US2286624A - Aluminum base alloy - Google Patents

Description

Patented Jun 16,1942

UNITED STATES PATENT" OFFICE ALUMINUM BASE ALLOY LouisW. Kempf and Walter A. Dean, Lakewood, I Ohio, assignors' to Aluminum Company of. America, Pittsburgh, Pa., a corporation of Pennsylvama No Drawing. Application December 30, 1941,

- Serial No. 424,891

2 Claims. (01.75-147) This invention relates to aluminum base alloys that are especially adapted for use at elevated to deformation at elevated temperatures, especially at the highentemperatures found in the newer aircraft motors. A particular object is to provide an alloy which possesses a high thermal vated' temperatures. Another object is to provide an aluminum vbase alloy which has a higher I modulus of elasticity than. the common commercial aluminum base alloys now in use.

We have discovered that aluminum base alloys containing from about 20 to 40 per cent beryllium,

from 1 to 8 percent magnesium, and the balance substantially aluminumpossess the aforementioned properties. More particularly, we have found that some of the alloys within this range possess a tensilestrength at elevated temperatures which is considerabLv greater than that of alloys' heretofore used for such service. Furthermore, this increase in strength is accompanied by a lower density than aluminun a relatively high thermal conductivity, and high modulus of elasticity. It is this combination of properties, especially that of high strength and relatively high thermal conductivity, that makes the alloys outstanding and particularly useful for such articles as valve push rods, pistons, and the like which are highly stressed at elevated teinperatures' g The highfmodulus of elasticity of these alloys 55 at temperaturefor'one half hour,-" and finally alloys, together with the corresponding properties of certain well known aluminum base alloys which have been used heretofore for high temperature service, are given in Table I below. The

first of the two alloys whichcontained no beryllium, i. e. the Al-Cu-Ni-Mg alloy, may be considered as being typical of prior aluminum base. .alloys designed for service at elevated tempera tures. The balance of the-composition of, each- 10 of the alloys appearing in the table was alumi- The berylliumcontaining alloys were cast as' ingots and extruded num and the usual impurities.

in the form of rods.

The thermal conductivity and tensile property tests were made on specimens taken from these extruded rods'while the tensile property determinations on the first two alloys containing no beryllium weremadeon specimens taken from forged rods, and; the specimens of the third alloy 'were sand cast. The difference in' fabricating conductivity along with the high strength at eleat elevatedtemperatures of any aluminum base alloy known heretofore but it also has a relatively low thermal conductivity. This alloy was tested in the form of a sand casting because that is the form in which it .has been employed for :10 high temperature service. The specimens of the other two alloys without beryllium received the conventional solution heat treatment and artificial aging, while those of the third alloy (sand cast) wereheated at 600 F. for three'h'ours,

before being subjected to any of the treatments and tests herein described in order to duplicate the condition of the alloys in many commercial applications. The tensile test bars of all the wrought alloys weresubjected to a short time test-at elevated temperatures consisting of' first stabilizing the bars by heating them for 16 hours at 700 F. This preliminary treatment served to accelerate any changes which would have 00- curred on exposure to a lower temperature over a long period of vtime. We have found from a number of other tests that such preliminary sta-.

bilizing treatment'for a relatively, short period of m time at a temperature higher than encountered inservice affects properties to a comparable ex- .50 tent as more extended periods at the temperature of service operation; Following the preliminary stabilizing treatment the,bars -'were cooled to room temperature-andjthnj.reheated to the testing temperature, inthis'cais'e' 00- E, held temperatures.

broken in tension at 600 F. in the usual manner. The sand cast alloy specimens were first heated at 700 F. for four hours to stabilize them and then cooled in air to 600 F., at which tempera.- ture they were held for 99 days before being tested.

The thermal conductivity values were calculated from electrical resistivity measurements made at room temperature. The calculations .were based on the well recognized Wiedemannsuch as exist in internal combustion engines.

The test specimens for electrical resistivity measurements were in the same temper as that of the specimens used for tensile tests prior to the stabilizing treatment.

TABLE I Tensile properties at 600 F. and

for two other aluminum base alloys in the solution and artificially aged condition, as described above. As is well recognized, such a difference in condition of the alloys would not affect the The superiority of the beryllium-containing 20 alloy over the other alloys is readily apparent from these data. From other determinations we have made at elevated temperatures we have thermal conductim'ty at room temperature Alloy composition 4 Thermal Tensile Elongation conductivity strength in 2 inches (0. G. S. 1

Be Mg Cu Ni 7 Si Mn units) Percent Percent Percent Percent Percent Percent Lbs/in. Percent "perature far exceeds that of the two wrought aluminum base compositions but is not much above that of the cast alloy. The lower elongation values of the beryllium alloys also indicate a greater resistance to deformation at elevated These tensile properties therefore indicate that these alloys are much better adapted for service at such high temperatures as 600 F. than the two wrought aluminum base alloys which have been widely employed heretofore for thatpurpose. It is to be observed also that the thermal conductivity of the alloy containing beryllium is identical with that of one of the wrought alloys used for comparison and is close to the conductivity value of the other alloy, while it significantly exceeds that of the cast alloy. It is the low thermal conductivity of the cast alloy which has seriously restricted its commercial use. The cast alloy does illustrate.

how, in the past, strength at elevated temperatures has been achieved at the expense of thermal conductivity while in the case of the wrought ductivity of the cast alloy.

.Modulus of elasticity determinations at room temperature were made on the above alloy containing 21.43 per cent beryllium, in the as-extruded condition, and compared with the values found that the alloys containing'beryllium retain this superiority by a wide margin. The relatively high modulus at elevated temperatures means that structures made from such alloys are much more resistant to distortion and hence may be expected to give longer service.

. It has been our experience that a substantial amount of beryllium must be present in the alloys to obtain the combination of a high modulus, a relatively high thermal conductivity, and l bination of properties desired, but if more than 40 per cent is employed, the alloy becomes very diflicult to work. The presence of magnesium in the alloy considerably enhances the strength at elevated temperatures. We have found that at least 1 per cent of this element is desirable to' achieve this purpose, while on the other hand, if more than 8 per cent is used, fabricating difficulties are encountered. Alloys which contain from 22 to 30 per cent beryllium and 1.5 to 5 per cent magnesium are preferred because they possess the most satisfactory I combination of strength and workability.

The expression balance substantially aluminum," as used hereinabove' and in the appended claims, means that small amounts of the usual impurities as well as other elements may be present in the alloys without affecting the high temperature properties described above. The presence of any elements which substantially impair the strength and thermal conductivity properties of 'these alloys at elevated temperatures is therefore excluded from the scope of this invention.

75' In referring to certain properties of our alloys at elevated temperatures, we mean that these properties are particularly outstanding in the range of 400 to 600 F., however, the advantageous properties of our alloys'are not confined to that temperature range.

The examples of the beryllium-containing alloys given hereinabove are for the purpose of illustrating our invention and are not to be regarded as limiting its scope. Other alloy compositions within the range set forth above posabout 20 to 40 per cent beryllium, 1 to 8 per cent 1 magnesium, and the balance substantially a1uminum, said alloy being characterized by a high tensile strength at elevated temperatures combined with a relatively high thermal conductivity.

' 2. An aluminum base alloy consisting of from 22 to 30 per" cent beryllium, 1.5 to 5.0 per cent magnesium, and the balance substantially aluminum, said alloy being characterized by a high tensile strength at elevated temperatures combined with a relatively high thermal conductivity.

LOUIS w. KEMPF. WALTER A. DEAN.