Aluminum Base Bearing Alloy and Bearing Composite

(1) Field of the Invention

This invention relates to aluminum base bearing alloys and bearing materials and, particularly, to aluminum base bearing alloys in which a specified number of coarse intermetallic precipitates of a specified size are dispersed in the aluminum matrix.

(2) Background of the Invention

Aluminum alloys are widely used as bearings in internal combustion engines, e.g., as connecting rod bearings and crankshaft bearings in automobile and marine engines. These bearings are resistant to corrosion in the engine environment and thus are highly advantageous for such use.

Materials employed as bearings in internal combustion engines are required to withstand high loads and high temperatures. Much effort has been directed in recent years, therefore, to providing aluminum base bearing alloys having high resistance to seizure, fatigue and wear under the conditions encountered in these engines.

U.S. Pat. No. 4,153,756 discloses Al-Sn base bearing alloys having a low degree of softening and, consequently, high fatigue strength, under high temperature conditions. The alloy is provided by adding chromium or zirconium to a basic alloy consisting of 10 to 30 wt.% tin and the remainder aluminum. Copper or both copper and beryllium can also be added to the alloy.

Al-Sn base bearing alloys comprising 3.5-35 wt.% of Sn, 0.1-1.0 wt.% of Cr and 1-10 wt.% in total of one or more members selected from Si, Cr, Mn, Sb, Ti, Zr, Ni and Fe, the remainder being aluminum, are also disclosed in the prior art as having high fatigue strength and, additionally, good wear resistance.

G. C. Pratt and C. A. Perkins in a paper entitled "Aluminum Based Crankshaft Bearings for the High Speed Diesel Engine", SAE Technical Paper Series 810199 (1981), describe the development of an engine bearing lining alloy for the high speed diesel engine and which has the composition Si (11 wt.%), Cu (1 wt.%) and the remainder of Al. The alloy is described as having superior seisure resistance as compared to lead bronze on test rigs under conditions of sparse lubrication and controlled misalignment. It is further described that the casting procedure adopted and the subsequent processing of the alloy to bimetal ensure that the silicon particles are restricted to a few microns in size and that the silicon particle size in an important factor in determining the extent of compatibility (defined in the paper as the resistance of a bearing alloy to local welding on to a steel counterface); the highest degree being obtained only when there is a complete absence of coarse particles. The casting procedure and processing of the alloy to bimetal, however, are not described.

The mere inclusion of silicon or of other elements which form precipitates in an aluminum base bearing alloy, however, does not ensure that the bearing alloy will possess consistently superior resistance to seizure, fatigue and wear under the severe loads and temperature conditions encountered in modern internal combustion engines and, particularly, in automobile engines which have shafts made of spheroidal, or nodular, graphite cast iron or other coarse material.

It is an object of the present invention, therefore, to provide an aluminum base bearing alloy and bearing composite which consistently possesses a superior balance of seizure, fatigue and wear resistance under high loads and high temperatures and when used as a bearing surface with nodular cast iron materials.

According to the present invention this and other objects are obtained by providing an aluminum base bearing alloy which contains from 1 to 11% of at least one of Mn, Fe, Mo, Ni, Zr, Co, Ti, Sb, Nb and Cr and 1.5-35 wt.% Sn and in which at least 5 particles of intermetallic compounds which have a nodular, or "lumpy" shape and a diameter of at least 5 .mu.m areprovided per 3.56.times.10.sup.-2 (mm).sup.2 of cross-sectional area of the bearing alloy. The bearing alloy of the invention may optionally include up to 8 wt.% of Pb, In, Tl, Cd or Bi and up to 2 wt.% of Cu or Mg.

In other embodiment of the invention a bearing material is provided by pressure welding the bearing alloy to a backing steel.

FIG. 1 is a graph showing the range of seizure unit loads (seizure resistance) of aluminum base bearing alloys according to the present invention in which the number and size of particles of intermetallic compounds were controlled within specified limits; the seizure unit loads being shown as a function of the weight percent of the alloys of the elements which form the intermetallic compounds.

FIG. 2 is a graph showing the range of seizure unit loads of alloys having the same compositions as the alloys of FIG. 1 but in which the number and size of particles of the intermetallic compounds were not controlled within the specified limits.

FIG. 3 is a graph showing the range of fatigue unit load (fatigue resistance) of the same alloys of FIG. 1; the fatigue unit loads also being shown as a function of the weight percent of the alloys of the elements which form intermetallic compounds in the aluminum matrix.

FIG. 4 is a graph showing the differences in the range of wear amount of the alloys of FIG. 1 and the alloys of FIG. 2.

FIG. 5 is a graph showing the seizure unit loads of an aluminum base bearing alloy according to the present invention and of a conventional Al-Sn(20)-Cu(1) alloy as a function of surface roughness of a supported shaft.

FIG. 6 is a graph showing the seizure unit loads of an aluminum base bearing alloy according to the present invention and of a conventional Al-Sn(20)-Cu(1) alloy as a function of lubricating oil temperature.

FIG. 7 is a graph showing the seizure unit loads or an aluminum base bearing alloy according to the present invention and of a conventional Al-Sn(20)-Cu(1)alloy as a function of the type of supported shaft.

FIG. 8 is a graph showing the wear amount of an aluminum base bearing alloy according to the present invention and of a conventional Al-Sn(20)-Cu(1) alloy as a function of time.

The aluminum base bearing alloy according to the present invention containing from about 1 to 11 wt.% of at least one element selected from Mn, Fe, Mo, Ni, Zr, Co, Ti, Sb, Nb and Cr and from 1.5 to 35 wt.% of Sn. The elements Mn, Fe, Mo, Ni, Zr, Co, Ti, Sb, Nb and Cr form intermetallic compounds in the aluminum base alloy of the invention. Intermetallic compounds are compounds of two or more metals and have a distinctive crystallographic structure and definite composition or composition range. Examples of the intermetallic compounds formed in the alloys of the present invention include MnAl.sub.4, MnAl.sub.6, FeAl.sub.3, MoAl.sub.3, NiAl.sub.3, ZrAl.sub.3, Co.sub.2 Al.sub.9, TiAl.sub.3, AlSb, NbAl.sub.3 and CrAl.sub.7. It is not possible to investigate all possible forms of the intermetallic compounds.

Amounts of less than about 1 wt.% of the elements which form the intermetallic compounds will not allow the formation of the coarse precipitates required to achieve the outstanding resistance to seizure, fatigue and wear of the bearing alloys of the present invention. Amounts of more than about 11% of these elements are not desirable since the strength of the alloys is reduced and the seizure, fatigue and wear resistance of the alloys also decrease. When two or more of these elements are contained in the alloy of the invention, the combined amount is from 1 to 11 wt.%, the minimum content of each element being 0.1 wt.%. Alloys containing a total of from about 2 to 5 wt.% of the elements which form the intermetallic compounds have an outstanding balance of seizure, fatigue and wear resistance properties.

The intermetallic compounds are present in the form of precipitates in the aluminum matrix of the alloys of the invention. At least 5 particles per 3.56.times.10.sup.-2 (mm).sup.2 of cross-sectional area of the bearing alloys must be in the form of coarse precipitates, i.e. nodular particles, having a diameter of at least 5 .mu.m. As employed herein to define the shape of the particles, the term "nodular" is intended to means an irregular, rounded lump as opposed to flat flake-like or needle-like particles. Nodular particles are required since flat or needle-like particles are brittle and may tend to disintegrate during use and adversely affect the antiseizing properties of the bearing alloy.

Also, as employed herein, the term "diameter" is intended to refer to the maximum dimension of a particle when viewed in a cross-sectional area of the alloy.

At least 5 nodular particles of the intermetallic compounds having a diameter, or size, of at least 5 .mu.m must be provided in a cross-sectional area of the bearing alloy of 3.56.times.10.sup.-2 (mm).sup.2. This area is chosen for convenience and is based on the viewing area of the microphotography equipment of the inventors. The number of particles per unit area can be modified by employing the appropriate conversion factors. For example, the above-described particle number/area limitation corresponds to 1.4.times.10.sup.8 particles per m.sup.2. It is also noted that the number of particles per cross-sectional area of the bearing alloy is that determined in a horizontal cross-section of a sheet of the alloy, i.e. a cross-section that is parallel to the surface of the sheet (and viewed in a direction perpendicular to the surface thereof), when prepared according to a process as described below. The size of particles measured in a vertical cross-section of a sheet of the alloy is typically smaller than that measured in a horizontal cross-section. It is further noted that the quantity limitations described above may not be fulfilled on the surface of a sheet of the alloy directly after its machining.

If the nodular particles of the intermetallic compounds are less than 5 .mu.m, the bearing alloy will not have exceptional seizure or wear resistance. On the other hand, there is a practical upper limit of about 40 .mu.m for the size of the particles because with bearing alloys containing particles above about 40 .mu.m machining of the bearings becomes difficult.

To achieve the desired levels of seizure resistance the number of the nodular particles having a size of at least 5 .mu.m per 3.56.times.10.sup.-2 (mm).sup.2 of cross-sectional area of the alloy must be at least 5. The maximum number of particles of any given size is of course limited by the amount of the elements forming the intermetallic compound particles that is contained in the alloy. Typically, the desired properties of the aluminum base bearing alloys of the present invention are achieved when the number of particles of the intermetallic compounds having a size of at least 5 .mu.m and, particularly, having a size between 5-40 .mu.m is less than the maximum number of particles possible based on the amount of these elements contained in the alloy; the balance of the elements forming precipitates of the intermetallic compounds of less than 5 .mu.m.

Seizure resistance of the bearing alloy according to the present invention increases as the size of the nodular intermetallic compound particles increases. On the other hand, the number of larger particles is limited because the larger particles tend to decrease the fatigue resistance of the bearing alloys. To obtain bearing alloys having the highest seizure resistance, the bearing alloy of the invention will preferably contain, per 3.56.times.10.sup.-2 (mm).sup.2 per cross-section area, at least 5 particles having a diameter of 5 .mu.m or higher and at least 5 particles greater than 10 .mu.m in diameter. More preferably, the alloy will contain, per said unit area, at least 5 particles having a size of 5 .mu.m to 10 .mu.m, at least 5 particles having a size greater than 10 .mu.m and up to 20 .mu.m and at least 1 particle greater than 20 .mu.m (and less than about 40 .mu.m for the reasons noted above). However, for bearing alloys according to the present invention having the highest level of seizure resistance, the fatigue resistance is somewhat lower as may be seen by referring to the data of the tables presented below.

The improved seizure resistance of the bearing alloy according to the present invention in which the shape, size and number of precipitates of intermetallic compounds are controlled according to the limitations described above is believed to be due to the ability of the particles to prevent the well-known adhesion phenomenon between aluminum base bearings and the shafts supported thereby and to reduce the surface roughness of the shafts. Aluminum tends to rub off onto a rotating shaft and to adhere thereto and there is a tendency for the aluminum on the shaft to adhere to that of the bearing. It is believed that the nodular particle of the intermetallic compounds tend to remove the aluminum from the shaft.

The apparent ability of the aluminum base bearing according to the present invention to reduce the surface roughness of the shaft to be supported by the bearing is most pronounced with the use of shafts having a hard and coarse surface and, particularly, with the use of nodular graphite cast iron shafts. With conventional aluminum base bearings, grinding burrs which are formed around the graphite particles on the surface of the shaft tend to grind, or wear, the surface of the bearing material. The nodular precipitates contained in the alloy according to the present invention, however, are believed to smooth over the roughness of the surface of the nodular shafts because of the number and size of these hard precipitates. It is noted that the improved wear resistance and antiseizing property of the bearing alloy according to the present invention is not obtained unless the shape, size and number of the precipitates are controlled as noted above.

The seizure resistance, fatigue resistance and wear resistance of the bearing alloy according to the present invention are determined according to tests under dynamic loads which are described more fully below in conjunction with specific embodiments of the alloy. These properties cannot be defined easily in quantitative terms because the values depend on the design of the testing equipment and test conditions and such equipment and conditions are not standard in the art.

The aluminum base bearing alloy according to the present invention also contains tin in an amount of 1.5-35 wt.% The tin is added mainly for the purpose of lubrication. An amount of tin of greater than 35%, although improving conformability and low friction properties, reduces the hardness and strength of the bearing alloy. On the other hand, if the amount of tin is less than 1.5%, the bearing alloy is too hard and does not sufficiently conformability. The addition quantity of the tin within the range of 1.5-35 wt.% can be determined according to the expected use of the bearing alloy. Generally, the amount of tin is low if the bearing is to be employed under large loads. For use under light loads, the amount of tin can be higher. The seizure resistance of the bearing alloy of the present invention is also increased as the amount of tin is increased.

In other embodiments of the aluminum base bearing alloy according to the present invention, the alloy, in addition to containing at least one of the elements selected from Mn, Fe, Mo, Ni, Zr, Co, Ti, Sb, Nb and Cr, and tin, may also contain at least one element from at least one of the following groups of elements: (a) lead (Pb), indium (In), thallium (Tl), cadmium (Cd) and bismuth (Bi) and (b) copper (Cu) and magnesium (Mg). The elements Pb, In, Tl, Cd and Bi can be used alone or in any combination and are employed in a total amount of from about 0.5-8 wt.%. Cu and Mg are employed alone or in a mixture thereof and are employed in an amount of from about 0.1-2 wt.%.

The addition of 0.5-8 wt.% of Pb, In, Tl, Cd and/or Bi to the bearing alloy improves the conformability and seizure resistance of the bearing material. An amount of less than 0.5 wt.% does not appreciably affect the properties of the alloys. Amounts greater than 8 wt.% are not desirable since they tend to decrease the melting point of the Sn and it is difficult to prepare the alloy because of a non-uniform distribution of these elements in the aluminum matrix. Additionally, the total addition quantity of these elements together with the addition quantity of the tin should not be more than about 35 wt.% since the fatigue resistance of the bearing alloy decreases as the combined amounts of these elements increases.

Cu and/or Mg can be contained in the bearing alloy of the invention in an amount of 0.1-2 wt.%. The Cu and/or Mg have an important effect on the hardness of the aluminum matrix and reduce the lowering of the hardness at high temperatures. The hardness of the alloy is increased as the amount of Cu and/or Mg is increased within this range whereas the seizure resistance decreases. Therefore, the amount of Cu and/or Mg employed is chosen so as to obtain a desired balance between the hardness and seizure resistance of the bearing alloy. An increase in the hardness of the alloy is not obtained with amounts of Cu and/or Mg of less than 0.1 wt.%. Amounts of these metals of more than 2.0 wt.% reduce the rolling property of the bearing alloy and lower the anti-corrosiveness. Furthermore, the Mg exists as a solid solution in the aluminum matrix and is liable to deposit during the annealing if the amount thereof is more than 2.0 wt.%.

The aluminum matrix of the bearing alloy according to the present invention preferably has a Vickers hardness (Hv) of from about 30 to 60. If the aluminum matrix is too soft, the load capacity of the bearing is insufficient and when a load is applied to the bearing, the particles of the intermetallic compounds are pushed into the surface. If the aluminum matrix is too hard, when a shaft contacts the bearing surface, the intermetallic compound particles may be removed from the surface and will not become embedded again but will roll between the shaft and the bearing and cause excessive wear.

The aluminum base bearing alloy according to the present invention is prepared by melting aluminum in a gas furnace and adding desired amounts of at least one of Mn, Fe, Mo, Ni, Zr, Co, Ti, Sb, Nb and Cr and of Sn and, depending on the desired properties of the alloy, the optional elements such as Pb, In, Cu, and the like, to the molten aluminum according to the conventional techniques. The molten alloy is cast and the cast alloy is then subjected to steps of peeling; repeated (if necessary) rolling and annealing to obtain a sheet of the alloy of desired thickness; slitting; annealing; sanding; brushing and the like to obtain bearing alloy pieces. These pieces are then applied to backing steel sheets by conventional pressure welding techniques to obtain bimetal pieces which are then subjected to annealing and coiling. These annealed pieces can then be worked into plain bearings. The foregoing steps employed in the process of the present invention are, per se, known in the art relating to the preparation of aluminum base bearings and are disclosed, for example, in U.S. Pat. Nos. 3,078,563; 3,093,885; 3,104,135; 3,167,404; 3,300,836; 3,300,838 and 3,384,950. The processes for preparing aluminum base bearings disclosed in these patents are incorporated herein by reference.

Control of the size and number of the precipitates of the intermetallic compounds in the bearing alloy so as to meet the limitations described above, i.e., at least 5 particles having a size of at least 5 .mu.m, may be obtained by controlled annealing of the cast alloy according to conditions not previously disclosed in the art. Specifically, in the process employed in the present invention, during the rolling and annealing of the cast alloy, annealing is carried out at a temperature of 280.degree.-550.degree. C. for 1.5 to 6 hours. Following slitting, annealing is carried out at a temperature of greater than 350.degree. C. and up to 500.degree. C. for 1.5 to 6 hours followed by controlled cooling at a rate of less than 200.degree. C. per hour. Following bonding to the backing steel by pressure welding, annealing is carried out at a temperature of 300.degree.-400.degree. C. for 1 to 2 hours.

The distinctions between the process employed in the present invention for obtaining the aluminum base bearing alloy and bearing composite wherein the alloy contains precipitates of intermetallic compounds of specified shape, size and number and the prior art processes may be better understood by referring to Table 1.

As noted previously, the aluminum base bearing composite according to the present invention is prepared by pressure welding the aluminum base bearing alloy according to the present invention to a backing steel according to conventional techniques and annealing the resultant composite at a temperature 300.degree. to 400.degree. C. for 1 to 2 hours. The aluminum base bearing composite according to the present invention can be used as a bearing for internal combustion engines under conditions of high load without the need of a lead overlayer, or overplate, which is required for conventional aluminum base bearings.

The present invention may be better understood by referring to the accompanying drawings in light of the following description and data.

The seizure resistance, fatigue resistance and wear resistance data presented below and shown in the drawings were measured under conditions as described in Table 2. The seizure resistance test measures the static load necessary to cause seizure at constant oil temperature. The fatigue resistance test measures fatigue limits under dynamic load conditions and under forced lubrication with lubricant oil of a constant temperature and by using a quenched shaft material rotating at 3000 rpm and with 10.sup.7 times repetition of stressing. The wear resistance tests measure the amount of wear of a bearing material at a constant load with respect to a shaft rotating at a constant speed.

Aluminum base bearing alloys according to the present invention were prepared by a process as described above employing the conditions listed in Table 1. Each of the alloys contained, in addition to aluminum, 15 wt.% Sn, 3 wt.% Pb and 0.5 wt.% Cu, and one of the elements Mn, Fe, Mo, Ni, Zr, Co, Ti, Sb and Nb (Mn.about.Nb). Alloys containing an amount of the individual elements Mn.about.Nb, with the amount of each of these elements varied as shown in Table A, were prepared. Each of the alloys was annealed at a temperature of 350.degree. C. for 1.5 hours in step (5) of the process (refer to Table 1). Annealing and cooling conditions in step (8) of the process (refer to Table 1) were controlled as listed in Table A so that each of the alloys contained from 30-35 nodular particles of the intermetallic compounds having a size of 5-10 .mu.m, from 8-11 nodular particles having a size of more than 10 and up to 20 .mu.m, and 2-4 nodular particles having a size of more than 20 and up to 40 .mu.m, the balance of the particles being less than 5 .mu.m.

The seizure resistance of each of these alloys was measured employing the conditions for seizure tester A listed in Table 2. The range of seizure unit loads obtained are within the shaded curve shown in FIG. 1. I,e., the seizure unit load of an alloy containing 1 wt.% of any one of the nine elements Mn.about.Nb falls within the shaded portion of the curve above 1 wt.% on the abscissa of the graph of FIG. 1. It is also noted that aluminum base alloys containing the amounts of Sn, Pb and Cu described above and varying amounts of Cr and prepared employing the conditions listed in Table A are also expected to have seizure resistance values within the curve of FIG. 1.

For comparison, alloys of the same composition as those of Table A were prepared according to the same process but without control of the size and number of the intermetallic compound particles. Seizure resistance of these alloys determined in the same manner as those of the alloys of Table A fall within the curve shown in FIG. 2.

It may be seen by referring to the data of FIGS. 1 and 2 that the aluminum base bearing alloys of the present invention in which the shape, size and number of particles of intermetallic compounds are controlled, have far better seizure resistance than similar alloys prepared according to prior art procedures where there is no control of particle formation.

The fatigue resistance of the alloys of Table A was measured according to the conditions for fatigue tester B listed in Table 2. The range of fatigue unit load data is illustrated in FIG. 3. It is seen that the fatigue resistance of the alloys of the invention remains relatively constant as the content of the elements which form the intermetallic particles is varied within the range of 1-5 wt.%.

The wear resistance of the alloys of Table A was measured according to the conditions listed for wear tester C in Table 2. The range of wear data for these alloys are shown in FIG. 4 (labelled "Control"). The wear resistance of alloys of the same composition but produced without control of the intermetallic compound particles was similarly determined and the data are also shown in FIG. 4 (labelled "No control").

The aluminum base bearing alloys according to the present invention are seen to be markedly superior in wear resistance.

Aluminum base bearing alloys according to the present invention having the composition; Mn 3 wt.%, Sn 15 wt.%, Pb 3 wt.%, and Cu 0.5 wt.%, with the balance being aluminum, were prepared according to the process described above and employing the conditions for the process employed in the present invention described in Table 1. Annealing conditions in step (B), Table 1, were varied to produce alloy samples A-1to A-3, B-1 to B-3, C-1 to C5, D-1 to D-3 and E-1 containing the distribution of nodular particles of intermetallic compounds of Mn set forth in Table B.

FIG. 8 compares the wear (resistance) of alloy sample C-2 of Table B with that of a conventional Al-Sn(20)-Cu(1) alloy as a function of time. The data of FIG. 8 was obtained employing the conditions of wear tester G of Table 2. The alloy of the invention is seen to have superior resistance to wear.

To demonstrate the outstanding seizure resistance of the aluminum base bearing alloys according to the present invention having different Mn contents, bearing alloys having an Sn content of 15 wt.%, Pb content of 3 wt.%, and Cu content of 0.5 wt.%, and Mn content as shown in Table C, the balance being Al, were prepared according to the process described above and employing the conditions of the process employed in the present invention listed in Table 1. The annealing conditions (step (8), Table 1) were varied to produce nodular intermetallic compounds particles of Mn having the number and size distribution listed in Table C.

The data of Table C demonstrates that for each of the Mn contents, the seizure resistance of the alloy is increased as the number and size of the intermetallic compound particles increase.

Bearing alloys of the present invention having the compositions and nodular intermetallic compound particle distribution shown in Table D were prepared employing the process conditions listed in Table 1. For comparison, alloys containing Si but in which particle formation was not controlled (Sample Nos. 48 and 49) and conventional Al-Sn-Cu alloys (Sample Nos. 50 and 51) were also prepared. The data of Table D show that aluminum base bearing alloys of the present invention containing the elements Pb, In, Tl, Cd, Bi, Cu and Mg, alone and in combination, in addition to Sn and at least one of the elements Mn, Fe, Mo, Ni, Zr, Co, Ti, Sb, Nb and Cr, also possess outstanding seizure resistance which is remarkably superior to that of the comparison alloys.

Although the present invention has been described in conjunction with certain preferred embodiments thereof, it is to be understood that the invention is not intended to be limited to these embodiments but, instead, is to include all those embodiments within the scope and spirit of the appended claims.