US3253965A

US3253965A - Thermal treatment of aluminum base alloy articles

Info

Publication number: US3253965A
Application number: US308085A
Authority: US
Inventors: Charles B Criner
Original assignee: Aluminum Company of America
Current assignee: Howmet Aerospace Inc
Priority date: 1963-09-11
Filing date: 1963-09-11
Publication date: 1966-05-31
Anticipated expiration: 1983-05-31
Also published as: NL6410481A; BE652897A; DE1300302B; GB1023674A

Description

United States Patent 3,253,965 THERMAL TREATMENT OF ALUMINUM BASE ALLOY ARTICLES Charles B. Criner, Southbury, Conn., assignor to Alumiuum Company of America, Pittsburgh, Pa., :1 c0rporation of Pennsylvania No Drawing. Filed Sept. 11, 1963, Ser. No. 308,085 7 Claims. (Cl. 148-11.5)

This application is a continuation-in-part of my application Serial No. 98,295, filed March 27, 1961, now abandoned.

This invention relates to improving the tensile strength and resistance to corrosion of articles of certain thermally treated aluminum base alloys and it is particularly concerned with the interposition of a cold working step before the final thermal treatment of the articles.

It is Well known that aluminum base alloys containing certain elements, which are soluble in solid aluminum, respond to the thermal treatment, known as solution heat treatment, which brings about a solution of at least a portion of undissolved elements or intermetallic compounds. When such alloys are rapidly cooled, i.e. quenched, from the solution heat treating temperature, a metastable condition is created. If the alloys in that condition are subjected to a further thermal treatment, by heating to a temperature somewhat above room temperature, at least a part of the dissolved elements or compounds is precipitated with a resultant increase in strength and hardness as compared to the untreated alloy or even the alloy which has only received a solution heat treatment.

It has also been recognized that aluminum base alloys containing magnesium in combination with other ele ments, which have a substantial solubility in solid aluminum, can be cold worked following the quenching and before the precipitation treatment with a resultant in crease in the tensile and yield strengths, as compared to the alloys which have not received such a cold working. In referring to cold work it is to be understood that this refers to any of the operations such as rolling, pressing, drawing, and the like which effect a reduction in cross sectional thickness of the alloy article where those operations are conducted at room temperature or close to that temperature and the article becomes work hardened. The term cold work also embraces operations which create work hardening strains with little or no reduction in cross sectional thickness. For example, flattening or stretching of a warped article introduces work hardening strains. Alloys of the foregoing type respond relatively rapidly to the precipitation treatment, either with or without any intermediate cold working, and consequently have presented no difiiculty in attaining a high strength. In such alloys that have been artificially aged the cold working does not improve the resistance to corrosion.

Aluminum base alloys composed essentially of aluminum and 4 to 7% by weight of copper and free from magnesium do not follow the pattern of those alloys referred to hereinabove in the response to the precipitation treatment and the increase in strength attained by that treatment where cold working precedes precipitation. Furthermore, this difference in response adversely affects the resistance to corrosion in the absence of cold work. As a consequence, alloys of this type have enjoyed but limited use although they possess advantageous properties in other respects than strength at room temperature and resistance to corrosion.

My invention is directed .to improving the strength and resistance to corrosion of articles of precipitation hardened aluminum-copper alloys of the foregoing type Patented May 31, 1956 ice and has as its primary object the provision of a method for treating articles of such alloys. It is also an object of this invention to provide a method of treating a wrought article or a portion thereof composed of an essentially binary aluminum-copper alloy whereby a higher strength and a better resistance to corrosion is developed than in the same alloy which has not received any cold work before the precipitation hardening treatment.

I have discovered that the strength, particularly the yield strength, of articles of essentially binary alloys of aluminum and 4 to 7% copper and substantially free from magnesium and zinc can be considerably improved and the resistance to corrosion simultaneously increased by interposing a cold working step between solution and the precipitation treatments, the amount of cold work being closely related to the temperature and length of the precipitation treatment. By means of this treatment it has been possible to develop a tensile strength on the order of 69,000 p.s.i. and a yield strength of 53,000 p.s.i.

with an elongation of 11% as compared to a tensile strength of 60,000 p.s.i., a yield strength of 43,000 p.s.i. and an elongation of 11% when the same alloy is treated in the conventional manner without any intermediate cold work. It has also been found that the resistance to corrosion of the alloy article is improved as compared with the same composition which has been given a precipitation treatment to develop the maximum strength and hardness but without any intervening cold work.

The type of alloy which is improved by my process, as mentioned above, is one that is composed essentially of aluminum and from 4 to 7% by 'Weight of copper and free from magnesium and zinc except as they occur as impurities. The alloy should contain at least 4% copper in order to attain a high strength while on the other hand more than 7% introduces problems in working the alloy and the increased copper content does not produce a significant increase in strength. when the alloy is cold Worked and precipitation hardened. Other elements which are substantially insoluble in solid aluminum may be present in relatively small amounts. For example, at least one of the group of hardening elements consisting of manganese, titanium, vanadium, zirconium, molybdenum, tungsten, chromium, boron, nickel, cobalt, tantalum and niobium can be present in the following amounts: 0.15 to 1.2% manganese, 0.05 to 0.20% vanadium, 0.05 .to 0.30% zirconium, and 0.1 to 0.25% each of titanium, molybdenum, tungsten, chromium, boron, nickel, cobalt, tantalum and niobium. The total amount of these elements with. the exception of manganese, vanadium and zirconium, should not exceed about 0.25%. If the machinability of the alloy is to be improved at least one of the group of low melting point elements consisting of lead and bismuth may be added in amounts of 0.1 to 0.75% each, the total not exceeding 1.5%. Neither of the foregoing elements, as far as is known, adversely affect the physical properties of the alloy, and being substantially insoluble in solid aluminum they do not interfere with the solution and precipitation of the copper.- By the same token the alloys are substantially free from elements such as magnesium and zinc which are soluble in solid aluminum, except as they may occur as impurities. The magnesium impurity content should not exceed 0.02% and the zinc content should not be over 0.25%

The usual iron and silicon impurities can be tolerated but it is advisable to restrict the iron to a maximum of 0.5% and the silicon content should not exceed 0.3%.

The first step in my process consists of a solution heat treatment which consists of heating the alloy to a temperature between 900 and 1050 F. and holding within this range for a period of time, on the order of A to 12 hours, to effect substantially complete solution of the copper. At the end of the holding period the alloy is to be quickly cooled to much lower temperature, generally room temperature or one not far removed from room temperature. The chilling may be accomplished in any one of several conventional ways as by quenching in water, or by a water spray or even an air blast, if the article is not too thick.

Upon attaining room temperature or close to it, the alloy article is strain hardened either by a reduction in cross section by rolling, pressing, drawing or by other known metal working methods or simply by flattening or stretching the article, for example, the work which is done in straightening a warped product. To gain a substantial increase in strength it is necessary to effect a reduction in cross section and generally this should be on the order of at least 1% to gain the desired improvement. More than 20% does not offer any advantage. The amount of cold work that can be employed depends upon the nature of the article and the ease of making reductions, thus, only a relatively small reduction can be made on extrusions and tubing by stretching, but sheet can be reduced by a much larger amount. Generally from 1 to cold work is preferred since most of the advantages are obtained within this range and because in some cases, the size of the product precludes greater amounts of cold work.

The relationship is more precisely expressed in equations related to the yield strength and maximum resistance to corrosion. The equation relating to the yield strength is as follows:

5 ing the cold worked product to a temperature between 300 and 400 F. for a period of 1 to 48 hours, the choice of particular conditions being determined from the equations stated above. At temperatures below 300 F. the

length of time required to gain the desired values becomes too long for practical purposes while above 400 F. the opposite condition prevails, the period of exposure is so short that control becomes difficult.

The alloys to which the above-described treatment is applied should be in the wrought condition, as distinguished from castings. The wrought forms may be produced by any of the conventional methods such as by rolling, forging, extrusion, pressing, and the like. In many instances these are performed at elevated temperatures and hence are referred to as hot working operations as distinguished from those performed at room temperature.

Although the description of the process thus far has involved the treatment of an entire article, it has been found that portions of an article may be treated with benefit by the process. For example, in welding members of this alloywith filler metal of a similar composition, or

even where only the filler metal is of this type of alloy, the welded structure is solution heat treated, quenched, the welded area cold worked and finally precipitation hardened. In this manner the weld bead and worked area adjoining it are improved.

where 1 t=time required to reach maximum yield strength T =precipitation hardening temperature C W=percent cold work 51 B (0.01T) [1 +0.01(CW)0.25]

The calculated length of time represents the minimum that should be used in practice. Exceeding this period by two hours, for example, has but a slight effect on the yield strength, generally less than 3,000 psi. The resistance to corrosion of a product which has been so treated is adequate for many applications.

To insure a maximum resistance to corrosion, with some sacrifice in yield strength, the minimum period of precipitation hardening should be determined in accordance with the following equation: 55

where t=time required to reach solution potential of 800 millivolts T=precipitation hardening temperature CW: percent cold work The maximum resistance to corrosion is here considered to be attained when the solution potential of the alloy has a value of not less than 800 millivolts as determined in a standard aqueous solution of 3 /2 NaCl containing 0.3% by volume of H 0 as measured against a standard calomel half cell.

- r In respect to the relationship of cold work to precipita- My invention is illustrated in the following examples.

Example 1 An alloy nominally composed of aluminum, 6.5% copper, 0.25% manganese, 0.1% vanadium, 0.15% zirconium 40 and 0.05% titanium was melted, cast into ingot form, hot

rolled to plate thickness, annealed and cold rolled to sheet 0.064 inch in thickness. Samples taken from the sheet were given a solution heat treatment at 1000 F. for /2 hour and quenched in cold water. One portion of the samples (A) was heated for 36 hours at 375 F. to produce precipitation hardening. A second portion (B) was cold rolled with a reduction in thickness of 1.5% and then heated to 350 F. for 18 hours which exceeds the minimum time determined from Equation 2. A third portion (C) was cold rolled with a reduction of 10% and precipitation hardened by heating to 325 F. for 14 hours which is close to the time calculated from Equation 2. The average tensile properties of each of these groups is given in Table I below.

TABLE I.TENSILE PROPERTIES OF AlCu-MnVZr-Ti ALLOY The improvement in yield strength produced by the cold Work is especially notable.

Samples of the sheets were also exposed to standardized corrosion tests wherein some of the samples were placed under a stress equivalent to 75% of the yield strength while other samples were not subjected to any stress. Both stressed and unstressed samples were alternately immersed and raised from an aqueous solution of 3.5% NaCl over a period of 12 weeks. At the conclusion of the 12 week period the samples were subjected to a tensile test and the loss in strength as compared to that of samples of the original material was noted. The percentage loss in strength of the specimens is given in Table II below.

TABLE II.-PERCENT LOSS IN TENSILE STRENGTH FROM CORROSION Unstressed Stressed of an alloy consisting essentially of aluminum and 4 to 7% by weight of copper, and free from magnesium and zinc except as impurities, said method comprising heating said articles to between 900 and 1050 F. for a period It is to be seen that the cold work not only increased the strength of the alloy but that the resistance to corrosion was improved appreciably which is unique among solution heat treated and precipitation hardened aluminum base alloys.

Example 2 The benefit of cold working another type aluminumcopper alloy is illustrated in the case of an alloy nominally composed of aluminum, 5.5% copper, 0.5% lead and 0.5% bismuth. An ingot of this alloy was hot rolled to rod form 2 inches in diameter. Sections were cut from the rod, solution heat treated at 975 F. for 2 /2 hours and quenched in cold water. One group (D) was heated to 320 F. and held for 14 hours while the second group (B) was cold drawn with a reduction in cross section of 20% before beingheated to 320 F. and held at that temperature for 14 hours which is close to the time determined according to Equation 2. The average tensile properties of the samples taken in a longitudinal direction are given in Table III below.

TABLE IIL-TENSILE PROPERTIES OF Al-Cu-Pb-Bi ALLOYS Tensile Yield Percent Group Strength, Strength, Elongap.s.i. p.s.i. tion Specimens from the respective bars were exposed to same alternate immersion test described above in both the stressed and unstressed conditions. The losses in tensile strength are given in Table IV.

TABLE IV.PERCENT LOSS IN TENSILE STRENGTH FROM CORROSION Unstressed Stressed and resistance to corrosion of wrought articles composed 75 where t=time required to reach maximum yield strength T=precipitation hardening temperature C W=percent cold work.

2. The method according to claim 1 wherein the alloy also contains at least one of the group of hardening elements composed of 0.15 to 1.5% manganese, 0.05 to 0.20% vanadium, 0.05 to 0.3% zirconium, and 0.01 to 0.25% of titanium molybdenum, tungsten, chromium, boron, nickel, cobalt, tantalum and niobium, the total amount of said elements except for manganese, vanadium and zirconium not exceeding 0.25

3. The method according to claim 1 wherein the alloy also contains at least one of the low melting point elements of the group composed of lead and bismuth in amounts of 0.1 to 0.75% each, the total not exceeding 1.5%.

4. The method according to claim 1 wherein the cold working is within the range of l to 10%.

5. The method of improving both the yield strength and resistance to corrosion of welded articles in the welded area wherein the filler metal is composed of an alloy consisting essentially of aluminum and 4 to 7% by weight of copper, and free from magnesium and zinc except as impurities, said method comprising heating said filler metal in said welded area to a temperature between 900 and 1050 F. or a period of A1 to 12 hours, quenching said heated filler metal, cold working the filler metal and the area immediately adjacent thereto from 1 to 20% and thereafter heating said cold worked metal to a temperature between 300 and 400 F. for a period of 1 to 48 hours to induce precipitation hardening, and for a minimum length of time needed to develop a maximum yield strength, said time being determined from the equation:

where t=time required to reach maximum yield strength T=precipitation hardening temperature CW=percent cold work 51 B=Ant11g (0.O1T) [1+0.01(CW)- 6. The method of improving the resistance to corrosion and the yield strength of wrought articles composed of an aluminum base alloy consisting essentially of aluminum and 4 to 7% by weight of copper, and free from magnesium and zinc except as impurities, said method comprising heating said articles to between 900 and 1050 F. for a period of A to 12 hours, quenching said articles, cold working said quenched articles from 1 to 20% and thereafter heating said cold worked articles to between 300 and 400 F. for a period of 1 to 48 hours to induce precipitation hardening, and for a minimum length of time required to attain a solution potential of 800 millivolts, said time being determined from the equation:

where t=time required to reach solution potential of 800 millivolts T=precipitation hardening temperature CW=percent cold Work.

7. The method according to claim 6 wherein the cold working is within the range of 1 to 10% References Cited by the Examiner UNITED STATES PATENTS 2,706,680 4/1955 Criner 75139 FOREIGN PATENTS 456,721 5/ 1949 Canada. 458,636 8/1949 Canada. 443,909 3/ 1936 Great Britain. 738,070 10/ 1955 Great Britain.

OTHER REFERENCES Physical Metallurgy of Aluminum Alloys, ASM, 1949, pp'. 37, 204, 205

DAVID L, RECK, Primary Examiner.

HYLAND BIZOT, Examiner.

H. F. SAITO, Assistant Examiner.

Claims

1. THE METHOD OF IMPROVING BOTH THE YIELD STRENGTH AND RESISTANCE TO CORROSION OF WROUGHT ARTICLES COMPOSED OF AN ALLOY CONSISTING ESSENTIALLY OF ALUMINUM AND 4 TO 7% BY WEIGHT OF COPPER, AND FREE FROM MAGNESIUM AND ZINC EXCEPT AS IMPURITIES, SAID METHOD COMPRISING HEATING SAID ARTICLES TO BETWEEN 900 AND 1050*F. FOR A PERIOD OF 1/4 TO 12 HOURS, QUENCHING SAID ARTICLES, COLD WORKING SAID QUENCHED ARTICLES FROM 1 TO 20% AND THEREAFTER HEATING SAID COLD WORKED ARTICLES TO A TEMPERATURE BETWEEN 300 AND 400*F. FOR A PERIOD OF 1 TO 48 HOURS TO INDUCE PRECIPITATION HARDENING, AND FOR A MINIMUM LENGTH OF TIME NEEDED TO DEVELOP A MAXIMUM YIELD STRENGTH, SAID TIME BEING DETERMINED FROM THE EQUATION: COMPLEX FRACTION WHERE