US2388563A - Thermal treatment for aluminum base alloys - Google Patents

Description

' Patented Nov. 6, 1945 manual. mam-mam roa ALUMINUM mss rumors Joseph A; .Bomert,

Nook, In, .Tarentum, and Theodore W. Carnegie, Pa, asslgnors to Aluminum Company of America, Pittsburgh, Pa... a corporation of Pennsylvania No Drawing. Application July 10, 1948,

Serial No. 494,244

Claims.

This invention relates to a thermal treatment for aluminum base alloys and more particularly to a thermal treatment for stabilizing the working characteristics and the tensile properties of certain annealed precipitation hardening aluminum base alloys. -7 I According to the usual practice of fabricating wrought articles of aluminum base alloys by cold working operations, suitable alloy stock within usually has been previously hot worked, and perhaps cold worked, is given a series of reductions in cross sectional area by rolling, forging or the like at a, temperature somewhat below the recrystallization temperature of the alloy, whereby the metal becomes progressively harder and consequently'more difficult to work. To alleviate this hardened condition and thereby to make the stock satisfactory for further cold working operations, the material is, given an annealing treatment, i. e. heated at a temperature high enough and for a time long enough to cause substantially complete recrystallization of the cold worked metal. The alternatecold working operations and annealing treatments are continued until the final cold worked size and shape of the articles are reached. Althoughin this procedure each cold working operation generally follows the annealing treatment within a relatively short period of time, i. e. within a period of about four or live days following the annealing treatment, this is not always possible nor convenient and the material in the annealed condition may remain at room temperature for long periods of time, sometimes as lon as many months.

In the period between the annealing treatment and the next succeeding working operation, the workability as well as the tensile properties of many aluminum base alloys remains substantially unchanged and is satisfactory for further working operations irrespective of the length of this intervening period, but in age hardening aluminum base alloys containing substantial amounts of magnesium and one or more of the elements copper, silicon and zinc, the workability may progressively decrease throughout this period until further working of the alloy without an additional annealing treatment becomes exceedingly diflicult, if not impossible. Whether the aluminum alloy containing magnesium and one or more of the elements copper, silicon and zinc will age harden following the annealing treatment depends upon the amount of these age hardening constituents resent in the material and also upon the conditions under which a particular lot of material is annealed and thereafter cooled to: room temperature. In other words, if the age hardening constituents are present in amounts greater than those which are soluble at room temperature and if the annealing conditions are such as to cause retention of more of the age. hardening constituents in solid solution than are'soluble at room temperature, then that material will age harden upon standing at room temperature, resulting in a progressive diminution of the workability of the material. Consequently, if the .periodfor which this material stands at room temperature is more than a few days, it has been necessary heretofore to reann'eal the material before the next working operation to remove the effect of age hardening on theworkability of the material. Since an addi tional annealing treatment increases the cost of production and since the facilities for giving these alloys this additional annealing treatment are not always readily available immediately preceding the working operation, it is desirable to minimize the change in workability and tensile properties occurring in these alloys following the annealing treatment.

An object of this invention is to provide a method of assuring relatively high stability in the working characteristics of the aforementioned annealed age hardening aluminum base alloys, however heated and cooled in the annealing procedure. Another object of this invention is to provide a method of lengthening the period during which the aforementioned annealed precipi ,tation hardening aluminum base alloys can be satisfactorily worked on a commercial scale.

And still another object is to provide a method,

of assuring relatively high stability inthe mechanical properties of these alloys in the annealed condition.

The term cold worked as used throughout the specification and appended claims includes all material having cold working strains, how? ever produced, whether the strains are the. result of working the material either above; at. or below the recrystallization temperature of the material. The word annealed is used to describe the condition of material which has been coldworked and then heated at a temperature sufilciently high and for a period sufficiently long to relieve substantially all work hardening strains. The term age hardening alloy," when employed herein, refers to an alloy containing agehardem ing constituents in amounts greater than that soluble at room temperature which has been annealed and cooled from the annealing temperature in such manner thatit will age harden if left standing in that condition at room temperature, resulting in an increase in tensile properties and hardness and a decrease in workability. The term "aluminum base alloy" as herein employed refers to alloys containing at least 75 per cent aluminum by weight.

This invention is predicated upon the discovery that the workability and tensile properties of cold worked and annealed aluminum base alloys containing magnesium and at least one of the elements from the group consisting of copper, silicon and zinc, in amounts sufllcient to render the alloys age hardenable, can be rendered relatively stable for long periods of time by heatingthe annealed alloys to a temperature between.

about 400 and 600 F. and maintaining them at that temperature for a period of time between about ,5 and 24 hours. The complete thermal treatment necessary to soften cold worked alloys of this type so that they possess good working characteristics and to retain the good working characteristics indefinitely consists in first annealing the alloys at a temperature between about 650 and 800 F. for a suflicient period of time to relieve substantially all work hardening strains, generally for a time between 1 minute and 20 hours, cooling them to a temperature between 400 and 600 F. and holding them at that temperature for /2 to 24 hours, and finally cooling in air to room temperature; or cooling them to room temperature at the end 01' the annealing treatment and thereafter heating them at a temperature between 400 and 600 F. for A to 24 hours. erably given within a short period of time following annealing, but the beneficial eifect of the treatment is not materially decreased if the alloy is stored for a considerable length of time before this treatment is applied. The time required to anneal the aluminum base alloys depends somewhat on the process employed. For example, if a large pile of sheet is annealed in an air heated furnace, the annealing time may be as long as 20 hours. If, on the other hand, the metal is heated as a single piece in a salt bath, then as little time as 1 minute is sufllcient to soften the material completely. Furthermore, the manner of heating the alloys in the lower temperature treatment has a corresponding en'ect on the time required to complete the treatment.

Although our treatment is applicable to all annealed age hardening aluminum base alloys containing magnesium and one or more of the elements copper, silicon or zinc in amounts greater than that soluble at room temperature, it is particularly applicable to aluminum base alloys containing about 3 to 10 per cent zinc, 0.75 to 4 per cent magnesium, and0.05 to 3 per cent copper since they are especially prone to age hardening after being annealed. In addition to these three alloying ingredients, 0.1 to 1.5 per cent manganese may be present either with or without a small amount of one or more of the grain refining elements titanium, vanadium, zirconium, chromium, cobalt, tungsten, columbium, boron, and tantalum. Other groups of alloys to which our treatment is especially applicable are those containing 3 to 5 per cent copper, 0.5 to 2 per cent magnesium and 0.1 to 1.5 per cent manganese, and those containing 0.05 to 1.0 per cent copper, 0.1 to 3 per cent magnesium and 0.01 to 2 per cent silicon. Small amounts of other alloying elements such as the grain refining elements mentioned above may be added to alloys of either Our low temperature treatment is prefassasss group without modifying the eifect of our treatment on the workability and tensile properties.

Our treatment is eifectlve in preventing progressive diminution of workability in any age 5 hardening aluminum base alloy of the above mentioned compositions. The particular method, such as batch or continuous processes, by which the material is annealed has little or no noticeable influence on our treatment subsequently given the material. The conditions, however, which may occur during any one of the annealing processes, such as the material being cooled rapidly from the annealing temperature, may

' have considerable influence on the age harden- 1 ing characteristics of the material and consequently considerable efiect on the particular conditions required by our treatment to stabilize the workability of the material.

As an example oi the operation of our invention, an alloy containing 5.25 per cent zinc,.2.l per cent magnesium, 1.5 per cent copper, 0.4 per cent manganese, 0.3 per cent iron, 0.12 per cent titanium, 0.25 per cent silicon, and balance aluminum, was cold rolled to sheet 0.064 inch in thickness and finally annealed at 650 F. Tensile tests were made on four groups of identical specimens from this material, the groups being designated and treated as follows:' 1. Tested immediately after annealing; 2. Stored 6 months at room temperature before being tested; 3. Heated shortly after the annealing treatment at 450 F. for 8 hours, cooled to room temperature, and then immediately tested; 4. Heated shortly after the annealing treatment at 450 F. for 8 hours,cooled to room temperature, and then stored for 6 months at room temperature before being tested. The average values of the tensile properties of the respective groups are given in Table I below:

Table I Tensile Yield Per cent Group strength, strength lbsJsq. in. lbsJsq. 115.

From these examples it is apparent that the tensile properties of the specimens in group 2 changed greatly in 6 months, whereas the properties of the specimens in group 4 by the present treatment were rendered relatively stable over the same period of time. The lower values for the specimens in groups 3 and 4 as compared to those of group 1 are not regarded as being significant for annealed material. The relatively low strength values of groups 1, 3 and 4 are indicative of greater workability than for group 2. While a slight change in the tensile properties may occur during the 6 mmiths period in the alloys given our treatment, these changes are not great enough to seriously affect the workability oi the alloys. Since the changes in tensile properties noted above, as well as those observed in many other cases, are of a minor order, the stabilizing treatment is considered to substantially prevent change in the tensile properties and workability of alloys of the type herein described.

We claim:

1. A process-for stabilizing the workability and tensile properties of age hardening type or aluminium base alloys containing magnesium and at least one of the elements copper. siliconand zinc that have been annealed and cooled under as'easss conditions that ave not substantially eliminated the susceptibility of said alloys to age. hardening, comprising heating said alloys in the annealed condition at a temperature between about 400? and 600 F. for a period of time between about and 24 hours and long enough to remove substantially all tendency for the alloys to age harden.

' 2. A process for stabilizing the workability and tensile properties oi aluminum base alloys containing from 3 toper cent zinc, 0.75 to 4 per. cent magnesium, 0.05 to 3 per cent copper that have been annealed and cooled under conditions that have not substantially the susceptibility or said alloys to age hardening, comprising heating said alloys in the annealed condi- 600 I". for a period or time between V and 24 hours and long enough to remove substantially all-- tendency for the alloys to age harden, and finally cooling the alloys to room temperature.

said alloys at a temperature between 650 and 800 F., under conditions of annealing and cooling 7 that do not substantially eliminate the'susceptlbilityof said alloys to age hardening, then heating saidalloys at a temperature within the range of about 400 to 600 F. for a period or time between about /2 and 24 hours and long enough to remove substantially all tendency for the alloys to age harden, and finally cooling the alloys to room temperature. 1

7.. A process for annealing and stabilizing the workability and tensile properties of cold worked aluminum base alloys containing from 3 to 10 per tion at atemperature between about 400 and per. cent? copper, comprising annealing said alloys 3 until substantially all work hardening strains are relieved underconditions-of annealing and cooling 3. A process for stabilizing the workability and i tensile properties oi aluminum'base alloys" containing irom3 to 5per cent copper, 0.5to2 per' cent magnesium and 0.1 to' 1.6 per cent manganese that have been annealed and cooled under conditions that have not substantially eliminated the susceptibility of said alloys to age hardening, comprising heating said alloys in the annealed condition at a temperature between about 400 and 600 F. for a period or time between about 56 to 24 hours and long enough to remove'substantially all tendencyi'or the alloys to age harden,

and finally cooling the alloys to-room temperature 4. A process for annealing and stabilizing the workability and tensile properties or 'cold worked that donot substantially eliminate the susceptibility of said alloys to agehardening, thereupon cooling said alloys to a temperature between about 400 and600 F., holding them at a temperature within said range of temperatures for a period of time between about and 24 hours and long enough to'remove substantially all tendency for the" alloys to age harden, and thereafter cooling the alloys to room temperature.

8. A process for stabilizing the tensile properties of aluminum base alloys containing from 3 to 10 per cent zinc, 0.75 to 4 per cent magnesium, 0.05 to 3 per cent copper that have been annealed and'cooled under conditions that have notsubstantially eliminated the susceptibility of said aluminum base alloys containing from 3 to 10 per cent zinc, 0.75 to 4 per cent magnesium,,0.05 to 3 per cent copper, comprising annealing said alloys until substantially all work hardening strains are relieved'under conditions of annealing and cooling that do not substantiallynliminate the susceptibility or said alloys to age hardening, then heating said alloys at a temperature within the range 01' about 400 to 600 F. for a period of time between V and 24 hours and long enough to remove substantially all tendency age harden.

V 5. A process for annealing and stabilizing the workability and tensile properties of cold workedaluminum base alloys containing from .3 tom per cent zinc, 0.75 to 4 per cent magnesium, 0.05 to 8 per cent copper, comprising annealing said alloys until substantially all work hardening strains are relieved under conditions of annealing and cooling that do not substantially eliminate the susceptibility of said-alloys to age hardening, cooling them to room temperature, then heating said alloys at a temperature within the range-of about 400 to' 600 F. for a period oi time between about and 24 hours and long enough to remove sub-' stantially all tendency for the alloys to age harden, and finally cooling the alloys to room temperature.

6. A process forannealing and stabilizing the workability and tensile properties or cold worked alloys to age hardening, comprising heating said alloys in the annealed condition at about 450 F. for about 8 hours, and thereafter cooling the alloys to room temperature.

9. Aprocess-Ior annealing and stabilizing the r workability and tensile properties of cold worked for the alloys to aluminum base alloys containing magnesium and at least one of the elements copper, silicon and zinc, comprising annealing said alloys until substantially all work hardening strains are relieved under conditions of annealing and cooling that do not substantially eliminate the susceptibility 9f said alloys to age hardening, then heating said alloys at a temperature within the range of about 400 to 600 F. for a period of time between about and 24 hours and long enough to remove substantialiy all tendency for the alloys to age harden. s 10. A process for annealing and stabilizing the workabilityand tensile properties of cold worked aluminum base alloys containing from 3 to 5 per aluminum basealloys containing from 3 to 10 per zinc, 0.75 to 4 per cent magnesium, 0.05 to 3 per JOSEPH A. NOCK, JR.

'rnnoponn w. nossER'rQ 3' cent copper, comprising continuously annealingcent zinc, 0.75 to 4 per cent magnesium, 0.05 to 3 Pat zgnt No r 2 ,5 88 565 CERTIFICATE OF comcnon; I

' November -6, l91 .5'.' I JOSEPH A. NOCK, JR., ET AL.

It is Hereby pertified that error lippeara in thq printeq, specification I of the abdve nu mbe rad patent i'equiringcorretion as. fol1ow:i:' Page "1,11rgt column, line 9, for the' wordl'withl-n" read which; and thatthe said Lettars-Patent shouldbe read with thi'correctiqn therein that the same may confrm to the record pi the cas in the Patent offic'e.

Signed and Segled th1s 29th day of Jaz mary, R. D. 1914.6.

Leslie Frazer (seal? first A s1staqt Commissioner of Patents.