US2092012A - Aluminum alloy - Google Patents

Description

Patented Sept. 7, 1937 PATENT OFFICE 2,092,012 ALUMINUM ALLOY Joseph A. Nook, Jr.,

Sager, New Aluminum Company Pa., a corporation of N0 Drawing.

Kensington,

Tarentum, and George F.

Pa., assignors to of America, Pittsburgh,

Pennsylvania Application November 25, 1936,

Serial No. 112,760

4 Claims.

This invention relates to aluminum base alloys and it is particularly directed to improving the properties of certain alloys which contain the intermetallic compound, MgaSi, as the principal hardening constituent.

Aluminum base alloys containing from about" 0.5 to 3 per cent of the intermetallic compound MgzSl and lesser amounts of copper and chromium have been found to possess exceptionally good cold working properties after thermal hardening treatment. In this respect they differ from many other aluminum base alloys which have received similar hardening treatment. Because of these favorable characteristics the alloys have .a wide field of application but their utility is limited under special conditions sometimes encountered in service where the corrosive attack is particularly severe, and where they are under high stress. It is particularly desirable that under these conditions the alloys should have than ordinary resistance to corrosive attack. Although these alloys are as resistant to corrosion as many other aluminum base alloys, it is nevertheless very desirable in special appli-v cations that they should have a resistance to stress corrosion which is above the average for aluminum base alloys.

The term stress corrosion as herein employed refers to a type of corrosion in which the rate of penetration is appreciably accelerated by the existence of a stress within the metal, especially where the stress is applied externally. This type of attack is generally distinguishable from that which ordinarily occurs on unstressed articles both in rate and character. The two types of attack are not necessarily related since it has been found that a treatment or alloy addition may effectively inhibit ordinary corrosive attack and yet fail to afford protection against corrosion under stress.

It is an object of our invention to improve the resistance to stress corrosion of aluminum-magnesium silicide-copperchrcmium alloys of the composition herein Set/forth without impairing their existing advantageous properties especially their working qualities. A further object is to accomplish this purpose in a simple direct manner without recourse to special treatment.

Our invention is predicated upon the discovery that the addition 'of from 1 to 3 per cent zinc to an alloy composed of aluminum, 0.5 to 3 per cent of the intermetallic compound MgaSi, 0.1 to 0.75 per cent copper and 0.1 to 1 per cent chromium, materially improves the resistance to stress corrosion. The use of zinc in the alloys within the aforesaid limits has no significant effect upon the cold working characteristics of the alloy nor does it impair the strength and hardness of the alloy in the solution heat treated and precipitation-hardened or aged condition. As a matter of fact the addition of zinc tends to increase the strength of the alloy as compared to that of an alloy of the same composition except for the omission of the zinc. The presence of zinc in the alloy offers an additional advantage in that it combines with any magnesium in excess of the amount required to form the compound MgzSi, thus improving the resistance to ordinary corrosion. magnesium is allowed to been found that the resistance to ordinary corrosion will be impaired which is obviously undesirable.

We have found that the alloys containing the zinc are practically as workable as those without They may be readily cold rolled drawn or shaped either before or after the hardening thermal treatment. It is thus possible to in excess of that consumed in forming the com-' pound. Under commercial operating conditions it is not always possible to achieve this condition, generally because of slight variations in the silicon content of the base metal. Hence it is often desirable to add not more than about 0.1 per cent magnesium in excess of that which would ordinarily be required to combine with the estimated amount of silicon. In this way, all of the silicon will be combined with the magnesium and. thus insure a. maximum amount of MgaSi which in turn will promote the development of the maximum strength throughout thermal treatment. Any magnesium over and above that required to form MgzSi will unite with zinc to form a noninjurious compound. Magnesium and silicon have a greater afiinity for each'other than exists between magnesium and zinc, and hence even though zinc is present in the alloy, yet the MgzSi compound will be formed first. I

We have found that aluminum base alloys containing the compound MgzSi, and the elements copper, chromium and zinc in the proportions mentioned hereinabove, are especially well adapted to use in the manufacture of cold worked articles. In these alloys, the compound MgzSi should be the predominant alloying constituent aside from the aluminum, as well as being the principal hardener. The copper and chromium only supplement the hardening effect of MgaSi. To obtain the optimum combination of properties in this type of alloy the following proportions of added alloying ingredients are preferred:

' 1.5 to 2.5 per cent MgzSi,

per, 0.25 to 0.75

per cent zinc 0.15 to 0.4 per cent copper cent chromium, and 1.5 to 2.5

The heat treatment to which the alloy should be subjected and hardness to develop its maximum strength is of the conventional type, namely,

heating at 500 to 530C. until substantially complete solid solution is alloy is quenched or otherwise quickly cooled to obtained whereupon the room temperature. This treatment produces a Uniform stress-cantilever specimens of the foregoing alloys were stressed at their yield st per cent of reng'th by fastening the base end The term aluminum as used herein and in the appended claims includes metal containing the per cent.

We claim:

to 0.75 per cent of copper, 0.1 to 1 per cent of chromium, and 1 to 3 per cent of zinc.

per cent of zinc.

4. An aluminum base alloy consisting of aluminum,'1.5 per cent of the intermetallic compound MgzSi, 0.2 per cent of copper, 0.25 per cent of chromium, and 1.5 per cent of zinc.

' JOSEPH A. NOCK, JR-

GEORGE Ff. SAGER,