JP5588884B2 - Magnesium alloy forged piston manufacturing method and magnesium alloy forged piston - Google Patents

Description

  The present invention relates to a method for manufacturing a magnesium alloy forged piston for manufacturing a piston by forging a magnesium alloy material, and a magnesium alloy forged piston obtained by this manufacturing method.

  In recent years, forged pistons formed by forging are being adopted as pistons for internal combustion engines. Forged pistons are superior in strength and wear resistance at high temperatures. If a forged piston is used, the explosion pressure is increased to increase the output, and the piston skirt is made smaller (or thinner) to reduce the weight. Can be.

Further, in recent years, with the growing awareness of global environmental conservation, there has been an increasing demand for improving the fuel efficiency of automobiles, and the development of lightweight materials for automobiles has been strongly demanded. Magnesium alloys have the lowest density among metal materials currently in practical use, and many forged pistons using magnesium alloys have been developed (see, for example, Patent Documents 1 and 2).
In piston forging such as these, it is usually formed by one blow in order to shorten the forging process. For this purpose, a material substantially equal to the inner diameter of the forging die is introduced into the die, and the outer shape of the material is set. It is a normal method to restrain and form a metal flow to the skirt part and the boss part.

  Here, in the piston, the top surface portion is exposed to the highest temperature during operation of the piston, and the heat load is severe. Therefore, it is necessary to improve the strength of the top surface portion. Regarding the improvement of the strength of the forged material, it is known that the strength is improved by increasing the forging reduction rate (increasing the amount of strain) (for example, see Patent Document 3).

Japanese Patent Laid-Open No. 09-263871 JP 2008-036710 A Japanese Patent No. 2763175

However, the conventional techniques have the following problems.
From the description of Patent Document 3, it is possible to increase the strength by increasing the strain. However, according to the study by the present inventors, in the normal forging process, it is possible to increase the distortion of the top surface portion of the forging material. It is difficult to improve the strength of the top surface portion.

  Specifically, the flow of the material metal during forging is due to the restraint of the cylindrical mold, the top surface part becomes dead metal and no significant distortion is applied, and the metal on the outer periphery moves in the skirt direction or boss part. It will flow toward you. Therefore, the strength of the top surface portion is lower than that of the skirt portion, and the excellent characteristics cannot be fully exhibited.

  This invention is made | formed in view of the said problem, and makes it a subject to provide the manufacturing method of a magnesium alloy forge piston and magnesium alloy forge piston which can fully improve the yield strength of the top surface part of a piston. .

  In order to solve the above-mentioned problem, a magnesium alloy forged piston according to the present invention (hereinafter referred to as “piston” as appropriate) is a method for producing a magnesium alloy forged piston manufactured by forging, and is a round made of magnesium alloy. In a forging process for forging a bar-shaped ingot material, a value obtained by dividing the outer diameter d by the inner diameter D when the inner diameter of the piston forging die is D and the outer diameter of the ingot material is d (d / D) is 85% or less, and forging is performed under the condition that the temperature of the ingot material at the time of forging is 250 to 400 ° C. The value of d / D is shown as a percentage (the same applies hereinafter).

  According to such a manufacturing method, in the forging process, by defining d / D to a predetermined value, the metal flows in the radial direction, and the processing distortion of the top surface portion of the forged material (piston) increases. Further, by forging under the condition that the temperature of the ingot material is 250 to 400 ° C., the forging material (piston) is not cracked, and the processing distortion of the top surface portion of the forging material (piston) increases. By these actions, the strength of the top surface portion of the piston is improved.

Moreover, it is preferable to forge on the conditions that the temperature of the said ingot raw material at the time of the said forging becomes 250 degreeC or more and less than 350 degreeC.
According to such a manufacturing method, the processing distortion of the top surface portion is easily increased, and the strength of the top surface portion of the piston is easily improved.

  A method for producing a magnesium alloy forged piston according to the present invention is a method for producing a magnesium alloy forged piston produced by forging, wherein a forging process for forging a round bar-shaped ingot material made of magnesium alloy performs rough forging. Then, in the forging step, when the inner diameter of the rough forging die used for the rough forging is D and the outer diameter of the ingot material is d, the outer diameter d The value (d / D) divided by the inner diameter D is 85% or less, and the portion of the rough forging die that is in contact with the top surface portion of the magnesium alloy forged piston is convex, and Forging is performed under the condition that the temperature of the ingot material during the rough forging is 250 to 400 ° C.

  According to such a manufacturing method, in the forging process, the metal flows in the radial direction by prescribing d / D at the time of rough forging, and the top surface portion of the rough forged material (and the forged material (piston)). Processing distortion increases. In addition, by forming a portion where the top surface portion of the piston of the rough forging die contacts with a convex shape, strain is concentrated on the top surface portion of the rough forging material (and the forging material (piston)), and the top surface portion Processing distortion increases. Furthermore, by forging under the condition that the temperature of the ingot material at the time of rough forging is 250 to 400 ° C., cracking of the rough forged material does not occur, and the top surface portion of the rough forged material (and forged material (piston)) Processing distortion increases. By these actions, the strength of the top surface portion of the piston is improved.

Moreover, it is preferable to forge on the conditions that the temperature of the said ingot raw material at the time of the said rough forging becomes 250 to 350 degreeC.
According to such a manufacturing method, the processing distortion of the top surface portion is easily increased, and the strength of the top surface portion of the piston is easily improved.

Further, at the time of the forging or the rough forging, a value (d / D) obtained by dividing the outer diameter d by the inner diameter D is preferably 60% or more and less than 70%.
According to such a manufacturing method, by further setting d / D within this range, forging defects and buckling deformation are less likely to occur, and processing distortion of the top surface portion is more easily improved, and the top surface portion of the piston is improved. Strength becomes easier to improve.

  The magnesium alloy forged piston according to the present invention is obtained by the method for producing a magnesium alloy forged piston described above.

  According to such a configuration, since the piston is manufactured by the above manufacturing method, the strength of the top surface portion is excellent.

The magnesium alloy forged piston material may be a Mg-Gd-Zn alloy or a Mg-Gd-Zn-Zr alloy.
According to such a configuration, the piston of Mg—Gd—Zn alloy or Mg—Gd—Zn—Zr alloy has excellent heat resistance.

According to the method for manufacturing a magnesium alloy forged piston according to the present invention, the yield strength of the top surface portion can be sufficiently improved, and a piston excellent in the strength of the top surface portion can be manufactured.
The magnesium alloy forged piston according to the present invention has a high yield strength of 250 MPa or more at the top surface portion, and is excellent in strength at the top surface portion.

It is a schematic diagram for demonstrating the 1st forge method in the forge process of the manufacturing method of the magnesium alloy forge piston which concerns on this invention. It is a schematic diagram for demonstrating the 2nd forge method in the forge process of the manufacturing method of the magnesium alloy forge piston which concerns on this invention. It is a schematic diagram of the magnesium alloy forged piston according to the present invention.

  Hereinafter, a magnesium alloy forged piston manufacturing method and a magnesium alloy forged piston according to the present invention will be described in detail with reference to the drawings.

First, a forging device used in a method for manufacturing a piston according to the present invention will be described with reference to FIGS. In the present embodiment, the forging materials 15 and 25 in FIGS. 1 and 2 will be described as being magnesium alloy forging pistons.
≪Forging equipment≫
As shown in FIG. 1, the forging device 10 includes a piston forging die 11 for arranging the ingot material 1 and a punch 12 for forging the ingot material 1 arranged in the piston forging die 11. ing. The forging device 10 is not particularly limited as long as it satisfies a predetermined condition in a forging process described later.
The piston forging die 11 is used in the first forging method in the forging process, and the piston forging die 11 is a cylindrical die. Therefore, the inside of the mold is circular in plan view.

As shown in FIG. 2, the forging device 20 includes a rough forging die 21 for placing the ingot material 2 and a punch 22 for rough forging the ingot material 2 arranged in the rough forging die 21. I have. The forging device 20 is not particularly limited as long as it satisfies a predetermined condition in a forging process described later.
The rough forging die 21 is used in the second forging method in the forging process, and is a portion (that is, a rough portion manufactured in the previous stage) that becomes the top surface portion A of the magnesium alloy forged piston 25 to be finally manufactured. The part a which the forged material 24 or the forged material 25 (the part which becomes the top surface portion A of the piston 25 in FIG. 2) contacts has a convex shape. By this convex shape, as will be described later, the processing distortion of the top surface portion A increases, and the strength of the top surface portion A is further improved. In addition, about the shape of the other rough forging die 21, since it is the same as that of the description in the said piston forging die 11, description is abbreviate | omitted here.
The rough forging material 24 formed by rough forging is finish forged by a forging device (not shown), but the configuration of the forging device used for finish forging is substantially the same as the configuration of the piston forging die 11. The description is omitted here.

≪Manufacturing method of magnesium alloy forged piston≫
A magnesium alloy forged piston manufacturing method (piston manufacturing method) according to the present invention is a piston manufacturing method manufactured by forging, and includes a forging step. Furthermore, you may include an ingot raw material preparation process, a homogenization heat treatment process, and a tempering process process as needed. Hereinafter, each step will be described.

<Ingot material production process>
The ingot material production step is a step of producing a round bar-shaped ingot material made of a magnesium alloy.
The ingot material production process includes a melting and casting process in which a magnesium alloy is melted and cast to produce an ingot, and a cutting process in which the ingot is cut into a predetermined length.

  The magnesium alloy to be used is not particularly limited and may be any alloy type. For example, Mg—Zn alloy, Mg—Al alloy, Mg—Re alloy, Mg—Re—Zn alloy, etc. Is mentioned. Here, Re is composed of one or more of Y, Sm, Gd, Er, Ce, La, Nd, and Ho, and the content is 2 to 15% by mass. Moreover, in these alloys, Zr as a crystal grain refiner may be contained about 0-1 mass%. Specific examples thereof include an Mg—Gd—Zn alloy, an Mg—Gd—Zn—Zr alloy, and a WE54 alloy.

  In the melting and casting step, a long round bar-shaped ingot having a diameter of 50 to 70 mm is produced from a molten metal in which a magnesium alloy having a predetermined composition is melted. The method for melting and casting the magnesium alloy is not particularly limited, and a conventionally known method may be used. For example, it can be melted using a vacuum induction furnace and cast using a continuous casting method or a semi-continuous casting method.

  In the cutting step, the ingot (long round bar-shaped ingot) is cut into a length required as a forging material by a cutting machine to obtain an ingot material (round bar material). Note that the ingot may be peeled before this cutting.

<Homogenization heat treatment process>
The homogenization heat treatment step is a step of performing homogenization heat treatment on the ingot material.
By subjecting the ingot material to homogenization heat treatment, the intermetallic compound crystallized at the time of casting is diffused and dissolved to homogenize the structure. The homogenization heat treatment may be performed in a heating furnace at a temperature below the solidus, for example, at 400 to 500 ° C. for several hours to several tens of hours. By performing the heat treatment under these conditions, sufficient homogenization can be performed. For the homogenization heat treatment, an air furnace, an induction heating furnace, a glass stone furnace or the like is appropriately used. However, it is omitted if it is not particularly necessary.

<Forging process>
The forging process is a process of forging a round bar-shaped ingot material made of a magnesium alloy. In addition, when performing the said homogenization heat processing, the ingot raw material by which the homogenization heat processing was carried out is forged.
The ingot material is heated to the forging start temperature. When the homogenization heat treatment is performed, the ingot material once cooled to room temperature after the homogenization heat treatment is reheated to the forging start temperature. Then, hot forging is performed by forging using a mechanical press or forging using a hydraulic press, and the piston is forged.
Here, in the present invention, the following two methods can be used in the forging step.

[First forging method]
The first forging method is forging by one forging, that is, one blow. As shown in FIG. 1, forging is performed while applying stress to the ingot material 1 in a predetermined direction (direction indicated by a white arrow in the figure), and finally, for example, as shown in FIG. 3. A shaped piston 30 is obtained. This forging is performed by lowering the punch 12 after placing the ingot material 1 in the piston forging die 11 in the forging device 10.
In this forging, when the inner diameter of the piston forging die 11 is D and the outer diameter of the ingot material 1 is d, the value obtained by dividing the outer diameter d by the inner diameter D (d / D) is 85% or less. And it forges on the conditions that the temperature of the ingot raw material 1 at the time of forging becomes 250-400 degreeC.

(D / D: 85% or less)
By making the outer diameter d of the ingot material 1 significantly smaller than the inner diameter D of the piston forging die 11, the metal flows in the radial direction during forging. Thereby, the process distortion of the top surface part A of the forging material (piston) 15 increases, and the intensity | strength of the top surface part A improves. However, if d / D exceeds 85%, the effect of increasing strain (increasing strength) cannot be obtained. Therefore, d / D is set to 85% or less. Preferably, it is 80% or less.
The outer diameter d of the ingot material 1 is smaller than the inner diameter D of the piston forging die 11, so that the processing distortion of the top surface portion A increases and the strength characteristics are improved. (See FIG. 3) Forging defects such as slow-through are likely to be generated. Furthermore, the ingot material 1 is buckled and deformed during forging, making forging difficult. Therefore, d / D is preferably 50% or more. Furthermore, d / D is preferably 60% or more and less than 70%. When d / D is within this range, forging defects and buckling deformation are less likely to occur, and the processing distortion of the top surface portion A is more likely to be improved.

(Ingot material temperature: 250-400 ° C)
In order to increase the processing strain of the top surface portion A and improve the strength, it is necessary to control the forging temperature within a predetermined range. Specifically, the temperature of the ingot raw material 1 at the time of forging shall be 250-400 degreeC. If the temperature of the ingot material 1 is less than 250 ° C., the forged material (piston) 15 is cracked. On the other hand, if it exceeds 400 ° C., the effect of increasing the strain (increasing strength) cannot be obtained. Therefore, the temperature of the ingot raw material 1 at the time of forging shall be 250-400 degreeC. Preferably, it is 250 degreeC or more and less than 350 degreeC. Within this temperature range, the effect of increasing strain is more likely to be improved.

[Second forging method]
In the second forging method, as shown in FIG. 2, the forging process is a process for performing forging after rough forging. As shown in FIG. 2, forging is performed while applying stress to the ingot material 2 in a predetermined direction (direction indicated by a white arrow in the figure), and finally, for example, as shown in FIG. A shaped piston 30 is obtained.
Rough forging is a step (rough forging step) in which the ingot material 2 is placed in the rough forging die 21 in the forging device 20 and then pressed by lowering the punch 22 (rough forging step). Molded. Finish forging is a process of pressing the rough forged material 24 by placing the punch in a forging device (not shown) and then lowering the punch (finish forging process). The top surface part A of 24 is shape | molded flatly, and the forging material (piston) 25 is manufactured. These rough forging and finish forging are performed consistently without reheating during the process. However, in some cases, reheating may be performed.

  In this forging, when the inner diameter of the rough forging die 21 used for rough forging is D and the outer diameter of the ingot material 2 is d, a value obtained by dividing the outer diameter d by the inner diameter D (d / D ) Is 85% or less, and the portion that becomes the top surface portion A of the magnesium alloy forging piston 25 in the rough forging die 21 (that is, the portion that becomes the top surface portion A of the rough forging material 24 or the forging material 25) is in contact. Forging is performed under the condition that a is convex and the temperature of the ingot material 2 during rough forging is 250 to 400 ° C.

(D / D: 85% or less)
Similarly to the first forging method, when d / D exceeds 85%, the distortion increasing effect (high strength) of the top surface portion A of the rough forged material 24 and the forged material (piston) 25 cannot be obtained. Therefore, d / D is set to 85% or less. Preferably, it is 80% or less.
For the same reason as in the first forging method, d / D is preferably 50% or more. However, by using the rough forging die 21 to produce the rough forged material 24 that is a material having an intermediate shape, generation of forging defects in finish forging is suppressed. Furthermore, d / D is preferably 60% or more and less than 70%. When d / D is within this range, forging defects and buckling deformation are less likely to occur, and the processing distortion of the top surface portion A is more likely to be improved. The conditions for finish forging are basically the same as those for rough forging, but it is preferable to lower the forging temperature than for rough forging in order to improve the strength.

(The part where the part which becomes the top part touches is a convex shape)
In the rough forging die 21, the top surface portion A of the rough forging material 24 (that is, the top surface portion A of the forging material (piston) 25) is formed by forming a convex portion at the portion a that contacts the top surface portion A of the piston 25. ) Is concentrated, and the processing distortion of the top surface portion A increases. Thereby, the strength of the top surface portion A is further improved.
The convex shape of the rough forging die 21 is designed so that the processing strain at the top surface portion A of the piston 25 is 2.5 or more, which is an equivalent strain value by plastic flow analysis. By designing the convex shape so that the equivalent strain value is 2.5 or more, a distortion increasing effect (high strength) can be achieved by making the portion a convex. The equivalent distortion value can be calculated by, for example, FORGE3D (manufactured by Transvaler).

(Ingot material temperature: 250-400 ° C)
Similar to the first forging method, if the temperature of the ingot material 2 during rough forging is less than 250 ° C., cracks occur in the rough forged material 24. On the other hand, if it exceeds 400 ° C., the effect of increasing the strain (increasing strength) cannot be obtained. Therefore, the temperature of the ingot raw material 2 shall be 250-400 degreeC. Preferably, it is 250 degreeC or more and less than 350 degreeC. Within this temperature range, the effect of increasing strain is more likely to be improved.

<Refining process>
The tempering process is a process of tempering the forged forged material. Specifically, the forged forged material is subjected to age hardening treatment, or the forged material is subjected to solution treatment, followed by quenching treatment, and then age hardening treatment.
In order to improve strength, toughness, and corrosion resistance after forging, it is preferable to perform tempering treatment such as T5, T6, T7, and the like. T5 is an artificial age hardening treatment after cooling from high temperature processing. T6 is an artificial age hardening treatment that obtains maximum strength after solution treatment and quenching treatment. T7 is an excessive age hardening treatment performed under conditions exceeding the artificial age hardening treatment conditions for obtaining the maximum strength after solution treatment and quenching treatment. In other words, the tempering treatment step consists of an age hardening treatment step, or a solution treatment step, a quenching treatment step, and an age hardening treatment step.

  The solution treatment may be performed under normal conditions, for example, a condition of holding in a temperature range of 400 to 570 ° C. for 1 to 7 hours. For the solution treatment, an air furnace, an induction heating furnace, a glass stone furnace or the like is appropriately used. The quenching process may be performed under normal conditions.

The age hardening treatment may be performed by selecting the conditions for the tempering treatment such as T5, T6, T7 and the like from normal conditions, for example, a temperature range of 170 to 400 ° C. and a holding time range of 1 to 6 hours. Note that an air furnace, an induction heating furnace, an oil bath, or the like is appropriately used for the age hardening treatment.
The tempering process at T6 and T7 has been described as being performed after forging here, but may be performed before forging.

  In carrying out the present invention, other steps may be included between or before and after each step as long as they do not adversely affect each step. For example, a machine that appropriately performs machining or surface treatment necessary as a piston after a foreign substance removing process for removing foreign substances such as dust or a forging process (before and after the tempering process if a tempering process is performed) You may include a process process, a surface treatment process, etc. As the machining, for example, a groove for holding a compression ring, an oil ring or the like, a hole for passing a piston pin (piston pin hole), or the like may be formed depending on the use of the piston.

≪Magnesium alloy forged piston≫
The magnesium alloy forged piston (piston) according to the present invention is obtained by the above-described method for producing a magnesium alloy forged piston. An example of the piston of the present invention will be described with reference to FIG.
As shown in FIG. 3, the piston 30 is located at the top thereof, and a piston side wall (piston wall) constituting a top surface portion (piston head) 31 facing the combustion chamber and a sliding surface (surface contacting the cylinder block). ) 32. The piston 30 further includes a boss portion (piston boss) 33 serving as a bearing for the piston pin.

  A groove 34 for holding a compression ring and an oil ring is formed in the upper portion (portion close to the top surface portion 31) of the piston side wall 32. The skirt portion (portion below the piston pin) of the piston side wall 32 is formed thin to reduce weight and is called a skirt portion (piston skirt) 35.

  Since this piston 30 is manufactured by the above manufacturing method, the proof stress of the top surface portion 31 is as high as 250 MPa or more, and is excellent in fatigue resistance (durability). When the piston 30 is manufactured by the manufacturing method of the present invention, the proof strength of the skirt portion 35 is higher than or equal to the proof strength of the top surface portion 31. The yield strength can be measured, for example, according to JIS Z 2241. Examples of the alloy that is a material of the piston 30 include an Mg—Gd—Zn alloy, an Mg—Gd—Zn—Zr alloy, and a WE54 alloy. The piston 30 is made of these alloys. it can.

And since piston 30 has high intensity | strength and fatigue resistance, it uses suitably for the internal combustion engine for various transport apparatuses, such as a passenger car, a bus, a truck, a motorcycle, a tractor, an airplane, a motor boat, and a civil engineering vehicle. Can do.
In addition, the said piston 30 showed an example of the form of the piston of this invention, and this invention is not limited to the said form.

As described above, according to the method for manufacturing a piston of the present invention, a piston with improved proof strength of the top surface portion can be obtained. Here, the strength (proof strength) required for the piston is 250 MPa or more at the top surface portion, but depending on the type of magnesium alloy, it is possible to manufacture a piston having a relatively high strength at the top surface portion by a normal manufacturing method. Conceivable. However, there is no manufacturing method for improving the yield strength of the top surface portion with respect to an alloy that reduces the yield strength of the top surface portion (an alloy that is less than 250 MPa) when a normal production method is used. According to the present invention, even with such a magnesium alloy type, a piston having a proof stress of 250 MPa or more at the top surface portion can be manufactured.
Further, the piston of the present invention has a characteristic that the proof stress of the top surface portion is 250 MPa or more and has a particular difference from the conventional piston.

As mentioned above, although the form for implementing this invention has been described, the Example which confirmed the effect of this invention is demonstrated concretely compared with the comparative example which does not satisfy | fill the requirements of this invention below. In addition, this invention is not limited to this Example.
In addition, as a material, for example, there is no particularly significant difference in the desired effect in the present invention in any of Mg—Gd—Zn alloy, Mg—Gd—Zn—Zr alloy, WE54 alloy, and the like. Here, a Mg—Gd—Zn—Zr alloy was used as a representative.

<Production of test material>
First, an ingot of an Mg-9% Gd-4% Zn-0.7% Zr (mass%) alloy was melted and continuously cast to produce a round bar-shaped ingot material. This ingot material was cut into a predetermined size to obtain a round bar material. The round bar material was heat treated under the condition of holding at 500 ° C. for 2 hours and then holding at 400 ° C. for 1 hour, and then hot forging was performed under the conditions shown in Table 1 to produce a test material piston.
Note that hot forging was performed by two methods, that is, only forging forging (1 blow) and forging forging after rough forging (indicated by the presence or absence of rough forging in Table 1). In the rough forging die, the portion that is in contact with the portion that becomes the top surface portion of the piston was made into a convex shape under the same conditions in the production of the test material.

<Evaluation method>
The yield strength of the top surface of the piston of the test material was measured according to JIS Z 2241. Further, the equivalent strain value of the top surface portion was calculated by plastic flow analysis using FORGE 3D (manufactured by Transvaler).
And the thing whose yield strength of a top surface part was 250 Mpa or more was set as the pass, the thing less than 250 Mpa, or the thing in which the crack generate | occur | produced was made into the failure.
These results are shown in Table 1. In Table 1, d / D indicates that the inner diameter of the piston forging die or the rough forging die is D, and the outer diameter of the ingot material (round bar material) is d. It is a value divided by D, and the forging temperature is the temperature of the ingot material (round bar material) at the time of forging (at the time of rough forging when rough forging is performed). Further, those not satisfying the scope of the present invention are indicated by underlining the numerical values.

  As shown in Table 1, in Examples 1 to 12, in order to satisfy the scope of the present invention, the yield strength of the top surface portion was 250 MPa or more, and the strength of the top surface portion was excellent. In addition, Example 1 in which the casting temperature has a more preferable value is particularly excellent in the strength of the top surface portion compared to Example 2 in which the d / D is the same and the casting temperature deviates from a more preferable value. It was. Further, Example 1 in which d / D has a more preferable value is particularly superior in strength at the top surface portion as compared with Example 4 in which d / D deviates from a more preferable value under the same casting temperature. It was.

On the other hand, since Comparative Examples 1-9 did not satisfy the scope of the present invention, the following results were obtained.
In Comparative Examples 1 and 2, since d / D and the forging temperature exceeded the upper limit values, the yield strength of the top surface portion was inferior. In Comparative Examples 3 to 5, since d / D exceeded the upper limit value, the yield strength of the top surface portion was inferior. In Comparative Examples 6 and 7, the forging temperature exceeded the upper limit value, so the yield strength of the top surface portion was inferior. In Comparative Examples 8 and 9, since the forging temperature was less than the lower limit value, the test material was cracked.

  In addition, the test material of the comparative example 4 assumes the conventional piston described in patent document 1,2. As shown in the present embodiment, the conventional piston does not satisfy a certain level in the above evaluation. Therefore, this example objectively revealed that the piston according to the present invention is superior to the conventional piston.

  The present invention has been described in detail with reference to the embodiments and examples. However, the gist of the present invention is not limited to the above-described contents, and the scope of right is widely interpreted based on the description of the claims. Must. Needless to say, the contents of the present invention can be widely modified and changed based on the above description.

1, 2 Ingot material 10, 20 Forging device 11 Piston forging die 12, 22 Punch 15, 25 Forging material (piston)
21 Rough forging die 24 Rough forging 30 Piston (magnesium alloy forged piston)
31 Top surface portion 32 Piston side wall 33 Boss portion 34 Groove 35 Skirt portion A Top surface portion a Site where the top surface portion of the piston is in contact D Inner diameter of piston forging or rough forging die d Outer diameter of ingot material

Claims (8)

A method for producing a magnesium alloy forged piston produced by forging,
In the forging process of forging a round bar-shaped ingot material made of magnesium alloy, when the inner diameter of the piston forging die is D and the outer diameter of the ingot material is d, the outer diameter d is the inner diameter D. A method for producing a magnesium alloy forged piston, comprising forging under a condition that a divided value (d / D) is 85% or less and a temperature of the ingot material during forging is 250 to 400 ° C.   2. The method for producing a magnesium alloy forged piston according to claim 1, wherein forging is performed under a condition in which a temperature of the ingot material during the forging is 250 ° C. or more and less than 350 ° C. 3. A method for producing a magnesium alloy forged piston produced by forging,
The forging process to forge a round bar-shaped ingot material made of magnesium alloy consists of a process for rough forging and then forging,
In the forging step, when the inner diameter of the rough forging die used for the rough forging is D and the outer diameter of the ingot material is d, a value obtained by dividing the outer diameter d by the inner diameter D (d / D) is 85% or less, and the portion of the rough forging die that comes into contact with the top surface portion of the magnesium alloy forged piston has a convex shape, and the temperature of the ingot material during the rough forging Forging a magnesium alloy forged piston, wherein forging is performed under a condition of 250 to 400 ° C.   4. The method for producing a magnesium alloy forged piston according to claim 3, wherein forging is performed under a condition that a temperature of the ingot material at the time of the rough forging is 250 ° C. or more and less than 350 ° C. 5.   The magnesium alloy forged piston according to any one of claims 1 to 4, wherein a value (d / D) obtained by dividing the outer diameter d by the inner diameter D is 60% or more and less than 70%. Manufacturing method.   A magnesium alloy forged piston obtained by the method for producing a magnesium alloy forged piston according to any one of claims 1 to 5.   The magnesium alloy forged piston according to claim 6, wherein the alloy that is a material of the magnesium alloy forged piston is an Mg—Gd—Zn-based alloy.   The magnesium alloy forged piston according to claim 6, wherein the alloy that is a material of the magnesium alloy forged piston is an Mg—Gd—Zn—Zr alloy.

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