AL-BASED BEARING ALLOY

Abstract

An Al-based bearing alloy includes 1 to 15 mass % of Si. In the Al-based bearing alloy, an average of A/a is greater than 1 and equal to or less than 4, where A represents a distance between adjacent Si particles residing on a sliding-side surface, and a represents a length of a major axis of the Si particles.

Description

TECHNICAL FIELD

The present invention relates to an Al-based bearing alloy containing Si.

BACKGROUND

An Al-based bearing alloy such as an Al—Sn bearing alloy containing approximately 20 mass % of Sn, and Al—Sn—Si bearing alloy containing approximately 10 mass % of Sn, and 3 mass % of Si are being used in slide bearings for automobiles and industrial machines in general. The Al-based bearing alloy, when being used as a slide bearing, is normally bonded with a metal backing made of a steel sheet. Si contained in the Al-based bearing alloy is a hard particle and thus, smoothens the protrusions of the countershaft when it contacts the countershaft (lapping effect). Further because the hard particle plays a sacrificial role in contacting the countershaft, the Al matrix becomes difficult to scrape off, thereby improving wear resistance.

Recent automobile engines are designed to go through the start-stop cycle on a frequent basis for better mileage. Further, housings such as connecting rods, into which slide bearings are installed, are becoming lighter in weight. Thinning, which is a typical approach for obtaining a lighter housing, reduces the strength of the housing and makes the housing deformation prone. Housing is thus, easily deformed by forces such as the dynamic load of the counter shaft. As a result, the Al-based bearing alloy (slide bearing), which supports the counter shaft, also becomes deformation prone and consequently fatigue prone.

One possible solution is proposed in patent document 1 which teaches an Al-based bearing alloy containing Si such that small Si particles and large Si particles are mixed in an suitable ratio to achieve improvement in both wear resistance and fatigue resistance.

RELATED ART

Patent Document

Patent Document 1: JP 2003-119530 A

SUMMARY OF THE INVENTION

Problems to be Overcome

Further improvements in wear resistance and fatigue resistance need to be sought to withstand today's rigorous usage environment.

The present invention is based on the above described background and one object of the present invention is to provide an Al-based bearing alloy with further improvements in wear resistance and fatigue resistance.

Means to Overcome the Problem

While conducting research based on an Al-based bearing alloy containing 1 to 15 mass % of Si, the inventors came to focus on the relation between the size of Si particles and the distance between the adjacent Si particles. As they pursued their analysis, they came to a conclusion that the size of the Si particles and the distance between the adjacent Si particles are closely related to wear resistance and fatigue resistance which has lead to the present invention.

The Al-based bearing alloy according to claim 1 of the present invention include 1 to 15 mass % of Si, wherein an average of A/a is greater than 1 and equal to or less than 4, where A represents a distance between adjacent Si particles residing on a sliding-side surface, and a represents a length of a major axis of the Si particles.

The following describes how A/a mentioned above is obtained. In summary, the image of the Si-containing Al-based bearing alloy structure is captured on the sliding-side surface and the captured image is put through software analysis to obtain the required parameters.

FIG. 1 is an image produced by the analysis showing idealized distribution of Si particles 1. The lines drawn between the adjacent Si particles 1 are Voronoi boundaries. Length a of the major axis of Si particle 1 shown in FIG.1 is assumed as the longer side of a circumscribing quadrangle surrounding Si particle 1. Then, distance A between the gravitational centers of the adjacent Si particles 1 is divided by length a of the major axis of Si particle 1. This is repeated for each Si particle 1 within a given area of observation to obtain the average A/a.

If the average of A/a is greater than 1 and equal to or less than 4, a judgment is made that Si particles 1 are evenly distributed with suitable spacing within the Al matrix. When in such even distribution, Si particle 1 is expected to exert lapping effect and wear resistance as well as fatigue resistance which are properties of a hard particle inherently possessed by Si particle 1.

If the average of A/a is greater than 4, the Al matrix occupies relatively greater percentage of the structure and thus, degrades the lapping effect and the wear resistance in particular. In contrast, if the average of A/a is equal to or less than 1, the Al matrix occupies relatively less percentage of the structure and thus, the ductility required in a bearing alloy is degraded which causes a sudden and significant degradation in conformability and degradation in fatigue resistance.

The Al-based bearing alloy of the present invention is manufactured as follows.

First, a material of Al, Si, and required additives are formed into a billet sheet of Al-based bearing alloy by typically using a continuous caster. Then, the billet is rolled repeatedly to a predetermined thickness. The billet is rolled at least twice at a high rolling reduction. More specifically, the first roll is carried out at a rolling reduction ranging from 40 to 80%. The second roll is carried out at a rolling reduction ranging from 30 to 70%. The rolling reduction of the (n+1)th roll is preferably lower than the rolling reduction of the nth roll. By rolling the Al-based alloy twice or more at a high rolling reduction, the crystal grains within the Al-matrix serving as the parent phase of the Al-based bearing alloy is destroyed. The destruction of the crystal grains is defined as a state in which the crystal grain boundaries of Al are excessively dense and hence cannot be distinguished from one another in the cross sectional sample of the structure obtained by etching. The rolling, when carried out to the extent to destroy the crystal grains, can be considered to have caused the Si particles to be evenly distributed within the Al matrix.

Then, Al is re-crystallized through heat treatment. It is thus, assumed that the Si particles are distributed within the Al matrix as evenly as possible and that the Si particles are shaped in a suitable form.

In the Al-based bearing alloy according to claim 2 of the present invention, an average aspect ratio of the Si particles residing on the sliding-side surface is equal to or greater than 1 and equal to or less than 2.5.

The aspect ratio of the Si particle is given by dividing the longer side (major axis) of the circumscribing quadrangle of Si particle 1 shown in FIG.1 by the shorter side (minor axis). The Si particle approximates a circle or a square as the aspect ratio approximates 1 and the Si particle becomes more and more elongate as the aspect ratio becomes greater.

The shape of the Si particle is closely related to the fatigue resistance of the bearing alloy. The aspect ratio of the Si particle indicates the degree of anisotropy in the shape of the Si particle. The Si particle with a large aspect ratio has relatively larger degree of anisotropy in its shape and is subjected to force with priority. Thus, Si particles with relatively larger degree of anisotropy render the bearing alloy to be deformation prone and therefore tend to degrade the fatigue resistance of the bearing alloy. The average aspect ratio of the Si particles is preferably 1 or more and 2.5 or less.

The Al-based bearing alloy of the present invention may be manufactured by repeating the rolling of the billet for obtaining the predetermined thickness by carrying out the first roll at a rolling reduction of 50 to 70%, and the second roll at a rolling reduction of 40 to 60%. The (n+1)th roll is preferably lower than the rolling reduction of the nth roll by 10%. For instance, if the rolling reduction of the nth roll is 60%, the rolling reduction of the (n+1)th roll is to be set to 50%.

According to claim 3 of the present invention, a given specific Si particle residing on the sliding-side surface and a closest Si particle adjacent to the specific particle in the thickness direction are spaced from one another such that the closest Si particle resides within radius r being taken from a gravitational center of the specific Si particle and being given by:

r=B×(A/a)(μm)

a/2<B≦20

where A represents a distance from the specific particle to the adjacent Si particle on the sliding-side surface, and a represents a length of a major axis of the specific Si particle.

This is explained with reference to the image shown in FIG.2. The direction indicated by D denotes the direction of thickness. The distribution of Si particles 11, 12, 13, and 14 on surface 10 of the sliding side and the distance to the closest adjacent Si particles 21, 22, and 23 in the thickness direction is closely related to wear resistance. More specifically, when distance from given specific Si particles 11, 12, 13, and 14 to their closest adjacent Si particles in the thickness direction is equal to or greater than a predetermined distance, wear resistance is further improved. In contrast, if the distance from each of the given specific Si particles 11, 12, 13, and 14 to their closest adjacent Si particles 21, 22, and 23 is too far, wear resistance tends to degrade. Thus, the Si particles 21, 22, and 23 each being in the closest adjacency in the thickness direction to the given specific Si particles 11, 12, 13, and 14 have been located within region 30 defined by predetermined radius r measured from each of the given specific Si particles 11, 12, 13, and 14. As a result, destruction of the material can be prevented while relaxing the progression of wear.

Each of the closest adjacent Si particles 21, 22, and 23 need not be entirely within but may be partly within region 30 defined by radius r measured from the specific particles 11, 12, and 14.

The distribution of the Si particles in the thickness direction can be controlled as follows.

The produced billet is rolled at least twice. More specifically, the first roll is carried out at the rolling reduction ranging from 40 to 80% and preferably from 50 to 70%. The second roll is preferably carried out at the rolling reduction which amounts to 60 to 95% of the rolling reduction employed in the first roll. The crystal grains of the Al matrix within the Al-based alloy is thus destroyed to control the location of the Si particles adjacent to one another in the thickness direction. The (n+1)th roll is carried out at the rolling reduction which amounts to 60 to 95% of the rolling reduction employed in the nth roll.

The coefficient B was defined to be greater than a/2 and equal to or less than 20, however, for the convenience of measurement, the value of a which is required to specify the value of a/2 is set to the average value obtained from each of length a of the major axis within the predetermined area of observation.

The closest adjacent Si particle is preferably within the range surrounded by 2a/3≦r≦15×(A/a) in view of improving wear resistance and fatigue resistance. The value a for specifying the value of 2a/3 is set to the average value obtained from each of length a of the major axis within the predetermined area of observation.

Further, radius r for defining region 30 is preferably equal to or greater than 2 μm and equal to or less than 50 μm in the manufacturing point of view.

Still further, the gravitational distance between a given specific Si particle and the closest adjacent Si particle being 5 μm or greater is advantageous in view of fatigue resistance and being 30 μm or less is advantageous in view of wear resistance.

The Al-based bearing alloy according to claim 4 of the present invention includes one or more of:

(1) a total of 0.1 to 7 mass % of one or more elements selected from the group of Cu, Zn, and Mg;

(2) a total of 0.01 to 3 mass % of one or more elements selected from the group of Mn, V, Mo, Cr, Co, Fe, Ni, W; and

(3) a total of 0.01 to 2 mass % of one or more elements selected from the group of B, Ti, and Zr.

The components (1) to (3) are limited to the above described composition based on the following reasoning.

The selective elements (Cu, Zn, Mg) given in (1) are additive elements for improving the strength of the Al matrix and may be forcibly incorporated by solid solution into the Al matrix through solution heat treatment and may be allowed to precipitate fine compounds by aging. This effect cannot be expected in content less than 0.1 mass % and will produce bulky compounds in content greater than 7 mass %. Total content preferably ranges from 0.5 to 6 mass %.

The selective elements (Mn, V, Mo, Cr, Co, Fe, Ni, W) given in (2) are additive elements for improving fatigue resistance and may be incorporated alone by solid solution into the Al matrix or may be crystallized as a multi-element intermetallic compound. This effect cannot be expected in content less than 0.01 mass %. In view of conformability required in a bearing alloy, content of 3 mass % or less is preferable. Preferable content ranges between 0.02 to 2 mass %.

The selective elements (B, Ti, Zr) given in (3) do not contribute to the production of Al—Si—Fe type intermetallic compounds but are incorporated by solid solution into the Al-matrix to improve the fatigue strength of the bearing alloy. This effect cannot be expected in content less than 0.01 mass %. In view of brittleness required in a bearing alloy, content of 2 mass % or less is preferable. Preferable content ranges between 0.02 to 0.5 mass %.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 An image of the anaylsis based on idealized distribution of Si particles.

FIG. 2 An image of the analysis based on idealized distribution of Si particles viewed in the cross section taken along the thickness direction.

FIG. 3 A side view schematically illustrating the roll step.

FIG. 4 A chart indicating the wear test conditions.

FIG. 5 A chart indicating the fatigue test conditions.

FIG. 6 A chart indicating the test results of EXAMPLES and COMPARATIVE EXAMPLES.

EMBODIMENTS OF THE INVENTION

To verify the effect of the Al-based bearing alloy according to the present invention, samples of slide bearings (EXAMPLES and COMPARATIVE EXAMPLES) using the Al-based bearing alloy containing Si were prepared and wear test and fatigue test were conducted on them.

A method of manufacturing EXAMPLES is as described below.

First, a billet of Al-based bearing alloy containing Si was cast with a continuous caster. More specifically, using the composition shown in the chart of FIG. 6 as a dissolving material for producing the Al-based bearing alloy, a sheet of Al-based alloy billet was obtained, which was approximately 15 mm thick.

Then, the billet was cold rolled multiple times to a predetermined thickness (e.g. 1 mm) to obtain a thin sheet of Al-based bearing alloy. The rolling step is carried out by passing the sheet of billet 111 between a pair of upper and lower rollers 112 and 113 while applying pressure with the rotating rollers 112 and 113 as shown in FIG. 3. In order to destroy the crystal grain of the Al matrix serving as the parent phase of the Al-based alloy, the present embodiment rolls billet 111 at least twice at a high rolling reduction. The rolling reduction of the first roll is set approximately to 70% and the rolling reduction of the second roll is set approximately to 50% slightly lower than the first roll.

Then, the obtained Al-based bearing alloy was roll bonded with a steel sheet constituting the metal backing to obtain the bearing forming sheet. Prior to the roll bonding of the Al-based bearing alloy, the bonding surface of the steel sheet may be controlled to surface roughness of maximum height ranging from 5 to 40 μm for securing bonding force. Annealing is carried out after the roll bonding for bonding enhancement and eliminating strain. Then, the obtained bearing forming sheet is machined into a semi-cylindrical form to obtain a semi-cylindrical bearing serving as EXAMPLES.

The method of manufacturing COMPARATIVE EXAMPLES differs from the method of manufacturing the EXAMPLES in the following respects. After the formation of 15 mm billet of Al-based alloy with the continuous caster using the materials similar to those of the EXAMPLES, the billet is repeatedly rolled in the rolling step to the predetermined thickness (1 mm), and at this instance, the maximum rolling reduction is set to 25% or less as was done conventionally. Then, the obtained Al-based bearing alloy was roll bonded with a steel sheet constituting the metal backing to obtain the bearing forming sheet as was done for the EXAMPLES to manufacture the bearing forming sheet. Annealing is carried out after the roll bonding for bonding enhancement and eliminating strain. Then, the obtained bearing forming sheet is machined into a semi-cylindrical form to obtain a semi-cylindrical bearing serving as COMPARATIVE EXAMPLES.

Using an optical microscope, images of the Al-based bearing alloy structures were captured on the sliding-side surface and on the cross section of the Al-based bearing alloy taken along the thickness direction. Then, the captured images were analyzed using an analysis software (Product Name: Image-Pro Plus (Version 4.5), made by Planetron, Inc.) to obtain measurements of length a of the major axis of each Si particle and distance A between the neighboring Si particles based upon which the average A/a was obtained. Further, measurements of the length of the major axis and the minor axis were obtained for each Si particle based upon which the average of the aspect ratio (major axis/minor axis) was obtained. Results show that the average of A/a of COMPARATIVE EXAMPLE was greater than 4. In contrast, the average A/a of EXAMPLES was greater than 1 but equal to or less than 4. The measurements were obtained from an observation area of 300 μm×400 μm.

EXAMPLES and COMPARATIVE EXAMPLE were further screened through wear and fatigue tests. The conditions employed in the wear test are indicated in FIG. 4 and the conditions employed in the fatigue test are indicated in FIG. 5. In the wear test, static load was applied on the inner surface of the bearing in which state the start and stop cycle was repeated for a predetermined time period whereafter wear amount (μm) was measured. The wear resistance was evaluated based on the results of the foregoing. In the fatigue test, dynamic load was applied on the inner surface of the bearing and the maximum specific load (MPa) tolerable without fatiguing within the predetermined test time was evaluated as the fatigue resistance. The results are indicated in FIG. 6.

First, EXAMPLE 8 is compared with COMPARATIVE EXAMPLE 1 to consider the impact of A/a on wear resistance and fatigue resistance. In EXAMPLE 8, the average of A/a was A/a=3.8. EXAMPLE 8 further showed maximum specific load without fatiguing of 80 MPa and wear amount of 18 μm. In contrast, in COMPARATIVE EXAMPLE 1, the average of A/a was A/a=4.3. COMPARATIVE EXAMPLE 1 further showed maximum specific load without fatiguing of 60 MPa and wear amount of 25 μm. It can be understood from the comparison of EXAMPLE 8 and COMPARATIVE EXAMPLE 1 that A/a being equal to or less than 4 is superior in terms of wear resistance and fatigue resistance as compared to A/a being greater than 4. It has been verified that EXAMPLES 1 to 8 in which the average A/a is greater than 1 but equal to or less than 4 has improved wear resistance and fatigue resistance as compared to COMPARATIVE EXAMPLE 1.

Next, EXAMPLES 7 and 8 are compared to consider the impact of the aspect ratio of Si particles on fatigue resistance. In EXAMPLE 7, the aspect ratio of Si particle was 2.3. EXAMPLE 7 further showed maximum specific load without fatiguing of 90 MPa. In contrast, in EXAMPLE 8, the aspect ratio of Si particle was 2.6 and maximum specific load without fatiguing was 80 MPa. It can be understood from the comparison of EXAMPLE 7 and EXAMPLE 8 that the aspect ratio of the Si particles being equal to or less than 2.5 is superior in terms of fatigue resistance as compared to the aspect ratio being greater than 2.5. It has been verified that setting the aspect ratio of the Si particles to range from 1 to 2.5 improves the fatigue resistance.

Next, EXAMPLES 5 and 6 are compared to consider the impact of the Si particles adjacent to one another in the thickness direction on wear resistance. In EXAMPLE 5, an Si particle being adjacent in a thickness direction to a given specific Si particle within the sliding-side surface and being located within radius r=B×(A/a) measured from the given Si particle was “PRESENT”. In EXAMPLE 5, the wear amount indicating the wear resistance measured 12 μm. In contrast, in EXAMPLE 6, an Si particle being adjacent in a thickness direction to a given specific Si particle within the sliding-side surface and being located within radius r=B×(A/a) measured from the given Si particle was “ABSENT”. In EXAMPLE 6, the wear amount indicating the wear resistance measured 15 μm. It can be understood from the comparison of EXAMPLE 5 and EXAMPLE 6 that Si particles being adjacent to one another in the thickness direction being “PRESENT” is superior in terms of wear resistance as compared to such Si particles being “ABSENT”. It has been verified that the presence of Si particles being adjacent to one another in the thickness direction within radius r improves the wear resistance.

As described above, it has been verified from the results of the wear and fatigue tests that the EXAMPLES are superior in terms of wear resistance and fatigue resistance as compared to the COMPARATIVE EXAMPLE.

Claims

1. An Al-based bearing alloy comprising:

1 to 15 mass % of Si,

wherein an average of A/a is greater than 1 and equal to or less than 4,

where A represents a distance between adjacent Si particles residing on a sliding-side surface, and a represents a length of a major axis of the Si particles.

2. The Al-based bearing alloy according to claim 1, wherein an average aspect ratio of the Si particles residing on the sliding-side surface is equal to or greater than 1 and equal to or less than 2.5.

3. The Al-based bearing alloy according to claim 1, wherein a given specific Si particle residing on the sliding-side surface and a closest Si particle adjacent to the specific particle in a thickness direction are spaced from one another such that the closest Si particle resides within radius r being taken from a gravitational center of the specific Si particle and being given by: where A represents a distance from the specific particle to the adjacent Si particle on the sliding-side surface, and a represents a length of a major axis of the specific Si particle.

r=B×(A/a)(μm)

a/2<B≦20

4. The Al-based bearing alloy according to claim 1 comprising:

one or more of: (1) a total of 0.1 to 7 mass % of one or more elements selected from the group of Cu, Zn, and Mg; (2) a total of 0.01 to 3 mass % of one or more elements selected from the group of Mn, V, Mo, Cr, Co, Fe, Ni, W; and (3) a total of 0.01 to 2 mass % of one or more elements selected from the group of B, Ti, and Zr.

5. The Al-based bearing alloy according to claim 2 comprising:

one or more of:

(1) a total of 0.1 to 7 mass % of one or more elements selected from the group of Cu, Zn, and Mg;

(2) a total of 0.01 to 3 mass % of one or more elements selected from the group of Mn, V, Mo, Cr, Co, Fe, Ni, W; and

(3) a total of 0.01 to 2 mass % of one or more elements selected from the group of B, Ti, and Zr.

6. The Al-based bearing alloy according to claim 3 comprising:

one or more of:

(1) a total of 0.1 to 7 mass % of one or more elements selected from the group of Cu, Zn, and Mg;

(2) a total of 0.01 to 3 mass % of one or more elements selected from the group of Mn, V, Mo, Cr, Co, Fe, Ni, W; and

(3) a total of 0.01 to 2 mass % of one or more elements selected from the group of B, Ti, and Zr.