US2083576A - Heat treatment of aluminum alloys - Google Patents

Description

Patented June 15, 1937 UNITED STATES PATENT OFFICE Joseph A. Nook, Jr., Tarentum, Pa., Aluminum Company of America,

assig'nor to Pittsburgh,

Pa., a corporation of Pennsylvania No Drawing. Application September 20, 1935, Serial No. 41,452

15 Claims.

This invention relates to the thermal treatment of certain aluminum base alloys and it is particularly concerned with a preliminary aging of alloys which require artificial aging.

It is well known that the physical properties of certain aluminum base alloys are improved through subjection to a so-called solution heat treatment and a subsequent aging at room temperature or slightly elevated temperatures. In the making of articles from alloys treated in this manner it frequently becomes necessary to straighten or shape the articles before the thermal treatment is completed. When this must be done, the operation usually has been performedimmediately after quenching from the solution heat treating temperature and before aging occurs to an appreciable extent because the alloy is then more ductile than after it has been aged. It has been ascertained, however, that this cold working of the alloys before they are aged affects their ultimate properties, the tendency being to lower their strength and hardness. The cold working appears to disturb the type of precipitation induced by normal aging as well as the rate of aging. It has also been observed that the greater the deformation of the heat treated article, the more pronounced is the reduction in physical properties of the aged alloy which indicates the existence of an abnormal condition. Even a small deformation such as flattening a warped sheet introduces sufficient strain in the metal to disturb the precipitation with a consequent diminution in physical properties. Inasmuch as it is often necessary to straighten heat treated products that have been warped, or to bend the sheet, rod or plate into some desired form, it has not been possible to secure in these articles the maximum strength of which the alloy is capable when it is aged. Aluminum base a1- loys that must be artificially aged, that is, heated to slightly elevated temperatures to induce precipitation, have been found to be particularly susceptible to this adverse effect of cold working prior to aging.

It is accordingly an object of my invention to provide a method for preventing a reduction in the physical properties of artificially aged aluminum base alloys that are cold worked a small amount after solution heat treatment and before 50 receiving the final aging treatment. Another object is to provide a method whereby articles can be shaped from solution heat treated and quenched aluminum base alloys prior to the' final aging without adversely affecting the physi- 55 cal properties of the final product. Still another object is to control the character of the precipitate in aluminum base alloys that arecold worked after receiving a solution heat treatment.

My invention is predicated upon the discovery that a preliminary aging of solution heat treated aluminum base alloys that must be artificially aged to develop their maximum strength and hardness prevents any subsequent deleterious effect of a limited amount of cold work upon the physical properties of the fully aged alloy. The preliminary aging appears to induce an initial precipitation of the constituents that were dissolved by the previous solution heat treatment. Precipitation produced in this manner overbalances or destroys the effect that is normally created by cold working a quenched alloy prior to aging. My invention is confined to the treatment of aluminum base alloys which are reheated to a temperature between about 200 and 350 F, after solution heat treatment to secure the desired physical properties. Some aluminum base alloys which age spontaneously at room temperature can also be aged at an elevated temperature, but such alloys are excluded from the purview of the present invention since it is essential that absolute control be exercised over the progress of the aging. The term artificial aging is here employed in the same sense in which it is used in the art, namely, heating the quenched alloy above room temperature for varying periods of time until the desired degree of hardness is o btained. Y

In order to secure the benefit of the preliminary aging the alloy should not be cold worked more than about 5 per cent between the preliminary and final aging treatments. The amount of cold'work is to be measured by the reduction in cross sectional area of the piece of metal being deformed. Larger reductions tend to work harden the alloy and introduce disturbances in the structure of the metal which nullify the effect of the preliminary aging. I am. aware that heat treated aluminum base alloys have been cold worked prior to final aging but in all cases the reductions made amounted to per cent or more and represented a definite part in the fabricating schedule of bringing the wrough product to the desired finish size. Deformations of such magnitude as pointed out above lie wholly outside the scope of the present invention because they create a condition in the alloy which considerably mitigates if it does not actually destroy the beneficial effect of the preliminary aging. My invention is applicable where warped sheets are straightened or where the sheet, rod

or plate is bent into the desired shape. In such instances the cold work introduced into the metal is incidental to the purpose of the operaemployed refers to the bending of a sheet, rod

or plate, into a desired form, or the drawing of a sheet as in the form of a pan.

A further advantage to be gained where articles are shaped after the sheet, rod or plate has been preliminarily aged lies in the elimination of an annealing operation. Ordinarily it is desirable to anneal metal stock previous to shaping in order to render it soft enough to take the cold work without excessive hardening. In

the practice of my invention the article is shaped after solution heat treatment and preliminary aging, the metal being sufliciently soft at this stage to readily permit working. After shaping the article is aged. If the customary practice of annealing the stock prior to shaping were followed the metal would be first annealed, then shaped, heat treated and finally aged. In heat treating the article after it has been shaped there is danger of warping it when quenching from the elevated temperature. According to my practice all danger of subsequent warpage is avoided since the article has already been quenched and the working operation straightens any warped areas.

I have found that aluminum base alloys containing between about 2 and 7 per cent copper and 0.005 to 0.1 per cent tin are particularly benfited by my improved aging treatment. They do not age spontaneously at room temperature .but must be artificially aged to develop their maximum strength and hardness. Those alloys containing between about 4 and 6 per cent copper, and 0.03 to 0.07 per cent tin have been found to be especially Well adapted to the production of wrought articles. Other elements than copper and tin may be present in the alloy without destroying its essential aging characteristics or rendering the preliminary aging treatment ineffective. From about 0.05 to 2 per cent of cadmium may be added to the base 'alloy to permit quenching in hot water, as described in co-pending application, Serial No. 35,132. One or more of the elements selected from the group composed of chromium, molybdenum, tungsten and titanium may be used in amounts of from 0.05 to 0.5 per cent. Manganese, which also belongs to this group, may be added to the alloy in amounts of from 0.1 to 1.25 per cent. The total amount of all of these elements should in no case exceed about 1.5 per cent, and not less than 0.1 per cent is preferred where two or more elements chromium, molybdenum, tungsten, ti-' tanium and manganese. The total amount of nickel, silicon and zinc should not exceed about 3 per cent, and the minimum quantity of each should not be less than 0.1 per cent. Magnesium is to be avoided as an alloying constituent and no more than about 0.01 per cent may be allowed as an impurity because this element introduces an undesired variation in the aging qualities of the alloy.

In the practice of my invention the alloys in the form of semi-finished articles such as rod, sheet, or plate are first subjected to the customary solution heat treatment, that is, they are held at a temperature between about 900 and 960 F. for a period of from about 5 minutes to 10 or 12 hours depending on the size of the load being treated and the heating medium surrounding the metal, whether it is a fused salt bath or a gaseous atmosphere as in the usual type of furnace. When the desired degree of solution has been obtained, the alloy is quickly cooled to ordinary temperatures by quenching in water, other liquids, or by any other suitable cooling means. Instead of working the alloy at this stage as has been done heretofore, or applying the full aging treatment, I submit the article to a preliminary aging treatment at a temperature of between about 200 and 350 F. for a limited period of time, only long enough to initiate precipitation of the dissolved constituents. A period of 30 minutes to 24 hours is usually required to effect this preliminary aging. The actual time needed to treat a particular alloy depends on the temperature used since it is obvious that heating at 212 F., for example, does not produce precipitation as rapidly as heating at 350 F. It is not generally practicable to employ temperatures below 200 F. and if a temperature higher than 350 F. is used, it is difficult to control the size and distribution of precipitated particles; which in turn, affects the ease of cold working. v

After the preliminary aging has been effected and the article cooled to room temperature, the straightening or shaping operation is to be performed as described hereinabove. Following this the-articles are again heated to a temperature between about 200 and 350 F. and held at the desired temperature for a period of from 4 to 40 hours. The length of time needed to complete the aging of the alloy varies with the size of the load treated and the temperature of aging, a longer time being required where the load is large, or the temperature is in the lower portion of the permissible range. This treatment is termed the final aging since the precipitation of the dissolved constituent in excess of that normally soluble at room temperatures is completed as nearly as possible consistent with commercial operating conditions, thereby attaining substantially the maximum strength and hardness of which the alloy is capable when subjected to thermal treatment alone.

The efiect of cold working a heat treated and quenched aluminum base alloy before it is aged, and the influence of a preliminary aging in overcoming this effect are well illustrated in the following example. done by rolling, but the eifectis the same as though the sheet were stretched or shaped, as is well recognized by those skilled in the art. The alloy employed for the test was composed of aluminum, about 4.5 per cent copper, 0.8 per cent manganese, 0.8 per cent-silicon and 0.05 per cent tin. This 'alloy was melted, cast and rolled to sheet form in accordance with the usual commercial practice. The sheets were heat treated at about 970 F. for 15 minutes and quenched in The cold work in this case was,

water. One portion of the lot was then aged in the customary manner without any intervening cold working operation. A second portion of the heat treated sheets was cold rolled with a reduction of 1 per cent in thickness, and finally aged at 320 F. for 12 hours. A third portion was first aged at 320 F. for 1 hour, then cold rolled with 1 per cent reduction in thickness, and finally aged at 320 F. for 12 hours. Tensile test speci- 0 mens were taken from each group for the determination of physical properties. The average results of these tests are given in the table below, the groups being designated A, B, C, corresponding respectively to the first, second and third portions of the lot of heat treated sheet described above.

It is apparent from these results that even as small a reduction as 1 per cent in thickness secured by cold working produces a marked decrease in strength. It is also to be noted that the preliminary aging restores the strength to substantially the same value as that found in the unworked sheet.

Although I can give no comprehensive explanation of the improvement obtained through preliminary aging, it appears from the results of tensile teststhat cold working a heat treated and quenched aluminum base alloy induces a precipitation of the dissolved constituents. thermore appears that the precipitate is not uniformly distributed but is largely confined to the atomic planes along which slippage has occurred during the deformation of the alloy. The preliminary aging on the other hand causes an incipient random precipitation which is not disturbed by subsequent cold working with the result that substantially the same physical prop erties are obtained as found in the normally aged and unworked alloy.

The term aluminum as employed in the appended claims refers to the metal of commercial purity used in the production of aluminum base alloys, said aluminum containing the usual impurities.

I claim:

1. A method of improving the physical properties of aluminum base alloys that are cold worked between solution heat treatment and artificial aging, said method comprising partially artificially aging the alloy, cold working it less than 5 per cent, and finally completing the aging.

2. A method of improving the physical properties of aluminum base alloys that must be artificially aged after solution heat treatment to develop maximum strength and hardness where the alloys are cold worked less than 5 per cent between the solution and aging treatments, said method comprising partially artificially aging the alloy after solution heat treatment, cooling to room temperature, cold working less than 5 per cent, and finally completing the aging.

3. A method of improving the physical properties of aluminum base alloys that are cold worked between solution heat treatment and artificial aging, said method comprising partially aging the alloy at about 200 to 350 F. after the It fursolution heat treatment, cold working less than about 5 per cent, and finally completing the aging at 200 to 350 F.

4. A method of improving the physical properties oi solution heat treated magnesium-free aluminum base alloys containing from about 2 to 7 per cent copper, and 0.005 to 0.1 per cent tin where the alloy article is straightened or shaped subsequent to solution heat treatment and prior to artificial aging, said method comprising preliminary aging the alloy at 200 to 350 F. for 30 minutes to 24 hours after the solution heat treatment, cooling the alloy to substantially room temperature, performing the straightening or shaping operation and thereafter completing the aging treatment by holding the alloy at 200 F. to 350 F. for a period of 4 to 40 hours.

5. A method of improving the physical properties of magnesium-free aluminum base alloys containing from about 2 to 7 per cent copper, and

0.005 to 0.1 per cent tin, said alloy being cold worked less than 5 per cent subsequent to solution heat treatment and prior to artificial aging, said method comprising preliminarily aging the alloy at 200 to 350 F. for 30 minutes to 24 hours after the solution heat treatment, cooling the alloy to room temperature, cold working less than 5 per cent, and thereafter completing the aging treatment by holding the alloy at 200 to 350 F. for a period of about 4 to 40 hours.

6. A method of improving the physical properties of magnesium-free aluminum base alloys containing from about 4 to 6 per cent copper, and 0.03 to 0.07 per cent tin, said alloy being cold worked less than 5 per cent subsequent to solution heat treatment and prior to artificial aging, said method comprising preliminarily aging the alloy at 200 to 350 F. for 30 minutes to 24 hours after the solution heat treatment, cooling the alloy to room temperature, cold working less than 5 per cent, and thereafter completing the aging treatment by holding the alloy at 200 to 350 F. for a period of about 4 to 40 hours.

. 7. A method of improving the physical properties of magnesium-free aluminum base alloys containing from about 2 to 7 per cent copper and 0.005 to 0.1 per cent tin, said alloy being cold worked less than 1 per cent subsequentto solution heat treatment and prior to artificial aging, said method comprising preliminarily aging the alloy at 200 to 350 F. for 30 minutes to 24 hours after the solution heat treatment, cooling the alloy to room temperature, cold working less than 1 per cent, and thereafter completing the aging treatment by holding the alloy at 200 to 350 F. for a period of about 4 to 40 hours.

8. A method of improving the physical properprising preliminarily aging the alloy at 200 to 350 F. for 30 minutes to 24 hours after the solution heat treatment, cold working the alloy less than 5 per cent, and finally completing the aging by heating to 200 to 350 F. for 4 to 40 hours.

9. A method of improving the physical properties of magnesium-free aluminum base alloys containing from about 2 to 7 per cent copper, 0.005 to 0.1 per cent tin and 0.1 to 3 per cent of alloying metal selected from the group composed of nickel,

silicon and zinc, said alloy being cold worked less than 5 per cent subsequent to solution heat treatment and prior to artificial aging, said method comprising preliminarily aging the alloy at 200 to 350 F. for 30 minutes to 24 hours after the solution heat treatment, cold working the alloy less than 5 per cent and finally completing the aging by heating to 200 to 350 F. for 4 to 40 hours.

10. A method of improving the physical proper-: ties of magnesium-free aluminum base alloys con-f taining from about 2 to '7 per cent copper, 0.005 to 0.1 per cent tin, and 0.1 to 1.25 per cent manganese, said alloy being cold worked less than 5 per cent subsequent to solution heat treatment and prior to artificial aging, said method comprising preliminarily aging the alloy at 200 to 350 F. for 30 minutes to 24 hours after the solution heat treatment, cold working the alloy less than 5 per cent, and finally completing the aging by heating to 200 to 350 F. for 4 to 40 hours.

11. A method of improving the physical properties of magnesium-free aluminum base alloys containing from about 2 to 7 per cent copper, 0.005% 0.1 per cent tin, 0.1 to 1.25 per cent manganese, and 0.1 to 3 per cent of alloying metal selected from the group composed of nickel, silicon and zinc, said alloy being cold worked less than 5 per cent subsequent to solution heat treatment and prior to artificial aging, said method comprising preliminarily aging the alloy at 200 to 350 F. for 30 minutes to 24 hours after the'solution heat treatment, cold working the alloy less than 5 per cent and finally completing the aging by heating to 200 to 350 F. for 4 to 40 hours.

12. A method of improving the physical properties of magnesium-free aluminum base alloys containing from about 2 to 7 per cent copper, 0.005 to 0.1 per cent tin, from about 0.05 to 1.5 per cent of alloying metal selected from the group of elements composed of chromium, molybdenum, tungsten, titanium, and manganese, and 0.1 to 3 per cent of alloying metal selected from the group composed of nickel, silicon and zinc, said alloy bein cold worked less than about 5 per cent subsequent to solution heat treatment and prior to artificial aging, said method comprising preliminarily aging the alloy at 200 to 350 F. for 30 minutes to 24 hours after the solution heat treatment, cold working the alloy less than 5 per cent and finally completing the aging by heating to 200 to 350 F. for 4 to 40 hours.

13. A method of improving the physical properties of magnesium-free aluminum base alloys containing from about 2 to 7 per cent copper, 0.05 to 2 per cent cadmium, and 0.005 to 0.1 per cent tin, said alloy being cold worked less than 5 per cent subsequent to solution heat treatment and prior to artificial aging, said method comprising preliminarily aging the alloy at 200 to 350 F. for 30 minutes to 24 hours after the solution heat treatment, cooling the alloy to room temperature, and cold working less than 5 per cent, and thereafter completing the aging treatment by holding the alloy at 200 to 350 F. for a period of about 4 to 40 hours.

14. A method of improving the physical properties of magnesium-free aluminum base alloys containing from about 2 to 7 per cent copper, 0.05 to 2 per cent cadmium, 0.005 to 0.1 per cent tin, and 0.05 to 1.5 per cent of alloying metal selected from the group composed of manganese, chromium, molybdenum, tungsten and titanium, said alloy being cold worked less than 5 per cent subsequent to solution heat treatment and prior to artificial aging, said method comprising preliminarily aging the alloy at 200 to 350 F. for 30 minutes to 24 hours after the solution heat treatment, cold working the alloy less than 5 per cent, and finally completing the aging by heating to 200to 350 F. for 4 to 40 hours.

15. A method of improving the physical properties of magnesium-free aluminum base alloys containing from about 2 to 7 per cent copper, 0.05 to 2 per cent cadmium, 0.005 to 0.1 per cent tin, and 0.1.to 3 per cent of alloying metal selected from the group composed of nickel, silicon, and zinc, said alloy being cold worked less than 5 per cent subsequent to solution heat treatment and prior to artificial aging, said method comprising preliminarily aging the alloy at 200 to 350 F. for 30 minutes to 24 hours after the solution heat treatment, cold working the alloy less than 5 per cent, and finally completing the aging by heating to 200 to 350 F. for 4 to 40 hours.

JOSEPH A. NOCK, JR.