US2185452A - Method of heat treating magnesium base alloys - Google Patents

Description

- 10 alloys.

Patented Jan. 2, 1940 PATENT OFFICE OF HEAT TREATING MAGNESIUM BASE ALLOYS Robert T. Wood, Cleveland, Ohio, asslgnor, by

niesne assignments, to Magnesium Development Corporation, a corporation of Delaware No Drawing, Application September 1, 1931. Serial No. 161,947

3 Claims. (Cl. 148-213) This invention relates to magnesium base alloys and it is especially concerned with accelerating the heat treatment of certain cast alloys.

Magnesium base alloys which contain aluminum, zinc and manganese as the chief added alloying ingredients have been used for some time in the production of castings because of their good casting characteristics and relatively high strength as compared to other magnesium base Although castings of I these alloys are suitable for many purposes in the as-cast form yet it has been found that their strength can be materially increased by subjecting them to a solution heat treatment followed by rapid cool-- ll ing to room temperature. It ordinarily requires a heating for a number of hours to secure the desireddegree of solution of the soluble constitucuts in the east alloys. In some instances, as for example in the case of cast magnesium base go alloys containing from 1 to'12 per cent aluminum, 0.3 to 5 per cent zinc and 0.05 to 0.3 per cent manganese, it has been found necessary to employ a two-step thermal treatment consisting of an initial heating at about 630 F. for several hours .5 followed by heating at about 720 F. for alonger P riod 'of time, to obtain the requisite solution of the alloy ingredients. Exposure of the castings to higher temperatures as a means of reducing the time of heat treatment has been considered to go be impractical because of the danger of intergranular melting and oxidation with consequent reduction in the mechanical properties of the castings.

It is an object of my invention to provide a a means of hastening the solution heat treatment of the foregoing type .of cast magnesium base alloys. Another object is to avoid the necessity for using the two-step thermal treatment which has heretofore been the standard practice in'treata ing castings of this type. Still another-object is toobtain substantially the same strength in solution heat treated cast magnesium base alloys containing aluminum, zinc and manganese in a..

much shorter time than has been considered pos- 5 sible heretofore. I My invention is based upon the discovery that the addition of from about 0.03 to 0.3 per cent calcium to cast magnesium base alloys containing from about 1 to 12 per cent aluminum, 0.3 to 5 50 per cent zinc, and 0.05 to 0.3 per cent manganese permits a more rapid solution heat treatment of thepastings at a highertemperature. than has hitherto been regardedas' being a safe practice. In the case of castings containing calcium it is u only necessary to use one solution heat treatment instead of the customary two-step process. Furthermore it hasbeen found that neither the presence of calcium nor the shorter heat treatment reduces the strength of the castings as compared to similar castings-containing no calcium and 5 treated by the two-step method.

Although the addition of calcium is beneficial in hastening the heat treatment of alloy castings containing aluminum, zinc and manganese in amounts mentioned above, it is especially advantageous in alloys containing from 4 to 8 per cent aluminum, 2 to. 4 per cent zinc and 0.1 to 0.3 per cent manganese. It is also my preferred practice to'add from 0.03 to 0.1 per cent calcium to these alloys as well as those containing the larger 15 amounts of aluminum, zinc and manganese.

The alloys may be made in the usual manner through the use of rich alloy hardener or by the addition of the pure metal. The alloys should be melted and cast under the conditions usually prevailing in magnesium alloy foundries, that is, the'melt should be protected against oxidation and the sand molds should be treated to prevent a burning of the metal. The alloys may be cast within the customary range of pouring temperatures.

The castings should be heat treated by subjecting them to a temperature of 720 to 780 F. for a period of 5 to hours followed by a rapid cooling to below 400 F., generally to room tem- 30 perature. If desired they may be artificially aged in the usual manner after the foregoing thermal treatment. In my preferred practice the castings are heated at 750 F. for 10 hours and then rapidly cooled toroom temperature. I have found that it is necessary to heat alloys of the foregoing type to at'least 120 F. to secure a solution of the soluble constituents within a reasonable length of time. At temperatures above 780? F., however, there is danger of incipient melting and/pr in- 40 tergranular oxidation with consequent weakening of the cast article. As to the length of time requird for heating the castings, it has been my experience that at least 5 hours is needed to obtain a substantial degree of solution and that a 5 complete solution is obtained within 30 hours.

Although the presence of 0.03 to 0.3 per cent calcium is useful in reducing the time required for heat treatment of the cast alloys by reason of the fact that higher temperatures can be employed, yet in greater concentrations than 0.3 per cent this-element hinders or may even prevent the solution of the MgaAlz constituent in the alloy. I have observed, for example, if more than 0.3 per cent calcium is present that the solubility of the intermetallic compound, MgaAlz, is so reduced that a satisfactory solution and increase in strength of the alloy can not be obtained within ordinate to the advantage gained in reducing the I time required for the solution heat treatment. In

other words, the increase in the rate of solution gained by using a higher heat treating temperature than has been heretofore employed in commercial practice more than offsets the solution impeding effect of 0.03 to 0.3 per cent calcium.

An example of the benefit derived from using calcium in cast magnesium base alloys of the type herein described, is found in the case of a wellknown commercial alloy. This alloy has a nominal composition of 6 per cent aluminum, 3 per cent zinc, 0.2 per cent manganese, and balance magnesium. According to the usual practice, this alloy in cast form is heat treated for 5 to 6 hours at 630 F., and then at 720 F. for 12 to 14 hours. When so treated the casting may be expected to have a tensile strength of 36,000 pounds per square inch, a yield strength of 10,000 pounds per squareinch and an elongation of 12 per cent in two inches. If 0.05 per cent calcium is added to the alloy and the casting is heated at 750 F. for 10 hours it will develop an average tensile strength of 36,000 pounds per square inch, a yield strength of 10,000pounds per square inch and an elongation of 12per cent in two inches. Should the alloy without calcium be exposed to the same kind of treatment it would be subject to intergranular melting and oxidation to the detriment of the mechanical properties. It will thus be seen that the shorter period of heat treatment in no way reduces the mechanical propertiesor renders the castings inferior to those subjected to the older two-step thermal treatment.

Having thus described my invention and a particular embodiment thereof,

I claim:

1. A method of producing, in a casting of a magnesium base alloy consisting of l to 12.per cent aluminum, 0.3 to 5 per cent zinc, 0.05 to 0.3 per cent manganese, 0.03 to 0.3 per cent calcium, and the balance magnesium, properties commensurate with those obtained by a two-step solution heat treatment, the method comprising subjecting the casting to a single step solution heat treatment at a temperature between 720 and 780 F. for 5 to 30 hours, and rapidly cooling said heat treated casting to a temperature below 400 F.

2.'A method of producing, in a casting of a magnesium base alloy consisting of 4 to 8 perv cent aluminum, 2 to 4 per cent zinc, 0.1 to 0.3 per cent manganese, 0.03 to 0.3 per cent calcium, and the balance magnesium, properties commensurate with those obtained by a two-step solution heat treatment, the method comprising subjecting the casting to a single step solution heat treatment at a temperature between 720, and,780 F. for 5 to 30 hours, and rapidly cooling said heat treated casting to a temperature below 400 F.

3. A method of producing, in a casting of a aluminum, 3 per cent zinc, 0.2 per cent manganese, 0.05 per cent calcium, and the balance- 7 magnesium, properties commensurate with those hours, and rapidly cooling said heat treated casting to a temperature below 400 F.

ROBERT T. wool).

magnesium base alloy consisting of. 6 per cent.