Manufacturing method for cylinder blocks for outboard motors

Abstract

To provide a manufacturing method of a cylinder block for an outboard motor, capable of subjecting a cylinder block body for an outboard motor, obtained by insert-casting a cylinder sleeve made of cast-iron with an aluminum alloy to an anodic oxidation treatment and a sealing treatment while preventing the cast-iron of the cylinder sleeve from being dissolved.SOLUTION: A manufacturing method of a cylinder block for an outboard motor includes an insert-casting step of integrally casting a cylinder sleeve 20A and a cylinder block body 10A, in which a cylinder-shaped cylinder sleeve has formed therein a thinned part 21 at an end on a crank-shaft side. The insert-casting step includes: performing casting in a state of engaging a positioning pin 30A onto an inner peripheral face 23 of the cylinder sleeve; and forming an aluminum alloy as a base material of the cylinder block body even on an inner peripheral face 22 of the thinned part. The manufacturing method of a cylinder block for an outboard motor further includes performing an anodic oxidation treatment and a sealing treatment in a state of pressing with an elastic jig 40 an area where boundaries 11B, 11D each between cast-iron of the cylinder sleeve and the aluminum alloy as a base material thereof are included on an inner peripheral face of a cylinder bore, followed by removing the aluminum alloy formed on the inner peripheral face of the thinned part.SELECTED DRAWING: Figure 7B

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a cylinder block for an outboard motor.

Background Art

[0002] In industrial internal combustion engines, cooling water is circulated to control the temperature of the internal combustion engine in order to maintain performance. Generally, for automotive cooling water, water added with ethylene glycol or the like is used, and when it decreases, it is necessary to replenish the shortage. On the other hand, in engines such as outboard motors, seawater (salt water) that is easily available in the usage environment is taken in and used as cooling water.

[0003] However, since each member of an outboard motor is composed of many types of metal materials, a potential difference occurs due to the difference in the metal materials of each member in contact with seawater, and the metal material with a lower natural potential dissolves in seawater, which can cause corrosion. In particular, the aluminum alloy used for the cylinder block of an outboard motor is a metal that is more likely to corrode than other materials.

[0004] In order to prevent the occurrence of such corrosion, for example, Patent Document 1 describes that a metal such as zinc having a lower potential than the aluminum alloy is used as a sacrificial anode and provided in a seawater contact portion of the cylinder block, for example, a cooling water flow path called a water jacket. [[ID=2T]]

[0005] On the other hand, generally, as a method for improving the corrosion resistance of an aluminum alloy, an anodic oxidation treatment for forming an anodic oxide film on the surface of the aluminum alloy has been conventionally known. In the anodic oxidation treatment, aluminum is oxidized to form a porous anodic oxide film, but this porosity contributes to a decrease in corrosion resistance. Therefore, in order to improve the corrosion resistance, a sealing treatment for closing the pores is performed after the anodic oxidation treatment.

Prior Art Documents

Patent Documents

[0006] [Patent Document 1] Japanese Patent Publication No. 2004-232614 [Overview of the project] [Problems that the invention aims to solve]

[0007] The area around the combustion chamber of an outboard motor, where the cylinder block is located, experiences extremely high temperatures, making it highly susceptible to corrosion. Therefore, it is desirable to form an anodic oxide coating not only on the inner surface of the water jacket of the cylinder block, but also on the joint surface with the cylinder head.

[0008] However, the cylinder block for outboard motors has a structure in which a cylindrical cylinder sleeve made of cast iron is cast into the base material, an aluminum alloy, on the inner circumferential surface of the cylinder bore. When the entire cylinder block with such a structure is immersed in an anodizing solution for anodizing treatment, there is a problem that the cast iron of the cylinder sleeve dissolves, causing pitting corrosion. Therefore, the inventors applied a masking agent to the entire inner circumferential surface of the cylinder bore and performed anodizing treatment, but the masking agent partially peeled off, and the cast iron underneath dissolved, so this problem was not solved.

[0009] Therefore, in view of the above problems, the present invention aims to provide a method for manufacturing an outboard motor cylinder block, which has a configuration in which a cylindrical cylinder sleeve made of cast iron is cast and encased in an aluminum alloy, and which can be subjected to anodizing treatment and subsequent sealing treatment without the cast iron of the cylinder sleeve melting. [Means for solving the problem]

[0010] To achieve the above objective, the present invention provides a method for manufacturing a cylinder block for an outboard motor, in one aspect, a casting process in which a cylinder block body is integrally cast with a cylindrical cylinder sleeve made of cast iron, wherein the cylinder block body has a cylinder bore and a water jacket formed around the cylinder bore at the joint surface where it is joined to the cylinder head, the cylindrical cylinder sleeve is cast into the inner circumferential surface of the cylinder bore, the cylindrical cylinder sleeve has a weight-reducing portion formed at the crankshaft end, which is recessed radially outward from the inner circumferential surface of the cylinder sleeve, and casting is performed with a positioning pin fitted into the inner circumferential surface of the cylindrical cylinder sleeve, The process includes: a casting step in which an aluminum alloy, which is the base material of the cylinder block body, is formed on the circumferential surface; a step of pressing with an elastic jig an area on the inner circumferential surface of the cylinder bore that includes an area encompassing the boundary on the cylinder head side between the cast iron of the cylindrical cylinder sleeve and the aluminum alloy, which is the base material of the cylinder block body, and an area encompassing at least a part of the annular portion of the surface of the aluminum alloy formed on the inner circumferential surface of the weight-reducing portion; a step of performing an anodizing treatment while the area is pressed to form an anodic oxide film on the surface of the aluminum alloy portion of the cylinder block body; a step of performing a sealing treatment to seal the surface of the anodic oxide film; and a processing step of removing the aluminum alloy formed on the inner circumferential surface of the weight-reducing portion.

[0011] Furthermore, in another embodiment, the method for manufacturing a cylinder block for an outboard motor according to the present invention is a casting process in which a cylinder block body is integrally cast with a cylindrical cylinder sleeve made of cast iron, wherein the cylinder block body has a cylinder bore and a water jacket formed around the cylinder bore at the joint surface where it is joined to the cylinder head, the cylindrical cylinder sleeve is cast into the inner circumferential surface of the cylinder bore, and the positioning pin fitted into the inner circumferential surface of the cylindrical cylinder sleeve has a weight-reducing portion formed at the crankshaft end, which is recessed radially inward from the outer circumferential surface of the positioning pin, and the weight-reducing portion is formed by casting with the positioning pin fitted into the inner circumferential surface of the cylindrical cylinder sleeve. The process includes: a casting step in which an aluminum alloy, which is the base material of the cylinder block body, is formed on the outer circumferential surface of the part; a step of pressing with an elastic jig a region on the inner circumferential surface of the cylinder bore that includes a region encompassing the boundary on the cylinder head side between the cast iron of the cylindrical cylinder sleeve and the aluminum alloy, which is the base material of the cylinder block body, and a region encompassing at least a part of the annular portion of the surface of the aluminum alloy formed on the outer circumferential surface of the weight-reducing part; a step of performing an anodizing treatment while the region is pressed to form an anodic oxide film on the surface of the aluminum alloy portion of the cylinder block body; a step of performing a sealing treatment to seal the surface of the anodic oxide film; and a processing step of removing the aluminum alloy formed on the outer circumferential surface of the weight-reducing part. [Effects of the Invention]

[0012] Thus, according to the present invention, since the region encompassing the boundary on the cylinder head side between the cast iron of the cylinder sleeve and the aluminum alloy, which is the base material of the cylinder block body, and the region encompassing at least a portion of the annular part of the surface of the aluminum alloy formed in the weight-reducing portion are pressed with an elastic jig, anodizing treatment and subsequent sealing treatment can be performed without melting the cast iron of the cylinder sleeve. [Brief explanation of the drawing]

[0013] [Figure 1]This is a perspective view of the cylinder head side showing an example of an outboard motor cylinder block manufactured by the method for manufacturing an outboard motor cylinder block according to the present invention. [Figure 2] Figure 1 is a magnified image showing delamination occurring on the inner circumferential surface of the cylinder bore in the cylinder block for an outboard motor. [Figure 3] Figure 1 is a perspective view of the crankshaft side of the cylinder block for an outboard motor. [Figure 4] Figure 3 is an enlarged perspective view showing the corners near the cylinder bore of the cylinder block for an outboard motor. [Figure 5] Figure 1 is a cross-sectional perspective view showing the inside of the cylinder bore of an outboard motor cylinder block. [Figure 6] Figure 1 is a cross-sectional front view showing the inside of the cylinder bore of an outboard motor cylinder block. [Figure 7A] This is a partial cross-sectional front view of the cylinder block body of an outboard motor, illustrating one embodiment of the manufacturing method for an outboard motor cylinder block according to the present invention. [Figure 7B] This is a partial cross-sectional front view of the cylinder block body of an outboard motor, illustrating one embodiment of the manufacturing method for an outboard motor cylinder block according to the present invention. [Figure 7C] This is a partial cross-sectional front view of the cylinder block body of an outboard motor, illustrating one embodiment of the manufacturing method for an outboard motor cylinder block according to the present invention. [Figure 8] Figures 7A to 7C illustrate a modified example of the manufacturing method for the cylinder block of an outboard motor, and show a partial cross-sectional front view of the cylinder block body of the outboard motor. [Figure 9A] This is a partial cross-sectional front view of the cylinder block body of an outboard motor, illustrating another embodiment of the method for manufacturing a cylinder block for an outboard motor according to the present invention. [Figure 9B] This is a partial cross-sectional front view of the cylinder block body of an outboard motor, illustrating another embodiment of the method for manufacturing a cylinder block for an outboard motor according to the present invention. [Figure 9C] This is a partial cross-sectional front view of the cylinder block body of an outboard motor, illustrating another embodiment of the method for manufacturing a cylinder block for an outboard motor according to the present invention. [Figure 10] This is a partial cross-sectional front view of a cylinder block for an outboard motor, which illustrates a modified example of a method for manufacturing the cylinder block for an outboard motor shown in FIGS. 9A to 9C. [Figure 11] This is an enlarged image showing the inside of the cylinder bore of the cylinder block body for an outboard motor according to the embodiment.

Embodiments for Carrying Out the Invention

[0014] Hereinafter, with reference to the accompanying drawings, an embodiment of a cylinder block for an outboard motor and a method for manufacturing the same according to the present invention will be described.

[0015] The method for manufacturing a cylinder block for an outboard motor according to the present embodiment includes a casting process of casting a cylinder block body integrally with a cylindrical cast iron cylinder sleeve, a pressing process of pressing a region including the boundary between the cast iron and the aluminum alloy on the inner peripheral surface of the cylinder bore of the cylinder block body for an outboard motor with an elastic jig, an anodic oxidation process of performing anodic oxidation treatment on the surface of the aluminum alloy portion of the cylinder block body for an outboard motor in a state where the region is pressed to form an anodic oxidation film, and a sealing process of performing a sealing treatment on the anodic oxidation film in a state where the region is pressed to seal the anodic oxidation film.

[0016] First, the cylinder block for an outboard motor manufactured by this method will be described. As shown in FIGS. 1 to 6, the cylinder block 10 for an outboard motor has a plurality of cylinder bores 11 and a water jacket 13 formed around these cylinder bores 11 on a joint surface 10T joined to a cylinder head (not shown). In these figures, the cylinder block 10 is shown with three cylinder bores 11 arranged in the horizontal direction, but in an actual outboard motor (not shown), the cylinder block 10 is mounted with the cylinder bores 11 arranged in the vertical direction. Also, the number of cylinder bores 11 may be one for a single-cylinder engine.

[0017] In particular, as shown in Figures 5 and 6, a cylindrical cylinder sleeve 20 made of cast iron is cast into the base material, an aluminum alloy, on each inner surface of the cylinder bore 11. In this casting method, the entire inner surface of the cylinder bore 11 is not usually covered by the cylinder sleeve 20, and the base material, the aluminum alloy, is exposed on the inner surface at the end 11A on the joint surface 10T side. Therefore, a boundary 11B exists on the inner surface of the cylinder bore 11 between two different metallic materials, the aluminum alloy and the cast iron.

[0018] Such a cylinder block 10 has an anodic oxide coating formed on its entire surface to prevent corrosion. However, if the entire cylinder block 10 is immersed in the treatment solution for anodizing, the cast iron cylinder sleeve 20 will dissolve. Therefore, it is necessary to apply a masking agent to the cylinder sleeve 20 portion on the inner circumferential surface of the cylinder bore 11 and form an anodic oxide coating only on the aluminum alloy portion. However, even when covered with a masking agent and subjected to anodizing, as shown in the photograph in Figure 2, the treatment solution penetrates at the boundary 11B between the cylinder sleeve 20 and the aluminum alloy, causing peeling of the masking agent B. It has been confirmed that the cast iron is dissolved in the portion of the cylinder sleeve 20 where the masking agent has peeled off.

[0019] Therefore, the inventors devised a way to prevent the intrusion of the treatment liquid, as would occur with a masking agent, by placing an elastic jig inside the cylinder bore 11 and pressing not only the surface of the cylinder sleeve 20 but also the area encompassing the boundary 11B between the cylinder sleeve 20 and the aluminum alloy. However, the cylinder block 10 has a more complex shape on the crankshaft side, shown in Figure 3, than on the cylinder head side, shown in Figure 1. Near the crankshaft end 11C of the cylinder bore 11, there are many corners 14, such as reinforcing ribs protruding as shown in Figure 4, and since the elastic jig comes into contact with these corners 14, there was a risk of wear on the elastic jig on the crankshaft side of the cylinder bore 11.

[0020] In this cylinder block 10, in order to perform anodic oxidation treatment without melting the cast iron of the cylinder sleeve 20, this embodiment first involves a casting process in which the cylinder block body 10A is cast integrally with the cylinder sleeve 20.

[0021] In the casting process, as shown in Figure 7A, the casting cylinder sleeve 20A is placed inside the casting mold (not shown) of the cylinder block body via a positioning pin 30A. The casting cylinder sleeve 20A has a weight-reducing section 21 on the crankshaft side that is thinner than the thickness on the cylinder head side. The inner circumferential surface 22 of this weight-reducing section 21 is recessed radially outward compared to the inner circumferential surface 23 on the cylinder head side and is parallel to the inner circumferential surface 23 on the cylinder head side.

[0022] Furthermore, the positioning pin 30A has a cylindrical shape and is fitted into the cylinder sleeve 20A for casting. Therefore, the inner circumferential surface 23 of the cylinder sleeve 20A on the cylinder head side is in close contact with the outer circumferential surface 31 of the positioning pin 30A, but a space is formed between the inner circumferential surface 22 of the weight-reducing portion 21 and the outer circumferential surface 31 of the positioning pin 30A. With the positioning pin 30A fitted into the cylinder sleeve 20A in this state, molten aluminum alloy is poured into the mold and the cylinder block body 10A is cast.

[0023] The thickness of the weight-reducing portion 21 of the casting cylinder sleeve 20A is the same as or greater than the final thickness of the cylinder sleeve 20. The difference between the thickness of the cylinder head side of the casting cylinder sleeve 20A and the thickness of the weight-reducing portion 21 should be sufficient to create enough space for the molten metal to flow in; for example, 0.5 mm or more is preferred, and 0.8 mm or more is more preferred.

[0024] As a result, the cylinder block body 10A is cast with the encasing cylinder sleeve 20A encased inside. Furthermore, since molten metal flows into the space formed between the inner circumferential surface 22 of the weight-reducing portion 21 of the encasing cylinder sleeve 20A and the outer circumferential surface 31 of the positioning pin 30A, aluminum alloy is also present on the inner circumferential surface 22 of the weight-reducing portion 21 of the encasing cylinder sleeve 20A. Therefore, two boundaries are formed between the aluminum alloy and cast iron inside the cylinder bore: a boundary 11B on the cylinder head side and a boundary 11D on the crankshaft side. The inner circumference of the crankshaft-side boundary 11D will be pressed by an elastic jig during the anodizing process described later. To avoid wear of the elastic jig by the corner on the crankshaft side of the cylinder bore at this time, the crankshaft-side boundary 11D is formed at a distance sufficiently far from the crankshaft-side end face 11C of the cylinder bore. In other words, this distance corresponds to the length of the weight-reducing portion 21 of the cylinder sleeve 20A for casting along the longitudinal axis of the cylinder bore. The length of the weight-reducing portion 21 is not limited to this, as it depends on the size of the cylinder block being manufactured, but for example, it is preferably 15 mm or more, and more preferably 18 mm or more. On the other hand, the longer the length of the weight-reducing portion 21, the more difficult it becomes to control the amount of molten metal to be poured in. Therefore, from the standpoint of material cost and work efficiency, the length of the weight-reducing portion 21 is preferably 25 mm or less, and more preferably 22 mm or less.

[0025] Next, as shown in Figure 7B, a pressing process is performed in which the region encompassing the boundaries s11B and 11D between the cast iron and aluminum alloy on the inner circumferential surface of the cylinder bore 11 is pressed with an elastic jig 40. By pressing the region encompassing the boundaries 11B and 11D in this manner and then performing the subsequent anodic oxidation treatment in that state, it is possible to prevent the treatment liquid from seeping into the boundaries 11B and 11D between the cast iron and aluminum alloy.

[0026] The region encompassing the boundaries 11B and 11D between the cast iron and the aluminum alloy may, for example, on the cylinder head side, be the region from the end of the inner circumferential surface of the cylinder bore 11 on the joining surface 10T side. On the crankshaft side, it may be the region up to a part of the surface of the aluminum alloy formed on the inner circumferential surface 22 of the weight-reducing portion 21 of the cast-wrapped cylinder sleeve 20A. Specifically, although not limited to these, it may be the region from the boundary 11D between the cast iron and the aluminum alloy to a position 5 mm, preferably 10 mm, toward the crankshaft.

[0027] The material for the elastic jig 40 can be any material that has enough elasticity to uniformly press over the entire inner surface of the cylinder bore 11, such as rubber or thermoplastic elastomer. Examples of rubber include silicone rubber, nitrile butadiene rubber (NBR), styrene rubber (SBR), butyl rubber (IIR), fluororubber (FKM), ethylene propylene diene rubber (EPDM), and chloroprene rubber. Examples of thermoplastic elastomers include polyester (TPC), polyurethane (TPU), and polyvinyl chloride (TPVC). In particular, since the elastic jig will come into contact with the treatment liquid in the subsequent anodic oxidation process, it is preferable that it has excellent chemical resistance, and for example, the use of silicone rubber is more preferable.

[0028] The dimensions of the elastic jig 40 are such that, for example, the outer diameter is larger than the inner diameter of the cylinder bore 11, and it is also preferable that the outer diameter be such that it can be attached to and detached from the inner circumferential surface of the cylinder bore 11. The length is preferably such that it covers the area encompassing the boundaries 11B and 11D between the cast iron and the aluminum alloy. However, the elastic jig 40 is not limited to a single jig as shown in Figure 7B, and two jigs may be used in combination. For example, as shown in Figure 8, the first elastic jig 40A may be inserted from the cylinder head side of the cylinder bore 11 to press the area of ​​the inner circumferential surface of the cylinder bore 11 that encompasses the boundary 11B between the cast iron and the aluminum alloy on the cylinder head side, and then the second elastic jig 40B may be inserted from the crankshaft side of the cylinder bore 11 to press the surface of the aluminum alloy formed on the inner circumferential surface 22 of the weight-reducing portion 21 of the casting cylinder sleeve 20A. The second elastic jig 40B does not necessarily have to press against the boundary 11D between the aluminum alloy and cast iron on the crankshaft side, but only needs to press against a region of the aluminum alloy surface formed on the inner circumferential surface 22 of the weight-reducing portion 21 that includes at least a portion of the annular section. Specifically, although not limited to this, it may be, for example, the region from 5 mm to 20 mm from the boundary 11D between the cast iron and aluminum alloy on the crankshaft side toward the crankshaft.

[0029] Furthermore, a masking agent may be applied to the inner surface of the cylinder bore 11 before pressing with the elastic jig 40. A commercially available masking agent for metal surface treatment can be used as the masking agent.

[0030] In the pressing process, it is preferable to use, for example, an inflatable elastic bag or balloon equipped with a gas supply unit (not shown) as the elastic jig 40. First, the elastic bag is placed inside the cylinder bore 11, and gas is supplied from the gas supply unit to inflate the elastic bag, thereby pressing the region encompassing the boundaries 11B and 11D between the cast iron and the aluminum alloy. As such an elastic bag, for example, a jig whose elastic bag portion can be inflated and deflated by gas pressure, commercially available under the name "air picker," can be used. Note that when combining two jigs as shown in Figure 8, the first elastic jig 40A and the second elastic jig 40B may be connected by a gas supply pipe (not shown), thereby allowing the two elastic bags to be inflated at the same time.

[0031] Then, as shown in Figure 7B, with the elastic jig 40 positioned inside the cylinder bore 11, the cylinder block body 10 is immersed in the processing solution and electrolytic treatment is performed to form a porous anodic oxide film 50 on the surface of the cylinder block body 10. Through electrolytic treatment, the aluminum alloy, which is the base material of the cylinder block body 10, dissolves, and the dissolved aluminum combines with oxygen in the processing solution to form an aluminum oxide anodic oxide film 50 on the surface of the aluminum alloy portion of the cylinder block body 10.

[0032] For the anodic oxidation treatment, either an acidic bath such as sulfuric acid, oxalic acid, phosphoric acid, or chromic acid, or a basic bath such as sodium hydroxide, sodium phosphate, or sodium fluoride may be used. The electrolytic treatment is performed by applying a voltage with the cylinder block body 10 as the anode and an electrode plate made of titanium or carbon (not shown) as the cathode.

[0033] The thickness of the anodic oxide film formed in the anodizing process is not particularly limited, but is preferably 1 to 60 μm, and more preferably 3 to 20 μm.

[0034] During the anodizing process, when the aluminum in the base material of the cylinder block body 10 is oxidized and a film is formed, the aluminum changes into aluminum oxide, causing volume expansion. In other words, about half of the formed anodic oxide film is a penetrating film that has penetrated deep into the surface of the aluminum alloy base material, and the remaining half of the film is a growing film that has grown from the surface of the base material.

[0035] On the other hand, since the inner circumferential surface of the cylinder bore 11 is covered by the elastic jig 40, the dissolution of the cast iron of the cylinder sleeve 20 can be prevented without contact with the processing liquid. Furthermore, even if the processing liquid seeps in from between the elastic jig 40 and the inner circumferential surface of the cylinder bore 11 and an electrolytic reaction occurs with the aluminum alloy, as described above, the anodic oxide film grows on the elastic jig 40 side, eliminating even the slight gap with the elastic jig and further suppressing the seepage of the processing liquid. Therefore, even if the processing liquid seeps into the cylinder sleeve 20 beyond the boundaries 11B and 11D between the aluminum alloy and the cast iron, the dissolution of the cast iron of the cylinder sleeve 20 can be prevented. Furthermore, on the crankshaft side, the elastic jig 40 does not necessarily have to press against the boundary 11D between the aluminum alloy and the cast iron. For example, as shown in Figure 8, when using two jigs in combination, if the second elastic jig 40B presses against a part of the annular portion of the aluminum alloy surface formed on the inner circumferential surface 22 of the weight-reducing portion 21 of the cylinder sleeve 20A for casting, it is possible to prevent the cast iron of the cylinder sleeve 20 from dissolving due to the seepage of the processing liquid.

[0036] To further explain the anodic oxidation process, DC electrolysis and AC / DC superimposed electrolysis can be used as electrolytic methods. Regardless of which electrolytic method is used, the anodic oxidation film formed is an anodic oxidation reaction that includes a penetration film and a growth film, as described above, but the properties of the resulting anodic oxidation film differ.

[0037] Anodized films formed by the DC electrolytic method have cells that grow linearly while eroding perpendicular to the base material surface. When a large amount of impurities or additives (such as silicon) are present in the aluminum alloy, cells do not grow near the surface around the impurities or additives, and depressions occur on the surface where the impurities or additives are deposited, resulting in an anodized film with a high surface roughness. In addition, an anodized film with large variations in film thickness is formed.

[0038] Anodized films formed by the AC / DC superposition electrolysis method have a structure in which cells form spherical or elliptical shapes almost continuously at a height less than twice the cell diameter, and these cells cluster together to form a grape cluster-like structure. Therefore, in anodized films formed by the AC / DC superposition electrolysis method, the ratio of the volume of pores contained within the cells to the volume of the cell walls is low. In contrast, in anodized films formed by the DC electrolysis method, the cells are formed in a continuous cylindrical shape, so the ratio of the volume of pores contained within the cells to the volume of the cell walls is high.

[0039] Furthermore, in an anodized film formed by the AC / DC superimposed electrolysis method, even if impurities or additives that inhibit cell growth are present during the cell growth process, the cells grow while avoiding and incorporating these impurities or additives. As a result, cell growth is not inhibited by impurities or additives, and the film has a nearly uniform thickness on the surface of the base material. Thus, in an anodized film formed by the AC / DC superimposed electrolysis method, the cell growth direction is finely bent in random directions relative to the surface of the base material. At points where the direction changes, resistance to invading water is generated, preventing water from penetrating to the base material. Therefore, it has higher corrosion resistance than an anodized film formed by the DC electrolysis method.

[0040] Next, the cylinder block body 10, on which the anodic oxide film 50 has been formed on its surface, is immersed in a sealing treatment solution or coated with the sealing treatment solution while the elastic jig 40 is still in place, thereby performing a sealing treatment process. This seals the pores in the porous anodic oxide film 50, improving the corrosion resistance of the anodic oxide film 50. It is preferable to perform a pretreatment such as washing with water before the sealing treatment to prevent the anodic oxide treatment solution that has adhered to the surface from mixing with the sealing treatment solution, and to remove any remaining treatment solution in the pores of the anodic oxide film 50.

[0041] The sealing process can employ known methods such as the hydrothermal method, boiling water method, nickel acetate method, and lithium hydroxide method. As an example, the lithium hydroxide method, which exhibits a self-healing function, will be described. An aqueous solution containing lithium ions is used as the sealing solution. As the lithium ions or the chemical that serves as the lithium ion source, lithium hydroxide, lithium sulfate, lithium chloride, lithium silicate, lithium nitrate, lithium carbonate, lithium phosphate, lithium hydroxide, etc., can be used. Of these, lithium hydroxide, lithium carbonate, and lithium silicate are preferred because their aqueous solutions exhibit basic properties. However, lithium silicate is highly toxic and poorly soluble in water, making it impractical; therefore, lithium hydroxide and lithium carbonate are more preferable. As mentioned above, there are various sealing processes, and while pitting corrosion of the cast iron sleeve occurs when electrolysis is applied during anodic oxidation, mild surface rust occurs during the sealing process. If this surface rust is acceptable, the elastic jig 40 can be removed and the sealing process can be performed.

[0042] In the lithium hydroxide method, the surface and pores of the anodic oxide film 50 are slightly dissolved by the sealing solution. Then, the dissolved film components react with the sealing solution components and precipitate as sealing products. These sealing products block the pores of the film, and the surface layer of the film transforms into a dense sealing product layer, completely sealing the pores of the film. Therefore, sealing products of lithium, aluminum, and oxygen are present on the surface and inside the pores of the anodic oxide film 50.

[0043] After performing the sealing process in this manner, the elastic jig 40 is removed from inside the cylinder bore 11, and a machining process is performed to remove the aluminum alloy formed on the inner surface of the weight-reducing portion 21 of the casting cylinder sleeve 20A, as shown in Figure 7C. For removing the aluminum alloy from the weight-reducing portion 21, it is preferable to first remove most of the aluminum alloy by cutting, and then remove the aluminum alloy by honing until the weight-reducing portion 21 of the casting cylinder sleeve 20A beneath the aluminum alloy is completely exposed.

[0044] In addition to removing the aluminum alloy from the weight-reducing portion 21, the inner circumferential surface 23 of the casting-fit cylinder sleeve 20A on the cylinder block side is also subjected to similar machining and honing. This makes the thickness of the casting-fit cylinder sleeve 20A on the cylinder block side the same as the thickness of the weight-reducing portion 21. Thus, a cylinder sleeve 20 with a smooth inner circumferential surface can be obtained on the inner surface of the cylinder bore 11, from the cylinder block side to the crankshaft side end 11C.

[0045] By performing the casting process, pressing process, anodizing process, sealing process, and processing process in this manner, it is possible to manufacture an outboard motor cylinder block 10 with a sealed anodized coating on the aluminum alloy portion of the cylinder block without melting the cast iron of the cylinder sleeve 20. In particular, the joint surface 10T of the cylinder block and the inner circumferential surface of the water jacket 13 are close to the combustion chamber, subjected to high temperatures, and come into contact with or are highly likely to come into contact with cooling water (seawater), making them prone to corrosion. Therefore, forming a sealed anodized coating can prevent corrosion from occurring in the outboard motor cylinder block. Especially when using inexpensive ADC or AC aluminum alloys, corrosion resistance is a concern, making the formation of a sealed anodized coating highly advantageous.

[0046] In the embodiment of the manufacturing method for an outboard motor cylinder block shown in Figures 7A to 7C, a weight-reducing portion 21 is formed in the casting cylinder sleeve 20A, and in the processing step, the thickness of the casting cylinder sleeve 20A on the cylinder block side is cut and honed until it is the same as the thickness of the weight-reducing portion 21. However, the present invention is not limited to this, and for example, the embodiment of the manufacturing method for an outboard motor cylinder block shown in Figures 9A to 9C can also be adopted.

[0047] As shown in Figure 9A, the positioning pin is a positioning pin 30B with a weight-reducing portion 33 on the crankshaft side, and the casting cylinder sleeve is a casting cylinder sleeve 20B with a uniform thickness from the cylinder head side to the crankshaft side. The positioning pin 30B has a frustoconical weight-reducing portion 33 on the crankshaft side, with a diameter that gradually decreases compared to the diameter on the cylinder head side. That is, the outer circumferential surface 34 of this weight-reducing portion 33 is recessed radially inward compared to the outer circumferential surface 35 on the cylinder head side, and is inclined with respect to the outer circumferential surface 35 on the cylinder head side.

[0048] Furthermore, the positioning pin 30B is fitted into the cylinder sleeve 20B for casting, so that the inner circumferential surface 24 of the cylinder sleeve 20B for casting is in close contact with the outer circumferential surface 35 of the positioning pin 30B on the cylinder head side, but a space is formed between it and the outer circumferential surface 34 of the weight-reducing portion 33. With the positioning pin 30B fitted into the cylinder sleeve 20B in this state, molten aluminum alloy is poured into the mold and the cylinder block body 10B is cast.

[0049] The dimensions of the weight-reducing portion 33 of the positioning pin 30B should be such that the space between it and the casting cylinder sleeve 20B is large enough for molten metal to flow into. Therefore, it is preferable that the difference between the diameter of the positioning pin 30B on the cylinder head side and the minimum diameter of the weight-reducing portion 33 be within the same numerical range as the difference in thickness of the casting cylinder sleeve 20A described in the embodiments of Figures 7A to 7C. The shape of the weight-reducing portion 33 is not limited to a frustoconical shape, and may be a cylindrical shape with a diameter smaller than that of the cylinder head side. Similarly, in Figure 7A, the weight-reducing portion 21 of the casting cylinder sleeve 20A is not limited to a shape with uniform thickness, and may be a shape that gradually increases in thickness towards the cylinder head side.

[0050] By using such a cast-encased cylinder sleeve 20B and positioning pin 30B, the cylinder block body 10B is cast with the cast-encased cylinder sleeve 20B encased within it. Furthermore, since molten metal flows into the space formed between the inner circumferential surface 34 of the weight-reducing portion 33 of the positioning pin 30B and the outer circumferential surface 24 of the cast-encased cylinder sleeve 20B, an aluminum alloy is also present on the crankshaft side of the inner circumferential surface 24 of the cast-encased cylinder sleeve 20B. Therefore, in this embodiment as well, two boundaries are formed between the aluminum alloy and cast iron in the cylinder bore: a boundary 11B on the cylinder head side and a boundary 11D on the crankshaft side. The crankshaft-side boundary 11D is formed at a distance sufficiently far from the crankshaft-side end face 11C of the cylinder bore. That is, this distance corresponds to the length of the weight-reducing portion 33 of the positioning pin 30B along the longitudinal axis of the cylinder bore. The length of the weight-reducing portion 33 is preferably within the same numerical range as the length of the weight-reducing portion 21 of the casting cylinder sleeve 20A described in the embodiments of Figures 7A to 7C.

[0051] Next, as shown in Figure 9B, a pressing step is performed in which the region encompassing the boundary 11B and 11D between the cast iron and aluminum alloy on the inner circumferential surface of the cylinder bore 11 is pressed with an elastic jig 40. The pressing step is the same as in the embodiments shown in Figures 7A to 7C, so a detailed explanation is omitted here.

[0052] Then, as shown in Figure 9B, with the elastic jig 40 placed inside the cylinder bore 11, the cylinder block body 10B is immersed in the processing liquid and electrolytic treatment is performed to form a porous anodic oxide film 50 on the surface of the cylinder block body 10B. Furthermore, the cylinder block body 10B with the anodic oxide film 50 formed on its surface is subjected to a sealing treatment while the elastic jig 40 is still in place. The anodic oxidation treatment and sealing treatment are the same as in the embodiments shown in Figures 7A to 7C, so a detailed explanation is omitted here.

[0053] Then, the elastic jig 40 is removed from inside the cylinder bore 11, and a machining process is performed to remove the aluminum alloy formed on the inner surface of the weight-reducing portion 33 of the positioning pin 30B, as shown in Figure 9C. For removing the aluminum alloy from the weight-reducing portion 23, it is preferable to first remove most of the aluminum alloy by cutting, and then remove the aluminum alloy by honing until the cylinder sleeve 20B for casting underneath the aluminum alloy is completely exposed.

[0054] In this embodiment, since the thickness of the casting cylinder sleeve 20B is the same as or slightly thicker than the final cylinder sleeve 20, the processing step requires significantly less material removal than in the embodiments shown in Figures 7A to 7C, thereby improving manufacturing efficiency. In addition, in the embodiments shown in Figures 9A to 9C, the elastic jig 40 may be used in combination with two other jigs. For example, as shown in Figure 10, by inserting the first elastic jig 40A from the cylinder head side of the cylinder bore 11 and pressing the area encompassing the boundary 11D between the cast iron and aluminum alloy on the crankshaft side, and simultaneously inserting the second elastic jig 40B from the cylinder head side and pressing the area encompassing at least a portion of the annular part of the aluminum alloy surface formed on the outer circumferential surface of the weight-reducing portion 33 of the positioning pin 30B, the dissolution of the cast iron of the cylinder sleeve 20 due to seepage of the processing liquid can be prevented in the same way as described above. [Examples]

[0055] A cast iron cylinder sleeve for casting, having a 20mm length cutout, was prepared, and a cylinder block was fabricated by casting this cylinder sleeve using aluminum alloy ADC12 material. It was then confirmed that the boundary between the cast iron and the aluminum alloy on the crankshaft side of the cylinder bore of this cylinder block was formed 20mm from the crankshaft side end of the cylinder bore.

[0056] Then, an air picker was inserted into the cylinder bore as an elastic fixture, and the air picker was inflated so that the area encompassing the boundary between the cast iron and the aluminum alloy was sufficiently pressed. Next, this cylinder block was subjected to anodizing. The anodizing was carried out by a DC electrolytic method, in which the cylinder block was immersed in a sulfuric acid bath at a temperature of 20°C and a concentration of 200 g / L, with a current density of 1.5 A / dm². 2 A voltage was applied for 20 minutes. As a result, an anodic oxide film of 5-15 μm was formed on parts other than the cylinder bore. The air pressure of the air picker was reduced to remove it from the cylinder bore, and the inner surface of the cylinder bore on the crankshaft side of this cylinder block was observed. A photograph of that observation is shown in Figure 11.

[0057] As shown in Figure 11, an anodic oxide film 50 was formed on the inner surface of the cylinder bore at a position away from the boundary between the cast iron cylinder sleeve 20 and the aluminum alloy 10. Furthermore, no melting of the cast iron due to the anodic oxidation treatment was observed in the cast iron cylinder sleeve 20. [Explanation of Symbols]

[0058] 10 Cylinder block for outboard motor 10A, 10B Cylinder Block Body 10T Cylinder head mating surface 11 Cylinder bore 11B, 11D Boundary between aluminum alloy and cast iron 13 Water Jacket 20 Cylinder Sleeves 20A, 20C Cylinder sleeve for casting 21. Weight reduction section 30A, 30B Positioning pins 33. Weight reduction section 40 Elastic fixtures

Claims

1. A method for manufacturing a cylinder block for an outboard motor, A casting process for integrally casting a cylinder block body with a cylindrical cast iron cylinder sleeve, wherein the cylinder block body has a cylinder bore and a water jacket formed around the cylinder bore at the joint surface where it is joined to the cylinder head, the cylindrical cylinder sleeve is cast onto the inner surface of the cylinder bore, the cylindrical cylinder sleeve has a weight-reducing portion formed at the crankshaft end, recessed radially outward from the inner surface of the cylinder sleeve, and casting is performed with a positioning pin fitted onto the inner surface of the cylindrical cylinder sleeve, thereby forming the aluminum alloy, which is the base material of the cylinder block body, on the inner surface of the weight-reducing portion. The process involves pressing an elastic jig on the inner circumferential surface of the cylinder bore, encompassing the region that includes the boundary on the cylinder head side between the cast iron of the cylindrical cylinder sleeve and the aluminum alloy that is the base material of the cylinder block body, and the region that includes at least a portion of the annular part of the surface of the aluminum alloy formed on the inner circumferential surface of the weight-reducing portion. The process involves applying pressure to the aforementioned region and performing an anodizing treatment to form an anodic oxide film on the surface of the aluminum alloy portion of the cylinder block body, A step of sealing the surface of the anodic oxide film by performing a sealing treatment, A machining step to remove the aluminum alloy formed on the inner circumferential surface of the weight-reducing portion. A method for manufacturing a cylinder block for an outboard motor, including the invention of the cylinder block for an outboard motor.

2. A method for manufacturing a cylinder block for an outboard motor, A casting process for integrally casting a cylinder block body with a cylindrical cast iron cylinder sleeve, wherein the cylinder block body has a cylinder bore and a water jacket formed around the cylinder bore at the joint surface where it is joined to the cylinder head, the cylindrical cylinder sleeve is cast onto the inner circumferential surface of the cylinder bore, and a positioning pin fitted into the inner circumferential surface of the cylindrical cylinder sleeve has a weight-reducing portion formed at the crankshaft end, recessed radially inward from the outer circumferential surface of the positioning pin, and by performing casting with the positioning pin fitted into the inner circumferential surface of the cylindrical cylinder sleeve, the aluminum alloy, which is the base material of the cylinder block body, is also formed on the outer circumferential surface of the weight-reducing portion, and The process involves pressing an elastic jig on the inner circumferential surface of the cylinder bore, encompassing the region that includes the boundary on the cylinder head side between the cast iron of the cylindrical cylinder sleeve and the aluminum alloy that is the base material of the cylinder block body, and the region that includes at least a portion of the annular portion of the surface of the aluminum alloy formed on the outer circumferential surface of the weight-reducing portion. The process involves applying pressure to the aforementioned region and performing an anodizing treatment to form an anodic oxide film on the surface of the aluminum alloy portion of the cylinder block body, A step of sealing the surface of the anodic oxide film by performing a sealing treatment, A machining step to remove the aluminum alloy formed on the outer surface of the weight-reducing portion. A method for manufacturing a cylinder block for an outboard motor, including the invention of the cylinder block for an outboard motor.

3. A method for manufacturing a cylinder block for an outboard motor according to claim 1 or 2, wherein in the step of pressing with the elastic jig, the elastic jig is an elastic bag body equipped with a gas supply unit, the elastic bag body is placed inside the cylinder bore, and gas is supplied from the gas supply unit to inflate the elastic bag body, thereby pressing the region encompassing the boundary and the region encompassing the annular portion with the elastic bag body.

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