US2249353A - Method of working aluminum and product thereof - Google Patents

Description

July 15, 1 941.

7 T. L. FRITZLENN METHOD OF WORKING ALUMINUM AND Pnobuc'r THEREOF Filed Aug. 23, 1959 n 6 W 9. MM a m Z n -sound surface on the product.

Patented July 15, 1941 METHOD OF WORKING ALUMINUM AND PRODUCT THEREOF Thomas L. Fritzlen, West Lafayette, Incl, assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania.

Application August 23, 1939, Serial No. 291,538

11 Claims.

This invention relates to a method for improving the workability of that class of aluminum base alloys which respond to solution heat treatment for increasing their mechanical .properties, and it relates more specifically to such method in the art of extrusion. The invention is particularly concerned with methods of extruding aluminum base alloys of this class, especially those containing copper as the main solid solutionforming element. The term aluminum base alloys as used in this specification and in the appended claims means alloys containing more than about 80 per cent aluminum.

According to the usual practice of extruding these alloys, cast ingots or billets of the alloys are heated to the proper extrusion temperature and extruded into the desired shapes. The extrusion is accomplished by means of an extrusion press which generally comprises a heated cylinder for receiving the heated ingot, a die at one end of said cylinder which has an opening of the desired shape, and a ram at the other end which forces These factors also to some extent indirectly affect the quality of such products. I will briefly mention several of the more important of these factors, since an understanding of their significance is necessary for a proper appreciation of the advantages of my invention.

It is evident that economic considerations favor an increase in extrusion speed, which speed is usually referred to in terms of the progression of extruded materialfrom the die in linear feet per'minute. As this speed is increased, however, there arises an increasing tendency towards what 1 is known as pickup, which may be defined as a tendency of the extruded metal to adhere or bond to the bearing surface of the die. Thisnot only decreases'extrusion speed but also causes grooves and surface irregularities on the extruded shapes; Generally speaking, the extrusion press is of ample capacity to force the'metal through the die at greater speeds than actually employed, but the operating speed is necessarily llmited to that which will produce an acceptable The attainment of a desired extrusion-speed may be prevented by another surface condition, usually termed surface cracking. Under some conditions minute cracks may appear, which grow larger as extrusion speed is increased until the product eitherruptures completely, or is otherwise unsatisfactory for commercial purposes.

For these and other reasons it has been found I that there exist definite maximum speeds at which alloys of the type herein under consideration can be satisfactorily extruded, and one of the problems of this art has been to develop means for permitting an increase in the extrusion speeds and for improving the efliciency of the process while maintaining a high standard of quality for the product.

It has been found that the structure of ingots in the as-cast condition commonly used for making extruded products is characterized by highly cored dendritic grains with a considerable amount of eutectic network in the interstices of the dendrites, said eutectic network being composed of both soluble and insoluble constituents,

and that this structure offers resistance to work- 'ing and to plastic flow. To acertain extent, of

course, this varies with the composition of the alloy. v,

It is a principal object of this invention to provide a process for treating the aforementioned ingots of aluminum base'alloys having a. highly Another object is to provide such a process of extrusion by which pickup is minimized, and in which the conventional extrusion presses may be used. Another-object is to provide a process whereby alloys may be extruded at increased speeds yet not inconsistent with satisfactory commercial quality of the product. Another ob- 'ject is to provide a process whereby surface cracking is substantially eliminated; Other objects and advantages will be apparent from thefollowing explanation of the invention.

The invention resides in the discovery that by subjecting the aluminum base alloy ingots to.

an extended heating and a subsequent slow cooling to a temperature at which practically no further structural change occurs and-then re-' heating to the extrusion temperature, a change in the structure of the alloy occurs which is accompanied by a decrease in resistance to plastic deformation which permits extrusion at higher speeds and/or lower temperatures. The

altered structure, resulting from the thermal treatment, is characterized by a generaldistribution throughout the alloy of a large number of relatively, large precipitated particles of the soluble constituents both within the grains and at the grain boundaries. By relatively large precipitated particles is meant precipitated particles larger than those formed when the alloy is subjected to the well known solution heat treatment and precipitation hardening methods, wherein it is heated, quenched and aged to impart hardness and strength thereto. I have found that ingots possessing such a microstructure can be extruded at greater speeds and at lower temperatures than ingots which have been only heated up to the extrusion temperature in the usual manner. I have also found that at any stated temperature the billet can be extruded at higher speed,while still of good quality and free from cracks.

Describing in more detail the process of the present invention, the cast ingots are homogenized by heating them at a temperature and for a period of time required to dissolve substantially all of the soluble constituents. The term homogenize as herein employed refers to the condition where a substantial portion of the soluble constituents of the alloy have been dissolved and is usually accompanied by a virtual disappearance of the cored dendritic structure. This operation causes undissolved particles of the soluble elements located at the grain boundaries and in the eutectic network-to go into solid solution. The effect of this heating step upon the structure is readily apparent from a comparison of the accompanying drawing, in which:

Fig. 1 is a photomicrograph at 100:: of an aluminum base alloy containing 4.0 per cent copper, 0.5 per cent manganese, and 0.5 per cent magnesium, which has been cast and cooled by conventional methods. The highly cored nature of the dendritic structure is exhibited by the darker portions of the grains near their edges, and a large amount of eutectic substance at the grain boundaries is clearly visible;

Fig. 2 is a photomicrograph at 100K of the same alloy after it had been heated according to this invention, at a temperature of from 905 to 935 F. for eight hours and subjected to a drastic quench to retain the structure existing at said temperature, and shows the homogenization of the structure, as exhibited by a reduction of the shaded areas of the grains, and by the reduction of the amount of eutectic substance at the grain boundaries.

In general, aluminum base alloys containing about 2.5 to about 6.0 per cent copper, from about 0.1 to about 1.0 per cent manganese, and from about 0.1 to about 2.0 .per cent magnesium, require heating for a period of several hours depending on the size of the ingots, usually about eight hours, at a temperature of from about 890 F. to about 950 F. after the metal has reached a uniform temperature within this range to produce the desired degree of solution as shown in Fig. 2, although some solution is obtained in less time. The alloys may be heated at these temperatures for longer periods than eight hours, depending on the relative amount of copper, magnesium and silicon present, the size of the ingot being heated, and the size of the load where a batch of ingots are treated at one time. Forother alloys containing magnesium, zinc, or silicon as the chief added alloying element, for example, the period of heating to dissolve the soluble constituents may differ from that required for the foregoing alloys containing copper as the chief added alloying element. Furthermore, the composition of the alloysolution in the form of the aforementionedlarge particles uniformly distributed throughout the alloy. It is preferred to continue the cooling down to a temperature at which substantially no further precipitation occurs in order to secure maximum precipitation. In most cases this condition has been reached when the alloy has been cooled to close to ordinary temperatures. In the case of copper-containing alloys, such as those referred to above, this precipitate is mainly 011A]:-

The photomi-crograph of Fig. 3 is of the same alloy at 1002! as that of Figs. 1 and 2 after it has been heated to the condition shown in Fig. 2 and slowly cooled to room temperature. Fig. 3 shows clearly the condition of the alloy after this precipitation step. The uniformly precipitated particles and the substantial reduction in the amount of eutectic substance at the grain boundaries is apparent.

When it is desired to extrude one of the billets so treated, it is heated to suitable extrusion temperature and extruded immediately thereafter in the conventional extrusion press. Generally speaking, I prefer to maintain the extrusion temperature below about 750 F. but this depends upon the character of the alloy being extruded.

Investigation has shown that when using initial pressures of 4000 to4400 pounds per square by the conventional method for certain specificsections. Further comparative tests have shown that the practice of this invention makes it possible to extrude this type of aluminum base alloy, using standard extrusion equipment, at considerably lower extrusion pressures than is possible under conventional extrusion practice for the same temperature conditions, with less pickup, better surface appearance, and higher extrusion speeds.

I claim:

1. A thermal treatment for an ingot of an aluminum base alloy containing solid solutionforming elements which comprises heating said ingot, prior to working. at a temperature and for a time sufiicient to effect solution of a material part of said solid solution-forming elements in the alloy, cooling the ingot at a relatively slow rate to cause precipitation of a substantial portion of said solid solution-forming elements from solution in the form of relatively large particles uniformly distributed throughout the alloy, said cooling being continued until the ingot reaches a temperature at which substantially no further structural change occurs in the alloy, and then reheating said ingot to a suitable extrusion tem-- perature.

2. A process for extruding an ingot of an forming elements and having a cored dendritic structure, which comprises heating said ingot at a temperature and for a time suflicient to effect solution of a material part of said solid solutionforming elements in the alloy, cooling the ingot at a relatively slow rate to cause precipitation of a substantial portion of said solid solution forming elements from solution in the form of relatively large particles uniformly distributed throughout the alloy, heating said ingot at a relatively rapid rate to a suitable extrusion temperature, and immediately thereafter extruding said ingot to the desired shape.

3. A process for extruding an ingot of an aluminum base alloy containing solid solution-forming elements and having a cored dendritic structure, which comprises heating said ingot at a temperature and for a time suiiicient to efiect solution of a material part of said solid solutionforming elements in the alloy, cooling the ingot at a relatively slow rate to cause precipitation of a substantial portion of said solid solutionforming elements from solution in the form of relatively large particles uniformly distributed throughout the alloy, said cooling being continued until the ingot reaches a temperature at which substantially no further structural change occurs in the alloy, heating said ingot at a relatively rapid'rate to a suitable extrusion temperature, and immediately thereafter extruding said ingot to the desired shape. I

4. A process for extruding an ingot of an aluminum base alloy containing solid solution-forming elements including copper and having a cored dendritic structure, which comprises heating said ingot at a temperature and for a time suflicient to effect solution of a material part of said solid solution-forming elements in the alloy, cooling the ingot at a relatively slow rate to cause precipitation of a substantial portion of said solid solution-forming elements from solution in the form of relatively large particles uniformly distributed throughout the alloy, heating said ingot at a relatively rapid rate to a suitable extrusion ytemperature, and immediately thereafter extrudyfing said ingot to the desired shape.

a time sufflcient to effect solution of a material part of said solid solution-forming elements in the alloy, cooling the ingot at a relatively slow rate to cause precipitation of a substantial portion of said solid solution-forming elements from solution in. the form of relatively large particles uniformly distributed throughout the alloy, heating said ingot at a relatively rapid rate to a I about 0.1 per cent to about 2.0 'per cent magnesium and having a cored dendritic structure, which comprises heating said ingot at a temperature of from about 890 F. to about 950 F. for a time sumcient to effect solution of a material part of said solid solution-forming elements in the alloy, cooling the ingot at a relatively slow rate to cause precipitation of a substantial portion of said solid solution-forming elements from solution in the form of relatively large particles uniformly distributed throughout the alloy, heating said ingot at a relatively rapid rate to a suitable extrusion temperature, andimmediately thereafter extruding said ingot to the desired shape.

7. An ingot of an aluminum base alloy containing. solid solution-forming elements and being capable of being extruded at higher speeds or at lower pressuresthan heretofore employed with similar alloys,-said ingot being characterized by an internal structure at the hot working temperature resulting from a thermal treatment comprising heating said ingot after casting and before working thereof at a temperature and for a time sufficient to eflect solution of a material part-of said solution-forming elements in the alloy, cooling the ingot at a relatively slow rate to cause precipitation of a. substantial portion of said solid solution-forming elements from solution in the form of relatively large particles uniformly distributed throughout the alloy, and then rapidly heating the ingot to a suitable working temperature.

8. A process for working an ingot of an aluminum base alloy containing solid solution-forming elements, which comprises heating said ingot at a temperature and for a time suflicient to effect solution of a material part of said solid solutionforming elements in the alloy, cooling the ingot at a relatively slow rate to cause precipitation of a substantial portion of said solid solutionforming elements from solution in the form of relatively large particles uniformly distributed throughout the alloy, and then working said in-' got containing said large precipitated particles to the desired shape.

9. A process for working an ingot of an aluminum base alloy containing solid solution-forming containing said large precipitated particles to rate to cause precipitation of a substantial portion of said solid solution-forming elements from solution in the form of relatively large particles uniformly distributed throughout the alloy, said cooling being continued until the ingot reaches a temperature at which substantially no further structural change occurs in the alloy, and then rapidly reheating said ingot to a suitable working temperature without substantial solution of said large precipitated particles.

tion-forming elements, copper and magnesium,

in amounts offrom about 2.5 to 6 per cent copper.

- and from about 0.1 to 2 per cent magnesium, and

precipitation of a substantial portion of said solid solution-forming elements from solution in the form of relatively large particles uniformly distributed throughout the alloy, rapidly reheating said ingot to a suitable hot working temperature and thereafter working said ingot to the desired shape.

THOMAS L. FRI'IZLEN.

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