Aluminum-Tin Base Bearing Alloy and Composite

1. Field of the Invention

The present invention relates to an aluminum-tin (Al-Sn) base bearing alloy which is prepared by adding tin to an aluminum matrix and to a bearing material which is made by applying the Al-Sn base bearing alloy to a backing steel by pressure welding. More particularly, the Al-Sn base bearing alloy of the invention is characterized in that the bearing alloy is improved in several properties thereof by adding various kinds of additive elements. That is, the fatigue strength is much improved by reducing the lowering of the hardness at high temperature and, especially, by avoiding the coarsening of tin particles. Furthermore, the wear resistance of the bearing alloy is also raised in order to improve the durability relative to the shaft to be supported which has a hard and coarse surface. Accordingly, in the case that the bearing alloy of the present invention is used for the bearing devices around the crankshafts of internal combustion engines which require severe conditions, remarkable advantages can be expected.

2. Description of the Prior Art

In recent years, the automobile internal combustion engines are required to be made compact and to have high power. Further, as the countermeasure to the regulation of exhaust gas, they must be provided with blow-by-gas recirculation devices or the like. Therefore, the use conditions for the bearing materials in the internal combustion engines have become severe at high loads and high temperatures. Under such severe conditions, the conventional bearing materials are liable to cause fatigue failure and abnormal wearing which cause several troubles.

In connection with the shafts to be brought into engagement with the bearing materials, there is a tendency to use, in place of the hitherto produced forged shafts, less expensive shafts made of spheroidal graphite cast iron or other coarse material in order to reduce the production costs. Therefore, improvements in the wear resistance, seizure resistance and fatigue resistance at high temperatures are required much more.

Exemplified as the Al-Sn base alloys used for making the bearings of internal combustion engines in the prior art are:

Al (balance)--Sn(3.5-4.5%)--Si(3.5-4.5%)--Cu(0.7-1.3%);

Al (balance)--Sn(4-8%)--Si(1-2%)--Cu(0.1-2%)--Ni(0.1-1%);

Al (balance)--Sn(3-40%)--Pb(0.1-5%)--Cu(0.2-2%)--Sb(0.1-3%)--Si(0.2-3%)--Ti(0 .01-1%);

Al (balance)--Sn(15-30%)--Cu(0.5-2%); and

Al (balance)--Sn(1.23%)--Pb(1.5-9%)--Cu(0.3-3%)--Si(1-8%),

in which the values in parenthesis denote the percentages by weight of the component materials.

When these conventional alloys are used for the bearings of automobile internal combustion engines under severe conditions as described above, fatigue failure is sometimes caused to occur in a short time if the engines are continuously operated under heavy loads. This is considered to be due to the fact that the temperature of the lubricant oil in an internal combustion engine becomes very high during the continuous full-load running thereof, for example, the temperature of the lubricant oil in an oil pan reaches 130.degree. C.-150.degree. C., so that the temperature of the sliding surfaces of bearings also becomes very high. As the result, since the eutectic point of such alloy is about 225.degree. C. or so, the hardness of the alloy rapidly becomes low under the high temperature conditions, which causes fusion and migration of the Sn component and the fatigue strength is resultantly lowered. The inventors of the present invention have prepared an alloy, the hardness of which is not lowered at high temperatures and the Sn component of the alloy is hardly altered. The alloy was worked into the shapes of bearings for internal combustion engines and they were subjected to fatigue tests under dynamic loads at high oil temperatures. As a result, the improvement in fatigue strength was recognized, which substantiated the above-mentioned consideration.

Further, in addition to the lowering of the fatigue strength due to the loss of hardness at high temperatures as mentioned above, the coarsening of tin particles in the texture of the conventional Al-Sn alloys also causes lowering of the fatigue strength. That is, the aluminum bearing material is generally formed by applying the Al-Sn alloy to a backing steel through pressure welding, in which an annealing step is required after the pressure welding in order to improve the adhesive strength between both metals. The annealing is generally done at a temperature below the point at which an Al-Fe intermetallic compound deposits and the higher the treating temperature is and the longer the treating time is, the larger the adhesion strength becomes. As a matter of fact, when the conventional Al-Sn alloy is placed in a high temperature condition during annealing, the coarsening of aluminum grain boundaries and tin particles is disadvantageously caused to occur in the alloy texture. That is, when the conventional aluminum bearing alloy is subjected to annealing in order to improve the adhesive strength to the backing steel, the coarsening of tin particles is brought about, which results in the lowering of the fatigue strength of the Al-Sn alloy at high temperatures.

Further, these conventional Al-Sn bearing alloys may not have sufficient wear resistance. Especially, when shafts having hard and coarse surfaces, such as those made of spheroidal graphite cast iron, are brought into engagement with the bearing alloys, the wear resistance is greatly lowered and fatigue failures are liable to occur and this has been a serious problem.

The present invention aims to reduce or eliminate one or more of the above-described disadvantages in the conventional Al-Sn base bearing alloys. Accordingly, one object of the present invention is to provide an Al-Sn base bearing alloy which exhibits a relatively small loss of hardness at high temperatures, and as a result, has a relatively high fatigue strength.

Another object of the present invention is to provide an improved Al-Sn base bearing alloy in which the coarsening of the tin particles is avoided or moderated during the annealing step or during use under high temperature conditions, which results in a higher fatigue strength.

A further object of the present invention is to provide an Al-Sn base bearing alloy having a relatively high wear resistance, especially when employed with shafts which are made of hard and coarse materials such as spheroidal graphite cast iron that is used for making the crankshafts of internal combustion engines.

Yet a further object of the present invention is to provide a bearing material which is made by applying the above bearing alloy to the surface of a backing steel sheet and bearings for internal combustion engines which are made of the above bearing material.

According to the present invention, the first Al-Sn base bearing alloy consists essentially of: 7-35% of Sn; 0.1-1.0 wt. % of Cr; 1-10 wt. % in total of one or more members selected from the group consisting of W, Ce, Nb, V, Mo, Ba, Ca and Co; and the balance is aluminum. In order to improve the bearing characteristics, the Al-Sn base bearing alloy of the invention can further contain 0-3 wt. % in total of Cu and/or Mg, and/or 0-9 wt. % in total of one or more members selected from the group consisting of Pb, Bi, Tl, Cd and In. In connection with the above additives, the addition of 0.1-1.0 wt. % of Cr is effective especially in reducing the lowering of the hardness at high temperatures, and also in reducing the coarsening of the Sn particles. Further, it is considered that the specified additive group exhibits a notable effect in improving the wear resistance.

A second Al-Sn base bearing alloy, according to the present invention, consists essentially of 3-7% by weight of Sn; 0.1 to 1.0% by weight of Cr; from 1-10% by weight in total of one or more elements selected from the group consisting of Si, Mn, Sb, Ti, Zr, Ni, Fe, W, Ce, Nb, V, Mo, Ba, Ca and Co; 0.1-0.8% by weight in total of Cu and/or Mg; 0-9% by weight in total of one or more constituents selected from the group consisting of Pb, Bi, Tl, Cd and In; and the remainder is Al.

A third Al-Sn base bearing alloy, according to the present invention, consists essentially of 3-35%, preferably 7-35%, by weight of Sn; 1-10% by weight in total of two or more constituents selected from the group consisting of Cr, Si, Mn, Sb, Ti, Zr, Ni, Fe, W, Ce, Nb, V, Mo, Ba, Ca and Co, with the proviso that said constituents include at least 1% by weight of Cr; 0-3% by weight in total of Cu and/or Mg; 0-9% by weight in total of one or more constituents selected from the group consisting of Pb, Bi, Tl, Cd and In; and the remainder is Al.

A fourth Al-Sn base bearing alloy, according to the present invention, consists essentially of 3-7% by weight of Sn; 1-10% by weight of two or more constituents selected from the group consisting of Cr, Si, Mn, Sb, Ti, Zr, Ni, Fe, W, Ce, Nb, V, Mo, Ba, Ca and Co, with the proviso that said constituents include at least 1% by weight of Cr; 0.1-0.8% by weight in total of Cu and/or Mg; from 0-9% by weight in total of one or more constituents selected from the group consisting of Pb, Bi, Tl, Cd and In; and the balance is Al.

The objects, features and advantages of the present invention will be more clearly understood from the following detailed description, examples and accompanying drawings, in which:

FIG. 1 shows graphic patterns of the changes in friction torques with the increases of loads;

FIGS. 2, 4, 6 and 8 are graphs showing the changes in wear losses of alloys with the increases of loads applied to steel shafts which are in contact with the alloys; and

FIGS. 3, 5, 7 and 9 are graphs showing the changes in wear losses of alloys with the increase of loads applied to the shafts made of spheroidal graphite cast iron which are in contact with the alloys.

The Al-Sn base bearing alloy of the present invention is prepared by adding the above-mentioned Sn, Cr, and the other foregoing specified components to the matrix of aluminum.

In connection with the quantity of tin, the conformability and lubricating property may generally be improved with the increase of tin; however, the hardness is lowered. Therefore, the load carrying property of the alloy when employed as a bearing becomes low. To the contrary, when the quantity of tin becomes small, the load carrying property is increased; however, the alloy is too hard for use as the bearing material and the conformability becomes worse. In the conventional art, the upper limit of the tin content was generally about 15% and the lower limit was about 3%. In the present invention, the tin content is restricted, as set forth above, in which the conformability is good enough. In order to separate and disperse the tin component, the upper limit of tin has been set to about 15% in the prior art as described above. That is, when more than 15% of tin is contained, the tin particles in the alloy cannot be dispersed independently in aluminum and they exist in a continuous state, which lowers the hardness. In the present invention, however, owing to the effect of the addition of other elements, up to 35% of tin can be added without causing any practical disadvantage. The addition quantity of tin in the range of 3 to 35 wt. % can be determined properly in compliance with expected uses. Generally speaking, the addition quantity of tin is made small when a bearing receives a large load, while for light duty purposes, much tin can be added to the bearing alloy. In another viewpoint on the quantity of tin, when there is fear of seizure, the addition quantity of tin may be increased, while if there is no fear of seizure, the quantity of tin may be reduced. However, in recent cases, the temperature of the bearing becomes often high due to the high temperature oil and this causes the deformation of the bearing which is followed by the occurrence of seizure and fatigue. Therefore, the addition quantity of tin may be determined from the viewpoint to reduce the deformation at high temperatures.

The addition of chromium (Cr) is effective in that the hardness of the alloy is raised to prevent the alloy from softening at high temperatures and thus the coarsening of tin particles is not caused to occur even in annealing. In the first place, the effects to raise the hardness and to avoid the softening of the alloy at high temperatures will be described. When the quantity of chromium is less than 0.1 wt. %, the improvement in high temperature hardness cannot be expected. If the addition quantity of chromium exceeds 1.0 wt. %, the Al-Cr inter-metallic compound cannot be dispersed finely and evenly as described later on; therefore, the effect of the addition becomes low. More particularly in connection with the improvement in the high temperature hardness, the chromium forms a solid solution in aluminum which raises the recrystallization temperature of the aluminum and, in addition, the solid solution itself improves the hardness of the aluminum matrix. At the same time, the hardness of the alloy containing chromium becomes higher even when it is subjected to rolling several times, which is contrasted to casting. With the rise of the recrystallizing temperature of aluminum, the bearings of engines that are exposed to high temperatures can maintain their mechanical properties. Especially, the lowering of hardness at high temperature can be reduced and the softening of bearings in a high temperature range can well be avoided, thereby improving the durability of bearings. Further, the Al-Cr inter-metallic compound that is deposited over the limit of solid solution, has a Vickers hardness of more than 370 so that the dispersion of such the compound aids the bearing alloy in maintaining the hardness at high temperature. Therefore, the dispersion of such the inter-metallic compound in a proper quantity gives a good effect. The preferable range of the quantity of chromium is, as described above, 1.0 wt. % or less, and if the quantity of chromium is within such the range, fine and uniform deposition of the inter-metallic compound is formed and it increases the hardness of the bearing alloy.

The effect of the addition of chromium to avoid the coarsening of tin particles will be described in the following. The coarsening of tin particles is a phenomenon owing to the migration of aluminum grain boundaries and tin particles in a high temperature condition of the Al-Sn alloy. Since the chromium is precipitated as the above-mentioned Al-Cr inter-metallic compound which is finely dispersed in the aluminum alloy matrix, this inter-metallic compound inhibits directly the migration of aluminum grain boundaries and, at the same time, it obstructs the growth of aluminum crystal grains. Therefore, the migration of tin particles is also hindered and, as the result, the coarsening of tin particles can be avoided. This is related to the fact that the finely divided tin particles are retained as they stand during the repeat of rolling and annealing, and the above-described various effects can be obtained. Such a phenomenon can be observed when the quality of tin is small; however, the effect becomes larger when the quantity of tin is relatively large (more than about 10 wt. %). Especially, in the case that the quantity of tin exceeds about 15 wt. % in which the tin particles exist in a continuous state, the effect of addition becomes notable. Further, even when the quantity of tin is less than 10 wt. %, the effect of the addition of chromium is, of course, expected sufficiently according to use conditions and utilities of the alloy.

Furthermore, the liquidation of tin particles having a low melting point of about 232.degree. C. can be prevented effectively under high temperature conditions because the tin particles are well maintained in a finely divided state in the aluminum matrix. From this viewpoint, the effect of the prevention of lowering in hardness will be understood.

In the above passage, the effect for preventing the coarsening of tin particles in the annealing step is described. This effect can also be expected in the working condition of the bearing materials in which the temperature is equal to the annealing condition. Accordingly, the fatigue strength in practical uses can also be improved with the inhibition of softening.

In order to improve mainly the wear resistance, it is effective to add one or more members of (1) the group of the specified additives such as tungsten (W), cesium (Ce), niobium (Nb), vanadium (V), molybdenum (Mo), barium (Ba), calcium (Ca) and cobalt (Co) for the first alloy of the invention, or (2) the members of (1) plus silicon (Si), manganese (Mn), antimony (Sb), titanium (Ti), zirconium (Zr), nickel (Ni) and iron (Fe) for the second alloy of the invention, or (3) the members of (2) plus chromium (Cr) for the third and fourth alloys of the invention. The addition quantity of each of these elements is within the range of a trace to 10 wt. %, while the total quantity of these elements is not mere than 10 wt. %, and preferably is in the range of 1 to 6 wt. %, which quantity may be determined in accordance with the purpose of use. The reason for the above restriction is as follows. The precipitated substances (or crystallized substances, the same shall apply hereinafter) of these elements are dispersed in the aluminum matrix; therefore, the wear resistance can be improved. If the addition quantity of the specified additive is less than 1 wt. %, the effect of addition cannot be exhibited. On the contrary, if the addition quantity is more than 10 wt. %, the precipitated substance becomes too much, so that the adaptability to rolling becomes worse and the repeating of rolling and annealing becomes difficult. Further, the formation of the fine particles of tin is inhibited. In order to eliminate these undesirable effects completely, the preferable upper limit is made 6 wt. % or so.

In the case of the third and fourth alloys of the invention, which employ the additive group (3), wherein chromium is one of the additives, the additives include chromium in the range of 1 to 10 wt. % plus at least one other of said additives.

The forms of precipitation of these specified additives are the precipitated substance of each added element, those of inter-metallic compounds between the added elements, those of inter-metallic compounds of aluminum and added elements, and those of inter-metallic compounds of aluminum and the inter-metallic compound of added elements. The wear resistance can be improved by the precipitated substances in any forms of the above.

The Vickers hardnesses of these precipitated substances reach several hundreds, so that the precipitated substances are very hard and the wearing of bearings caused by the friction with shafts can be reduced notably by the precipitated substances. Accordingly, a quite good result can be obtained when a proper quantity of the precipitated substance exists in the aluminum matrix. The range of the proper quantity is 1 to 10 wt. % as described above, and if the quantity of the precipitated substance is in this range, the precipitated substance can be dispersed uniformly and the wear resistance can effectively be improved without causing any ill effects such as the lowering of conformability.

The effect to improve the wear resistance is remarkable when the bearing supports a shaft having a hard and coarse surface. The performance of a bearing generally depends upon the hardness and coarseness of the material to be supported to a great extent. For example, when the conventional Al-Sn base bearing material is used for supporting a shaft made of spheroidal graphite cast iron, the properties of the bearing such as the antiseizing property and the wear resistance are markedly lowered. Since the shafts made of spheroidal graphite cast iron can be produced at low cost, such shafts have been widely employed recently in place of the steel shafts. In the iron matrix of the shaft, soft graphite particles are scattered. Therefore, when the shaft surface is scraped, blade-like grinding burrs are formed around the particles of graphite. When the shaft having such grinding burrs is slid relative to the bearing under a heavy load in which the coarsenesses of the shaft and the bearing and the thickness of the oil film between them are equal to each other, the bearing surface which is softer than the shaft is ground. If such condition is continued, the surface of the bearing becomes coarse and the clearance between the bearing and the shaft becomes large, which causes breakage of the oil film or the lack of an oil film. As a result, the direct contact between the shaft and the bearing (the metal-to-metal contact) is caused to occur, which results in the occurrence of seizure of both the parts.

While in the alloy according to the present invention, the precipitated substance which is formed in the aluminum matrix by the addition of one or more members of the above specified additives, is harder than the above-mentioned grinding burrs of the shaft of spheroidal graphite cast iron. Therefore, the precipitated substance removes the above-mentioned grinding burrs from the surface of the shaft and, in addition, the metal transfer and adhesion of the precipitated substance is hard to occur. Therefore, the course of wearing of the bearing surface can be suppressed within a relatively short time to cause the formation of a stable oil film. As a result, in relation to the shaft made of the spheroidal graphite cast iron, the wear resistance and the antiseizing property of the bearing can be improved.

By the way, among the group (1) of specified additives, the most desirable ones are Mo and Co, then Ce, and next Nb, W and V and further, Ba and Ca. This order is taken in view of the degree of uniform dispersion of the inter-metallic compounds with aluminum or other elements and the easiness in casting. Since Mo and Co are a little worse in anticorrosiveness, when anticorrosiveness is especially required, in the use, it is necessary to consider that the addition quantities of them may be reduced and other elements are employed.

Among the group (2) or group (3) of specified additives, silicon is most preferably used because the silicon itself is hard, the inter-metallic compounds between the silicon and other elements are also hard, and the Al-Sn alloy containing silicon is excellent in its casting property. In view of the above properties, the group of chromium, manganese and antimony are of next importance to the silicon. The rank of the above chromium, manganese and antimony is followed by titanium, zirconium and nickel because the latter elements are somewhat inferior in the casting property. The element iron can be used if the anticorrosiveness is not a problem.

In addition to the above-described components of the present invention, the first and third bearing alloys of the invention can further contain from zero to 3 wt. % in total of copper (Cu) and/or magnesium (Mg). The copper and/or magnesium are added in view of the fact that they reduce the lowering of the hardness at high temperatures. The preferable range of the addition of them is 0.1 to 2.0 wt. %. When the addition quantity of them is less than 0.1 wt. %, the rise of hardness cannot be expected so much, while if the addition quantity is more than 3.0 wt. %, the alloy becomes too hard to reduce the rolling property and the anticorrosiveness is lowered. Further, with regard to magnesium, the addition of more than 3 wt. % increases the hardness; however, the rise in hardness during the roll working becomes too large. Therefore, satisfactory rolling cannot be performed and it becomes difficult to obtain a quite fine tin texture. Furthermore, the magnesium existing as a solid solution in the aluminum matrix is liable to deposit during the annealing. Therefore, the reinforcing of the aluminum matrix by the solid solution cannot be expected owing to the deposition of the excess magnesium. Therefore, the preferable ratio of addition is not more than 2.0 wt. %. Further, the effect of the addition of copper and/or magnesium is exhibited when the chromium is added simultaneously, and the effect to raise the hardness at high temperatures cannot be expected when only the copper and/or magnesium are added. In other words, if copper and/or magnesium are added to the aluminum matrix, the hardness in the rolling is much raised, which is remarkable as compared with the case in which other elements are added to the aluminum matrix. It is to be noted, however, that the aluminum matrix containing the copper and/or magnesium is easily softened at about 200.degree. C. Therefore, it cannot be expected to maintain the hardness at high temperatures. On the contrary, when the copper and/or magnesium are added together with the chromium, the hardness which is raised during the rolling by the effect of the addition of the copper and/or magnesium is not so much lowered by the annealing, which is brought about by the addition of the chromium. This hardness can be maintained under high temperature conditions. Therefore, as compared with the prior art alloys, the bearing alloy of the present invention has a higher hardness at high temperatures, which results in the improvement in the fatigue strength. When the copper and magnesium are simultaneously added, it is desirable that the total quantity of them is not more than 3 wt. %, while the quantity of copper itself is not more than 2 wt. %.

In the second and fourth alloys of the invention, there is contained from 0.1 to 0.8 wt. % in total of copper (Cu) and/or magnesium (Mg), in view of the low amounts of tin (Sn), i.e., from 3 to 7 wt. %, used in the second and fourth alloys of the invention.

Further, in all the bearing alloys of the present invention, the property as a slidable metal containing tin can be further improved by adding from zero to 9 wt. % in total of one or more members selected from the group consisting of lead (Pb), bismuth (Bi), indium (In), thallium (Tl) and cadmium (Cd). The effect of the addition of lead, bismuth, indium, thallium and cadmium is exhibited when they are added together with chromium. In the prior art, it has been considered that these elements are added to Al-Sn base alloys and the addition has been practiced in some cases. However, when only these elements are added to the Al-Sn base alloy, they form alloys so that the disadvantage that the melting point of tin becomes low cannot be avoided. Thus the fusion and the migration of tin at low temperatures is liable to occur in the prior art Al-Sn base alloy, which causes the growth of tin particles into larger and coarse ones. When such alloy is employed as a bearing material, partial fusion and scraping are caused to occur under continuous heavy load running. On the contrary, the tin particles are made fine by the addition of chromium and such texture is maintained at high temperatures in the bearing alloy of the present invention. Therefore, even when one or more of the above lead, bismuth, indium, thallium and cadmium are added to the alloy, the lubricating property of tin can be improved without causing the above troubles in the prior art. Further, the bearing alloy of the invention can be used for a bearing in which a high fatigue strength is required. In addition, it becomes possible to improve the conformability of the bearing material. As described above, the addition quantity of one or more of the lead, bismuth, indium, thallium and cadmium having the above effects is in the range of up to 9 wt. % in total. Among these elements, lead and indium are most preferable which are followed by bismuth and cadmium, and then thallium. This depends upon the fact that lead and indium are most flowable under pressure so that the sliding property and conformability become good. The next, bismuth and cadmium, are somewhat harder and higher in melting points as compared with the lead and indium. The last, thallium, has similar properties as those of lead and indium. However, the production quantity of thallium is small and it is expensive. Further it is desirable that the total addition quantity of lead, bismuth, indium, thallium, cadmium and tin is not more than 35 wt. %. Further, one or more of lead, bismuth, indium, thallium and cadmium can be added together with the above-mentioned copper and/or magnesium, in which the lowering of the high temperature hardness can be reduced and, at the same time, the lubricating property of the tin can be improved.

The above described Al-Sn base bearing alloy is mainly used as the sliding bearings of automobile internal combustion engines or the like, in which the bearing alloy is generally applied to backing steel sheets by pressure welding and, in order to increase the adhesive strength, the annealing is done after the pressure welding. However, in the prior art Al-Sn base alloys, the lowering of the hardness, the fusion of tin particles and so forth are caused to occur because the migration of aluminum grain boundaries and tin particles in the alloy texture is brought about which causes the coarsening of the tin particles. While in the present invention, the migration of aluminum grain boundaries and the growth of aluminum crystal particles are effectively avoided by the precipitated substance of Al-Cr inter-metallic compound which is generated in the pressure welding and annealing steps. Therefore, the bearing alloy of the present invention is free from the above ill influences of annealing and, as a result, the adhesion strength between the Al-Sn base alloy and backing steel sheets can be made high by raising the temperature of annealing. Since the above fact can be applied to the case in which the bearing alloy of the present invention is placed under the circumstances which correspond to the temperature of annealing, it is quite meaningful that the fatigue strength can be improved by the prevention of softening. Furthermore, the improvement in the wear resistance is also observed, especially, and the bearing alloy is quite effective when it is used for the shafts made of spheroidal graphite case iron.

Taking all the properties into consideration such as the adaptability to casting and rolling, the adhesiveness to backing steel sheet, the workability, the antiseizing property, the wear resistance and the sliding characteristic, the most preferably composition of the first bearing alloy of the present invention is 7.5-25 wt. % of Sn; 0.1-0.7 wt. % of Cr; 1.0-6.0 wt. % of the specified additive group (1); 0.1-2.0 wt. % of Cu and/or Mg; 0.5-5.0 wt. % of Pb, Bi, In, Tl and/or Cd; and the remainder is Al. When the above elements of Pb, Bi, In, Tl and/or Cd are not added, the quantity of Sn may be increased to 10-30 wt %. Further, the addition of Cu and/or Mg can be omitted.

In the following, the present invention will be described further in detail by several examples.

The following Table A shows the compositions of the alloys (specimens) 1 to 16 of the first alloy of the present invention and comparative alloys (specimens) 17 to 21.

In the preparation of the alloys 1 to 16, an aluminum material was melted in a gas furnace and, in accordance with the formulae of Table A, the base alloys of Al-Cr, Al-Cu, Al-Mg and Al-specified additives were dissolved into the molten aluminum, respectively. After that, Sn and Pb, Bi, In, Tl and Cd were added and degassing was performed. Then, metal mold casting was performed, which was followed by repeated rolling and annealing (350.degree. C.) to obtain specimens. The high temperature hardnesses of the specimens were then measured. In the next step, these specimens were subjected to rolling and after that, the alloy specimens were fixed to backing steel sheets by pressure welding to obtain bimetallic specimens. These were then subjected to annealing and worked into plain bearings and the fatigue resistance under dynamic loads of them were tested. In like manner as the above, alloys 17 to 21 for comparative tests, were also prepared and they were subjected to the same tests.

Table B shows the results of measurement of Vickers hardnesses of several alloys at an ordinary temperature and at 200.degree. C., the results of fatigue test under dynamic loads, and the results of seizure tests with steel shafts and spheroidal graphite case iron shafts. The above fatigue tests were carried out by applying each alloy with 10.sup.7 times repetition of loads under the following conditions and measured the intensity of loads at which the fatigue occurs, that is, the pressure at fatigue limit by that number of repetitions.

In the above seizure tests, the loads at seizure were measured with increasing loads by 50 Kg/cm.sup.2 at every 20 minutes under the following test conditions. The following material (1) in contact with the bearing was used as a steel shaft and the material (2) in contact with the bearing was used as a spheroidal graphite cast iron shaft.

As will be understood from Table B, the alloys of the present invention have higher hardnesses in the high temperature range as compared with the comparative alloys.

Further, as compared with the comparative alloys, the alloys of the present invention gave relatively good results in view of fatigue resistance. Further, in the seizure tests with using spheroidal graphite cast iron shafts, the alloys of the present invention gave excellent results.

Shown in FIG. 1 are the changes in friction torques with the increases in load in connection with the alloy 5 and the comparative alloys 18 and 20. In these tests, S 55 C quenched shaft materials were used at the revolution speed of 1,000 rpm with the forced lubrication at a constant oil temperature (140.degree. C.) and the change of friction torque with the increase of load was measured with using an oscillograph. According to the graphs of FIG. 2, peaks of friction torques are caused to occur with the increase of load to the prior art alloy 18 and the values are largely fluctuated. In the case of the alloy 20, the friction torque increases gradually without the formation of any peak and the dome-shaped changes in friction torque are observed at the ceasing of the increase of load. While in the case of the alloy 5 of the present invention, the friction torque is gradually increased with the increase of the load and any undesirable change in the friction torque is not observed. Therefore, it will be understood that the alloys of the present invention are excellent in conformability and are free from the seizure. In other words, the fluctuating peak waves in the prior art alloy 18 mean that the oil film on the sliding surface is partially broken which causes solid contact and when such contact is repeated, the whole breakage (seizure) is brought about. The alloy 5 of the present invention which is free from such wave forms, is advantageous in view of the conformability and antiseizing property.

Shown in FIG. 2 are the results of friction tests, in which the alloys 1, 4 and 7 of the present invention and the comparative alloys 20 and 21 are compared. In FIG. 3, the results of other friction tests are shown, in which the alloys are the same as those of FIG. 2 and the shafts made of spheroidal graphite cast iron (surface roughness: 1 .mu.m, hardness: Hv 200-300) were used under the same test conditions. From FIG. 2, it will be understood that the wear losses of the alloys 1, 4 and 7 of the present invention are quite small as compared with those of the comparative alloys 20 and 21. As compared with the cases of FIG. 2, the differences in wear losses between the alloys 1, 4 and 7 and the comparative alloys 20 and 21 are large in FIG. 3.

The following Tables C and D and FIGS. 4 and 5 present corresponding data for the second alloys according to the invention.

The following Tables E and F and FIGS. 6 and 7 present corresponding data for the third alloys according to the invention.

The following Tables G and H and FIGS. 8 and 9 present corresponding data for the fourth alloys according to the invention.

By the way, it should be noted that in the composition of the alloys of the present invention, the aluminum way of course contain a trace quantity of impurities which cannot be eliminated by the ordinary refining technique.

Although the present invention has been described in connection with the preferred examples thereof, many variations and modifications will now become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein but only by the appended claims.