US3379583A - Heat treatment of aluminum alloys - Google Patents

Description

United States Patent 3 379,583 7 HEAT TREATMENT OF ALUMINUM ALLOYS Wolfgang Gruhl, Bad Godesberg, and Giinther Scharf,

Bonn, Germany, assignors to Vereinigte Aluminium- Werke Aktiengesellschaft, Bonn, Germany No Drawing. Filed Mar. 26, 1965, Ser. No. 443,067 Claims priority, application Germany, Apr. 9, 1964, V 25,762; Feb. 5, 1965, V 27,691 9 Claims. (Cl. 148-11.5)

ABSTRACT OF THE DISCLOSURE The hot deformability of aluminum alloy bodies of the type AlMgSi is improved by annealing such aluminum alloy body at a temperature between 500 and 590 C., and slowly cooling the thus-annealed aluminum alloy body at a rate of up to about 20 C. per hour at least down to a temperature of between about 450 C. and 300 C.

The present invention relates to the heat treatment of aluminum alloys and, 'more particularly, the present invention is concerned with the heat treatment of continuous or semi-continuous aluminum alloy castings which are to be used as extrusion and rolling ingots.

The aluminum alloys with which the present invention is concerned are alloys consisting essentially of aluminum, magnesium and silicon, primarily alloys which consist of between about 0.4 and 1.4% magnesium, 0.30 and 1.6% silicon, the balance being aluminum but may also include between 0.1% and 1% manganese and/ or between 0.001 and 0.3% chromium. However, special alloys which include diiierent quantitative ranges may also be treated according to the present invention, such as for instance, aluminum alloys having a magnesium content of between 2.3 and 3.5%. The alloys may also contain relatively small proportions of copper, for instance up to about 0.5% and, in addition, the aluminum of the alloy may contain the conventional impurities of for instance iron. In addition, the alloys which are treated according to the present invention may also contain relatively small proportions of zinc, such as 0.2%. The above described alloys will be referred to in the description and the claims as aluminum alloys of the type AlMgSi.

Extruded profiles of aluminum alloys of the type AlMgSi are of progressively increasing importance for building purposes. When heat-hardened such profiles of the above composition have high strength characteristics and, after anodic oxidation, such profiles have a smooth surface of light color. In order to produce such profiles in the most economical manner, extrusion thereof should be carried out at the highest speed possible. The extrusion speed, however, is limited by the heat deformation characteristics of the cast ingots.

It has been proposed to improve the extrudability of such cast ingots of the above described aluminum alloy composition by subjecting the same to a homogenizing annealing' process. It is the purpose of such annealing to eliminate the liquation of alloying components by diffusion and thereby to produce an even and homogenous structure. According to this suggestion, alloys of the type described above are conventionally annealed for between 6 and 48 hours at temperatures of between 530 and 590 C. and thereafter cooled by exposure to air.

It'is an object of the present invention to provide an improved heat treating process for the above described aluminum alloys, which will permit to increase the extrusion speed of the same.

It is another object of the present invention to provide a method for extruding aluminum alloy bodies of the 3,379,583 Patented Apr. 23, 1968 above described composition, wherein the annealed aluminum bodies are subjected to a controlled cooling which will increase the extrusion speed of the thus-treated aluminum alloy bodies.

It is yet a further object of the present invention to provide a method for improving the hot deformability of aluminum alloys of the above described compositions.

Other objects and advantages of the present invention will become apparent from a further reading of the description and of the appended claims.

With the above and other objects in view, the present invention contemplates a method of heat treating aluminum alloy bodies of the type AlMgSi which comprises the steps of annealing the aluminum alloy bodies at a temperature between 500 C. and 590 C., and slowly cooling the thus-annealed aluminum alloy bodies at a rate of about 20 C. per hour, and not exceeding 50 C. per hour.

The present invention also contemplates a method of heat treating and extruding an aluminum alloy body of the type AlMgSi, which method comprises the steps of annealing the aluminum alloy body at a temperature between 500 C. and 590 C., slowly cooling the thusannealed aluminum alloy body at a rate of about 20 C. per hour, and not exceeding 50 C. per hour to a temperature of between 450 C. and 300 C., further cooling the aluminum alloy body at a faster rate to a temperature below 300 C., thereafter heating the thus-cooled aluminum alloy body to an elevated temperature of between about 350 C. and 530 C., and subjecting the thusreheated aluminum alloy body to extrusion at the elevated temperature.

Thus, the present invention is based on the surprising finding that with respect to aluminum alloys of the type- AlMgSi, the manner in which the cooling from the homogenizing temperature is carried out has a considerably greater influence with respect to the subsequent extrudability or possible extrusion speed of the ingots than the temperature and the length of time for which the homogenization heat treatment or annealing has been carried out.

In this connection it is noteworthy that already in the continuously cast or semi-continuously cast ingots of the above described aluminum alloys, the main alloying components, namely magnesium and silicon are to a very considerable portion dissolved in the mixed crystal, due to the quick cooling upon casting into water. A short-time heating of the cast ingots to a homogenization temperature of between 550 and 590 C. will cause complete dissolution of the entire content of Mg Si. If thereafter the ingot is cooled in conventional manner by exposure to air, the Mg Si is precipitated in part in finely subdivided form, while the residual part of the Mg Si remains in solution. Upon subsequent heating to extrusion temperatures of be- I tween about 450 C. and 530 C., the Mg Si particles which were previously precipitated and which due to the quick air cooling are of relatively small size, will be quickly dissolved again so that at the start of the extrusion process a more or less homogenous mixed crystal is present. The same condition is obtained by quickly cooling the ingot after the same has been subjected to homogenizing heating or annealing, for instance by spraying with water, or by extruding the annealed cast ingot without intermediate cooling.

Thorough investigations have shown that an ingot in which all of the Mg Si is present in a solid solution has much poorer extrusion characteristics than an ingot which also after being heated to extrusion temperature still contains residual undissolved Mg Si. Thus, it has been found advantageous, in accordance with the present invention, to control the cooling after annealing or homogenizing heating of the cast ingot in such a manner that as much Mg Si as possible will precipitate; and that the precipitate will be coarse so that it will not be completely dissolved upon subsequent heating to extrusion temperatures and short time maintenance of the extrusion temperature.

The desired structure of the cast aluminum alloy ingot is obtained according to the present invention by a method of heat treating continuously or semi-continuously cast extrusion or rolling ingots of aluminum alloys of the type AlMgSi, by annealing the ingots within a temperature range of between 500 and 590 C., preferably between 530 and 580 C., and thereafter cooling the thus-annealed ingots in a slow manner, namely so that the temperature drop will amount to not more than about 50 C. per hour, and preferably not more than 20 C. per hour. This slow cooling is particularly important while the annealed ingot is still at a relatively high temperature. The slow cooling is easily accomplished, for instance by allowing the annealed ingots to remain in the annealing furnace or under a suitable hood. In any event, the speed of cooling is to be so controlled according to the present invention that the temperature drop at the beginning, i.e., until the temperature has dropped to between 450 C. and 250 C., will be not more than 50 C. per hour and preferably about 20 C. per hour.

The slow cooling according to the present invention may be continued until the temperature of the annealed ingot has dropped to 250 C. or below or, a stepwise cooling may be carried out so that the slow cooling by no more than 50 C. per hour and preferably about 20 C. per hour will be carried out until the temperature has dropped from the annealing temperature to between about 450 C. and 300 C. and preferably to between 400 C. and 300 C. and thereafter, in the second cooling step, the further cooling may be carried out at a faster rate of more than 150 C. per hour to a temperature of 200 C. and below.

The heat treatment, including the slow cooling after annealing is carried out according to the present invention with aluminum alloys containing silicon and magnesium as alloying elements and which may also contain copper, zinc, manganese and chromium, in addition to the conventional impurities of the aluminum.

Broadly, the present invention encompasses the treating of aluminum alloys within the range indicated in the table below.

Minimum, Percent Maximum, Percent Balance The aluminum should be of a purity of at least about 99.7%.

It has been found that the first method, namely the slow cooling to 250 C. or below will result in a very considerable increase in the speed at which the thustreated ingots can be extruded while still maintaining sufficiently high strength characteristics for many practical applications, particularly with respect to yield strength and tensile strength, while the second method, namely the twostep cooling, beginning at a slow rate and terminating at a faster rate will permit maintenance of about the same or only slightly reduced extrusion speeds as compared with the first method while at the same time resulting in an extruded product of improved strength characteristics.

The faster cooling after a temperature of between 450 and 300 C. has been reached can be carried out in conventional manner by exposure of the ingot to air, or by blowing air or another suitable gas against the ingot, as well as by spraying the ingot with water or immersion of the ingot in a Water bath.

Furthermore, it has been found that the length of time for which the annealing or homogenizing heating is carried out has only a very insignificant influence on the heat deformability characteristics of the cast ingot, provided that the cooling from the annealing temperature is carried out in the slow manner described above. Thus, ingots which were homogenized for one hour at 580 C. and then cooled to 200 C. at a rate of cooling which did not exceed 20 C. per hour could be extruded equally well as ingots which were subjected to homogenizing treatment at 580 C. for 48 hours and then cooled in the same man ner. Thus, by slow cooling in accordance with the present invention, the time required for the homogenizing heat treatment or annealing and thus the costs thereof can be greatly reduced.

The same holds true for the second method, namely the two-step cooling. Thus, ingots which were homogenized for one hour at 580 C. and then cooled to 300 C. at a maximum cooling speed of 20 C. per hour, and thereafter quickly further cooled by being sprayed with water, could be equally well extruded as ingots which were homogenized at 5 C. for 48 hours, and then similarly subjected to the two-step cooling process.

The following examples are given as illustrative only of the present invention, without, however, limiting the invention to the specific details of the examples.

Example I Several cast ingots of varying composition were homogenized in an air furnace at 5 80 C. for varying periods of time and subsequently cooled in diiferent manners. Thereafter, the ingots having a diameter of 80 mm. were heated by induction heating to an extrusion temperature of 480 C., maintained for five minutes at such extrusion temperature and then extruded at a constant or unchanging extrusion force of 170 metric tons.

The following alloys were tested:

Percent Mg: A, 0.45; B, 0.69; C, 0.90. Percent Si: A, 0.40; B, 0.57; C, 0.69.

The balance of the alloys consisted of aluminum with conventional impurities.

Four ingots were differently heat treated in the following manner:

Ingot (1): Homogenization for 48 hours, followed by air cooling down to C. and below, at a rate of 80 to 100 C.;

Ingot (2): Homogenization for 48 hours, followed by air cooling to 370 C., maintenance of 370 C. for 3 hours and subsequent further air cooling down to 100 C. and below, at a rate of 80 to 100 C.;

Ingot (3): Homogenization for 48 hours, cooling in the annealing furnace at a rate of 20 C. per hour down to 200 C.;

Ingot (4): Homogenization for 1 hour followed by cooling in the furnace at a rate of 20 C. per hour down to a temperature of 200 C.

The average extrusion speed which could be obtained with ingots treated as described above, the extrusion speed being expressed in meters per minute, were as follows:

A 4.7 B 2 3.3 C 2 2.7 A2: 6.5 B2: 5.6 Ca: A 11.7 B 2 12.3 C 12.3 A 2 11.8 B 1 12.3 C 11.8

It is immediately apparent that the ingots which were cooled in the annealing furnace, i.e. the ingots which were slowly cooled at a rate of 20 C. per hour could be extruded at much greater speed than the ingots which were quickly cooled and, furthermore, that it did not make any appreciable difference whether the ingots had been annealed for one hour or for 48 hours. While with respect to the quickly air cooled ingots the specific composition of the same seems to be of considerable influence with respect to the extrudability, the slowly cooled ingots show the same favorable extrudability irrespective of the specific composition. Thus, the heat treatment according to cooled with air. The extrusion pressure was maintained at an even 170 T.

Strength Values of the Profiles Extruded of the Respective Cast Iugots Extrusion Treatment of the Cast Ingots Speed, 5 Days Storage at 16 h./160 0.,

mJmin. room temperature kp./mm.

kp./mm.

a0. 2 B 00. 2 (TB (1) Air cooling 4. 7 7. 3 16. 18. 0 24. 5 (2) Cooling down to 200 C. at a rate of C./h., followed by air cooling 11. 8 5. 4 13. 3 13. 2 19. 5 (3) Cooling down to 400 C. at a rate of 20 C./h. followed by water spray cooling 10. 9 7. 4 15. 8 17. 8 24. 7 (4) Cooling down to 300 C. at a rate of 20 C./h. followed by water spray cooling 11. 2 6. 4 15. 1 7. 4 23. 4

aluminum alloys with higher contents of magnesium and silicon as aluminum alloys with lower contents of these alloying constituents. Thereby, namely by the higher content of these alloying constituents, more favorable, higher strength characteristics can be achieved at the same high extrusion speed.

These experiments were intentionally carried out with extrusion at relatively low pressure and relatively low temperature, in order to be able to measure exactly the maximum extrusion speed which could be obtained under the various conditions of these experiments.

The following examples of aluminum alloy compositions which may be treated according to the present invention, are given as illustrative only without limiting the invention to the specific compositions.

Example (percent Balance Example II The table hereinbelow will serve for comparison of the extrusion speed which is obtained with a given aluminum alloy of the type AlMgSi by exposing the annealed ingot either to quick air cooling or to several variations of the slow cooling in accordance with the present inven tion.

The test described in the present examples were carried out with cast ingots of an aluminum alloy containing 0.45% magnesium, 0.40% silicon, the balance being aluminum having a purity of 99.7% and containing the conventional impurities.

The ingots were subjected to a homogenizing heat treatment of 570 C.il0 C. metal temperature for a period of one hour. The ingots were then cooled as indicated in the table below and thereafter subjected to induction heating up to an extrusion temperature of 480 C. The ingots were then maintained at such extrusion temperature for two minutes and thereafter extruded by means of a 315 T extrusion press. The extruded profile was then The results summarized in the table above show that the cast ingot treated according to experiment 1 could be extruded only at the low speed of 4.7 meters per minute, while according to experiment 2 the extrusion speed rose to 11.8 m./rnin. The extrusion speed according to experiments 3 and 4 is only slightly different from that achieved according to experiment 2, however, according to experiments 3 and 4 a significant increase in yield strength and tensile strength is achieved.

Similar results are also achieved with other alloys of the type AlMgSi, and also with alloys of this type which contain up to about 0.5% Cu, and particularly also with alloys which contain larger proportions of Mg and Si, whereby in the case of larger proportions of Mg and Si, it is preferred to limit the slow cooling to the temperature range between the annealing temperature and a lower range of between 450 and 300 C.

As pointed out further above, the experiments decribed herein were carried out at a relatively low extrusion temperature and extrusion pressure in order to make it possible to obtain accurate determinations of the various extrusion speeds. By carrying out experiments under industrial, operational condition, regularly extrusion speeds of more than 50 m./min. could be obtained by following the method of the present invention, i.e. by starting with slow cooling of the annealed ingot. Furthermore, more than extrusions could be carried out without requiring cleaning of the extrusion apparatus. Each extrusion yielded a profile of about 30 in. length. This is a further proof that ingots which are heat treated according to the present invention, i.e., which are slowly cooled after having been annealed, lend themselves excellently for hot deformation and possess highly desirable surface characteristics.

The dilference in theextrusion speed which has been determined in the experiments described hereinabove, is useful as a general measure for the resistance to heat deformation.

Consequently, results which are analogous to those obtained by the extrusion experiments hereinabove can also be obtained by different methods of heat deformation. Thus, for instance, the hot rolling characteristics of the ingots are improved according to the present invention in a similar manner as the extrudability of the same.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be secured by Letters Patent is:

1. A method of deforming a heated aluminum alloy body of the type AlMgSi, comprising the steps of annealing said aluminum alloy body at a temperature between 500 C., and 590 C.; slowly cooling the thus annealed aluminum alloy body at a slow rate not exceeding 50 C. per hour to a temperature not exceeding 450 C.; thereafter cooling the thus slowly cooled annealed aluminum alloy body at a rate faster than said slow rate; heating the thus first slower and than faster cooled annealed aluminum alloy body to an elevated temperature between 350 C. and 530 C.; and subjecting the thus reheated aluminum alloy body to hot deformation at said elevated temperature.

2. A method as defined in claim 1, wherein said slow rate of cooling is between 20 and 50 C., and said slow cooling is continued until the temperature of said aluminum alloy body has dropped to between 450 C. and 300 C.

3. A method as defined in claim 1, wherein said aluminum alloy body is slowly cooled at a rate of at least 20 C. per hour to a temperature below 350 C.

4. A method according to claim 1 wherein said annealing is carried out in an annealing furnace, and said annealed aluminum alloy body is retained in said annealing furnace during said slow cooling of the same.

5. A method according to claim 1, wherein said annealed aluminum alloy body is retained under a hood during said slow cooling of the same.

6. A method as defined in claim 1, wherein said aluminum alloy body is an extrusion or rolling ingot formed by continuous or semicontinous casting of said aluminum alloy.

7. A method as defined in claim 1, wherein said slow cooling is carried out at a rate ,of up to about 20 C. per hour until the temperature of said annealed aluminum alloy body has dropped to between about 450 C. and 300 C., and thereafter cooling is continued at a rate faster than 20 C. per hour.

8. A method as defined in claim 1, wherein said slow cooling is carried out at a rate of up to about 20 C. per hour until the temperature of said annealed aluminum alloy body has dropped to between about 400 C. and 300 C., and thereafter cooling is continued at a rate faster than 20 per hour.

9. A method as defined in claim 7, wherein said coolby continuous or semicontinuous casting of said aluminum alloy body with a cooling fluid.

References Cited UNITED STATES PATENTS 2,249,353 7/1941 Fritzlen 148159 X 2,381,714 8/1945 Beck 14811.5 X 3,113,052 12/1963 Schneck 148-11.5 3,219,492 11/1965 Anderson et al 148-115 3,222,227 12/1965 Baugh et a1. 14811.5

HYLAND BIZOT, Primary Examiner.

DAVID L. RECK, Examiner.

W. W. STALLARD, H. F. SAITO, Assistant Examiners.