US2286625A - Aluminum base alloy - Google Patents

Description

Patented June 16, 1942 ALUMINUM BASE ALLOY 1 Louis W. Kempf and Walter A. Dean,- Lakewood,- Ohio, assignors to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application December 30, 1941,

' Serial No. 424,892

i 2 Claims. (01. 75-142) This invention relates to aluminum that are especiallyadapted for temperatures.

Most of the applications where aluminum base alloys are employed involve exposure to the usual base alloys use at elevated atmospheric temperature range. However, there.

are places where it is necessary to use aluminum alloy parts at elevated temperatures, for example, in internal combustion engines.

the range of 400 to 600 F. The demand for light alloys which can be employed at elevated temperatures has been increased by the demand for more powerful motors for aircraft. It is an object of this invention to provide an alloy com position which exhibits high strength and resistance to deformation at elevated temperatures, especially at the higher 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 alloy which possesses a high fatigue strength at elevated temperatures. Still another object is to provide an aluminum base alloy having a higher modulus of elasticity than the common aluminum base alloy now in use.

We have discovered that aluminum base alloys containing from about 20 to 40 per cent beryllium, from 0.1 to 3.0 per cent magnesium, from 0.5 to 5.0 per cent copper, and the balance substantially aluminum possesses the aforementionedxproperties. More particularly, we have found that some of the alloys within this range possess tensile strengths at elevated temperatures which are considerably greater than that of alloys heretofore used for such service. Furthermore, this increase in strength is accompanied by a lower density than aluminum, a rela-.-

, tively high thermal conductivity, a high modulus of elasticity, and high fatigue strength. This Some of the parts are exposed to temperatures withinnecting rods. Our alloys'may be used in either cast or wrought form but we prefer to use them in wrought form.

The tensile properties and fatigue strength at a an elevated temperature of some examples of our alloys at an elevated temperature and the thermal conductivity at room temperature of one of these 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 Tables I and 11 below. The first of the two alloyswhich 3 contained no beryllium, i. e. the AlCuNiMg alloy may be considered as being typical ofprior aluminum base alloys designed for service at elevated' temperatures. The balance of the compositionof each of thealloysappearing in the tables was aluminum and the usual impurities.

The beryllium-containing alloys were cast as ingots and extruded into the form of rods.

The thermal conductivity and tensile property determinations were made on specimens taken from the extruded rods while the test specimens of the first two aluminum base alloys containing no beryllium in Table I were taken from forged rods, and the specimens of the last alloy were sand cast. The difierence in fabricating practices used in making the wrought material is considered to have no significant eifect upon thetest results reported here. The last alloy .in the Table Lpossesses about the highest tensile strength at elevated temperatures of any aluminum base alloy known heretofore but it also has a relatively low thermal conductivity. This 'alcombination of properties, especially that of high tensile and fatigue strength and relatively high thermal conductivity, makes the alloys 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 structures having a greater resistance to distortion under a load with a given section thickness or the same resistance toidisto'rtion' loy was tested in the form of a sand casting because that-isthe form in which it has been employed for 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) were heated at 600 .F. for three hours before being subjected to any of the treatments andvtests herein described in order to duplicate the condition of the alloys in many commercial applications. Specimens of the first beryllium-containing alloy given in Table I below likewise received a-solution heat treatment and artificial aging. All of the test bars of the wrought alloys for tensile strength determinawith a lighter section as compared to structures made fromalloys having lower modulus values. The highfatigue' strength of the alloys renders them particularly useful for such articles as contions were subjected to a short time test at elevated temperatures consisting of first stabilizing them byheating-them for 16 hours at 700 F. This preliminary treatment served to accelerate any changes which would have occurred on exposure to a lower temperature over a long'period of time. We have found from a number of other 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 for tensile strength determinations were cooled to room temperature, then reheated to the testing temperature, in this case 600 F., held at this temperature for one-half hour, and then 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 temperature 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 Wiedemanm 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 close to those at elevated temperatures such 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.

The fatigue strength at 500 F; was determined on specimens which had previously received a solution heat treatment and artificial aging. The number of cycles which the test bars withstood prior to failure at the indicated loads is given below in Table II.

TABLE I also indicate a greater resistance to deformation at elevated temperatures. These tensile properties therefore indicate that the alloys containing beryllium are much better adapted for service at such high temperatures as 600 F. thanthe two wrought aluminum base alloys which have been employed heretofore for that purpose. It is also to be observed that the thermal conductivity of the alloy containing 27.8 per cent beryllium is very close to that of the other two wrought alloys used for comparison, 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 alloys described above, a relatively high thermal conductivity has been associated with a low strength. It is the combination of high tensile and fatigue strengths and a relatively high thermal conductivity which characterizes our alloy. 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, and is also high with respect to the thermal conductivity of the cast alloy. Again it is to be noted that the fatigue strength of the beryllium-containing alloys considerably exceeds that of the aluminum base alloys with which they are compared.

Modulus of elasticity determinations were made at room temperature on the preceding alloy containing 23.2? per cent beryllium and compared with the values for two'other aluminum base alloys described above. All three alloys were in thesolution heat treated and artificially aged condition when tested. The test results are given below in Table III.

Tensile properties at 600 F. and thermal conductivity at room temperature Alloy composition Thermal Tensile Elongation conductivity strength in 2 inches (0. G. S Be Mg Cu Ni Si Mn units) Percent Percent Percent Percent Percent Percent Lbalsq. in ercent I 27. 61 0. 3. 97 14, 000 9. 0 27.80 1.36 3. 29 17,200 7.5 0.33 0.5 4.0 2.0 6,000 75.0 0.37 l. 0 0. 9 0.9 12. 5 5, 600 54. 0 0.32 6.0 1.5 1.0 13,700 10.5 0.20

TABLE II 55 TABLE III Fatigue strength at 500 F, Modulus of elastrczty at room temperature Alloy composition Alloy composition Cycles to failure at Modulus Be Mg Cu Ni Si Be Mg cu Ni Si 9,000 1Il1)s./sq. 20,000hl1bs./sq. 1 Percent Percent Percent Percent Percent Lila/sq. i 1|. 23. g5 g3 16 (D0 000 I Per- Per- Per- Percent Percent cent cent cent 0 9 000 27 35 03 ii. 388 38g 8 ;g 0001000 721000 The superiority of the beryllium -containing alloy over the other alloys is readily apparent It will be noted that the tensile strength of the beryllium-containing alloys at the elevated temperature 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-containing alloys 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 relatively high thermal conductivity, a high resistance to -fatigue, a high strength at elevated temperasistance to fatigue at elevated temperatures and together with copper renders the alloy susceptible to improvement by thermal treatment. We have found that at least 0.10 per cent of magnesium is desirable to achieve this purpose, while on the other hand, if more than 3.0 per cent is used, fabricating difficulties are encountered. The addition of copper increases strength and resistance to fatigue at elevated temperatures and also servesto increase the susceptibility of the alloys to improvement in strength by conventional thermal treatments. At least 0.5 per cent copper is desirable to achieve thispurpose, while if more than 5.0 per cent is used, fabrication becomes diflicult. Alloys which contain from 22 to 30 per cent beryllium, 0.5 to 1.5 per cent magnesium, and 2.0 to 4.0 per cent copper 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 may be present in the alloys without alfecting 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.

Y 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 properties at elevated temperatures.

We claim:

1. An aluminum base alloy consisting of from about 20 to 40-.per centsberyllium, 0.1 to -3.0 per cent magnesium, 0.5 to 5.0 per cent copper, and the balance substantially aluminum, said alloy being characterized by high tensile and fatigue strengths at elevated temperatures combined with a relatively high thermal conductivity.

2. An aluminum base alloy consisting of from a 22 to 30 per cent beryllium, 0.5 to 1.5 per cent magnesium, 2 to 4.0 per cent copper, and the balance substantially aluminum, said alloy being characterized by high tensile and fatigue strengths at elevated temperatures combined with a relatively high thermal conductivity.

' LOUIS W. KEMPF.

WALTER A. DEAN.