US3598577A - Aluminum base alloy - Google Patents

Abstract

AN ALUMINUM BASE ALLOY, SUITABLE FOR CASTING USEFUL ARTICLES CONTAINING ALUMINUM AND 3 TO 6% COPPER, 2 TO 5% ZINC, 0.2 TO 1.5% MAGNESIUM, 0.2 TO 0.6% MANGANESE, AND GRAIN REFINING ADDITIONS, EXHIBITS, WHEN SOLUTION HEAT TREATED, QUENCHED AND ARTIFICIALLY AGED, HIGH ROOM AND ELEVATED TEMPERATURE STRENGTH WHICH MAY BE COUPLED WITH SUBSTANTIAL IMMUNITY TO STRESS CORROSION CRACKING.

Description

United States Patent 3,598,577 ALUMINUM BASE ALLOY Edward E. Stonebrook, Cleveland, Ohio, assignor to Aluminum Company of America, Pittsburgh, Pa. N0 Drawing. Filed Aug. 23, 1967, Ser. No. 662,584 Int. Cl. C22c 21/00 US. Cl. 75-141 Claims ABSTRACT OF THE DISCLOSURE An aluminum base alloy, suitable for casting useful articles containing aluminum and 3 to 6% copper, 2 to 5% zinc, 0.2 to 1.5% magnesium, 0.2 to 0.6% manganese, and grain refining additions, exhibits, when solution heat treated, quenched and artificially aged, high room and elevated temperature strength which may be coupled with substantial immunity to stress corrosion cracking.

BACKGROUND OF THE INVENTION Aluminum base casting alloys of various compositions are known and are characterized, for the most part, by only moderate strength typified "by tensile strengths of 30,000 to 45,000 p.s.i. and yield strengths of 25,000 to 40,000 p.s.i. For the most part these casting alloys are characterized by relatively low elongation, generally less than 3% and often around 1 to l /2%, which seriously hinders their acceptance for many applications where their tensile properties would otherwise render them acceptable. As is known, higher elongation imparts flexibility so that a cast part can accommodate unforeseen strains which so often occur, inter alia, when joining the parts into assemblies. The lack of sufficient elonga tion has constituted a serious impediment to the acceptance of castings in many applications where they would otherwise present considerable economies over forged or machined parts.

STATEMENT OF THE INVENTION The invention contemplates an alloy particularly suited for casting, consisting essentially of aluminum, 3 to 6% copper, 2 to 5% zinc, 0.2 to 1.5% magnesium and 0.2 to 0.6% manganese, by Weight. The alloy also contains for grain refinement 0.005 to 0.4% titanium, and preferably up to 0.008% boron. Where less than 0.2% titanium is present, it is preferable to include 0.001% or more boron. The following maximum limits are placed on impurities: iron 0.15%, silicon 0.10%, others 0.05% each, the other impurities moreover not to exceed 0.15% total. Copper, zinc and magnesium are principally present to improve strength and elongation. Preferred limits for these constituents are 4 to 4.5% copper, 2.5 to 3.5% zinc and 0.25 to 0.5% magnesium. Manganese serves to improve the yield strength without impairing elongation or resistance to stress corrosion. Manganese is preferably maintained above 0.3% and, for best results is kept between 0.35 and 0.5 Titanium and boron are preferably present in respective amounts of 0.15 to 0.4% and 0.001 to 0.005%. The following maximum limits on impurities are preferred: iron 0.06%, silicon 0.03%, others 0.03% each, the other impurities moreover not exceeding 0.1% total. The aforementioned composition ranges must be strictly followed in order to realize the improved properties. The copper, zinc, magnesium: and manganese constituents, particularly, combine to provide these benefits.

Useful articles in the form of castings produced in the improved alloy, when solution heat treated, quenched and artificially aged, exhibit high strength and where desired, substantial immunity to stress corrosion cracking. For instance, the alloy, when cast and properly heat ice treated, as described in more detail hereinafter, is capable of achieving tensile and yield strengths as high as, respectively, 70,000 and 60,000 p.s.i., and higher, and an elongation, in 2 inches, of 10%. The alloy is consistently capable of achieving tensile and yield strengths of 65,000 and 55,000 p.s.i., respectively, and an elongation of 8%. The alloy is also capable of substantial immunity to stress corrosion cracking, although the heat treatment required therefor slightly lowers the tensile and yield strengths consistently achievable to about 60,000 and 50,000 p.si., respectively, such strength nevertheless being very desirable when a substantial immunity to stress corrosion cracking is achieved. The improved alloy is also capable of achieving elevated temperature tensile strength levels of over 40,00 p.s.i. at 350 F. and over 30,000 p.s.i. at 450 F., together with elongations of over 12 and 17% at these respective temperatures after hours exposure thereto.

The improved alloy can be cast employing the well known sand casting process. The constitutents for the alloy may be blended in any conventional fashion. Because of the limits on impurities, it is important to start with suitably high purity aluminum, preferably 99.9% or purer. In making castings it is highly advisable to use care with respect to gating, risers, chilling, and melt handling so as to keep to a minimum the content of gas and other contaminants. It is also advisable to minimize turbulence when filling the mold with molten alloy. The chilling is preferably such as to provide progressive solidification. These practices are all well known in the foundry art, generally being considered good foundry practices, and need not be elaborated upon here.

To achieve higher strength and improve other prop erties, the castings are solution heat treated, quenched and artificially aged. The solution heat treatment is carried out at a temperature of at least 950 F. for a sufficient time to place all of the soluble constituents in solution. A preferred solution heat treatment is one where the casting is heated for 2 to 8 hours at a temperature of 900 to 940 F. followed by 12 to 48 hours at a temperature of at least 970 F. but low enough to avoid melting. The casting is then quenched, preferably in water and preferably to a temperature of F. or lower, for example to room temperature. Thereafter, the casting can be aged to fully develop the desired properties. Aging treatments can conveniently vary somewhat over a range of 275 to 400 F. for 5 to 48 hours depending on the properties considered most important for any given application. In general, where a combination of high strength and elongation is of maximum importance, the castings are aged at a temperature of 275 to 350 F. for 5 to 48 hours. Sand cast tensile test specimens when so aged have demonstrated tensile and yield strengths as high as, respectively, 70,000 and 60,- 000 p.s.i., and higher, and an elongation, in 2 inches of 10%. Sand cast test specimens of the preferred composition, when so aged, can consistently achieve minimum tensile and yield strengths of at least 65,000 and 55,000 p.s.i., respectively, and an elongation in 2 inches of at least 7%. Where, on the other hand, immunity to stress corrosion cracking is of prime importance, aging is effected at a higher temperature, 350 to 400 F. The use of the higher temperature reduces the aging time to 2 to 8 hours. A preferred treatment is 3 to 5 hours at 360 to 380 F. While this treatment imparts substantial immunity to stress corrosion, it slightly lowers the tensile properties, although cast tensile specimens so treated can consistently achieve tensile and yield strengths of 60,000 and 50,000 p.s.i., respectively, and an elongation of at least 5% in 2 inches.

EXAMPLE 1 Standard test specimens /2 inch in diameter with 2% inch long reduced section were separately cast in sand molds without chills. The alloy composition of the specimens was approximately 4.3% Cu, 3% Zn, 0.35% Mg, 0.4% Mn, 0.25% Ti, 0.005% B, the balance aluminum and, as impurities, 0.03% Fe and 0.02% Si. The specimens were solution heat treated for 5 hours at 920 F. followed by 30 hours at 970 F. and then quenched in water to substantially room temperature. The quenched specimens were divided into two groups. The first group was aged at a relatively low temperature of 320 F. for 12 hours. The other group was aged at a higher temperature of 375 F. but for a shorter time of 5 hours. Tensile properties of these specimens were determined as described in A.S.T.M. specification E8-65T on Tension Testing of Metallic Materials. In addition, stress corrosion tests were performed utilizing constant deflection methods to specimens stressed to 75% of the yield strength and alternate immersion in a 3 /2% sodium chloride solution as the test environment. In this type of tests the materials which are sufficiently susceptible to stress corrosion to cause problems in service display a high percentage of failures in less than 30 days of exposure. Materials having generally adequate resistance characteristically display no failures in the first 30 days of exposure with an increasing number of failures as the remainder of the normal 84-day exposure proceeds. Materials that repeatedly do not fail in the 84-day period under the test conditions are considered to have substantial immunity to stress corrosion. Table I illustrates a comparison of the respective properties of the specimens aged at the lower and higher temperatures with respect to strength and susceptibility to stress corrosion.

TABLE I Tensile Aluminum Percent Susceptibility Aging strength, strength, elongation to stress temperature p.s.i. p.s.i. in 2 inches corrosion Low 70, 000 60, 000 10 Susceptible. High 68, 000 58. 000 8 Immune.

EXAMPLE 2 The subject of elevated temperature properties is often of interest concerning an aluminum base alloy especially where strength is of importance. Several cast tensile specimens of the improved alloy containing aluminum, 4.3% Cu, 3% Zn, 0.4% Mg, 0.39% Mn, 0.24% Ti and, as impurities, 0.02% Fe, 0.01% Si were subjected to tensile tests at 350 and 400 F. The specimens were solution heat treated, quenched and aged by heating to 375 F. for 5 hours. After 100 hours at test temperature the specimens exhibited tensile strength levels of 46,000 and 35,000 p.s.i., respectively, at the 350 and 400 F. test temperatures. The elongations at these respective temperatures were 16% and 21%. It can be seen that the elevated temperature characteristics of the improved alloy are not only comparable to but exceed those exhibited in an identical test on a known premium strength casting alloy identified by the commercial designation C355-T 61 which exhibited corresponding 350 and 400 F. tensile strength levels of 39,000 and 27,000 p.s.i. and slightly lower elongation. This known premium strength casting alloy has a nominal composition of 5% silicon, 1.3% copper and 0.5% magnesium and is generally considered to exhibit good elevated temperature strength characteristics. This comparison alloy, by the way, was cast in a permanent mold which provided for higher properties than would have been obtained with a sand mold.

The foregoing examples, and for the most part this description, describes the improved alloy in terms of properites demonstrated by cast tensile specimens since such provides the most clear-cut results insofar as defining the actual strength capabilities of the alloy. It is known that tensile properties of an alloy vary from one cast shape to another because of casting variables, and that there are also marked variations in test specimens removed from ditferent portions of a given casting. The following examples clearly demonstrate that cast articles and products of commercially useful configuration produced in the improved alloy exhibit tensile strengths of at least 45,000, specifically from about 57,000 up to about 67,000 p.s.i. yield strength of at least 35,000, specifically from about 50,000 up to about 55,000 p.s.i. and elongations of at least 3% and up to 9% in 2 inches.

EXAMPLE 3 A hollow cylinder about one foot long and 8 inches in diameter was cast using the improved alloy containing aluminum, 4.2% Cu, 3.1% Zn, 0.42% Mg, 0.37% Mn, 0.23% Ti, 0.03% Fe, 0.07% Si. Several such cylinders were cast using sand molds and cores. After removal from the mold and rough cleaning, the castings were solution heat treated 5 hours at 920 F. followed by 24 hours at 970 F. The castings were then quenched by immersion in water at F. and then divided into two groups. One group was aged at 320 F. for 20 hours and the other group was aged 5 hours at 375 F. From several of these castings test specimens were removed from the top and side-wall areas.

Tensile tests of the machined test bars gave the results EXAMPLE 4 A multivane impeller having a base diameter of about 4 inches and a height of a little over 3 inches was cast using the same metal as that for Example 3. Several such castings were made using permeable plaster molds with suitably placed chills. These castings were solution heat treated, quenched and aged as described in Example 1. Tensile tests of standard bars machined from these castings gave the results indicated in Table III.

TABLE III Tensile Yield strength, strength, Elongation, Aging temperature p.s.i. p.s.i. percent EXAMPLE 5 Aircraft bracket castings were made in green sand molds using the improved alloy containing aluminum, 4.4% Cu, 3.0% Zn, 0.45% Mg, 0.41% Mn, 0.06% Ti, 0.001% B, 0.02% Fe, 0.02% Si. The casting was L shaped with sections having an I form. Two such castings were solution heat treated 5 hours at 920 F. followed by 24 hours at 970 F. and quenched by immersion in water at l50 F. One casting was aged 20 hours at 320 F. and the other 5 hours at 375 F. Twelve machined tensile bars removed from each casting gave the results shown in Table IV.

In five chilled areas, the casting aged at 375 F. exhibited properties as follows:

Tensile Yield strength, strength, Elongation, p.s.i. p.s.i. percent Maximum 61, 900 51, 900 9. Average 60, 200 50, 900 7. 0

From the foregoing it is apparent that the applicant has provided an improved alloy having very high strength at both room and elevated temperatures which may be coupled with substantial immunity to stress corrosion cracking. The alloy may be made into castings, and these may, if desired, be subsequently worked.

What is claimed is:

1. An aluminum base alloy consisting of 3 to 6 percent copper, 2 to percent Zinc, 0.2 to 1.5 percent magnesium, 0.35 to 0.5 percent manganese, 0.005 to 0.4 percent titanium, the balance aluminum, and not more than the following amounts of impurities and other elements: iron 0.15 percent, silicon 0.10 percent, others 0.05 percent each, the others moreover not exceeding 0.15 percent to tal, the alloy in the as cast condition being characterized by the ability to achieve a tensile strength of 70,000 p.s.i., a yield strength of 60,000 p.s.i. and an elongation of percent.

2. The alloy in accordance with claim 1 which contains 4 to 4.5 percent copper, 2 to 5 percent Zinc, 0.2 to 0.6 percent magnesium, 0.35 to 0.5 percent manganese.

3. An article comprising a cast member composed of an aluminum base alloy consisting of 3 to 6 percent copper, 2 to 5 percent zinc, 0.2 to 1.5 percent magnesium, 0.35 to 0.5 percent manganese, 0.005 to 0.4 percent titanium, up to 0.008 percent boron, the balance aluminum and not more than the following amounts of impurities and other elements: iron 0.15 percent, silicon 0.10 percent, others 0.05 percent each, the others moreover not exceeding 0.15 total, said alloy in the as cast condition exhibiting the ability to achieve high strength at room and elevated temperature and high resistance to stress corrosion cracking.

4. An article comprising a cast member composed of an aluminum base alloy consisting of 4 to 4.5 percent copper, 2 to 5 percent zinc, 0.2 to 0.6 percent magnesium, 0.35 to 0.5 percent manganese, 0.15 to 0.4 percent titanium, up to 0.008 percent boron, the balance aluminum and not more than the following amounts of impurities and other elements: iron 0.15 percent, silicon 0.10 percent, others 0.05 percent each, the others moreover not exceeding 0.15 total, said alloy in the as cast condition exhibiting the ability to achieve high strength at room and elevated temperature and high resistance to stress corrosion cracking.

5. A cast article composed of an aluminum base alloy consisting of 4 to 4.5 percent copper, 2 to 5 percent zinc, 0.2 to 0.6 percent magnesium, 0.35 to 0.5 percent manganese, 0.15 to 0.4 percent titanium, up to 0.008 percent boron, the balance aluminum and not more than the following amounts of impurities and other elements: iron 0.06 percent, silicon 0.03 percent, others 0.03 percent each, the others moreover not exceeding 0.1 percent total, said cast article when solution heat treated, quenched and artificially aged exhibiting a tensile strength of at least 45,000 p.s.i., a yield strength of at least 35,000 psi and an elongation of at least 3 percent.

References Cited UNITED STATES PATENTS 3,475,166 10/1969 Raffin 143X 2,076,570 4/1937 Kempf et a1. 75141 2,240,940 5/1941 Nook 75141X 2,301,759 11/1942 Stroup 75141X 3,287,185 11/1966 Vachet 14832.5X 3,414,406 12/1968 Doyle et al. 148159X FOREIGN PATENTS 544,439 4/ 1942 Great Britain 75--141 476,930 12/1937 Great Britain 75-141 598,192 2/1948 Great Britain 75141 650,905 3/ 1951 Great Britain 75-141 CHARLES N. LOVELL, Primary Examiner US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 598, 577 Dated August 10, 1971 Inventr(S) Edward E. Stonebrook It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 2, line 11 Change "strength" to --strengths--.

Col. 2, line 15 Change "40,00" to "40,000".

C01. 2, line Change "constitutents" to c0nst1tuents--.

Col. 2, line 43 Change "F." to -F.--.

Col. 4, line 16 Change "strength" to --strengths-.

C01. 4, line 64 Change "12." to --l2.5-.

Signed and sealed this 11th day of January 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Acting Commissioner of Patents RM pomso (1069) USCOMM-DC 60376-P69 U15. GOVERNMENT FRINTiNG OFFICE. 1969 QJ6G-]Jl