Parts formed by injection molding and manufacturing method thereof

Abstract: In a half-molten alloy including a large amount of a liquid phase portion, that is, half-molten alloy with less than 50% of solid phase rate, there is a tendency for a solid phase portion (3b) to concentrate into the central portion in the direction of thickness, that is the internal portion. In order to enhance corrosion resistance at a portion where high corrosion resistance is particularly required in parts molded by half-molten injection, the above tendency is utilized. By utilizing the tendency, a layer ( d ) consisting of a liquid phase portion (3a) is partially formed at the half-molten state on a surface portion (6c) where high corrosion resistance is required.

Description: The present invention relates to a manufacturing method of parts molded by injection half-molten alloy material including a solid phase portion and a liquid phase portion into a mold and to parts molded by injection half-molten alloy material including a solid phase portion and a liquid phase portion into a mold. In this context "half-molten" means "semi-molten" or "semi-solid". WO 90/09251 discloses a half-molten injection molding method and mentiones that when half-molten injection molding is applied to Al-Mg alloys, liquid phase portion which has relatively large amount of Al-component tends to exist on the surface of the molded parts. Further, the attached Figure 1 shows a cross-sectional view of a typical texture in comparison to this piece molded by half-molten injection molding, where the surface layer, which is partially formed in a pre-determined portion of said molded parts, consists of the liquid phase portion. Generally, parts manufactured by die casting or gravity casting (casting by slowly pouring molten raw material into a cast) with aluminum and magnesium alloy as raw materials have virtually homogeneous chemical composition on the surface and inside, and the material characteristic rarely changes. Therefore, characteristics such as wear resistance and corrosion resistance required on a surface of molded parts are usually different from high flexibility or the like required for an internal portion of the parts, and it is considered difficult to attain both characteristics simultaneously. In contrast, a technique has been proposed to partially provide wear resistance to molded parts, where a rigid porous material such as ceramic fiber or the like is located at a predetermined position inside a mold and a molten alloy is poured into the mold and pressed inside the mold to compound the porous material with the molded parts. Moreover, it is a well-known technique which enables SiC (silicon carbide) particles to be concentrated at a particular portion with high density by setting a filter at a predetermined position inside a mold, pouring a molten alloy into the mold and pressing the molten alloy inside the mold, where large particles such as non-metal material or the like are scattered, to be molded (Japanese Patent Application Laid-Open No. 3-5063). Furthermore, a method has been suggested where magnesium alloy material is half molten to have a solid phase rate of 60% or less, injected to a mold to form a cast product, and then a plasticizing process is performed thereon to form a molded product (Japanese Patent Application Laid-Open No. 6-297127). In the foregoing injection molding with half-molten particulate metal (hereinafter referred to as half-molten injection molding), half-molten alloy includes both a solid phase portion and a liquid phase portion, each of which has different chemical compositions and has the following characteristics. That is: 1 ○ In aluminum-magnesium (hereinafter referred to as Al-Mg) magnesium alloys, the solid phase portion has a small amount of aluminum (hereinafter referred to as Al) component, and the liquid phase portion has a large amount of Al component; 2 ○ In aluminum-silicon (hereinafter referred to as Al-Si) aluminum alloys, the solid phase portion has a small amount of silicon (hereinafter referred to as Si) component, and the liquid phase portion has a large amount of Si component. In the above described technique for partially providing wear resistance to molded parts, since the porous material needs to be preliminary heated or to be maintained at more than a predetermined temperature in order to be located in a mold, such processing causes to reduce production efficiency. Further, in the half-molten injection molding, compositions of the material is different in the solid phase portion and the liquid phase portion. It is possible to change characteristics of the material on the surface and inside portion of molded parts by varying the arrangement of the solid phase portion and liquid phase portion. However, there has been no art suggested to positively achieve the above. For instance, when the half-molten injection molding is applied to Al-Mg magnesium alloys, a liquid phase portion which has relatively large amount of Al component tends to exist in the surface of molded parts. Although this characteristic can be utilized to provide corrosion resistance to the surface, there has been no art suggested to constructively arrange Al component included in the liquid phase portion to a portion where high corrosion resistance is required. Therefore, corrosion of parts could not be further prevented. SUMMARY OF THE INVENTION The present invention has been made in consideration of the above situation, and has as its object to provide parts formed by injection molding with half-molten material and manufacturing method thereof for constructively arranging a liquid phase portion to those portions that require high corrosion resistance such as a surface of parts formed by half-molten injection molding, in order to enhance corrosion resistance and wear resistance, and to readily obtain molded parts having different material characteristics between a surface portion and an internal portion. In order to solve the above problem and attain the foregoing objective, the present invention provides a manufacturing method of parts molded by injecting half-molten alloy material including a solid phase portion and a liquid phase portion into a mold according to claim 1. Further, parts molded by half-molten injection molding according to the present invention are specified in claim 9. Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. Fig. 1 is a cross-sectional view illustrating typical texture of a comparison test piece molded by half-molten injection molding; Fig. 2 is a photomicrograph showing a cross-section of actual texture of the comparison test piece formed by the half-molten injection molding; Fig. 3 is a schematic view showing a main part of a half-molten injection molding machine according to an embodiment of the present invention; Fig. 4 is a cross-sectional view showing a method of manufacturing a corrosion test piece applying the manufacturing method according to a first embodiment; Fig. 5 is a cross-sectional view cut along the A-A line in Fig. 4; Fig. 6 is a chart showing results of a salt spray test (SST) experimented upon the corrosion test piece which is manufactured by the method according to the first embodiment; Fig. 7 is a cross-sectional view illustrating a molding method of the corrosion test piece which applies half-molten injection molding according to a second embodiment; Fig. 8 is a cross-sectional view cut along the B-B line in Fig. 7; Fig. 9 is a chart showing two types of test pieces on which a heating process T6 complying with the Japanese Industrial Standard (JIS) is performed, each of which is left without a finishing process, and also on which a polishing process is performed using an emery paper with surface roughness of #600; and a graph showing results of the salt spray test (SST) in corrosion loss on the surface of the two types of the test pieces; Fig. 10 is a chart showing two types of test pieces on which the heating process T6 is performed, each of which is left without a finishing process, and also on which a polishing process is performed using an emery paper with surface roughness of #600; and a graph showing results of the salt spray test (SST) in average erosion depth on the surface of the two types of the test pieces; Fig. 11 is a chart showing chemical compositions for four types of Al-Mg magnesium alloys which is molded by the conventional injection method with various Al component, and on which a tension test and an impact test are to be experimented; Fig. 12 is a graph showing results of the tension test and impact test experimented upon the four types of alloy shown in Fig. 11; Fig. 13 is a cross-sectional view illustrating a molding method of a wheel for an automobile applying the half-molten injection molding according to the first embodiment; Fig. 14 is an elevational view of a wheel for an automobile molded in accordance with the first embodiment where a mechanical process has been performed thereupon; Fig. 15 is a cross-sectional view of Fig. 14; Fig. 16 is a cross-sectional view showing a molding method of a wheel for an automobile applying the half-molten injection molding according to the second embodiment; Fig. 17 is an elevational view of a wheel for an automobile molded in accordance with the second embodiment where a mechanical process has been performed thereupon; Fig. 18 is a cross-sectional view of Fig. 17; Fig. 19 is a chart showing chemical compositions for four types of Al-Mg magnesium alloys which is molded by the injection method according to the first and second embodiments with various Al component, and on which a tension test and an impact test are to be experimented; Fig. 20 is a chart showing results of the corrosion test and impact test experimented upon the four types of alloy shown in Fig. 19; Fig. 21 is a graph illustrating a state of Al-Si aluminum alloys; Fig. 22 is a chart showing chemical compositions of Al-Si aluminum alloys; and Fig. 23 is a graph showing results of a wear test experimented upon a surface and inside portion of the aluminum alloy having the chemical compositions shown in Fig. 22, which is molded according to the present embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described in detail in accordance with the accompanying drawings. 〈Principle of Manufacturing Method〉 First, principle of the manufacturing method for parts molded by half-molten injection molding according to the present embodiment will be explained. Fig. 1 shows a cross-sectional view of-typical texture of a comparison test piece molded by half-molten injection molding. Fig. 2 is a photomicrograph showing a cross-section of actual texture of the comparison test piece formed by the half-molten injection molding. In relation to a half-molten alloy with a large amount of liquid phase, that is, a half-molten alloy having 50% or less of a solid phase rate { =solid phase quantity/(solid phase quantity + liquid phase quantity) }, the solid phase portion and the liquid phase portion are arranged relatively homogeneously in the thickness direction of thin molded parts (5 mm or thinner) such as those molded by a normal die casting. However, for thick molded parts, the solid phase portion tends to concentrate towards the center, that is the internal portion, in the thickness direction. This is caused by a phenomenon that attributes to the difference in fluidity between the solid phase portion and liquid phase portion in the mold. The parts molded by the half-molten injection molding according to the present embodiment is molded by utilizing the above described phenomenon. Inventors of the present invention have discovered that the phenomenon is influenced by the relationship between a particle size of a solid phase and a thickness of molded parts in a half-molten state, and that the smaller the particle size of the solid phase is, as compared to she thickness of the molded parts, the more tendency it has for the solid phase portion to concentrate in the internal portion. Note that the particle size of the solid phase is an average size of all the particles included in the solid phase portion. 〈Configuration of Half-Molten Injection Molding Machine〉 Fig. 3 shows a schematic view of a main part of the half-molten injection molding machine according to the present embodiment. A brief description on a screw-type half-molten injection molding machine utilized in the present embodiment will be provided with reference to Fig. 3. In the figure, a screw-type injection molding machine 1 rotates a screw 2 to send raw material 3 to a heating cylinder 4, and the raw material 3 is stirred by the screw 2, sufficiently mixed and heated to be brought into a half-molten state. As the half-molten raw material 3 is pushed forward to the front of the screw 2, the pressure pushes the screw 2 to retreat. As another method of retreating the screw without using the pressure of the raw material, the screw can be forced to retreat with arbitrary speed. A high-speed injection mechanism 5 detects the retreat when the screw 2 retreats for a predetermined length, stops the rotation of the screw, and at the same time, stops the retreat of the screw. The quantity of the raw material 3 can be determined by setting the retreated distance of the screw 2. By pushing the screw 2 forward by the high-speed injection mechanism 5, the half-molten raw material 3 is injected from a nozzle 9 to a mold 6. The material 3 is magnesium pellet which will be described later, and sent from a hopper 8 to the cylinder 4. Argon gas is filled in a path 7 connecting the hopper 8 to the cylinder 4. Oxidation of the raw material (such as magnesium pellet) is prevented by disposing the raw material in the argon atmosphere. According to the above described screw-type molding machine 1, raw material can be homogeneously heated in a heating zone | inside the heating cylinder 4 by virtue of the screw 2 stirring the material and mixing sufficiently. [First Embodiment of Manufacturing Method for Parts Molded By Half-Molten Injection Molding] Next, as a first embodiment, descriptions will be provided for a manufacturing method of parts molded by half-molten injection molding without utilizing a filter (in a second embodiment, with a filter), but by manipulating the size and arrangement of particles of the solid phase. Fig. 4 illustrates a method of manufacturing a corrosion test piece applying the manufacturing method according to the first embodiment. Fig. 5 shows a cross-section cut along the A-A line in Fig. 4. Fig. 6 shows results of-a salt spray test (SST) experimented upon the corrosion test piece which is manufactured by the method according to the first embodiment. Referring to Figs. 4 to 6, the corrosion test piece used in the first embodiment is molded by injecting half-molten material from the nozzle 9 into the mold 6, with satisfying the following conditions. Results of the salt spray test experimented upon a comparison test piece which is manufactured by the conventional injection molding and results of the salt spray test experimented upon another comparison test piece manufactured by die casting, are shown in Fig. 6 for a comparison purpose. (Conditions for Manufacturing) Material: AZ91D alloys complying with the American Society for Testing and Materials (hereinafter referred to as ASTM Standard) (Salt Spraying Condition) Salted water: 5wt% NaCl (sodium chloride) Temperature: 35°C Duration: 1000 hours (Method of Manufacturing) Injection molding by the conventional method: A test piece is molded with a solid phase rate of approximately 25% utilizing the injection molding machine shown in Fig. 3, to obtain the particle size of a solid phase of approximately 100 to 150 µm. Injection molding according to the present embodiment: A test piece is molded so that a solid phase rate is approximately 25%. When material pellets are produced by machine processing, material pellets which has been plasticized before the process is utilized to obtain finely granulated particle size of the solid phase, that is approximately 50 to 80 µm. Die Casting: A test piece is molded by a regular cold-chamber-type die casting machine. 〈Results of Corrosion Test〉 As shown in Fig. 6, according to the manufacturing method of the first embodiment, corrosion resistance is improved by granulating the particles of the solid phase of an alloy as a material more finely than the conventional material. 〈Method of Finely Granulating Solid Phase Particles〉 Hereinafter, an explanation will be given for the method of finely granulating the solid phase particles to less than one-fifty of the thickness of molded parts. A particle size of the solid phase obtained at the time of heating half-molten alloy material depends upon a particle size of the pelletized crystal. In other words, the smaller the size of crystal, the smaller the size of a solid phase particle becomes. Therefore, the solid phase particle can be finely granulated by performing plasticizing process (e.g. rolling process, forging process or the like) on solid alloy as a base material, which is the alloy material before cutting into pellets. Further, granulating of the crystal particle can be realized by adding CaCN2 (calcium cyanide) or Sr (strontium) at the time of producing the solid alloy as a base material. Moreover, Sr (strontium) effectively prevents the solid phase particle from gradually coarsening, which results from alloy material staying inside an injection molding machine and being kept in a half-molten state for a long period of time. [Second Embodiment of Manufacturing Method for Parts Molded By Half-Molten Injection] Next, as a second embodiment, descriptions will be provided for a manufacturing method of parts molded by half-molten injection molding utilizing a filter. Fig. 7 illustrates a molding method of the corrosion test piece applying half-molten injection molding according to the second embodiment, and Fig. 8 is a cross-sectional view cut along the B-B line in Fig. 7. In the second embodiment, attention has been given on the following points: 1 ○ In Al-Mg magnesium alloys, a solid phase portion has a small amount of Al component, and a liquid phase portion has large amount of Al component; 2 ○ In Al-Si aluminum alloys, a solid phase portion has a small amount of Si component, and a liquid phase portion has a large amount of Si component. In order to improve corrosion resistance and wear resistance by constructively arranging the liquid phase portion to those portions that require high corrosion resistance and wear resistance such as a surface portion, a filter 12 which partitions the mold 6 into cavities 6a and 6b is utilized (see Fig. 7). The filter 12 is a porous material (e.g. foamed nickel) of which pore is smaller than a particle size of a solid phase portion, that is about 80 µm. The filter 12 traps a solid phase portion of half-molten metal material injected from the nozzle 9 and passes only a liquid phase portion to the cavity 6b. 〈Results of Corrosion Test〉 Next, corrosion between a corrosion test piece formed by the half-molten injection molding according to the second embodiment and the comparison test piece formed by the conventional half-molten injection molding will be compared. The test piece used in the second embodiment and the comparison test piece are existing magnesium alloy AZ91D, that is identical to the first embodiment, and a portion 6c of the cavity 6b is an evaluation surface of the corrosion test. The comparison test piece is formed by half-molten injection molding utilizing a mold without the filter 12 shown in Fig. 7. As has been described in the "Description of the Related Art," a liquid phase portion 3a (see Fig. 1) which has relatively large amount of Al component tends to gather in the surface of molded parts when formed by half-molten injection molding. Accordingly, a layer having a thickness d of few µm to 400 µm, solely consisting of the liquid phase portion 3a, is formed on the evaluation surface 6c, and a layer consisting of both she liquid phase portion 3a and solid phase portion 3b is formed in the internal portion, as illustrated in Figs. 1 and 2. Further, since the solid phase portion 3b is trapped by the filter 12 in the test piece according to the second embodiment, the cross-section of its texture contains only the liquid phase portion 3a. Fig. 9 is a chart showing two types of test pieces on which the heating process T6 complying with JIS is performed, each of which is left without a finishing process, and also on which a polishing process is performed using an emery paper with surface roughness of #600; and a graph showing results of the salt spray test (SST) in corrosion loss on the surface of the two types of the test pieces. Fig. 10 is a chart showing two types of test pieces on which the heating process T6 is performed, each of which is left without a finishing process, and also on which a surface polishing process is performed using an emery paper with surface roughness of #600; and a graph showing results of the salt spray test (SST) in average erosion depth on the surface of the two types of the test pieces. As can be seen from Figs. 9 and 10, when the experiment is carried out without the filter, results from both tests show that the surface of the test piece having no finishing process has a better result than the test piece with the polishing process using the #600 roughness emery paper. This is due to the polishing process which causes to surface the texture having low aluminum component, which is low in corrosion resistance, formed inside of the test piece by each of the molding. Note that the process T6 is a heating process that executes an artificial aging process after a solution treatment. When the test results of the two types of test pieces are compared, the test piece molded according to the second embodiment, where the texture with a large amount of aluminum component is constructively distributed in the surface portion, is superior in both of the cases, with no finishing process and with the polishing process. By employing the half-molten injection molding according to the second embodiment for Al-Mg magnesium alloys, the surface of molded parts has better corrosion resistance, high rigidity and improved internal flexibility. Moreover, when Al-Si aluminum alloys are employed, the surface of molded parts achieves improved wear resistance and improved internal flexibility. [Application to Automobile Wheel] Next, descriptions will be provided for a case where the half-molten injection molding according to the first and second embodiments is applied to mold a wheel for an automobile. Generally speaking for an automobile wheel where a rim, a hub and a spoke are integrally formed, the less the wheel weighs, the more improved the driving stability is. Therefore, demands are increasing lately for wheels made of an aluminum alloy or of a magnesium alloy. The surface portion of an automobile wheel requires corrosion resistance. Particularly when a wheel made of a magnesium alloy is manufactured by a casting method such as die casting or the injection molding such as the present embodiment, aluminum-manganese (hereinafter referred to as Al-Mn) magnesium alloys (such as AM60 alloy complying with ASTM Standard) is utilized, since its impact resistance characteristic is important. From a corrosion resistance point of view, AZ91D alloy complying with ASTM Standard which has a large amount of aluminum contents is preferable; however, impact resistance thereof is considerably low. In practice, there is no alloy which satisfies all corrosion resistance, high rigidity characteristic such as yield strength or tensile strength, and flexibility. In view of this situation, the present embodiment selects alloy components appropriate for an automobile wheel, as described below. Fig. 11 shows chemical compositions for four types of Al-Mg magnesium alloys which is molded by the conventional injection molding with various Al components, and on which a tension test and an impact test are to be experimented. Fig. 12 shows results of the tension test and impact test experimented upon the four types of alloys shown in Fig. 11. Referring to Figs. 11 and 12, among alloys including aluminum (Al), manganese (Mn) and zinc (Zn), the aluminum component has the most influence over physical characteristics and corrosion resistance, and all the characteristics dramatically deteriorates when aluminum content rises above 7wt%. In order to achieve an impact value that is higher than the value necessary for a wheel (7J/cm 2 in Fig. 12), it is preferable to set Al contents to less than 7%; however, when Al contents are low, tension strength deteriorates, which results in low rigidity and particularly influences wear resistance on the surface clamped by a nut. Accordingly, it is necessary to partially increase Al contents to increase rigidity in particular portions. In the present embodiment, the characteristics of the aluminum is taken into consideration, and alloy components are specified to satisfy the foregoing functional elements as molded parts, by employing the half-molten injection molding described in the first and second embodiments to mold an automobile wheel. [Application Example of the First Embodiment] Next, an application example for molding an automobile wheel employing the half-molten injection molding according to the first embodiment will be described. Fig. 13 shows a molding of a wheel for an automobile applying the half-molten injection molding according to the first embodiment. Fig. 14 shows an elevational view of an automobile wheel where a mechanical process has been performed. Fig. 15 is a cross-sectional view of Fig. 14. Note that the following embodiments are also applicable to a clutch drum of an automatic transmission or engine pistons, in addition to an automobile wheel. Generally, an automobile wheel requires strength and corrosion resistance as a whole, as well as wear resistance on the surface clamped by a nut. When the first embodiment is applied, a wheel can be molded to have a liquid phase portion concentrated to the surface of the wheel, as shown in Fig. 13. Therefore, it is possible to enhance strength as a whole (such as flexibility and impact strength) by hardening only a nut clamping surface 20a of a wheel 20 in Fig. 13. When Al-Mg magnesium alloys are utilized, Al density increases, and when Al-Si aluminum alloys are utilized, Si density increases; and either of the cases can enhance rigidity of the nut clamping surface 20a. [Application Example of the Second Embodiment] Next, an application example for molding an automobile wheel employing the half-molten injection molding according to the second embodiment will be described. Fig. 16 illustrates a molding of an automobile wheel applying the half-molten injection molding according to the second embodiment. Fig. 17 is an elevational view of an automobile wheel where a mechanical process has been performed. Fig. 18 is a cross-sectional view of Fig. 17. As shown in Fig. 16, when the second embodiment is applied, the filter 12 is located at a hub portion of the molded parts, which would become the nut clamping surface 30a of the hub portion of the wheel, in order to prevent wear on the nut clamping surface at the time of clamping a nut on the hub portion of the automobile wheel. Since the solid phase portion is filtered, the nut clamping surface 30a is formed solely with the liquid phase portion. Therefore, it is possible to enhance strength as a whole (such as flexibility and impact strength) by hardening only the nut clamping surface 30a of a wheel 30 in Figs. 17 and 18. When Al-Mg magnesium alloys are utilized, Al density increases, and when Al-Si aluminum alloys are utilized, Si density increases; and either of the case can enhance rigidity of the nut clamping surface 30a. Further, application of a rigid material for the filter can strengthen the base material when the filter is left inside the molded parts. For instance, metal or ceramic porous material can be located at a position on which surface is clamped by a nut, so that it can function as a filter and also can be utilized as a reinforcement material after being molded to prevent wear. [Effect of Applying the Present Embodiment to Automobile Wheel and Selection of Alloy] Fig. 19 shows chemical compositions for four types of Al-Mg magnesium alloys on which a tension test and an impact test are to be experimented. Fig. 20 shows results of the corrosion test and impact test experimented upon the four types of alloy shown in Fig. 19. Note that the specification of the molded parts is set as follows. Wheel disc: minimum thickness of 5 mm (thickness in spoke portion is 15 mm) Size of a solid phase particle: 80 µm The test results shown in Fig. 20 are based on an automobile wheel molded with the four types of Al-Mg magnesium alloys shown in Fig. 19. A corrosion resistance test is performed on the test pieces taken from P1 (Fig. 14) and P2 (Fig. 17) of a disc surface and the Charpy impact test is performed on the internal portion of the spoke. Figs. 19 and 20 demonstrate how Al contents affect the corrosion resistance and physical characteristics of each alloy. Fig. 20 shows that the alloy having high corrosion resistance and high impact resistance is "No. 5" and "No. 6" alloys in Fig. 19, and indicates that the range of 6.5wt% to 7.5wt% of Al contents is preferable. When a filter is utilized as shown in Fig. 16, Al contents may be more than 7.5wt% since a solid phase portion can be arbitrary arranged without considering a cross-sectional thickness of molded parts, but no higher than 10wt% since it also causes to increase the Al contents in the solid phase portion. [Relationship with Silicon Contents] Next, the relationship with silicon contents will be described. Fig. 21 illustrates a state of equilibrium of a liquid phase portion and a solid phase portion included in Al-Si aluminum alloys based on a temperature, weight % (wt%) and atomic % (at%) of silicon contents. As shown in Fig. 21, a dotted line | 1 denotes variance of the liquid phase (hereinafter referred to as liquidus | 1 ), and a solid line | 2 denotes variance of the solid phase (hereinafter referred to as solidus | 2 ). An intersection point Q of the liquidus | 1 and the solidus | 2 denotes an eutectic point (hereinafter referred to as eutectic point Q). Further, an area A 1 between the liquidus | 1 and the solidus | 2 denotes an area where aluminum alloy is half molten. Values in parenthesis indicated near the liquidus | 1 solidus | 2 and near the eutectic point Q denote silicon contents by weight % (wt%) and values outside the parenthesis denote silicon contents by atomic % (at%). The Si contents at the eutectic point Q is 11.3at% and 11.7wt%, that is about 12wt%. In a half-molten state where a fusing point of eutectic compositions is the lowest, the eutectic compositions become liquid phase and arranged in the surface portion. The solid phase portion having a small amount of Si contents is arranged in the internal portion of the parts, providing flexibility. In order to have the above configuration, Si contents must be less than about 12wt% (if Si contents is less than 12wt%, compositions of the internal portion of the parts includes large amount of Si contents). Moreover, when Si contents is less than about 6wt%, it becomes difficult to compose the surface portion with an eutectic composition or a composition having a large amount of Si contents. Accordingly, when Al-Si aluminum alloys are utilized in the above described first and second embodiments, a layer having a large amount of Si is formed in a liquid phase portion particularly when Si contents are at least 6 to 12wt%, resulting an increase in rigidity in the surface portion and flexibility in the inside portion. Fig. 22 shows a chemical composition of Al-Si aluminum alloys. Fig. 23 shows results of a wear test experimented upon a surface and inside portion of aluminum alloy having the chemical compositions shown in Fig. 22, which is molded according to the present embodiment. The Al-Si aluminum alloys having the chemical composition of Fig. 22 are half molten to the solid phase rate of 30%, stirred, injected to a mold, and the wear resistance test is experimented with the following test conditions. (Test Conditions) Wear test method: ring-on-disc type Ring material: Scr420 complying with the JIS Disk material: aluminum alloy material manufactured according to the present embodiment (with T6 heating process performed) Surface pressure: 190kg/cm 2 lubrication oil: equivalent to engine oil 5W30 complying with Society of Automotive Engineers (SAE) number Temperature: 100 °C Sliding distance: 5000 m As shown in Fig. 23, by including silicon to the disc material manufactured according to the present embodiment, the surface portion shows better wear resistance compared to the internal portion. As set forth above, according to the manufacturing method of parts formed by the half-molten injection molding, a layer consisting of a liquid phase portion is partially molded in a predetermined portion of molded parts, which is molded by injecting half-molten alloy material consisting of a solid phase portion and a liquid phase portion into a mold. By virtue of the foregoing feature, it is possible to constructively arrange a liquid phase portion to those portions that require high corrosion resistance such as a surface portion of the parts formed by the half-molten injection molding, in order to improve corrosion resistance and wear resistance, and readily obtain molded parts having different material characteristics between the surface and inside the parts. Furthermore, a layer consisting of the liquid phase portion can be partially molded at a predetermined portion of molded parts by placing a filter material in a predetermined position inside the mold and trapping the solid phase portion at the time of injecting half-molten alloy material. By virtue of this, a liquid phase portion can be assuredly arranged to those portions where high corrosion resistance is particularly required, such as the surface of the parts formed by the half-molten injection molding. Also corrosion resistance and wear resistance can be enhanced by the above feature. The present invention is not limited to the above embodiments and various changes and modifications can be made within the spirit and scope of the present invention. Therefore, to appraise the public of the scope of the present invention, the following claims are made.

Claims: A manufacturing method of parts molded by injecting semi-molten alloy material including a solid phase portion and a liquid phase portion into a mold, a layer of the parts consisting of an outer layer being formed by the liquid phase portion and an inner layer being formed by the solid phase portion and the liquid phase portion, a thickness of said molded parts including said outer and inner layers being set so that it is more than fifty times the particle size of the solid phase. The method according to claim 1, characterized in that said alloy material is a magnesium alloy to which strontium is added in advance, and/or on which plasticizing process is performed in a state of solid alloy, and then chipped and semi-molten. The method according to claim 1 or 2, characterized in that said alloy material is magnesium alloy including a predetermined portion 6% to 10% of aluminium by weight, and a layer including a large amount of aluminum is formed in a predetermined portion of said molded parts. The method according to claim 3, characterized in that said magnesium alloy includes 6,5% to 7,5% of aluminum by weight, a layer including a large amount of aluminum is formed at a surface portion of the molded parts, and a layer including a small amount of the aluminum is formed inside the molded parts. The method according to claim 1, characterized in that said alloy material is aluminum alloy including a predetermined portion 6% to 12% of silicon by weight, and a layer including a large amount of silicon is formed in a predetermined portion of said molded parts. The method according to a preceding claim, characterized in that a filter material is located at an outer surface side in said mold when the semi-molten alloy material is injected, and a layer consisting of the liquid phase portion is partially formed in said inner layer of said molded parts by trapping the solid phase portion with said filter material. The method according to claim 6, characterized in that said filter material is a porous material having pores smaller than an average size of a particle in said solid phase portion. The method according to claim 6 or 7, characterized in that the filter material remaining inside the molded parts to become a reinforcement material. Parts molded by injecting semi-molten alloy material including a solid phase portion and a liquid phase portion into a mold, a layer of the parts consisting of an outer layer being formed by the liquid phase portion and an inner layer being formed by the solid phase portion and the liquid phase portion, a thickness of said molded parts including said outer and inner layers being set so that it is more than fifty times the particle size of the solid phase. The molded parts according to claim 9, characterized in that said alloy material is magnesium alloy including a predetermined portion 6% to 10% of aluminum by weight, and a layer including a large amount of aluminum is formed in a predetermined portion of said molded parts. The molded parts according to claim 10, characterized in that said magnesium alloy includes 6,5% to 7,5% of aluminum by weight, a layer including a large amount of aluminum is formed at a surface portion of the molded parts, and a layer including a small amount of the aluminum is formed inside the molded parts. The molded parts according to claim 9, characterized in that said alloy material is aluminum alloy including a predetermined portion 6% to 12% of silicon by weight, and a layer including a large amount of silicon is formed in a predetermined portion of said molded parts. The molded parts according to one of claims 9 to 12, characterized by a filter material (12) arranged to be located in an outer surface side in said mold (6) and to trap the solid phase portion. The molded parts according to one of claims 9-13, characterized in that said part is a wheel composing an automobile wheel. The molded parts according to claim 13, characterized in that the filter material remaining inside the molded parts to become a reinforcement material. The molded parts according to claim 13 or 15, characterized in that said filter material is a porous material having pores smaller than an average size of a particle in said solid phase portion.

Inventors: HIRABARA SHOJI, SAKAMOTO KAZUO, SAKATE NOBUO, YAMAMOTO YUKIO, HIRABARA, SHOJI, SAKAMOTO, KAZUO, SAKATE, NOBUO, YAMAMOTO, YUKIO
Applicants: MAZDA MOTOR, MAZDA MOTOR CORPORATION
Application year: 1996
Main classification: B22D19
IPC codes: B22D17, B01D35, C22C23, B22D21, C22C21, C22C1