Patent Application
20240218521
The present application is based on, and claims priority from JP Application Serial Number 2022-211894, 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.
For example, WO 2004/062837 discloses a magnesium-based composite material in which a reaction product of magnesium alloy coarse grains and fine powders are dispersed in a magnesium alloy matrix. Examples of the fine powders include γ-alumina, and examples of the reaction product include MgO, Al3Mg2, Mg17Al12, and MgAl2O4. Further, the magnesium-based composite material in which such a reaction product is dispersed is considered to have excellent mechanical properties, excellent wear resistance, a low friction coefficient, and the like.
In particular, γ-alumina does not have sufficient chemical stability, but reacts with Mg and the like through solid-phase reaction synthesis to produce a reaction product. When the reaction product is dispersed in the magnesium alloy matrix, the above-described effects are exhibited.
However, in the magnesium-based composite material described in WO 2004/062837, it is not easy to control a composition and a structure of the reaction product, and it is difficult to stabilize physical properties of the magnesium-based composite material to be produced. Therefore, it is an object to improve mechanical properties such as high specific strength and specific rigidity, electrical properties such as a low electrical conductivity, and thermal properties such as a low linear expansion coefficient in a well-balanced manner for the magnesium-based composite material.
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 particle portion dispersed in the matrix portion and containing α-Al2O3 as a main component. In a cross section, a total area fraction of the particle portion in a range of 500 μm square around a point at a depth of 1 mm from a surface is 0.5% or more and 30.0% 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; alumina particles adhering to a surface of the metal body and containing α-Al2O3 as a main component; and a bonding portion interposed between the metal body and the alumina particles. An average particle diameter of the alumina particles is 20.0 μm or less, and a ratio of the alumina particles to a total of the metal body and the alumina particles is 1.0 mass % or more and 20.0 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, alumina particles adhering to a surface of the metal body and containing α-Al2O3 as a main component, a binder, and a dispersion medium; a stirring step of stirring the mixture; and a drying step of adhering the alumina particles to the 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 alumina particles is 20.0 μm or less, and in the mixture, a ratio of the alumina particles to a total of the metal body and the alumina particles is 1.0 mass % or more and 20.0 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 alumina particles 14 and between the alumina particles 14 to bond them. In the embodiment, as shown in
By performing thixotropic molding using the thixotropic molding material 10 including the bonding portion 13, the alumina particles 14 are prevented from falling off. Therefore, a semi-molten material of the metal body 11 and the alumina particles 14 are likely to be uniformly mixed in the heating cylinder 7. Accordingly, the alumina particles 14 are uniformly dispersed in the semi-molten material. As a result, a thixotropically molded product in which α-Al2O3 dispersed in a matrix portion is uniformly distributed can be produced.
The alumina particles 14 have high yield strength and elastic modulus derived from α-Al2O3. Therefore, in the thixotropically molded product in which the alumina particles 14 are distributed, high specific strength and specific rigidity derived from Mg can be further enhanced. Since the alumina 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, the specific strength and the specific rigidity of the obtained thixotropically molded product can be further enhanced.
Further, the alumina particles 14 have high insulation resistance and a low thermal expansion derived from α-Al2O3. Therefore, a low electrical conductivity and a low linear expansion coefficient are imparted to the thixotropically molded product in which the alumina particles 14 are dispersed.
The interposed particles 15 act to enhance wettability between the alumina 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 alumina 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 contains both 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, M1A, 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 alumina particles 14. In the embodiment, as shown in
The alumina particles 14 are dispersed in the semi-molten material when subjected to the thixotropic molding. The alumina particles 14 are less likely to vaporize during the thixotropic molding, and can be prevented from causing molding defects.
The alumina particles 14 contain α-Al2O3 as a main component. Containing α-Al2O3 as a main component refers to that, when the elemental analysis is performed on the alumina particles 14, a content of one of Al 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 Al and O in the alumina particles 14 may be higher than other elements, and is preferably more than 50 atomic %, and more preferably 60 atomic % or more. In addition, α-Al2O3 (alpha-type alumina) is a compound having a corundum-type crystal structure. On the other hand, in addition to α-Al2O3, γ-Al2O3 (gamma-type alumina) having a defect spinel crystal structure, X—Al2O3 (chi-type alumina) having a distorted defect spinel crystal structure, and the like can be used. Distinction between α-Al2O3 and γ-Al2O3 or X—Al2O3 can be made by, for example, crystal structure analysis by an X-ray diffraction (XRD) method.
α-Al2O3 has yield strength and elastic modulus higher than those of alumina having other crystal structures, such as γ-Al2O3 or X—Al2O3. Therefore, in the thixotropically molded product produced using the thixotropic molding material 10 containing the alumina particles 14, high specific strength and specific rigidity derived from Mg can be further enhanced.
In addition, the alumina particles 14 have high insulation resistance and low thermal expansion derived from α-Al2O3. Therefore, in the thixotropically molded product produced using the thixotropic molding material 10, the dispersion of the alumina particles 14 imparts a low electrical conductivity and a low linear expansion coefficient.
Alumina having a crystal structure such as γ-Al2O3 or X—Al2O3 has water absorption higher than that of α-Al2O3. Therefore, when the alumina particles 14 contain γ-Al2O3, X—Al2O3, or the like, gas is generated during the thixotropic molding, resulting in molding defects. In addition, γ-Al2O3, X—Al2O3, and the like react with Mg during the thixotropic molding to produce a reaction product. Since the reaction product does not have sufficient functions as a reinforcing material and other functions, the reaction product cannot bring about the above effects.
An average particle diameter of the alumina particles 14 is 20 μm or less, preferably 0.5 μm or more and 20 μm or less, and more preferably 1 μm or more and 10 μm or less. By setting the average particle diameter of the alumina particles 14 within the above range, when the alumina particles 14 adhere to the surface of the metal body 11 and is subjected to the thixotropic molding, the alumina particles 14 can be uniformly distributed, and the alumina 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 alumina particles 14 are satisfactorily dispersed can be produced.
When the average particle diameter of the alumina particles 14 is less than the above lower limit value, the particle diameter of the alumina particles 14 is too small, and thus the alumina particles 14 may be less likely to function as a reinforcing material that enhances mechanical properties of the thixotropically molded product. On the other hand, when the average particle diameter of the alumina particles 14 is more than the above upper limit value, the alumina particles 14 may be likely to fall off from the surface of the metal body 11.
As shown in
The average particle diameter of the alumina particles 14 is a value obtained by measuring the particle diameters of the alumina particles 14 from an observation image of the alumina 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 alumina 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 alumina particles 14.
A ratio of the alumina particles 14 to a total of the metal body 11 and the alumina particles 14 is 1.0 mass % or more and 20.0 mass % or less. By setting the ratio of the alumina particles 14 within the above range, the alumina 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 alumina 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 α-Al2O3. That is, in addition to further enhancing high specific strength and specific rigidity derived from Mg, a thixotropically molded product to which a low electrical conductivity and a low linear expansion coefficient are added is obtained. The ratio of the alumina particles 14 is preferably 3.0 mass % or more and 17.0 mass % or less, and more preferably 5.0 mass % or more and 15.0 mass % or less.
When the ratio of the alumina particles 14 is less than the above lower limit value, it is not possible to sufficiently obtain the effect of enhancing the specific strength and the specific rigidity of the produced thixotropically molded product and decreasing the electrical conductivity and the linear expansion coefficient. On the other hand, when the ratio of the alumina particles 14 is more than the above upper limit value, since the alumina 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 alumina particles 14. In this case, a content of the substance other than the alumina particles 14 may be less than the content of the alumina particles 14 in terms of mass ratio, and is preferably 30.0 mass % or less, and more preferably 10.0 mass % or less of the alumina particles 14.
The alumina 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.
A method of producing the alumina particles 14 is not particularly limited. Examples of the method of producing the alumina particles 14 include a Bayer method, a thermal decomposition method of aluminum alum, a thermal decomposition method of ammonium aluminum carbonate, a water pyrotechnic discharge method of aluminum, and a vapor phase oxidation method.
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 alumina particles 14. Since such interposed particles 15 are minute, the interposed particles 15 easily enter between the metal body 11 and the alumina particles 14 or between the alumina particles 14. It is considered that the interposed particles 15 strongly interact with both the metal body 11 and the alumina 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 alumina 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 alumina particles 14 to the surface of the metal body 11.
Since the interposed particles 15 more firmly fix the metal body 11 and the alumina particles 14, the alumina 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 alumina particles 14 are likely to be uniformly mixed. Accordingly, the alumina 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 alumina 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 alumina 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.
In addition, the silicon oxide acts to enhance the wettability between the alumina 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 alumina particles 14 in the thixotropic molding material 10, the interposed particles 15 are interposed between the alumina 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 alumina 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 30.0 mass % or less, and more preferably 10.0 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 alumina 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 alumina particles 14. Accordingly, the interposed particles 15 are particularly likely to enter between the metal body 11 and the alumina particles 14 and between the alumina 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 alumina 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 alumina particles 14 or between the alumina 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 10.0 parts by mass or less, more preferably 1.0 parts by mass or more and 8.0 parts by mass or less, and still 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 alumina 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 alumina 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 the mechanical properties of the thixotropically molded product may decrease, or an effect due to the addition of the alumina particles 14 may decrease.
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 10.0 mass % or less, and more preferably 5.0 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 a resin. In this case, the bonding portion 13 also brings about the above-described bonding action. An addition amount of the resin is the same as the addition amount of the interposed particles 15 described above. When the interposed particles 15 and the resin are used in combination, a total addition amount thereof is preferably within the above-described range of the addition amount of the interposed particles 15. However, the resin may vaporize during the thixotropic molding to cause air bubbles to form in the thixotropically molded product. Therefore, from the viewpoint of being unlikely to vaporize, it is preferable that the bonding portion 13 contains the interposed particles 15.
As described above, the thixotropic molding material 10 according to the first embodiment includes the metal body 11, the alumina particles 14, and the bonding portion 13. The metal body 11 contains Mg as a main component. The alumina particles 14 adhere to the surface of the metal body 11, and contain α-Al2O3 as a main component. The bonding portion 13 is interposed between the metal body 11 and the alumina particles 14. Further, the average particle diameter of the alumina particles 14 is 20.0 μm or less. The ratio of the alumina particles 14 to the total of the metal body 11 and the alumina particles 14 is 1.0 mass % or more and 20.0 mass % or less.
In such a thixotropic molding material 10, α-Al2O3 as a main component of the alumina particles 14 has high yield strength and elastic modulus. In addition, refinement of the Mg crystals is achieved with the alumina particles 14. Therefore, in the thixotropically molded product produced using the thixotropic molding material 10, high specific strength and specific rigidity derived from Mg can be further enhanced. Further, the alumina particles 14 have high insulation resistance and low thermal expansion derived from α-Al2O3. Therefore, in the thixotropically molded product produced using the thixotropic molding material 10, the dispersion of the alumina particles 14 imparts a low electrical conductivity and a low linear expansion coefficient.
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 the average particle diameter of the alumina particles 14, or a resin.
In such a thixotropic molding material 10, the bonding portion 13 penetrates between the metal body 11 and the alumina particles 14 and between the alumina particles 14, and acts to bond them. Accordingly, a thixotropically molded product in which the alumina particles 14 are satisfactorily dispersed can be produced.
The inorganic material is preferably a silicon oxide. The silicon oxide combines with magnesium to form a compound, and can enhance the mechanical properties of the thixotropically molded product. In addition, the silicon oxide acts to enhance the wettability between the alumina particles 14 and the semi-molten material of the metal body 11 during the thixotropic molding. Accordingly, it is possible to produce a thixotropically molded product which is denser and excellent in mechanical properties.
The metal body 11 preferably contains at least one of Al and Zn. Containing at least one of Al and Zn lowers a melting point of the thixotropic molding material 10 and improves fluidity of the semi-solidified product during the thixotropic molding. As a result, moldability during the thixotropic molding is enhanced, and thus dimensional accuracy of the produced thixotropically molded product can be enhanced.
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 alumina particles 14, a binder, and a dispersion medium is prepared. The mixture is a dispersion liquid in which the metal body 11, the alumina 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 alumina 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 alumina particles 14, and the interposed particles 15.
As described above, the average particle diameter of the alumina particles 14 contained in the thixotropic molding material 10 is 20 μm or less. Accordingly, in the stirring step S104 to be described later, an adhesion rate of the alumina 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 alumina particles 14 are satisfactorily dispersed can be obtained.
As described above, the ratio of the alumina particles 14 to the total of the metal body 11 and the alumina particles 14 is 1.0 mass % or more and 20.0 mass % or less. By setting the ratio of the alumina particles 14 within the above range, the alumina 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 alumina 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 alumina particles 14 can adhere to the surface of the metal body 11 via the binder. A part of the alumina particles 14 may directly adhere to the surface of the metal body 11 without the binder interposed therebetween. At this stage, the alumina 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 alumina particles 14, and aggregation of the interposed particles 15 can be prevented.
In the drying step S106, the mixture is dried. Accordingly, the alumina particles 14 adhering to the surface of the metal body 11 via the binder 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 alumina 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 alumina 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 alumina 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 alumina 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 alumina particles 14 containing α-Al2O3 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 alumina 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 alumina particles 14 is 20.0 μm or less. In the mixture, the ratio of the alumina particles 14 to the total of the metal body 11 and the alumina particles 14 is 1.0 mass % or more and 20.0 mass % or less.
According to such a production method, since the metal body 11 and the alumina 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 alumina particles 14 are less likely to fall off. In such a thixotropic molding material 10, the alumina 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, in the thixotropically molded product produced using the thixotropic molding material 10, high specific strength and specific rigidity derived from Mg can be further enhanced. In addition, it is possible to produce the thixotropic molding material 10 with which a thixotropically molded product having a low electrical conductivity and a low linear expansion coefficient derived from α-Al2O3 can be produced.
Next, a thixotropically molded product according to a third embodiment will be described.
The thixotropically molded product 100 shown in
The matrix portion 200 contains Mg as a main component. The particle portions 300 contain α-Al2O3 as a main component. An area fraction Si of the particle portions 300 in a cross section of the thixotropically molded product 100 is 0.5% or more and 30.0% or less.
In such a thixotropically molded product 100, high specific strength and specific rigidity derived from the matrix portion 200 are further enhanced by the particle portions 300. In addition, a low electrical conductivity and a low linear expansion coefficient derived from the particle portions 300 are also imparted to the thixotropically molded product 100.
When the thixotropic molding material 10 used for the production of the thixotropically molded product 100 contains the interposed particles 15 containing a silicon oxide, Mg2Si or MgO is generated by a reaction between the silicon oxide and Mg. Mg2Si or MgO functions as a reinforcing material for enhancing rigidity of the thixotropically molded product 100. At least a part of Mg2Si or MgO is distributed to be adjacent to the particle portions 300. Accordingly, wettability between the 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.
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 and thermal 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 particle portions 300 contain α-Al2O3 as a main component. Containing α-Al2O3 as a main component refers to that, when the elemental analysis is performed on the particle portions 300, a content of one of Al 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 Al and O in the 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 particle portions 300 in the qualitative and quantitative analysis, the 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 particle portions 300 may contain additives or impurities other than α-Al2O3.
In the observation image of the cross section of the thixotropically molded product 100 shown in
At this time, the area fraction S1 is 0.5% or more and 30.0% or less as described above. By setting the area fraction S1 within the above range, the above-described effect by the particle portions 300, specifically, the effect of further enhancing high specific strength and specific rigidity derived from Mg can be obtained. A low electrical conductivity and a low linear expansion coefficient derived from α-Al2O3 are added to the thixotropically molded product 100. Therefore, when the area fraction S1 is less than the above lower limit value, the particle portions 300 are insufficient, and thus the above-described effect cannot be obtained. On the other hand, when the area fraction Si is more than the above upper limit value, the particle portions 300 are excessive, and the specific strength of the thixotropically molded product 100 decreases. The area fraction S1 is preferably 1.0% or more and 10.0% or less, and more preferably 1.5% or more and 7.0% or less.
The area fraction S1 in the range A is calculated as follows. First, a range of the 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 particle portions 300 to the entire area of the range A is calculated. The proportion is defined as the area fraction S1.
An average particle diameter of the particle portions 300 is preferably 1.0 μm or more and 20.0 μm or less, and more preferably 3.0 μm or more and 15.0 μm or less. Accordingly, the 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.
Although not shown, the thixotropically molded product 100 may contain Mg2Si as described above. Mg2Si has a tensile elastic modulus (Young's modulus) higher than that of the matrix portion 200. Therefore, Mg2Si functions as a reinforcing material for enhancing the rigidity of the thixotropically molded product 100. Accordingly, the thixotropically molded product 100 has higher rigidity.
In addition, Mg2Si also has 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.
Although not shown, the thixotropically molded product 100 may contain MgO as described above. MgO has a tensile elastic modulus (Young's modulus) higher than that of the matrix portion 200. Therefore, MgO functions as a reinforcing material for enhancing the rigidity of the thixotropically molded product 100. Accordingly, the thixotropically molded product 100 has higher rigidity.
In addition, MgO has 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, MgO also has a function of inhibiting abnormal growth of Mg2Si in a branch shape or a needle shape. Due to the function, when Mg2Si is precipitated, Mg2Si tends to have an isotropic shape, and an increase in average aspect ratio is prevented.
In addition, as described above, MgO also contributes to enhancing the wettability between the particle portions 300 and the matrix portion 200. Accordingly, the thixotropically molded product 100 can be made denser.
Next, a 20 diffraction pattern obtained when crystal structure analysis by an X-ray diffraction method is performed on the thixotropically molded product 100 will be described. The 2θ diffraction pattern is a pattern created by plotting detection results of X-rays diffracted by the thixotropically molded product 100 when a horizontal axis is a diffraction angle 2θ and a vertical axis is a diffraction intensity.
The 2θ diffraction pattern shown in
In the 2θ diffraction pattern, the Mg peak is located near a diffraction angle 2θ of 32° to 33°, and the Al2O3 peak is located near a diffraction angle 2θ of 25° to 27°. Here, an intensity of the Mg peak is defined as I(Mg), and an intensity of the Al2O3 peak is defined as I(Al2O3). The 2θ diffraction pattern obtained from the thixotropically molded product 100 preferably satisfies I(Mg)>I(Al2O3). Accordingly, a quantitative balance between the matrix portion 200 and the particle portions 300 is improved. The peak intensity refers to a height from a background to a peak top of the 2θ diffraction pattern.
The peak intensity I(Mg) is preferably 3 times or more, more preferably 10 times or more and 5000 times or less, and still more preferably 50 times or more and 1000 times or less the peak intensity I(Al2O3). Accordingly, both the effect of the matrix portion 200 and the effect of the particle portions 300 can be achieved at a higher level. As a result, the thixotropically molded product 100 having particularly good specific strength, specific rigidity, electrical conductivity, and linear expansion coefficient can be implemented.
A tensile elastic modulus (Young's modulus) of the thixotropically molded product 100 is preferably 44 GPa or more, more preferably 46 GPa or more, and still more preferably 48 GPa or more. The thixotropically molded product 100 having a tensile elastic modulus within the above range has particularly high specific rigidity. Since such a thixotropically molded product 100 is lightweight and has high rigidity, the thixotropically molded product 100 is suitable for, for example, a component used for a transportation device such as an automobile and an aircraft, a component used in a mobile device such as a mobile terminal and a notebook computer, and a movable component such as a robot arm.
The tensile elastic 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 tensile elastic modulus of the thixotropically molded product 100. The tensile elastic 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.
The tensile strength of the thixotropically molded product 100 is preferably 180 MPa or more, and more preferably 190 MPa or more. Further, a 0.2% proof stress of the thixotropically molded product 100 is preferably 155 MPa or more, and more preferably 165 MPa or more.
The thixotropically molded product 100 in which the tensile strength and the 0.2% proof stress are within the above ranges has particularly high specific strength. Since such a thixotropically molded product 100 is lightweight and has high strength, the thixotropically molded product 100 is suitable for, for example, a component used for a transportation device such as an automobile and an aircraft, a component used in a mobile device such as a mobile terminal and a notebook computer, and a movable component such as a robot arm.
The tensile strength and the 0.2% proof stress of the thixotropically molded product 100 are 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. In addition, in a stress-strain curve obtained by measurement, a stress corresponding to a point of 0.2% strain is defined as the 0.2% proof stress.
A breaking elongation of the thixotropically molded product 100 is preferably 0.5% or more, and more preferably 0.7% or more and 2.5% or less. Such a thixotropically molded product 100 is resistant to brittle fracture, and is therefore excellent in durability and handleability.
The breaking elongation of the thixotropically molded product 100 is a maximum value of the tensile strain in the case of measuring the tensile strength described above.
Vickers hardness of a surface of the thixotropically molded product 100 is preferably 65 or more, more preferably 70 or more, and still more preferably 80 or more. When the Vickers hardness is within the above range, it is possible to obtain the thixotropically molded product 100 having high surface hardness and being resistant to scratches and the like. The Vickers hardness of the surface of the thixotropically molded product 100 is measured according to a Vickers hardness test method defined in JIS Z 2244:2009. A measurement load is 200 gf.
In addition, the electrical conductivity of the thixotropically molded product 100 is preferably 9% IACS or less, and more preferably 8% IACS or less. Such a thixotropically molded product 100 has a relatively low electrical conductivity among metal materials. Therefore, an induced current caused by, for example, electromagnetic induction is less likely to flow, and thus the thixotropically molded product 100 is suitable for a member used in a site where a change in magnetic field occurs, such as a motor case.
The electrical conductivity of the thixotropically molded product 100 is measured according to, for example, volume resistivity and a conductivity measurement method of a nonferrous metal material defined in JIS H 0505:1975.
The linear expansion coefficient of the thixotropically molded product 100 is preferably 20×10−6/K or less, more preferably 19×10−6/K or less, and still more preferably 18×10−6/K or less. Since the thixotropically molded product 100 having such a linear expansion coefficient has a linear expansion coefficient lower than that of Mg alone, a dimensional change due to a temperature change is small.
The linear expansion coefficient of the thixotropically molded product 100 is measured according to, for example, a method of measuring linear expansion coefficient of a metal material defined in JIS Z 2285:2003.
As described above, the thixotropically molded product 100 according to the third embodiment includes the matrix portion 200 and the particle portions 300. The matrix portion 200 contains Mg as a main component. The particle portions 300 are dispersed in the matrix portion 200 and contain α-Al2O3 as a main component. Further, in the cross section of the thixotropically molded product 100, the total area fraction Si of the particle portions 300 in a range of 500 μm square around a point at a depth of 1 mm from the surface is 0.5% or more and 30.0% or less.
According to such a configuration, the thixotropically molded product 100 in which high specific strength and specific rigidity derived from the matrix portion 200 are further enhanced by the particle portions 300 is obtained. In addition, a low electrical conductivity and a low linear expansion coefficient derived from the particle portions 300 are also imparted to the thixotropically molded product 100. As a result, the thixotropically molded product 100 having high specific strength and specific rigidity, a low electrical conductivity, and a low linear expansion coefficient is obtained.
In addition, when a 20 diffraction pattern of the thixotropically molded product 100 is obtained by X-ray diffraction, the obtained 20 diffraction pattern has an Mg peak assigned to a (100) plane of Mg and an Al2O3 peak assigned to a (012) plane of α-Al2O3. Further, the 2θ diffraction pattern preferably satisfies I(Mg)>I(Al2O3), where I(Mg) is an intensity of the Mg peak, and I(Al2O3) is an intensity of the Al2O3 peak.
In the thixotropically molded product 100 from which such a 20 diffraction pattern is obtained, a quantitative balance between the matrix portion 200 and the particle portions 300 is improved. That is, it is possible to obtain the thixotropically molded product 100 in which the high specific strength, the specific rigidity, and the like derived from the matrix portion 200 are further enhanced while preventing the ratio occupied by the particle portions 300 from becoming too large, and the low electrical conductivity and the linear expansion coefficient are added.
In addition, the thixotropically molded product 100 preferably has a tensile elastic modulus of 46 GPa or more, tensile strength of 180 MPa or more, an electrical conductivity of 9% IACS or less, and a linear expansion coefficient of 20×10−6/K or less.
In such a thixotropically molded product 100, various properties such as high specific strength, high specific rigidity, a low electrical conductivity, and a low linear expansion coefficient become more remarkable. Therefore, for example, the thixotropically molded product 100 is suitably used for a member that requires high mechanical properties, that is to be coupled to a component made of another material, and that requires reduction of an eddy current.
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, an α-Al2O3 powder as alumina particles, a silica 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. In addition, the alumina particles were added in a sufficient amount to sufficiently coat the magnesium alloy chip.
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. 3, 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.
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 3. In Sample No. 20, 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 4.
In Tables 1 to 4, 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”.
For the thixotropic molding material of each sample No., a charge-in amount of the alumina particles during production and an amount of the alumina particles adhering to the metal body after production (adhesion amount) were obtained. Then, a mass ratio of the adhesion amount to the charge-in amount was calculated as the adhesion rate of the alumina particles. Calculation results are shown in Tables 1 and 2.
In each Example shown in Table 1, it is found that the adhesion rate of the alumina particles can be sufficiently enhanced by setting the average particle diameter of the alumina particles within a predetermined range. The reason is that, when the average particle diameter of the alumina particles is too large, the alumina particles tend to fall off from the metal body due to their own weight.
In each Example shown in Table 2, it is found that adhesion rate of the alumina particles can be sufficiently enhanced by setting the addition amount of the silica powder within a predetermined range. In particular, it is found that, when the addition amount of the silica powder serving as a binder is within the predetermined range, the adhesion rate of 40% or more can be secured, and production efficiency of the thixotropic molding material can be easily enhanced.
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.
The breaking elongation, the electrical conductivity, and the linear expansion coefficient of the obtained thixotropically molded product were measured. Measurement results are shown in Tables 3 and 4.
In each Example shown in Table 3, since the addition amount of the alumina particles is within the predetermined range, sufficient breaking elongation is obtained. In particular, when the addition amount of the alumina particles is set to 2.0 mass % to 10.0 mass %, the breaking elongation of 1% or more is obtained. In addition, it is found that the electrical conductivity and the linear expansion coefficient can be sufficiently reduced by setting the addition amount of the alumina particles within the predetermined range.
In each Example shown in Table 4, it is found that, when the addition amount of the silica powder is within the predetermined range, a decrease in the breaking elongation is reduced within an allowable range as compared with the Comparative Example (Sample No. 24) in which the silica powder is not added. However, when the addition amount of the silica powder is more than 3.0 parts by mass, the decrease in breaking elongation is within the allowable range but is slightly large. It is found that this is because Mg2Si, MgO, and the like derived from the silica powder are excessively precipitated in the thixotropically molded product, thereby decreasing the breaking elongation.
First, a magnesium alloy chip as a metal body, an α-Al2O3 powder having an average particle diameter of 5 μm as alumina particles, a silica 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 Materials 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.
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.
Next, the obtained thixotropic molding material was charged into an injection molding machine and subjected to the thixotropic molding to obtain a thixotropically molded product of Sample No. 31. As the injection molding machine, a magnesium injection molding machine JLM75MG manufactured by The Japan Steel Works, Ltd. was used.
A thixotropically molded product was obtained in the same manner as in the case of Sample No. 31 except that the addition amount of the alumina particles is set such that the area fraction S1 of the particle portions in the thixotropically molded product is the value shown in Table 5.
In Table 5, among the thixotropically molded products 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”.
The thixotropically molded product of each sample No. was cut, and a cut surface was observed with an optical microscope. Then, the matrix portion and the particle portions were identified from the observation image.
Next, the area fraction Si of the particle portions was calculated. Calculation results are shown in Table 5.
In addition, a 2θ diffraction pattern was obtained for the thixotropically molded product of each sample No. by an X-ray diffraction method. Then, I(Mg)/I(Al2O3) was calculated as an XRD peak intensity ratio. Calculation results are shown in Table 5.
Tensile strength, a tensile elastic modulus (Young's modulus), an electrical conductivity, and a linear expansion coefficient were measured for the thixotropically molded product of each sample No. Measurement results are shown in Table 5.
In each Example shown in Table 5, it is found that since the area fraction Si of the particle portions derived from the alumina particles is within a predetermined range, the tensile strength is larger than that of each Comparative Example. On the other hand, it is found that in each Comparative Example, since the area fraction S1 of the particle portions is zero or excessively large, the tensile strength is relatively small.
In addition, in each Example shown in Table 5, it is found that the tensile elastic modulus (Young's modulus) can be enhanced and the electrical conductivity and the linear expansion coefficient can be sufficiently reduced by setting the area fraction S1 of the particle portions derived from the alumina particles within the predetermined range.
Although not shown in Table 5, when only the resin is used as the binder, the tensile strength is decreased as compared with the case of using the interposed particles. The result is considered to be due to the fact that the resin is vaporized during the thixotropic molding and denseness of the thixotropically molded product is decreased.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2022-211894 | Dec 2022 | JP | national |
