Patent Application
20240218522
The present application is based on, and claims priority from JP Application Serial Number 2022-211891, filed Dec. 28, 2022, the disclosure of which is hereby incorporated by reference herein in its entirety.
The present disclosure relates to a thixotropically molded product, a thixotropic molding material, and a method of producing a thixotropic molding material.
Magnesium has properties of a low specific gravity and good electromagnetic wave shielding properties, good vibration damping capability, good machinability, and good biosafety. Based on such a background, magnesium alloy components are beginning to be used in products such as automobile components, aircraft components, mobile phones, and notebook computers.
A thixotropic molding method is known as a method of producing components made of magnesium. The thixotropic molding method is a molding method in which a molding material is heated in a cylinder to bring the material into a solid-liquid coexistence state in which a liquid phase and a solid phase coexist, then thixotropy is developed by rotation of a screw, and the obtained semi-solidified product is injected into a mold. According to such a thixotropic molding method, since fluidity of the semi-solidified product is enhanced by heating and shearing, a thin component or a component having a complicated shape can be formed as compared with a die casting method.
For example, WO 2006/003899 discloses production of a magnesium alloy product by a casting method in which injection molding such as die casting or thixotropic molding is performed.
JP-A-2019-131857 discloses production of an Mg-based composite material in which nitride particles having an average diameter of 0.05 μm or more are dispersed in an Mg parent phase by warm or hot strain imparting processing such as extrusion processing, forging processing, rolling processing, and drawing processing. Further, JP-A-2019-131857 discloses that an Mg-based composite material having excellent strength characteristics due to particle dispersion in a parent phase and having high frictional wear characteristics can be obtained.
When a magnesium alloy product is applied to a mechanical component or the like, it is necessary to consider mechanical strength required for the mechanical component and sliding between mechanical components. By increasing the mechanical strength, existing mechanical components can be replaced with the magnesium alloy product. Since friction is repeatedly generated in a sliding portion, high wear resistance is required for a sliding mechanical component. However, the magnesium alloy product described in WO 2006/003899 has insufficient wear resistance and has room for improvement. On the other hand, the Mg-based composite material described in JP-A-2019-131857 is intended to improve frictional wear characteristics. However, since the Mg-based composite material described in JP-A-2019-131857 is not a thixotropically molded product, there are problems that it is not easy to form the Mg-based composite material into a desired shape and there is room for further improvement in wear resistance.
A thixotropically molded product according to an application example of the present disclosure includes: a matrix portion containing Mg as a main component; and a first particle portion dispersed in the matrix portion and containing c-BN or w-BN as a main component. An average particle diameter of the first particle portion in a cross section is 10.0 μm or less, and an area fraction of the first particle portion in the cross section is 2.0% or more and 20.0% or less.
A thixotropic molding material according to an application example of the present disclosure includes: a metal body containing Mg as a main component; boron nitride particles adhering to a surface of the metal body and containing c-BN or w-BN as a main component; and a bonding portion interposed between the metal body and the boron nitride particles. An average particle diameter of the boron nitride particles is 10.0 μm or less, and a ratio of the boron nitride particles to a total of the metal body and the boron nitride particles is 2.0 mass % or more and 15.0 mass % or less.
A method of producing a thixotropic molding material according to an application example of the present disclosure includes: a preparation step of preparing a mixture containing a metal body containing Mg as a main component, boron nitride particles containing c-BN or w-BN as a main component, a binder, and a dispersion medium; a stirring step of stirring the mixture; and a drying step of adhering the boron nitride particles to a surface of the metal body via the binder by removing at least a part of the dispersion medium from the stirred mixture. An average particle diameter of the boron nitride particles is 10.0 μm or less, and in the mixture, a ratio of the boron nitride particles to a total of the metal body and the boron nitride particles is 2.0 mass % or more and 15.0 mass % or less.
Hereinafter, a thixotropically molded product, a thixotropic molding material, and a method of producing a thixotropic molding material according to the present disclosure will be described in detail based on embodiments shown in the accompanying drawings.
First, an example of a thixotropic molding method of producing a thixotropically molded product will be described.
The thixotropic molding method is a molding method in which a pellet-shaped or chip-shaped raw material is heated in a cylinder to bring the material into a solid-liquid coexistence state in which a liquid phase and a solid phase coexist, then thixotropy is developed by rotation of a screw, and the obtained semi-solidified product is injected into a mold. According to such a thixotropic molding method, since fluidity of the semi-solidified product is enhanced by heating and shearing, a thin component or a component having a complicated shape can be formed as compared with, for example, a die casting method.
As shown in
A material containing magnesium as a main component is used as the thixotropic molding material 10. The hopper 5 may be charged with other materials together with the thixotropic molding material 10.
Next, a thixotropic molding material according to a first embodiment will be described.
The thixotropic molding material 10 shown in
The metal body 11 contains Mg as a main component. As shown in
The bonding portion 13 penetrates between the metal body 11 and the boron nitride particles 14 and between the boron nitride particles 14 to bond them. In the embodiment, as shown in
By performing thixotropic molding using the thixotropic molding material 10 having the bonding portion 13, the boron nitride particles 14 are prevented from falling off. Therefore, a semi-molten material of the metal body 11 and the boron nitride particles 14 are likely to be uniformly mixed in the heating cylinder 7. Accordingly, the boron nitride particles 14 are uniformly dispersed in the semi-molten material. As a result, a thixotropically molded product in which c-BN or w-BN dispersed in a matrix portion is uniformly distributed can be produced.
The boron nitride particles 14 have high yield strength and elastic modulus derived from c-BN or w-BN. Therefore, in the thixotropically molded product in which the boron nitride particles 14 are distributed, high specific strength and specific rigidity derived from Mg can be further enhanced. Since the boron nitride particles 14 are uniformly dispersed, it is possible to inhibit enlargement of Mg crystals precipitated in a process of solidification during the thixotropic molding. Accordingly, refinement of the Mg crystals can be achieved, and a movement of dislocation can be prevented. As a result, specific strength and specific rigidity of the obtained thixotropically molded product can be further enhanced.
Further, the boron nitride particles 14 have high hardness derived from c-BN or w-BN. Therefore, high wear resistance is imparted to the thixotropically molded product in which the boron nitride particles 14 are dispersed.
The interposed particles 15 act to enhance wettability between the boron nitride particles 14 and the semi-molten material of the metal body 11 during the thixotropic molding. Accordingly, in the produced thixotropically molded product, affinity and adhesion between a site derived from the boron nitride particles 14 and a site derived from the metal body 11 can be enhanced. As a result, it is possible to produce a thixotropically molded product which is denser and has enhanced mechanical strength.
The metal body 11 is, for example, a section obtained by machining or cutting an Mg-based alloy cast with a mold or the like. A method of producing the metal body 11 is not limited thereto.
A material containing Mg as a main component is used as a constituent material of the metal body 11. Containing Mg as a main component refers to that, when elemental analysis is performed on the metal body 11, a content of Mg is the highest in terms of atomic ratio. For the elemental analysis, for example, a qualitative and quantitative analysis based on an energy dispersive X-ray spectroscopy (EDX) is used. The content of Mg in the metal body 11 may be higher than that of other elements, and is preferably more than 50 atomic %, more preferably 70 atomic % or more, and still more preferably 80 atomic % or more.
The metal body 11 may contain various additive components other than Mg. Examples of the additive components include lithium, beryllium, calcium, aluminum, silicon, manganese, iron, nickel, copper, zinc, strontium, yttrium, zirconium, silver, tin, gold, and rare earth elements, and one or a mixture of two or more of these components is used. Examples of the rare earth elements include cerium.
In particular, the additive component preferably contains at least one of aluminum and zinc, and more preferably both contains aluminum and zinc. Accordingly, a melting point of the thixotropic molding material 10 decreases, and fluidity of the semi-solidified product during the thixotropic molding is improved. As a result, moldability during the thixotropic molding is enhanced, and thus dimensional accuracy of the produced thixotropically molded product can be enhanced.
A content of aluminum in the metal body 11 is, for example, preferably 5.0 mass % or more and 13.0 mass % or less, and more preferably 7.0 mass % or more and 11.0 mass % or less. A content of zinc in the metal body 11 is, for example, preferably 0.3 mass % or more and 3.0 mass % or less, and more preferably 0.5 mass % or more and 2.0 mass % or less.
In addition to aluminum and zinc, the additive component preferably contains at least one selected from the group consisting of silicon, manganese, yttrium, strontium, and rare earth elements. Accordingly, mechanical properties, corrosion resistance, wear resistance, and thermal conductivity of the thixotropically molded product can be enhanced.
A composition of the metal body 11 may be a composition of a magnesium alloy defined in various standards. Examples of such a magnesium alloy include AZ91A, AZ91B, AZ91D, AM60A, AM60B, AS41A, AZ31, AZ31B, AZ61A, AZ63A, AZ80A, AZ91C, AZ91E, AZ92A, AM100A, ZK51A, ZK60A, ZK61A, EZ33A, QE22A, ZE41A, MIA, WE54A, and WE43B of the American Society for Testing and Materials (ASTM) standard. Among these, AZ91A, AZ91B, or AZ91D are preferably used. These magnesium alloys are useful because of well-balanced moldability, mechanical properties, and the like, and are excellent in corrosion resistance.
The additive component may be present in a form of a simple substance, an alloy, an oxide, an intermetallic compound, and the like in the metal body 11. The additive component may be segregated or uniformly dispersed in a crystal grain boundary of a metal structure such as Mg or an Mg alloy in the metal body 11.
An average particle diameter of the metal body 11 is not particularly limited, and is preferably 0.5 mm or more, and more preferably 1.5 mm or more and 10.0 mm or less. By setting the average particle diameter within the above range, generation of bridges and the like in the heating cylinder 7 of the injection molding machine 1 can be prevented.
The average particle diameter of the metal body 11 is an average value of diameters of circles having an area same as a projected area of the metal body 11. The average value is calculated based on 100 or more randomly selected metal bodies 11.
An average aspect ratio of the metal body 11 is preferably 5.0 or less, and more preferably 4.0 or less. In the metal body 11 having such an average aspect ratio, a filling property in the heating cylinder 7 is enhanced and temperature uniformity during heating is improved. As a result, a thixotropically molded product having high mechanical properties and high dimensional accuracy can be obtained.
The average aspect ratio of the metal body 11 is an average value of aspect ratios calculated from major axis/minor axis in a projection image of the metal body 11. The average value is calculated based on 100 or more randomly selected metal bodies 11. The major axis is a maximum length that can be taken in the projection image, and the minor axis is a maximum length in a direction orthogonal to the major axis.
The metal body 11 may be subjected to any surface treatment as necessary. Examples of the surface treatment include a plasma treatment, a corona treatment, an ozone treatment, an ultraviolet irradiation treatment, and a roughening treatment.
The coating portion 12 includes the plurality of boron nitride particles 14. In the embodiment, as shown in
The boron nitride particles 14 are dispersed in the semi-molten material when subjected to the thixotropic molding. The boron nitride particles 14 are less likely to vaporize during the thixotropic molding, and can be prevented from causing molding defects.
The boron nitride particles 14 contain c-BN or w-BN as a main component. Containing c-BN or w-BN as a main component refers to a state in which, when elemental analysis is performed on the boron nitride particles 14, a content of one of B and N is the highest and a content of the other is the second highest in terms of atomic ratio, and when crystal structure analysis is performed on the boron nitride particles 14, a main crystal structure is a cubic crystal or a wurtzite hexagonal crystal. For the elemental analysis, for example, a qualitative and quantitative analysis based on an energy dispersive X-ray spectroscopy (EDX) is used. For the crystal structure analysis, for example, an X-ray diffraction method, or an electron diffraction method using a transmission electron microscope is used. A total content of B and N in the boron nitride particles 14 may be higher than other elements, and is preferably more than 50 atomic %, and more preferably 60 atomic % or more.
An average particle diameter of the boron nitride particles 14 is 10 μm or less, preferably 0.2 μm or more and 9 μm or less, and more preferably 0.5 μm or more and 8 μm or less. By setting the average particle diameter of the boron nitride particles 14 within the above range, when the boron nitride particles 14 adhere to the surface of the metal body 11 and is subjected to the thixotropic molding, the boron nitride particles 14 can be uniformly distributed, and the boron nitride particles 14 are less likely to fall off from the surface of the metal body 11. As a result, a thixotropically molded product in which sites derived from the boron particles nitride 14 are satisfactorily dispersed can be produced.
When the average particle diameter of the boron nitride particles 14 is less than the above lower limit value, since the particle diameter of the boron nitride particles 14 is too small, for example, in a stirring step during the thixotropic molding, aggregation of the boron nitride particles 14 and adhesion of the boron nitride particles 14 to a container used in the stirring step are likely to occur, and dispersibility of the boron nitride particles 14 may be reduced. In addition, since the particle diameter of the boron nitride particles 14 is too small, the boron nitride particles 14 may not sufficiently improve the wear resistance of the thixotropically molded product. On the other hand, when the average particle diameter of the boron nitride particles 14 is more than the above upper limit value, the boron nitride particles 14 may be likely to fall off from the surface of the metal body 11. In addition, the boron nitride particles 14 are likely to become a starting point of breakage, and the mechanical strength of the thixotropically molded product may decrease.
The average particle diameter of the boron nitride particles 14 is a value obtained by measuring the particle diameters of the boron nitride particles 14 from an observation image of the boron nitride particles 14 magnified and observed with a microscope and averaging 100 or more pieces of measurement data. As the microscope, for example, a scanning electron microscope or an optical microscope is used. The particle diameters of the boron nitride particles 14 are intermediate values between a length of the major axis and a length of the minor axis in the observation image of the boron nitride particles 14.
A ratio of the boron nitride particles 14 to a total of the metal body 11 and the boron nitride particles 14 is 2.0 mass % or more and 15.0 mass % or less. By setting the ratio of the boron nitride particles 14 within the above range, the boron nitride particles 14 are less likely to fall off from the metal body 11. In addition, a ratio of the site derived from the metal body 11 to the site derived from the boron nitride particles 14 can be optimized in the produced thixotropically molded product. Accordingly, in the thixotropically molded product, a good balance can be achieved between the characteristics derived from Mg and the characteristics derived from c-BN or w-BN. That is, it is possible to obtain a thixotropically molded product in which high specific strength and specific rigidity derived from Mg are further enhanced. In addition, good wear resistance can be imparted to the thixotropically molded product. The ratio of the boron nitride particles 14 is preferably 3.0 mass % or more and 12.0 mass % or less, and more preferably 5.0 mass % or more and 10.0 mass % or less.
When the ratio of the boron nitride particles 14 is less than the above lower limit value, specific strength and specific rigidity of the produced thixotropically molded product cannot be sufficiently enhanced. In addition, the wear resistance of the thixotropically molded product cannot be sufficiently enhanced. On the other hand, when the ratio of the boron nitride particles 14 is more than the above upper limit value, since the boron nitride particles 14 are excessive, the mechanical strength of the produced thixotropically molded product decreases, leading to a decrease in specific strength and also a decrease in elongation.
The coating portion 12 may contain a substance other than the boron nitride particles 14. In this case, a content of the substance other than the boron nitride particles 14 may be less than the content of the boron nitride particles 14 in terms of mass ratio, and is preferably 30 mass % or less, and more preferably 10 mass % or less of the boron nitride particles 14.
The boron nitride particles 14 may be subjected to any surface treatment as necessary. Examples of the surface treatment include a plasma treatment, a corona treatment, an ozone treatment, an ultraviolet irradiation treatment, a roughening treatment, and a coupling agent treatment.
In the embodiment, the bonding portion 13 includes the interposed particles 15 in the form of particles. As shown in
The interposed particles 15 have an average particle diameter smaller than that of the boron nitride particles 14. Since such interposed particles 15 are minute, the interposed particles 15 easily enter between the metal body 11 and the boron nitride particles 14 or between the boron nitride particles 14. It is considered that the interposed particles 15 strongly interact with both the metal body 11 and the boron nitride particles 14 since the interposed particles 15 have a very large specific surface area. Examples of the interaction include an intermolecular force such as a hydrogen bond and a Van Der Waals force, and an anchor effect caused by an aggregate of the interposed particles 15 entering irregularities present on the surface of the metal body 11. In addition, hydroxy groups are often present in a high density on surfaces of the interposed particles 15 made of an inorganic material. The hydroxy group forms a hydrogen bond with the metal body 11 and the boron nitride particles 14, which is considered to be a driving force for the interaction. Due to such interaction, the bonding portion 13 has a function of fixing the boron nitride particles 14 to the surface of the metal body 11.
Since the interposed particles 15 firmly fix the metal body 11 and the boron nitride particles 14, the boron nitride particles 14 are less likely to fall off. Therefore, when the thixotropic molding material 10 is charged into the heating cylinder 7 during the thixotropic molding, the semi-molten material of the metal body 11 and the boron nitride particles 14 are likely to be uniformly mixed. Accordingly, the boron nitride particles 14 and the interposed particles 15 can be uniformly dispersed in the thixotropically molded product.
The interposed particles 15 are particles made of an inorganic material. Examples of the inorganic material include oxides such as silicon oxide, aluminum oxide, and zirconium oxide, various nitrides, and various carbides. Among these, the oxide in particular has hydroxy groups in a high density on the surfaces of the interposed particles 15. The hydroxy group forms a hydrogen bond with the metal body 11 and the boron nitride particles 14, which is considered to be a driving force for the interaction. Due to such interaction, the bonding portion 13 has a function of fixing the boron nitride particles 14 to the surface of the metal body 11.
The oxide is less likely to vaporize and is less likely to have a bad influence on the characteristics of the thixotropically molded product even when incorporated into the thixotropically molded product. Therefore, occurrence of molding defects due to vaporization is prevented, and a thixotropically molded product having excellent characteristics is obtained.
The oxide is particularly preferably a silicon oxide. The silicon oxide combines with magnesium to form a compound and functions as a reinforcing material that reinforces the mechanical properties of the thixotropically molded product. Therefore, by using the interposed particles 15 containing the silicon oxide, it is possible to obtain the thixotropic molding material 10 with which a thixotropically molded product having excellent mechanical properties while preventing the occurrence of molding defects due to the vaporization can be produced.
The silicon oxide acts to enhance wettability between the boron nitride particles 14 and the semi-molten material of the metal body 11 during the thixotropic molding. That is, since the interposed particles 15 are present adjacent to the boron nitride particles 14 in the thixotropic molding material 10, the interposed particles 15 are interposed between the boron nitride particles 14 and the metal body 11, thereby enhancing the affinity therebetween. Accordingly, in the produced thixotropically molded product, adhesion between the site derived from the boron nitride particles 14 and the site derived from the metal body 11 can be enhanced. As a result, it is possible to produce a thixotropically molded product which is denser and excellent in mechanical properties.
In the present specification, the “silicon oxide” refers to a substance represented by a composition formula of SiOx (0<x≤2).
Containing the silicon oxide as a main component refers to that, when the elemental analysis is performed on the interposed particles 15, a content of one of Si and O is the highest and a content of the other is the second highest in terms of atomic ratio. For the elemental analysis, for example, a qualitative and quantitative analysis based on an energy dispersive X-ray spectroscopy (EDX) is used. A total content of Si and O in the interposed particles 15 may be higher than that of other elements, and is preferably more than 50 atomic %, more preferably 60 atomic % or more, and still more preferably 80 atomic % or more.
The interposed particles 15 may contain impurities in addition to the inorganic material described above. An allowable amount of the impurities is preferably 30 mass % or less, and more preferably 10 mass % or less of the interposed particles 15. Accordingly, inhibition of the effect due to the impurities is sufficiently reduced.
When the interposed particles 15 contain the silicon oxide, the silicon oxide may be crystalline, and is preferably amorphous. So-called amorphous silica is distributed under names of colloidal silica, fumed silica, or the like, and has a stable quality. Therefore, by using the interposed particles 15 containing the amorphous silica, it is possible to form the bonding portion 13 containing few coarse particles. As a result, the thixotropic molding material 10 that enables stable thixotropic molding is obtained.
As described above, it is sufficient that the average particle diameter of the interposed particles 15 is smaller than the average particle diameter of the boron nitride particles 14. Specifically, the average particle diameter of the interposed particles 15 is preferably 20% or less, more preferably 10% or less, and still more preferably 5% or less of the average particle diameter of the boron nitride particles 14. Accordingly, the interposed particles 15 are particularly likely to enter between the metal body 11 and the boron nitride particles 14 and between the boron nitride particles 14. The specific surface area of the interposed particles 15 is also particularly large.
The lower limit value may not necessarily be set, and is preferably 0.01% or more, more preferably 0.05% or more, and still more preferably 0.10% or more of the boron nitride particles 14 from the viewpoint of easy aggregation of the interposed particles 15 and difficulty in handling of the interposed particles 15.
The average particle diameter of the interposed particles 15 is preferably 1 nm or more and 100 nm or less, more preferably 10 nm or more and 80 nm or less, and still more preferably 20 nm or more and 60 nm or less. When the average particle diameter is within the above range, the interposed particles 15 are particularly likely to enter between the metal body 11 and the boron nitride particles 14 or between the boron nitride particles 14. The specific surface area of the interposed particles 15 is also particularly large. Further, when the average particle diameter is within the above range, aggregation of the interposed particles 15 is prevented.
The average particle diameter of the interposed particles 15 is a value obtained by measuring an intermediate value between a length of a major axis and a length of a minor axis of the interposed particles 15 as the particle diameter from an observation image of the interposed particles 15 magnified and observed with a microscope and averaging 100 or more pieces of measurement data. As the microscope, for example, a field emission scanning electron microscope or a transmission electron microscope is preferably used.
An addition amount of the interposed particles 15 is preferably 0.5 parts by mass or more and 12.0 parts by mass or less, more preferably 3.0 parts by mass or more and 11.0 parts by mass or less, and still more preferably 5.0 parts by mass or more and 10.0 parts by mass or less, when a total mass of the metal body 11 and the boron nitride particles 14 is 100 parts by mass. By setting the addition amount of the interposed particles 15 within the above range, it is possible to sufficiently secure a bonding function by the interposed particles 15 and to prevent generation of the excess interposed particles 15. When the addition amount of the interposed particles 15 is less than the above lower limit value, the interposed particles 15 are insufficient, and thus the boron nitride particles 14 may fall off from the metal body 11 depending on a surface state or the like of the metal body 11. On the other hand, when the addition amount of the interposed particles 15 is more than the above upper limit value, the interposed particles 15 are excessive, and thus mechanical properties of the thixotropically molded product may decrease, an effect due to the addition of the boron nitride particles 14 may decrease, or aggregates of the interposed particles 15 may be generated.
The interposed particles 15 may be subjected to any surface treatment as necessary. Examples of the surface treatment include a plasma treatment, a corona treatment, an ozone treatment, an ultraviolet irradiation treatment, a roughening treatment, and a coupling agent treatment.
The bonding portion 13 may contain a substance other than the interposed particles 15. In this case, a content of the substance other than the interposed particles 15 may be less than the content of the interposed particles 15 in terms of mass ratio, and is preferably 10 mass % or less, and more preferably 5 mass % or less of the interposed particles 15.
Examples of the substance other than the interposed particles 15 include a resin. The resin increases a bonding force of the bonding portion 13. In addition, by using the interposed particles 15 and the resin in combination, it is possible to obtain the above-described effect while reducing an amount of the resin to be used.
Examples of the resin include various resins such as a polyolefin, an acrylic resin, a styrene-based resin, a polyester, a polyether, polyvinyl alcohol, polyvinyl pyrolidone, and a copolymer thereof, waxes, alcohols, higher fatty acids, fatty acid metals, higher fatty acid esters, higher fatty acid amides, nonionic surfactants, and silicone-based lubricants.
Examples of the polyolefin include a polyethylene, a polypropylene, and an ethylene-vinyl acetate copolymer. Examples of the acrylic resin include polymethyl methacrylate and polybutyl methacrylate. Examples of the styrene-based resin include a polystyrene. Examples of the polyester include polyvinyl chloride, polyvinylidene chloride, polyamide, polyethylene terephthalate, and polybutylene terephthalate.
The resin may be a mixture containing at least one of the above components and another component, or may be a mixture containing two or more of the above components.
Among these, the resin preferably contains waxes, and more preferably contains a paraffin wax or a derivative thereof. The wax has good binding capacity.
Examples of the waxes include natural waxes such as a plant wax, an animal wax and a mineral wax, synthetic waxes such as a synthetic hydrocarbon, a modified wax, a hydrogenated wax, a fatty acid, an acid amide, and an ester.
Examples of the plant wax include a candelilla wax, a carnauba wax, a rice wax, a Japan wax, and jojoba oil. Examples of the animal wax include a beeswax, lanolin, and spermaceti. Examples of the mineral wax include a montan wax, ozokerite, and ceresin.
Examples of the synthetic hydrocarbon include a polyethylene wax. Examples of the modified wax include montan wax derivatives, paraffin wax derivatives, and microcrystalline wax derivatives. Examples of the hydrogenated wax include hardened castor oil and hardened castor oil derivatives. Examples of the fatty acid include 12-hydroxystearic acid. Examples of the acid amide include stearic acid amides. Examples of the ester include phthalic anhydride ester.
When the bonding portion 13 contains a resin, the interposed particles 15 may be omitted. That is, the bonding portion 13 may be made of only resin. In this case, the bonding portion 13 also provides the above-described bonding action.
An addition amount of the resin is preferably 0.5 parts by mass or more and 12.0 parts by mass or less, and more preferably 3.0 parts by mass or more and 6.0 parts by mass or less, when a total mass of the metal body 11 and the boron nitride particles 14 is 100 parts by mass. By setting the addition amount of the resin within the above range, an excess amount of the resin can be reduced, and the function of the bonding portion 13 can be sufficiently secured while preventing the generation of a large amount of gas during the thixotropic molding.
As described above, the thixotropic molding material 10 according to the first embodiment includes the metal body 11, the boron nitride particles 14, and the bonding portion 13. The metal body 11 contains Mg as a main component. The boron nitride particles 14 adhere to the surface of the metal body 11, and contain c-BN or w-BN as a main component. The bonding portion 13 is interposed between the metal body 11 and the boron nitride particles 14. The average particle diameter of the boron nitride particles 14 is 10.0 μm or less. The ratio of the boron nitride particles 14 to the total of the metal body 11 and the boron nitride particles 14 is 2.0 mass % or more and 15.0 mass % or less.
In such a thixotropic molding material 10, c-BN or w-BN as a main component of the boron nitride particles 14 has high yield strength and elastic modulus. In addition, refinement of the Mg crystals is achieved with the boron nitride particles 14. Therefore, a thixotropically molded product produced using the thixotropic molding material 10 has high mechanical strength. Further, the boron nitride particles 14 have high hardness derived from c-BN or w-BN. Therefore, the thixotropically molded product produced using the thixotropic molding material 10 has high wear resistance.
The bonding portion 13 is formed of the interposed particles 15 which are made of an inorganic material and have an average particle diameter smaller than that of the boron nitride particles 14, or a resin.
In such a thixotropic molding material 10, the bonding portion 13 penetrates between the metal body 11 and the boron nitride particles 14 and between the boron nitride particles 14, and acts to bond them. Accordingly, a thixotropically molded product in which the boron nitride particles 14 are satisfactorily dispersed can be produced.
Next, a method of producing a thixotropic molding material according to a second embodiment will be described. In the following description, a method of producing the above-described thixotropic molding material 10 will be described as an example.
The method of producing the thixotropic molding material 10 shown in
In the preparation step S102, a mixture containing the metal body 11, the boron nitride particles 14, a binder, and a dispersion medium is prepared. The mixture is a dispersion liquid in which the metal body 11, the boron nitride particles 14, and the binder are dispersed using a sufficient amount of dispersion medium. Here, the binder contains the above-described interposed particles 15.
The dispersion medium is not particularly limited as long as the dispersion medium does not modify the metal body 11, the boron nitride particles 14, and the interposed particles 15. Examples of the dispersion medium include water, isopropyl alcohol, and acetone. In this step, a mixture produced in advance may be prepared. By containing water in the dispersion medium, it is possible to introduce hydroxy groups in a higher density to the surfaces of the metal body 11, the boron nitride particles 14, and the interposed particles 15.
As described above, the average particle diameter of the boron nitride particles 14 contained in the thixotropic molding material 10 is 10 μm or less. Accordingly, in the stirring step S104 to be described later, adhesion of the boron nitride particles 14 to the metal body 11 can be enhanced. As a result, the thixotropic molding material 10 having a stable quality can be produced, and finally, a thixotropically molded product in which the boron nitride particles 14 are satisfactorily dispersed can be obtained.
As described above, the ratio of the boron nitride particles 14 to the total of the metal body 11 and the boron nitride particles 14 is 2.0 mass % or more and 15.0 mass % or less. By setting the ratio of the boron nitride particles 14 within the above range, the boron nitride particles 14 are less likely to fall off from the metal body 11. In addition, a ratio of the site derived from the metal body 11 to the site derived from the boron nitride particles 14 can be optimized in the produced thixotropically molded product.
In the stirring step S104, the mixture is stirred. For stirring, for example, a method of using a stirring rod or a stirring bar, or a method of shaking a container containing a mixture in a state of being covered with a lid is used. By such stirring, the boron nitride particles 14 can adhere to the surface of the metal body 11 via the interposed particles 15. A part of the boron nitride particles 14 may directly adhere to the surface of the metal body 11 without the interposed particles 15 interposed therebetween. At this stage, the boron nitride particles 14 may adhere to the surface of the metal body 11 with a weak adhesive force. In addition, by stirring, aggregation of the metal body 11, aggregation of the boron nitride particles 14, and aggregation of the interposed particles 15 can be prevented.
In the drying step S106, the mixture is dried. Accordingly, the boron nitride particles 14 adhering to the surface of the metal body 11 via the interposed particles 15 adhere to the metal body 11 more firmly. For example, when hydroxy groups present on the surfaces of the interposed particles 15 and hydroxy groups present on the surfaces of the metal body 11 or the boron nitride particles 14 are bonded to each other by a weak adhesive force due to a hydrogen bond or the like, dehydration shrinkage occurs through this step, and the boron nitride particles 14 and the metal body 11 are bonded to each other by a stronger adhesive force. For example, silanol groups are present on the surfaces of the interposed particles 15. Through this step, the dehydration shrinkage occurs in the silanol groups, siloxane bonds are generated between the interposed particles 15, and the interposed particles 15 act like an adhesive. In this way, the boron nitride particles 14 are fixed to the metal body 11. When a resin is added to the mixture, the resin is melted by heating in the drying step S106 and solidified, and the boron nitride particles 14 are fixed.
In addition, the dispersion medium contained in the mixture can be sufficiently removed by drying. Accordingly, a vaporized component is sufficiently removed, and the thixotropic molding material 10 capable of preventing occurrence of molding defects due to vaporization during thixotropic molding is obtained. According to such a thixotropic molding material 10, it is possible to produce a dense thixotropically molded product with few pores.
For drying, a method of heating the mixture, a method of exposing the mixture to a gas, or the like is used. Among these, in the case of heating the mixture, for example, the entire container containing the mixture may be heated using a hot bath or the like. In the drying step S106, all the dispersion medium in the mixture may be removed, or a part of the dispersion medium may remain without being removed.
As described above, the thixotropic molding material 10 is obtained. When the mixture contains a resin, a degreasing treatment may be performed on the thixotropic molding material 10 after the drying step S106.
As described above, the method of producing the thixotropic molding material according to the embodiment includes the preparation step S102, the stirring step S104, and the drying step S106. In the preparation step S102, a mixture containing the metal body 11 containing Mg as a main component, the boron nitride particles 14 containing c-BN or w-BN as a main component, a binder, and a dispersion medium is prepared. In the stirring step S104, the mixture is stirred. In the drying step S106, the boron nitride particles 14 adhere to the surface of the metal body 11 via the binder by removing at least a part of the dispersion medium from the stirred mixture. Further, the average particle diameter of the boron nitride particles 14 is 10.0 μm or less. In the mixture, the ratio of the boron nitride particles 14 to the total of the metal body 11 and the boron nitride particles 14 is 2.0 mass % or more and 15.0 mass % or less.
According to such a production method, since the metal body 11 and the boron nitride particles 14 are more firmly fixed to each other via the binder, it is possible to produce the thixotropic molding material 10 in which the boron nitride particles 14 are less likely to fall off. In such a thixotropic molding material 10, the boron nitride particles 14 and the binder can be uniformly dispersed during the thixotropic molding. As a result, refinement of the Mg crystals can be achieved in the entire thixotropically molded product. Accordingly, a thixotropically molded product produced using the thixotropic molding material 10 has high mechanical strength. In addition, the boron nitride particles 14 have high hardness derived from c-BN or w-BN. Therefore, the thixotropically molded product produced using the thixotropic molding material 10 has high wear resistance.
Next, a thixotropically molded product according to a third embodiment will be described. In the following description, a thixotropically molded product produced using the interposed particles 15 containing a silicon oxide will be described as an example.
The thixotropically molded product 100 shown in
The matrix portion 200 contains Mg as a main component. The first particle portions 300 contain c-BN or w-BN as a main component. The second particle portions 400 contain Mg2Si as a main component. The third particle portions 500 contain MgO as a main component. The second particle portions 400 and the third particle portions 500 are sites derived from the interposed particles 15. Further, an area fraction of the first particle portions 300 in a cross section of the thixotropically molded product 100 is 2.0% or more and 20.0% or less.
In such a thixotropically molded product 100, refinement of the Mg crystals can be achieved with the first particle portions 300. Accordingly, high specific strength and specific rigidity derived from the matrix portion 200 can be further enhanced.
In addition, the second particle portions 400 and the third particle portions 500 also contribute to the refinement of the Mg crystals. At least a part of Mg2Si or MgO is distributed to be adjacent to the first particle portions 300. Accordingly, wettability between the first particle portions 300 and the matrix portion 200 can be enhanced. As a result, the thixotropically molded product 100 is dense and is particularly excellent in mechanical properties.
Further, the boron nitride particles 14 have high hardness derived from c-BN or w-BN. Therefore, high wear resistance is imparted to the thixotropically molded product in which the boron nitride particles 14 are dispersed. Accordingly, when the thixotropically molded product is applied to, for example, a sliding mechanical component, a wear amount in a sliding portion can be reduced. As a result, the mechanical component to which the thixotropically molded product is applied has excellent durability of the sliding portion.
The matrix portion 200 contains Mg as a main component. Containing Mg as a main component refers to that, when elemental analysis is performed on a cross section of the matrix portion 200, a content of Mg is the highest in terms of atomic ratio. For the elemental analysis, for example, a qualitative and quantitative analysis based on an energy dispersive X-ray spectroscopy (EDX) is used. The content of Mg in the matrix portion 200 may be higher than that of other elements, and is preferably more than 50 atomic %, more preferably 70 atomic % or more, and still more preferably 80 atomic % or more. During identification of the matrix portion 200 in the qualitative and quantitative analysis, the matrix portion 200 can be distinguished based on a contrast with other sites or a color tone in, for example, an observation image from a scanning electron microscope or an optical microscope. The matrix portion 200 may contain additives or impurities other than Mg.
The matrix portion 200 occupies the highest area fraction in the cross section of the thixotropically molded product 100. Therefore, the matrix portion 200 has a dominant influence on the mechanical properties of the thixotropically molded product 100. Accordingly, high specific rigidity and high specific strength of Mg are reflected in the thixotropically molded product 100.
The matrix portion 200 is a site derived from the metal body 11, for example, when the matrix portion 200 is produced using the above-described thixotropic molding material 10. In this case, the matrix portion 200 is a site having a composition substantially the same as that of the metal body 11.
An average particle diameter of the Mg crystals in the thixotropically molded product 100 is preferably 1.0 μm or more and 8.0 μm or less, more preferably 2.0 μm or more and 7.0 μm or less, and still more preferably 3.0 μm or more and 6.0 μm or less.
When the average particle diameter of the Mg crystals is within the above range, a movement of dislocation is particularly less likely to occur at a grain boundary of the Mg crystals. Therefore, the mechanical strength and the rigidity of the thixotropically molded product 100 can be particularly enhanced.
The Mg crystals can be identified on an image by performing crystal orientation analysis (EBSD analysis) on a cut surface of the matrix portion 200. Accordingly, an intermediate value between a length of a major axis and a length of a minor axis of the Mg crystals identified on the image can be set as a particle diameter of the Mg crystals. The average particle diameter of the Mg crystals can be obtained by averaging 100 or more measured particle diameters.
The first particle portions 300 contain c-BN or w-BN as a main component. Containing c-BN or w-BN as a main component refers to that, when the elemental analysis is performed on the first particle portions 300, a content of one of B and N is the highest and a content of the other is the second highest in terms of atomic ratio. For the elemental analysis, for example, a qualitative and quantitative analysis based on an energy dispersive X-ray spectroscopy (EDX) is used. For the crystal structure analysis, for example, an X-ray diffraction method, or an electron diffraction method using a transmission electron microscope is used. A total content of B and N in the first particle portions 300 may be higher than other elements, and is preferably more than 50 atomic %, and more preferably 60 atomic % or more. During the identification of the first particle portions 300 in the qualitative and quantitative analysis, the first particle portions 300 can be distinguished based on a contrast with other sites or a color tone in, for example, an observation image from a scanning electron microscope or an optical microscope. The first particle portions 300 may contain additives or impurities other than c-BN or w-BN.
In the observation image of the cross section of the thixotropically molded product 100 shown in
At this time, the area fraction described above is 2.0% or more and 20.0% or less. By setting the area fraction within the above range, the above-described effect by the first particle portions 300, specifically, the effect of further enhancing high specific strength and specific rigidity derived from Mg can be obtained. Therefore, when the area fraction is less than the above lower limit value, the first particle portions 300 are insufficient, and thus the above-described effect cannot be obtained. On the other hand, when the area fraction is more than the above upper limit value, the first particle portions 300 are excessive, and the specific strength of the thixotropically molded product 100 decreases. The area fraction is preferably 3.0% or more and 15.0% or less, and more preferably 4.0% or more and 10.0% or less.
The area fraction in the range A is calculated as follows. First, a range of the first particle portions 300 is extracted by image processing in the range A. For the image processing, for example, image analysis software OLYMPUS Stream or the like can be used. A magnification of the observation image is preferably 300 times or more. Next, a proportion of the area of the first particle portion 300 to the entire area of the range A is calculated. The proportion is defined as the area fraction.
An average particle diameter of the first particle portions 300 is 10.0 μm or less, and preferably 0.5 μm or more and 8.0 μm or less. Accordingly, the first particle portions 300 are less likely to become a starting point of breakage, and the mechanical strength of the thixotropically molded product 100 can be enhanced without impairing the rigidity of the matrix portion 200.
The average particle diameter of the first particle portions 300 is calculated as follows. First, a length of a major axis and a length of a minor axis of each of the first particle portions 300 included in the range A are determined. Next, an intermediate value between the length of the minor axis and the length of the major axis is determined. An average value of the intermediate values calculated in this manner is the average particle diameter of the first particle portions 300.
An average aspect ratio of the first particle portions 300 is preferably 4.0 or less, more preferably 3.0 or less, and still more preferably 2.0 or less. When the average aspect ratio of the first particle portions 300 is within the above range, anisotropy of structures of the first particle portions 300 is reduced. Therefore, the mechanical strength and the rigidity of the thixotropically molded product 100 can be isotropically enhanced.
The average aspect ratio of the first particle portions 300 is calculated as follows. First, a length of a major axis and a length of a minor axis of each of the first particle portions 300 included in the range A are determined. Next, a ratio of the length of the major axis to the length of the minor axis is referred to as an “aspect ratio”. An average value of the aspect ratios calculated in this manner is the average aspect ratio of the first particle portions 300.
The second particle portions 400 contain Mg2Si as a main component, and have a particulate shape. Mg2Si has a tensile elastic modulus (Young's modulus) higher than that of the matrix portion 200. Therefore, the second particle portions 400 function as a reinforcing material for enhancing the rigidity of the thixotropically molded product 100. Accordingly, the thixotropically molded product 100 has higher creep resistance.
Containing Mg2Si as a main component refers to that, when elemental analysis is performed on cross sections of the second particle portions 400, a content of Mg is the highest and a content of Si is the second highest in terms of atomic ratio. For the elemental analysis, for example, a qualitative and quantitative analysis based on an energy dispersive X-ray spectroscopy (EDX) is used. A total content of Mg and Si in the second particle portions 400 may be higher than other elements, and is preferably more than 50 atomic %, and more preferably 60 atomic % or more. During the identification of the second particle portions 400 in the qualitative and quantitative analysis, the second particle portions 400 can be distinguished based on a contrast with the matrix portion 200 and other sites or a color tone in, for example, an observation image from a scanning electron microscope or an optical microscope. The second particle portions 400 may contain additives or impurities other than Mg2Si.
The second particle portions 400 also have a function of preventing coarsening of the Mg crystals contained in the matrix portion 200. Therefore, in the thixotropically molded product 100, refinement of the Mg crystals in the matrix portion 200 is achieved. Accordingly, the thixotropically molded product 100 has high mechanical strength.
An average particle diameter of the second particle portions 400 is preferably 10.0 μm or less, more preferably 0.1 μm or more and 8.0 μm or less, and still more preferably 0.3 μm or more and 5.0 μm or less. Accordingly, the second particle portions 400 can effectively contribute to the refinement of the Mg crystals and can be prevented from becoming a starting point of breakage. As a result, specific strength and specific rigidity of the thixotropically molded product 100 can be particularly enhanced.
The average particle diameter of the second particle portions 400 is calculated as follows. First, a length of a major axis and a length of a minor axis of each of the second particle portions 400 included in the range A are determined. Next, an intermediate value between the length of the minor axis and the length of the major axis is determined. An average value of the intermediate values calculated in this manner is the average particle diameter of the second particle portions 400.
A cross-sectional shape of the second particle portion 400 is not particularly limited and may be any shape, and an average aspect ratio thereof is preferably less than 2.0, and more preferably 1.5 or less. Accordingly, the second particle portions 400 are less likely to become a starting point that causes a crack in the thixotropically molded product 100, and thus an effect of enhancing the mechanical strength of the thixotropically molded product 100 is obtained. In addition, the effect of preventing coarsening of the Mg crystals is more remarkable.
An aspect ratio of the second particle portions 400 is a ratio of a length to a width of the second particle portions 400 in the cross section of the thixotropically molded product 100. The length is a maximum length that can be taken in a cross section of the second particle portion 400, and the width is a maximum length in a direction orthogonal to a direction of the maximum length. Then, ten second particle portions 400 are randomly extracted from the observation image of the cross section of the thixotropically molded product 100, and an average value of the aspect ratios of the second particle portions is the average aspect ratio. For example, an optical microscope or an electron microscope is used to acquire the observation image.
In addition, as described above, the second particle portions 400 also contribute to enhancing the wettability between the first particle portions 300 and the matrix portion 200. Accordingly, voids and the like are less likely to be generated between the first particle portions 300 and the matrix portion 200, and the thixotropically molded product 100 can be made denser.
The third particle portions 500 contain MgO as a main component, and have a particulate shape. MgO has a tensile elastic modulus (Young's modulus) higher than that of the matrix portion 200. Therefore, the third particle portions 500 function as a reinforcing material for enhancing the rigidity of the thixotropically molded product 100. As a result, the thixotropically molded product 100 has higher rigidity.
Containing MgO as a main component refers to that, when elemental analysis is performed on cross sections of the third particle portions 500, a content of one of Mg and O is the highest and a content of the other is the second highest in terms of atomic ratio. For the elemental analysis, for example, a qualitative and quantitative analysis based on an energy dispersive X-ray spectroscopy (EDX) is used. A total content of Mg and O in the third particle portions 500 may be higher than other elements, and is preferably more than 50 atomic %, and more preferably 60 atomic % or more. During the identification of the third particle portions 500 in the qualitative and quantitative analysis, the third particle portions 500 can be distinguished based on a contrast with the matrix portion 200 and other sites or a color tone in, for example, an observation image from a scanning electron microscope or an optical microscope. The third particle portions 500 may contain additives or impurities other than MgO.
In addition, the third particle portions 500 have a function of preventing coarsening of the Mg crystals contained in the matrix portion 200. Therefore, in the thixotropically molded product 100, refinement of the Mg crystals in the matrix portion 200 is achieved. Accordingly, the thixotropically molded product 100 has high mechanical strength.
Further, the third particle portions 500 also have a function of inhibiting abnormal growth of the second particle portion 400 in a branch shape or a needle shape. Due to the function, the second particle portion 400 tends to have an isotropic shape, and an increase in the average aspect ratio is prevented.
In addition, as described above, the third particle portions 500 also contribute to enhancing the wettability between the first particle portions 300 and the matrix portion 200. Therefore, at least a part of the third particle portions 500 are preferably adjacent to the first particle portions 300. Accordingly, voids or the like are less likely to be generated between the first particle portions 300 and the matrix portion 200, and the thixotropically molded product 100 can be made denser.
Further, when the third particle portions 500 are exposed to the surface of the thixotropically molded product 100, lubricity of the sliding portion is enhanced due to a solid lubrication effect. Accordingly, frictional resistance can be reduced, and durability of the sliding portion can be enhanced.
An average particle diameter of the third particle portions 500 is preferably smaller than the average particle diameter of the second particle portions 400, and is more preferably 1% or more and 60% or less, and still more preferably 3% or more and 40% or less of the average particle diameter of the second particle portions 400. Accordingly, the third particle portion 500 exerts the above-described function without becoming a starting point of breakage or inhibiting the function of the first particle portions 300 or the second particle portions 400. As a result, specific strength and specific rigidity of the thixotropically molded product 100 can be particularly enhanced.
The average particle diameter of the third particle portions 500 is calculated as follows. First, a length of a major axis and a length of a minor axis of each of the third particle portions 500 included in the range A are determined. Next, an intermediate value between the length of the minor axis and the length of the major axis is determined. An average value of the intermediate values calculated in this manner is the average particle diameter of the third particle portions 500.
A total area of the second particle portions 400 and the third particle portions 500 in the cross section of the thixotropically molded product 100 is preferably 1.0 or more and 30.0 or less, more preferably 2.0 or more and 25.0 or less, and still more preferably 3.0 or more and 20.0 or less, when the area of the first particle portions 300 is 100. Accordingly, the second particle portions 400 and the third particle portions 500 contribute to the refinement of the Mg crystals without impairing the effect of the first particle portions 300. That is, it is possible to improve a balance of an existence ratio of the first particle portions 300, the second particle portions 400, and the third particle portions 500. As a result, the mechanical properties and the wear resistance of the thixotropically molded product 100 can be particularly enhanced.
When the first particle portions 300, the second particle portions 400, and the third particle portions 500 are identified on the image in the range A, in particular, it is possible to identify using a difference in a color space on the image acquired by the optical microscope. For example, when a color space of the image is set to a HSV color space, a hue of the first particle portions 300 is in a range of 120 deg or more and 210 deg or less, and a hue of the second particle portions 400 is in a range of 15 deg or more and 85 deg or less. Therefore, the first particle portions 300 and the second particle portions 400 can be distinguished from each other by using a difference in hue. Since a hue of the third particle portions 500 is also different from the hues of the first particle portions 300 and the second particle portions 400, the third particle portions 500 can be distinguished on the image.
Depending on imaging conditions, a color degree of the first particle portions 300 is 10% or more and 45% or less, and a color degree of the second particle portions 400 is 3% or more and 60% or less. Further, brightness of the first particle portions 300 is 55% or more and 80% or less, and brightness of the second particle portions 400 is 0% or more and 80% or less.
In a case of acquiring an image for obtaining the area of each particle portion, the following conditions are given as the imaging conditions.
In the cross section of the thixotropically molded product 100, among aggregates including at least one of the first particle portions 300, the second particle portions 400, and the third particle portions 500, one having a particle diameter exceeding 30 μm is referred to as an “aggregated portion”. As long as the particle diameter of the aggregated portion is more than 30 μm, a combination of types of the particle portions is optional.
Such an aggregated portion may inhibit uniform dispersion of each particle portion, and is preferably as small as possible. On the other hand, depending on an existence ratio of each particle portion, it may be unavoidable that the aggregated portion is formed in probability.
Therefore, in the thixotropically molded product 100, a ratio of the number of aggregated portions to the total number of measured particles (aggregated portion number density) is preferably 1000 ppm or less, more preferably 800 ppm or less, and still more preferably 500 ppm or less. Accordingly, even when the aggregated portion is included, uniform dispersion of the particle portions is less likely to be inhibited. As a result, the strength and the rigidity of the thixotropically molded product 100 can be sufficiently enhanced, and the wear resistance derived from the first particle portions 300 can be sufficiently enhanced. When the aggregated portion number density is more than the above upper limit value, the uniform dispersion of the particle portions may be inhibited depending on the particle diameter of each particle portion.
The particle diameter of the aggregated portion is measured as follows. First, each particle portion is identified on the image based on a contrast and a color tone. Next, when the above-described aggregate is present, an intermediate value between the length of the major axis and the length of the minor axis is obtained for the aggregate. When the intermediate value is the particle diameter of the aggregate and the particle diameter of the aggregate is more than 30 μm, the aggregate is defined as the “aggregated portion”.
The aggregated portion number density is calculated as follows. First, in the range A of the cross section of the thixotropically molded product 100, a “total number of particles” obtained by summing the number of particles of the first particle portions 300, the number of particles of the second particle portions 400, and the number of particles of the third particle portions 500 included therein is determined, and the number of particles of the aggregated portion is determined. Next, a proportion of the number of particles in the aggregated portion to the total number of particles is calculated. A calculation result is the aggregated portion number density.
In a case of acquiring an image for obtaining the aggregated portion number density, the following conditions are given as the imaging conditions.
Tensile strength of the thixotropically molded product 100 is preferably 170 MPa or more and 350 MPa or less, and more preferably 200 MPa or more and 300 MPa or less. A Young's modulus of the thixotropically molded product 100 is preferably 42 GPa or more and 80 GPa or less, and more preferably 44 GPa or more and 70 GPa or less.
The thixotropically molded product 100 in which the tensile strength and the Young's modulus are within the above ranges has particularly high specific strength and specific rigidity, and can be stably produced. Since such a thixotropically molded product 100 is lightweight and is excellent in mechanical properties, the thixotropically molded product 100 is suitable for, for example, a component used for a transportation device such as an automobile and an aircraft, and a component used in a mobile device such as a mobile terminal and a notebook computer.
The tensile strength of the thixotropically molded product 100 is measured as follows. First, a test piece is cut out from the thixotropically molded product 100. Examples of the test piece include a No. 13 test piece defined in JIS. Next, the test piece is attached to a tensile tester, and a stress corresponding to a maximum force applied to the test piece at 25° C. is calculated. The obtained stress is defined as the tensile strength of the thixotropically molded product 100.
The Young's modulus of the thixotropically molded product 100 is measured as follows. First, a test piece is cut out from the thixotropically molded product 100. Next, the test piece is attached to a tensile tester, and a tensile load is applied to the test piece at 25° C. Next, an amount of change in tensile strain when the tensile load is varied and an amount of change in tensile stress when the tensile load is varied are calculated. Then, a ratio of the latter amount of change to the former amount of change is calculated, and the obtained ratio is defined as the Young's modulus of the thixotropically molded product 100. The Young's modulus of the thixotropically molded product 100 may be a value measured by a method other than the above-described measurement method, for example, a resonance method or an ultrasonic pulse method.
Vickers hardness of the surface 101 of the thixotropically molded product 100 is preferably 75 or more, more preferably 80 or more, and still more preferably 90 or more.
When the Vickers hardness is within the above range, it is possible to implement the thixotropically molded product 100 having high surface hardness, resistance to scratches and the like, and particularly good wear resistance. The upper limit value is not particularly limited, and is preferably 200 or less from the viewpoint of avoiding remarkable wear of a counterpart material sliding on the thixotropically molded product 100.
The Vickers hardness of the surface 101 of the thixotropically molded product 100 is measured in a region within a depth of 500 μm from the surface 101 of the cross section of the thixotropically molded product 100 according to a Vickers hardness test method defined in JIS Z 2244:2009. A measurement load is 5 kgf (49 N), and a load holding time is 10 seconds. For the hardness tester, for example, a multi-Vickers hardness tester FLV-50ARS-F manufactured by FUTURE-TECH CORP. is used.
A thermal conductivity of the thixotropically molded product 100 is preferably 52 W/(m·K) or more, more preferably 54 W/(m·K) or more, and still more preferably 57 W/(m·K) or more. The thixotropically molded product 100 having such a thermal conductivity can also be applied to, for example, a site where heat dissipation is required.
The thermal conductivity of the thixotropically molded product 100 is measured by, for example, a laser flash method.
As described above, the thixotropically molded product 100 according to the third embodiment includes the matrix portion 200 and the first particle portions 300 dispersed in the matrix portion 200. The matrix portion 200 contains Mg as a main component. The first particle portion 300 contains c-BN or w-BN as a main component. Further, in the thixotropically molded product 100, the average particle diameter of the first particle portions 300 in the cross section of the thixotropically molded product is 10.0 μm or less. The area fraction of the first particle portions 300 in the cross section is 2.0% or more and 20.0% or less.
According to such a configuration, the first particle portions 300 function as a reinforcing material, and refinement of the Mg crystals can be achieved. Accordingly, the thixotropically molded product 100 is obtained in which high specific strength and specific rigidity derived from the matrix portion 200 are further enhanced by the first particle portions 300. In addition, the thixotropically molded product 100 has high wear resistance derived from the first particle portions 300. Further, since the thixotropically molded product 100 is produced by thixotropic molding, shape accuracy and dimensional accuracy are excellent.
The average particle diameter of the first particle portions 300 in the cross section of the thixotropically molded product 100 is preferably 0.5 μm or more and 8.0 μm or less.
Accordingly, the first particle portions 300 are less likely to become a starting point of breakage, and mechanical strength of the thixotropically molded product 100 can be enhanced without impairing the rigidity of the matrix portion 200.
The thixotropically molded product 100 according to the third embodiment includes the second particle portions 400 and the third particle portions 500. The second particle portion 400 contains Mg2Si as a main component. The third particle portion 500 contains MgO as a main component.
The second particle portions 400 and the third particle portions 500 contribute to the refinement of the Mg crystals. At least a part of Mg2Si or MgO is distributed to be adjacent to the first particle portion 300. Accordingly, the wettability between the first particle portion 300 and the matrix portion 200 can be enhanced. As a result, the thixotropically molded product 100 is dense and is particularly excellent in mechanical properties.
A site in which at least one of the first particle portions 300, the second particle portions 400, and the third particle portions 500 are aggregated to a size exceeding the particle diameter of 30 μm in the cross section is defined as an aggregated portion. Further, a total of the number of particles in the first particle portions 300, the number of particles in the second particle portions 400, and the number of particles in the third particle portions 500 is defined as the total number of particles. A proportion of the number of aggregated portions to the total number of particles is defined as the aggregated portion number density. At this time, the aggregated portion number density is preferably 1000 ppm or less.
Accordingly, even when the aggregated portion is unavoidably included, uniform dispersion of each particle portion is less likely to be inhibited. As a result, the strength and the rigidity of the thixotropically molded product 100 can be sufficiently enhanced, and the wear resistance derived from the first particle portions 300 can be sufficiently enhanced.
The total area of the second particle portions 400 and the third particle portions 500 in the cross section of the thixotropically molded product 100 is preferably 1.0 or more and 30.0 or less, when the area of the first particle portions 300 is 100. Accordingly, the mechanical properties and the wear resistance of the thixotropically molded product 100 can be particularly enhanced.
The Vickers hardness of the surface 101 of the thixotropically molded product 100 is preferably 75 or more. Accordingly, it is possible to implement the thixotropically molded product 100 having high surface hardness, resistance to scratches and the like, and particularly good wear resistance.
The thixotropically molded product, the thixotropic molding material, and the method of producing the thixotropic molding material according to the present disclosure are described above based on the shown embodiments, and the present disclosure is not limited to the above-described embodiments. For example, the thixotropic molding material and the thixotropically molded product according to the present disclosure may be those obtained by adding any components to the above-described embodiments. The method of producing the thixotropic molding material according to the present disclosure may be a method in which any step is added to the above-described embodiments.
Next, specific Examples of the present disclosure will be described.
First, a magnesium alloy chip as a metal body, a c-BN powder as boron nitride particles, a silica (silicon oxide) powder as interposed particles, and IPA (isopropyl alcohol) as a dispersion medium were mixed to obtain a mixture. A chip of 4 mm×2 mm×1 mm made of an AZ91D alloy manufactured by Nippon Material Co., Ltd. was used as the magnesium alloy chip. The AZ91D alloy is an Mg-based alloy containing 9 mass % of Al and 1 mass % of Zn. Colloidal silica (amorphous silica) obtained by colloidal dispersion in IPA was used as the silica powder. Production conditions for other thixotropic molding materials are as shown in Table 1.
Next, the mixture was stirred. A method of shaking a container containing the mixture was used for stirring.
Next, the stirred mixture was heated and dried. Accordingly, a thixotropic molding material was obtained.
A thixotropic molding material was obtained in the same manner as in the case of Sample No. 1 except that the production conditions for the thixotropic molding material were changed as shown in Table 1. In Sample No. 8, a paraffin wax as resin was used as a binder instead of the silica powder.
A thixotropic molding material was obtained in the same manner as in the case of Sample No. 1 except that addition of the c-BN powder and addition of the silica powder were omitted, that is, only the magnesium alloy chip was subjected to the thixotropic molding.
A thixotropic molding material was obtained in the same manner as in the case of Sample No. 1 except that the addition of the c-BN powder was omitted, that is, only the magnesium alloy chip and the silica r were subjected to the thixotropic molding.
A thixotropic molding material was obtained in the same manner as in the case of Sample No. 1 except that silicon carbide particles were used instead of the boron nitride particles and the production conditions for the thixotropic molding material were changed as shown in Table 1. In Sample No. 13, a paraffin wax as resin was used as a binder instead of the silica powder.
A thixotropic molding material was obtained in the same manner as in the case of Sample No. 1 except that the production conditions for the thixotropic molding material were changed as shown in Table 2. In Sample No. 25, an alumina (aluminum oxide) powder was used instead of the silica powder.
In Tables 1 and 2, among the thixotropic molding materials of respective sample Nos., those corresponding to the present disclosure are indicated by “Examples”, and those not corresponding to the present disclosure are indicated by “Comparative Examples”.
A sieve having an opening of 600 μm and a wire diameter of 500 μm was used to shake the thixotropic molding material of each sample No. Next, a weight was measured before and after shaking, and a weight loss amount was calculated. Then, a weight loss rate was calculated by dividing the calculated weight loss amount by an original weight. Based on the weight loss rate, adhesion of the boron nitride particles in the thixotropic molding material and the particles instead of the boron nitride particles was evaluated. Evaluation results are shown in Tables 1 and 2.
In each Example shown in Table 1, it is found that the adhesion of the boron nitride particles can be sufficiently enhanced by setting the addition amount of the boron nitride particles within a predetermined range. On the other hand, it is found that, when the addition amount of the boron nitride particles is too large, a large amount of boron nitride particles that cannot come into contact with the metal body are generated, resulting in a decrease in adhesion.
In each Example shown in Table 2, it is found that the adhesion of the boron nitride particles can be sufficiently enhanced by setting the average particle diameter of the boron nitride particles within a predetermined range. On the other hand, it is found that, when the average particle diameter of the boron nitride particles is too large, the boron nitride particles are likely to fall off from the metal body due to their own weight.
In each Example shown in Table 2, it is found that the adhesion of the boron nitride particles can be sufficiently enhanced by setting the addition amount of the binder within a predetermined range.
The thixotropic molding material of each sample No. was charged into an injection molding machine and subjected to thixotropic molding to obtain a thixotropically molded product. As the injection molding machine, a magnesium injection molding machine JLM75MG manufactured by The JAPAN STEEL WORKS, LTD. was used.
Next, the thixotropically molded product of each sample No. was cut, and a cut surface was observed with an optical microscope. Further, based on an observation image, the matrix portion, the first particle portions, the second particle portions, and the third particle portions were identified, and an average particle diameter was calculated. An average aspect ratio of the first particle portions was calculated.
Next, an area fraction of the first particle portions was calculated. A ratio of the total area of the second particle portions and the third particle portions to the area of the first particle portions was calculated. The above calculation results are shown in Tables 3 and 4.
Wear resistance of the thixotropically molded product of each sample No. was evaluated by the following procedure.
First, the thixotropically molded product was polished in advance to a thickness of about 100 μm from a surface.
Next, the thixotropically molded product was set in a reciprocating plane wear test device such that a polished surface was a test object. Suga Wear Tester NUS-ISO-3 manufactured by Suga Test Instruments Co., Ltd. was used as the test device. In addition, #240 waterproof abrasive paper was used as test paper. In addition, as test conditions, a pressing load was set to 30 N, and a cumulative friction count was set to 400 times.
Next, a wear test in which the test paper was reciprocated with respect to the thixotropically molded product was performed using the test device. Thereafter, a weight change rate was calculated by dividing an amount of change in a weight of the thixotropically molded product before and after the test by a weight of the thixotropically molded product before the test.
Further, when the weight change rate obtained for the thixotropically molded product of sample No. 10 was set to 100, a relative value of the weight change rate obtained for the thixotropically molded product of each sample No. was calculated as a “normalized weight change rate”. The calculated normalized weight change rates are shown in Tables 3 and 4 as evaluation results of wear resistance.
Tensile strength and tensile elastic modulus (Young's modulus) of the thixotropically molded product of each sample No. were measured. Measurement results are shown in Tables 3 and 4.
Vickers hardness of the thixotropically molded product of each sample No. was measured. Measurement results are shown in Tables 3 and 4.
It is found that the thixotropically molded product of each Example shown in Table 3 has good wear resistance and high tensile strength as compared with the thixotropically molded product of each Comparative Example. In addition, in the thixotropically molded product of each Example, the tensile elastic modulus (Young's modulus) and the Vickers hardness are also good.
Further, it is found that, when the interposed particles are used as the binder, the wear resistance and the mechanical properties of the produced thixotropically molded product are improved as compared with the case where the resin is used as the binder.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2022-211891 | Dec 2022 | JP | national |
