US3232796A - Treatment of aluminum-magnesium alloy - Google Patents

Description

United States Patent 3,232,796 TREATMENT OF ALUMINUM-MAGNESIUM ALLOY William A. Anderson, Verona, Pa., assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Filed Mar. 21, 1962, Ser. No. 181,432 14 Claims. (Cl. 148--12.7)

This application is a continuation-impart of my application Serial No. 154,868, filed November 24, 1961, now abandoned.

This invention relates to improving the resistance to stress corrosion of certain aluminum-magnesium alloys in the cold worked condition.

Aluminum-magnesium alloys which are adapted to being worked generally contain less than 8% by weight of magnesium, and often minor amounts of other elements. This type does not respond to solution and precipitation hardening treatments to a sufficient degree to effect an increase in strength to warrant the cost thereof and hence reliance is placed upon cold working as a means of developing a strength above that of a casting or annealed wrought product. While these alloys have given satisfactory service in many instances it has been found that under some severe corrosive conditions, those which are in the cold worked or strain hardened temper do not possess adequate resistance to stress corrosion. The term stress corrosion as here employed refers to the combined effect of corrosion and sustained high tensile stresses, and includes the condition where corrosion is accelerated by stress. The stresses which are considered here are of external origin as distinguished from those existing within the metal body although the latter may be a contributing factor in the end result.

It is well known that a relatively large amount of magnesium is soluble in solid aluminum, under equilibrium conditions and that the commercial aluminum-magnesium alloys in the as-fabricated condition generally contain more magnesium in solution at room temperature than indicated on the constitutional diagram of the aluminummagnesium system. Such super-saturation may be the result of chilling conditions existing during casting of the ingot and may be further aided by the intermediate annealing operations incident to the production of wrought articles. If the alloys in that condition are subsequently cold worked it has been observed that there is a distinct tendency for the magnesium-containing constituent to precipitate at room temperature over a period of time and that such a precipitate lowers the resistance to stress corrosion. The precipitate is generally concentrated at the grain boundaries and consequently corrosion occurs under corrosive conditions because of a difference in solution potential between the grain boundary precipitate and the body of the grain. This invention is directed to altering the distribution of any precipitate and, more particularly, minimizing the concentration of it at the grain boundaries.

It is an object of this invention to provide a method of treating aluminum-magnesium alloy articles which materially improves their resistance to stress corrosion.

Another object is to provide a method of making a cold worked aluminum-magnesium alloy article which possesses a high resistance to stress corrosion.

Still another object is to provide a cold worked article of aluminum-magnesium alloy which has a high resistance to stress corrosion.

These and other objects and advantages will become apparent from the following description and examples.

I have discovered that aluminum-magnesium alloy articles can be made, which in the cold worked condition not only possess the strength which has characterized such articles in the past, but which are substantially free from stress corrosion. This result is accomplished through a sequence of working and heating steps as more particularly described below. From the standpoint of the internal structure of the article, the aluminum-magnesium constituent which is present in the form of finely divided particles of microscopic size is uniformly distributed throughout the metal body and within the grains or grain fragments rather than being localized at the grain boundaries. The several steps in my process and their sequence are shown in the accompanying flow diagram.

My invention is applicable to those aluimnum-magnesium alloys which contain from 4 to 8% by weight of magnesuim which exceeds the amount that is soluble in aluminum at room temperature under equilibrium conditions. At least 4% magnesium must be present to provide the desired level of strength but more than 8% makes working too difficult to be economical from a commercial standpoint. In the preferred practice of my invention the magnesium content of the alloys should be between 4.5 and 7%. While the binary alloy can be used, it is frequently desirable to include small amounts of other elements, for example, an element of the group composed of 0.1 to 1% manganese and 0.1 to 0.25% chromium. Zinc may be included in amounts of 0.1 to 1%. To refine the grain size of the alloy at the ingot stage it may be advisable to add from 0.05 up to 0.2% titanium. Also, to minimize oxidation of the molten metal it is helpful to add from 0.001 to 0.05% beryllium to the alloy. Up to 0.2% copper, up to 0.5% iron and up to 0.5% silicon can be tolerated as impurities.

The alloy may be melted, cast in ingot form and hot worked in accordance with conventional practices employed in the art. More specifically, the ingots should be heated to between 800 and 1050 F. and then hot worked. Generally, rolling is the operation best adapted to produce stock for cold working but instead it may be forged, extruded, pressed or otherwise deformed by pressure. It is essential in any case that the hot working be carried only far enough to allow at least a further reduction in cross section of 30% by cold working. A greater reduction by cold working is generally preferred, however.

Before the hot Worked product is cold worked, it is usually advisable to subject the product to an intermediate annealing treatment to remove any work hardening strains introduced by the hot working operation. The intermediate annealing generally involves heating the worked article to between 600 and 800 F. and holding within that temperature range for one to four hours. This treatment along with the preheating the ingot preparatory to hot working causes a substantial solution of any undissolved particles of aluminum-magnesium constituent. Moreover, the rate of cooling from the treating temperatures is usually sufliciently rapid to retain a large part of the con stituent in solid solution. Neither the hot. working operation mentioned above nor the cold working described be low substantially alters the amount of constituent held in solution at the conclusion of the working operation. It is important in any case that at least a major portion of the magnesium be in solution. The thermal treatment incident to working the alloy articles generally produces solution of undissolved particles so that a separate solution treatment is not required. It is essential, however, that the alloy body receive some form of thermal treatment at a high enough temperature to dissolve at least a part of any undissolved magnesium-containing constituent.

The hot worked product, which may or may not have received an intermediate annealing treatment, is cold worked with a reduction in cross section of at least 20%. The cold working may be done by rolling, drawing, pressing or any of the other methods adapted to deform the metal and produce strain hardening. It is to be understood that in referring to cold work that the term as used herein not only encompasses strain hardening at room temperature but also strain hardening at somewhat elevated temperatures where what is known as equivalent cold working is produced. Thus the alloys may be deformed at temperatures up to 500 F. and still obtain the strain hardening which is essential to the process of the invention. If the reduction in cross section is less than 20%, there is inadequate strain hardening and fragmentation of the grains, particularly if the product has been subjected to an intermediate annealing treatment.

Larger amounts of cold work are preferred in order to obtain further fragmentation of the grains, but there is no upper limit other than that imposed by the dimensions of the finished product.

The foregoing cold worked article is next subjected to thermal treatment to produce the desired precipitation of the aluminum-magnesium constituent to improve the resistance to stress corrosion. The treatment consists of heating the article to a temperature between 400 and 525 F. for a period of from 2 to 24 hours. The temperature employed is related to the magnesium content, the relationship being a substantially direct one. Thus, alloys containing 4% magnesium should be treated at about 400 F. while the alloys containing the maximum amount of magnesium, i.e. 8%, should be treated at about 525 F. and those having between 4 and 8% magnesium are heated to intermediate temperatures as determined by the magnesium content. The temperature range is critical in that at temperatures below 400 F. the precipitate tends to concentrate at the grain boundaries instead of being uniformly distributed. The upper temperature limit will in any case be determined by the magnesium content of the alloy in relationship to the production of a uniform precipitate. Thus, in the case of an alloy containing 8% magnesium, the precipitation temperature should not exceed 525 F. Although it may not be possible to eifect complete precipitation of all the dissolved magnesium in excess of that which is soluble at the temperature of treatment, nevertheless a major portion of the amount which can be precipitated is taken out of solution. The period of treatment is related to the degree of precipitation desired, a longer period generally being used where as nearly complete precipitation as possible at a given temperature is desired and where the temperature is in the lower portion of the range.

The article which has received the precipitation treatment is again cold worked, with a reduction of at least 10%. It is the cold working at this stage which imparts the work hardening and resultant increase in strength which is desired since the precipitation treatment relieves at least some of the previous work hardening strains. To achieve the highest strength the reduction in cross section should be in the neighborhood of 75 to 80%.

To stabilize the strength of the cold worked article it may be given a further thermal treatment at a relatively low temperature which slightly reduces the strength but effectively deters any age softening, a change that often occurs in cold worked aluminum-magnesium alloys. The treatment for articles of the type described above should consist of heating to 150 to 350 F. for 0.5 to 10 hours.

The cold worked alloy articles produced in the foregoing manner show a uniformly distributed precipitate throughout the alloy, there being no concentration at the grain boundaries. These articles have shown an exceptionally high resistance to stress corrosion, and in many cases have been found to be free from stress corrosion.

While the strength of the articles of course varies with the magnesium content and extent of the final cold working step I have obtained tensile strengths within the range of 55,000 to 70,000 p.s.i., yield strengths of 35,000 to 55,000 p.s.i. and elongation values of 10 to These tensile properties compare favorably with those of the same or similar alloys which have been cold worked in the usual manner.

My invention is illustrated in the following examples.

Example 1 An alloy consisting of aluminum, 5.50% magnesium, 0.77% manganese, 0.11% chromium and the usual impurities was melted and cast by conventional practice in the form. of a slab type of ingot which was adapted to being rolled into plate and sheet. The ingot was heated to 915 F. and hot rolled to sheet having a thickness of 0.188 inch. The hot rolled sheet was annealed at 650 F. which removed any previous work hardening strains and then cold rolled to sheet 0.113 inch in thickness which represented a reduction in thickness of 40%. The cold rolled product was subjected to a special precipitation treatment consisting of heating it to 450 F. and holding at that temperature for four hours following which it was further cold rolled to a thickness of 0.057 inch, which represented a reduction of 50%. To stabilize the tensile properties the sheeet was heated to 250 F. and held at that temperature for four hours after which it was cooled to room temperature. Samples were taken from the sheet for tensile and stress corrosion tests. The average tensile strength was found to be 64,200 p.s.i., the yield strength 52,300 p.s.i., and the elongation 10.0%. A portion of the samples were given a sensitizing treatment to stimulate long exposure to room temperature and the coincident precipitation of dissolved constituents. The sensitizing treatment consisted of heating the samples at 212 F. for one week. The corrosion test consisted of stressing samples of both the as-fabricated and sensitized materials under constant deflection to a stress equivalent to of the yield strength and alternately immersing them in a 3.5% NaCl aqueous solution over a period of 950 days. No failures occurred in any of the stressed specimens in either the sensitized or nonsensitized condition.

Example 2 An alloy consisting of aluminum with associated impurities, 6.25% magnesium, 0.51% manganese, 0.11% chromium and the usual impurities was also melted and cast in the same manner as in the preceding example. The ingot was heated to 920 F. and hot rolled to plate having a thickness of 0.25 inch. The plate was annealed at 750 F. and cold rolled to 0.102 inch thick sheet, again annealed at 750 F. and cold rolled to sheet 0.061 inch in thickness which represented a reduction of 40% from the thickness of the annealed sheet. The sheet product was heated to 450 F. and held at that temperature for 12 hours after which it was cold rolled with a reduction of 20% to a thickness of 0.049 inch. Finally, it was stabilized by heating to 250 F. for a period of two hours. The average tensile properties of the stabilized sheet specimens were tensile strength 56,700 p.s.i., yield strength 38,900 p.s.i., and elongation 15.0%. These values are lower than those in the preceding example because of the smaller reduction in thickness following the last intermediate anneal even though the magnesium content was slightly higher. A portion of the sepcimens were sensitized for the corrosion test which in this case consisted of bending both as-fabricated and sensitized specimens over a radius of inch and stressing the specimens by constant deflection across a 3 inch span between supports. The stressed spcimens were alternately immersed in a 3.5% NaCl aqueous solution over a period of 349 days. No failures occurred in the specimens in either condition.

Example 3 Another alloy consisting of aluminum, 4.91% magnesium, 0.48% manganese, 0.11% chromium and the usual impurities was melted and cast into an ingot as in the preceding examples. The ingot was preheated, hot rolled and cold rolled to sheet 0.061 inch in. thickness as in Example 2. The cold rolled sheet was given a precipitation treatment by heating it to 450 F. for four hours following which it was further cold rolled with a reduction of 20%. This product was stabilized by heating it to 250 F. for a period of two hours. The sheet specimens treated in this manner were found to have an average tensile strength of 55,950 p.s.i., a yield strength of 41,950 p.s.i. and an elongation of 12.0%. These values reflect the effect of a lower magnesium content than in Example 2. The corrosion test employed consisted of stressing tensil test specimens in the as-fabricated and sensitized conditions by constant deflection to a stress equivalent to 75% of the yield strength and alternately immersing them in a 3.5% NaCl aqueous solution over a period of 166 days. No failures occurred in specimens in either the as-fabricated or sensitized condition.

Example 4 The performance of an alloy similar to that in Example 1 which did not receive the special precipitation treatment is shown in the following test. The alloy consisted of aluminum, 5.33% magnesium, 0.80% manganese, 0.11% chromium and the usual impurities and was cast in the same manner as the alloy in Example 1. The ingot was preheated, hot rolled, annealed and cold rolled according to the same schedule as that followed in Examples 2 and 3 which resulted in a sheet 0.061 inch in thickness. The cold rolled sheet was stabilized by heating it for four hours at 250 F. The sheet treated in that manner had a tensile strength of 59,200 p.s.i., a yield strength of 45,400 p.s.i. and an elongation of 11.5% which reflects the effect of a smaller reduction in thickness after the last intermediate anneal than in Example 1. The resistance to stress corrosion was tested by exposing specimens in the stabilized and sensitized conditions to alternate immersion in a 3.5% NaCl aqueous solution where the specimens were placed under a stress equivalent to 75% of the yield strength. The stabilized speci mens failed in 46 days while those that had been sensitized failed within 7 days thus amply demonstrating the benefit derived from the percipitation treatment.

Having thus described my invention and certain examples thereof, I claim:

1. The method of improving the resistance to stress corrosion of cold worked aluminum-magnesium alloys consisting essentially of aluminum and 4 to 8% by weight of magnesium comprising the steps of hot working a preheated body of said alloy having at least a major portion of the magnesium in solution, and thereafter cold working said worked product with a reduction in cross section of at least 20%, heating said cold worked product to a temperature within the range of 400 to 525 F. and holding it within that temperature range for a period of 2 to 24 hours whereby a substantially uniformly distributed precipitate of aluminum-magnesium constituent is produced, cooling the so-treated product to room temperature and cold working it with a reduction in cross section of at least 2. The method according to claim 1 wherein the temperature within the precipitation temperature range of 400 to 525 F. is substantially directly related to the magnesium content of the alloy, such that as the magnesium content increases from 4 to 8%, the temperature of the precipitation treatment increases in the same proportion between 400 and 525 F.

3. The method according to claim 1 wherein the tensile properties of final cold worked product are stabilized by heating it to a temperature between 150 and 350 F. for a period of 0.5 to 10 hours.

4. The method according to claim 1 wherein the alloy contains from 4.5 to 7% magnesium.

5. The method according to claim 1 wherein the alloy also contains at least one of the elements of the group composed of 0.1 to 1% manganese and 0.1 to 0.25% chromium.

6. The method according to claim 1 wherein the alloy also contains 0.1 to 1% zinc.

7. The method according to claim 1 wherein the alloy also contains from 0.05 up to 0.2% titanium.

8. The method according to claim 1 wherein the alloy also contains from 0.001 to 0.05% beryllium.

9. The method according to claim 1 wherein the hot worked product is annealed at 600 to 800 F. before it is cold worked in the first-mentioned cold working step.

10. The method of improving the resistance to stress corrosion of cold worked aluminum-magnesium alloys consisting essentially of aluminum and 4 to 8% by weight of magnesium comprising heating a body of said alloy to a high enough temperature to produce solution of undissolved magnesium, shaping said body by hot deformation thereof, cold working said body with a reduction in cross section of at least 20%, heating said cold Worked product to a temperature within the range of 400 to 525 F. for a period of 2 to 24 hours whereby a uniformly distributed precipitate of aluminum-magnesium constituent is produced, cooling to room temperature and cold working with a reduction in cross section of at least 10%.

11. The method according to claim 10 wherein the tensile properties of the final cold worked product are stabilized by heating to a temperature between and 350 F. for aperiod of 0.5 to 10 hours.

12. The method according to claim 10 wherein the first-mentioned heating step consists of heating the alloy body to between 800 and 1050 F.

13. The method according to claim 10 wherein the hot deformed product is annealed at 600 to 800 F. before it is cold worked in the first-mentioned cold working step.

14. The method of improving the resistance to stress corrosion of cold worked aluminum-magnesium alloys consisting essentially of aluminum and 4 to 8% by weight of magnesium comprising the steps of preheating a body of the alloy to a temperature within the range of 800 to 1050 F. until at least a major portion of the magnesium is in solution, hot working said preheated body, annealing said hot worked product at 600 to 800 F., cold working the annealed product with a reduction in the cross section of at least 20%, reheating the cold worked product to a temperature within the range of 400 to 525 F. for a period of 2 to 24 hours whereby a uniformly distributed precipitate of aluminum-magnesium constituent is produced, cooling to room temperature and cold working with a reduction in cross section of at least 10%.

References Cited by the Examiner UNITED STATES PATENTS 1,926,057 9/1933 Nock et a1. 148-115 2,063,022 12/1936 Beck 148-115 2,841,512 7/1958 Cooper 148-115 3,031,299 4/1962 Criner 148-115 DAVID L. RECK, Primary Examiner.

RAY K. WINDHAM, HYLAND BIZOT, Examiners.

Claims (1)

1. THE METHOD OF IMPROVING THE RESISTANCE TO STRESS CORROSION OF COLD WORKED ALUMINUM-MAGNESIUM ALLOYS CONSISTING ESSENTIALLY OF ALUMINUM AND 4 TO 8% BY WEIGHT OF MAGNESIUM COMPRISING THE STEPS OF HOT WORKING A PREHEATED BODY OF SAID ALLOY HAVING AT LEAST A MAJOR PORTION OF THE MAGNESIUM IN SOLUTION, AND THEREAFTER COLD WEORKING SAID WORKED PRODUCT WITH A REDUCTION IN CROSS SECTION OF AT LEAST 20%, HEATING SAID COLD WORKED PRODUCT TO A TEMPERATURE WITHIN THE RANGE OF 400 TO 525* F. AND HOLDING IT WITHIN THAT TEMPERATURE RANGE FOR A PERIOD OF 2 TO 24 HOURS WHEREBY A SUBSTANTIALLY UNIFORMLY DISTRIBUTED PRECIPITATE OF ALUMINUM-MAGNESIUM CONSTITUENT IS PRODUCED, COOLING THE SO-TREATED PRODUCT TO ROOM TEMPERATURE AND COLD WORKING IT WITH A REDUCTION IN CROSS SECTION OF AT LEAST 10%.