US1985420A - Magnesium base alloy - Google Patents

Description

Patented Dec. 25, 1934 UNITED STATES PATENT OFFICE MAGNESIUM BASE ALLOY No Drawing.

Application May 18, 1934,

Serial No. 726,289

sciaims.

This invention relates to new and improved magnesium base alloys and it is particularly concerned with that class of alloys which contain no aluminum. r

' Magnesium base alloys have been extensively employed in structures where it is essential to combine light weight with a moderate strength.

.The alloys have been used in both'cast and wrought form, the particular form used for a given application being determined by the type of service expected and the design of the article.v

Specific alloys-have, furthermore, been developed that are particularlyadapted to casting or working since each of these methods of fabrication has peculiarities for which allowance must be 1 made if the best results are to be obtained. Casting alloys have received considerable attention because of the difficulties encountered in working magnesium and its alloys. An alloy used for 2 castings that possesses a high strength compared with other cast magnesium base alloys'is one containing aluminum as the sole or chief alloying ingredient. The aluminum content is generally in excess of '1 per cent. Such an alloy casts well 5 and is furthermore susceptible to improvement in strength through'solution heat treatment and in In spite of these advantageous properties,

however, the alloy can not be used in the presence of salt water without being severely at- 9 tacked with consequent deterioration and ultimate failure. This limitation seriously affects the field of utility of the alloy since there are many potential applications for magnesium base alloys in naval craft or other structures subjected to salt water and spray. Another drawback of magnesium-aluminum alloys lies in the greater susceptibility to corrosive attack in alloys that have been heat treated. The eifort to attain a maxi- "mum strength is attended by an even lower corle rosion resistance than possessed by the alloy in the as cast condition.

It is an object of my invention to produce a magnesium base casting alloy that has a corrosion resistance superior to that of the commercial magnesium-aluminum alloy and yet possesses a moderately high strength. Another object is to make an alloy that can be strengthened by heat treatment without deleterious eifect upon the corrosion of tin,'4 per cent of zinc, 0.3 per cent of man- 1 to 15 per cent of tin, 0.1 to 10 per cent of zinc,

0.1 to 2 per cent of manganese and 0.1 to 1.5 per cent of silicon. An alloy of this composition possesses suiiicient strength for allo'rdinary purposes where magnesium base alloys may be used. I have 5 found that such an alloy is much superior to magnesium-aluminum alloys in resisting attack by salt water, particularly in the heat treated and aged condition. The heat treatment commonly employed to improve the strength of magnesium- 10 aluminum alloys is effective in improving my alloy also. The customary aging treatment may likewise be used to effect a still further increase in strength.

While the alloy within the range of composition above stated possesses the described properties of strength and corrosion resistance, I have found it advantageous under most conditions to use from about 4 to 10 per cent of tin, 2 to 7 per cent of zinc, 0.2 to 1 per cent of manganese and 0.1 to 0.75 per cent of silicon. An alloy composition that illustrates my invention and represents preferred proportions of the several elements is one that consists of about 5 per cent ganese .and 0.2 per cent of silicon; the balance being magnesium of commercial purity. This alloy in the form of a sand casting had a tensile strength of about 20,900 pounds per square inch, 2. yield strength of about 9,500 pounds per square inch and an elongation of about 4 per cent in 2 inches. After heat treating this sand cast alloy for 16 hours at 482 C. and quenching, the tensile strength-was increased to about 24,000 pounds per square inch and the elongation to 5.2 per cent, the yield strength remaining practically unchanged. Upon aging the heat treated. alloy for 8 hours at C., the tensile strength was further increased to about 26,400 pounds per square inch, the yield strength to 16,900 pounds per square inch and the elongation diminished to 3.5 per cent. In comparison, typical mechanical properties of the magnesium-aluminum alloy containing 8 per cent aluminum, the amount commonly used in casting alloys, are as follows. In the as cast condition it has a tensile strength of about 22,000 pounds per square inch, and an elongation of about 2 per cent. 'When heat treated the alloy has a strength in the neighborhood of 32,000 pounds per square inch and an elongation of 6 per cent. If aged after heat treatment, the tensile strength remains practically unaltered but the yield strength is considerably increased, and the elongation drops toaboutiipercent. 55

While the magnesium-aluminum alloy has as high or a higher strength than alloys of my invention under normal conditions, the superiority of the alloys herein disclosed is conspicuous under corrosive attack by salt water. In a severe corrosion test comprising subjecting specimens of the alloy to a spray of 20% NaCl solution for more than a month, an alloy composed of aboutv 5 percent tin, 4 per cent zinc, 0.3.per cent manganese and 0.2 per cent silicon, the balance being magnesium, in the as cast condition lost only about 28 per cent in strength. In the heat treated condition it lost only about 20 per cent in strength while the heat treated and aged alloy lost only 12 per cent in strength. Under similar conditions a magnesium-aluminum allow of the kind described herein above loses from 20 to 25 per cent in strength. The absence of any deleterious effect of heat treating and aging on the corrosion resistance of the magnesium base alloys containing tin, zinc, manganese and silicon is contrary to the usual experience with aged alloys, and my alloys for this reason afford a peculiar advantage in 'ofiering the maximum attainable strength combined with a maximum corrosion resistance. Such a fortuitous combination of properties is of considerable importance, particularly on aircraft operating on or near salt water.

To secure the maximum strength in my alloys, they should be heated to a temperature above about 400 C. and below the point of incipient fusion for a sufllciently long period of time in order to get the practical maximum amount of room temperature. increase the strength of the alloy, it may be heated at a temperature of about C. for

several hours.

The alloy may be compounded in the manner generally employed in making magnesium base I alloys, preferably by adding the alloying constituents either as metallic elements or as rich alloys with magnesium.

The term magnesium as used herein and in I the appended claims refers to the commercially obtainable metal containing the usual impurities I claim:

1. A magnesium base alloy composed of magnesium and about 1 to 15 per cent of tin, 0.1 to 10 per cent of zinc, 0.1 to 2 per cent of manganese and 0.1 to 1.5 per cent of silicon.

2. A magnesium base alloy composed of magnesium and about 4 to 10 per-cent of tin, 2 to 7 per cent of zinc, 0.2 to l per cent of manganese and 0.1 to 0.75 per cent of silicon.

3. A magnesium base alloy consisting of magnesium and about 5 percent of tin, 4 per cent of zinc, 0.3 per cent of manganese and 0.2 per cent of silicon.

ROY E. PAINE.