Sliding material

Abstract

To solve the problem that when an amount of Si to be added to aluminum alloy is increased, an area occupied by a region having comparatively high Si concentration is increased, the region having comparatively high Si concentration is relatively harder than other region, when unintended external force is applied to the region, cracks may occur with the region having high Si concentration as a starting point.SOLUTION: A sliding material includes an aluminum alloy to which Si is added, where the aluminum alloy has been annealed, wherein an amount of Si to be added is 7.0 to 12.6 mass%, and in the aluminum alloy, in its observation visual field, 5 area% or more of an Si rich region having Si concentration of 17 mass% or more exists.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an improvement of a sliding material, particularly a sliding material made of an aluminum alloy.

Background Art

[0002] There are cases where the outermost layer (sliding layer) of a sliding member such as a bearing of an automobile is formed of a sliding material made of a soft metal. As such a sliding material, an aluminum (Al) alloy containing tin (Sn) and added with silicon (Si) has been proposed (Patent Document 1). Generally, Si is added to an aluminum alloy to improve wear resistance, and Sn is added to impart conformability.

[0003] The region containing Si in the aluminum alloy becomes a harder region than other regions. When this contacts the rotating shaft, it smoothens the protrusions of the rotating shaft, and since the contact between this and the rotating shaft avoids wear of the aluminum alloy, it can be said that Si contributes to so-called wear resistance. In recent years, for improving fuel efficiency in engines for internal combustion engines, engine start-stop operations such as GO-STOP are frequently performed. When the engine stops, the oil film of the engine oil breaks, and solid contact occurs between the rotating shaft and the bearing, so wear resistance of the bearing sliding surface is required. As described above, generally, by containing a large amount of Si, wear resistance is improved. Please also refer to Patent Documents 2 to 6 related to the present invention.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

[0005] When a sliding layer is formed from an aluminum alloy sliding material with a high Si content simply to improve wear resistance, the following problems arise. The Si added to the aluminum alloy does not dissolve uniformly in the aluminum alloy, but rather, regions with relatively high Si concentrations are scattered around the aluminum alloy, with the Si phase precipitated as a nucleus. If the amount of Si added to the aluminum alloy is increased, the area occupied by these regions with relatively high Si concentrations increases. Although the regions with relatively high Si concentrations are relatively harder than other regions, when an unexpected external force is applied to the sliding layer, there is a risk that cracks will occur in the sliding layer, originating from these hard regions with high Si concentrations. In other words, when adding Si to an aluminum alloy used as a sliding material, it is necessary to control the trade-off between wear resistance and crack suppression so as to satisfy the sliding characteristics of the sliding layer formed from the sliding material. [Means for solving the problem]

[0006] As a result of extensive research to solve the above problems, the inventors have found that wear resistance and crack suppression can be optimized by adjusting not only the amount of Si added but also the region in the aluminum alloy where the Si concentration is relatively high. A sliding material comprising an aluminum alloy to which Si is added, the aluminum alloy having been subjected to annealing, The amount of Si added is 7.0 to 12.6 mass %, The aluminum alloy has a Si-rich region with a Si concentration of 17 mass % or more, which accounts for 5 area % or more in an observed field of view.

[0007] When the sliding material of the first aspect defined in this way forms a sliding layer applied to, for example, a bearing for an automobile, it has excellent sliding characteristics such as wear resistance and crack suppression maintenance. The addition amount of Si is preferably 7.0 to 12.6% by mass. By setting the addition amount of Si within such a range, suitable hardness and wear resistance as a sliding material can be ensured. If it exceeds 12.6% by mass, the aluminum alloy is likely to crack. The reason for defining the Si concentration in the Si-rich region in the aluminum alloy to be 17% by mass or more is as follows. Figure 1 shows the relationship between the Si concentration (mass%, the same hereinafter) and its hardness.

[0008] From the results of Figure 1, it can be seen that when the Si concentration exceeds 17% by mass, its hardness increases dramatically. Note that the graph in Figure 1 is obtained by casting the molten aluminum alloy added with 8% by mass of Si into a plate shape in the same manner as in the examples described later, performing rolling treatment, and then annealing the sample in a hot air drying furnace under the conditions of 500 °C × 8 hours to prepare a sample. The hardness HV on the vertical axis of the cross-section of the sample is measured with a nanoindenter (model DUH-211S manufactured by SHIMADZU Corporation), and the Si concentration on the horizontal axis is analyzed with an electron probe microanalyzer (EPMA) (model JXA-8530F manufactured by JEOL Ltd.).

[0009] By setting the ratio of the region where the Si concentration is 17% by mass or more, that is, the Si-rich region, to occupy 5% by area or more in the aluminum alloy, sufficient wear resistance can be imparted to the sliding material. The upper limit of the area% of the Si-rich region where the Si concentration is 17% by mass or more is defined by the addition amount of Si and the annealing conditions. For example, when the annealing conditions are 430 to 570 °C for 5 to 10 hours and the addition amount of Si is 7.0 to 12.6% by mass, the upper limit of the area% of the Si-rich region is 25% by area. The method for measuring the area% of the region where Si is 17% by mass or more will be described later.

[0010] It has been conventionally known that it is preferable to add Sn to an aluminum alloy in order to ensure the conformability of the sliding material. However, according to the study by the present inventors, it has been found that the addition of Sn tends to promote the precipitation of Si by annealing. In other words, even if the energy consumption in the annealing process is reduced and the manufacturing cost is suppressed, the area percentage of the Si-rich region where the Si concentration is 17% by mass or more can be sufficiently obtained. However, in the sliding material defined in the first aspect, the addition amount of Sn is preferably 6% by mass or less. By setting the addition amount of Sn to 6% by mass or less, the sliding material is provided with a desired hardness and can maintain wear resistance. Also, cracking can be suppressed.

[0011] Next, a manufacturing method suitable for obtaining the sliding material of the first aspect described above will be described. In the sliding material defined in the first aspect, the area percentage of the hard Si-rich region is defined. The area of the Si-rich region in the aluminum alloy containing Si depends on the amount of Si in the aluminum alloy, annealing conditions, etc.

[0012] According to the study by the present inventors, the sliding material defined in the first aspect can be obtained by the following manufacturing method. A preparation step of preparing a molten metal of an aluminum alloy with the addition amount of Si being 7.0 to 12.6% by mass, A casting step of casting a plate-shaped workpiece from the molten metal, A rolling step of rolling the plate-shaped workpiece, An annealing step of annealing the rolled workpiece, which is a manufacturing method of a sliding material comprising: In the annealing step, a manufacturing method of a sliding material in which the Si-rich region having a Si concentration of 17% by mass or more is 5% by area or more.

[0013] In the above manufacturing method, the annealing conditions can be appropriately adjusted according to the characteristics of the material, the annealing environment, etc. However, according to the study by the present inventors, it is preferable that the temperature is 430°C to 570°C.

Brief Description of the Drawings

[0014]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0015] Figure 2 shows a cross-section of the sliding member 1 according to an embodiment of the present invention. This sliding member 1 has a structure in which a sliding layer 5 is laminated on the upper surface of a backing layer 3. The backing layer 3 is formed of a general-purpose steel material, and the sliding layer 5 is an aluminum alloy containing Si. An intermediate layer made of aluminum can also be interposed between the backing layer 3 and the sliding layer 5. The backing layer 3 and the sliding layer 5 are prepared in a flat state and pressure-bonded, and after heat treatment, they are machined into a cylindrical shape or a semi-cylindrical shape to form a bearing (sliding member).

[0016] The sliding member 1 is manufactured through a casting process, a rolling process, and an annealing process. In the casting process, a molten metal (700 - 900 °C) of an aluminum alloy containing 7.0 - 12.6 mass% Si or an aluminum alloy further containing Sn is prepared, and this molten metal is cast into a plate shape. For this casting, a roll caster method can be adopted. A roll caster is equipped with a pair of rollers, a molten metal supply nozzle for supplying an aluminum alloy molten metal between the rollers, and a cooling device for cooling the pair of rollers.

[0017] In this example, the molten aluminum alloy was cooled at a rate of 80 - 130 °C / second by a pair of rollers to obtain a billet (workpiece) with a thickness of about 6 mm. The distribution of the Si concentration of this workpiece was obtained as follows. The Si concentration is the value of the concentration when the cross-section of the workpiece is subjected to area analysis in a range of 45 μm × 45 μm (200 × 200 pixels) at 2000 times the observation field of view using an electron probe microanalyzer (EPMA). The analysis conditions are as follows.

[0018] Analytical instrument: FE-EPMA (JXA-8530F of JEOL Ltd.) Analysis method: Area analysis using WDS Observation field of view: 45 μm × 45 μm (200 × 200 pixels) Probe diameter: 40 nm to 1000 nm (appropriately selected according to the sample and measurement settings, and when there are multiple samples, measure with the same probe diameter) Analytical elements (crystals used): Al (TAP), Sn (PETH), Fe (LIF), Cu (TAPH), Si (TAP) Crystals used: ↑ Acceleration voltage: 15 kV Irradiation current: 3×10 -8 A Scan direction: One-way scan

[0019] In the area analysis results obtained by the above EPMA, the concentration of each analytical element is represented by hue for each element. However, in order to relatively compare the concentrations of each element, the hue range for each concentration is set, and binarization is performed using the Si concentration result. In binarization, the threshold is determined so that the area where the Si concentration is 17% or more and the other areas are separated. In the example of Fig. 3, the area where the Si concentration is 17% or more is shown in white. Perform particle size analysis on the binarized image and measure the area ratio of the area selected by binarization (the area where the Si concentration is 17% or more). For binarization and particle size analysis, the analysis application ver. 3.8.0.0 of the LASER MICROSCOPE manufactured by KEYENCE was used. The area % of the area where the Si concentration is 17 mass% or more in Fig. 3(A1) obtained by image processing the cross-section of the workpiece after the casting process as described above was 0.4 area%.

[0020] In the rolling process, the obtained plate-shaped aluminum alloy is rolled. The rolling method is not particularly limited.

[0021] In the annealing process, the rolled workpiece is heat-treated (annealed) under the conditions of 430°C to 570°C for 5 to 10 hours to promote the crystallization of Si. The result of EPMA measurement - binarization processing of the cross-section of the workpiece (i.e., the sliding material) that has undergone such an annealing process is shown in Fig. 3(A2). Note that the example in Fig. 3(A2) is an annealing temperature of 430°C × 8 hours and Sn is not added. From the results of Fig. 3(A2), it can be seen that the area ratio of the region where the Si concentration is 17% by mass or more has increased compared to Fig. 3(A1).

[0022] The area percentage of the region where the Si concentration in Fig. 3(A2) is 17% by mass or more is 6.4 area%. In this specification, the region where the Si concentration after annealing is 17% by mass or more is referred to as the Si-rich region. This Si-rich region is the region where the Si concentration observed by the above EPMA analysis is 17% by mass or more. As this Si-rich region, in addition to the Si phase in the eutectic, the Si region containing Al also corresponds. Fig. 3(B2) shows the state with 3% by mass of Sn added. Other conditions such as the annealing conditions are the same as those in Fig. 3(A2). In Fig. 3(B2), the area percentage of the region where the Si concentration is 17% by mass or more is 8.3 area%.

[0023] Note that Fig. 3(B1) shows the state before annealing of the one with 3% by mass of Sn added. In Fig. 3(B1), the area percentage of the region where the Si concentration is 17% by mass or more is 0.6 area%. From the results of Fig. 3, it can be seen that by adding Sn, the area percentage of the region where the Si concentration is 17% by mass or more increases.

[0024] Here, it is preferable that the Si-rich regions are evenly dispersed. Such dispersion is obtained by sufficiently stirring the molten aluminum alloy. This is because the Si crystallized in the casting process is inevitably dispersed. The particle size of the region where the Si concentration is 17% by mass or more can be adjusted according to the amount of Si added and the annealing conditions. According to the study by the inventors, it is preferable that the particle size is 1 μm to 20 μm.

[0025] Examples of the sliding material of the present invention are shown in Table 1 below. This sliding material was obtained through the above-described casting process, rolling process, and annealing process. The workpieces that cracked during the above processes are not subject to the wear test. The wear amount in Table 1 was obtained as follows. A wear test was conducted by rotating a cylindrical shaft while pressing the bottom surface of the cylindrical shaft vertically against a flat plate-shaped sliding material, and the plate thicknesses (at 12 locations) of the sliding portions of the sliding material before and after the test were measured to confirm how much the wall thickness changed as the wear amount.

[0026] The test conditions are as follows. Peripheral speed: 0.1 m / second Surface pressure: 5 MPa (constant) Oil type: Neutral oil Lubrication method: Dropwise 20 ml / min Oil temperature: 80°C Test piece: Plate-shaped Test time: 5 hours Test shaft: S55C annealed Test shaft roughness: Ra0.1 finish

[0027]

Table 1

[0028] From the wear test results in Table 1, it is recognized that the wear resistance of those with a wear amount of 30 μm or less is good. As a result, it is preferable that the addition amount of Si is 7.0 to 12.6% by mass (see Example 5 and Example 6). Assuming that the blending amount of Si is as described above, excellent wear resistance can also be obtained when the addition amount of Sn is 6% by mass or less (see Example 7 and Example 4). Good wear resistance is ensured when the annealing temperature is in the range of 430°C to 570°C (see Example 7 and Example 18).

[0029] Although excellent wear resistance is ensured when the area ratio of the Si-rich region with a Si concentration of 17 mass% or more is in the range of 5 area% to 23.1 area% (see Example 4 and Example 12), according to the study by the inventors, it is considered that excellent wear resistance can also be obtained when the area ratio is in the range of 5 area% to 25 area%. Also, from the results of the examples, it can be seen that the area ratio of the Si-rich region with a Si concentration of 17 mass% or more can be controlled by adjusting the annealing temperature. It can be seen that increasing the addition amount of Sn promotes the precipitation of Si (see Example 4, Example 10, and Example 7 where the annealing temperature is the same at 430°C). Note that when the addition amount of Sn exceeds 6.0 mass%, the wear resistance decreases and there is a risk of cracking.

[0030] This invention is not limited to the description of the embodiments and examples of the above invention. Various modifications that can be easily conceived by those skilled in the art without departing from the description of the claims are also included in this invention.

Explanation of Reference Signs

[0031] 1 Sliding member 3 Backing layer 5 Sliding layer

Claims

1. A sliding material comprising an aluminum alloy added with Si and annealed for the aluminum alloy, wherein the addition amount of the Si is 7.0 to 12.6% by mass, and in the aluminum alloy, there is a Si-rich region having a Si concentration of 17% by mass or more in an observation field of view in an area of 5% or more. The sliding material.

2. The sliding material according to claim 1, wherein Sn of 6% by mass or less is further added.

3. A sliding member including a sliding layer made of the sliding material according to claim 1.

4. A preparation step of preparing a molten metal of an aluminum alloy with the addition amount of Si being 7.0 to 12.6% by mass, a casting step of casting a plate-shaped workpiece from the molten metal, a rolling step of rolling the plate-shaped workpiece, and an annealing step of annealing the rolled workpiece. A manufacturing method of a sliding material, wherein in the annealing step, a Si-rich region having a Si concentration of 17% by mass or more is 5% by area or more. The manufacturing method of the sliding material.

5. The manufacturing method according to claim 4, wherein in the casting step, the plate-shaped workpiece is formed using a roll caster.

6. The manufacturing method according to claim 5, wherein the annealing temperature in the annealing step is 430°C to 570°C.

7. The manufacturing method according to claim 4, wherein in the preparation step, Sn of 6% by mass or less is further added.

8. A manufacturing method of a sliding member including a rolling step of forming the sliding material according to claim 1 into a plate shape and rolling it into a backing layer.

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