US2752240A - Aluminum base alloy bearing - Google Patents

Description

ALUMINUM BASE ALLOY BEARING Alfred W. Schluchter, Dearborn, Mich., assignor to General Motors Corporation, Detroit, Mich, a corporation of Delaware No Drawing. Application December 27, 1952, Serial No. 328,266

7 Claims. (Cl. 75148) This invention relates to an aluminum base alloy and particularly to an improved alloy of this type which is especially suitable for use as a bearing material in the as-cast condition.

Aluminum and most of its alloys are generally quite unsuitable for use in bearings for ferrous metal machine parts because the aluminum tends to adhere to, or combine with, the ferrous metal, thereby causing scoring or seizing. I have found, however, that by suitable combination of alloying constituents, this difticulty can be overcome and a bearing alloy produced having not only anti-friction properties but other characteristics especially suitable in a bearing material.

Many aluminum base alloys, such as the type disclosed in Patent No. 2,238,399, which issued April 15, 1941 in the name of Alfred W. Schluchter, are satisfactory bearing materials in most respects. However, most of these alloys do not possess sufficient hardness, especially in the as-cast state, to enable them to be satisfactorily used for many purposes. In the past, many metals have been added to aluminum to improve its hardness, but most of these elements, such as iron, also impair the frictional properties of aluminum and aluminum base alloys. Likewise, many of the alloys thus produced cannot be satisfactorily used in the as-cast condition.

Accordingly, a principal object of the present invention is to provide an inexpensive aluminum base bearing alloy which possesses satisfactory hardness and high score resistance. A further object of this invention is to provide an aluminum base alloy which has highly desirable frictional properties when used as a bearing in the as-cast condition and which also may be heat treated to further increase its hardness.

In accordance with my invention, therefore, the foregoing and other objects and advantages are attained to a particularly high degree in an aluminum base alloy containing minor proportions of silicon, bismuth and chromium. Inasmuch as an alloy of this composition is a stronger metal than the aluminum alloys generally heretofore used for bearing purposes, solid bearings may be formed from it and no backing of steel or similar metals is necessary. If desired, a bearing formed from my alloy may be advantageously provided with a thin overlay of lead or a lead base alloy. Examples of these overlays include the lead-tin and lead-indium alloys which are used for this purpose and in which lead is the major constituent.

The alloy formed in accordance with the present invention is characterized by greater hardness and a correspondingly longer fatigue life than related aluminum base alloys heretofore used. As a result of this hardness, solid bearings made from this alloy retain their original shapes better than many of the bearings which up to the present time have been formed of softer alloys. The former do not take a set at temperatures to which they are normally subjected, and they undergo a negligible amount of shrinkage after extensive use. This type of nite States Patent inexpensive bearing alloy is found to be particularly valuable for low speed applications where it may be used in as as-cast condition. Hence it may be satisfactorily employed as a cast bell end bearing in fractional horsepower motors.

Accordingly, I have found that satisfactory bearing properties are obtained with an alloy comprising, by weight, approximately 0.2% to 10% silicon, 0.1% to 5% bismuth, 0.1% to 0.8% chromium, and the balance substantially all aluminum. Various incidental impurities may be included in this alloy in the usual small amounts without any substantial detrimental effects. Thus the term aluminum, as used herein, embraces the usual impurities which are found in aluminum ingots of commercial grade or which are introduced during the handling operations incident to ordinary melting practice. For example, iron, which together with silicon is found in commercial aluminum, may be present in amounts not greater than approximately 0.5% without causing any harmful results. For optimum results I have found that an alloy should be used which consists essentially of ap proximately 2% to 5% silicon, 0.3% to 2% bismuth, 0.2% to 0.35% chromium, and the balance substantially all aluminum.

Under severe test conditions, alloys having the above compositions show excellent anti-friction properties so that bearings formed of this alloy do not score or gall when in contact with a rotating steel shaft, and neither the shaft nor the bearing show an appreciable amount of wear after long and severe use. I have also found that the resistance of this alloy to cracking or crumbling is extraordinarily high.

The inclusion of silicon in my aluminum base bearing alloy improves its frictional properties and also increases its strength. Hence, in order to obtain a high degree of score resistance and adequate strength, it is necessary that the alloy have a silicon content of at least 0.2%, and for best results it is desirable to use at least 2% silicon. More than approximately 10% silicon should not be included in the alloy, however, because of casting difficulties; and the resultant bearing is too brittle for practical applications if the silicon content is excessive. Furthermore, while an increased silicon content improves score resistance, the addition of silicon in amounts greater than about 5% provides only slight additional beneficial properties in this respect. Accordingly, best results are obtained for most purposes when the silicon content is kept within the preferred range of approximately 2% to 5%.

In the present aluminum base bearing alloy, the bismuth greatly improves the frictional properties of the alloy and permits it to be used in the as-cast condition. However, an excessively high bismuth content impairs the ductility of this alloy to too great an extent, and hence the bismuth should not be included in amounts greater than approximately 5%. A greater bismuth content results in an alloy which not only is too brittle for most applications but which also presents casting problems. On the other hand, if the bismuth content is below 0.1%, the bearing alloy will have poor frictional properties and will be unsuitable for use as a bearing. In view of these factors, it is generally preferred to maintain the bismuth content between 0.2% and 1.5%. Even relatively high percentages of bismuth within the aforementioned ranges, although lowering the ductility of the formed alloy some- What, are not particularly detrimental in the manufacture of bearings since bearings formed in accordance with the invention may be used in the as-cast condition and require very little ductility.

The chromium confers hardenability, high strength and machinability to the resultant alloy. While the hardness Patented June 26, 1956 I may be substantially reduced if the chromium content is too low, the addition of approximately 0.3% chromium is all that is necessary in order to obtain a completely satisfactory degree of hardness. Moreover, a chromium content of only 0.2% chromium increases the hardness of the alloy considerably and makes it suitable for many bearing applications. I

The addition of chromium in amounts greater than about 0.8%, however, reduces the ductility of the resultant alloy to too great an extent, a high ductility being particularly necessary if the material is to be used as a wrought alloy. It is also not feasible to add more than 0.8% chromium because increasing the chromium content above this amount raises alloy costs by greatly in creasing the difficulty in casting and fabrication of the cast parts. Too high a temperature is required to place and hold greater quantities of chromium in solution in the liquid state, the chromium segregating out unless the temperature of the melt is raised excessively. The resultant formation of hard spots in the alloy prevents the obtaining of a uniform casting. A chromium content below 0.1%, on the other hand, is insutficient to confer the necessary hardness and strength to the alloy. Furthermore, the score resistance of the alloy is slightly improved as the chromium content is increased. As a result of the above considerations, 1 have found that a chromium content within the preferred range of approximately 0.2% to 0.35% provides excellent results in all respects.

An example of the above alloy which possesses the aforementioned desirable characteristics to an outstanding degree, therefore, is one consisting of 4% silicon, 1% bismuth, 0.3% chromium, and the balance all aluminum and incidental impurities.

In order to obtain the high degree of resistance to pounding, such as is encountered in a bearing, it is preferable that the alloy have a physical structure typified by the absence of continuous networks of relatively brittle eutectic mixtures. Unlike many of the aluminum base bearing alloys heretofore used, alloying the above composition does not involve problems due to vaporizing of any of the constituents, and close controls are not required because all of the elements used have relatively low vapor pressures. The temperature of the melt may be raised as high as approximately 1600 F. before difiiculty is encountered because of vaporization of bismuth, the element in the present alloy having the highest vapor pressure. Simple, conventional alloying procedures may be employed in the present instance, therefore, with intermediate alloys, such as aluminum-silicon and aluminum-chromium alloys, being used to introduce the silicon and chromium. The bismuth may then be added to the melt, which is subsequently stirred and cast in the desired form, usually in metal or graphite molds.

Cast articles having a metallographic structure showing a continuous network of segregated metal compounds may be improved as to strength and fatigue resistance by suitable heat treatment. For example, I have found that a solution treatment at a temperature between approximately 900 F. and 1050 F. for a period of eight to fifteen hours is particularly effective. Upon removing the alloy from the furnace following the solution treatment, it is preferable to cool it immediately by quenching in water. This treatment serves to increase the ductility of the alloy to an appreciable extent.

A precipitation treatment may thereafter be employed to provide the alloy with even greater hardness. This process is preferably carried out by heating the article for five to ten hours at a temperature in the range between approximately 300" F. and 400 F., a precipitation treatment at 370 F. for eight hours being particularly satisfactory. The alloy then may be again cooled, preferably in water, and suitably machined. Such a heat treating process results in an article which is approximately three or four times as hard as it was in the as-cast condition and whose fatigue strength is proportionally improved.

The specific gravity of the above-described alloy is about one-third that of a tin-bronze bearing alloy, and has much greater resistance to fatigue or to cracking under the pounding action to which many types of bearings are subjected. This property renders such an alloy suitable as a bearing for use under extreme conditions, tests on such bearings indicating the relative absence of wear, either of the hearing or the shaft. In addition, the alloy appears to be resistant to corrosion by acid constituents of lubricating oils which attack many other bearing compositions.

It is to be understood that, while the invention has been described by means of certain specific examples, the scope of the invention is not to be limited thereby except as defined in the following claims.

I claim:

1. A hearing formed of an alloy consisting essentially of approximately 0.2% to 10% silicon, 0.1% to 5% bismuth, 0.1% to 0.8% chromium, and the balance substantially all aluminum.

2. A hearing formed of an alloy having high antifrietion properties and fatigue resistance in the as-cast condition, said alloy consisting of approximately 2% to 5% silicon, 0.3% to 2% bismuth, 0.1% to 0.8% chromium, and the balance aluminum plus incidental impurities.

3. A bearing as in claim 2 in which a surface thereof is provided with a thin overlay of a metal of a class consisting of lead and lead base alloys.

4. A bearing characterized by high anti-friction properties and resistance to disintegration under impact and to attack by acids developed in lubricating oils, said bearing being formed of an alloy consisting essentially of 2% to 5% silicon, 0.1% to 5% bismuth, 0.2% to 0.35% chromium, iron not in excess of 0.5%, and the balance substantially all aluminum.

5. A hearing formed of an alloy consisting of 0.2% to 10% silicon, 0.1% to 5% bismuth, 0.1% to 0.8% chromium, and the balance aluminum.

6. A hearing formed of an alloy having high antifriction properties and fatigue resistance in the as-cast condition, said alloy consisting of 2% to 5% silicon, 0.3% to 2% bismuth, 0.1% to 0.8% chromium, and the balance aluminum plus incidental impurities.

7. A hearing characterized by high score resistance formed of a heat treatable alloy consisting of 2% to 5% silicon, 0.3% to 2% bismuth, 0.2% to 0.35% chromium, iron not in excess of 0.5%, and the balance aluminum.

References Cited in the file of this patent UNITED STATES PATENTS 1,940,922 Rainer Dec. 26, 1933 2,026,541 Kempf et al. Jan. 7, 1936 2,026,543 Kempf et al. Jan. 7, 1936 2,026,557 Kempf et a1. Jan. 7, 1936 2,076,578 Kempf et al. Apr. -13, 1937 2,238,399 Schluchter Apr. 15, 1941 2,325,071 Murray July 27, 1943 2,531,910 Hensel et al Nov. 28, 1950

Claims (1)

1. A BEARING FORMED OF AN ALLOY CONSISTING ESSENTIALLY OF APPROXIMATELY 0.2% TO 10% SILICON, 0.1% TO 5% BISMUTH, 0.1% TO 0.8% CHROMIUM, AND THE BALANCE SUBSTANTIALLY ALL ALUMINUM.