US2240489A - Aluminum alloys - Google Patents

Description

Patented May 6, 1941 ALUMINUM ALLOYS Walter Bonsack, South Euclid, Ohio, assignor to The National Smelting Company, Cleveland, Ohio, a corporation of Ohio No Drawing. Application April 12, 1939,

Serial N0. 267,467

4 Claims.

This invention relates to aluminum alloys, and more particularly to aluminum alloys of superior strength and ductility-at ordinary and elevated temperatures.

In the making of parts for various machines where strength and weight of the parts are important factors, it is desirable to use alloys of aluminum which possess relatively high elastic limits and relatively high ultimate strength and ductility, and which are readily machinable.

An aluminum alloy knownto the trade as the fY alloy," and containing 3.5% to 4.5% copper, 1.2% to 1.8% magnesium, 1.7% to 2.3% nickel, a maximum of .7% silicon, and a maximum of .1% zinc, has been used for this purpose. It is desirable that the strength, ductility, machinability, and particularly the elastic limit of alloys of the Y type be increased, especially at elevated temperatures, so thatsuch alloys may be utilized to more advantage for the manufacture of motor parts, and internal combustion engine parts, such as pistons, cylinder heads, and the like.

An object of this invention is to provide an aluminum base alloy of relatively high tensile strength, proportional limit and ductility, which properties are maintainable at higher temperatures, such as those encountered in use in the parts of internal combustion engines and the like.

It is also an object of thisinvention to provide an alloy with such improved properties, without any substantial decrease in thermal conductivity.

Another object is to provide an aluminum base alloy of increased fatigue strength.

It is a further object of this invention to provide an aluminum base alloy wherein the hardness, tensile strength and. elastic limit may be. further enhanced by heat treatment.

It has frequently been considered that more than .1% zinc is deleterious to aluminum alloys of the Y type in causing hot shortness and weakness at elevated temperatures. It has been found,

I however, that a substantial quantity of zinc may be advantageously present in aluminum base alloys of the Y type, if a sufilcient quantity of mag-' nesium is present and the other ingredients of the alloy are also present in suitable proportions.

It has been found that an aluminum base alloy of the Y type having a relatively small amount of silicon, and having zinc and magnesium present in proper proportions related to each other, and also to the silicon, will produce an alloy that may be readily cast and have improved physical properties for use both at ordinary and elevated temperatures, and which may have these proper ties improved by heat treatment.

My improved aluminum alloy may contain copper, nickel, zinc, a small amount of silicon, and magnesium in an amount to provide an excess over that which combines with the silicon that may be present. manganese may be present as impurities.

The copper and nickel function in the improved alloys in the same manner as in the Y alloy. The copper content of my new alloy is preferably approximately that of the Y alloy, but it may be present in amounts of ,between 3% and 5%.

The nickel content may be slightly less than that of the Y alloy, for the reason that the improved properties may be obtained with a somewhat lower nickel content. It is desirable to use less nickel as it tends to decreasethe fluidity and castability of the alloy, and also, on account of the cost of the nickel. The improved alloy may contain from about 1% to about 2.5% of nickel, but the preferred properties are obtained with around 1.5% of nickel.

Silicon is ordinarily present in aluminum alloys, and its presence in small quantities is not objectionable in the improved alloy. In the absence of calcium it combines with magnesium to form magnesium silicide (MgzSi), a compound which may be maintained in solid solution in aluminum alloys in an amount up to about 1.85%. This quantity of Mg2Si corresponding to'.7% silicon is non-deleterious to the alloy, and acts as a hardening constituent. A larger amount of silicon than .7% would react with further magnesium, forming more MgzSi than desirable and decreasing the amount of magnesium present for combination with other constituents. Consequently, it is desirable to limit the silicon content of the alloys to a value below .7%, as more MgZSi than 1.85% does not have a beneficial efi'ect.

However, if a given alloy contains a little more than .7% silicon, the quantity of MgzSi formed .may be reduced to about 1.85% or below by add- A small amount of iron and make the alloy sluggish. It is therefore desirable that the silicon content of the improved alloys be held below about1% even when calcium is present.

Zinc cooperates with the magnesium, which is uncombined with any calcium or silicon present, to form with the aluminum a ternary compound. This compound in small amounts I greatly increases thestrength and hardness of the alloy, and has a tendency to decrease its hot shortness. An excess of zinc over and'above that which cooperates with magnesium to form a ternary compound greatly 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 Suiflcient to react to form a ternary compound with magnesium and aluminum. The most desirable properties are obtained with the magnesium present in slight excess, and, when the zinc becomes excessive, a rather sharp decline in ultimate strength and elastic limit occurs.

Magnesium adds to the hardness and machining qualities of the alloy and should be present in an amount sufiicient to. combine with the available silicon to form the ternary compound with the zinc and aluminum present, and to provide a small excess of magnesium which may combine with aluminum so that suincientmagnesium is present to replenish losses that may occur when the alloy metal is remelted. The ternary compound, which is said by some experts to have a composition approximated by the formula MgaZneAls, gives powerful hardening and strengthening properties.

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 temperature. Thus, for maximum strength of the solution heat treated alloys the ternary compound, if represented by the above formula, may constitute as much as about 4% of the metal, whereas less of the ternary is preferred in alloys which are quenched upon removal from the mold and heat treated at a low temperature.

The zinc content should range from about .8% to 3%. With zinc present in an amount of about 2.5%, to form about 3.5% MgvZnsAla, the alloy has great strength, especially when given a solu-,

tion heat treatment. When the alloy is to be given only an aging treatment at relatively low temperatures, the zinc content should preferably range between .8% and 1.6%, with a preferred content of about 1.2% zinc. Too much zinc .or too much ternary compound, in addition to copper, nickel and MgzSi, causes the alloy to become brittle, and tends to decrease the ductility considerably.

The magnesium content in any given alloy thus depends on its silicon and zinc content. The quantity neccessary for combination with silicon can be calculated from the content of free silicon on the basis of the formula MgzSi. The quantity of magnesium for cooperating with the zinc may be computed from the zinc content on the basis of the formula MgaZnsAls. It is then preferred to add an additional .2% or .3% magnesium to form a surplus to replenish the :magnesium lost in melting or processing the alloy. Thus, for example, if an alloy contains .'7% silicon and 1.5% zinc, the magnesium required to form MgzSi will be 1.16 The magnesium required to 3% of Zn. .8% to about 1.6% of zinc, should have betweenv Y pounds per square inch and is the stress necescombine with zinc to form the ternary compound, believed to have the iormula MgvZmAls, would be .6%, and the excess should be about .2%, making a total magnesium content of about 1.96% for the most desirable results. Too much magnesium makes the alloy sluggish, decreases the castability, and lowers the thermal conductivity.

The quantity of magnesium present in the alloys may then vary from a low limit of about 35%, in alloys containing but .8% of Zn and a negligible amount of silicon, to a high limit of about 3% in an alloy containing 31% silicon and The preferred alloys, having about about 35% and about 2.2% of magnesium present, depending on their silicon and zinc content.

As illustrations of my improved alloy the following specific examples are given:

Example I Example II An alloy containing 4% Cu, 1.44% Mg, 2% Ni, .3% Si, 1.2% Zn, .5% Fe present as an impurity, with the balance substantially pure aluminum, was cast as in Example I, annealed at 950 F., quenched in hot water and aged 8 hours at 400 F. The tensile strength and the elongation of the heat treated bars was 60,000 lbs/sq, in. and about 1%. respectively.

The alloy of Example I contains only sumcient magnesium to combine with the silicon to form MgzSl and to combine with the zinc and som of the aluminum to form a ternary compoundsaid to have the formula MgrZnoAlz. The alloy of Example 11 on the other hand contains an excess of magnesium over and above that which may combine with the silicon and zinc to form M22Si and MgvZnaAla respectively. The higher tensile strength of the alloy of Example 11 is illustrative of th eflectiveness of this small excess of magnesium.

The annealing of alloys at temperatures around 950 F., as required by the solution treatment above, is not always practical, and it has been found that when used at elevated temperatures the alloys so treated have a tendency to lose their improved properties more rapidly than non-solution treated alloys, with the result that lower values are often obtained. The disadvantages relative to solution treatment may be overcome and properties better adapted to high temperature use may be obtained by quenching the alloy castings directly from the mold into hot water, and then aging them at relatively low temperatures, such as about to 250 C.

The following table illustrates the comparative tensile strengths, elongations, and proportional limits of a. typical high strength Y alloy and a typical improved alloy. It may be noted that the proportional limit is expressed in sary to produce .0l% permanent strain.

The improved alloy with respect to which the following data was compiled and which illustrates the present invention, contained 1.17% zinc, .5l% silicon, 4.25%copper, 1.7% magnesium, 1.6% nickel, .5% iron and .2% manganese (the latter two elements being present as impurities), with the balance substantially all aluminum. Both alloys were cast into suitable test bars, the pouring temperature being between 1250 F. and 1300 F. and the mold temperature being 800 F. The bars were quenched directly from the mold into'hot water and aged at the temperatures and for the times indicated below:

AGING TEMPERATURE 200 C.

l 'f f f Teiiligile site a: Fiongation s. s in. ercent Aging time (um/sq q p (hours) Y- Improved Y- Improved Y- Improved alloy alloy alloy alloy alloy alloy AGING TEMPERATURE 225 C From the above data it is apparent that the preferred properties are obtained by aging the alloys about eight hours or more at 200 C. It

' is also apparent that although maintained higher temperatures affect both alloys somewhat adversely, the properties of the improved alloy are consistently much better than those of the conventional Y-alloy.

The improved alloys even though containing.

mirably suited for Diesel engine pistons and other severe uses.

As used in the appended claims the expression "balance substantially all aluminum is intended to include minor quantities of impurities which may be present in the alloy.

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 comprising 3% to 5% copper, 1% to 2.5% nickel, less than 1% silicon, .8% to 1.6% zinc, about 35% to about 2.2% magnesium, with the balance substantially all aluminum.

2. An aluminum base alloy having high strength at elevated temperatures, comprising 3% to 5% copper, about .35% to 3% magnesium, about .8% to 3% zinc, 1% to 2.5% nickel, less than 1% silicon, with the balance substantially all aluminum, the magnesium being present in sufiicient amount to combine in the ratio represented by the formula MgzSi with all silicon uncombined with any calcium present, and to combine with all the zinc to form a ternary compound of aluminum, magnesium and zinc with no substantial excess.

3. An aluminum base alloy having high strength at elevated temperature, comprising 3% to 5% copper, 1% to 2.5% nickel, .8% to 1.6% zinc, less than .7% silicon, about .35% to about 2.2% magnesium, with the balance substantially all aluminum, the magnesium being present in sufiicient amount to combine in the ratio represented by the formula MgzSl with all silicon uncombined with any calcium present, and to combine with all the zinc to form a ternary eompound of aluminum, magnesium and 4. An aluminum base alloy comprising about 3.5% to 4.5% copper, about 1.5% nickel, about .8% to 1.6% zinc, less than 37% silicon, magnesium in the amount of 35% to 2.2%, to react with the silicon which is uncombined with any calcium present and form MgzSi, to react with all'the zinc to form a ternary compound of aluminum, magnesium and zinc, and to provide an excess no larger than about .3 or .4%, with the balance substantially all aluminum.

WALTER BONSACK.