US2286627A - Aluminum base alloy - Google Patents

Description

Patented June is, 1942 ALUMINUM BASE ALLOY 1 Louis W. Kempf and Walter A. Dean, Lakewood, Aluminum Company of America, Pittsburgh, Pa., a corporation of Ohio, assignors to Pennsylvania No Drawing. Application December 30,1941, Serial No. 424,894

2 Claims. (Cl. 75-147) 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 parts at elevated temperatures, for example, in internal combustion engines. parts are exposed to temperatures within 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 obiect of this invention to provide an alloy composition 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. Another object is to provide an aluminum base alloy having a higher modulus of elasticity than the common aluminum base alloys now in use.

We have discovered that an aluminum base alloy containing from about 20 to 4-0 per cent beryllium, from 0.25 to 3 per cent magnesium, from 0.5 to 4 per cent silicon, and the balance substantially aluminum possesses the aforementioned properties. 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 the alloys heretofore used in such service.

Some of the Furthermore, this increase in strength is accompanied by a lower density than aluminum, a relatively high thermal conductivity, a high fatigue strength, and a high modulus of elasticity. This combination of properties, especially that of high tensile and fatigue strength and relatively high thermal conductivity, makes the alloys particuwith a lighter section as compared to structures made from alloys having lower modulus values. The high fatigue strength of our alloys renders .a long period of time.

number of other tests that such preliminary stathem particularly useful for such articles as connecting 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 an elevated temperature of some examples of our alloys, and the thermal conductivity at alloysdesigned for service at elevated temperatures. The balance of the composition of each of the alloys appearing in the tables was aluminum and the usual impurities. The beryllium-containing alloys were cast as lngots'and extruded into the form of rods.

The thermal conductivity and tensile property determinations were madeon specimens taken from extruded'rods, while the test specimens of.

the other aluminum base alloys were taken from forged rods. The difference in fabricating practices used in making the rods' is considered as having no significant effect upon the test results reported here. 'The specimens without berylliumreceived 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 beryllium-containing alloys were. given a solution heat treatment and artificial aging. All of the test bars for tensile strength determinations were first stabilized by heating them for 16 hours at700 F. This preliminary treatment served to accelerate any changes which would have occ red on exposure to a lower temperature over We have found from a bilizing treatment, for a relatively short period 'of time at a temperature higher than encountered in service afiects 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 thermal conductivity of the alloy '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 toiailure at the indicated load is given in Table II below.

' TABLE I- temperature for one-halt hour, and then broken 26.75 per cent beryllium is definitely superior to that of the two alloys containing no beryllium. It is the combination of high tensile and fatigue strengths and a 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. Again it is to be noted that the fatigue strength oi the beryllium-containing alloys exceeds that of the aluminum base alloys with which it is compared.

Modulus of elasticity determinations were made at room temperature on two alloys containing 23.36 and 26.85 per cent beryllium, respectively, and on four common aluminum base alloys containing no beryllium. The alloy containing 23.36 per cent beryllium was tested in the as-extruded condition, while the. other alloys were given a solution heat treatment and artiflcial aging. As is well recognized, such a dinerence in condition of the alloys would not ailect the modulus values. The test results are given belowin Table III.

Tensile properties at 600 F. and thermal v conductivity at room temperature Alloy composition Thermal Tensile Elongation conductivity strength in 2 inches (0. G. 8. Be Mg Cu Ni Si Cr units Percent Percent Percent Percent Percent Percent Lballq. in. Percent TABLE II Fatigue strength at 500 F.

Alloy composition Cycles to failure at- I 5300 0000 20,000 Ba M! Ni I 9' lbs./sq. m. lbs/sq. in. lbs./sq. in.

It will be noted that the tensile strength of the T m beryllium-containing alloys at the elevated temperature exceedsthat of the four aluminum base compositions used for comparison, even the first two alloys that have been employed in service at elevated temperatures. The lower elongation values of the beryllium-containing alloys also indicate a greater'resistance to deformation at elevated temperatures. These tensile proper-.

ties therefore indicate that these alloys are much better adapted for service: at such high temperatures as 600 F. than the two aluminum base alloys which have been employed heretofore for that purpose. It is also to be observed that the containing Modulus of elasticity at room temperature The superiority of the beryllium-containing alloy overthe other alloys is readily apparent from these data. From other determinations we have had 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 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 to obtain the combination of properties described above. A minimum of about 20 per cent of this element has been found to be necessary to obtain such a combination of properties, but that if more than 40 per cent is employed, the alloy becomes very difiicult to-work. The presence of magnesium in the alloy considerably enhances the strength and resistance to fatigue at elevated temperatures. We have found that at least 0.25 per cent of this element is necessary to achieve this purpose, while on the other hand, if more than 3 percent is used, fabricating difliculties are encountered. The addition of silicon serves to increase the susceptibility of the alloys to improvement in strength by conventional thermal treatments, the greatest benefit being obtained from using from 0.5 to 4 per cent of this element. Alloys which contain from 22 to 30 per cent beryllium, 0.5 to 1.5 percent magnesium, and 0.? to 1.5 per cent 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 silicon are preferred because they loys given hereinabove are ,with a relatively present in the alloys without afiecting 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 are therefore excluded-from the scope of this invention.

In referring. to certain properties of our alloys at elevated temperatures, we mean that these properties areparticularly 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 alfor the purpose of illustrating our invention and are not to be re,- garded as limiting its scope. Other alloy compositions within the range set forth above possess equally satisfactory properties at elevated temperatures.

We claim: a v

1. An aluminum base alloy consisting of from about 20 to 40 per cent beryllium, 0.25 to 3 per cent magnesium, 0.5 .to 4 per cent silicon, 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 adapted for service at elevated temperatures 22 to 30 per cent beryllium, 0.5 to 1.5 per cent magnesium, 0.7 to '1.5 per cent silicon, and the balance substantially aluminum, said alloy being characterized by high tensile and fatigue strengths at elevated temperatures combined high thermal conductivity.

LOUIS W. KEMPF. WALTER A. DEAN.

consisting of from