US2389583A - Thermal treatment - Google Patents

Description

. Patented Nov. 2c, 1945 Robert Thomas Wood and Marvin E. Gantz,

Cleveland, Ohio, asslgnors to AluminumCompany of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application December 16, 1944,

' Serial No. 568,561

4 Claims. (Cl. l e-21.3)

This invention relates to methods ofv subjecting magnesium base alloys to thermal treatment by which the physical properties of such alloys may be enhanced without the formation in the alloy of substantial-amounts of excessively large or uneven grains caused by'the growth of the metal grains during thermal treatment. The invention is particularly concerned with the thermal treatment of magnesium base alloy castings.

The magnesium base alloys herein referred'to are those containing 80 per cent. by weight, or more, of magnesium,.the balance being alloying elements added to the magnesium to enhance the properties of that metal or to add thereto new properties or to balance or overcome natural deilciencies in the metal. The present invention, however, is directed to that class of these magnesium alloys which contains an alloying element capable of forming a solid solution with the maB- nesium and which, when the temperature of the alloy is properly adjusted, will precipitate, in part at least, from said solid solution.

In the art of improving magnesium base alloys it is known to add to the alloy a solid solution forming element and to thereafter treat the alloy by first heating the alloy to temperatures within 30 to 40 F., or closer, of the point at which in.- cipient fusion of the alloy will first occur. and to then cool the alloy, more or less quickly, to retain a part or all of the solid solution forming element in solution. If a given concentration oi the solid solution forming element is placed in solution in magnesium by such a process, and in amount that will not, under the equilibrium conditions obtaining, be retained in solid solution at room temperature, cooling of the alloy will cause precipitation of a portion of the solid solution forming element. This precipitation may, under many circumstances, be insufllcient to eifect useful changes in the properties of the alloy, and in such cases it is customary to reheat the alloy to relatively low temperatures in order to hasten such precipitation. This procedure of reheating is commonly referred to inthe art as artificial ageing.

These well known thermal treatments are usedto improve the tensile and compressive propertiesof the magnesium alloys, and the alloying v taining solid solution elements in such amount that, thermal treatment of the alloy at temperatures slightly below the point of incipient fusion, followed by cooling, with or without later artiflci'al ageing, will bring improvement of the prop- 5. erties of the alloy.

When such alloys are placed in the formof castings, 'the conditions of chill in the castin mold will produce in some sections or the casting very fine grains. This is particularly true when thecastings are made in molds employing chilled surfaces. When the casting'is later subjected to the thermal treatment above described, a condition is sometimes encountered where these grains tend .to grow or become excessive in size, and

16 therefore the result of the thermal treatment is 20 of magnesium base alloys of the class containing solid solution forming element has developed this dimculty on solution heat treatment may be readily determined by etching a machined surface ofthe casting with acid or, ii the surface is not machined, by rough polishing of the surface followed by etching or by any other of the common methods of detecting size inequalities in 'the grain structure of cast metal. This invention is useful wherever said difllculty appears.

It is the object of this invention to provide a modified form of thermal treatment which will produce in those magnesium base alloys of the type above described, which are susceptible to grain growth during thermal treatment, the

physical properties desired but will minimize the tendency of grain growth of the alloy during the thermal treatment.

It is common practice in the thermal treatment ofsuch alloys to heat them at the solid solution 40 forming temperature, usually within 40 F. oi. the incipient fusion point of the alloy, until substantial solution of the solid solution forming element has been achieved or until there has been placed into solution sufllcient of the solid solution formin element to achieve the results desired. The time which this heating will take will depend, as is well known in the art, upon the alloy and the thickness of the sections in the casting being heated. Normally, magnesium base alloy castings are subjected to such treatment for from 8 to 20 hours, and in few instances is the time of treatment less than 4 hours. In the practice of this invention this heat treatment is modified, the modiflcation consisting in achieving the same purpose, 1. e., substantially complete solution of the solid 2 a sea sea I 5 solution forming element. or sufilcient solution for the purposes desired, by a series of short, interrupted thermal treatments no one of which is in itself capable of placing the desired amount of solid solution forming element in solid solution 5 in the ma nesium, but all of which, seriatim,

.achieve the same result, in so far as solution of solid solution forming element is concerned, as did the previously used treatments. Thus, the invention consists in achieving substantial or desired solution of element in the magnesium by a stepby-step treatment, each heating step achieving only partial solution, except for the final step in which the desired amount of solid solution formingelement isfinally placed into solid solution in the magnesium, there being in the preferred and commercial positive practice of the invention at least .two partial solution steps prior to the final solution heat treatment step. Thus, for instance,

the alloy casting, instead of being heated for from 5 to 20 hours at the required temperature until solution is complete, is first heated at that temperature for a period insufiicient to completely cause the desired solution, then cooled, then reheated again for a period which may be insufficient to cause the desired solution and then cooled, and the cycle repeated until final solution is achieved. The'number of times that the heating and cooling cycle is to be applied, in accordance with this'invention, is a minimum oftwo, but this minimum is not recommended except in the heat treatment of magnesium base alloy castings which are not particularly prone to rapid or severe grain growth. That is to say there are magnesium base alloy castings of the type herein described which are susceptible to grain growth when heat treated in cast form but exhibit this susceptibility to 'a relatively small degree because the grains initially formed during casting are not small but'are, in-

stead, relatively large or because the amount of 40 chilling during the casting process has not been great. In such cases a minimum of two cycles might be used but the results are not reliable and the invention was developed and is preferably practiced for sound commercial results on the basis that the cycle be performed at least three times, i. e., a solution heat treatment followed by cooling followed by another solution heat treatment followed by cooling and then followed by a third and final heat treatment followed by final cooling. The final cooling step is preferably rapid in order to insure the retention of the solid solution forming elements within the solid solution. Where the casting isprone to rapid growth in grain during the thermal treatment, four or even five applications of the cycle are desirable.

Thus, for example, in an alloy in which heating must take place for a total time of, say, 16 hours in order to place the desired amount of solid solution forming element in solution, the practice of this invention may be carried out as follows: If

the alloy casting is prone to grain growth, the cycle may be repeated three times, the first heating taking about 4 hours, the next about 5 hours and the last about 7 hours, and each heating being foltation to be artificially induced, after th r a 7 treatment at temperatures near the incipient point of fusion of the alloy. In more particularly describing the invention, reference will be had to the outstanding commercial example of such an alloy, that being a magnesium base alloy in which aluminum and line are the solid solution forming elements, but it will be understood that the principles of this invention, as herein set forth and defined in the claims, are equally applicable to any magnesium base alloy which contains other solid solution forming elements and which, after thermal treatment, tends to precipitate said element from solid solution on cooling, or in which this tendency may be induced in substantial amount by artificial ageing, all to the end that the tensile and compressive properties of the metal are changed or enhanced.

In the magnesium alloys containing aluminum as a solid solution forming element, the aluminum must usually, from a practical standpoint, be present in amounts of about 6 per cent, or more, by weight before substantial amount of precipitation will take place ter thermal treatment at elevated temperature However, some precipitation takes place when the alloy contains over 2.0 to 2.5 per cent by weight of aluminum. The reason for this is that when the aluminum content is much less than 2.5 per cent, cooling of the thermally induced solid solution to room'temperature will not cause a tendency to precipitation, because approximately that amount of aluminum is solu le in magnesium at room temperature and, as s' well known in the art, recipitation does not occur and cannot be induced except where the solid solution formed by the high temperature thermal treatment is unstable at some temperature between the temperature at which the solid solution is formed and room tempera- 'ture. By "unstable" is meant that the solubility of the solid solution forming element in the solvent magnesium decreases as room temperature is approached. Therefore, when aluminum is the solid solution forming element, 2.0 to 2.5 per cent by weight of aluminum is roughly the dividing line between the formation of solid solutions in which precipitation may occur or be induced and the formation of solid solutions in which precipitation does not occur and cannot be induced. In the case of other solid solution forming elements the percentage of element necessarily present to form a solid solution which is unstable at lower temperatures will vary, but in the case of any such metal can be easily ascertained from information available in the art, such as equilibrium diagrams depicting the equilibrium conditions in solid state between magnesium and such metals. There is a great variety of magnesium alloys containing 2.0 to 2.5 per cent, or more, of aluminum to which this invention may be applied and there are likewise a substantial number of alloys containing 6 per cent by weight or more of aluminum. The magnesium alloy may contain no other element thanthe aluminum, or it may contain other alloying elements added to enhance oralter other properties of the alloy, for example, manganese,

silicon, calcium, iron, beryllium, or it may contain other solid solution forming elements than aluminum, for example, zinc, tin, cadmium, lead, or silver.

An example of a commercial alloy which has given substantial difilculty in cast form during heat treatment is an alloy commonly known as AM260 which contains approximately 9 per cent of aluminum, 2 per cent of zinc, and 0.2 per cent asaasss I 3' cooling to room temperature followed by artificial- T ageing, while satisfactory, may be economically temperature but, in any event, to a temperature 5 at which the solid solution formed by the thermal treatment is unstable. In the case of a casting made of such an al1oy, which because of its highly chilled condition and large areas of small grains was very prone to growth of grain during thermal treatment, the grain growth which normally'occurred in such standard treatments was substantially inhibited by treatment which consisted in heating for4 hours at 775 F., cooling to about 400 F., reheating for 4 hours at 775 F., cooling to about 400 F., reheating for 5 hours at 775 F., and cooling rapidly toroom temperature. The temperature of 775 F. used in the solution heat treatment in the example Just given is but slightly below and well within 40 F. of the point at which incipient fusion in the alloy would take place. The cooling to 400 F. was a cooling to a point .below that at which the solid solution formed by the heating was stable. The importance lies not in the specific temperature to which g the alloy is cooled but in cooling it to a temperature lying below the point at which the solid solution as formed by each thermal treatment in the repetition of the cycles of heating and cooling employed remains stable. In other words, 0001- ing must, in every case, be to a point which is below that temperature at which the solid solution begins to decompose into two phases one of which is referred to as the precipitate and which represents the excess solute thrown out of the solid solution at a given temperature. The amount of precipitation which will take place during this cooling step, in which the heat treated casting is cooled prior to a subsequent solid solution treatment step, will depend, for any given alloy, upon the temperature to which the casting is cooled and.

the rate at which it is cooled to that temperature,

as wellas the time at which it remains'at a precipitation temperature. For example, it the casting be cooled quickly to room temperature the tendency to precipitation will exist but in many cases substantial precipitation can only be induced by reheating the casting to a relatively low temperature, say 200 to 400 F. which is, of

course, a typical and'well known step termed artificial ageing. On the other hand, it the casting were cooled slowly to room temperature, say over a period of two to three hours, the precipitate might be formed in such amount that this low temperature reheating would not be neces '7 mg said cycle of heating and cooling at least '1 sary. Some magnesium base alloys when cooled below the point at which the solid solution is' stable have a greater tendency to precipitation than do others and it is well known in the art toso adjust the rate of cooling, or to so apply 00 reheating at low temperatures (artificial ageing) as to induce the desired degree ofpreclpitation.

In the practice of the present invention, it is essential that the casting be cooled between successive solution heat treatment steps to a tem- Where the castings are removed fromthe furnace ture of thermal treatment and room temperature and 'merely allowed to cool in the atmosphere, the rate of cooling may be satisfactory in the summertime but too rapid for best results in cold weather ii the foundry in which operations take wasteful because there is removed from the casting, between successive solution heat treatments.

large amounts of heat. The same result may be.

obtained without this excessive loss of heat by cooling the casting slowly in a furnace to a point- Just below the temperature at which substantial precipitation takes place and then-reheating the casting for the nextsolution heat treatment and V in such case :only, a minimum amount of heat is.

wasted and the amount of precipitate formed is. controlled by the rate of cooling and the time that the casting is at a temperature at which precipltation can take lace. For example, in the case of the specific alloy above mentioned, one satisfactory method of cooling between successive solution heat treatments is to cool in the furnace at anydesired rate from the solution'heat treat-' ment temperature to about 600 F: and to then cool to 400 F. over a period of two to four hours.

While it may be equally satisfactory to cool from the solution heat treatment temperature to 200 F. or to room temperature at faster rates, it is generally desirable to control the cooling in order that consistent commercial results will be obtained, particularly if the foundry is exposed to weather conditions and therefore subject to substantial changes in air temperature.

By the commercial use of this invention, as above described, it has been possible to produce castings in which the tendency to grain growth during heat treatment has been so completely suppressed as to not ail'ect the tensile and compressive properties of the completed casting.

This application is a continuation-in-paft of the application of Robert Thomas Wood and Marvin E. Gantz, Serial No. 552,200, filed August 31, 1944.

Having thus described our invention, we claim: 1. Theimproved method of heat treating castings, made of magnesium base alloys of the class containing solid solution element in amount which will, upon thermal treatment, form with the magnesium a solid solution unstable at some temperature between the temperature of said thermal treatment and room temperature and which alloys when so heated exhibit a tendency to form large grains, which consists in heating said casting to a temperature at which solid solution is formed between the magnesium and the solid solution forming element'for a time suitlcient to cause only a portion of the desired solid solution to form, cooling the casting, and repeattwice, the temperature to which said casting cooled between successive heatingsteps being a temperature at which the solid solution formed.

during the previous heating step is unstable whereby precipitation .will result, the time of any of said successive heating steps being such that that step used alone would place in solid solution only a portion of the desired amount of solid solution forming element.

2. In the method of treating castings made of magnesium base alloys of the class containing solid solution element in amount which will, upon v thermal treatment, form solid solutions which are unstable at a temperature between the temperapl i op Cooling t I em temper ture or mg during thermal treatment which consists in applying thermal treatment in a series of at least three interrupted beatings and cooling the casting aiter each heating, the total time at heat oi the casting during said heatings being suiilcient in total to place the desired amount of solid solution forming element or elements into solid solution with the magnesium, and the temperature to which the casting is cooled between successive heatings being a temperature below the point at which the solid solution formed by the preceding heating is unstable whereby precipitation will result, the timeof any of said successive heating steps being such that that step used alone would place in solid solution only a portion of the desired amount or solid solution forming element.

3. The method of treating castings made of magnesium base alloys of the class containing solid solution element in amount which will, up-

on thermal treatment, form with the magnesium I a solid solution unstable at some temperature between the temperature of said thermal treatment and room temperature, which consists of a cycle of heating said castings to a temperature within 40 F. of the lowest temperature of incipient fusion of the alloy for a time sumcient to cause only a portion of the desired solid solution to form and cooling the casting, and repeating said cycle of heating and cooling at least once, the temperature to which said casting is cooled in each cycle being a temperature at which the solid solution a,ssa,ses

formed during the previous heating cycle is unstable whereby precipitation will result and the heating time in each cycle taken alone being insumclent to cause formation of the total desired solid solution. 4. In the method of treating castings made of magnesium base alloys orthe, class containing solid solution element in amount which will, upon thermal treatment at temperatures less than 40 )3. below the temperature 01' incipient fusion of the alloy and subsequent cooling, form solid solutions which are unstable at a temperature between the aforesaid temperature of thermal treatment and room temperature, the improvement of inhibiting grain growth in the casting during treatment which consists in' applying the thermal treatment in a series of interrupted heatings, and cooling the casting after each heating, the total time at heat of the casting during said heatings being sumcient in total to place the desired amount of solid solutibn forming element or elements into solid solution with the magnesium and temperature to which the casting is cooled between said heatings being a temperature below the point at which the solid solution formed by each individual heating would be unstable, whereby precipitation takes place between each individual heating.

' ROBERT THOMAS WOOD.

MARVIN E.. GANTZ.