US2070833A - Method of treating aluminium alloy and product - Google Patents

Description

Patented- Feb. 16, 1937 UmrsoffsTArEs PATENT OFFICE.

' METHOD OF TREATING ALUMINIUM ALLOY AND PRODUCT Fred Keller and Charles Kensington, Pa., assignors to M. Craighead, New

Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing.

8 Claims.

This invention relates to aluminum base alloys containingmagnesium, silicon and copper, and

' it is especially concerned with this type of alloy and less than '4 per cent each of magnesium and silicon have been widely used in recent years for.

structural purposes because: of their excellent mechanical working characteristics, and superior corrosion resistance combined with a moderate strength when heat treated. Such alloys must be given I eu e require as. high a'strength as possible which ,is-flconsistent with the other desired properties,

I the] articles made from these alloys are usually heat treated. Although the strength ofxheat -ltreated' aluminum-magnesium-silicon alloys is satisfactory for many purposes, it isnevertheless lower than that normally obtained in the well known aluminum-copper type of duralum-in alloy in the heat treated condition. Thisdeficiency has served to limit the utility of the alloy for certain types of service where ease in mechanical working and a high degree of corrosion resistance are of prime importance and must be combined with a high strength. Various elements have been added'to the simple ternary alloy to overcome the foregoing disadvantage, but these additions in general tend to lower the corrosion resistance and hence have been used only sparingly where this propertyis an important consideration. In the production of aluminum-magnesiumsilicon alloys some attempts have beenmade to restrict the quantityof magnesium and silicon employed to therelative proportions found.- in the intermetallic] compound, magnesium. silicide,

j which has the formula :Mgzsi. Where this has beendone; theamount of MgzSl has been generallylimited to less than 1 per cent in order to avoid an excess of the compound beyondthe limit of its solid solublity in aluminum. It has been generally a believedthat more than 1 per cent'of MgzSi was neither necessary: nor desirable from the standpointer-improved mechanicalproperties and coro,

rosion'resistance."

,Oneof theobjects of our, inventionis to improve thestrength of heat treated aluminummagnesium-silicon alloys without substantially altering their corrosion resistance and workabil ity. A further object is to-so control the proporabout 1.5 to 3 per cent so-c'alled solution and aging heat treatpments to developztheir full strength and. since st of the applications where these alloys find 500: (1., at least. 1.5

Application October 25, 1934, Serial No. 750,020

tions of magnesium and silicon as to insure a saturated solid solution of MgzSi in' aluminum at the temperature of heat treatment. Another object is to increase the strength of the alloy beyond that obtainable from the presence of MgzSi alone. Still another object is to control the grain size of the alloy in order toobtain a fine grained structure in the heat treated product.

Our' invention is predicated upon the discovery that the addition of from 0.1 to 4per cent of copper to an aluminum base alloy containing of MgzSi yields a product which exhibits the workability and corrosion resistance found in aluminum-magnesium-silicon alloys but at the same, time possesses a much higher strength in the heat treated condition. We have found that in order to obtain this result the magnesium and silicon must be added in substantially the proportion existing in the compound magnesium silicide, MgzSi, and that there must be at least 0.5 per cent of this compound in excess of the maximum amount which is soluble in aluminum at the particular heat treating temperature employed. Contrary to what has heretofore been believed concerning the eflect of an extra amount of a solublecons'tituent over and beyond the limit of solid solubility, it has been discovered that an excessoi Mgzsi does not reduce the corrosion resistance or adversely affect the working quality of the alloy. The

' added amount of this constituent serves to insure maximum solution when the alloy is heat treated. The most recently determined limits of solid solubility of MgzSi' in aluminumvary between about 1 per cent at 500 C. and 1.85 per cent at the eutectic temperature of 595 C. The eutectic temperature is lowered when copper is added to the alloy, but the solid solubility of MgzSi remains virtually unchanged as far as has been ascertained.

The amount of MgzSi WhlCh'WOllld be requiredto yield an excess of at least 0.5 per cent beyond the maximumoua'ntity which is soluble at the heat treating temperature may be readily determined through reference toan equilibrium diagrannor 'for all practical purposes it maybe obtained by assumingthat the solubility increases linearily from 1 to about 1.85 per cent within the temperature range between 500 C. and the melting point of the eutectic. 'For example,if the solution heat treating temperature should be per cent MgzSi would'be alloy I having the desired required-to'produce an temperature of 520 C.

characteristics. or if a should be employed, at least about 1.7 per cent of Mgzsi would be needed to provide an excess of 0.5 per cent of this compound beyond the limit of solid solubility. Within the range of heat treating temperature used in commercial practice we prefer to have from about 1.5 to 2 per cent of MgzSi in the alloy. As much as 2 per cent MgzSi in an alloy heat treated at 520 C. is not deleterious to the properties of the alloy and under some circumstances it may be desirable to have more than the minimum 0.5 per cent excess constituent present.

The copper content of the alloy serves, to increase the strength beyond that of an alloy containing only MgzSi as a solution and age hardening constituent. While from 0.1 to 4 per cent copper may be advantageously employed it has been found that from 1 to 3 per cent of this element yields the most satisfactory results in combination with MgzSi. The use of copper in conjunction with MgzSi has produced an alloy having much better working qualities than the common aluminum-copper alloys. The corrosion resistance of our improved alloy is also superior to that of the familiar aluminum-copper type of heat treated alloys. I

Although the foregoing alloys are satisfactory for many purposes it has nevertheless been found that they are prone to develop a coarse grained structure upon being annealed or heat treated after cold working. A structure of this character is very undesirable under certain conditions, particularly where the alloy is to be drawn into final shape, because the coarse grained material develops a rough surface with an unsightly appearance. This difficulty may be overcome, we have found, through the addition of relatively small amounts of certain related elements which are members of a larger and well recognized group of elements. It has been ascertained that the presence of from about 0.1 to 1 per cent of at least one of the class of chemically and physically related elements chromium, molybdenum, tungsten and uranium in alloys of the class described hereinabove will prevent the formation of coarse grains, with consequent improvement in the drawing quality of the alloy. If more than one of these elements is added the total amount should not exceed about 1 per cent.

The heat treatment herein referred to comprises heating the alloy at a temperature be tween about 500 C., the lowest practicable limit for commercial operation, and the temperature of incipient fusion of the lowest melting point eutectic. Since diffusion of the soluble constituents increases with the temperature it is the usual commercial practice to use as high a temperature as possible and yet avoid any incipient fusion of any alloy constitutents. In any case the article being treated is held at the elevated temperature for a sufficient length of time to dissolve virtually the maximum possible amount of the soluble constituents, MggSi and copper. The time required to accomplish this result may vary between 5 minutes and 20 hours depending on the nature of the article. Upon attaining this condition the article is rapidly cooled to a much lower temperature such as by quenching in hot or cold water. This state of solid solution which normally does not exist at room temperatures is preserved by the rapid cooling. Although there is some increase in strength bythis treatment as compared to its strength prior to heating yet the full ormaximum strength is only attained by a subsequent aging, which usually comprises reheating to a temperature between about 140 and 200 C. for a period time ranging from 3 to 36 hours. Since the increase in strength is de pendent upon first obtaining a solid solution and later causing the dissolved material to precipitate in very finely divided form, it is essential that a maximum solution be effected in order that there be an abundant uniformly distributed precipitate. According to prevailing theories regarding the hardening of heat treated aluminum base alloys of this character, the ultimate strength of the article is directly connected with the form and distribution of the precipitate from a supersaturated solid solution which is unstable at ordinary temperatures.

A preferred composition which exhibits the properties referred to hereinabove consists of aluminum, about 1.6 per cent MgzSi, 1 per cent copper and 0.25 per cent chromium. An alloy of this composition when heat treated at 515 C., quenched and aged 18 hours at 160 0., had a tensile strength of about 56,600 pounds per square inch, a yield strength of 45,500 pounds per square inch, and an elongation of 15.5 per cent in two inches. When the copper content was increased to 2 per cent, the other constituents remaining unchanged, and the alloy heat treated and aged as in the former case, a tensile strength of 62,200 pounds per square inch was obtained, also a yield strength of 48,700 pounds per square inch, and an elongation of 16 per cent. By way of contrast, a similar alloy containing no copper, after heat treating and aging, had a tensile strength of 38,000 pounds per square inch, a yield strength of 33,000 pounds per square inch, and an elongation of 15 per cent. The benefit to be derived from the addition of copper to the alloy thus becomes apparent. In the foregoing alloys about 1.1 per cent of MgzSi is soluble at the temperature of heat treatment but an excess of 0.5 per cent of this constituent was employed to develop the maximum strength. The alloys containing copper were found to fabricate as easily as the well known a-luminum-magnesium-silicon alloys.

The herein described alloys may be compounded in any manner known to those skilled in the art, but it is our preferred practice to add a "rich alloy to the molten aluminum to incorporate the desired alloying ingredients.

The term aluminum as here employed refers to the metal of commercial quality which contains the usual impurities of iron and silicon.

We claim:

1. A method of improving aluminum base alloys composed of aluminum, magnesium silicide and about 0.1 to 4per cent copper, said method comprising adding at least 0.5 per cent magnesium silicide in excess of the amount soluble at the temperature of heat treatment, heating the alloy to a temperature above 500 C. but below the temperature of incipient fusion until substantially the maximum amount of soluble constituents has been dissolved, and rapidly cooling from the elevated temperature to ordinary temperatures.

2. A method of improving aluminum base alloys composed of aluminum, magnesium silicide and about 0.1 to 4 per cent copper, said method comprising adding at least 0.5 per cent magnesium silicide in excess of the amount soluble at the temperature of heat treatment, heating the alloy to a temperature above 500 C. but below the temperature of incipient fusion until substantiallythe maximum amount of soluble constituents has beendissolved, and rapidly cooling from the elevated temperature to ordinary'temperatures and'aging by reheating to a temperature between about 140 and 200 C.

3. A method of improving aluminum base al-- loys composed of aluminum, magnesium silicide, from about 0.1 to 4 per cent copper and from about 0.1 to 1 per cent of at least one of the group of metals, chromium, molybdenum, tungsten and uranium, the total amount of said chromium, molybdenum, tungsten, and uranium being not in excess of 1 per cent comprising adding at least 0.5 per cent magnesium silicide in excess of the amount soluble at any heat treating temperature lying betwen about 500 C. and the temperature of incipient fusion, heating the alloy to the desired temperature within said range until substantially the maximum amount of soluble constituents has been dissolved, and rapidly cooling the alloy to ordinary temperatures.

4. A method of improving aluminum base alloys composed of aluminum, from about 1.5 to 3 per cent of magnesium silicide, from about 0.1 to 4 per cent of copper, and from about 0.1 tol per cent'of at least one of the group of metals, chromium, molybdenum, tungsten and uranium,

the total amount of said chromium, molybdenum,

' tungsten, and uranium being not in excess of 1 per cent, comprising heating the alloy at a temperature, between about 500 C. and the temper-, ature of incipient fusion, such that at least 0.5 per cent of magnesium silicide remains undissolved beyond the maximum amount soluble at the temperature of heat treatment, and rapidly cooling the alloy to ordinary temperatures.

5. A heat treated and aged aluminum base alloy I containing from about 1.5 to 3 per cent of magnesium silicide, from about 0.1 to 4 per cent of copper and from about 0.1 to 1 per cent of at least one of the class of elements, chromium, molybdenum, tungsten and uranium, the total amount of said chromium, molybdenum, tungsten, anduranium being not in excess of 1 per cent, characterized by having at least 0.5 per cent magnesium silicide in excess of the amount soluble at a heat treating temperature between about .cent magnesiumsilicide, from about 0.1 to 4 per cent copper and from about 0.1 to 1 per cent of at least one of the group of metals, chromium, molybenum, tungsten and uranium comprising heating to such a temperature between 500 C. andthe temperature of incipient fusion that at least 0.5 per cent of magnesium silicide remains undissolved, rapidly cooling the alloy to ordinary temperatures, and thereafter aging the alloy for 3 to 36 hours at to 200 C.

FRED KELLER. CHARLES M. CRAIGI-EAD.