US3475166A

US3475166A - Aluminum base alloy

Info

Publication number: US3475166A
Application number: US791506*A
Authority: US
Inventors: Joseph Raffin
Original assignee: Electronic Specialty Co
Current assignee: Electronic Specialty Co
Priority date: 1969-01-15
Filing date: 1969-01-15
Publication date: 1969-10-28
Anticipated expiration: 1986-10-28
Also published as: NO128827B; BE744380R; DE2001712A1; DE2001712B2; GB1289621A; CA917961A; JPS5128562B1; ES375477A2; NL7000498A; FR2031126A6

Description

United States Patent Ofiice 3,475,166 Patented Oct. 28, 1969 3,475,166 ALUMINUM BASE ALLOY Joseph Rafiin, Temple City, Calif., assignor to Electronic Specialty Co. No Drawing. Continuation-impart of applications Ser. No. 497,557, Oct. 18, 1965, and Ser. No. 561,747, June 30, 1966. This application Jan. 15, 1969, Ser. No.

Int. Cl. C22c 21/02; C221? N04 US. Cl. 75141 14 Claims ABSTRACT OF THE DISCLOSURE This application is a continuation-in-part of application Ser. No. 497,557, filed Oct. 18, 1965 and now abandoned and of copending application Ser. No. 561,747, filed June 30, 1966, now abandoned.

This invention relates to aluminum alloys, and, more particularly to high strength aluminum casting alloys and methods for producing them.

Aluminum alloy castings with high physical strength have long been needed, not only to replace more expensive high strength aluminum parts made by forging, extruding, cold rolling, and machining, but to handle more intricate design requirements. Aluminum casting alloys for casting parts are available but the strength of such parts has been well below that obtainable with machined plates and billets, machined forgings, and wrought assemblies.

At the present time, aluminum casting alloy number 356 is widely used, but it does not have sufficient strength for many design requirements. Aluminum alloys 195 and 357 are also used, but they likewise fall short of the tensile and yield strengths required for many high strength aluminum parts. A few special purpose aluminum casting alloys such as Tens 50, APM and NA222 and experimental alloys such as ST60 and M710 have been used to get relatively high strengths. APM, for example, has a nominal composition of about 4%5% copper, about .3% magnesium, and traces of titanium, silicon, and iron and can be processed to give tensile strength around 45,000 p.s.i., yield strength of around 30,000 p.s.i. and elongation of 5% to The copper in the aluminum casting alloy adds to the alloy strength, but

it increases the susceptibility of the alloy to stress corrosion.

This invention provides an improved aluminum base alloy which is virtually free of stress corrosion problems, and from Which castings can be made with ultimate tensile strength in excess of 70,000 p.s.i., yield strength in excess of 60,000 p.s.i., and about 4% to 10% or more elongation. Moreover, properties as high as 65,000 p.s.i. tensile strength, 55,000 p.s.i. yield strength and 8% elongation can be guaranteed in commercial castings with chills, and 60,000 p.s.i. tensile, 50,000 p.s.i. yield and 3% elongation can be obtained consistently on sand castings without chills. Such properties are comparable to those normally achieved only with aluminum forgings rather than aluminum castings. The alloy forming the core of this application, therefore, has surpassed all competitors in providing a satisfactory high strength aluminum casting alloy.

Not only are the properties of the present aluminum alloy better than any others available but the properties of the castings in accordance with an exemplary embodimerit of this invention remain high at elevated temperatures. For example, at 500 F. test results show over 33,000 p.s.i. ultimate tensile strength, over 32,000 p.s.i. yield strength and 14% elongation. Even at 600 F., the castings have shown over 19,000 p.s.i. tensile, over 19,- 000 p.s.i yield and about 16% elongation. This is to be contrasted with most aluminum casting alloys which lose virtually all of their strength at such elevated temperatures.

Briefly, the alloy of this invention contains, in addition to its aluminum base, essentially from about 3.5% to about 6.0% copper, and from about .05% to about 3.0% silver and up to about 1% manganese. For optimum results, the copper is present in the amount of from about 4.7% and about 5.3% and the silver is present in the amount of from about .40% to about 1.0% and the alloy includes from about .15 to about .40% magneslum.

The percentages referred to throughout the specification and claims are percentages by weight. The term up to as used herein means from 0% up to the des- -ignated percentage.

If the copper content varies below 3.5%, the strength of the alloy is detrimentally effected. With low manganese the preferred lower limit for copper is about 4.2%. Optimum properties have been obtained with from 4.7 to 5.3% copper.

The silver apparently improves the dispersion of copper throughout the alloy to increase its strength and counteracts the tendency of the aluminum alloy to undergo stress corrosion caused by the high percentage of copper. The amount of silver can be increased substantially above 1% without adversely affecting the physical properties of the alloy. However, since silver is an expensive metal, an amount above 1%, or even above .7%, unnecessarily adds to the cost of the alloy without significantly changing its physical properties or its ability to reduce stress corrosion.

The tensile strength, yield strength, and elongation are further improved if zinc in the amount of up to 4.0% is added, good results being achieved with about 1.0% to about 3.0% zinc. Strength is also increased by the addition of a relatively small amount of magnesium in the range of about .15 to .4%. The best properties have been observed when the magnesium content is maintained between .2 and 3% Titanium is beneficial in assuring fine grain structure in the alloy which is important for successful heat solution treatment in accordance with the method of this invention. The titanium may be present in the amount of about .15% to about .7 and preferably is about .20% to about 30%. In some cases, the titanium is kept at the lower limit, and more is added when the alloy is remelted as this improves the grain structure.

Silicon is kept below .15% in order to avoid burning and iron is kept below .15% so that the alloy will properly respond to the heat treatment. For optimum results, both silicon and iron are kept below about .1% and preferably below about .05

Boron addition is not essential in making the alloy, but generally a small amount in the range of .001% to .05% should be added when the alloy is remelted to improve the grain structure.

The manganese content of the alloy may be varied from up to about 1% by weight without detrimentally affecting the alloy. Additions of from about .2 to .8% manganese improve the elevated temperature properties of the alloy and for that reason are preferred. The best combination of properties has been obtained when the manganese content has been maintained at about 0.3%.

Elements such as molybdenum and cerium are preferably kept less than about 3% each. Chromium is kept below about .5%

The broad and narrow ranges and an exemplary composition of the alloy of this invention are given in the following table.

TABLE Percent by weight Broad Narrow Exemplary Elements Range Range Value Copper 3. 5-6. 0 4. 7-5. 3 4. 9 Silver 05-3. 0 40-1. 0 60 Titanium 15-. 7 20-. 30 25 Silicon (maximum). 0-. 15 10 05 Iron (maximum)- 0. 0-. 03 Nil Boron 001-. 05 001-. 01 002 Manganese 0-1 8 .3 Molybdenum. 0-. 3 3 Nil Cerium 0. 3 3 Nil Chromium. 0-. 5 3 Nil Others (each 0. 05 05 Nil Others (tota1). 0. 15 15 Nil Zinc 0-4. 0 1. 0-3. 0 2. 0 Magnesium 0. 15-. 4 30 24 luminum Balance Balance Balance A typical melt of the alloy was prepared as follows: about seventy-five pounds of returns (gates and risers from previous castings to be remelted) is melted down with about fifty pounds of high-purity aluminum (99.8% to 99.99% pure aluminum) and about four pounds of an aluminum-titanium master alloy (5% titanium, balance aluminum) in a silicon carbide crucible in a gas-fired furnace. Temperature control was assured by a chromelalumel thermocouple and a potentiometer. After reaching about 1300 F., 2.75 pounds of electrolytic copper and .33 pound of silver were added. If zinc were to be included, it would have been added with the copper and silver. After the metals dissolved, the crucible was filled with an additional forty-five pounds of returns from previous melts to provide a composition within the ranges given in the above table. When the temperature reached 1300 F. again, nitrogen was bubbled through the melt with a graphite pipe to remove any deleterious gases, such as hydrogen produced by the decomposition of moisture, and the temperature was allowed to rise to 1400 F. About .50 pound of an aluminum-titanium-boron alloy (5% titanium, 1% boron, and the balance being substantially all aluminum) was added, then about .18 pound of pure magnesium. A check was made to see if some hydrogen gas was dissolved in the metal, and if the check was positive, additional nitrogen was bubbled in until a negative check was obtained.

About .10 pound of a grain refiner (a mixture of two parts of titanium-potassium fluoride with one part of potassium borofluoride) was then added to the melt, and after waiting at least ten minutes and when the pouring temperature of about 1325 F. to about 1425 F. (depending on the shape and size of the casting) was reached, the melt was poured into a mold, including a test bar mold and a sample for chemical analysis. A pouring temperature of 1375 F. is suitable for a wide range of parts. Too low a pouring temperature results in lower mechanical properties.

In a variation of this melting procedure which has also been used successfully, about one ounce of hexachlorethane pills per 150 pounds of metal were plunged into the melt when it reached 1300 F. to remove traces of sodium which may be present. Chlorine gas can be used for the same purpose. Then the magnesium and the aluminum-titanium-boron alloy were added. After skimming the melt, the grain refiner was added and nitrogen was bubbled through the melt until a check showed that the metal was free of gas. At the same time, the temperature was raised to between 1375 F. and 1400 F. The preferred pouring temperatures are the same irrespective of which of the two melting procedures is used.

A waterless sand casting mold is preferred. Natural bonded sand is also suitable, and synthetic sands can be used but they often induce gas pick-up by reaction between the metal and the moisture of the sand.

The cast alloy was then subjected to a solution heat treatment in an electric, drop quench furnace by heating the casting from three to eight hours at 980 F. to 1000 F. The casting was then quenched in water at a temperature not exceeding 130 F. Quenching sometimes causes warping of the cast part, which is straightened in a press or with a plastic or wooden mallet. After straightening the casting as required during the next three hours, it was age-hardened for eight to twenty hours at 280 F. to 340 F.

The purpose of the solution heat treatment is to dissolve the copper-rich compound deposited around the aluminum-rich matrix during the solidification of the alloy without causing the melting of any compound. The temperature and duration of the solution heat treatment is chosen after consideration of the size, shape, and thickness of the casting to obtain practically complete dissolution of the eutectic in the matrix which is checked by micrographic examination.

The purpose of the quenching is to keep the supersaturated solid solution of the copper rich phase and other intermetallics in the aluminum matrix. Quenching should be as quick and as drastic as possible without producing stress cracks. Quenching with the alloy at 1010 F. made cracks in castings, even in small parts. Quenching with the alloy at 1000 F. did not make cracks in test bars of alloy, but it made some light surface cracks in a few areas of complex castings. Quenching with the alloy at 995 F. caused cracks in heavily chilled complex castings, while the same unchilled castings had none. Quenching with the alloy at 985 F. did not cause any cracks in castings even up to five feet in length. Consequently, the alloy of this invention is preferably at 985 F., when it is quenched even if solution heat treatment is carried out at 985 F. or 1000 F. In such a case, the temperature is preferably reduced to about 985 F. prior to quenching. Parts made of the alloy ten to fifteen inches long with wall thickness of one-fourth to three-fourths inch were quenched at 995 F.'without cracking. The temperature of the water is preferably no greater than F., and quenching in water at room temperature appears to improve stress corrosion resistance.

In general, a solution heat-treatment time of about five hours has been suflicient for parts two and one-half inches thick. A solution heat treatment temperature in the range of 985 to 1000 F. produced satisfactory results, with optimum results being obtained by reaching 995 F. during two to three hours of a five-hour cycle. A typical solution heat treatment was one hour at 985 F., followed by three hours at 995 F., followed by one hour at 985 F. for a total of five hours.

Castings not larger than 15" x 15" and not thicker than A" may be satisfactorily solution heat-treated by heating the parts five hours at 995 F. Smaller castings on the order of about 8" x 1" x /2" can be heat treated at 1000 F. for about four hours.

The parts are aged to precipitate the copper compound, with subsequent hardening of the alloy. The temperature and duration of the aging is determined by the properties most desired. The tensile strength of the cast alloy generally improves with increased time and temperature up the maximum aging and then begins decreasing as the alloy is overaged. Generally, the ductility of the alloy decreases as the tensile strength increases. Increased impact strength is obtained by aging at a lower temperature for a longer period, e.g., room temperature for at least five days, but yield strength is lower. Aging the alloy at 320 F. for about twenty hours produced very stable material which did not change in time and which also had high resistance to stress corrosion. Aging at the higher temperature of 340 F. was successfully done in less time, but at the expense of losing a few percentage point in elongation. An alloy with acceptable well-balanced physical properties is obtained by aging at 295 F.

A typical heat treatment for a casting such as a strut for an aircraft landing gear is as follows: one hour at 985 F., three hours at 995 F., and one hour at 985 F. for a total of five hours solution heat treatment; quench within five seconds in water at room temperature, and hold the casting twenty-four hours at room temperature; thereafter age twenty hours at 320 F.

intermetallic compound CuAl This compound has to be dissolved during the heat treatment. Its solubility increases with temperature which probably accounts for the fact that this invention uses a temperature range of 975 to 1000 F. for heat treatment instead of 940 F. to 970 F. as is used for conventional 195 aluminum alloy.

Analysis of another test bar showed the following composition:

Experiments have shown that best properties are obtained when the copper is between about 4.7% and about 5.3%. For example, identical heat-treated test bars, differing within each set only by copper content, had the following tensile properties:

Set No. 1 Set No. 2 Set No. 3 Set N0. 4 Set No. 5

Cu (percent weight) 4. 20 4. 75 4. 75 5. 75 5. 25 5. 75 3. 2 5. 0 5. 0 6. 0 Ultimate (p.s.i.) 43, 000 58, 000 62, 000 56, 000 61, 000 58,000 53, 400 67, 500 59, 700 55, 200 Yield (p.s.i.). 26 000 35, 000 42 000 33, 000 44, 000 38, 000 47, 200 60, 600 50, 300 53, 400 Elongation, percent 1 2.0 5. 5 4. 0 8. 0 5. 5 4. 4 1. 2

Best results have been obtained by slowly raising the temperature of the casting to the solution treating temperature in a series of stages. The castings are first heated to a temperature of 940 F. and maintained at that temperature for a period of eight hours. The temperature of the heat treating furnace is then raised to about 960 F. and again maintained at that temperature for eight hours. The temperature of the furnace is then raised another 20 F to 980 F. and the castings are maintained at this solution heating temperature for another eight hour period. The final solution treating temperature is selected on the basis of the alloying content of the. casting, i.e. the amounts of silver, magnesium, manganese, etc., added to the melt. Generally as the alloy content of the casting increases the final solution heat treating temperature should be reduced. The solution treating temperature should be high enough to dissolve the copper-rich phase but must not cause melting of any of the intermetallic compounds. The aging process is both time and temperature dependent. If the lower aging temperatures are employed, the aging times should be increased. For example, it has been found that good results are obtained when the castings are aged for twenty hours at 310 F.

Results of mechanical tests on coupons machined from castings made in accordance with the above and following currently available high quality techniques to promote progressive directional solidification were in the following range: ultimate tensile strength, 59,450 to 70,150 p.s.i.; yield strength (by .2% offset method), 49,500 to 64,450 p.s.i.; and elongation, 5% to 17%.

Chemical compositions of the coupons determined by chemical analysis varied as follows:

Element: percent by weight Copper 4.74 to 5.55. Magnesium .20 to .31. Titanium .22 to .28. Silver .54 to .61. Manganese Up to .8. Silicon-iron Nil. Aluminum Balance.

The alloy of this invention includes a high quantity of copper, part of which contributes to the formation of the Set No. 1

Mg (percent weight) 19 29 38 48 Ultimate (p.s.i 64, 500 70, 600 70, 400 30, 000 Yield (p.s.1.) 56, 000 66, 400 67, 400 Elongation, percent- 5. 5 3. 0 2. 5 5

The .48% magnesium test bar showed some burning. The best range for magnesium is about .20% to 30%, and as indicated by the above example, this range appears to increase the ultimate tensile strength and yield by about 10%.

Elemental silver is added to the alloy because it increases the mechanical strength of the alloy and increases the resistance of the alloy to stress corrosion. The mechanical strength of the alloy is improved by the addition of as little as 0.2% silver. In the range of .4% to 1.5% silver, the alloy is substantially free of stress corrosion. The mechanical strength appears to be optimum at about 0.5% silver but is little diminished when the silver content is as high as 3.0%. The effect of silver on the properties is shown by the following exemplary sets of test bars:

A third set has varying magnesium as well, but still shows that a high percentage of silver has no detrimental effect on the tensile strength.

Set No. 3

Ag (percent weight) Zinc, when added in amounts between about 1.0% and 3.0% also substantially improves the strength as is evidenced by the following exemplary set of test results.

Set No.1

Zn, percent Nil 1.0 2.0 3.0 4.0 Ultimate (p.s.i.) 64,700 68,000 72,200 72,700 69,000 Yield (p.s.i.) 56,400 57,800 63,900 64,200 65,200 Elongation, percent 5.0 9. 5.0 5.0 2.0

Titanium is a good grain refiner. The range of .20%

to 30% titanium produces a fine grain in the alloy, which facilitates required dispersion of the copper throughout the alloy during solution heat treatment, with the result that castings can be made which are much stronger than castings made with previous aluminum casting alloys. There seems to be no strength gained by adding more titanium and the elongation drops as the alloy gets richer in titanium. This is shown by the following set of test bars:

Set No. 1

Ti (percent wei ht) 24 39 54 69 Ultimate (p.s.i 59, 700 59, 400 60, 500 200 Yield (p.s.i.) 50, 300 50, 000 51, 000 53, 400 Elongation, percent 4. 4 4. 2 4. 2 3. 0

Set No. 2

. 15 15 16 65 59, 700 45, 000 000 50, 300 35, 400 Elongation, percent 2. 8. 5 4. 2. 0

To determine the effect of manganese on the strength properties of the alloy, the quantity of manganese was varied up to about 1% and the other components of the alloy were held substantially constant as shown in Table 1.

TABLE 1 Copper, percent 4.11 3. 91 4. 81 3. 55 Iron, percent 04 05 07 04 Magnesium, percent- 36 29 27 28 Silicon, percent... .002 015 003 016 Zinc, percent- 08 08 10 08 Manganese, percen 28 48 63 94 Titanium, percent 21 23 Boron, percent 023 027 019 028 Aluminum 1 Remainder.

These alloys were cast as chilled plates in said molds and their tensile properties varied as shown in Table 2.

TABLE 2 The test results shown in all the foregoing examples were obtained on standard test bars /2 in diameter with 2%" long reduced section cast in sand mold without chills. The results from each set are not directly comparable with those of other sets because of difierences in other variables between sets.

Among other elements that might be added to the alloy, some were very detrimental, some slightly detrimental, and others had no effect or possibly light beneficial efiects on the alloy.

Cadmium at .30% caused severe burns and cracks during the heat treatment with complete loss of strength and elongation.

Sodium, calcium, and lithium at .02% caused reduction of 10% to 20% of the ultimate tensile strength and 30% to 40% reduction of the elongation with fiaws in the test bars of the alloy.

Cobalt at 30% caused reduction of 20% of the ultimate tensile strength and 30% reduction of the elongation with coarsening of the grain.

Tin at .005 did not affect the properties of the alloy, but its association with .005 of bismuth caused severe burns and cracks during the heat treatment.

Antimony at .005 caused a 10% reduction of ultimate tensile strength and a similar reduction in elongation.

Chromium at .25 and molybdenum at .25 caused a Slight increase of ultimate tensile strength. At .50% molybdenum, there was a slight decrease of the tensile strength and no significant change for chromium in this range. Nickel and cerium, each at 30% had no appreciable effect on the propuerties of the alloy. Zirconium at .25% caused a slight decrease of tensile strength.

I claim:

1. An aluminum base alloy comprising by weight from about 3.5 to about 6.0% copper, from about .05 to 3.0% silver, from about .15 to about .4% magnesium, up to about 1% manganese, less than about .05 silicon less than .15% iron and the remainder aluminum, said alloy being characterized by yielding sand castings which in the solution treated and aged condition have yield strengths in excess of 50,000 p.s.i., ultimate tensile strengths in excess of 60,000 p.s.i., elongations of at least 5% and high resistance to stress corrosion.

2. An aluminum base alloy as defined in claim 1 wherein said copper is from 4.7 to 5.3%, said silver is from .4 to 1% and said magnesium is from about .2 to about .3%.

3. A casting produced from the alloy defined in claim 1 and heat treated to have a yield strength in excess of 50,000 p.s.i., a tensile strength in excess of 60,000 p.s.i., an elongation of at least 5% and a high resistance to stress corrosion.

4. An aluminum base alloy consisting essentially of from about 3.5 to about 6.0% copper, from about 0.05 to about 3% silver, from about .15 to about .4% magnesium as a strengthening agent, up to 1% manganese, less than about 0.5% silicon, less than about .05% iron and the balance aluminum.

5. An aluminum base alloy as defined in claim 4 wherein said copper is from 4.7 to 5.3%, said silver is from .4 to 1%, said magnesium is from .2 to 3%, and said manganese is from .2 to .8%.

6. A casting produced from the alloy defined in claim 5 said casting being heat treated to have a yield strength of at least 50,000 p.s.i., a tensile strength of at least 60,000 p.s.i. and an elongation of at least 5%.

7. An aluminum base alloy comprising by weight from about 3.5 to about 6.0% copper, from about .05 to about 3.0% silver, from about .15 to about .4% magnesium as a strengthening agent, from about .2 to .8% manganese, from about .15 to .7% titanium as a grain refiner, less than about .05% silicon, less than .15% iron and the remainder aluminum, said alloy being characterized by yielding sand castings which in the solution treated and aged condition have yield strengths in excess of 50,000 p.s.i., ultimate tensile strengths in excess of 60,000 p.s.i., elongations of at least 5% and a high resistance to stress corrosion.

8. An aluminum base alloy casting in the solution heat treated-aged condition produced from an aluminum base alloy comprising as essential elements from about 4.2 to about 6.0% by weight copper, from about .05 to about 3.0% by weight silver, up to 1% by weight manganese, less than .1% by weight silicon, less than .15% by weight iron with the balance aluminum, said casting being characterized by having a tensile strength in excess of 60,000 p.s.i., a yield strength in excess of 50,000 p.s.i. and at least elongation at room temperature and a high tensile strength and yield strength and a high percentage elongation at elevated temperatures.

9. An aluminum base alloy consisting essentially of in percent by weight:

Percent Copper 4.2-6 Silver .41 Magnesium .15-.4 Silicon less than .05 Iron less than .10 Manganese 0-1 Others total .15 Aluminum Balance 10. A high strength heat treated casting formed from the alloy of claim 9.

11. An aluminum base alloy comprising in Weight percent:

12. An aluminum base alloy casting in the solution heat treated-aged condition produced from an aluminum base alloy comprising as essential elements from about 4.2 to about 6% by weight copper, from about .05 to about 3% by weight silver, from about .15 to about .4% by Weight magnesium as a strengthening agent, less than .1% silicon, less than .15% iron with the balance aluminum, said casting being characterized by having a tensile strength in excess of 60,000 p.s.i., a yield strength in excess of 50,000 p.s.i. and at least 5% elongation.

13. An aluminum base base alloy comprising by weight from about 3.5 to about 6.0% copper, from about .05 to 3% silver, from about .15 to about .4% magnesium, up to about 1% manganese, up to about 4% zinc, less than about .05% silicon, less than .15% iron and the remainder aluminum.

14. A casting produced from the alloy defined in claim 13, said casting being heat treated to have a yield strength of at least 50,000 p.s.i. and a tensile strength of at least 60,000 p.s.i.

References Cited UNITED STATES PATENTS 1,099,561 6/1914 McAdams 145 1,860,947 5/1932 Pacz 75-139 2,240,940 5/1941 Nock 75-141 2,381,219 8/1945 Baron 75139 2,459,492 1/ 1949 Bradbury 14832.5 3,414,406 12/1968 Doyle et al 75-141 X FOREIGN PATENTS 309,586 3/1930 Great Britain. 650,905 3/ 1951 Great Britain.

CHARLES N. LOVELL, Primary Examiner U.S. Cl. X.R.

@52 3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,475,166 Dated October 28, 1969 Inventor(s) Joseph Raffin It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 5 line 5 after "up" insert to- Col. 7 line 57 "said" should be -sand-; Col. 8 line 22, "propuerties should be --properties-- Col. 8 line 28, "less than about .05 silicon" should be -less than about .051, silicon,--; Col. line 481, "less than about 0.570 silicon should be less than about .0570 silicon-- L-.9 SEALED FEB 1 7 1970 ml.) Attem M. Fletcher, Ir,

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