,000 lbs./sq. in., together with an elongation of almost 8%, is obtained. In an aged casting maximum elongation and substantially maximum strength are obtained when the iron content is about 1% or so.

Whenthe quantity of ternary compound is increased, these maximum values may be obtained with a somewhat lower iron content, or higher maximum strength isobtained with the same .iron content, the castability of the alloy may be also somewhat increased. It will be seen that, since the iron permits one to obtain exceptionally high elongation, combined with high tensile strength, without the necessity of even as much heat treatment as quenching from the mold, the

In theabove examplesof alloy of the present invention it is to be noted thatexcellent tensile strength andhardness are obtainable in a relatively short time byagingat room temperature. A very astounding fact has been discovered, however in connection with these alloys, namely, that thetensile strength may increase up to approximately 50% of its initial value by aging at room temperature for relatively long periods of i time, such as a few months. The same improvement in tensile strength can, of course, be obtained relatively quickly by aging at temperaturesabove room temperature. The improvementoi properties is illustrated by the following table showing the improvement in an alloy containing a small percentage of silicon, about 6% ternary compound, about 1% 1 iron, andabout' .2% titanium. The test bars were chill cast, quenched from the mold, and tested after aging at room temperature for the period indicated.

Table 2 Tensile Hardness .Agmg time Elongation strength Rockwell E Per cent None i2 31, 200 56 3 days... i 7. ii 36, 700 76 1 week.-. 7. 3 38, 300 80 a 2 weeks.. 6. 6 30, 700 82 3 weeks 6. 6 40, 000 84 4 week 6. 3 40, 300 83 5 weeks. 6. 6 40, 900 85 2 months 6. 0 41, 350 85. 3 months.-. 4. 8 41, 600 88. 4 monthsi ,4. 8 42, 500 87. 6 months... 5. 2 43, 400 89; 6 months 5. 1 i3, 700 89.

alloy is especially useful for many purposes, such as large castings or Iorgings, wherein it is ditficult to heat. treat or quench.

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 whenthe quantity of ternary compound is suificiently low, so that the alloy may be drawn or i rolled into structural members.-

If the alloycontains uncombined silicon, about 1.75% magnesium is required to combine with each percent of uncombined silicon to form magnesium silicide (MgzSi) beforeany ternary compound will be formed. For example, it 2% of the ternary compound on the basis of AlMgaZm be silicon, thedesired' in an alloy having .3% 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 willbe about .5%, makinga total of about 1%.

The magnesium and zinc in an alloy containing 37% free silicon and 20% AhMg'rZm would be about 7% and 12%, respectively. The alloys described herein include aluminum, magnesium and zinc, the zinc being present in the amounts 0! 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 oi the present invention it is not necessary to solution heat treat for improvement in properties.

The alloy of the present invention have good fatigue and tensile strength 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 suflicient 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

aluminum, magnesium and zinc in the alloy, such magnesium as is necessary to combine with the u aniuncombined silicon is not to be considered as part of the magnesium necessary to form th 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 to 2% iron, and about ,15% L 1.5% silicon, the percentage of zinc in the alloy being about 1.2% to 12%, the amount of magnesium uncombined with silicon being about 35% to about 45% of the zinc content, such percentage of magnesium being within the range of about -5% to with the balance substantially' all aluminum and minor impurities.

2. An aluminum base alloy containing magnesium, zinc, about to iron, and about 15% to 1.5% silicon, the percentage of zinc in the to 7.2%, the amount of alloy being about 2.4%

magnesium uncombined with silicon being about to about of the zinc content, such percentage of magnesium being within the range of about .5% to 6%, with the balance substantially all aluminum and minor impurities.

3. An aluminum base alloy containing magnesium, zinc, about .6% to 1.5% iron, and about .15% to 1% silicon, the percentage of zinc in the alloy being about 1.2% to 12%, the amount of magnesium uncombined with silicon being about 35% to about 45% of the zinc content, such percentage of magnesium being within the range of about .5% to 7%, with the balance substantially all aluminum and minor impurities.

4. An aluminum base alloy containing magne- Sium, Zinc, ab t to 2% iron, and silicon in an amount up to 1.5%, the percentage of zinc in the alloy beingfrom 1.2% up to about; 6%, the amount of magnesium uncombined with silicon being about 35% to about 45% of the zinc content, such percentage of magnesium being within the range of about .5% to 6%, with the balance substantially all aluminum and minor impurities.

WALTER BONSACK.