US2240940A - Aluminum alloy - Google Patents

Description

Patented May 6, 1941 Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application September 28, 1940, Serial No. 358,925

3 Claims.

This invention relates to aluminum base alloys, and it is particularly concerned with wrought alloys that receive a solution heat'treatment and are artificially aged. This application is a continuation-in-part of my copending applicationSerial No. 309,392, filed December 15, 1939.

It has been known. that aluminum base alloys containing more than per cent of zinc, especially those that also contain copper, magnesium and/or silicon, possess relatively highstrength in the wrought form. These alloys, however, sufier from several disadvantages, as compared to alloys containing copper and magnesium as the chief added alloying elements but no zinc,

amount of zinc is used, increased difficulty in of. time. Another object is to provide an aluminum base alloy of this type which can be readily hot worked. Still another object is to provide a wrought solution heat treated. and artificially aged aluminum base alloy containing zinc which exhibits substantially no acceleration of corrosion under stress.

a My invention is predicated upon the discoverythat aluminum base alloys composed oi. aluhot working, a lower resistance to corrosion, and

under certain conditions a susceptibility to 'a form of corrosion known as stress cracking.

The early alloy compositions containing 15 per,

cent or more of zinc were later modified by the addition of such elements as copper, magnesium, and silicon, and some reduction in the amount of zinc. While the reduction in the amount of zinc and the addition of other alloying elements reduced the above named disadvantages, they were not entirely eliminated.

Some of the most satisfactory high strength aluminum base alloys containing less than 15 percent zinc together with additions of magnesium, copper, and manganese,. arethose disclosed in U. S. Patent 1,924,729 to L. J. Weber.

structures which are designed to utilize the maximum loading characteristics ofthe structural material, an intensive search has been made in the field of high strength aluminum basealloys to find compositions that are relativehr free from susceptibility to acceleration of corrosion under stress.

The principal object of my invention is to provide an improved wrought aluminum base alloy containing zinc as the principal added alloying component, which possesses both high strength and a high degree of resistance to corrosion under stress. A further object is to provide an age hardened alloy of this type having stable mechanical properties over a long period minum, 4 to 6 per cent zinc, 0.75 to 2.5 per cent magnesium, 0.1 to 2 per cent copper, 0.1 to l'per cent manganese, and a small amount of one or more of the grain refining elements titanium, boron, zirconium, molybdenum, tungsten, cobalt, chromium, and vanadium, avoid to a large extent the above named disadvantages usually associated with aluminum base alloys containing substantial amounts of zinc. By the expression grain refining elements, I mean that these elements refine the grain structure of the alloys in either the cast or wrought condition or both. At least one of the group of grain refiningelements should be employed in the following amounts: 0.02 to 0.25 per cent titanium, 0.005 to 0.1 per cent boron, 0.01 to 0.15 per cent zirconium, 0.02/to 0.25 per cent molybdenum, 0.02 to 0.2 per cent tungsten, 0.02 to 0.2 per cent cobalt, 0.05 to 0.5 per cent chromium, and 0.02. to 0.2 per cent vanadium. Alloys falling within the foregoing range possess exceptionally high stable mechanical properties and resistance to corrosion when thermally treated in the-conventional manner to improve their strength and hardness. I have also discovered that when these alloys are artificially agedafter solution heat treatment they are remarkably resistant to the acceleration of corrosion under the influence of stress. Although many lower strength aluminum base a1- loys possess a satisfactory resistance to ordinary corrosion, it has been observed that it is difiicult to obtain a high degree of resistance to acceleration of corrosion under stress in alloys having 1 very high strength. By the expression acceleration of corrosion under stress}? I mean that upon exposure to the same corroding medium there is substantially no increase in the susceptibility to loss of strength in an alloy article under external stress as compared to the susceptibility to lossin strength in the same article under no external str'ess.

in a duralumin type of alloy. This property is of value in commercial heat treating operations,

- since variations in heating conditions can easily occur, and it is therefore desirable to use an alloy which is not too sensitive to such variations. In

the case of my improved alloy, the solution heat treatment may be given within. a range of .820 to 1000 F. The period of time required to secure the desired solution of soluble constituents will, of course, vary somewhat with the temperature and with the mass of material being heated, butthis is a matter that can be easily determined under the particular conditions existing in commercial operation.

Another advantage possessed by my alloy is I that it may be quenched from the solution heat treating temperature in a variety of quenching media without/great eifect upon the ultimate mechanical properties of the alloy. In other words, my-"alloy is not as sensitive to variations in a quenching procedure, especially if the copper-content does not exceed about 1 per cent, as aremany of the aluminum base alloys. In a number of casesv an air blast quench can be used 1 whereas this cannot be employed in quenching processes, such as rolling, forging or extruding.

Several examples may be cited to illustrate the properties obtainable in my improved alloys. The A alloys tested had the following chemical comduralumin type of alloys ifmaximum strength and resistance to corrosion is desired.

As mentioned hereinabove, my improved alloy must be artificially aged in order to attain high stable mechanical properties and maximum re-i sistance to corrosion under stress. Although my I alloy will spontaneously age at room temperature after beingquenched from'the solution heat treating temperatureand exhibit a continuous increase in strength over a long period of time, I have found that the resistance to acceleration of corrosion under stress decreases with prolonged aging at room temperature which is, of course, highly undesirable. If, however, the alloy is artificially aged instead of aging at room temperature, the desired resistance to corrosion is obtained. The artificial aging treatment should consist of reheating the quenched alloy to a temperature between 225 and 340 F. and holding it at that temperature for a period of 4 to hours. In general, I have found that aging at a temperature of 275 F. for 8 to 12 hours produces a very satisfactory combination of properties.

In the manufacture of articles from sheet, it is often necessary to cold form the sheet. In the case of the solution heat treated and aged ,duralu min type alloys, the age hardening occurs at room temperature and progresses so rapidly after the alloy has been quenched that it is difficult to cold form the alloy. In contrast to such a condition, my alloys age harden but slowly after having been quenched from the solution heat treating temperature with the result that they may be readily cold formed over a muchlonger period after quenching. After the cold forming operation the alloys are artificially aged in the manner described above. I have also found that even the hours at 320 F. Following these treatments,

artificially aged material can be readily cold formed as compared to other age hardened aluminum base alloys of theduralumin'type.

' In the manufacture of my alloys, aluminum is used'which contains the usual impurities of iron and silicon. In general, I prefer to use metal containing less than about 0.4 percent total of iron and silicon. However, it is possible to allow as much as 0.75 per cent silicon and 0.5 per cent iron. Where the alloys are-cast by a continuous process, the silicon should not exceed 0.2 per cent.

Through use of the foregoing thermal treatments, I have been able to consistently obtain in positions:

TABLE I Chemical composition, per cent Magne- Cop- Manga- 8111- v Tita- Anoy Zmc sium per nese con Iron nium All of the alloys were rolled to sheet 0.064 inch in thickness in the usual manner and thermally treated as follows: -Sheets from alloys C, D, E, F,

and G were heated in anair furnace at 920 F.

for 20 minutes, quenched in cold water and aged at 275 F. for 12 hours. were heated at 970 F. for 20' minutes, quenched in cold water and aged 12 hoursat 320 F.', while sheet from alloy B was heated at 970 F. for 20 minutes, quenched in cold water and aged for 18 tensile'test specimens were taken from the sheets of the several alloys for the several tests. The

average mechanical properties of these alloys were as follows: i

' TABLE II Mechanical properties Tensile Yield Elonga- Anoy strength strength tion Lbs/sq. in. Lba/rq. in. Percent Additional tensile test specimens were subjected to an alternate immersion test for 48 hours consisting of elevating from and lowering I the 'specimens'into an aqueous solution of 5 per cent sodium chloride and 0.3 per cent hydrogen peroxide. One group of specimens from each alloy were exposed to the test in an unstressed condition, while the other group were stressed as simple beams in an amount equal to per cent of'the yield strength. At the conclusion of the 48 hour period, the specimens were'removed and their mechanical properties determined. A comparison was made between the properties of the corroded specimensand the original properties of the several alloys. The change brought about by corrosion is expressed Sheets from alloy A,

in the table below in terms of the per cent lost with respect to the original tensile strength and elongation values.

TABLE III Tensile test specimens from the material described above were subjected to the 48 hour alternate immersion test referred to hereinabove, one portion of the test specimen being exposed 5 in the unstressed condition and the other porlosses tion being stressed 75 per cent of the yield v strength. The per cent, change in properties Unstrcssed Stressed caused by corrosion are given in the table below. A TABLE V lggg ggg Corrosion losses -25 -7 -29 Unstressed Stressed l6 -3 -18 1 -50 -6 -54 Alloy Quench .25 Tensile Elonga- Tensile Elonga- -7 1 7 strength tion strength tion 38 -4 38 75 -10 -sa Air blast -14 -67 -24 88 --.do -5 -7a -4 -70 It will be observed that there is little diflerqggfigfiggg :2 :2; :2 :22 ence in loss of tensile strength between the Oil n stressed and unstressed specimens. This indie cates that the applied stress had substantially no It will be observed that alloy C which conefl'ect upon the resistance to corrosion, and hence it may be said that there has been no acceleration of corrosion by stress.

The efiect of the copper content of the alloy upon the resistance to corrosion and acceleration of corrosion under stress when a less drastic quenching medium than cold water is used, is shown in the following tests. For this purpose, sheets from alloys C, D, E, and G were used. The sheets from alloys C, D, and E were heated in an air furnace at 920 F. for minutes and quenched in a high velocity air blast, the air being at room temperature, and aged for 12 hours at 275 F. The G alloy material was likewise heated in an air furnace at 920 F. for 20 minutes, one portion being quenched in boiling water, and another portion quenched in a commercial quenching oil at room temperature.

Material from both portions were aged for 12 hours at 275 F. The mechanical properties of these alloys in this condition were as follows:

TABLE IV Mechanical properties tained more than 1 per cent copper suffered proportionately greater losses than the other alloys containing less than 1 per cent copper. It is also evident that stressing the latter alloys containing less than 1 per cent copper had substantially no eifect upon the resistance to corrosion.

Having thus described my invention, I claim:

1. A wrought, heat treated, and artificially aged aluminum base alloy composed of from 4 to 6 per cent zinc, 0.75 to 2.5 per cent magnesium, 0.1 to 2 per cent copper, 0.1 to 1 per cent manganese, and at least one of the group of grain refining elements consisting of 0.02 to 0.25 per cent titanium, 0.005 to 0.1'-per cent boron, 0.01 to 0.15 per cent zirconium, 0.02 to 0.25 per cent molybdenum,- 0.02 to 0.2 per cent tungsten, 0.02 to 0.2 per cent cobalt, 0.05 to 0.5 per cent chromium, and 0.02 to 0.2 per cent vanadium, the balance being aluminum.

2. A wrought, heat treated, and. artificially aged aluminum base alloy composed of from 4 to 6 per cent zinc, 0.75 to 2.5 per cent magnesium, 0.1 to 2 per cent copper, 0.1 to 1 per cent manganese, 0.05 to 0.5 per cent chromium, and 0.02 to 0.25 per cent titanium, the balance being aluminum.

3. a wrought, heat treated, and artificially aged aluminum base alloy composed of from 4 to 6 per cent zinc, 0.75 to 2.5 per cent magnesium, 0.1 to 2 per cent copper, 0.1 to 1 per cent manganese, and 0.02 to 0.25 per cent titanium, the balance being aluminum.

- JOSEPH A. NOCK, JR.