Abstract:
An aluminum alloy bearing for use in a low-rigidity housing, includes a bearing aluminum alloy, and a steel back metal bonded to the bearing aluminum alloy. The steel back metal contains 0.16 to 0.35 wt. % of carbon, and the steel back metal has a hardness of Hv 200 to 280 and has an elastic limit of not less than 40 kgf/mm 2 . For producing the aluminum alloy bearing, the back metal material is pressure-bonded to the bearing aluminum alloy in such a manner that the rolling reduction of the back metal material is 38 to 50%.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to an aluminum alloy sliding bearing comprising a steel back metal and an aluminum bearing alloy, and also to a method of producing such a sliding bearing. More specifically, the present invention relates to an aluminum alloy sliding bearing which has an excellent fretting resistance to overcome the fretting of the bearing resulting from a lightweight and low-rigidity construction of a housing due to a small-size and lightweight construction and a high-speed and high-power design of a recently-developed internal combustion engine, and the present invention also relates to a method of producing such a sliding bearing. The term &#34;fretting&#34; means a surface damage developing when slight vibrations are present on a contact surface. 
     Various kinds of aluminum alloy sliding bearings are known. Generally, such an aluminum alloy sliding bearing has a bearing alloy layer (for example, of Al-Sn type, Al-Sn-Si type, Al-Zn type, or Al-Zn-Si type) formed on a surface of a back metal (JIS G3141 SPCC, SAE 1010, or the like). This back metal is usually made of low carbon steel having the carbon content of not more than 0.15%. Such low carbon steel has been used in order to reduce a deformation resistance encountered when plastically working the steel into a half-shell type configuration or a configuration with a flange. 
     However, in recent automobile engines, a high power output design by a high engine speed has been intended, and therefore an inertia force of a connecting rod is increased. 
     Therefore, in order to decrease the inertia force, the connecting rod is reduced in thickness to reduce its weight, which further reduces the rigidity of the connecting rod. 
     Also, there is a tendency that an engine block is made of an aluminum alloy in order to achieve a lightweight design, and as a result the engine block, like the connecting rod, has a low rigidity. 
     When the housing is thus reduced in rigidity, the bearing must have an increased amount of interference for mounting the bearing on the housing; otherwise the close contact between the bearing and the housing would be adversely affected at high temperatures or under a high load, which may often result in damage, such as fatigue and seizure, due to fretting or migration. The interference can be obtained by making the diameter of the bearing slightly greater than the diameter of the housing. Namely, the fixing of the bearing relative to a bore of the housing depends on such interference. Therefore, the amount of the interference is determined depending on the material, strength, etc., of the bearing and/or the housing. The term &#34;migration&#34; means that for example, a plating layer on the outer surface of the bearing is displaced out of place, or that foreign matters in a lubricant are moved to the outer surface of the bearing and/or the bore in the housing. 
     With respect to features of the conventional bearings, attention has been directed mainly to the properties of the bearing alloy, and with respect to the back metal, attention has been directed to its workability and its ability of bonding with the alloy. 
     However, with the high performance design of the recent automobile internal combustion engines, the total bearing performance, including not only the bearing properties of the bearing alloy but also the properties of the back metal, has now been required. 
     Particularly, recently, the engine tends to produce a high power output by a high engine speed, and the connecting rod tends to receive an increased inertia force due to such high engine speed. 
     Under the circumstances, the connecting rod tends to be reduced in thickness to reduce its weight in order to reduce the inertia force of the connecting rod, which further lowers the rigidity of the connecting rod. 
     Therefore, at high temperatures or at a high engine speed, the connecting rod is deformed under the influence of the heat and the inertia force because of its low rigidity. As a result, the close contact between the bearing and the housing is adversely affected, so that damage, such as fatigue and seizure, due to fretting and migration has been often encountered. 
     The engine block also increasingly tends to be made of an aluminum alloy in order to achieve its light-weight constriction. The aluminum alloy has a high thermal expansion coefficient of 23×10 -6  /°C., and the engine block is expanded at high temperatures, so that the close contact between the engine block with the outer surface of the metal of a sliding bearing is adversely affected, thereby causing damage due to fretting, as is the case with the connecting rod. 
     When the housing is thus reduced in rigidity, there is encountered damage, such as fatigue and seizure, due to fretting or migration. 
     In order to overcome these problems, it is necessary to increase the amount of the interference available when mounting the bearing so that the bearing can follow the deformation of the housing at high temperatures or under a high load; however, in the conventional bearing having the back metal made of low carbon steel, the strength of the back metal is low, and when the bearing is mounted on the housing under a high mounting stress, the back metal undesirably deformed beyond its elastic limit to cause permanent strain. Therefore, the intended object can not be achieved. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide a novel aluminum alloy bearing which achieves excellent fretting resistance and migration resistance by the use of a high-rigidity and high-strength back metal having sufficient strength and toughness to withstand a high clamping force so that the bearing can be suitably used in a low-rigidity housing, such as a connecting rod reduced in rigidity for reducing an inertia force, and an aluminum block for lightweight purposes. Another object is to provide a method of producing such an aluminum alloy bearing. 
     According to the present invention, there is provided an aluminum alloy bearing for use in a low-rigidity housing, comprising a bearing aluminum alloy; and a steel back metal bonded to the bearing aluminum alloy; the steel back metal containing 0.16 to 0.35 wt.% of carbon, the steel back metal having a hardness of Hv 200 to 280 and having an elastic limit of not less than 40 kgf/mm 2 . 
     Preferably, a flash plating layer having a thickness of 0.1 to 5 μm is formed on an outer surface of the steel back metal, the flash plating layer being made of a material selected from the group consisting of Sn, Pb and alloys thereof. 
     Preferably, the bearing aluminum alloy consists, by weight, of 3 to 40% Sn; 0.1 to 10% Pb; 0.2 to 5% Cu; 0.1 to less than 3% Si; 0.1 to less than 3% Sb, all of which constituents are ones indispensable to the alloy; at least one optional component of 0.01 to 3% selected from the group consisting of Mn, V, Mg, Ni, Co, Mo, Zr, Nb, Ti, and B; and the balance Al and incidental impurities. 
     Preferably, the bearing aluminum alloy consists, by weight, of 1 to 10% Zn; 1 to 12% Si; 0.1 to 5% Cu; 0.1 to 5% Pb, all of which constituents are ones indispensable to the alloy; at least one optional component of 0.05 to 5% selected from the group consisting of Mg, Ni, Mn, V, Co, Cr and Sr; and the balance Al and incidental impurities. 
     Damages apt to occur in the low-rigidity housing, such as fatigue and seizure due to a fretting abrasion and migration, can hardly be overcome with the conventional back metals. Therefore, in the present invention, the back metal with a higher carbon content is used, and its rigidity is enhanced by work hardening, thus improving and increasing the strength of the back metal. 
     The material for the steel back metal used in the present invention is selected from the group consisting of S20C, S30C, S17C, S22C, S25C, S28C and S33C (defined in JIS G4051). 
     Preferably, the thickness of this material is 0.8 to 20 mm. Preferably, the thickness of the aluminum bearing alloy is 0.2 to 3 mm. 
     The reasons for the limitation of the carbon content and the rolling reduction specified in the appended claims directed to the aluminum alloy bearing for the low-rigidity housing, as well as effects thereof, will now be described. 
     (1) Most of back metal materials used in the conventional bearings are made of low carbon steel having the carbon content of not more than 0.15%, because the plastic workability, obtainable when working such material into a half-shell type configuration or a configuration with a flange, is considered to be important. 
     The bearing for a low-rigidity housing need to have an increased amount of interference available when mounting the bearing on the housing, so as to absorb deformation of the housing. At this time, if the strength of the bearing is low (that is, the back metal is made of low carbon steel whose carbon content is not more than 0.15% as described above), the assembling stress surpasses the resisting force of the back metal, and the back metal is undesirably deformed beyond its elastic limit to cause permanent strain and yielding. 
     If the carbon content exceeds 0.35%, the toughness is adversely affected although the mechanical strength is improved. This reduces the plastic workability. 
     Therefore, the carbon content should be in the range of 0.16 to 0.35 wt.%. 
     (2) Generally, a steel material, when subjected to a plastic working, is increased in strength and hardness, and is reduced in elongation, and reduces toughness to be brittle. 
     If the rolling reduction of the stock steel strip is less than 37%, the intended mechanical strength is not obtained, and causes permanent strain and yielding, as described above. On the other hand, if the stock steel strip is subjected to a high rolling reduction more than 50%, the elongation is reduced, and the toughness is lowered, thus adversely affecting the mechanical strength. 
     Therefore, the rolling reduction of the steel strip at the time of the pressure bonding should be in the range of 38 to 50%. 
     (3) The flash plating reduces a load occurring when press-fitting the bearing of a bush type. Also, the flash plating relieves the concentration of stress, and reduces the friction coefficient of the contact surface. Therefore, the flash plating is effective against fretting. 
     If the thickness of the flash plating layer is 
     less than 0.1 μm, its effect is low, and also if this thickness is more than 5 μm, its effect is also low, and in contrast such thickness is harmful, and constitutes a factor in the occurrence of migration. 
     Therefore, the thickness of the flash plating layer should be in the range of 0.1 to 5 μm. 
    
    
     DESCRIPTION OF THE INVENTION 
     The invention will now be illustrated by way of the following Examples: 
     EXAMPLE 1 
     Strips of steel (JIS G3141 SPCC; JIS G4051 S17C to S35C) having a width of 110 mm and a thickness of 2.24 mm were prepared as materials for back metals. Each steel strip and each of aluminum bearing alloys (having a thickness of 1 mm) shown in Table 5 was pressure-bonded together by rolling so as to provide a respective one of rolling reduction of the steel strips shown in Table 1. Then, each composite material thus produced was heat-treated at 350° C. for 4.5 hours, and then was worked into a semi-cylindrical shape having an outer diameter of 56 mm, a thickness of 1.5 mm and a width of 26 mm, thus forming a sliding bearing. 
     Next, for examining the mechanical strength of the steel back metals by way of comparison, tensile test pieces (JIS No. 5) were prepared therefrom, and various tests were conducted. The test piece was prepared by removing the aluminum alloy entirely from the sliding bearing, thus leaving the back metal alone. Results of the tests are shown in Table 1. 
     EXAMPLE 2 
     Each of four kinds of sliding bearings, that is, Sample Nos. 2, 4, 7 and 8 in Table 1, was mounted on an actual device under a bearing stress of 25 to 30 kgf/mm 2 , and a test was conducted. Results of the test are shown in Table 2. 
     Sliding bearing (that is, a bush type) were produced from three kinds of bimetals of Sample Nos. 1, 4 and 8, and each of the sliding bearing thus formed was press-fitted into an aluminum housing (Al-Si type housing) for the purpose of a press-fitting test. 
     The test metals of each Sample included ones with a 3 μm-thick flash plating layer and ones without a flash plating layer. 
     Results of the press-fitting test are shown in Table 3. 
     In order to examine the amount of a permanent strain in the test metals used for the press-fitting test, a thermal cycle test was conducted. 
     In this test, a thermal cycle between room temperature and -40° C. was repeated 10 times. The retention time for each of the room temperature and -40° C. was one hour. 
     For evaluation, the bushing was removed from the housing after the test, and the dimension of the bushing was measured, and the permanent strain was determined by a difference between its outer diameter after the test and its outer diameter before the pressfitting. 
     Results thereof are shown in Table 4. 
     (1) As is clear from the test results of Table 1, the mechanical strength is closely related to the carbon content and the rolling reduction, and the strength and hardness increase with the increase of the carbon content and the rolling reduction. 
     The products (Nos. 1 to 6) of the present invention are much improved in strength and hardness over the conventional products (Nos. 7, 8 and 9). 
     In the actual device test, fretting damage occurred in the conventional products (Nos. 7 and however, fretting was not found in the products (Nos. and 4) of the present invention. 
     In the case where there is involved a high bearing stress, that is, an increased amount of interference, in connection with a low-rigidity housing, the heretofore-used material of a low strength is deformed beyond the elastic limit, thus causing a permanent strain. 
     The products of the present invention have high strength and toughness, as well as an excellent ability to prevent the permanent strain from occurring in the housing, and therefore exhibit a superior fretting resistance. 
     (2) As is clear from Table 3, those products with the Sn-flash plating layer on the metal are around 25% lower is press-fitting load than those products without such flash plating layer, and are not subjected to scoring. 
     Therefore, since the amount of interference can be increased without causing any scoring, a fretting resistance can be improved. 
     (3) As is clear from the test results of Table 4, the products of the present invention have a smaller amount of the permanent strain than the conventional products. Namely, the products of the present invention have a good ability to follow the deformation of the housing, and therefore the close contact thereof with the housing is improved, thus providing superior resistance against the fretting and the migration. 
     The aluminum alloy sliding bearings of the present invention for the low-rigidity housing are higher in the strength of the back metal than the conventional products, and have an excellent ability to follow the deformation of the housing. Therefore, it is clear that the aluminum alloy bearings of the present invention have excellent fretting resistance and migration resistance. 
     
                                           TABLE 1__________________________________________________________________________    Rolling        0.2%  Propor-    reduction         Carbon             Tensile                   yield tional                               Elonga-Sample    of steel         content             strength                   strength                         limit tion HardnessKind No. strip (%)         (%) (kgf/mm.sup.2)                   (kgf/mm.sup.2)                         (kgf/mm.sup.2)                               (%)  (Hv10)__________________________________________________________________________Products1   38   0.20             67.5  65.1  65.1  9.0  223of   2   45   0.20             69.4  66.5  66.5  4.1  226present3   50   0.20             72.1  68.7  68.7  2.4  235invention4   38   0.30             76.0  73.5  73.5  3.5  2405   45   0.30             80.0  76.3  76.3  2.5  2496   50   0.30             82.3  79.5  79.5  1.0  255Conven-7   40   0.05             55.1  52.5  52.5  12.0 187tional8   50   0.05             57.0  54.8  54.8  10.1 193products9   60   0.05             62.3  59.5  59.5  7.5  199__________________________________________________________________________ 
    
     
                       TABLE 2______________________________________        Sample  Results of actualKind         No.     device test______________________________________Products of  2       No fretting damagepresent      4       No fretting damageinventionConventional 7       Fretting damage occurredproducts     8       Fretting damage occurred______________________________________ 
    
     
                                           TABLE 3__________________________________________________________________________            Outer Inner        Press-                                   Scoring* at        Bush-            diameter                  diameter of                           Inter-                               fitting                                   time of   Sample        ing of bushing                  housing  ference                               load                                   press-Kind    No.  No. (mm)  (39,000 = 0 μm)                           (mm)                               (kg)                                   fitting__________________________________________________________________________Products 1 with Sn         1  39.174                   0       0.174                               1685                                   Noneof present   flash         2  39.174                   0       0.174                               1695                                   Noneinvention   plating         3  39.175                  -1       0.176                               1700                                   None   without         4  39.175                   1       0.174                               2215                                   Occurred   Sn flash         5  39.174                   0       0.174                               2260                                   Occurred   plating         6  39.174                  -1       0.175                               2245                                   Occurred 4 with  7  39.175                  -1       0.176                               1685                                   None   Sn flash         8  39.175                  -1       0.176                               1685                                   None   plating         9  39.175                   1       0.174                               1690                                   None   without        10  39.175                   0       0.175                               2180                                   Occurred   Sn flash        11  39.174                   0       0.174                               2200                                   Occurred   plating        12  39.174                  -1       0.175                               2175                                   OccurredConven- 8 with Sn        13  39.173                  -4       0.177                               1690                                   Nonetional  flash        14  39.174                  -4       0.178                               1680                                   Noneproducts   plating        15  39.174                  -3       0.177                               1680                                   None   without        16  39.174                  -2       0.176                               2060                                   Occurred   Sn flash        17  39.175                  -2       0.177                               2100                                   Occurred   plating        18  39.175                   0       0.175                               2170                                   Occurred__________________________________________________________________________ *&#34;Scoring at time of pressfitting&#34; means that when the bearing of the bushing type is to be pressfitted (pressfixed) in the housing, the outer surface of the bearing and the housing are in abrasive contact with each other, thereby causing wear. 
    
     
                                           TABLE 4__________________________________________________________________________            Outer diameter  Amount of        Bush-            of bushing                    Outer diameter                            permanent   Sample        ing before test                    of bushing                            fatigue*Kind    No.  No. (mm)    after test (mm)                            (μ)__________________________________________________________________________Products 1 with Sn         1  39.174  39.156  18of present   flash         2  39.174  39.155  19invention   plating         3  39.175  39.157  18   without         4  39.175  39.158  17   Sn flash         5  39.174  39.155  19   plating         6  39.174  39.156  18 4 with  7  39.175  39.155  20   Sn flash         8  39.175  39.158  17   plating         9  39.175  39.160  15   without        10  39.175  39.156  19   Sn flash        11  39.174  39.158  16   plating        12  39.174  39.161  13Conven- 8 with Sn        13  39.173  39.133  40tional  flash        14  39.174  39.144  30products   plating        15  39.174  39.146  28   without        16  39.174  39.136  38   Sn flash        17  39.175  39.143  32   plating        18  39.175  39.145  30__________________________________________________________________________ *&#34;Amount of permanent strain&#34;  means the amount of deformation occurring when the bearing of the bushing type goes beyond its yield point, so that a sufficient stress to keep the bearing in close contact with the housing is not available. 
    
     
                       TABLE 5______________________________________Sample      Chemical components (wt. %)Kind    No.     Al     Si  Sn  Cu  Pb  Sb  Zn  Mn   V______________________________________Products of   1       Rem.   --  20  1.0 --  --  --  --   --present 2       Rem.   2.5 12  1.0 1.4 0.3 --  0.4  0.12invention   3       Rem.   6.0 --  1.2 1.0 --  4.0 --   --   4       Rem.   --  20  1.0 --  --  --  --   --   5       Rem.   2.5 12  1.0 1.4 0.3 --  0.4  0.12   6       Rem.   6.0 --  1.2 1.0 --  4.0 --   --Conven- 7       Rem.   --  20  1.0 --  --  --  --   --tional  8       Rem.   2.5 12  1.0 1.4 0.3 --  0.4  0.12products   9       Rem.   6.0 --  1.2 1.0 --  4.0 --   --______________________________________