PROCESS FOR PRODUCING SINTERED PRODUCT

Abstract: A method of manufacturing a sintered compact is disclosed herein. The method comprises the steps of: (1A) producing a green body containing metal powder, for example, by means of metal injection molding (MIM); (2A) compacting the green body by pressing it preferably by means of an isostatic pressing (2A); (3A) debinding the compacted green body; and (4A) sintering the debinded green body to obtain a sintered compact. The green body compacting step may be carried out during or after the debinding step or during the debinding step. Further, a step for performing machine working on the green body may be included.

Description: FIELD OF THE INVENTON The present invention relates to a method of manufacturing sintered compacts by sintering metal powder, and more particularly to a manufacturing method in which a green body of a predetermined shape containing a metal powder is prepared, and then the green body is subjected to debinding treatment and sintering treatment to produce sintered compacts. BACKGROUND ART As a process of manufacturing a metal product by sintering a green body containing metal powder, there is known in the conventional art a process named as "metal injection molding (MIM)" . In MIM, metal powder is mixed with an organic binder and then they are compounded to obtain a compound, and then injection molding is carried out using the compound. A green body prepared by MIM is subjected to a debinding treatment (binder removal treatment) in order to eliminate the organic binder, and then such green body is sintered. In order to ensure good forming properties during injection molding, a green body used in MIM must contain an organic binder in a fairly large amount. Therefore, the green body which has undergone the debinding treatment (that is binder removed green body) tends to have a number of pores. When such a binder removed green body having a number of pores is sintered, the following drawbacks will arise. (1) While density of the sintered compact is lowered, porosity of the sintered compact is high. This results in a sintered compact with low mechanical strength. (2) Relatively high sintering temperatures are required. Such high temperatures give a large load to the furnace, thus leading to disadvantages that requires expensive equipment and consumes large amounts of power. (3) It is impossible to obtain high dimensional precision. For example, when a green body has significant variation its thickness, the obtained sintered compact is likely to have a deformed shape. Metals Handbook Vol. 7, pages 495-500 discloses hot isostatically pressing the said debinded and sintered, MIM-prepared green bodies without containerisation. It is therefore an object of the present invention to provide a simpler method of manufacturing sintered compacts which can obtain sintered compacts having high density, or can obtain sintered compacts having excellent formability, that is, can obtain sintered compacts having high dimensional precision, and which can moderate sintering conditions such as lowering a sintering temperature to be employed or the like. Summary of the Invention The present invention is directed to a method of manufacturing a sintered compact, which comprises steps performed in order as follows : producing a green body containing metal powder; debinding the green body at least once; presintering the debinded green body; compacting the presintered green body by pressing it; and sintering the compacted presintered green body further to obtain a sintered compact. Provision of the step for compacting the green body by pressing it makes it possible to reduce pores present in the presintered compact and to increase the density thereof, thus enabling to obtain a sintered compact having a higher density and a higher mechanical strength. Further, this also makes it possible to moderate sintering conditions such as lowered sintering temperature, shortened sintering time and the like, thus leading to improved sinterability and reduced load to sintering furnace and the like. By the further sintering process, it becomes likewise possible to increase the density of the final sintered compact and to increase the mechanical strength thereof, as well as to improve dimensional precision of the final sintered compact without the need for isostatic pressing. Therefore, metal products having high quality can be manufactured Even if molding flaws such as pores would be formed during the production of the green body, such flaws are eliminated to bring the green body in good condition. Therefore, when a sintered compact is, formed from the green body through the subsequent debinding, presintering, compacting and further sintering treatments, it is possible to obtain a metal product having especially high qualities based on the sintered compact. Machine working may be performed on the compacted presintered green body between the presintered green body compacting step and the presintered green body sintering step. Since the machine working is performed on the presintered compact which has been compacted by pressing it, less variations occur in the shape and dimensions at the working area as compared with the case where such machine working would be performed on an uncompacted green body (debinded green body or presintered compact), and therefore it is possible to improve dimensional precision of the sintered compact. In addition, since. the machine working is carried out before the completion of the sintering step, that is prior to the main sintering process, hardness of a work is relatively low as compared with the case where such a machine working would be performed on a final sintered compact having high hardness, so that working can be made easily. Further, since workability is also excellent, the shape and dimensions of the working area can be easily controlled, thus leading to improved dimensional precision. It is preferred that the green body presintering step be carried out until diffusion bonding is made at least at contact points of particles of the metal powder. Conducting presintering in this way increases the shape stability. As a result, it becomes possible to reliably prevent various flaws of the green body (presintered compact) such as breaking, chipping, cracking and the like from occurring during the subsequent compacting step and the machine working process, thus improving handling ability thereof. Further, in the present invention, it is preferred that the pressing for compaction is carried out isotropically, in particular the pressing for compacting is carried out means of an isostatic pressing. In this way, it becomes possible to produce a green body and a sintered compact having uniform density with a simple method. In this case, it is preferred that the isostatic pressing is carried out at ambient temperature or temperature close thereto, because equipment for pressing can be simplified and no heat resistance property is required to waterproof coating film. In the present invention, it is preferred that the pressing is 1 to 100 t/cm 2 . This makes it possible to achieve sufficient compaction without requiring large-scale pressing equipment. Furthermore, in the present invention, it is preferred that the green body producing step is carried out by means of metal injection molding. This makes it possible to manufacture metal sintered products having a relatively small size and/or a complex and intricate shape, and having relatively high mechanical strength. Moreover, in the present invention, it is also preferred that the metal powder content of the green body just before the debinding treatment is 70 to 98 wt %. When using such a green body, it becomes possible to ensure good formability when the green body is produced, and to prevent shrinkage from being increased during sintering of the green body. In the present invention, it is also preferred that the metal powder for the green body is prepared in accordance with a gas atomisation method. Particles of meal powder produced by the gas atomisation method have a roughly spherical shape, so that it is possible to moderate a particle size of metal powder and pressing conditions. With this result, it becomes possible to enhance the mechanical strength of the obtained sintered compact. Brief Description of the Drawings Fig. 1 is a step diagram which shows a method of manufacturing sintered compacts according to the present invention; Fig. 2 is a step diagram which shows a method of manufacturing sintered compacts according to the present invention; Fig. 3 is an illustration which shows a sectional structure (internal metallographic structure) of a green body at a step of producing the green body; Fig. 4 is an illustration which shows a sectional structure (internal metallographic structure) of a green body (binder removed green body) after debinding treatment; Fig. 5 is an illustration which shows a sectional structure (internal metallographic structure) of a presintered compact after presintering treatment; Fig. 6 is an illustration which shows a sectional structure (internal metallographic structure) of a presintered compact after pressing; Fig. 7 is an illustration which shows a sectional structure (internal metallographic structure) of a sintered compact after main sintering treatment; Fig. 8 is an illustration which shows a sectional structure (internal metallographic structure) of a presintered compact after machine working; and Fig. 9 is an illustration which shows a sectional structure (internal metallographic structure) of a presintered compact after the main sintering treatment. Preferred Embodiments of the Invention Hereinafter, a method of manufacturing sintered compacts according to the present invention is described in detail with reference to the accompanying drawings. Fig. 1 is a step diagram which shows a first embodiment of the method of manufacturing sintered compacts according to the present invention; and Figs. 3 to 7 are illustrations of sectional structure (internal metallographic structure) of the green body, at each step. Hereinafter, a description of the first embodiment of the method of manufacturing sintered compacts will be given with reference to the drawings. [1G] Production of Green Body A method for producing a green body is not limited to any particular method, and a typical powder compacting process may be used. In this invention, however, metal injection molding (MIM) is preferably used. Metal injection molding has the advantages of being able to produce sintered metal products that are relatively small in size and that have complex and intricate shapes, and to give high mechanical strength thereto. Therefore, MIM is particularly preferred in this invention, because these advantages can be effectively realised in practising the present invention. Production of a green body by MIM is described below. First, a metal powder and a binder (organic binder) are prepared, and then they are compounded by a compounding machine to obtain a compound. No limitation is imposed upon the metal material for the metal powder (hereinbelow, referred to simply as "metal material"). For example, at least one of Fe, Ni, Co, Cr, Mn, Zn, Pt, Au, Ag, Cu, Pd, Al, W, Ti, V, Mo, Nb, Zr, Pr, Nd, Sm and the like; or alloys (mainly) containing at least one of these elements may be used as a constituent material for the metal powder. According to the present invention, the formability of the sintered compact can be improved as described above. Therefore, as for metal materials for the sintered compact, it is preferable (possible) to use any metal material by which a finally obtained sintered body can have a relatively high hardness or be difficult to process. Specific examples of such metal materials include Fe-base alloys such as stainless steels (e.g., SUS 304, SUS 316, SUS 317, SUS 329J1, SUS 410, SUS 430, SUS 440 and SUS 630), die steel, high speed tool steel and the like; Ti or Ti-base alloys; W or W-base alloys; Co-base cemented carbides; Ni-base cements; and the like. No limitation is imposed upon mean particle size of metal powder, but it is preferably smaller than 50 µm, and more preferably about 0.1 to 40 µm. Excessively large. mean particle size can result in low density of the sintered compact, depending on other factors. Further, no limitation is imposed upon the method for producing the metal powder. For example, a water atomisation method, a gas atomisation method, a reduction method, a carbonyl method, or a comminution method may be used to produce the metal powder. Examples of the binder include polyolefines such as polyethylene, polypropylene, ethylene-vinyl acetate copolymer and the like; acrylic resins such as polymethyl methacrylate, polybutyl methacrylate and the like; styrene resins such as polystyrene and the like; various resins such as polyvinyl chloride, polyvinylidene chloride, polyamide, polyester, polyether, polyvinyl alcohol, copolymers of the above and the like; various waxes; paraffin, higher fatty acids (e.g., stearic acid); higher alcohols; higher fatty acid esters; higher fatty acid amides; and the like. These may be used singly or in combinations of two or more. Plasticizers may also be added. Examples of the plasticizers include phthalic acid esters (e.g., DOP, DEP and DBP), adipic acid esters, trimellitic acid esthers, sebacic acid esthers and the like. These may be used singly or in combinations of two or more. In addition to the metal powder, binder and plasticizers, if required, various additives such as lubricants, antioxidants, debinding promotors, surface active agents and the like may be added during the compounding process. Conditions for compounding will vary depending on the component and particle size of the metal powder to be used, and the type and amount of the binder and additives to be added. An example of conditions is .a compounding temperature of 20 to 200°C and a compounding time of about 20 to 210 minutes. The obtained feedstock may be pelletized if necessary. Pellet size is set within the range of approximately 1 to 10mm, for example. The feedstock prepared in the above-mentioned manner, or the pellets produced from the prepared feedstock is subjected to injection molding with an injection molding machine to produce a green body having desired shape and dimensions. In this case, a green body having complex and intricate shape can be produced easily by selectively using a suitable die. The shape and dimensions of the green body to be produced should be decided upon taking into account the estimated shrinkage that the green body will experience during debinding and sintering treatment. Conditions for injection molding will vary depending on the component and particle size of the metal powder to the used, the type and amount of the binder, and other factors. As an example of conditions, the material temperature is preferably about 20 or 200°C and the injection pressure is preferably about 30 to 150 kgf/cm 2 . The sectional structure of the obtained green body 1 is shown in Fig. 3. As shown in this figure, the metal powder 20 and pores 30 are generally distributed uniformly throughout the binder 10. The content of the metal powder in the green body 1 after the pressing and prior to the debinding treatment is preferably about 70 to 98 wt%, and more preferably about 82 to 98 wt%. When the content of the metal powder is lower than 70 wt%, the green body experiences greater shrinkage with sintering, and therefore dimensional precision is deteriorated. Further, the porosity and C content of the sintered compact tends to increase. On the other hand, when the content of the metal powder exceeds 98 wt%, the relative content of the binder 10 becomes too small, resulting in poor fluidity during injection molding. This makes injection molding difficult or impossible, or results in inhomogeneous green body composition. Debinding Process for Debinding Green Body The green body produced in the foregoing step is then subjected to a debinding treatment (binder removing treatment). Debinding is effected by heat treatment under a non-oxidising atmosphere such as under a vacuum or reduced pressure state (1 x 10 -1 to 1 x 10 -6 torr, for example), or under an inert gas such as nitrogen gas, argon gas and the like. In this case, the conditions for debinding treatment are preferably about 0.5 to 40 hours at a temperature of about 150 to 750°C, and more preferably about 1 to 24 hours at a temperature of about 250 to 650°C. The debinding process by the heat treatment described above may be carried out in multiple steps (stages). Further, the debinding may also be carried out by means of some treatments other than the heat treatment. The sectional structure of the obtained debinded green body 2 is shown in Fig. 4, in which pores 40 are formed in areas where the binder 10 was formerly present. Presintering (Primary Sintering) The debinded green body 2 produced in the above-mentioned manner is sintered in a sintering furnace to effect presintering. In preferred practice, the presintering of the debinded green body 2 is continued until particles of the metal powder 20 undergo diffusion bonding, at least at the points of contact thereof. Conducting the presintering in this way increases the shape stability. As a result, it becomes possible to reliably prevent various flaws of the green body (presintered compact) such as breaking, chipping, cracking and the like from occurring in subsequent steps, particularly in the pressing step for compacting the green body, thus facilitating handling thereof. In particular, the use of metal powder produced by a gas atomisation method is preferred due to the following advantages. Namely, metal powder produced by a gas atomisation method includes particles which are roughly spherical in shape, and which have fewer surface irregularities (this results in weaker bonding strength between metal powder particles) than that produced by a water atomisation method. Thus, in the case where the green body which has not undergone the presintering treatment is subjected to the pressing, the particle size distribution of the metal powder must be relatively broad, or conditions such as pressure and the like must be regulated to optimal levels during pressing in order to prevent the flaws described earlier from occurring during pressing. However, the presintering treatment is highly effective in preventing flaws from occurring in the green body during pressing as described above, so that metal powder particle size and pressing conditions can be relaxed, that is, they can be selected from a broader range. Therefore, when the gas atomisation method is used in this embodiment to produce metal powder, the mechanical properties of the final sintered compact can be improved. For these reasons, this invention is particularly useful to the case where metal powder produced by a gas atomisation method is used. In this regard, it is needless to say that similar advantages can be obtained using metal powder produced by a water atomisation method and other methods and, therefore, it is possible to use powder produced by any one of these methods. In the case where the metal composition is, for example, Fe or Fe-base alloy, the sintering temperature during such a presintering is preferably about 700 to 1300°C, and more preferably about 800 to 1250°C. Further, in the case where the metal composition is Ti or a Ti-base alloy, the sintering temperature is preferably about 700 to 1200°C, and more preferably about 800 to 1150°C. Furthermore, in the case where the metal composition is W or W-base alloy, it is preferably about 700 to 1400°C, and more preferably about 800 to 1350°C. In this connection, it is to be noted that the sintering temperature during presintering may be changed (risen or lowered) with elapse of time within or outside of the range mentioned in the above, if desired. In the case where the sintering temperatures given in the above are employed, sintering time for presintering is preferably about 0.2 to 6 hours and more preferably about 0.5 to 4 hours. In the preferred practice, the sintering atmosphere is a non-oxidising atmosphere that does not contain hydrogen. This improves safety in sintering and contributes to reduced porosity in the sintered compact. The preferred sintering atmosphere is one under a reduced pressure (a vacuum) of 1 x 10 -2 torr or lower (more preferably to 1 x 10 -2 to 1 x 10 -6 torr), or under an inert gas such as nitrogen gas, argon gas. and the. like at 1 to 760 torr. The sintering atmosphere can be changed during the presintering. For example, the sintering atmosphere, which has been initially set to a reduced pressure (a vacuum) of 1 x 10 -2 to 1 x 10 -6 torr, may be changed into the atmosphere under the inert gases mentioned in the above during the presintering. The sectional structure of the obtained presintered compact (preseintered green body) 4a is shown in Fig. 5, in which the contact points of particles of the metal powder 20 undergo diffusion bonding, and therefore pores 40 are reduced in number. Pressing of Presintered Compact Pressure is applied to the green body (presintered compact 4a) produced in the foregoing step to effect compaction thereof. The pressing method is not limited into any particular way. Examples of pressing methods include a method in which the presintered compact is pressed in a predetermined direction, such as rolling or pressing; and a method in which the compact is pressurised isotropically, such as isostatic pressing. The latter method, particularly isostatic pressing, is preferred. Hereinafter, a description will be made with regard to the isostatic pressing. The isostatic pressing method includes cold isostatic pressing (CIP) which performs pressing at ambient temperature or temperature close to ambient temperature (5 to 60°C, for example); and hot isostatic pressing (HIP), which performs pressing under heating condition (80°C or above, for example). The former is preferred due to the simplicity of the equipment required. Further, since it is not necessary for a coating film to have heat resistance as described later, the former is especially preferred in the case where a green body having three-dimensional shape or having complex and intricate shape is used. In the isostatic pressing, first, the surface of the compact is covered with a coating film having liquid barrier properties (not shown in the drawings), and then the compact is placed in an isostatic pressing unit, where it is subjected to isostatic pressing. In the case of CIP, rubber material such as natural rubber, isoprene rubber and the like may be used for coating film. Further, the coating film may be formed, for example, by dipping. No limitations are imposed upon the pressure employed in this isostatic pressing (isotropic pressing). In preferred practice, the pressure is about 1 to 100 t/cm 2 , and more preferably about 3 to 80 t/cm 2 . Excessively low pressure may not give adequate effect (that is, reduction in porosity through compaction). On the other hand, if pressure is higher than the upper limit given above, it is impossible to achieve an improved effect. In addition, a pressure exceeding the upper limit given above has the drawback of requiring a larger machine, resulting in higher equipment costs. After pressing, the presintered compact 1a is produced in the above-mentioned manner will be in good condition, with molding flaws having been corrected. After the pressing, the coating film on the surface of the green compact may be peeled and removed. Typically, however, there is no need to provide separately a step of removing the coating film, since it can be eliminated by heat in the course of the subsequent debinding or sintering treatment. The sectional structure of the pressed presintered compact 4b is shown in Fig. 6. As shown in this figure, the presintered compact 4a which has been compressed by pressing has high density, and the number of pores 40 among the metal powder particles 20 further reduced as compared to the presintered compact 4a prior to the pressing. In this case, depending on conditions for the pressing, pores 40 can be significantly reduced in number so that pores 40 can be virtually eliminated. After the pressing, the coating film on the surface of presintered compact 4b may be peeled and removed. Typically, however, there is no need to provide separately a step for removing the coating film, since it can be eliminated by heat in the course of the subsequent main sintering process. Main Sintering (Secondary Sintering) The pressed presintered compact 4b produced in the above-mentioned manner is sintered in a sintering furnace to effect main sintering (final sintering) in order to produce a metal sintered compact. As shown in Fig. 7, the main sintering brings diffusion and grain growth of metal powder 20 to form crystal grains 50. The pores 40 disappear to form a sintered compact 4 which is dense overall, that is, a sintered compact 4 having high density and low porosity. In particular, since the number of pores 40 in the presintered compact 4b has been appreciably reduced through the pressing, the main sintering enables to provide a sintered compact 4 having higher density and lower porosity as compared with the case where no pressing has been carried out before the main sintering. In the case where the metal composition is, for example, Fe or Fe-base alloy, the sintering temperature during main sintering is preferably about 950 to 1400° C, and more preferably about 1100 to 1350° C. Further, in the case where the metal composition is Ti or a Ti-base alloy, the sintering temperature is preferably about 900 to 1350° C, and more preferably about 1000 to 1300° C. Furthermore, in the case where the metal composition is W or W-base alloy, it is preferably about 1100 to 1600° C, and more preferably about 1200 to 1500° C. In this case, the sintering temperature during the main sintering is preferably higher than that of the presintering. In general, higher sintering temperatures are advantageous since they can shorten sintering time. On the other hand, however, if sintering temperature is too high, a large load is given to the sintering furnace and sintering jig, so that the life span thereof is shortened due to wear and the like. However, in the present invention, the pressing process in the step [4G] causes particles of the metal powder 20 in the presintered compact to contact each other, which creates internal stress. Since such internal stress created by pressing is released when sintered, it becomes possible to cause diffusion of the metal at lower temperatures, which is advantageous in that sintering temperatures can be lowered and sintering time can be shortened. Such lower sintering temperatures contribute to improved sinterability, as a result facilitating sintering of metal compositions which were difficult to alloy in the past. In this connection, it is to be noted that the sintering temperature during the main sintering may be changed (risen or lowered) with elapse of time within or outside of the range mentioned in the above, if desired. In the case where the sintering temperatures given in the above are employed, sintering time for main sintering is preferably about 0.5 to 8 hours and more preferably about 1 to 5 hours. In preferred practice, the sintering atmosphere is a nonoxidizing atmosphere that does not contain hydrogen. This improves safety in sintering, and contributes to reduced porosity in the sintered compact. The preferred sintering atmosphere is one under a reduced pressure (a vacuum) of 1 × 10 -2 torr or lower (more preferably 1 × 10 -2 to 1 × 10 -6 torr), or under an inert gas such as nitrogen gas, argon gas and the like at 1 to 760 torr. The sintering atmosphere can be changed during the main sintering. For example, the sintering atmosphere, which has been initially set to a reduced pressure (a vacuum) of 1 × 10 -2 to 1 × 10 -6 torr, may be changed into the atmosphere under the inert gases mentioned in the above during the main sintering. The sintering atmosphere for the main sintering may be the same as or different from that for the presintering. Conducting presintering and main sintering under the conditions described above contributes to reduced porosity, that is, higher density of the sintered compact, and enables to obtain high dimensional precision. Further, performing the sintering in multiple step enables to carry out sintering treatment effectively, which results in a shorter sintering time, and realize to a high safety in sintering operation, thereby making it possible to improve productivity in manufacturing sintering compacts. Depending on a particular objective, the present invention may include a preliminary step coming before the step [1G], an intermediate step coming between the steps [1G] and [4G], or a post step coming after the step [4G]. For example, a step of pressing the green body may come between the steps [1G] and [2G], during the step [2G], or between the steps [2G] and [3G]. Fig. 2, is a step diagram which shows a second embodiment of the method of manufacturing sintered compacts according to the present invention; and Figs. 8 and 9 are illustrations of sectional structure (internal metallographic structure) of a presintered compact, at each step after machine working. This second embodiment is the same as in the first embodiment except that a presintered compact is machined after pressing it. Hereinafter, a description will be given with reference to the drawings. [1H] Production of Green Body Same as in the step [1G] (see Fig. 3). [2H] Debinding Process for Debinding Green Body Same as in the step [2G] (see Fig. 4). [3H] Presintering (Primary Sintering) Same as in the step [3G] (see Fig. 5). [4H] Pressing of Presintered Compact Same as in the step [4G] (see Fig. 6). [5H] Machine Working Process The Presintered compact 4b after pressing is subjected to a desired machine working. Examples of machine workings include drilling (as shown in Fig. 8), cutting, grinding, polishing, punching and the like. Any one or combination of two or more of the above may be used. Since the hardness of the presintered compact 4b after the pressing is lower than that of the sintered compact after the main sintering, these machine working processes may be carried out easily regardless of metal composition. In other words, workability for the presintered compact 4b after the pressing is excellent than that for the sintered compact after the main sintering. Therefore, when forming a hole 5 or the like, it is easy to control the shape and dimensions thereof, thus improving dimensional precision. This is advantageous in terms of working complex and intricate shapes, compared to working of sintered compacts after the main sintering . The presintered compact 4b has been compacted through the pressing. Therefore, when machine working (drilling) is performed on such a presintered compact 4b, there is less variation in the shape and dimensions of the hole 5 (in particular, less dimensional error in diameter and depth of the hole 5) in the completed sintered compact 4 in comparison with the case where machine working would be performed on a debinded green body or an uncompacted presintered compact, thus leading to improved dimensional precision. In this regard, the shape and dimensions of a hole 5 to be formed in the presintered compact 4b should be decided taking into account the estimated shrinkage that the presintered compact 4b will experience during the main sintering process (described later). Here, shrinkage of the final sintered compact 4 relative to the presintered compact 4b after pressing is less than shrinkage of the final sintered compact 4 relative to the debinded green body 2 or the presintered compact 4a prior to pressing, so dimensional error can be minimized by drilling the hole 5 in the presintered compact 4b after pressing thereof. That is, the dimensions of the hole 5 in the final sintered compact 4 will be closer to the target dimensions (design value), thus improving dimensional precision in this respect as well. The same manner as in the above is also applied to machine working processes other than drilling. [6H] Main Sintering Same as in the step [5G] (see Fig. 9). Depending on a particular objective, the present invention may include a preliminary step coming before the step [1H], an intermediate step coming between the steps [1H] and [6H], or a post step coming after the step [6H]. For example, a step of pressing the green body may come between the steps [1H] and [2H], during the step [2H], or between the steps [2H] and [3H]. EXAMPLES Hereinafter, specific examples of the manufacturing method of sintered compacts according to the present invention will be described. (Example 1g) As for metal powder, a stainless steel (SUS 316 / composition: Fe-18Cr-12Ni-2.5Mo alloy) powder of 9µm mean particle size was prepared by a gas atomisation method. A binder including 1.9 wt% of polystyrene (PS), 1.8 wt% of an ethylene-vinyl acetate copolymer (EVA) and 1.5 wt% of paraffin wax; and 0.8 wt% of dibutyl phthalate (plasticizer) were mixed with 94 wt% of the prepared metal powder. The mixture containing these components was compounded in a compounding machine under the conditions of 115°C x 1 hour. Then, the obtained feedstock was pelletized and classified to produce pellets of 3mm mean particle size. Using these pellets, metal injection molding (MIM) was performed with an injection molding machine to produce 200 cylindrical green bodies of 11.5mm diameter x 28.7mm height (target dimensions after sintering treatment were 10mm diameter x 25mm height). Molding conditions during the injection molding were mold temperature of 30°C and injection pressure of 110 kgf/cm 2 . The metal powder content of the produced green body was about 93.6 wt %. Next, the produced green bodies were subjected to a debinding treatment in a debinding furnace. This debinding treatment was carried out under the conditions of reduced pressure of 1 × 10 -3 torr at a temperature of 300° C for one hour, and then the temperature was raised to 500° C and such condition was being kept for another one hour. Next, the binder removed green bodies which had been obtained through the debinding treatment were sintered in a sintering furnace to effect presintering in order to produce presintered compacts. Sintering conditions during the presintering treatment were 1050° C × 1 hour under 1 × 10 -3 torr reduced pressure. Next, the produced presintered compact was cooled to ambient temperature, and then an isoprene rubber coating film (thickness 0.3 mm) was formed on the surface of each presintered compact in accordance with a dipping process. The presintered compacts coated with the coating film were set in an isostatic pressing machine ( produced by Rabushiki Kaisha Kobe Selkosho) and then subjected to an isostatic pressing (CIP). In this process, conditions were 22° C temperature , 6 t/cm 2 pressure. Next, the presintered compacts after the pressing were sintered in a sintering furnace to effect main sintering (final sintering) in order to produce sintered compacts. Sintering conditions during the main sintering were 1300°C × 2 hours in an Ar gas atmosphere. The coating film on each green body was eliminated in the course of the sintering treatment. (Example 2g) Sintered compacts were produced in the same manner as in Example 1g, except that conditions for isostatic pressing (CIP) were changed into 22°C temperature under 50 t/cm 2 pressure. (Example 3g) Sintered compacts were produced in the same manner as in Example 1g, except that conditions for isostatic pressing (CIP) were changed into 22°C temperature under 100 t/cm 2 pressure. (Example 4g) Sintered compacts were produced in the same manner as in Example 1g, except that sintering conditions during the presintering treatment were changed into 1100° C × 1 hour under 1 × 10 -3 torr reduced pressure. (Example 5g) Sintered compacts were produced in the same manner as in Example 2g, except that sintering conditions during the main sintering treatment were changed into 1250° C × 2 hours in an Ar gas atmosphere. (Example 6g) Sintered compacts were produced in the same manner as in Example 3g, except that sintering conditions during the presintering treatment were changed into 1130° C × 1 hour in an Ar gas atmosphere and that sintering conditions during the main sintering treatment were changed into 1300°C × 1.5 hours in an Ar gas atmosphere. (Comparative Example 1g) Sintered compacts were produced in the same manner as in Example 1g, except that isostatic pressing process for pressing the presintered compacts was omitted, and that sintering conditions during the main sintering treatment were changed into 1350° C × 2.5 hours in an Ar gas atmosphere. In this regard, the presintering and the main sintering were continuously conducted. (Example 7g) As for metal powder, Ti powder of 6 µm mean particle size was prepared by a gas atomization method. A binder including 2.1 wt% of polystyrene (PS), 2.4 wt% of an ethylene-vinyl acetate copolymer (EVA) and 2.2 wt% of paraffin wax; and 1.3 wt% of dibutyl phthalate (plasticizer) were mixed with 92 wt% of the prepared metal powder. The mixture containing these components were compounded in a compounding machine under the conditions of 115° C × 1 hour. Then, the obtained feed stock was pelletized and classified to produce pellets of 3 mm mean particle size. Using these pellets, metal injection molding (MIM) was performed with an injection molding machine to produce 200 cylindrical green bodies of 11. 2 mm diameter × 28 mm height ( target dimensions after sintering treatment were 10 mm diameter × 25 mm height). Molding conditions during the injection molding were mold temperature of 30° C and injection pressure of 110 kgf/cm 2 . The metal powder content of the produced green body was about 91.5 wt %. Next, the produced green bodies were subjected to a debinding treatment in a debinding furnace. This debinding treatment was carried out under the conditions of reduced pressure of 1 × 10 -3 torr at a temperature of 290°C for one hour, and then the temperature was raised to 450° C and such condition was being kept for another one hour. Next, the binder removed green bodies which had been obtained through the debinding treatment were sintered in a sintering furnace to effect presintering in order to produce presintered compacts. Sintering conditions during the presintering treatment were 1000° C × 1 hour under 1 × 10 -3 torr reduced pressure. Next, after cooling' the produced presintered compact to ambient temperature, a coating film was formed on the surface of each presintered compact in the same manner as in the above, and then the presintered compacts were set in the isostatic pressing machine described earlier and subjected to isostatic pressing (CIP) . In this pressing process, conditions were 27° C temperature, 15 t/cm 2 pressure. Next, the presintered compacts after the pressing were sintered in a sintering furnace to effect main sintering (final sintering) in order to produce sintered compacts. Sintering conditions during the main sintering were 1150° C × 2 hours in an Ar gas atmosphere. The coating film on each green body was eliminated in the course of the main sintering treatment. (Example 8g) Sintered compacts were produced in the same manner as in Example 7g, except that conditions for isostatic pressing (CIP) were changed into 27°C temperature under 40 t /cm 2 pressure. (Example 9g) Sintered compacts were produced in the same manner as in Example 7g, except that conditions for isostatic pressing (CIP) were changed into 27° C temperature under 80 t/cm 2 pressure. (Example 10g) Sintered compacts were produced in the same manner as in Example 7g, except that sintering conditions during the presintering treatment were changed into 1080° C × 0.8 hours under 1 × 10 -3 torr reduced pressure. (Example 11g) Sintered compacts were produced in the same manner as in Example 8g, except that sintering conditions during the' main sintering treatment were changed into 1100° C × 2 hours in an Ar gas atmosphere. (Example 12g) Sintered compacts were produced in the same manner as in Example 9g, except that sintering conditions during the presintering treatment were changed into 1050° C × 1 hour in an Ar gas atmosphere, and that sintering conditions during the main sintering treatment were changed into 1200°C × 1.5 hours in an Ar gas atmosphere. (Comparative Example 2g) Sintered compacts were produced in the same manner as in Example 7g, except that isostatic pressing process for pressing the presintered compacts was omitted, and that sintering conditions during the main sintering treatment were changed into 1220° C × 2.5 hours in an Ar gas atmosphere. In this regard, the presintering and the main sintering were continuously conducted. (Example 13g) As for metal powder, W powder of 3 µm mean particle size. Ni-powder of 2 µm mean particle size, and Cu powder of 12 µm mean particle size were respectively prepared by a reduction method. A binder including 1.2 wt% of polystyrene (PS), 1.4 wt% of an ethylene-vinyl acetate copolymer (EVA) and 1.3 wt% of paraffin wax; and 0.6 wt% of dibutyl phthalate (plasticizer) were mixed with 92 wt% of the W powder, 2.5 wt% of the Ni powder, 1 wt% of the Cu powder. The mixture containing these components were compounded in a compounding machine under the conditions of 100° C × 1 hour. Then, the obtained feed stock was pelletized and classified to produce pellets of 3 mm mean particle size. Using these pellets, metal injection molding (MIM) was performed with an injection molding machine to produce 200 cylindrical green bodies of 12.6 mm diameter × 31.5 mm height (target dimensions after sintering treatment were 10 mm diameter × 25 mm height). Molding conditions during the injection molding were mold temperature of 30° C and injection pressure of 110 kgf/cm 2 . The total content of the metal powder (including the W, Ni and Cu powder) of the produced green body was about 95 wt%. Next, the produced green bodies were subjected to a debinding treatment in a debinding furnace. This debinding treatment was carried out under the conditions of reduced pressure of 1 × 10 -3 torr at a temperature of 280°C for one hour, and then the temperature was raised to 500° C and such condition was being kept for 1.5 hours. Next, the binder removed green bodies which had been obtained through the debinding treatment were sintered in a sintering furnace to effect presintering in order to produce presintered compacts. Sintering conditions during the presintering treatment were 1200° C × 1.5 hours under 1 × 10 -3 torr reduced pressure. Next, after cooling the produced presintered compact to ambient temperature, a coating film was formed on the surface of each presintered compact in the same manner as in the above, and then the presintered compacts were set in the isostatic pressing machine described earlier and subjected to isostatic pressing (CIP). In this pressing process, conditions were 35° C temperature, 8 t/cm 2 pressure. Next, the presintered compacts after the pressing were sintered in a sintering furnace to effect main sintering (final sintering) in order to produce sintered compacts. Sintering conditions during the main sintering were 1350° C × 2 hours in an Ar gas atmosphere. The coating film on each green body was eliminated in the course of the sintering treatment. (Example 14g) Sintered compacts were produced in the same manner as in Example 13g, except that conditions for isostatic pressing (CIP) were changed into 35° C temperature under 30 t/cm 2 pressure. (Example 15g) Sintered compacts were produced in the same manner as in Example 13g, except that conditions for isostatic pressing (CIP) were changed into 35°C temperature under 65 t/cm 2 pressure. (Example 16g) Sintered compacts were produced in the same manner as in Example 13g, except that sintering conditions during the main sintering were changed into 1350° C × 1.5 hours in an Ar gas atmosphere. (Example 17g) Sintered compacts were produced in the same manner as in Example 14g, except that sintering conditions during the main sintering were changed into 1300° C × 2 hours in an Ar gas atmosphere. (Example 18g) Sintered compacts were produced in the same manner as in Example 15g, except that sintering conditions during the main sintering were changed into 1300°C × 1.5 hours in an Ar gas atmosphere. (Comparative Example 3g) Sintered compacts were produced in the same manner as in Example 13g, except that isostatic pressing process for pressing the presintered compacts was omitted, and that sintering conditions during the main sintering treatment were changed into 1400° C × 2.5 hours in an Ar gas atmosphere. In this regard, the presintering and the main sintering were continuously conducted. <QUALITY/PROPERTIES EVALUATION> The sintered compacts obtained in each of Examples 1g - 18g and in each of Comparative Examples 1g - 3g were cut along ifferent cutting planes to observe visually the cutting planes thereof . In each observation, no sintering flaws and other flaws were found on the cutting plane of each sintered compact. Namely, through the observations, it was found that the sintered compacts obtained in each of Examples 1g- 18g and in each of Comparative Examples 1g - 3g had good quality. Subsequently, each sintered compact was measured to determine relative density (which was represented from the equation "100 - porosity" [%]) and tensile strength [N/mm 2 ]. Measurement results are given in the attached Tables 1 to 3. As shown in each table, it has been found that the sintered compacts obtained in each of Examples 1g - 18g can have higher density and improved mechanical strength under the sintering conditions of lower sintering temperatures and shorter sintering times as compared with the sintered compacts prepared in each of Comparative Examples 1g - 3g of which presintered compacts were not pressurized. (Example 1h) 200 sintered compacts were produced in the same manner as in Example 1g, except that a hole of 5.1 mm diameter × 10.2 mm deep (target dimensions after main sintering were 5 mm diameter × 10 mm depth) was formed in the center of each presintered compact after pressing. (Example 2h) 200 sintered compacts were produced in the same manner as in Example 2g, except that a hole having the same dimensions as in Example 1h was formed in the center of of each presintered compact after pressing. (Example 3h) 200 sintered compacts were produced in the same manner as in Example 3g, except that a hole having the same dimensions as in Example 1h was formed in the center of each presintered compact after pressing. (Example 4h) 200 sintered compacts were produced in the same manner as in Example 4g, except that a hole having the same dimensions as in Example 1h was formed in the center of each presintered compact after pressing. (Example 5h) 200 sintered compacts were produced in the same manner as in Example 5g, except that a hole having the same dimensions as in Example 1h was formed in the center of each presintered compact after pressing. (Example 6h) 200 sintered compacts were produced in the same manner as in Example 6g, except that a hole having the same dimensions as in Example 1h was formed in the center of each presintered compact after pressing. (Comparative Example 1h) 200 sintered compacts were produced in the same manner as in Comparative Example 1g, except that a hole of 5.15 mm diameter × 10.3 mm deep (target dimensions after main sintering were 5 mm diameter × 10 mm depth) was formed in the center of each presintered compact after pressing (which had not undergone pressing). (Example 7h) 200 sintered compacts were produced in the same manner as in Example 7g, except that a hole of 5.1 mm diameter × 10.2 mm deep (target dimensions after main sintering were 5 mm diameter × 10 mm depth) was formed in the center of each presintered compact after pressing. (Example 8h) 200 sintered compacts were produced in the same manner as in Example 8g, except that a hole having the same dimensions as in Example 7h was formed in the center of each presintered compact after pressing. (Example 9h) 200 sintered compacts were produced in the same manner as in Example 9g, except that a hole having the same dimensions as in Example 7h was formed in the center of each presintered compact after pressing. (Example 10h) 200 sintered compacts were produced in the same manner as in Example 10g, except that a hole having the same dimensions as in Example 7h was formed in the center of each presintered compact after pressing. (Example 11h) 200 sintered compacts were produced in the same manner as in Example 11g, except that a hole having the same dimensions as in Example 7h was formed in the center of each presintered compact after pressing. (Example 12h) 200 sintered compacts were produced in the same manner as in Example 12g, except that a hole having the same dimensions as in Example 7h was formed in the center of each presintered compact after pressing. (Comparative Example 2h) 200 sintered compacts were produced in the same manner as in Comparative Example 2g, except that a hole of 5.15 mm diameter × 10.3 mm deep (target dimensions after main sintering were 5 mm diameter × 10 mm depth) was formed in the center of each presintered compact. (Example 13h) 200 sintered compacts were produced in the same manner as in Example 13g, except that a hole of 5 .1 mm diameter × 10.2 mm deep (target dimensions after main sintering were 5 mm diameter × 10 mm depth) was formed in the center of each presintered compact after pressing. (Example 14h) 200 sintered compacts were produced in the same manner as in Example 14g, except that a hole having the same dimensions as in Example 13h was formed in the center of each presintered compact after pressing. (Example 15h) 200 sintered compacts were produced in the same manner as in Example 15g, except that a hole having the same dimensions as in Example 13h was formed in the center of each presintered compact after pressing. (Example 16h) 200 sintered compacts were produced in the same manner as in Example 16g, except that a hole having the same dimensions as in Example 13h was formed in the center of each presintered compact after pressing. (Example 17h) 200 sintered compacts were produced in the same manner as in Example 17g, except that a hole having the same dimensions as in Example 13h was formed in the center of each presintered compact after pressing. (Example 18h) 200 sintered compacts were produced in the same manner as in Example 18g, except that a hole having the same dimensions as in Example 13h was formed in the center of each presintered compact after pressing. (Comparative Example 3h) 200 sintered compacts were produced in the same manner as in Comparative Example 3g, except that a hole of 5.15 mm diameter × 10.3 mm deep (target dimensions after main sintering were 5 mm diameter × 10 mm depth) was formed in the center of each presintered compact. <QUALITY/PROPERTIES EVALUATION> The sintered compacts obtained in each of Examples 1h - 18h and in each of Comparative Examples 1h - 3h were cut along different cutting planes to observe visually the cutting planes thereof . In each observation, no sintering flaws and other flaws were found on the cutting plane of each sintered compact. Namely, through the observations, it was found that the sintered compacts obtained in each of Examples 1h - 18h and in each of Comparative Examples 1h - 3h had good quality. Subsequently, each sintered compact was measured to determine relative density (which was represented from the equation "100 - porosity" [%]) and tensile strength [N/mm 2 ]. Measurement results are given in the attached Tables 4 to 6. Further, dimensional error in diameter and height of each sintered compact (that is, error with respect to target dimensions: which is represented as average value for 200 compacts); and dimensional error in diameter and depth of the hole formed in each sintered compact (that is, error with respect to target dimensions: which is represented as average value for 200 sintered compacts) were measured. Measurement results are presented in the attached Tables 4 to 6. As shown in each table, it has been found that the sintered compacts obtained in each of Examples 1h - 18h can have higher density and improved mechanical strength under the sintering conditions of lower sintering temperatures and shorter sintering times as compared with the sintered compacts prepared in each of Comparative Examples 1h - 3h of which presintered compacts were not pressurized. Further, it has been also found that the sintered compacts prepared in each of Examples 1h - 18h exhibit less dimensional error in the overall and in the hole and have high dimensional precision as compared with the sintered compacts prepared in each of Comparative Examples 1h - 3h of which presintered compacts were not pressurized. According to the invention described above, it is possible to obtain sintered compacts having improved sinterability and higher quality. In particular, it is possible to obtain sintered compacts having higher density and improved mechanical strength. Further, according to the present invention, sintering conditions can be moderated, in particular, lower sintering temperatures or shorter sintering times can be used, while still maintaining high quality, thereby facilitating to manufacture sintered compacts and reducing the load applied to the sintering furnace and sintering jig. In particular, when pressing for the green body is carried out during the debinding treatment, it is possible to effectively prevent flaws from being formed on the green body during the pressing. Further, when conducting the pressing after the presintering process, it is possible to effectively prevent flaws from being formed on the presintered compact during the pressing. Furthermore, according to the present invention, it is possible to stabilize shapes and dimension of the sintered compacts and increase dimensional precision. In particular, excellent workability can be achieved during machine working processes, and machining for hard metals and complex shapes which were not readily accomplished with conventional machining process can be made easily. In addition, machined areas have high dimensional precision. INDUSTRIAL UTILIZATION The method of manufacturing sintered compacts according to the present invention is suitable for manufacturing of various metal products such as exterior components of watches, accessories and other precious metal products, eyeglass frames, various machine components, tools, weights , golf club heads and other sports products, weapons, coins, medallions, and the like. The method is particularly suitable for manufacturing of products having complex shape and products which are required to have high dimensional precision.

Claims: A method of manufacturing a sintered compact, comprising steps performed in order as follows: producing a green body containing metal powder; debinding the green body at least once; presintering the debinded green body; compacting the presintered green body by pressing it; and sintering the compacted presintered green body further to obtain a sintered compact. A method of manufacturing a sintered compact as claimed in claim 1, further comprising a step of performing machine working on the compacted presintered green body between the presintered green body compacting step and the presintered green body sintering step. A method of manufacturing a sintered compact as claimed in claim 1 or claim 2, wherein the green body presintering step is carried out until diffusion bonding is made at least at contact points of particles of the metal powder. A method of manufacturing a sintered compact as claimed in any one of the preceding claims, wherein the pressing for the compacting is carried out isotropically. A method of manufacturing a sintered compact as claimed in any one of the preceding claims, wherein the pressing for the compacting is carried out by means of an isostatic pressing. A method of manufacturing a sintering compact as claimed in claim 5, wherein the isostatic pressing is carried out at ambient temperature or temperature close to ambient temperature. A method of manufacturing a sintered compact as claimed in any one of the preceding claims, wherein pressure during the pressing is 1 to 100 t/cm 2 . A method of manufacturing a sintered compact as claimed in any one of the preceding claims, wherein the green body producing step is carried out by means of metal injection molding. A method of manufacturing a sintered compact as claimed in any one of the preceding claims, wherein the metal powder content of the green body just before the debinding treatment is 70 to 98 wt%. A method of manufacturing a sintered compact as claimed in any one of the preceding claims, wherein the metal powder for the green body is prepared in accordance with a gas atomisation method.

Inventors: SAKATA MASAAKI, SHIMODAIRA KENICHI, SAKATA, MASAAKI, SHIMODAIRA, KENICHI, SAKATA, Masaaki, Injex Corporation, Shimodaira, Kenichi, Injex Corporation
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1999
Main classification: B22F3
IPC codes: B22F3