Manufacturing method for molded products and Mg-based composite chips for thixomolding

Abstract

To provide a Mg-based composite material chip for thixomolding capable of improving mechanical characteristics of a molding.SOLUTION: A Mg-based composite material chip for thixomolding comprises a Mg-based chip, and a fine particle powder modified on a surface of the Mg-based chip and having a component to produce a Mg compound, wherein a 50% particle diameter D50 of the maximum particle of the fine particle powder is 10 μm or smaller, and when the particle size of the maximum particle of the fine particle powder is defined as Lx and the particle size of the minimum particle thereof is defined as Ln, a relation of 0.1≤Ln / Lx≤1.0 is satisfied.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an Mg-based composite chip for thixomolding, a molded article, and a method for producing an Mg-based composite chip for thixomolding. [Background technology]

[0002] Magnesium is the lightest of all practical metallic materials and has excellent mechanical properties such as specific strength and bending rigidity. For this reason, molded products made of magnesium or magnesium alloys are used in a variety of fields, including transportation equipment such as automobiles, industrial equipment such as robot arms and machine tools, and electronic devices.

[0003] Methods for manufacturing parts made of magnesium or magnesium alloys include casting methods such as die casting and thixomolding, plastic processing methods, powder metallurgy, etc. Among these, thixomolding is a molding method in which chip-shaped raw material is heated in a cylinder to create a solid-liquid coexistence state where liquid and solid phases coexist, and then the solid phase is granulated by the rotation of a screw (shear force), reducing viscosity and increasing fluidity (a state in which thixotropy is expressed), and this state is then injection molded into a mold.

[0004] In recent years, from the perspective of environmental protection, including the reduction of CO2 emissions, there has been a demand for further weight reduction of various devices as mentioned above. Against this background, various methods for improving the mechanical properties, particularly strength, of magnesium and magnesium alloys have been investigated.

[0005] Patent Document 1 discloses a magnesium composite powder in which fine powder made of Si, SiO2, Al2O3 or Al is adhered to the surfaces of magnesium alloy coarse particles via an organic binder.

[0006] Patent Document 2 discloses a metal-based alloy material in which small pieces of Mg in the form of particles as a matrix metal material and alumina powder as a reinforcing material are mixed in a ball mill to adhere the reinforcing material to the matrix metal material, and a method for producing the same.

[0007] Patent Document 3 discloses a chip for injection molding in which the surface of a magnesium alloy chip having a magnesium content of 80% by weight or more is coated with carbon powder (carbon black) by mixing it with a mixer. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] International Publication No. 2004 / 062837 [Patent Document 2] Japanese Patent Application Publication No. 9-295122 [Patent Document 3] International Publication No. 2012 / 137907 Summary of the Invention [Problem to be solved by the invention]

[0009] However, in the conventional techniques described in Patent Documents 1 to 3, it was sometimes impossible to ensure a sufficient amount of metal powder adhered to the magnesium-based chips. If the amount of metal powder adhered to the magnesium-based chips is insufficient, the molded body molded by the thixomolding method may not have the desired Young's modulus and thermal conductivity. [Means for solving the problem]

[0010] In order to solve the above problems, one embodiment of the present invention provides an Mg-based composite material chip for thixomolding, which comprises an Mg-based chip and a fine powder having a component that produces an Mg compound and which is applied to the surface of the Mg-based chip, wherein the 50% particle diameter D50 of the largest particle of the fine powder is 10 μm or less, and where Lx is the particle diameter of the largest particle and Ln is the particle diameter of the smallest particle of the fine powder, the relationship 0.1≦Ln / Lx≦1.0 is satisfied.

[0011] A molded article according to one embodiment of the present invention is a molded article produced by thixomolding using an Mg-based composite material chip for thixomolding according to one embodiment of the present invention, wherein when a plurality of SEM images are taken of a cross section of the molded article and the area ratio and average particle size of the fine powder are measured for each of the SEM images, the degree of variation in the measured values of the area ratio of the fine powder in the plurality of SEM images is within 10%, and the maximum average particle size S MAX and the minimum average particle size S min The ratio of S MAX / S min is 5.0 or less.

[0012] One embodiment of the present invention relates to a method for producing Mg-based composite chips for thixomolding, which comprises a modification step in which a fine powder containing components that produce Mg compounds is applied to at least a portion of an Mg-based chip via a binder, and a heat treatment step in which the Mg-based chips modified with the fine powder are heat-treated at a temperature higher than the temperature at which the binder begins to lose weight due to combustion, evaporation, or vaporization, but below the melting point of the Mg-based chips and below a temperature at which the size of the Mg crystal grains does not change. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, an Mg-based composite material chip for thixomolding, a method for producing the Mg-based composite material chip for thixomolding, and a molded product according to one embodiment of the present invention will be described.

[0014] [Mg-based composite chip for thixomolding] The Mg-based composite chip for thixomolding according to this embodiment (hereinafter also referred to as Mg-based composite chip) comprises an Mg-based chip and a fine powder coated on the surface of the Mg-based chip. The fine powder has a 50% particle diameter D50 of 10 μm or less, and satisfies the relationship 0.1≦Ln / Lx≦1.0, where Lx is the maximum particle diameter and Ln is the minimum particle diameter.

[0015] The Mg-based chip is made of a magnesium alloy. The magnesium alloy is not particularly limited, but examples thereof include AZ31, AZ91, and AM60 according to the ASTM standard. Among these, AZ91 is preferred from the viewpoint of a balance between mechanical properties and castability.

[0016] The Mg-based chips of this embodiment are magnesium alloy chips, and can be obtained by cutting an ingot or billet, for example.

[0017] In this embodiment, the surfaces of the Mg-based chips are modified with a fine powder containing a component that produces an Mg compound. That is, the fine powder that has been modified on the surfaces of the Mg-based chips reacts with magnesium to produce a magnesium compound. By injection molding the Mg-based chips modified with such fine powder by the thixomolding method, the fine powder reacts with magnesium to produce a magnesium compound, and a magnesium-based composite material (molded body) can be produced.

[0018] The fine powder may be one or more selected from the group consisting of Si, SiC, SiO2, carbon, graphite, ZnO, MnO2, CaO, ZrO2, YO3, MgO, and diamond. For example, when Si is used as the fine powder, Mg2Si can be obtained as a molded body after injection molding.

[0019] The particle size of the fine powder's largest particle is 10 μm or less in terms of 50% particle size D50 (median diameter). The fine powder is applied to the Mg-based chip surface via a binder, and in order to increase the amount of fine powder adhered, the smaller the particle size of the fine powder is, the better. Therefore, the 50% particle size D50 of the fine powder is set to 10 μm or less, and preferably, D50 is 1 μm or less. In this way, by reducing the particle size of the fine powder, it is possible to firmly adhere the fine powder to the chip surface, thereby improving the adhesion rate of the fine powder. As a result, the mechanical properties of the resulting molded body, particularly Young's modulus and thermal conductivity, can be improved.

[0020] In this embodiment, when the particle size of the largest particle in the fine powder is Lx and the particle size of the smallest particle is Ln, the relationship 0.1≦Ln / Lx≦1.0 is satisfied. Here, "Ln / Lx" is an index representing the particle size variation in the fine powder. In other words, by setting this Ln / Lx within a certain range or more, the particle size variation of the fine powder can be suppressed.

[0021] If Ln / Lx is less than 0.1, the particle size variation of the fine powder will be large, which may result in a decrease in the adhesion rate of the fine powder. Furthermore, if the particle size variation of the fine powder is large, it may promote the detachment of the fine powder during molding by the thixomolding method, which may result in a deterioration in the properties of the molded body. Therefore, Ln / Lx is set to 0.1 or more, and preferably 0.5 or more. Meanwhile, since Lx represents the maximum particle size and Ln represents the minimum particle size, the upper limit of Ln / Lx is substantially 1.0.

[0022] Here, the particle shape of the fine powder of this embodiment may be spherical, polygonal, or a mixture of these shapes. The "particle size" of the fine powder in these various shapes refers to the maximum length of each shape. That is, when the fine powder has a spherical shape, the "particle size" refers to the diameter of the particle, and when it has a polygonal shape, the "particle size" refers to the length of the longest side of the particle. Furthermore, when the fine powder has a spherical shape, the "maximum particle size" refers to the diameter of the largest particle, and the "minimum particle size" refers to the diameter of the smallest particle. Furthermore, when the fine powder has a polygonal shape, the "maximum particle size" refers to the length of the longest side of the largest particle, and the "minimum particle size" refers to the length of the longest side of the smallest particle.

[0023] When the fine powder contains polygonal particles, it is preferable that the relationship 0.5≦Ln / Lx≦1.0 is satisfied. By using a fine powder within this range, the adhesion rate of the fine powder can be further improved, and the Young's modulus and thermal conductivity of the resulting molded body can be further improved. When the fine powder contains polygonal particles, the particle size of the largest particle of the polygonal particles is the aforementioned Lx, and the particle size of the smallest particle is the aforementioned Ln.

[0024] Furthermore, when the fine powder contains polygonal particles, the relationship between the length La of the longest side of the particle and the length Li of the shortest side preferably satisfies the relationship 0.5≦Li / La≦1.0. By using a fine powder within this range, the adhesion rate of the fine powder can be further improved, and the Young's modulus and thermal conductivity of the resulting molded body can be further improved.

[0025] The method for measuring the particle size of the fine powder and the method for calculating the maximum particle size and the minimum particle size will be described below. Particles are observed from multiple directions using an SEM to determine their general shape. The longest side (maximum particle size) and shortest side (minimum particle size) are then determined, and their respective lengths are derived from the SEM observation. For one fine powder, 20 or more particles (n≧20) are measured to determine the maximum particle size Lx and minimum particle size Ln, and the average Ln / Lx is calculated.

[0026] The 50% particle size D50 (median diameter) can be determined, for example, by measuring the volumetric particle size distribution using a laser diffraction / dispersion method and then using the cumulative distribution curve obtained from this particle size distribution. Specifically, the particle size at which the cumulative value from the smallest diameter side is 50% on the cumulative distribution curve is the 50% particle size D50 (median diameter).

[0027] The adhesion rate (coverage rate) of the fine powder is preferably 50 to 100% in terms of area percent relative to the surface of the Mg-based chip. In Mg-based composite chips, the fine powder is an element that contributes to improving the properties. Therefore, the greater the amount of fine powder adhesion on the surface of the Mg-based chip, the better. In particular, to increase Young's modulus and thermal conductivity, the adhesion rate of the fine powder is preferably 50% or more. From the viewpoint of improving the mechanical properties of the molded body, the adhesion rate of the fine powder may be 100%.

[0028] In this embodiment, a binder is used as a suitable means for adhering the fine powder to the surface of the Mg-based chips. However, if the Mg-based chips are fed into a molding machine with a large amount of binder remaining on their surface, the binder will decompose due to heating during molding, generating gas inside the cylinder. If the amount of gas generated is excessive, the pressure inside the cylinder will increase, which may cause the cold plug to come out. If the cold plug comes out, stable molding may become difficult. For this reason, it is desirable that the residual weight of the binder components before feeding into the molding machine be 1% or less of the weight of the binder immediately after the fine powder modification. Preferably, it is 0.5% or less from the viewpoint of the stability of continuous molding.

[0029] The residual weight of the binder component before the thixomolding process can be controlled by the heat treatment step in the manufacturing method described below. As will be explained in detail later, in the manufacturing method of Mg-based composite material chips of this embodiment, the fine powder is applied to the surface of the Mg-based chips via the binder, and then the binder is burned, evaporated, or vaporized by heat treatment, thereby removing the organic binder.

[0030] The coverage of the fine powder on the surface of the Mg-based chips is determined by the following method. The measurement method uses SEM-EDS analysis and image processing. First, the coating condition is ascertained using a metallurgical microscope. Based on the coating condition, multiple locations on the chip are observed using SEM-EDS, and elemental analysis is used to distinguish between areas where Mg is exposed and areas where it is covered. The areas of the exposed Mg and covered areas are determined using image processing, and the coverage rate is calculated using the following formula. Coverage rate = (covered area) / (exposed area of Mg + covered area)

[0031] In addition, in the X-ray diffraction pattern of the Mg-based composite chip, the maximum peak intensity of Mg, I m and the maximum peak intensity I of the fine powder a But, I m >I a Specifically, in the diffraction pattern obtained by X-ray diffraction (XRD) of the Mg-based composite chip, the maximum peak intensity I of Mg appears at a diffraction angle (2θ) of about 37°. m is the maximum peak intensity I of the fine powder a It is preferable that it is greater than .

[0032] The relationship between the peak intensities of Mg, the main component of the Mg-based composite chip, and the fine powder modifier in XRD is shown in Fig. m >I a However, in the Mg-based composite chip, the maximum peak intensity I m is the maximum peak intensity I of the fine powder a Below (I m ≦I a ), the Young's modulus of the compact becomes excessively high, making the compact more susceptible to breakage. m and the maximum peak intensity I of the fine powder a That is, I m >I a It is preferable that the relationship between the maximum peak intensity of Mg and the mand the maximum peak intensity I of the fine powder a The relationship between the above can be controlled by adjusting the amount of fine powder added.

[0033] where, the maximum peak intensity of Mg, I m and the maximum peak intensity I of the fine powder a is determined by X-ray diffraction (XRD) using CuKα radiation. Specifically, analysis can be performed under the following conditions: tube voltage: 45 kV, tube current: 40 mA, measurement step: 0.01°, scan speed: 0.2 seconds / step, 10°≦2θ≦80°.

[0034] The Mg-based composite chips for thixomolding described above enable the fine powder to adhere firmly to the chip surface by reducing the particle size and suppressing particle size variation, thereby improving the adhesion rate of the fine powder. As a result, the mechanical properties of compacts made using the Mg-based composite chips for thixomolding, particularly Young's modulus and thermal conductivity, can be improved. Furthermore, by using Mg-based composite chips with firmly attached fine powder, detachment of the fine powder during thixomolding can be suppressed, allowing the fine powder to be uniformly dispersed in the compact after molding. Furthermore, improving the adhesion rate of the fine powder not only improves the properties of the compact but also reduces the amount of detached fine powder during molding, thereby reducing manufacturing costs.

[0035] [Molded body] Next, a molded product using the above-mentioned Mg-based composite material chips for thixomolding will be described. The molded article according to this embodiment is a molded article produced by the thixomolding method using the above-mentioned Mg-based composite material chips for thixomolding. Specifically, a molded article can be obtained by introducing the above-mentioned Mg-based composite material chips for thixomolding into an injection molding machine and molding them by the thixomolding method. The material chips introduced into the injection molding machine are generally heated in a cylinder and simultaneously transported to a nozzle by a screw in the cylinder. In the thixomolding method, the material chips in a semi-molten state transported to the vicinity of the nozzle are injected into a mold to be molded.

[0036] In the molded product of this embodiment molded by such a thixomolding method, when a plurality of SEM images are taken of the cross section of the molded product and the area ratio and average particle size of the fine powder are measured for each SEM image, the degree of variation in the measured values of the area ratio of the fine powder in the plurality of SEM images is within 10%, and the maximum average particle size S MAX and the minimum average particle size S min The ratio of S MAX / S min is 2.0 or less.

[0037] By dispersing the fine powder uniformly in the molded product, i.e., by making the distribution uniform, the properties of the molded product, particularly its strength and Young's modulus, can be further improved. Specifically, the degree of variation in the measured value of the area ratio of the fine powder in the molded product is set to within ±10%. The "degree of variation in the measured value" is calculated using the following method.

[0038] First, multiple (N≧10) SEM images are taken at a constant magnification of the cross section and surface of the molded product. Next, the area ratio of fine powder is measured for each SEM image obtained, and the degree of variation (%) of all measurements is calculated. Specifically, the area of the Mg matrix and the area of the fine powder (including precipitates generated by reaction) region are determined from multiple (N≧10) SEM image processing images, with the area of the SEM image kept constant. In this embodiment, the area ratio of these areas is determined within ±10% and the maximum value S of the average particle size of the fine powder (including precipitates generated by reaction) is determined. MAX and the minimum value S min The ratio (S MAX / S min ) is 5.0 or less. By satisfying these requirements, excellent uniformity and dispersibility can be obtained in the distribution state of the fine powder. To further improve the properties of the molded product, it is preferable that the degree of variation is ±5%. In order to suppress the degree of variation in the measured value of the area ratio of the fine powder, it is effective to refine the particle size of the fine powder in the Mg-based composite material chips before molding, suppress the particle size variation of the fine powder (Ln / Lx), and improve the adhesion rate of the fine powder.

[0039] The molded article described above can be manufactured by injection molding using the Mg-based composite material chips according to this embodiment as a raw material, using the thixomolding method.

[0040] [Manufacturing method for Mg-based composite chips for thixomolding] Next, a method for producing an Mg-based composite material chip for thixomolding according to this embodiment will be described. The method for producing Mg-based composite material chips for thixomolding according to the present embodiment described above comprises a modification step in which a fine powder containing components that produce Mg compounds is applied to at least a portion of the Mg-based chips via a binder, and a heat treatment step in which the Mg-based chips modified with the fine powder are heat-treated at a temperature higher than the temperature at which the organic binder begins to lose weight due to combustion, evaporation, or vaporization, but below the melting point of the Mg-based chips. Each step will be described in detail below.

[0041] <Modification process> A fine powder containing a component that produces an Mg compound is applied to at least a portion of the Mg-based chips via a binder. The modification method may involve coating the Mg-based chips with a binder and then modifying the fine powder, or may involve mixing the fine powder in a binder solution in advance and then adhering the binder solution to the Mg-based chips. The following describes a method for modifying the fine powder after coating with a binder.

[0042] First, at least a portion of the Mg-based chips is coated with a binder. The Mg-based chips are preferably made of a magnesium alloy such as those described above. At least a portion of the Mg-based chips is coated with the binder. From the viewpoint of increasing the amount of fine powder attached, it is preferable that the coated area be the entire surface of the Mg-based chips.

[0043] As the binder, paraffin wax, silicone-based, colloidal silica-based, acrylic-based, PVA (polyvinyl alcohol)-based organic solvents, etc. can be used.

[0044] Examples of the coating method include a dipping method and a stirring method.

[0045] Next, the surface of the binder-coated Mg-based chips is surface-modified with fine powder containing components that produce Mg compounds. The main method for surface-modifying the Mg-based chips by attaching the fine powder to the surface is to sprinkle the fine powder on the binder-coated Mg-based chips and then stir and mix them.

[0046] <Heat treatment process> Next, the Mg-based chips modified with the fine powder are heat-treated at a temperature higher than the temperature at which the binder begins to lose weight due to combustion, evaporation, or vaporization, but below the melting point of the Mg-based chips. This heat treatment reduces the amount of binder remaining on the Mg-based chips. Therefore, the heat treatment temperature in the heat treatment step is set to a temperature higher than the temperature at which the binder begins to lose weight due to combustion, evaporation, or vaporization. For example, if paraffin wax is used as the binder, the heat treatment temperature is preferably 200°C or higher. On the other hand, if the heat treatment temperature is higher than the melting point of the Mg-based chips, the Mg-based chips themselves will melt. Therefore, the heat treatment temperature in the heat treatment step is set to a temperature below the melting point of the Mg-based chips but below the temperature at which changes in crystal grain size begin to occur. For example, if AZ91 is used as the Mg-based chips, the heat treatment temperature is preferably 400°C or lower.

[0047] In this way, by reducing the amount of binder remaining on the Mg-based chips through the heat treatment process, the amount of gas generated inside the cylinder during injection molding using the thixomolding method can be suppressed. If the Mg-based chips are fed into a molding machine with a large amount of binder remaining on their surfaces, the binder is decomposed by heating during molding, generating gas inside the cylinder. If the amount of gas generated is excessive, the pressure inside the cylinder may increase, causing the cold plug to come out. If the cold plug comes out, stable molding may become difficult. For this reason, it is desirable to reduce the amount of binder remaining on the Mg-based chips. From the perspective of molding stability, it is preferable to keep the residual weight of the binder after the heat treatment process to 1 wt% or less of the weight of the binder before the heat treatment process. A residual weight of 1 wt% or less can be achieved by appropriately adjusting the heat treatment time within the above-mentioned heat treatment temperature range.

[0048] In this embodiment, it is preferable to further include a separation and removal step after the heat treatment step, in which the Mg-based chips modified with the fine powder are sieved to separate and remove the fine powder detached from the Mg-based chips. After the heat treatment step, the Mg-based chips modified with the fine powder are sieved to remove weakly adhesive fine powder before molding, thereby further suppressing the detachment of the fine powder during molding. Furthermore, suppressing the detachment of the fine powder during molding enables continuous extrusion in injection molding. Furthermore, sieving the Mg-based chips modified with the fine powder also makes it possible to achieve uniform size of the fine powder.

[0049] By carrying out the above steps, the Mg-based composite material chip according to this embodiment can be manufactured. The manufacturing conditions for each step other than those described above may be determined appropriately within a range that does not impair the effects of the present invention.

[0050] As described above, according to this embodiment, it is possible to obtain Mg-based composite chips for thixomolding that can improve the mechanical properties of the molded body, particularly the Young's modulus and thermal conductivity. Furthermore, by using Mg-based composite chips to which fine powder is firmly attached, it is possible to suppress the detachment of the fine powder during molding by the thixomolding method, thereby enabling the fine powder to be uniformly dispersed in the molded body after molding. Furthermore, by improving the adhesion rate of the fine powder, it is possible to reduce the amount of detached fine powder during molding, which in turn reduces manufacturing costs. [Example]

[0051] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples in any way.

[0052] First, an ingot made of AZ91D (Al: 9%, Zn: 1%, Mn: 0.3%, balance: Mg) was machined to produce magnesium alloy chips (Mg-based chips) with a width of approximately 2 mm, length of approximately 4 mm, and height of approximately 1 mm.

[0053] Next, the obtained magnesium alloy chips were surface-modified with the fine powder shown in Table 1 using paraffin wax as a binder. Specifically, the fine powder was attached to the chip surface by stirring and mixing. The magnesium alloy chips with the surface modified with the fine powder were then heat treated at 320°C for 20 hours to remove the organic binder, producing Mg-based composite chips.The remaining organic binder on the chips after the heat treatment was measured by thermogravimetric analysis to determine the weight loss, and the remaining amount of organic binder was found to be 0.8% by weight compared to before the heat treatment.

[0054] Next, the obtained Mg-based composite material chips were introduced into a thixomolding injection molding machine (manufactured by The Japan Steel Works, Ltd.) and injection molding was carried out under the conditions of heater temperature: 400 to 650°C.

[0055] The resulting molded articles were subjected to the following evaluation tests.

[0056] (strength) Dumbbell test pieces were prepared and subjected to tensile tests, and the tensile strength was determined from the obtained stress-strain diagram.

[0057] (Young's modulus) The Young's modulus was calculated from the slope of the elastic region of the stress-strain diagram described above.

[0058] (thermal conductivity) The laser flash method was used. The laser irradiation surface (10 mm × 10 mm) and the measurement surface on the back side of the sample were coated with carbon spray, and the sample was then introduced into a laser flash analyzer (LFA457, manufactured by NETZSCH). In the laser flash method, one side of the sample was irradiated with a laser to heat it, and the temperature rise on the back side was measured with an infrared thermometer. The thermal diffusivity α (mm 2 / s) was calculated. Furthermore, the specific heat C (J / (g K)) was calculated from the irradiated laser energy and the temperature rise of the sample. These values, together with the weight and dimensions of the sample, gave the density ρ (g / cm 3 The thermal conductivity K (W / (m K)) was calculated as the product αCρ.

[0059] The above evaluation results are shown in Tables 1 and 2. As shown in Tables 1 and 2, the molded products of Examples 1 to 4, which contain Si fine powder, were able to improve Young's modulus without a decrease in strength as intended compared to the comparative example as the amount of Si added increased. The contribution of Young's modulus is thought to be due to Mg2Si precipitated by the reaction of Mg and Si. In addition, in Examples 5 to 7, the fine powders were SiC, SiO2, and graphite, respectively, and it was found that characteristic results were obtained according to the type of fine powder. That is, in the case of SiC in Example 5 and SiO2 in Example 6, as in the case of Si, there was no significant decrease in strength and the Young's modulus was improved. For SiC, the thermal conductivity was also improved. Furthermore, in the case of graphite in Example No. 7, since the Young's modulus of the graphite itself is low, the Young's modulus of the molded article also decreases, but on the other hand, the thermal conductivity increases. From the above, it was found that the target properties can be controlled by changing the type of fine powder.

[0060] [Table 1]

[0061] [Table 2]

Claims

1. Mg-based chips; A fine powder having a component for producing an Mg compound and having the Mg-based chip surface modified thereon; The fine powder has a maximum particle diameter D50 of 50% of 10 μm or less, A molded product manufactured by thixomolding using Mg-based composite material chips for thixomolding that satisfy the relationship 0.5≦Ln / Lx≦1.0, where Lx is the particle size of the largest particle and Ln is the particle size of the smallest particle of the fine powder, When a plurality of SEM images were taken of a cross section of the molded article and the area ratio and average particle size of the fine powder were measured for each of the SEM images, the degree of variation in the measured values of the area ratio of the fine powder in the plurality of SEM images is within 10%; A molded article, wherein the ratio S MAX / S min of the maximum average particle size S MAX to the minimum average particle size S min of the average particle sizes of the fine powder in each SEM image is 5.0 or less.

2. 2. The molded article according to claim 1, wherein the coverage of the surface of the Mg-based chips with the fine powder is 50% to 100% in terms of area %.

3. 3. The molded article according to claim 1, wherein the fine powder is spherical or polygonal particles, or a mixture of spherical and polygonal particles.

4. The fine powder contains polygonal particles, the particle size of the largest particle of the polygonal particles is the Lx, the particle size of the smallest particle is the Ln, and the relationship of 0.5≦Ln / Lx≦1.0 is satisfied. The molded product according to any one of claims 1 to 3.

5. In the X-ray diffraction pattern, the maximum peak intensity I of Mg ｍ and the maximum peak intensity I of the fine powder ａ But, I ｍ >I ａ The molded article according to any one of claims 1 to 4, characterized in that the following relationship is satisfied:

6. The fine powder is made of Si, SiC, SiO ２ , carbon, graphite, ZnO, MnO ２ , CaO, ZrO ２ , Y ２ O ３ 6. The molded article according to claim 1, wherein the powder contains one or more kinds selected from the group consisting of MgO and diamond.

7. a modifying step of modifying at least a part of the Mg-based chips with a fine powder having a component that forms an Mg compound via a binder; a heat treatment step of heat treating the Mg-based chips modified with the fine powder at a temperature higher than the temperature at which the binder begins to lose weight due to combustion, evaporation or vaporization, but below the melting point of the Mg-based chips, and below a temperature at which the size of the Mg crystal grains does not change.

8. 8. The method for producing Mg-based composite material chips for thixomolding according to claim 7, characterized in that the treatment temperature and time in the heat treatment step are adjusted so that the residual weight of the binder after the heat treatment step is 1% by weight or less of the weight of the binder before the heat treatment step.

9. 9. The method for producing Mg-based composite chips for thixomolding according to claim 7 or 8, wherein the binder is a paraffin wax, a silicone-based, a colloidal silica-based, an acrylic-based, or a polyvinyl alcohol-based binder.

10. The method for producing Mg-based composite material chips for thixomolding according to any one of claims 7 to 9, further comprising a separation and removal step of sieving the Mg-based chips modified with the fine powder after the heat treatment step, and separating and removing the fine powder detached from the Mg-based chips.

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