US1955991A - Aluminum alloy - Google Patents

Description

Patent ed Apr. 24, 1934 UNITED "STATES ALUMINUM ALLOY Ludwig J. Weber, New Kensington, Pa., assignor to Aluminum Company of America, Pittsburgh, Pa.,' a corporation of Pennsylvania No Drawing.

Application September 21, 1932,

, Serial No. 6 4,149 ionim, (01. 75-1) This invention relates to aluminum base alloys containing substantial amounts of magnesium and particularly to certain modifications of such alloys which have improved properties for specific purposes hereinafter to be described.

The'aluminum base alloys containing magnesium to which this specification and the appended claim refer are those containing to per cent by weight of magnesium, and no zinc, or if zinc is present, then in amounts not exceeding about-1 per cent by weight. It is a characteristic of these alloys that where the alloy is under a high stress, either external or internal in source, and is exposed to a corrosive condition, as for instance corrosive solutions, gases, etc., a peculiar cracking or structural failure may occur. This phenomenon is herein designated as stress cracking. The stress cracking of aluminum base alloys containing 5'to 15 per cent by weight of magnesium does not at all times occur under the conditions above noted and, in fact, its occurrence under mild corrosion conditions and usual conditions of stress is rare. Yet such stress cracking may occur and does more frequently occur where the stresses and the corrosion condition are severe. Because the type of failure known as stress cracking is more drastic in its detrimental action on the structure of the metal than are the. ordinary types of failure induced by corrosion, the elimination of diminution'of this type of failure is of great importance and it is to this end that the present invention is directed.

I have discovered that the addition of a small amount of copper to the abovementioned alloys will greatly diminish the possibility of stress cracking under service conditions. The effect of my discovery is readily demonstrated.' Take, for example, a condition of service exceedingly more severe than ordinary servicea specimen of an aluminum base alloy containing 5 to 15 per cent by weight ,of magnesium is put under a severe and definite stress by bending it. I The specimen is then placed in an aqueous solution containing about 5 per cent by weight of sodium chloride and about 0.3 per cent by weight of hydrogen peroxide. This is one of the severest corrosion tests known. Under these conditions stress 50 cracking occurs, on the average, at the end of 3 to 5 hours. If this same specimen contains, in accordance with my invention, a small amount of copper, the stress cracking does not appear, on the average and under the same conditions, .until 70 to 100 hours have elapsed, I

The amount of copper which has this remarkable efiect is 0.05 to 0.20 per cent by weight of thetotal alloy. No. further improvement is attained by the addition 'of copper in-amounts much greater than 0.20 per cent and, as I have found, further additions are detrimental in that the alloy becomes more susceptible to ordinary or pitting types of corrosion. Since one of the outstanding properties of aluminum base alloys containing 5 to 15 per cent of magnesium is the resistance of the alloy to pitting or ordinary corrosion, it is apparent that the addition of an element which causes this pitting is detrimental and, therefore, the copper content should not exceed more than 0.20 .per cent by weight of the total alloy.

As above indicated, the presence of zinc-in the alloy is undesirable in amounts greater than 1.0 per cent by weight and, in fact, I prefer that the alloy be substantially zinc-free. For some reason not known to me, the presence of zinc in alloys of the composition herein described places the alloy in such condition that the addition of the small amount of copper does not diminish the possibility of stress cracking. In the appended clairh the term substantially free from zinc defines an alloy which is zinc-free or contains zinc in amount not greater than 1 per cent by weight of the total alloy.

Although any good commercial grade of aluminum may be used successfully in making the alloys, I have found that the alloys are least prone to stress cracking when the aluminum used in their fabrication is of relatively high purity and contains 99.6 per cent or more of aluminum. I prefer, consequently,.to use metal of this purity.

The alloys may be manufactured by any convenient method, the most common of which comprises melting the aluminum and adding the alloying elements thereto in the desired proportions.

The alloys may be heated, aged or otherwise thermally treated in accordance with principles well known to the art to effect changes'in tensile and other properties.- The addition to the alloys of 0.05 to 0.20 per cent of copper does not. impair the tensile properties of the alloy and may even increase them as may be evidenced by the fact .that 'a heat-treated and artificially aged alu- Having thus described my invention, I claim:'

A-metallic alloy substantially free from zinc and containing 5 to 15 per cent by weight of magnesium and. 0.05 to 0.20 per cent by weight of.

copper the balance being substantially aluminum,

the aluminum used in compounding the alloy being at least 99.6 per cent pure aluminum.

LU'DWIG .J. WEBER.