US 3836405 A
An aluminum base alloy containing zinc, copper and magnesium together with manganese and with specially controlled composition limits exhibits very high strength when thermally treated to a condition having high resistance to stress corrosion cracking. Improved products of the alloy also exhibit low quench sensitivity and, accordingly, high strength even in very thick sections. A special aging treatment produces the optimum combination of strength and resistance to stress corrosion cracking properties.
Description (OCR text may contain errors)
Staley et a1.
[ ALUMINUM ALLOY PRODUCT AND METHOD OF MAKING  Inventors: James T. Staley, Lower Burrell;
Harold Y. Hunsicker, New Kensington; Robert H. Brown, Natrona Heights, all of Pa.
 Assignee: Aluminum Company of America, Pittsburgh, Pa.
 Filed: July 20, 1972  Appl. No.: 273,842
Related US. Application Data  Division of Ser. No. 60,343, Aug. 3, 1970,
[451 Sept. 17, 1974 Primary Examiner-W. W. Stallard Attorney, Agent, or Firm-Carl R. Lippert 5 7 ABSTRACT An aluminum base alloy containing zinc, copper and magnesium together with manganese and with specially controlled composition limits exhibits very high strength when thermally treated to a condition having high resistance to stress corrosion cracking. Improved abandoned products of the alloy also exhibit low quench sensitiv-  U S Cl 148/12 7 148/159 ity and, accordingly, high strength even in very thick  m 1/04 sections. A special aging treatment produces the opti-  Fie'ld 159 325 mum combination of strength and resistance to stress corrosion cracking properties.  References Cit d This invention relates to improved wrought products UNITED STATES PATENTS and to a method of their production.
3,133,839 5/1964 Thomas 148/127 6 Claims, 1 Drawing Figure 70 E Q) R u k 65 Q \1 o X l l l l 1 0 .5 I5 20 25 HOURS AGING PAIENIEB SEP 1 11914 HOURS AGING 0 7 Q IRQEWQK 6 .m QQWTA ALUMINUM ALLOY PRODUCT AND METHOD OF MAKING This is a division of application Ser. No. 60,343, filed Aug. 3, 1970 and now abandoned.
BACKGROUND OF THE INVENTION Aluminum alloys containing 3 to 8 percent Zn, 1.5 to 4 percent Mg and 0.75 to 2.5 percent Cu are known for their high strength to weight ratio which renders them highly suited for use in applications such as structural components for aircraft. Alloy 7075 is an example of this type of alloy and has achieved wide spread use in aircraft because of its very high strength and other desirable properties. Alloy 7075 contains 5.1 to 6.1 percent Zn, 2.1 to 2.9 percent Mg, 1.2 to 2 percent Cu, 0.18 to 0.40 percent Cr, the balance being aluminum together with incidental elements and impurities. In relatively thick members such as forgings a few inches thick, 7075 develops typical tensile and yield strength levels of, respectively, 80 and 68 ksi when artificially aged to its highest strength, which aging treatment usually contemplates an extended period of 20 hours or more at a relatively low aging temperature of about 225F. In this temper which is often referred to as a T6 temper, however, alloy 7075 and similar alloys are sometimes objectionable because of susceptibility to stress corrosion cracking which is improved by a twostep aging cycle as described in U.S. Pat. No. 3,198,676 where, in addition to the lower temperature aging treatment the members are subjected to a second treatment at a higher temperature of about 350 but for a comparatively short time which is usually under hours. This treatment, which produces what can be termed a T7 type temper, markedly improves resistance to stress corrosion cracking but results in a 15 percent loss in strength which, for many applications, is acceptable. This loss in strength is due to the fact that during the second, higher temperature, aging treatment while resistance to stress corrosion cracking is increasing up to the desired level, the strength is being diminished at a substantial rate as the second step proceeds. Attempts to optimize the strength and resistance to stress corrosion cracking by the most exacting controls during the second aging step have met with varying degrees of success. Guarantying the best strength levels together with high resistance to stress corrosion cracking often involves very high rejection rates which are obviously objectionable and which equally obviously increase the cost of the acceptable products.
The foregoing disadvantages of existing 7000 series alloys are overcome by the practice of the invention wherein special composition controls together with a higher temperature aging treatment which may or may not be preceded by a lower temperature aging treatment combine to produce very high resistance to stress corrosion cracking together with a very high strength level. In fact the improved products in accordance with the invention exhibit their maximum strength levels at the same temper at which they demonstrate high resistance to stress corrosion cracking which is a unique property in 7000 series alloys.
DESCRIPTION In this description reference is made to the drawing which is a graph of aging time versus strength;
In practicing the invention the desired combination of properties is achieved by carefully controlling the composition of the aluminum alloy which consists essentially of, by weight 4.5 to 8 percent Zn, 1.7 to 3.25 percent Cu, 1.4 to 2.6 percent Mg, the balance being aluminum and impurities and incidental elements. The copper to magnesium ratio must be at least 0.85 and preferably at least 1.0. The alloying elements are somewhat interrelated and the following further limits also apply thereto. The weight percent of Mg plus Cu should not exceed 5.4 percent and Mg plus 0.2 times Cu is at least 2. The total of all three principal alloying elements, Zn, Cu and Mg is between 8.75 percent and 12.25 percent. In addition to the foregoing the alloy contains 0.3 to 0.75 percent Mn. It is important in practicing the invention that chromium be carefully controlled and be present only in amounts of up to but not to exceed 0.04 percent. Impurities are preferably limited as follows: 0.35 percent Fe, 0.25 percent Si, 0.06 percent Ti and 0.02 percent V. The foregoing alloy surprisingly develops its maximum strength during relatively high temperature artificial aging treatments needed to develop high resistance to stress corrosion cracking in direct contrast to other commercial 7000 series type alloys of high strength which have their high strength levels diminished considerably during that thermal treatment.
The improved alloy is especially useful in the form of forgings, which may be of the hand or die forged type, rolled products such as sheet or plate or in the form of extruded products. The improved material exhibits a surprising lack of yield strength sensitivity to quench rate which enables the realization in members of substantial thickness of the same general strength levels achieved in members of relatively thin cross section. This is in contrast with other 7000 series alloy materials which exhibit substantial decreases in yield strength with increasing thickness in section.
In fabricating improved products there are no especially cumbersome or intricate procedures required. A body of the desired composition is provided which may be a continuously direct chilled cast ingot. The ingot is subjected to an elevated temperature of about 860F for a period sufficient to homogenize its internal structure and provide an essentially uniform distribution of the alloying elements. The alloy body is then subjected to hot working and, if desired, cold working operations to produce the desired product. As mentioned above these working operations may include forging, rolling, extrusion and other known metal working procedures useful in producing aluminum alloy products. Of course the conventional intermediate annealing or reheating operations can be employed if they facilitate ease of fabrication. The products may be of relatively thick cross section, for example, two to four inches or more in thickness, or they may have relatively thin sections of less than A inch. Irrespective of thickness the members will exhibit a uniformity of high yield strength foreign to other 7000 series alloy products where strength diminishes considerably with increasing thickness which strength loss is attributed to the quench sensitivity of these alloys.
The product is typically solution heat treated at a temperature of 860F'or higher for a sufficient time for solution effects to approach equilibrium and then quenched. Quenching may be accomplished in a number of ways in view of the surprising lack of quench sensitivity possessed by the improved products. For instance, the products can be quenched by spraying with cold water, immersion in room temperature water or immersion in relatively hot water of, for instance, 180F or even boiling water.
The solution heat treated and quenched product is then aged to develop its strength and other properties. Aging is accomplished by heating to a temperature of between 300 and 380F for a period of at least one hour but preferably not over 70 hours. To develop the best combination of properties, mechanical and stress corrosion performance, the minimum aging time is determined in accordance with the equation shown below although the minimum time never is less than one hour.
Log t= 11.9 X /T 450 l4.6
where f= time in hour s and T temperature, F.
While the invention contemplates a relatively high temperature aging treatment, this treatment may or may not be preceded by a lower temperature treatment at a temperature of, typically, between 200 and 270F. That is a two-step aging treatment of the type described in U.S. Pat. No. 3,198,676 may be employed provided the second aging step is in accordance with the above set forth equation with respect to time and temperature. However, the earlier aging step is not necessary and, surprisingly, the improved product can be aged to its optimum properties in a single aging step of relatively modest time which results in obvious economies in furnace utilization.
To illustrate the benefits of the invention and the importance of the herein described careful composition control and aging treatment the following examples proceed.
EXAMPLE 1 Two-inch thick plates of varying alloy composition were produced by casting large ingots which were hot rolled to produce the two-inch plate products. Before rolling, the ingots were homogenized at a temperature of about 880F and after rolling, the plates were solution heat treated at temperature around 890F and then quenched by immersion in room temperature water. The plates were first aged for 24 hours at 250F and then subjected to a second aging step at a temperature of 325F for varying amounts of time. The compositions of the materials tested are listed in Table I and the long transverse yield strength values are plotted versus the aging time at 325F in FIG. 1. The curve identifications in FIG. 1 correspond to the composition designations in Table I.
Referring to Table I and FIG. 1, material D is conventional 7075 alloy and it can be seen that the highest yield strength occurs with no aging time at 325F. As the 7075 plate is exposed to the higher temperature aging treatment its strength decreases at a considerable rate. Material A would be in accordance with the invention except that it contains excessive chromium and it too exhibits a decrease in yield strength during the higher temperature aging treatment with its maximum strength level being realized before any higher temperature aging treatment. Material B, on the other hand, is in accordance with the invention and, it is readily apparent from curve B in FIG. 1 that maximum strength values are achieved and maintained between about 5 and 6 and 12 hours in the 325 elevated temperature aging treatment, with quite high strength levels being maintained up to 24 hours and longer. This corresponds to the equation which indicates a minimum aging time of 5.6 hours for the 325F treatment to achieve the best combination of properties. It is during this higher temperature aging treatment that high resistance to stress corrosion resistance is achieved in all the materials listed in Table I although, it is quite clear, that this property must be traded off against strength for materials A and D, which are similar in this respect to the prior art, but not for material B which are representative of the invention.
EXAMPLE 5 Three-inch forgings were produced from an alloy in accordance with the invention by providing continuously cast bodies of the alloy, homogenizing at 860F and forging them into three-inch thick forgings at a temperature of about 650F after which the forgings were solution heat treated and quenched by immersion in cold water. The alloy composition was 5.92 percent Zn, 2.16 percent Mg, 2.41 percent Cu, 0.33 percent Mn and 0.01 percent Cr, balance essentially aluminum. Several forgings were aged under different conditions and long transverse tensile and short transverse stress corrosion tests were performed. The stress corrosion tests were performed for 16 months in an industrial atmosphere and the stresses applied ranged up to approximately '75 percent of the yield strength.
From Table IV it is clear that a first aging step at a relatively low temperature is not necessary in practicing the invention. On the contrary a single aging treatment at a relatively high aging temperature is sufficient. Nonetheless where desired the higher temperature aging treatment may be preceded by a lower temperature aging step without losing the advantages of the in vention. A single low temperature aging treatment, however, falls short both on yield strength and resistance to stress corrosion cracking. In addition it is worth noting that extended time periods during the elevated temperature aging treatment are not necessary. The equation set forth herein in the description indicates that a two hour time at a temperature of 350F is adequate for the three-inch forging and Table IV, treatment No. l verifies such is correct. Nonetheless much longer aging times may be employed without excessive strength loss which indicates that once the minimum time set forth above is reached the improved products can be maintained at the higher aging temperature without rapidly losing strength as was the case with prior 7000 type alloy products. This provides a time tolerance highly useful in large plant furnaces.
EXAMPLE 5 To illustrate the effect of quench rate, several alloy compositions in accordance with the invention were fabricated into thick plate and were aged for 24 hours at 250F plus 18 hours at 325F. Prior to aging some of the members were quenched by immersion in water at room temperature, CWQ, whereas others were quenched in hot water at 175F, HWQ. Table V sets forth the compositions tested together with the yield strength and long transverse percent elongation for each quench condition. In viewing the table the surprising improvement in elongation accompanied by minimal decrease in strength achieved with hot water quenching appears to be peculiar and unique to the improved product. This contrasts with the usual What is claimed is:
1. A method of producing an improved aluminum alloy product comprising 1. providing a body composed of an alloy consisting essentially of 4.5 to 8 percent Zn, 1.7 to 3.25 percent Cu, 1.4 to 2.6 percent Mg, the total of Zn Cu Mg being from 8.75 percent to 12.25 percent, Cu Mg being 5.4 percent maximum with 0.2 Cu Mg being 2 percent minimum, the Cu/Mg ratio being at least 0.85, and 0.3 to 0.75 percent Mn, chromium not to exceed 0.04 percent, the balance being aluminum and impurities and incidental elements,
2. working said body to produce said product,
3. solution heat treating said product,
4. quenching said product,
5. subjecting said product to an artificial aging treatment which includes an exposure at an elevated metal temperature to develop resistance to stress corrosion cracking.
2. In the method of producing a solution heat treated, quenched and artificially aged aluminum alloy wrought product in an aluminum alloy of the type containing zinc, magnesium and copper as the major alloying additions wherein the alloy is worked to a wrought condition, solution heat treated, quenched and artificailly aged, the improvement wherein said alloy is provided as a composition consisting essentially of 4.5 to 8 percent Zn, 1.7 to 3.25 percent Cu, 1.4 to 2.6 percent Mg, the total of Zn Cu Mg being from 8.75 percent to 12.25 percent, Cu Mg being 5.4 percent maximum with 0.2 Cu Mg being 2 percent minimum, the Cu/Mg ratio being at least 0.85, 0.3 to 0.75 percent Mn, chromium not to exceed 0.04 percent, the balance being aluminum and impurities and incidental elements, said improvement imparting to said wrought product the ability to achieve high resistance to stress corrosion cracking substantially in the condition of maximum strength and low sensitivity in strength to quench rate and good toughness.
3. In the method according to claim 2, the further improvement wherein the artificial aging treatment includes exposing said alloy to a temperature of between 300 and 380F for at least one hour, the minimum time being further determined by the following equation:
Log t= 11.9 X 1O /T 450 14.6
where t time in hours and T temperature in F 4. In the method according to claim 2 wherein the alloy composition is further controlled such that Fe does not exceed 0.35 percent, Si does not exceed 0.25 percent, V does not exceed 0.02 percent and Ti does not exceed 0.06 percent.
5. In the method according to claim 2 wherein the alloy composition is controlled such that the ratio Cu/Mg is equal to at least 1.
6. The method according to claim 1 wherein said artificial aging treatment exposure is at a metal temperature of between 300 and 380F and for a time of at least one hour the minimum time being further deter mined by the following equation:
Log t= 11.9 X l0 /T 450-14.6
where t= time in hours and T temperature in F U TED STATES PATENT @FFHQE QERTIFEQATE FF CORQ'EWN Eatent No. 3,836,405 Dated September 17, 19.74
I Inventor(s) James T. Sta ley Harold Y. Hunsicker and RoBert'I-i. Brown 1 It is certified that error appears in the above-identified patent and that said Letters Patentare hereby corrected as shown below:
Col. 3, line 1. 5 m t 11.9 X 10 /1: 450 44.6" should read 3 --Log t 11.9 X 1Q -l z-.6--.
T'T ES'G $1101. 3 after Insert Table I, as follows:
line 59 me ABLE I Material Q 11g 2 LI r Q01. 4, line 12 Change "Example 5" to -Example 2-.
5 Col. 4, line 64 Delete Example 5. through Col. 5,
line 10 fiigme and seale this 26th day of November 19%.
seam M. GIBSON JR. 0. SHALL DANN attesting- Officer Commissioner of Patents RM PO-1050 (10-69) USCOMM-DC 60376-5 69 31' U. 5. GOVERNMENT PRINTING OFFICE I969 0-365-334.