US2369354A - Aluminum base alloy - Google Patents

Description

v values.

' Patented Feb. 13, 1945 s, PATENT OFFICE ALUMINUM BASE Anton Louis W. Kemp! and Walter A. Dean, Lakewood,

Ohio, assignors America, Pittsburgh, Pa.,

I Pennsylvania to Alumin um Company ot a corporation of No Drawing. Application June Ct, 1942,

- Serial No. 346,140

(or. rs-sssl' 2 Claims.

This invention relates to aluminum base alloys that are especially adapted for use at elevated temperatures.

Most of the applications where aluminum base alloys are employed involve exposure to the usual atmospheric temperature range. However, there are places where it is necessary to use aluminum alloy articles at elevated temperatures, for example, in internal combustion engines. Some of the parts are exposed to temperatures within the range of 400 to 600 F. The demand for light alloys which can be employed at-such elevated temperatures has been increased by ,the demand for more powerful motors for aircraft, It is an object of this invention to provide analloy composition which exhibits high strength and resistance to deformation at elevated temperatures, especially at the high temperatures found in the newer aircraft motors. A particular object is to.

provide an alloy which possesses a high thermal conductivity along with the high strength at elevated temperatures. Another object is to provide an aluminum base alloy which has a higher modulus of elasticity than the common commer- 'cial aluminum base alloys now in use.

We have discovered that aluminum base alloys containing from about 15 to 50 percent beryllium, from 5 to 20 per cent silver, and the balance substantially aluminum possess the aforementioned properties. Mor particularly, we have found that some of the alloys in this range possess a tensile strength at elevated temperatures which is considerably greater than that of the alloys heretofore used for such service. Furthermore, this increase in strength is accompanied by a lower density than aluminum, 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 out-.

standing. and particularly useful for such articles as valve push rods, pistons, and the like which are highly stressed at elevated temperatures. The high modulus of elasticity of these alloys makes it possible to design a structure having a greater resistance to distortion under a load with a given section of thickness or the same resistance to distortion with a lighter section as compared to astructure made from alloys having lower modulus Our alloys may be used in either cast or wrought form, but we prefer to use them in wrought form.

The tensile properties at an elevated temperature of two of our improved alloys and the thermal conductivity at room temperature of one of treatment served to accelerate any changes which would have occurred on exposure to :a lower temperature over a long period of time. We have service, are given in Table l below. The first of the two alloys which contained 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 temperatures. The balance of the composition of each of the alloys appearing in the table was aluminum and the usual impurities. The beryllium-containing alloys were cast as ingots extruded in the form of rods.

The thermal conductivity and tensile property tests were made on specimens taken from these extrusion rods, while tensile property determinations on the two alloys containing no beryllium were made on the specimens taken from forged rods. The difierence in fabricating practices used in making the wrought material is considered to have no significant effect On the test results reported here. The specimens from these two alloys received the conventional solution heat treatment and artificial aging 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 alloys were subjected to short time test at elevated temperatures .consisting of a first stabilizing the bars by heating them for 16 hours at 700 F. This preliminary found from a number ofotl er tests that such preliminary stabilizing treatment for a relatively short period of time at a temperature higher than encountered in service affects properties to a comparable extent as more-extended periods at,

the temperature of service operation. Following the preliminary stabilizing treatment the bars were cooled at room temperature and then reheated to. the testing temperature, in this case 600 F., held at this temperature for /2 hour, and finally broken in tension at 600 F. in the usual manner.

The thermal conductivity values were calculated from electrical resistivity measurements made at room temperature. The calculations were based on the well recognized Wiedemann-Franz-Lorenz relationship between the thermal conductivity and electrical resistivity of metals.' It is generally true that there is such a small change in thermal conductivity of aluminum base alloys over the range of room temperature to about 600 F. that values at room temperature are very exist in internal combustion engines. The test specimens for electrical resistivity measurements were in the-same temper as that of the specimens 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 a used in tensile tests prior to the stabilizingtreat- 5 high strength at elevated temperatures. A miniment. mum ofabout per cent of this element has TABLE I Tensile properties at 600 F. and thermal coriductivity at room temperature Alloy composition Them'ml 5233 Percent conductivity 9 Percent Percent Percent Percent Percent -Percent 'lbsJsq. in. el'ongatmn. a g S Be Ag Cu Mg Ni Si It will be noted that the tensile strength of the been found necessary to vobtain combinations ofberyllium-containing alloys at the elevated temperature exceeds that of the two aluminum base compositions containing no beryllium. The lower elongation values of the beryllium-containing alloys also-indicate a greater resistance to deformation at elevated temperatures. These tensile properties therefore indicate that these alloys are' much better adapted for service at such elevated temperatures as 600 F. than the two wrought aluminum base alloys which have been widely employed heretofore for that purpose. It is'to be observed also that the thermal conductivity of the beryllium-containing alloys exceeds that of the other two alloys used for comparison.

' It is the combination of a high strength and relatively high thermal conductivity which characterizes our alloys. A thermal conductivity of 0.3 C. G. S. units or more is considered to be relatively high for alloys employed in service at elevated temperatures.

Modulus of elasticity determinations at'room temperature are made on the above alloy con-- taining 21.13 percent beryllium in the as-extruded condition and compared with the values for the other two aluminum base alloys in the solution heat treated and artificially aged condition as described above. .As is well recognized, such a difference in condition of the alloys would not afiect the modulus values. Test results are given below in Table II.

The superiority of the beryllium-containing alloy over the other com-positions is readily apparent from these data. From other determinations we have made at elevated temperatures, we have found that the alloys containing beryllium retain this superiority by a wide margin. The

relatively high modulus at elevated temperatines means that structures made from such al- .loys' are more resistant to distortion and hence maybeexpeetedto glvelonge rservice.

properties desired, but if more than 50 per cent is employed, the alloy becomes very .difficult to work.v The presence of silver in the alloy enhances the strength at elevated temperatures.

We have found that at least 5.0 per cent of this element is desirable to achieve this purpose, while on the other hand, if more than 20 per cent is used, fabricating difiiculties are encountered. Alloys which contain from 20 to 40 per cent beryllium and 5 to 15 per cent silver are preferred because they possess the most satisfactory 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 maybe present in the alloys without affecting the high temperature properties described above. The

presence of any elementslwhich substantially impair the strength and thermal conductivity prop- I 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 possess equally satisfactory We claim: v

1.- An' alumimiin base alloy consisting of from about 15 to 50 per cent beryllium, 5 to 20 per combinations of properties.

cent silver. and the balance substantially aluminum, 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 20 to 40 per cent beryllium, 5 to 15 per cent silver, and the balance substantially aluminum, said alloy being characterized by a high tensile strength at elevated temperatures combined with a relatively high thermal conductivity.-

' LQUIS W. KEMPF. WALTER A. DEAN;