US2381714A - Method of thermally treating aluminum base alloy ingots and product thereof - Google Patents

Description

Patented Aug. 7, 1945 METHOD OF THERMALLY TREATING ALU- MINUM BASE ALLOY INGOTS AND PROD- UCT THEREOF Paul C. Beck, Bay Village, Ohio, assignor to Aluminum Company of America, Pittsburgh, Pa.,

a corporation of Pennsylvania No Drawing. Application April 3, 1942,

, Serial No. 437,510

13 Claims.-

This invention relates to a method of improving the characteristics of hot worked aluminum base alloys. The term aluminum base alloys,"

These alloys contain one or more soluble components and are susceptible to improvement in tensile roperties by solution heat treatment and precipitation or age hardening. In common with other aluminum base alloys, precipitation hardening alloys may readily be shaped by a variety of both hot and cold working processes. This invention concerns itself, however, with the hot working of cast bodies of precipitation hardening alloys by such methods as extruding, rolling, pressing, and forging.

Hot working of cast bodies of precipitation hardening aluminum base alloys, particularlyin the temperature range 600 to 850 F., produces worked material which, upon solution heat treatment, frequently recrystaliizes into grains which are large and often extremely elongated. Upon subsequent precipitation hardening, substantial amounts of the dissolved constituents precipitate in the form ,of small particles along the boundaries of these grains. The resulting internal structure is known as lamellar, and may be readily identified -by the extremely rough, lamellar" fracture produced when the material is fractured. Since the fracture occurs substantially at the grain boundaries where precipitation has thus occurred, the resulting fracture, due to the large and often elongated nature of the grains, is characterized by a very uneven surface having a fibrous, jagged appearance very similar to that observed when a piece of wood is broken. This type of fracture is distinguished is substantially unrecrystallized.

While comparative tests made of lamellar and non-lamellar material indicate general similarity of tensile properties, the tensile strength of lamellar material in a direction transverse to the direction of metal working is somewhatv inferior. This weakness may account for the internal splitting which sometimes occurs when lamellar material is drilled. A further disadvantage of lamellar material presents itself upon fracture of the material, for the uneven nature of the broken edge necessitates an extensive grinding operation before the end can be joined to another piece of material by such an operation as welding, for example. For whatever the reason, there is a widespread preference among aluminum fabricators for stock which exhibits a smooth, silky edge upon fracture.

It is therefore an important object of this invention to provide a method of thermally treating cast bodies composed of precipitation hardenening aluminum base alloys whereby they may be hot worked into articles which can be subjected to recrystallization and precipitation hardening treatments without the production therein of a lamellar structure. It is a particular object to provide a thermal treatment which permits hot working the cast body at a temperature between about 600 to 850 F. so that, upon recrystallization of the wrought product, small, uniform grains are formed rather than those of a large and often elongated shape. Other objects will appear from the following and more specific description of the invention.

My invention is predicated upon the discovery that these objects may be achieved by first subjecting a cast body of a, precipitation hardening aluminum base alloy to a novel thermal treatment upon removal of the body from the mold while at a relatively high temperature. Ingots are representative of such bodies, and the term ingot," as hereinafter employed, embraces, as a matter of convenience, as-cast bodies in general which are intended for work affecting the crystalline structure. The cooling of the ingot in the mold is arrested at any temperature above 700 F. at which it may conveniently be removed therefrom without damage, and the hot ingot is then transferred to a furnace heated to between 600 and 800 F. before the temperature of the ingot falls below 700 F. The ingot is held in the I hot, as-cast body in the fashion described above.

The direct-soaking process is applicable to ingots of precipitation hardening aluminum base alloys produced by any casting method which will permit the removal of the ingot to the soaking furnace before its temperature falls below 700 F. Although the temperature of the furnace into which the material is placed. as indicated above, may be somewhat lower than 700 F., the elevated temperature of the furnace causes the material placed therein to cool at a much slower rate than would otherwise be the case were the ingots to be cooled in the mold or in air. It is impossible to obtain the desired result by treating material which has cooled below 100 F. before it is placed in the soaking furnace, nor can material which has cooled below this temperature be reheated and thereafter successfully treated by the direct-soaking process.

After direct-soaking, the ingot may be immediately hot worked or may be cooled to room temperature, a slow air cool being preferred, and stored for any length of time before reheating for subsequent hot working. In any event, the structure developed in the ingot by direct-soaking is such that the material may be subsequently hot worked in' one or more operations at any temperature ranging from about 600 to 850 F. without producing worked material which will recrystallize to form large grains.

When the ingot is hot worked immediatel after the soaking treatment, the hot working temperature obviously will not exceed the highest soaking temperature, that is, 800 F. If the ingot has previously been allowed to cool, it is generally reheated to a temperature at least as high as that at which the hot working is to be carried on and for a time sufilcient to insure a. uniform temperature throughout the ingot. However, in order to retain the desirable internal structure and properties imparted by directsoaking, it is important that this reheating temperature not exceed about 850 F., nor should the heating at this comparatively high temperature be carried on for more than about two hours. The hot working to the final, size of the article need not be continuous, but may be interrupted by a cooling to room temperature and a subse- 'quent reheating treatments are carried on within the foregoing temperature range of 600 to 850 F.-

The direct-soaking treatment described herein is particularly effective for those aluminum base alloys known as precipitation hardening alloys which undergo a solution heat treatment and precipitation or age hardening. These alloys contain about 0.1 to 12 per cent, but not more than a total of about 25 per cent, of one or more of such soluble elements as copper, silicon, magnesium, and zinc. Substantial amounts of these soluble components precipitate, either in the form of elements or as compounds such as CuAlz, MgzSi, and MgZn-z, as small particles along the boundaries of the grains or grain fragments during the precipitation hardening treatment. Precipitation hardening aluminum base alloys which I have found to be particularly benefited by the direct-soaking treatment are those having the following composition ranges, the percentages in each case being given in terms of weight per cent, with aluminum and normal impurities making up the balance of each alloy:

' (1) 2.5 to '7 per cent copper, 0.1 to 1.5 per cent manganese, with or without 0.1 to 2 per cent magnesium, and with or without 0.3 to 2 per cent silicon;

(2) 0.5 to 4 per cent magnesium and 0.5 to 4 per silicon;

(3) 3 to 15 percent silicon, 0.1 to 5 per cent copper, 0.1 to 3 per cent magnesium, and 0.2 to 5 per cent nickel;

(4) 5 to 15 per cent zinc, 0.5 to 3 per cent copper, 0.1 to 5 per cent magnesium, and 0.1 to 1.5 per cent manganese, with or without 0.05 to 0.5 per cent chromium.

Each of the foregoing alloys may also contain a total of about 0.05 to 1.5 or 2 per cent of one or more of the elements zirconium, tungsten, molybdenum, vanadium, titanium, uranium, iron, cobalt, beryllium, columbium, and tantalum. While specifically recited in connection with certain of the above-mentioned alloy compositions, one or more of the elements manganese, nickel, and chromium, in total amount of about 0.05 to 1.5 or 2 per cent, may also be added to any of these alloys.

The exact nature of the change in the internal structure of the ingot resulting from the directsoaking treatment is not wholly understood. However,'the observation has been made that the prolonged heating of the.ingot between 600 and 800 F. causes the precipitation of a large part of the soluble components and a coalescence of a substantial portion of this precipitate into relatively large aggregates, which result is not obtained either below 600 F. or above 800 F. These aggregates are much larger than the particles formed during the precipitation hardening treatment, for example. In a copper-containing aluminum base alloy, for example, the precipitated material is largely CuAlz. Apparently the desirable results obtained by directsoaking the ingot are in some way associated with the formation in the ingot of these large aggregates of precipitate, although other changes may occur which have not as yet been appreciated. In any event, the beneficial results attributable to the direct-soaking treatment are no longer observed when the ingot is subjected to any thermal treatment which redissolves these aggregates to any appreciable extent. Reheating the direct-soaked material at temperatures above 850 F. for any considerable interval, for example, will cause substantial re-solution.

As an example of the manner in which this invention finds application, molten metal consisting of about 4.2 per cent copper, 0.7 per cent silicon, 0.8 per cent manganese, 0.4 per cent magnesium, the balance being aluminum and normal impurities, was cast in the form of ingots which were removed from the mold and placed in a furnace controlled at 725 F. before the temperature of the ingots fell below 700' F. The ingots were kept in the furnace at this temperature for a period varying from 16 to 18 hours, after which they were removed from the furnace and cooled in air to room temperature. One of these ingots was thereafter placed in a furnace and reheated to a temperature of about 740 F. The hot ingot was then transferred to an extrusion press and extruded at a temperature of about 700 F. into 2% inch diameter rod. Another of the ingots was reheated to a temperaabout 800 F. into a 2% inch diameter rod. A

section was cut from each of these rods and both sections were given the usual solution and precipitation hardening treatments and were thereafter fractured. In each case, a silky, nonlamellar fracture resulted. Micrographic ex-' aminationof the specimens revealed that the recrystallized grains formed during the solution heat treatment were of a small size and were non-elongated in appearance.

duction in cross sectional area of 70 per cent.

These pressings were subjected to the same solution and precipitation hardening treatments, and each piece was fractured. In all cases a nonlamellar, silky fracture resulted, and micrographic examination again revealed the formation,

upon recrystallization, of small, uniform grains.

-Similarly,'sections of-the rod extruded at 800 F.

were reheated and pressed in like fashion at temperatures of 600, 700, and 800 F., and thereafter were subjected to solution-and precipitation hardening treatments and fractured. Again, all of th fractures were of the non-lamellar variety, and micrographic examination bore out the existence in the specimens of only small equiaxed grains. I

It may be noted that although the principal object f this invention was to produce material having a non-lamellar structure, the direct-soaking treatment disclosed herein also imparts other desirable characteristics to the metal. For example, it was observed that the direct-soaked ingots were easier to work and thus could be fabricated at lower pressures, and consequently lower cost, than ingots which had not been similarly treated. Further economiesof production also characterized the process in that the residual heat of the ingot in the as-cast condition is not lost;

" other ingot thermal treatments involve an initial cooling, frequently to room temperature, before the ingot is reheated to the desired treating temperature. These economies of production and fabrication have proven to be of considerable value from a practical operating standpoint.

I claim:

'1. The method of thermally treating a hot in-' got of a precipitation hardening aluminum base alloy upon removal from the ingot mold and preparatory to hot working within a temperature range such that the wrought product is capable of undergoing subsequent recrystallization, comprising removing said ingot from the mold and exposing it to a soaking treatment before the ingot temperature falls' below 700 F., and soaking the ingot at a temperature between 600 and 800 F. for a time sufllcient toproducerelatively large aggregates of precipitated constituents.

2. The method of thermally treating a hot ingot of a precipitation hardening aluminum base alloy upon removal from the inset. mold and preparatory to hot working within the range of 600 to 850 F., comprising removing said ingot from the mold and exposing it to a soaking treatment before the ingot temperature falls below 700 F., and soaking the ingot at a temperature between 600 and 800 F. for a time suffi'cient t produce relatively large agrgegates of precipitated constituents.

3. The method of thermally treating a hot ingot of a precipitation hardening aluminum base alloy upon removal from the ingot mold and preparatory to hot working within the range of 600 to 850 F., comprising removing said ingot from the mold and exposing it to a soaking treatment before the ingot temperature falls below 700 F., and soaking the ingot at a temperature between 600 and 800 F. for about 5 to 40 hours.

4. The method of thermally treating a hot ingot of a precipitation hardening aluminum base alloy upon removal from the ingot mold and preparatory to hot .working within the range of 600 to 850 F., comprising removing said ingot from the mold and exposing it to a soaking treatment before the ingot temperature falls below 700 F.,' and soaking the ingot at a temperature between 675 and 750 F. for about 10 to 24 hours.

5. The method of. thermally treating a hot ingot of a precipitation hardening aluminum base alloy upon removal from the mold and preparatory to hot working, comprising removing the ring it to a soaking furnace before the tempera-.

ture of the ingot falls below 700 F., and soaking said ingot between 600 and 800 F. for a period of about 5 to ,40 hours, and thereafter hot working said ingot.

.7. The method of producing hot worked precipitation hardening aluminum base alloy material which will be free from lamellar structure upon subsequent recrystallization andprecipitation hardening treatments, comprising removing the hot ingot from the mold, transferring it to a soaking furnace before the temperature of the ingot falls below 700 F., soaking said ingot between 600 and 800 F. for a period of about 5 to 40 hours, and immediately thereafter hot working the ingot at any temperature-between 600" and 800 F.

8. The method of producing hot worked precipitation hardening aluminum base alloy material which will be free from lamellar structure upon subsequent recrystallization and precipitation hardening treatments, comprising removing the hot ingot from the mold-transcipitation hardening treatments, comprising re-- moving the hot ingot from the mold, transferring it to a soaking furnace before the temperature of the ingot falls below 700 F., soaking the ingot in the furnace between 600 and 800 F. for a period of about 5 to 40 hours, and thereafter structure, said ingot being characterized by an internal structure resulting from a thermal treatment comprising removing the hot ingot 9 from the mold and transferring it to a soaking furnace before the temperature oi the ingot falls below 700 F., and soaking the ingot in the furnace between 600 and 800 F. for a time sumcient to prod ce. relatively large aggregates of precipitated constituents.

11. An ingot of a precipitation hardening aluminum base alloy, which can be hot worked at any temperature between 800 and 850 F. without the production in the worked and precipitation hardened material of a lamellar structure, said ingot being characterized by an internal structure resulting from a thermal treatment comprising removing the hot ingot from the mold and transferring it to a soaking furnace before the temperature of the ingot falls below 700 F., and soaking the ingot in the furnace between 600 and 800 F., for a period of about 10 to 24 hours.

12. An ingot of a precipitation hardening aluminum base alloy, which can be hot worked at any temperatur between 600 and 850 F. without the production in the worked and precipitation hardened material of a lamellar structure, said ingot being characterized by an internal structure resulting from a thermal treatment comprising removing the hot ingot from the mold and transferring it to a soaking furnace before the temperature of the ingot falls'below 700 and soaking the ingot in the furnace betweenv 600 and 800 F. for a period of about 5 to 40 hours.

13. Th method of thermally treating an ingot of an aluminum base alloy containing 2.5 to 7 per cent copper, 0.1 to 1.5 per cent manganese, 0.1 to 2 per cent magnesium, and 0.2 to 2 per cent silicon, comprising removing the hot ingot from the mold and transferring it to a soaking furnace before the temperature of the ingot falls below 700 F., and soaking the ingot in the furnace at a temperature between 600 and 800 F. for a period of time between about 5 and 40 hours.

PAUL C. BECK.