US2352990A - Method of making and treating magnesium castings - Google Patents

Description

' covered Patented July 4, 1944 DIETHOD OF MAKING AND TREATING MAGNESIUM CASTINGS Robert'Thomas Wood, Cleveland, Ohio, assignor to Aluminum Company of America,

Pittsburgh,

Pa., a corporation of Pennsylvania No'Drawing. Application June 3, 1943,

Serial No. 489,517

Claims.

This invention relates to methods of casting magnesium, a term which is used herein and in v aggregate, molten metal is cast therein and after solidification thereof the casting is removed from the mold. During such removal the mold is whol- 1y or partially destroyed and the aggregate reto be again used in subsequent formation of other molds. Such aggregate-bodied molds are somewhat lacking in chilling effect on the solidifying metal and it is often desirable or necessary to increase the rate of solidification in localized areas Where the mass of solidifying metal is great or-where a greater rate ofsolidification is necessary to insure that'solidification proceeds, at

the desired time, at the desired place in the casting. To this end it is customary to insert in the body of the'mold, near or on the face of the mold cavity, one or more pieces of metal. Such metallic pieces are commonly known as chills and are conventionally' made of copper or cast iron. These chills, having greater chilling power than the aggregate from whichthe mold is formed,

increase the rate of solidification of the molten metal and thus forms inthe chilled regions of the casting smaller grains than would have been formed if chills had not been used. This method of casting metal into aggregate-bodied molds in which are incorporated metallic chilling bodies is used in the casting of most metals and is widely used in the casting of magnesium.

It has been observed that when magnesium castings, madeby theabove described method, are ,heat treated to improve their characteristics such as, for instance, their strength or hardness, areas of large grain may develop in the chilled regions of the casting. In these areas the grain has grown relatively large as compared to the normal grain size of the casting established during the casting process. Such localized grain growth durin heat treatment is, Ibelieve, more prevalent in magnesium castings than in castings of other commercial metals. The development of these excessively large grains in the otherwise comparatively small grain casting unfavorably affects the useful properties of the metal, such as its strength, ductility and machinability.

The present invention is predicated upon the use, in the aggregate-bodied mold, of chills of'certain thermal conductivity. I have found that if the metallic body used as a chill is composed of a metal or alloy having a thermal conductivity substantially less than that of cast iron, 1. e. a thermal conductivity not greater than about 80 per cent of that of cast iron, the metallic bodies will fulfill their chilling function, but that the casting produced in such a mold will not have the same tendency to form large grains on heat treatment. It is understood, of course, that the mass of the metal chill used is sufiicient to perform the chilling function and that the present inventionis concerned with the thermal conductivity rather than the mass (thermal capacity) of the chill.

I have further found that the tendency to the formation of large grains on heat treatment is almost, if not entirely, suppressed if the chills employed have a thermal conductivity that does not exceed about 65 per cent of that of cast iron, and I accordingly prefer to employ chills having such conductivity. Any metal or alloy, stable at the temperatures employed, which fulfills the conductivity requirements outlined above may be employed in the practice of the invention, and

such metals and alloys may be easily selected either by reference to standard data on the thermal conductivity of metals and alloys or by determination of that conductivity by well known standard methods. It is, of course, desirable that the metal or alloy selected as a chill body have a melting point sufliciently high to withstand the temperatures developed in the mold when magnesium is cast therein. These temperatures may be as high as 820 C. or even higher; However, the location of the chill in the mold and also the quantity of'metal cast in the mold are compenthat an alloy containing per cent copper and 40 per cent nickel is particularly suitable for the practice of the invention. The melting point of this alloy is suificiently high and at room temperature its thermal conductivity measures to be about 50 per cent of that of cast iron. Chills made of it produceexcellent results. For example, castings of an alloy containing about per cent by-weight of aluminum and about 0.2 per cent by weight of manganese, the balance being magnesium, when made in sand molds containing conventional cast iron chills and then useful. However, beneficial results will be obtained by the practice of this invention whenever the heat treatment tends to develop in the magnesium casting areas of grain which are larger 6 than the normal size of the grain established heat treated for 40 hours at,810 F. develop areas areas. The practice of the invention is not,

however, limited to chills made of copper-nickel of any metal which hasa conduc vity not greater in the casting by the solidification ofthe metal in the mold.

I claim:

1. In that method of making and treating mago nesium castings which comprises casting and solidifying molten magnesium in a sand mold containing one or more chilling bodies and thereafter subjecting the casting thus produced to heat treatment at temperatures below the incipient melting point of the casting and above alloys and, aslabove stated, continplates the use than about 80 per cent of the tivity of cast iron:

Listed below are a number of alloys selected at random from standard tables showing the normal thermal conductivity of various materials. The conductivity values are expressed in terms of c. g. s. units (1. e., calories-per second per square centimeter pr degree centigrade where the distance between the opposed surfaces of the body being measured is one centimeter). The conductivity of cast iron is between 0.1 and about 0.115. The values expressed in the following tabulation will, of course, vary slightly with the c ndition of the metal (i. e., cast, wrought,

ermal conducetc.) nd with the temperature at which the measurement is made.

Thermal Composlmm conductivity 21-237 Ni 7-107 or 1-1.57 Cu. 0. 00.07 Mn,

1.25-3.57, 51, 0.23% 0., balance r0 f 0.000 17-19% 01', 7-97 Ni, 0.6% max. Mn, 0.75% max. Bi, 2.30% 21??? iffi'fiiulibt'i'ifi'si" l 0.25%, max. 0, balance f About 0.05

12-107 Cr, 0.47,, Mo, 0.30% s, 0 $57, Mn, 0 25% s1,

b 1 r 0. 040 0.054 0. 013' 0.052 0.041 03 c 157 Ni 227 8% ll N 8x01 smel cok z, i i

0, balance Fe 0:031 84% on, 4% Ni, 12% Mum- 0.053 22-28% 01', 12-10% Ni, 0.25% max. 0, balanee'Fe. 0.04 6-187 (in-0.107 max. 0 balance-Fe 0.07 25-00 0 Cr, 0.1 a max. (5, balance Fe 0.00

The-heat treatments of magnesium castings above referred to arethe normal thermal procabout 730 F., the step of minimizing the inducement of grain growth by 'said heat treatment which consists in providing as 'chilling bodies metallic bodies'having a thermal conductivity which is not more than about 80 per cent of the thermal conductivity of cast iron. e

.2. In that method of making and treating magnesium castings which'comprises casting and solidifying molten magnesium in a sand mold containing one or more chilling bodies and thereafter subjecting the casting thus produced to heat treatment at temperatures below the incipient melting point of the casting and above about 730 F., the step of minimizing the inducement of grain growth by said heat treatment which consists in providing as chilling bodies metallic bodies having a thermal conductivity which is not more than about 65 per cent .of the thermal conductivity of cast iron.

3., In that method of making and treating magnesium castings which comprises casting and solidifying molten magnesium in a mold composed of a non-metallieaggregate body containing one or more chilling bodies and thereafter subjecting the casting thus produced to heat treatment, the step of minimizing grain growth induced by said heat treatment whichconsists in providing as .chilling bodies metallic bodies composed of. an alloy consisting of about 60 per cent nickel and balance copper.

i 4. In that method of making and treating magnesium castings which comprises casting and solidifying molten magnesium in a mold composed of a non-metallic aggregate bodycontaining one or more chilling bodies and thereafter subjecting thecasting thus produced to heat treatment, the step of. minimizing grain growth induced by said heat treatment which consists in providing as chilling bodies metallic bodies havesses to which many magnesium bastings are subjected and which usually involve heating the casting to temperatures above about 200 F. but below those temperatures at which incipient melting would take place in the magnesium casting under treatment. The development of large grains which the practiceof this invention wholly or partially overcomes is usually encountered in those magnesium castings, produced in aggregate-bodied moldscontaining chills, which have been so treated at temperatures over about 730 F., and-it is in connection with the manufacture of castings which are to be heat treated in this higher range that the practice of my invention is particularly ing a thermal conductivity at least 20 per cent less than that of cast iron. 5. In that method of and treating magnesium castings which comprises casting and solidifying molten magnesium in a mold composed of a non-metallic aggregate body containing one or more chilling bodies and thereafter subjecting ',the casting thus produced to heat treatment, the step of'minimizing grain growth induced by said heat treatment which consists in providing as chilling bodies-metallic bodies having a thermal I conductivity which is notmore than about 65 per cent of the thermal conductivity of cast iron.

' ROBERT THOMAS WOOD.