CN1287080C - Piston for internal combustion engine and method of manufacturing the same - Google Patents

Abstract

A forged piston for an internal combustion engine, made of an aluminum alloy containing 6-25% silicon by mass, includes an oil ring groove portion (12) and a skirt portion (13). The ratio (A/B) of the average size (A) of the eutectic silicon grains contained in the oil ring groove portion to the average size (B) of the eutectic silicon grains contained in the front end portion (18) of the skirt portion is at least 1.5. The average size (A) is at least 4 μm. Due to the configuration that results in a smaller average size (B) and a larger average size (A), the skirt portion has excellent forgeability, while the oil ring groove portion has reliable machinability and improved wear resistance.

Description

Forged piston for internal combustion engine and method of manufacturing the same

Cross References to Related Applications

This application claims the filing date benefit of Provisional Application No. 60/308,110, filed July 30, 2001, under 35 U.S.C. §119(e)(1).

technical field

The invention relates to a forged piston for an internal combustion engine made of an aluminum-silicon alloy and to a method for producing the piston.

Background technique

Typically, pistons for internal combustion engines are produced by metal (permanent) mold casting. First, molten aluminum alloy is poured into a mold to thereby mold the aluminum into a piston preform. The formed preform is then subjected to the desired heat treatment, eg, intentional aging, and then the desired machining to produce a finished product.

Recently, in some cases, pistons for internal combustion engines are produced by forging. Continuous casting of molten aluminum-silicon alloy to thereby form a billet for extrusion; heat treatment (homogenization treatment) of the billet to obtain uniformity of internal stress due to segregation or shrinkage of solute elements during solidification distribution; and forming the formed blank into a small diameter round rod by extrusion. Alternatively, the molten aluminum-silicon alloy is continuously cast to thereby form a small diameter continuously cast rod; the formed cast rod is homogenized; and the rod is machined to form a small diameter round rod. The small-diameter round bar thus formed is cut into a workpiece to be worked as a forging material. The forged material is preheated and then forged into a piston preform using a hot forging machine. The preform is then heat treated, eg, artificially aged, and then machined to thereby produce a finished product (ie, a piston). Depending on the use of the piston, the head of the piston or the portion of the side wall of the piston between the upper cylinder ring and the head may be treated with an anti-corrosion passivated aluminum/alumina layer or coated for improved wear and heat resistance. layer forming process.

Recently, it has been increasingly required to improve the fuel economy of internal combustion engines used in, for example, automobiles. In order to meet this demand, attempts have been made to reduce the weight of automobile bodies and to develop lightweight engines. For example, aluminum is used to produce pistons for engines, and thin-walled pistons have been developed.

At the same time, there is an increasing demand for high-quality pistons that meet the requirements of high-performance engines.

When producing a piston by a conventional metal (permanent) mold casting method, it is difficult to reduce the skirt thickness due to technical limitations on the casting method. Typically, therefore, the cast piston is machined to thereby reduce the thickness of the skirt. When a piston is produced by casting, the metallographic structure of the piston becomes coarser due to the low solidification rate during casting, so that the obtained piston has good machinability. However, it is difficult to control the dimensional accuracy of the final product due to large variations in thickness and size between pistons formed by casting. In addition, internal defects such as cavities, micro-shrinkage cavities may occur in the piston formed by casting, thereby reducing its strength. Therefore, in order to increase the strength of the piston, the entire wall of the piston is thickened, and the thickness of its reinforcing ribs is increased, making the piston formed by casting unsuitable for use in high-performance engines. Furthermore, the performance variation among the pistons becomes large due to the thickening of the piston walls. From the above it can be seen that further improvements in pistons are required to produce an engine with reliable performance.

Meanwhile, when a piston is produced from a forging material by forging, since the forging material substantially contains no internal defects and the forging material has reliable mechanical properties, the thickness of the segments/parts constituting the piston becomes uniform. Pistons of reliable quality can thus be produced by forging. However, although the forging material is suitable for forging a thin, long part such as a skirt, the machinability of the material is not satisfactory due to the finer metallographic structure of the forging material. Chip manageability is compromised, for example because chips are produced in a continuous form rather than in pieces during machining, resulting in lower productivity. In addition, the surface roughness of the oil ring groove portion of a final piston product after machining is unsatisfactory. When continuous casting is applied, in order to prevent the occurrence of cracks due to solidification shrinkage stress generated in the ingot at the time of casting, restrictions are imposed on the composition of the alloy to be produced. It is therefore not possible to easily produce a desired composition for use as a forging material that can provide a piston for use with higher strength, higher wear resistance, and higher strength than required at high temperatures. alloy.

In view of the above, the present invention has been developed and it is an object thereof to provide a forged piston for an internal combustion engine comprising: a piston having improved machinability (e.g. reliable surface roughness and flatness) oil ring groove with dimensional accuracy), a head with excellent mechanical properties (for example, the head surface constituting the head and a piston pin portion have excellent mechanical strength properties at high temperatures), a head with excellent forgeability permanent skirt, and an oil ring groove section with reliable wear resistance.

Contents of the invention

The present invention provides a forged piston for an internal combustion engine made of an aluminum alloy containing silicon in a mass ratio of 6-25%, the piston comprising an oil ring groove portion and a skirt, wherein the oil ring The ratio (A/B) of the average size (A) of the eutectic silicon grains in the groove portion to the average size (B) of the eutectic silicon grains contained in the front end portion of the skirt is at least 1.5, and the The average size (A) is at least 4 μm.

The forged piston includes a ratio of the average size (C) of native silicon crystal grains contained in the oil ring groove portion to the average size (D) of native silicon crystal grains contained in the front end portion of the skirt ( Forged pistons with a C/D) of at least 1.3 and the mean dimension (C) of at least 15 μm.

In each forged piston, the aluminum alloy contains 0.3-7% by mass of Cu and 0.1-2% by mass of Mg.

Any of the aluminum alloys may contain Ni in a mass ratio of 0.1-2.5%.

The present invention also provides a method of manufacturing a forged piston for an internal combustion engine, comprising the steps of:

Unidirectional solidification casting is performed on a molten aluminum alloy containing silicon in a mass ratio of 6 to 25% to thereby produce an ingot serving as a forging material having a first surface and a second surface opposed to each other, wherein the the average size of the silicon grains in the surface is different from the average size of the silicon grains included in the second surface;

Preheating the forged material;

placing the forging material in a forging die such that the surface comprising silicon grains of larger average size faces the surface of the die corresponding to the piston head, thereby forging the forging material into a piston preform;

artificially aging the piston preform; and

The formed piston preform is machined to thereby manufacture a forged piston for an internal combustion engine.

In the manufacturing method, the unidirectional solidification casting comprises obtaining an average size (A) of eutectic silicon grains contained in the upper part of the ingot of at least 1.5 and contained in the part of the ingot near a cooling plate The ratio (A/B) of the average size (B) of the eutectic silicon grains and the average size (A) of at least 4.0 μm are cooled.

In this manufacturing method, unidirectional solidification casting includes: to obtain a cooling rate (E ) measured at a point e that is 5 mm downward from the top of the solidification mold and 5 mm inward from the side wall of the solidification mold to be 0.85 or less. ) to the cooling rate (F) measured at a point f 1 mm up from the bottom of the curing mold and 5 mm inward from the sidewall of the curing mold (E/F), where the cooling rate (E) is at least 0.5°C/sec.

In either production method, preheating is performed at a temperature within a range of 350°C to a difference obtained by subtracting 10°C from the solidus temperature (°C) of the aluminum alloy.

As described above, according to the present invention, it is possible to produce a forged piston for an internal combustion engine comprising an oil ring groove portion and a skirt, wherein the average amount of eutectic and native silicon grains contained in the front end portion of the skirt is The size is smaller, and the average size of the eutectic and native silicon grains contained in the oil ring groove portion is larger. Due to this configuration, the skirt has the same excellent malleability as that of a piston formed from a continuously cast rod of a small diameter, and thus the thickness of the skirt can be reduced. In addition, the oil ring groove portion has the same excellent debris manageability during milling as the oil ring groove portion of the piston formed by casting, and the oil ring groove has less surface roughness and has excellent wear resistance sex.

Description of drawings

Fig. 1 (a) is a vertical sectional view schematically showing an embodiment of a forged piston for an internal combustion engine according to the present invention, which includes a sectional view of a skirt;

Fig. 1 (b) is a vertical sectional view schematically showing the forged piston of Fig. 1 (a), which includes a sectional view of a piston pin hole;

Figure 2 is a schematic diagram of an example of a casting apparatus for unidirectional solidification casting;

Fig. 3 is an explanatory view showing points arranged in a mold of a unidirectional solidification apparatus, wherein cooling rates are measured at these points;

Figure 4(a) is a side view of a Compax milling cutter used for milling tests;

Fig. 4 (b) is the top view of cutter among Fig. 4 (a);

Fig. 4 (c) is the front view of cutter among Fig. 4 (a);

Fig. 5 is a schematic view showing a forging apparatus used in manufacturing the forged piston of the present invention.

Detailed ways

One embodiment of the forged piston for an internal combustion engine according to the present invention will be described below.

The forged piston for an internal combustion engine of the present invention includes a head surface having valve pockets, a skirt portion having a large thickness, a reinforcing rib, an oil ring groove portion and a piston pin hole.

FIG. 1 shows a cross-sectional view of an embodiment of a forged piston for an internal combustion engine according to the present invention. FIG. 1(a) is a vertical sectional view of the forged piston, which includes a skirt 13 in section. Fig. 1(b) is a vertical sectional view of the forged piston including a sectional view of a piston pin hole 14 into which a piston pin for connecting the piston to a connecting rod is inserted. The upper surface of the piston is a head surface 11 with valve pockets. The oil ring groove 12 is used as a groove matched with the piston ring. The oil ring groove must be arranged in a direction perpendicular to the peripheral wall of the piston, ie, in a direction perpendicular to the vertical direction. The skirt 13 serves as a guide for maintaining the position of the piston in the cylinder liner, and is required to have high strength and high wear resistance. In order to reduce the weight of the piston, the skirt is required to have a reduced thickness. The outline of a piston preform, ie a forged product, is drawn with a two-dashed line marked 15 . The outline of a machined piston final product is drawn with a solid line marked 16 . Reference numeral 17 denotes a reinforcing rib, and reference numeral 18 denotes a front end portion of the skirt portion 13 . The height of the front end of the skirt, measured from the bottom of the piston, is 40% of the overall height of the piston. Since considerable plastic flow occurs at the front end during forging, the front end is required to have excellent forgeability.

The forged piston for an internal combustion engine of the present invention is made of an aluminum alloy containing silicon in a mass ratio of 6-25%. The forged piston is characterized by the average size (A) of the eutectic silicon grains contained in the oil ring groove portion 12 and the average size (B) of the eutectic silicon grains contained in the front end portion 18 of the skirt 13 The ratio (A/B) is at least 1.5 (preferably at least 1.6), and the average size (A) is at least 4 μm, preferably at least 4.5 μm.

In the case where the average size (A) is less than 4 µm, when the oil ring groove portion is subjected to milling, the milling cutter and a chuck of a milling machine tend to be entangled with elongated chips. And the entangled debris can scratch the surface of the piston to be milled. In addition, debris builds up forming a filamentous pile at the bottom of the mill until the debris covers the entire chuck, rendering the mill inoperable, resulting in lower productivity. A forged piston milled using a chip-laden milling machine has a large surface roughness, which means that the quality of the forged piston is not satisfactory.

Since the oil ring groove portion 12 is continuous along the entire circumference of the piston, the generated elongated chips need to break themselves. Thus, good debris manageability is required.

In the forged piston for an internal combustion engine of the present invention, the average size of the eutectic silicon crystal grains contained in the oil ring groove portion is at least 4 μm. Therefore, when the oil ring groove portion is machined, chips generated are easily broken into small chips due to silicon crystals. Therefore, it is possible to prevent chips from becoming entangled in the milling cutter or chuck. In addition, since the accumulation of filamentous debris in the milling machine can be prevented, the manageability of this debris is greatly improved. Furthermore, since debris can be prevented from being entangled in the milling cutter or in the product being milled, the milled product has a stable surface roughness.

Since the average size (A) of eutectic silicon crystal grains contained in the oil ring groove portion is at least 4 μm, the oil ring groove portion has excellent wear resistance. Since the upper and lower surfaces of the oil ring groove rub against a piston ring during engine operation, the oil ring groove portion must have high wear resistance. A piston head surface (ie, a surface exposed to the combustion chamber of the engine) is heated by high temperature combustion gases generated during fuel combustion, and the temperature near the piston head increases. The portion of the oil ring groove disposed near the piston head and in contact with the inner wall of an engine cylinder operates under more severe conditions than the skirt portion. Therefore, the oil ring groove portion must have excellent wear resistance. When the average size (A) is less than 4 μm, the wear resistance of the portion becomes insufficient.

In the forged piston for an internal combustion engine of the present invention, the average size of the eutectic silicon crystal grains contained in the oil ring groove portion is at least 4 μm. Therefore, the oil ring groove portion has sufficient wear resistance without subjecting the vicinity of the oil ring groove to any treatment for improving wear resistance, such as a hard aluminum oxide layer, which is performed in a high-performance engine (hard-alumite) treatment or coating treatment with a wear-resistant coating agent. Therefore, in the present invention, since such costly processing is unnecessary, the cost per piston can be reduced, so that an inexpensive engine can be provided.

The ratio (A/B) of the average size (A) of the eutectic silicon grains contained in the oil ring groove portion to the average size (B) of the eutectic silicon grains contained in the front end portion of the skirt is preferably is at least 1.5, more preferably at least 1.6. In other words, the average size (B) is smaller than the average size (A), and the average size (B) is 0.67 times or less than the average size (A), for the reason described below.

The skirt 13 is not continuous along the entire circumference of the piston and is divided into segments by the piston pin section. Therefore, when the skirt is milled in the circumferential direction during machining, the milling of the skirt becomes discontinuous, and thus it is possible to prevent chips from being entangled in the milling cutter. Therefore, as long as the average size (B) of the eutectic silicon grains contained in the front end portion 18 of the skirt is 0.67 times or less than the average size (A), satisfactory chipping at the time of machining can be obtained. manageability.

In contrast, when the average size (B) exceeds 0.67 times the average size (A), excellent plastic flow of the skirt may not be obtained during hot forging although the wear resistance of the oil ring groove is maintained. For example, there may arise a problem that although the excellent plastic workability of the skirt portion is maintained, the wear resistance of the oil ring groove portion is impaired; or, although the oil ring groove portion has excellent wear resistance , but the plastic workability of the skirt is compromised. As a result, it has become difficult to provide a piston including an oil ring groove portion excellent in wear resistance and a skirt portion excellent in plastic workability.

However, in the forged piston for an internal combustion engine of the present invention, the average size (B) of the eutectic silicon grains contained in the front end portion of the skirt is that of the eutectic silicon grains contained in the oil ring groove portion 0.67 times or less than the average size (A). Therefore, even when the thickness of the skirt is reduced, cracks are not generated in the front end of the skirt placed in a forging die, and the plastic fluidity of the front end in the forging die is also improved. will be damaged. Therefore, since the thickness of the skirt can be reduced, the weight of the piston can be easily reduced. Since the reduced-thickness skirt can be formed by forging and because machining allowances required for machining can be reduced, the productivity and output of pistons based on (certain) materials can be increased.

In the forged piston for an internal combustion engine of the present invention made of an aluminum alloy preferably containing silicon in a mass ratio of 6-25%, the average size of eutectic silicon grains contained in the oil ring groove portion The ratio (A/B) of (A) to the average size (B) of eutectic silicon grains contained in the front end portion of the skirt is at least 1.5, and the average size (A) is at least 4 μm, contained in oil the ratio (C/D) of the average size (C) of native silicon crystal grains in the annular groove portion to the average size (D) of native silicon crystal grains contained in the front end portion of the skirt is at least 1.3, and The average size (C) is at least 15 μm.

The reasons for setting the ratio (A/B) to be at least 1.5 and the average size (A) to be at least 4 μm are as described above.

In some cases, an aluminum alloy containing 6-25% by mass of silicon has a metallographic structure in which native silicon crystal grains are dispersed in a eutectic silicon structure, depending on the cooling rate of the alloy.

In this case, the average size (C) of primary silicon crystal grains contained in the oil ring groove portion is preferably at least 15 μm, more preferably at least 17 μm. This can further improve the machinability and wear resistance of the oil ring groove portion. However, when the average size (C) is less than 15 μm, the effect of the native silicon crystal grains cannot be sufficiently obtained.

In the forged piston for an internal combustion engine of the present invention, the average size (A) of the eutectic silicon crystal grains contained in the oil ring groove portion is at least 4 μm, and the average size (A) of the primary silicon crystal grains contained in the portion ( C) At least 15 μm. Therefore, when the oil ring groove portion is subjected to machining, generated chips are easily broken into small chips due to silicon crystals. Therefore, it is possible to prevent chips from becoming entangled in the milling cutter or chuck. In addition, since the accumulation of filamentous debris in the milling machine can be prevented, the manageability of this debris is greatly improved. Furthermore, since debris can be prevented from being entangled in the milling cutter or in the product being milled, the milled product has a reliable surface roughness. Since the average size (C) of primary silicon crystal grains contained in the oil ring groove portion is at least 15 μm, the oil ring groove portion has improved wear resistance.

The ratio (C/D) of the average size (C) of native silicon crystal grains contained in the oil ring groove portion to the average size (D) of native silicon crystal grains contained in the front end portion of the skirt portion is preferably At least 1.3, more preferably at least 1.4, for the reasons described below. When the ratio (C/D) is at least 1.3, the wear resistance and machinability of the oil ring groove portion can be further improved, and the plastic fluidity of the front end portion of the skirt can be maintained. On the contrary, when the ratio (C/D) is less than 1.3, the wear resistance and machinability of the oil ring groove portion are impaired, or the plastic fluidity of the front end portion of the skirt is impaired.

Preferably, the aluminum alloy used to form the piston also contains 0.3-7% by mass (more preferably 0.4-6.5% by mass) of Cu and 0.1-2% by mass (more preferably 0.4-6.5% by mass) 0.15-1.8%) of Mg. The addition of these alloying elements enhances the hardness of the piston, as well as the mechanical strength properties of the piston including tensile strength, 0.2% yield strength, and fatigue resistance. Furthermore, since such a thin-walled piston can be produced, the weight of the piston can be reduced. When the amount of alloying elements falls below the lower limit value, the effects of these elements cannot be obtained. On the contrary, when the amount of the alloying element exceeds the upper limit value, the effect equivalent to the added element amount can no longer be obtained, but the material cost increases, and the forgeability of the piston is impaired.

Preferably, the aluminum alloy used to form the piston further contains Ni in a mass ratio of 0.1-2.5%, more preferably in a mass ratio of 0.2-2.0%, for the reasons described below. The addition of Ni enhances the strength of the piston at high temperatures and improves the durability of the oil ring groove portion provided near the piston head and in contact with the inner wall of an engine block under severe engine operating conditions. When the amount of Ni falls below the lower limit value, the effect of Ni cannot be obtained, and when the amount of Ni exceeds the upper limit value, the effect corresponding to the added amount of Ni cannot be obtained any more. Furthermore, when the amount of Ni increases, since Ni is an expensive element, the production cost increases.

An embodiment of a manufacturing method for a forged piston according to the present invention will be described below.

The manufacturing method of the present invention includes the steps of performing unidirectional solidification casting of a molten aluminum alloy containing silicon in a mass ratio of 6 to 25% to thereby produce a forging material having a first surface and a second surface opposed to each other. an ingot in which the average size of silicon grains contained in a first surface differs from the average size of silicon grains contained in a second surface; placing the forged material in a forging die so as to contain the larger average size the surface of the silicon grains facing the surface of the mold corresponding to the piston head to thereby forge the forging material into a piston preform; artificially aging the piston preform; and forming the piston preform For machining.

The piston produced by the method of the invention has the above-mentioned characteristics.

The manufacturing method will be described in detail below.

The wrought material is obtained by unidirectional solidification casting of an aluminum alloy used as a raw material. This manufacturing method employs, for example, a casting apparatus disclosed in JP-A HEI 9-174198 and shown in FIG. 2 .

In Fig. 2, reference numeral 201 denotes a cooling plate. A master mold 202 is arranged on the cooling plate 201 . On the master mold 202 is provided a container 203 for receiving molten aluminum alloy 207 supplied from, for example, a melting furnace (not shown). As shown in FIG. 2 , the bottom of the container 203 serves as the top of the mold 202 . The container 203 communicates with the main mold 202 through a molten metal inlet 204 . A blocking member 205 is disposed on the inlet 204 . Molten alloy is injected into the mold by lifting the stopper by means of a device (not shown) for moving the stopper vertically, the height/level of the injected molten alloy being shifted upward. After the injection of the molten alloy is completed, or after a predetermined period of time has elapsed, the stopper is moved downward to thereby stop the injection of the molten alloy. Reference numeral 208 denotes a cover, and numeral 209 denotes an electric furnace for keeping the molten alloy at a predetermined temperature. The cooling plate 201 is cooled by spraying water or the like thereon from a nozzle 210 provided under the cooling plate. Reference numerals 211 and 212 denote a housing and a drain outlet, respectively.

The molten aluminum alloy injected into the mold is cooled by cooling plates and solidifies unidirectionally towards the top of the mold. As a result, an ingot 206 is obtained. The metallographic structure of the ingot 206 is affected by the cooling rate. The higher the cooling rate, the smaller the size of the eutectic silicon grains and native silicon crystal grains (these grains may be collectively referred to as "silicon grains"). The lower the cooling rate, the larger the size of these silicon grains. When applying the casting apparatus described above, the part near the cooling plate obtains the highest cooling rate, while the part near the top of the mold obtains the lowest cooling rate. Therefore, the silicon crystal grains generated at the portion near the cooling plate by the solidification of the aluminum-silicon alloy become smaller, and the silicon crystal grains generated at the portion near the top of the mold by the solidification of the alloy become smaller. big. That is, an ingot having a metallographic structure in which the sizes of silicon crystal grains are graded can be obtained.

The aluminum alloy used as a raw material contains silicon in a mass ratio of 6-25%. When the silicon content is less than 6% by mass, the wear resistance is impaired, and when the silicon content exceeds 25% by mass, the wear resistance is no longer improved by a comparable amount of silicon. In addition, when the mass ratio content of silicon exceeds 25%, cracks are generated during forging, which means that forgeability is impaired. In addition, the service life of cutting tools is greatly shortened.

Preferably, in addition to silicon, the aluminum alloy also contains Cu in a mass ratio of 0.3-7% and Mg in a mass ratio of 0.1-2%, either alone or in combination. These elements age-harden the aluminum alloy, thereby enhancing the hardness and mechanical properties of the resulting piston. More preferably, the aluminum alloy contains Ag or Sc in a mass ratio of 1.5% or less.

Since a forged piston for an internal combustion engine is subjected to high temperature inside the engine due to heat generated by combustion of fuel, the piston is required to have a certain strength at high temperature. Therefore, the aluminum alloy preferably contains 0.1-2.0% by mass of Ni, an element known to increase strength at high temperatures. It is also effective to add Fe, Mn, Zr, Ti, W, Cr, V, Co, Mo, etc. singly or in combination.

The aluminum alloy preferably contains an element effective for reducing the size of eutectic silicon grains, such as Na, Ca, Sr or Sb. These elements may be added alone or in combination. The addition of such an element is advantageous because the negative effects of large sized eutectic silicon grains on forgeability and wear of machining tools can be prevented.

When native silicon crystal grains are produced, P is usually incorporated into the aluminum alloy to reduce the size of the native silicon crystal grains. However, when Na or Ca exists in the molten aluminum alloy, Na or Ca interferes with the action of P, resulting in failure to reduce the grain size of the native silicon crystals. Therefore, the upper limit of the mass ratio of Na or Ca in the aluminum alloy is 50 ppm. When the mass ratio of Na or Ca exceeds 50 ppm by mass, the size of primary silicon crystal grains becomes considerably large. As a result, forgeability is compromised and the life of the milling cutter is shortened.

The ingot used in the present invention can be produced from the above-mentioned molten alloy by cooling using the cooling plate so that the ingot has a metallographic structure in which the sizes of eutectic silicon grains and primary silicon crystal grains are graded This makes the grains smaller near the cooling plate and larger near the top of the mold.

When producing an ingot with such a grain size graded metallographic structure by, for example, unidirectional solidification casting, cooling is performed to obtain an average of at least 1.5 eutectic silicon grains contained in the upper part of the ingot. The ratio (A/B) of size (A) to the average size (B) of eutectic silicon grains contained in a portion of the ingot adjacent to a cooling plate, and the average size (A) is at least 4.0 μm.

In order to produce an ingot having a metallographic structure graded in the above-mentioned grain size, the cooling rate can be controlled as follows. For example, as shown in FIG. 3, during unidirectional solidification casting, cooling is performed to obtain a temperature of at least 0.5° C./sec when measured at point e 5 mm down from the top of the solidification mold and 5 mm inward from the sidewall of the mold. The cooling rate (E), and the cooling rate (E) measured from the point e and the cooling rate (E) measured from the point e measured from the bottom of the curing mold 1 mm up and 5 mm inward from the side wall of the mold to obtain 0.85 or less (F) ratio E/F.

When the cooling rate falls within the above range, a forged material having the above metallographic structure can be produced. When this forged material is forged into a piston for an internal combustion engine, the produced forged piston has excellent forgeability, machinability and wear resistance.

The ingot is generally in the shape of a disk having upper and lower surfaces parallel to each other. However, the ingot may have any shape according to the shape of the piston to be forged as long as the ingot has the metallographic structure of the above-mentioned graded grains. For example, the ingot may have a shape in which the upper and lower surfaces are non-parallel to each other, or a shape in which one or both of the upper and lower surfaces has non-parallel protrusions and depressions. The ingot having such a non-parallel shape is advantageous in that the load applied to a forging die can be reduced, and a piston having a complicated shape can be formed by forging.

If desired, the ingot can be machined prior to forging.

If desired, the ingot can be milled after forging to obtain a surface with the desired metallographic structure. When the outermost surface of the ingot contains silicon grains of an undesired average size, it is preferred to mill the ingot with a metallographic structure in which the average size of the silicon grains is graded until a layer containing silicon grains is obtained. Surfaces of silicon grains having a desired average size, thereby using the formed ingot as a forging material.

The forging material is preheated before forging. The preheating is performed at a temperature falling within a range from 350°C to a difference obtained by subtracting 10°C from the solidus temperature (°C) of the aluminum alloy. The forged material is preheated until the bulk temperature of the material reaches a temperature within the above range and thereafter forged. When preheating is performed at a temperature lower than 350°C, sufficient plastic flow cannot occur during hot forging of the forged material, and when preheating is performed at a temperature higher than the above-mentioned difference, forging may The material burnt (localized melting). When overburning occurs in the forged material, the strength of the forged product is greatly weakened, or defects due to local melting, such as holes and microshrinkage cavities, are generated in the product.

Since hot forging is usually performed on forging materials, the material is preheated and a forging die is also heated. The heating temperature is 100 to 400°C. The heating temperature is determined according to various forging parameters including the shape of the forged product, the type of forging equipment, and the type of alloy constituting the material to be forged. When the heating temperature is too low, the forged material is cooled together with the forging die, and the workability of the material is impaired, resulting in insufficient plastic flow of the material. Conversely, when the heating temperature is too high, the strength of the forging die decreases, and the die tends to be worn or broken. Therefore, an excessively high heating temperature is not preferable from the viewpoint of the service life of the forging die. Preferably, forging is performed after applying a lubricant to the forging die.

Die forging for forging materials. An example of forging equipment applied in the present invention will be described below with reference to FIG. 5 . The forging equipment includes a forging bed 101, an upper die 103 mounted on an upper die base 102, and a lower die 105 mounted on a lower die base 106. The forging die used in the present invention includes an upper die 103 , a lower die 105 and a knockout pin/ejector pin 107 . As shown in FIG. 5, the forging die to be used includes an upper die 103 for forming a piston head and a lower die 105 for forming a skirt. However, a forging die including a lower die for forming a piston head and an upper die for forming a skirt may be used. If desired, a lubricant applicator may be provided comprising means 108 for horizontally transporting an atomizer, an atomizer rotating means 109 and a lubricant nozzle 104 connected by a shaft 110 to the atomizer transport means 108.

In the present invention, the forging material is placed in the forging die such that the surface containing silicon grains of larger average size faces the surface of the die corresponding to the piston head. For example, when the above ingot is forged, the ingot is placed in a forging die such that the upper surface of the ingot faces the surface of the die corresponding to the piston head. When the ingot is placed in the forging die in a reverse fashion, the front end of the skirt is formed to contain silicon grains of larger average size and the oil ring groove portion is formed to contain silicon grains of smaller average size. silicon grains. In this case, therefore, the effects of the present invention cannot be obtained. That is, the average size of eutectic silicon crystal grains contained in the oil ring groove portion (a) becomes smaller than 4 μm, the average size of native silicon crystal grains contained in the oil ring groove portion becomes smaller than 15 μm, and the average size of primary silicon crystal grains contained in the oil ring groove portion becomes smaller than 15 μm, The ratio (A/B) of the average size (A) of the eutectic silicon grains in the ring groove portion to the average size (B) of the eutectic silicon grains contained in the front end portion of the skirt becomes smaller than 1.5, and The ratio (C/D) of the average size (C) of native silicon crystal grains contained in the oil ring groove portion to the average size (D) of native silicon crystal grains contained in the front end portion of the skirt portion becomes smaller than 1.3. As a result, the oil ring groove portion does not have excellent machinability and wear resistance, and the front end portion of the skirt does not have excellent plastic fluidity during forging.

In the manufacturing method of the present invention, the forging material is placed in the forging die, preferably so that the surface containing silicon grains of larger average size faces the surface of the die corresponding to the piston head, while the skirt portion The front end portion preferably contains eutectic silicon grains having an average size of 3 μm or less. The advantage of this average size is that the front end portion has good machinability during hot forging. That is, in the case where the front end portion of the skirt contains eutectic silicon grains having an average size of 3 μm or less, even when the thickness of the skirt is reduced during forging, the front end portion placed in the forging die Cracks will not be generated in the center, and the die charging characteristics of the front end portion will not be impaired.

The resulting forged piston preform may be machined. However, it is preferable to subject the piston preform to a heat treatment, such as artificial aging, because the mechanical properties of a preform made of an alloy containing Cu, Mg, Sc, Ag, etc. can be improved by heat treatment. In the artificial aging treatment, preferably, the piston preform is subjected to solid solution (solid solution) treatment, wherein the piston preform is subjected to water quenching immediately after heating at 400-500° C. for 0.2-10 hours, and then at 150- Temper at 250°C for 0.2-20 hours. Through this artificial aging treatment, the preform can obtain enhanced hardness, mechanical properties (eg, tensile strength and 0.2% yield strength) and fatigue strength.

Thereafter, the formed piston preform is subjected to processing including, for example, processing for forming a piston pin hole, milling of a piston surface, and processing for forming an oil ring groove, to thereby manufacture a final product (internal combustion Forged pistons for engines).

In the manufacturing method of the present invention, since the forging material is placed in the forging die, the surface containing silicon grains of larger average size faces the surface of the die corresponding to the piston head, contained in the oil ring groove portion The average size (A) of the eutectic silicon crystal grains of the eutectic silicon becomes at least 4 μm, the average size (C) of the primary silicon crystal grains contained in the oil ring groove portion becomes at least 15 μm, and the average size (A) is the same as that containing The ratio (A/B) of the average size (B) of the eutectic silicon crystal grains in the front end portion of the skirt becomes at least 1.5, and the average size ( C) The ratio (C/D) to the average size (D) of native silicon crystal grains contained in the front end portion of the skirt becomes at least 1.3. As a result, when the oil ring groove portion is milled, it is possible to prevent elongated chips from being entangled in the milling cutter and in the chuck of the milling machine. Scratches on the piston surface to be milled can thus be prevented. In addition, it prevents the accumulation of filamentous debris on the bottom of the mill and prevents the entire chuck from being covered with debris. Therefore, productivity can be improved.

The skirt is not continuous along the entire circumference of the piston, but is segmented by a piston pin hole into which a piston pin for connecting the piston to a connecting rod is inserted. Thus, the milling of the skirt is performed in a discontinuous manner, preventing entanglement of chips in the milling cutter. Therefore, as long as the average size (B) of the eutectic silicon crystal grains contained in the front end portion of the skirt is 0.67 times or smaller than the average size (A) of the eutectic silicon crystal grains contained in the oil ring groove portion, And the ratio (C/D) of the average size (C) of native silicon crystal grains contained in the oil ring groove portion to the average size (D) of native silicon crystal grains contained in the front end portion of the skirt is at least 1.3, it is possible to obtain satisfactory chip manageability during milling.

Since the reduced-thickness skirt can be formed by forging, machining (milling) for reducing the thickness of the skirt is not required, or a machining allowance required for machining can be reduced. Thus, the yield of pistons based on (certain) materials can be increased. In addition, productivity increases because the time required for machining is shortened.

Preferably, the forged material is subjected to a homogenization treatment prior to hot forging to enhance the forgeability of the material and artificial aging of a forged piston preform. In the homogenization process, the wrought material is heated at high temperature to evenly distribute an additional metal, such as Cu or Mg, in the aluminum matrix. The metal is added to the forged material to enhance the mechanical strength of the piston to be formed and to increase the strength of the piston used in an engine at a high temperature, and microsegregate during casting. Through this homogenization treatment, the forgeability of the forged material can be ensured, and the homogenization of the mechanical properties of the forged piston preform that has been subjected to the artificial aging treatment can be obtained. The homogenization treatment may be performed for 1 to 30 hours at a temperature ranging from 400° C. to the difference obtained by subtracting 10° C. from the solidus temperature (° C.) of an alloy to be applied.

Depending on the type of alloy to be used or the shape of the piston to be formed, the preheating of the forged material before forging can have an effect similar to that obtained by homogenizing the forged material. For example, when the forged material is preheated for one hour or more, an effect similar to that obtained by homogenization treatment can be obtained. Meanwhile, depending on the type of an alloy to be applied or the shape of the piston to be formed, heat treatment of the forged piston preform after forging can exert effects similar to those obtained by homogenizing the forged material. For example, when the forged piston preform is subjected to solution treatment for a long period of time during artificial aging treatment, effects similar to those obtained by homogenization treatment can be obtained.

The effects of the present invention will be described in detail below with reference to examples of applying a forged material formed by unidirectional solidification casting, a forged material formed by continuous casting, and a forged material formed by metal (permanent) mold casting.

Example 1 and 2:

Alloy 1 shown in Table 1 having a solidus temperature of 549° C. was subjected to unidirectional solidification casting by the apparatus shown in FIG. 2 to thereby form an ingot (outer diameter: 77 mm, thickness: 30mm) (Example 1). Alloy 1 shown in Table 1 having a solidus temperature of 528° C. was subjected to unidirectional solidification casting by the apparatus shown in FIG. 2 to thereby form an ingot (outer diameter: 110 mm, thickness : 30mm) (Example 2). Casting conditions are shown in Table 2. In each of Examples 1 and 2, a K-type thermocouple was placed at the position of the mold shown in FIG. 3 to measure the cooling rate of the ingot during solidification.

Each of the above-mentioned forged materials was subjected to homogenization treatment at 490° C. for 8 hours, and then forged into a piston preform by hot forging using the forging apparatus shown in FIG. 5 . The forging material is placed in the forging die of the forging equipment so that the upper surface of the forging material (ingot), that is, the surface of the ingot that does not face the cooling plate of the forging equipment is forged into a piston head, and the The bottom surface, ie the surface of the ingot facing the cooling plate of the forging apparatus, is forged as a skirt. Forging conditions are shown in Table 3.

In Example 1, the outer diameter of the forged piston preform produced was 78 mm, the thickness of the skirt of the preform was 3.5 mm, and the forging load during forging was 430 t. In Example 2, the outer diameter of the forged piston preform produced was 111 mm, the thickness of the skirt of the preform was 4 mm, and the forging load during forging was 670 t.

The formability of the skirt of each piston preform was evaluated by visual observation of cracks generated in the same direction as the plastic flow, underfill and hair crack generation due to insufficient plastic flow of the front end portion of the skirt during forging sex.

The formed piston preforms were artificially aged under the conditions shown in Table 4.

The hardness of the formed piston preform was measured using a Rockwell hardness tester. Measure the average size of eutectic silicon grains contained in the oil ring groove portion, measure the average size of eutectic silicon grains contained in the front end portion of the skirt, and calculate the ratio of the former average size to the latter average size . Also, measuring the average size of native silicon crystal grains contained in the oil ring groove portion, measuring the average size of native silicon crystal grains contained in the front end portion of the skirt, and calculating the former average size and the latter average size The ratio. For comparison, the maximum chord length (MAXLNG) of each silicon grain was measured using an image analyzer.

In the present invention, a prepared sample is observed under a microscope using a metallographic structure observation technique, and an image analysis processor is used to perform image analysis on the observed surface. The circle-equivalent diameter (the HEYWOOD diameter obtained by reducing the average cross-sectional area of grains found in the observed surface to the area of a circle and using the diameter of the circle as the grain diameter) The average value is taken as the average grain size.

The maximum chord length refers to the maximum length of a silicon grain measured using a vernier caliper. When two silicon crystal grains have the same HEYWOOD diameter, and one of the grains has a larger MAXLNG, the silicon crystal grain with the larger MAXLNG is in the shape of flakes or needles.

The oil ring groove portion of the formed piston preform was subjected to a milling test under the conditions shown in Table 5, and chips were evaluated in terms of shape and manageability. Then, the surface roughness of the inner wall of the oil ring groove was evaluated under the conditions shown in Table 6. Milling tests were carried out using a Compax milling cutter (synthetic diamond milling cutter) shown in FIG. 4 . Using a surface roughness meter, the surface roughness of the oil ring groove of the piston preform was measured in a direction parallel to the milling direction (ie, in a direction parallel to the surface of the piston head).

Then, a sample was obtained from the vicinity of the oil ring groove, and then a pin-on-disk wear test was performed on the sample at ambient temperature under the conditions shown in Table 7 to measure the wear amount of a pin.

Table 8 shows the results of the cooling rates measured above. Table 9 shows the average size of eutectic silicon crystal grains and native silicon crystal grains contained in the ingot. Table 12 shows the hardness (HRB) of the piston preform, the formability of the skirt, the shape of chips generated in the oil ring groove portion, the manageability of chips, the surface roughness of the inner wall of the oil ring groove, and from The wear amount of the sample obtained in the vicinity of the oil ring groove portion.

Compare example 1 and 2:

From a continuously cast rod (comparative example 1) with a diameter of 82 mm made from an alloy 1 shown in Table 1, and from a continuously cast rod with a diameter of 115 mm made from an alloy 2 shown in Table 1 ( Comparative example 2). Continuous casting was performed under the conditions shown in Table 10 using the air pressure hot top casting process disclosed in JP-B SHO 54-42847. In Comparative Example 1, the formed ingot was homogenized at 495° C. for 8 hours, and then cut to reduce the diameter to 77 mm. In Comparative Example 2, the formed ingot was similarly processed to reduce the diameter to 110 mm. Thereafter, each ingot was cut into a workpiece having a thickness of 30 mm, and the formed workpiece was used as a forging material. In the comparative examples 1 and 2, the forged materials were respectively forged under the same conditions as those in the examples 1 and 2. However, during forging, each forging material is placed into the forging die in a manner different from that described in Example 1 or 2. In Comparative Examples 1 and 2, the formed forged piston preforms were artificially aged and then machined under the same conditions as in Examples 1 and 2, respectively.

Table 11 shows the measurement results of the average size of eutectic silicon crystal grains and the average size of native silicon crystal grains. Table 12 shows the results of evaluations performed in a similar manner to that of Example 1 or 2.

Table 1 (mass percentage%) Si Cu Mg Ni Fe mn Ti Sr Ca P Alloy 1 9.6 3.0 0.46 - 0.18 - - 0.005 - - Alloy 2 19.2 1.1 1.10 0.9 0.44 0.40 0.13 - 0.002 0.010

Table 2 project Example 1 Example 2 1. The molten alloy temperature measured in the molten alloy container 740°C 800℃ 2. Mold and molten alloy vessel material Lumiboard Lumiboard 3. The height difference between the top of the mold and the liquid level of the molten alloy in the molten alloy container 150mm 150mm 4. Cool the temperature of the plate before pouring 100°C 200℃ 5. Cooling water volume 6L/min 7L/min 6. Molten alloy inlet diameter 12mm 10mm 7. Air temperature in electric furnace 770°C 820°C 8. The air temperature of the upper part of the mold and the side wall 680°C 700°C 9. Casting process: 1) pouring 2) cooling start conditions 3) cooling end conditions 4) cooling plate operation 5) material removal After 15 seconds the stopper was closed; water cooling was started at 500°C; water cooling was ended at 100°C; cooling plate was lowered at 150°C; natural fall. After 17 seconds stopper closed; water cooling started at 500°C; water cooling ended at 100°C; cooling plate lowered at 250°C; natural fall.

table 3 project Example 1 Example 2 1. Forging machine 630t-mechanical press 800t-mechanical press 2. Forging die temperature 1) Punch temperature 150°C 170°C 2) mold temperature 250°C 300℃ 3. Lubricating oil type Graphite lubricant same left 4. Preheating temperature of forging material 400°C 450°C

Table 4 project Example 1 Example 2 1. Solution treatment conditions 1) Heating temperature 495°C 505°C 2) Keep the temperature 2 hours 2 hours 2. Tempering conditions 1) Heating temperature 200℃ 180°C 2) Keep the temperature 6 hours 10 hours

table 5 Example 1 Example 2 1. Milling cutter type 2. Speed 3. Feed rate 4. Cutting depth 5. Milling oil type 6. Chip evaluation position Compax tool head (Nose) radius R2mm600rpm0.5mm/rev0.2mm water-soluble milling oil around the oil ring groove Same left 400rpm Same left Same left Same left Same left

Table 6 Example 1 Example 2 1. Milling cutter type 2. Speed 3. Cutting depth 4. Milling oil type 600rpm0.06mm/rev water-soluble milling oil as shown in Figure 4 same left 400rpm same left same left

Table 7 project content 1) Test equipment Abrasion tester TRI-S500 type (Takachiho Seiki) 2) Test method pin-on-disc method 3) Disc material ADC12 Molding Material 4) Lubricating oil mission oil 5) Applied load 5kgf 6) Slip rate 0.25m/sec 7) Sliding time 60 minutes 8) Pin shape φ7.9mm-h20mm

Table 8 Measuring position Measurements Example 1 Example 2 Cooling rate °C/s e E. 2.6 4.1 f f 6.2 5.5 ratio E/F 0.42 0.75

Table 9

Table 10 Comparative example 1 Comparative example 2 1. Ingot diameter 82mm 115mm 2. Header overhang length 10mm same left 3. Molten alloy temperature 720°C 820°C 4. Casting speed 200mm/min 180mm/min 5. Cooling water volume 15L/min 25L/min 6. Lubricating oil type castor oil same left 7. Lubricating oil flow rate 0.4cc/min 0.1cc/min 8. Gas type Air Air 9. Gas flow rate 0.5L/min 0.1L/min

Table 11

Table 12 Example 1 Example 2 Comparative example 1 Comparative example 2 1. Piston section hardness (HRB) 75 69 74 68 2. Skirt formability ○ ○ ○ ○ 3. The chip shape of the ring groove part Fragment shape Fragment shape elongated shape elongated shape 4. Debris manageability in the ring groove section ○ ○ x x 5. Surface roughness of the inner wall of the ring groove (Ra: μm) 0.9 1.1 3.5 5.4 6. The wear amount of the sample in the ring groove (μm) 19 3 34 7 overall evaluation ○ ○ x x

Formability Evaluation Criteria for Skirts project no yes 1. Skirt crack ○ x 2. Insufficiency of the front part of the skirt ○ x 3. Hairline cracks in the front part of the skirt ○ x

Evaluation of crumb shape

As clearly shown in Table 8, in Example 1 using the forged material formed by casting under the condition that the cooling rate measured at point e was 2.6°C/sec and the ratio (E/F) was 0.42, There are no primary silicon crystal grains in the forged material; the average sizes (HEYWOOD diameter) of the eutectic silicon grains contained in the oil ring groove portion and the front end portion of the skirt are 4.7 μm and 2.3 μm, respectively; the ratio ( A/B) is about 2. As clearly shown in Table 11, in Comparative Example 1 to which a small-diameter continuously cast rod formed of Alloy 1 was applied, eutectic silicon grains contained in the oil ring groove portion and in the front end portion of the skirt The average sizes of are 2.0 μm and 2.0 μm, respectively; the ratio (A/B) is 1, indicating that the piston preform has no metallographic structure in which the silicon grain size is graded along the thickness direction. The average size of the eutectic silicon grains contained in the oil ring groove portion of the piston preform of Comparative Example 1 was smaller than the average size of the eutectic silicon grains contained in the oil ring groove portion of the piston preform of Example 1. As clearly shown in Table 12, in both Example 1 and Comparative Example 1, the formability of the skirt was good. In Example 1, chips generated from the oil ring groove portion had a fragmented shape. In contrast, in Comparative Example 1, the chips generated from the oil ring groove portion had an elongated shape, from which it can be seen that the manageability of the chips was poor. In Comparative Example 1, the inner wall of the oil ring groove had a large surface roughness, and the oil ring groove portion had poor wear resistance. Generally, when the inner wall of the oil ring groove has a large surface roughness, the oil ring groove is worn by the piston ring, and the oil ring groove becomes enlarged. As a result, the piston fits obliquely with the oil ring groove, which causes problems such as wear on the inner wall of the cylinder. When the gap between the piston and the cylinder becomes large due to poor wear resistance of the oil ring groove portion, oil consumption becomes large, and the piston and the cylinder cannot fit each other in a sealing manner, thereby reducing engine output. Therefore, when the piston preform of Comparative Example 1 was formed into a piston, the formed piston was considered to have unsatisfactory characteristics.

A comparison between Example 2 and Comparative Example 2 reveals the following. In Example 2, the cooling rate measured at point e was 4.1°C/sec, and the ratio (E/F) was 0.75. In Example 2, the average size (A) of the eutectic silicon grains contained in the oil ring groove portion was 6.9 μm; the average size (B) of the eutectic silicon grains contained in the front end portion of the skirt was 2.8 μm. μm; the ratio (A/B) was 2.5; the average size (C) of primary silicon crystal grains contained in the oil ring groove portion was 23.9 μm; the average size (C) of primary silicon crystal grains contained in the front end portion of the skirt Dimension (D) is 15.7 μm; ratio (C/D) is (approximately) 1.5. As clearly shown in Table 11, in Comparative Example 2, the average size (A) of the eutectic silicon grains contained in the oil ring groove portion was 2.6 μm; the eutectic silicon contained in the front end portion of the skirt portion The average size (B) of the grains is 2.5 μm; the ratio (A/B) is (approximately) 1.0; the average size (C) of the primary silicon crystal grains contained in the oil ring groove part is 17.7 μm; The average size (D) of the native silicon crystal grains in the front end portion of the section was 18.3 μm; the ratio (C/D) was (approximately) 1.0, indicating that the piston preform had no silicon grain size graded along the thickness direction. Metallographic structure. As described above, the average size of the eutectic silicon crystal grains and native silicon crystal grains contained in the oil ring groove portion of the piston preform of Comparative Example 2 was smaller than that contained in the oil ring groove portion of the piston preform of Example 2. The average size of eutectic silicon grains and primary silicon crystal grains. In both Example 2 and Comparative Example 2, the formability of the skirt was good. In Example 2, chips generated from the oil ring groove portion had a fragmented shape. In contrast, in Comparative Example 2, debris generated from the oil ring groove portion had an elongated shape, whereby the manageability of the debris was poor, and the inner wall of the oil ring groove had a large surface roughness. In Comparative Example 2, since the average size of eutectic silicon crystal grains and primary silicon crystal grains contained in the oil ring groove portion is small, chips generated from the oil ring groove portion have an elongated shape, and the The manageability is poor, the inner wall of the oil ring groove has a large surface roughness, and the wear amount of the sample obtained from the oil ring groove part is relatively large.

Comparison example 3:

Piston preforms were produced from Alloy 1 and Alloy 2 shown in Table 1 by metal (permanent) mold casting. Comparative Example 3 applied two types of molds, ie, type A (thickness corresponding to a part of the skirt: 3.5 mm) and type B (thickness corresponding to a part of the skirt: 6 mm). These two dies are identical in shape to the forging dies used in Example 1. The Type A and Type B molds have the same inner diameter and shape.

Piston preforms were produced by casting from the Type A and Type B molds under the casting conditions shown in Table 13. The skirt soundness of the formed piston preform was evaluated. The results are shown in Table 14. Table 15 shows the average size measurements of eutectic silicon grains and native silicon crystal grains.

Table 13 Alloy 1 Alloy 2 1. Casting temperature 720°C 810°C 2. Mold temperature 400°C 420°C 3. Mold coating reagent Release agent (die coat) same left 4. Riser height 200mm same left

Table 14 skirt thickness Alloy 1 Alloy 2 3.5mm x x 6.0mm ○ ○

illustrate:

○: There is no underfill at the skirt

×: There is a defect due to insufficient casting of the skirt

Table 15 Measuring position Measurements HEYWOOD diameter MAXLNG Alloy 1 Alloy 2 Alloy 1 Alloy 2 Average size of eutectic silicon grains μm Oil ring groove part A 3.6 5.1 6.4 8.8 the front part of the skirt B 3.6 5.2 6.5 8.8 ratio A/B 1.0 1.0 1.0 1.0

The results in Table 14 show that when formed by metal (permanent) pattern casting from either of Alloys 1 and 2 - including a skirt having a thickness of 3.5 mm, i.e. the same thickness as that of the forged piston preform When casting a piston preform, an underfill occurs at the front end of the skirt due to insufficient pouring, and the resulting preform does not have a quality comparable to that of a forged piston preform. The results also show that the skirt has defect-free properties when the thickness of the skirt is 6 mm. As mentioned above, piston preforms formed by metal (permanent) mold casting do not have characteristics similar to those of forged piston preforms. Because the average size of the eutectic silicon crystal grains and primary silicon crystal grains contained in the oil ring groove portion of the piston preform of Comparative Example 3 was almost equal to that contained in the oil ring groove portion of the piston preform of Example 1 or 2 The average size of the eutectic silicon grains and native silicon crystal grains, it can be considered that the oil ring groove portion of the piston preform of Comparative Example 3 has good millability. However, in the case of Comparative Example 3, since the thickness of the skirt cannot be reduced, and a large amount of milling allowance is required, high productivity cannot be obtained.

Comparison example 4:

Alloy 2 shown in Table 1 was melted in a melting device, and the resulting molten alloy was poured into a cylindrical iron mold (outer diameter: 300 mm, overall length: 350 mm, inner diameter: 115 mm, depth: 250 mm), To thereby form a cylindrical ingot having an outer diameter of 115 mm (the length of the non-defective portion of the ingot not including holes: 150 mm). During casting, the temperature of the molten alloy was kept at 800°C, and the mold was preheated to 300°C. The formed cylindrical ingot was homogenized at 490° C. for 8 hours. Then, the periphery of the ingot was subjected to cutting work to remove defective portions, thereby forming a round bar having an outer diameter of 110 mm. The round bar was cut into a workpiece having a thickness of 30 mm, and the formed workpiece was used as a forging material.

Piston preforms were produced by hot forging with two types of forging dies of type C (thickness corresponding to a part of the skirt: 4.0 mm) and type D (thickness corresponding to a part of the skirt: 6 mm), whereby the The obtained pistons had the same outer diameter. The forged material is preheated to 450° C. before hot forging. The formability of the skirt portion of the formed piston preform was evaluated. Hot forging was performed under the same conditions as in Example 2 shown in Table 3. The results are shown in Table 16. A sample was obtained from the front end portion of the skirt of each preform, and the metallographic structure of the sample was observed to thereby measure the average size of eutectic silicon crystal grains and native silicon crystal grains. The results are shown in Table 17.

Table 16 skirt thickness 4mm 6mm Evaluation of the non-defectiveness of the skirt x ○ Note crack at skirt

Table 17 the front part of the skirt Average size of eutectic silicon grains 8.7μm Average size of primary silicon crystal grains 33.4μm

Average size: HEYWOOD diameter

The results showed that when the average size of the eutectic silicon grains or primary silicon crystal grains was large, the forgeability was lowered, and a finished product with excellent wear resistance and chip breakability could not be produced. The results also show that when the average size of the eutectic silicon grains or primary silicon crystal grains is large, as in the case of metal (permanent) mold casting, a skirt of greater thickness must be formed during forging and then The skirt must be machined to thereby reduce the thickness of the skirt.

Industrial Applicability

As described above, according to the present invention, it is possible to produce a forged piston for an internal combustion engine comprising an oil ring groove portion and a skirt, wherein eutectic silicon crystal grains and native silicon crystals are contained in the front end portion of the skirt The average size of the crystal grains is small, and the average size of these crystal grains contained in the oil ring groove portion is large. The skirt has excellent malleability as that of a piston formed from a small-diameter continuously cast rod, and thus the thickness of the skirt can be reduced. In addition, the oil ring groove portion has the same excellent debris manageability during milling as the oil ring groove portion of the piston formed by casting, and the oil ring groove has less surface roughness and has excellent wear resistance .

Forged pistons as described above cannot be produced by hot forging using conventional small diameter continuously cast rods or by metal (permanent) mold casting. However, the present invention makes it possible to provide a forged piston for an internal combustion engine with the advantages obtained by hot forging and metal (permanent) mold casting.

The forged piston of the invention is manufactured from a material obtained by unidirectional solidification casting by a manufacturing method combining hot forging and artificial aging.

In addition to the original additional elements (ie, Si, Cu and Mg), the aluminum alloy used in the present invention may also contain elements for further improving artificial age hardening properties, such as Ag or Sc, and an element for improving Heat-resistant elements, such as Fe, Ni, Ti, Cr, V, Zr, Mn, Co, Nb or Mo. These elements may be added individually or in combination. By adding such an element, the characteristics of the oil ring groove portion, the skirt portion, the head surface, and the piston pin portion can be improved more than other portions.

Therefore, an inexpensive high-performance forged piston can be provided, and a high-performance engine of uniform quality can be provided.

Claims (8)

1. one kind is the forging piston that is used for explosive motor that the aluminum alloy of the silicon of 6-25% is made by comprising mass ratio, this piston comprises an oil ring groove part and a skirt section, the ratio (A/B) of the average-size (B) of the Eutectic Silicon in Al-Si Cast Alloys crystal grain in the average-size (A) that wherein is included in the Eutectic Silicon in Al-Si Cast Alloys crystal grain in this oil ring groove part and the fore-end that is included in this skirt section is at least 1.5, and this average-size (A) is at least 4 μ m.

2. forging piston according to claim 1, it is characterized in that, the ratio (C/D) of the average-size (D) of the primary silicon crystal crystal grain in the average-size (C) that is included in the primary silicon crystal crystal grain in the oil ring groove part and the fore-end that is included in the skirt section is at least 1.3, and this average-size (C) is at least 15 μ m.

3. forging piston according to claim 1 and 2 is characterized in that, it is the Mg of 0.1-2% that described aluminum alloy comprises Cu and the mass ratio that mass ratio is 0.3-7%.

4. forging piston according to claim 1 and 2 is characterized in that, described aluminum alloy comprises the Ni that mass ratio is 0.1-2.5%.

5. a manufacturing is used for the method for the forging piston of explosive motor, comprises the steps:

To comprising mass ratio is that the molten aluminium alloy of the silicon of 6-25% carries out unidirectional curing casting to produce an ingot casting as the forged material with each other relative first surface and second surface thus, and the average-size that wherein is included in the silicon crystal grain in the first surface is different from the average-size that is included in the silicon crystal grain in the second surface;

This forged material is preheated;

This forged material is placed a forging die, and the surface corresponding to piston crown of this mould is faced on the feasible surface that comprises the silicon crystal grain of big average-size, thus this forged material is forged into a piston prefabricated component;

This piston prefabricated component is carried out artificial aging to be handled; With

The piston prefabricated component that forms is carried out machining, to make a forging piston that is used for explosive motor thus.

6. method according to claim 5, it is characterized in that, unidirectional curing casting be included as the average-size (A) that obtains to be at least the Eutectic Silicon in Al-Si Cast Alloys crystal grain in 1.5 the top that is included in ingot casting be included in this ingot casting near the ratio (A/B) of the average-size (B) of the Eutectic Silicon in Al-Si Cast Alloys crystal grain in the part of a cooling plate and be at least the average-size (A) of 4.0 μ m and the cooling carried out.

7. method according to claim 5, it is characterized in that, unidirectional curing casting comprises: for obtain be 0.85 or littler, from the downward 5mm in top that solidifies mould and from the sidewall of curing mold inwards the cooling rate (E) measured of the some e of 5mm with make progress 1mm and from the cooling carried out of the ratio (E/F) of the cooling rate (F) of the some f measurement of 5mm inwards of the sidewall of curing mold, wherein cooling rate (E) is at least 0.5 ℃/second in bottom from curing mold.

8. according to each described method in the claim 5 to 7, it is characterized in that the temperature in a scope that falls between the difference that deducts 10 ℃ of gained from 350 ℃ to the solidus temperature from aluminum alloy is carried out described preheating.

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