US1985421A - Magnesium base alloy - Google Patents

Description

Patented Dec. 25, 1934 PATENT OFFICE 1,985,421 MAGNESIUM BASE ALLOY Roy E. Paine, Cleveland, Ohio, assignor, by mesne assignments, to Magnesium Development Corporation, a corporation of Delaware No Drawing. Application May 18, 1934,

seriamoqzzazss 3Claims.

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

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. Specific alloys have, furthermore, been developed that are particularly adapted to casting or working since each of these methods of fabrication has peculiarities for which allowance must be made if the best results are to be obtained. Casting alloys have received considerable attention because of the difllculties encountered in working magnesium and its alloys. An alloy used for 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 7 per cent. Such an alloy casts well and is furthermore susceptible to improvement in strength through solution heat treatment and aging. In spite of these advantageous properties, however, the alloy can not be used in the presence of salt water without being severely attacked with consequent deterioration ,and ultimate failure. This limitation seriously aifects 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 efiort to attain a maximum strength is attended by an even lower corrosion resistance than possessed by the alloy in the as castcondition. e It is an object 'o1' 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 objectis to make an alloy that can be strengthenedv by heat treatment without deleterious effect upon the corrosion resistance. Still another object is to produce an alloy that can be easily made from readily available materials. W

My invention is predicated upon the discovery that a satisfactory casting alloy having good corrosion'resistance coupled with a strength suflicient for ordinary purposes can be made by adding ((11. va-i) from about 1 to 15 per cent of tin, 0.1 to'10 per cent of zinc, and 0.1 to 1.5 per cent of silicon to magnesium. I have found that such an alloy in the heat treated condition is much superior to the well known magnesium-alumiriumalloys in 7 the same condition in resisting attack by salt water.

While alloys within the above quoted range possess the desired properties of casting quality, strength and corrosion resistance, these qualities have been found to be most effectively combined in alloys containing from about 4 to 10 per cent of tin, 2 to 7 per cent of zinc and 0.1 to 0.75 per cent of silicon. A preferred alloy composition that illustrates the advantageous combination of properties above referred to is one composed of magnesium, about 5 per cent tin, 4 per cent zinc and 0.3 per cent silicon. A casting of this alloy when heat treated at 482 C. for 16 hours and quenched had a'tensile strength of about 26,500

lbs. per sq. in., a yield strength of about 9,000 lbs. per sq. in. and an elongation of 7 per cent in two inches. After aging the heat treated alloy at 150 C. for 8 hours the tensile strength was increased to about 34,100 lbs. per sq. in /the yield strength was raised to 22,700 lbs. per sq. in. and the elongation decreased to 3.4 per cent. A cast magnesium-8 per cent aluminum alloy on the other hand that has been heat treated has a tensile strength of about 32,000 lbs. 'per'sq. in. and an elongation of 6 per cent. It aged after heat treatment the tensile strength remains practically unchanged and the elongation is reduced to about 4 per cent. 1

. The presence of silicon is particularly beneficial in improving the-strength of the heat treated and aged alloy. A magnesium base alloy for example that contained 5 per cent tin and 4 per cent zinc with silicon present only as an impurity under similar heat treatment and aging such as disclosed above had a tensile strength of about 32,200 lbs. per sq. in., a yield strength of 19,200 lbs. per sq. in. and an elongation of 3.7 per cent. Compared to .the properties of the magnesiumtin-zinc-silicon alloy previously disclosed the improvement in yield strength is particularly noticeable. This increase; in stifiness is very desirable in certain applications where the rigidity of a light section is an important consideration.

The improved corrosion resistance of my alloy is readily apparent when compared to the behavior of the common magnesium-aluminum alloy. In a severe corrosion testcomprising alternately immersing specimens 01 several alloys in a 3% aqueous NaCl solution and exposing to the atmosphere for a period of about hours, the heat treated and aged magnesium-tin-zinc-silicon alloy previously referred to suffered no loss in strength whereas the magnesium-aluminum alloy lost over 80 per cent in strength. The corrosion resistance of a heat treated and aged alloy is the most critical test of an alloy since it is the general rule that thermally treated alloys are much more susceptible to corrosive attack than the untreated material.

The usual thermal treatment for improving the strength of magnesium base alloys may be employed in treating the alloys-described hereinabove. This treatment usually consists of heating to a temperature of over 400 C. and below the point of incipient fusion of the alloy for several hours to secure the practicable maximum solution. At the end of the heating period the alloy is rapidly cooled to a lower temperature. While such treatment serves to increase the strength to a marked degree it may be still further augmented by reheating the alloy to a lower temperature, about C. for example, for a period of several hours.

My alloys ay be compounded in any manner understood a a practiced by those skilled in the art, although preferably I add the alloying constituents either in the metallic form or as rich alloys with magnesium.

The term magnesium as used herein and in the appended claims refers to the metal of commercial purity with the usual impurities.

I claim:

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

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

3. A magnesium base alloy consisting of mag- 20