Rubber isostatic pressing RIP of powders for magnets and other materials

Transcription

1 Ž. Materials and Design ž / Rubber isostatic pressing RIP of powders for magnets and other materials Masato Sagawa, Hiroshi Nagata, Toshihiro Watanabe, Osamu Itatani Intermetallics Co. Ltd., 401 Erie, 22-1 Olagecho, Matsumuro, Nishikyo-ku, Kyoto , Japan Abstract A brief review is given about the development of rubber isostatic pressing Ž RIP. of powders for magnets and other materials that includes: principles of RIP; RIP apparatuses; recent progress; and advantages of RIP. As the recent progress, a new filling technique, air tapping Ž AT. and grid separation Ž GS., is presented. Owing to the development of the AT GS technique, the RIP process has become a promising new technology of powder compaction not only for magnets but also for many other materials including titanium alloys, various ceramics, cemented carbides, diamond powders, and eventually, ordinary metal powders for sintered machine parts Published by Elsevier Science Ltd. All rights reserved. Keywords: Rubber isostatic pressing; Net shape compaction powder; Powder filling; Rubber mold; Isostatic pressing 1. Introduction We have developed a powder compaction technique called rubber isostatic pressing Ž RIP. for producing green compacts of permanent magnet powders with improved orientation degree 1 3. We are now exploring RIP for use with other materials including titanium alloys, various ceramics, cemented carbides, diamond powders, and eventually, ordinary metal powders for sintered machine parts 4. Although RIP had been a known technique for 20 years or more, nobody had been serious to develop this technique for industrial uses before we started the development almost 10 years ago. The development of this kind is not such a one that is achieved in a flash. Accumulation of know-how is needed. The most important know-how is about the filling of powder into the cavity of the rubber mold. This paper reviews our development of the RIP process including the filling Corresponding author. Tel.: ; fax: technique as an accumulation of know-how developed until recently. Because the accumulation of know-how in RIP is growing rapidly now, some important knowhow developed recently is presented besides the contents published in our previous papers Principles of RIP The basic RIP arrangement is shown in Fig. 1. A rubber mold, with a cavity for powder filling, is inserted into a die. The powder-filled mold is then pressed by the upper and lower punches to form a compact. To prevent the rubber of the mold from sticking out from the clearances between the punches and the die, sealing rings made of rubber, harder than that of the mold, are mounted on the surfaces of the punches. A variation of the RIP arrangement is shown in Fig. 2. In this structure, called die float type rubber-mold-die set, the upper punch pushes down the top surface of the die while the height of the lower punch being inserted into the die is virtually fixed. As the die is pushed down, the distance between the two punches is $ - see front matter 2000 Published by Elsevier Science Ltd. All rights reserved. Ž. PII: S

2 244 ( ) M. Sagawa et al. Materials and Design powder from being disturbed during pressing. A pulsed field can be much stronger than a stationary field. Such a strong field can destroy agglomeration of the powder to give it a very high orientation degree, even though the filling density of it is very high. 3. If the filling density of the powder is high, the deformation of the powder during pressing is small. If the deformation of the powder is small, the degree of disturbance in orientation during pressing is small. Fig. 1. Principle arrangement of RIP. reduced to press the powder together with the rubber mold. This structure is conveniently employed in automated RIP apparatuses explained in Section 3. In RIP, the powder is pressed not only along the press axis, but in all lateral directions due to the deformation of the rubber mold. The deformation of the powder during pressing can be either isotropic or anisotropic, depending on the relationship between the size of the cavity and the thickness of the rubber wall of the mold in the lateral direction. The greater the wall thickness, the larger the deformation of the powder in the lateral direction with respect to its shrinkage along the press-axis. RIP can give the compacts of permanent magnet powder higher orientation degree than the ordinary die pressing because of the following reasons. 1. RIP can make the deformation of powder during pressing isotropic by using a rubber mold properly designed as explained above. If the powder is pressed isotropically, the orientation of powder given magnetically is not disturbed during pressing. 2. If the orientation of powder is not disturbed during pressing, the magnetic field for aligning the powder can be a pulsed field: it is not necessary to keep the magnetic field high to prevent the orientation of The orientation degree of a magnet is defined as the ratio of the remanence against the saturation magnetization. The orientation degrees of the NdFeB-sintered magnets achieved by various pressing methods are as follows: ADP TDP RIP 86 88% 90 93% 94 96% where ADP is axial die pressing and TDP transverse die pressing, both of which are the conventional pressing methods. In RIP, the pulsed magnetic field can be as high as 3 4 T, while the stationary field used in ADP and TDP can practically be 1.5 T at most. Most of the NdFeB and ferrite magnets are flat plates with various profiles such as rectangular, circular, fan-shaped, in which the alignment direction is perpendicular to the surface of the plates. To produce such magnets, ADP is superior to TDP from the viewpoint of net shape manufacturing, while TDP gives higher orientation than ADP. RIP gives a solution to dissolve this frustration keeping the pressing cycle unchanged: RIP is as good as ADP from the viewpoint of net shape manufacturing for the typical shapes of the magnets and at the same time gives the compacts of the magnet powder better orientation degree enabling the production of permanent magnets with higher performance than TDP. 3. RIP apparatuses Fig. 2. Spring-die-float type rubber-mold-die set. Multistation apparatuses are used to implement the RIP technology. The unit comprises at least four stations: filling, pressing, ejecting and cleaning station. Two types of automated RIP apparatuses, rotary motion type Ž R-RIP. and linear motion type Ž L-RIP., have been designed and built until now. In either type of presses, rubber-mold-die sets are transferred from station to station, and at each station, each processing operation is performed. In the R-RIP press, the same number of the rubber-mold-die sets as the number of the stations are installed, and in all stations, all the processing operations are performed at the same time.

3 ( ) M. Sagawa et al. Materials and Design In the L-RIP press, a single rubber-mold-die set is transferred from station to station, and each processing operation is performed in the order from filling to ejecting and cleaning. The number of the stations can be increased. This allows for a variety of other processing operations to be incorporated. An important example of this is magnetic alignment, which is used when producing anisotropic NdFeB- and ferrite-sintered magnets. It is easy to adopt the multistation system in RIP but not in the ordinary die pressing Ž DP.. This variance arises from the difference in the positioning accuracy required for the die with respect to the punches. Roughly speaking, RIP requires this accuracy in the range of 100 m in either case, if the type of Fig. 1 or Fig. 2 is employed for the rubber-mold-die sets, while DP requires it in the range of 10 m or less. It would be almost impossible to keep such an extremely high accuracy of positioning during operation, if the multistation system is adopted in DP presses. The advantages of adopting the multistation system are as follows: 1. because there is a large space in each station above the rubber-mold-die set, there is a large flexibility for the processing operation that can be applied to the powder; 2. the number of the processing operations applied to the powder can be chosen freely depending on the necessity; and 3. productivity can be increased in the case of R-RIP because all processing operations are performed at the same time in all stations. An example of R-RIP and L-RIP is shown in Figs. 3 Fig. 3. A 70-t RIP apparatus for NdFeB-sintered magnets. Ž. Fig. 4. A 500-t RIP apparatus not for magnets. and 4, respectively. The R-RIP press in Fig. 3 is used for producing NdFeB-sintered magnets. The press capability of this press is 70 t. The L-RIP press in Fig. 4 is designed and built for use with titanium alloys and ceramics. The press capability of this press is 500 t. The main part of either press is encapsulated so that all processing operations can be performed in a nitrogen atmosphere. 4. Recent progress 4.1. Filling technique In RIP, filling of powder into the cavity is crucial. To produce green compacts in good shape, the powder has to be filled to a density as high as its tap density and homogeneously throughout the cavity. We have encountered a filling technique called the air tapping Ž AT. method that allows one to achieve high and homogeneous filling of powder in the cavity of the rubber mold. The principle arrangement of the AT method is shown in Fig. 5. The procedure is as follows: Ž. 1 a weighed powder is poured into the hopper before mounting the cover on the hopper; Ž. 2 the cover is mounted on the hopper; Ž. 3 air in the internal space of the hopper and the cavity of the rubber mold is evacuated slowly by driving the aspirator; Ž. 4 air is introduced rapidly into the space of the hopper and the cavity; and Ž. 5 the steps Ž. 3 and Ž. 4 are repeated several times. A typical cycle of the air evacuation and introduction for AT is shown in Fig. 6. It should be noted that the terms slow and rapid are in a sense of comparison with each other: even the slow evacuation is done within a fraction of a second so that the total cycle of AT is completed within a couple of seconds. When AT is applied to the powder filled in the space of the hopper connected with the cavity of the rubber

4 246 ( ) M. Sagawa et al. Materials and Design Ž. Fig. 5. Principle arrangement of air tapping AT. Fig. 6. Typical cycle of air evacuation and introduction in AT. mold, rapid blows of air destroy bridges and cavities contained in the powder, and at the same time, push all the powder toward the bottom of the cavity. As a result, a state of high and homogeneous filling of powder is realized in the cavity. The apparent density of the powder can be increased to the so-called tap density that is defined as the apparent density of powder reached by applying mechanical tapping. Although AT allows one to achieve high and homogeneous filling throughout the cavity, it takes a long time to weigh the powder to be poured in the hopper each time before the AT filling. We have solved this problem by combining what we call grid separation Ž GS. with AT as shown in Fig. 7. In this method, the powder poured in the hopper is not weighed: the quantity of the powder has only to be larger than that to be filled in the cavity of the rubber mold. The hopper has a grid at its bottom opening. After the hopper is mounted on the rubber mold when the opening of the hopper is connected to the cavity Fig. 7Ž. 1, AT is driven and the powder is pushed down to fill the cavity Fig. 7Ž. 2. At this stage, the apparent density of the powder filled in the cavity reaches the tap density. Then the powder left in the hopper is separated from that filled in the cavity of the rubber mold by lifting the hopper upward Fig. 7Ž. 3. We have found that no powder drops from the hopper. This is because the grid sustains the powder that forms a soft cake after AT. In the next cycle of the powder filling, a weak mechanical vibration is exerted to the hopper to destroy the soft cake. This AT GS technique greatly improves the productivity of the RIP process because the weighing of the powder is not needed. It is enough to make a rough control of the quantity of the powder being poured in the hopper. Besides the improvement of the productivity of the RIP process, there are many interesting applications of this new filling technique. For example, if a curved grid is used, it is possible to fill powder in a cavity in such a manner that the top surface of the powder is curved as explained in Section 5. The AT GS technique is useful also as a general technique for Fig. 7. Air tapping Ž AT. and grid separation Ž GS.Ž. : 1 pouring powder into the hopper; Ž. 2 applying air tapping Ž AT.; and Ž. 3 separation of powder with a grid Ž GS..

5 ( ) M. Sagawa et al. Materials and Design the filling of various powders into cavities or containers Magnetic rubber For the net shape manufacturing of permanent magnets by RIP, we have found that the rubber mold has to be made from rubber containing iron powder. Without iron powder in the rubber of the rubber mold, compacts of permanent magnet powders tend to have a distortion like a beer barrel in producing a straight cylinder. Such a distortion occurs in the magnetic alignment of powder by applying a strong pulsed field to the powder. To prevent this type of distortion, iron powder is mixed in the liquid rubber from which the mold is made. The concentration of the iron powder in the rubber is adjusted so that the saturation magnetization of the magnetic rubber coincides with the saturation magnetization of the permanent magnet powder at its apparent density in the cavity of the rubber mold. In other words, the values that the saturation magnetization of iron and the permanent magnet alloy multiplied by the volume fractions of these powders in the rubber and in the cavity, respectively, have to be the same. 5. Advantages of the RIP process Besides the advantage that it enables remarkable improvement in the magnetic properties of the anisotropic sintered magnets such as NdFeB and ferrite magnets, the RIP process that we have developed has the following advantages as a powder compaction technique: 5.1. Shape An advantage of this process is that it allows a greater variety of shapes of compacts to be produced. As shown in Fig. 8, a variety of three-dimensional shapes such as helical gears and threads as well as long pipes and flat plates can be produced. In addition, RIP allows the production of the ultimate three-dimensional shape, sphere. Fig. 9 shows its arrangement in which a curved grid is used. By applying AT, powder is filled in the cavity of the rubber mold through the curved grid Fig. 9Ž. 1, and then the hopper is lifted from the rubber mold. As shown in Fig. 9Ž. 2, the top surface of the powder filled in the cavity is curved exactly as the grid is curved. We have confirmed repeatedly that the shape of the powder that is in the tap density is stable: no corruption occurs. Next, the top part of the rubber mold is put on as shown in Fig. 9Ž. 3, and RIP is performed. This new filling method combined with RIP enables the production of compacts with shapes that have no straight plane at all. RIP enables compaction of long pipes and solid rods. For the compaction of pipes, a metallic core is used. Because this metallic core supports the compact and prevents it from being broken during the release of pressure, pipes are easier than solid rods to produce by RIP. By selecting proper rubber material for the mold, solid rods with aspect ratio up to 10 can be produced by RIP. For pipes, this limit is much larger than that for solid rods. The development is currently in progress to expand further the capability for long parts. It should be noted that the RIP process has an advantage in also producing flat compacts as shown in Fig. 8. Green compacts of titanium powder produced by RIP.

6 248 ( ) M. Sagawa et al. Materials and Design Fig. 9. Air tapping Ž AT. and grid separation Ž GS. for producing a sphere compact: Ž 1. air tapping Ž AT.; Ž 2. separation of powder with a grid Ž GS.; and Ž. 3 putting the top part of the rubber mold. Fig. 8. Our filling technique, AT GS, enables a homogeneous filling of powder for such flat shapes, and RIP ensures a homogeneous compaction. Flat compacts produced by the process have very homogeneous green density throughout. Therefore, the distortion of the products during sintering can be suppressed. This is in contrast to the conventional filling followed by DP. Flat compacts produced in such a manner show large distortion during pressing Tolerances The development of the state-of-the-art filling technique, namely, AT GS has enabled us to achieve tolerances in weight of less than 1%, and in size of less than 0.5% for compacts consisting of approximately 100 g of steel powder. This is superior to CIP and also to DP for some shapes, such as long pipes and rods. The GS technique contributes to the improvement in the tolerance. Without this technique, the top part of the powder filled in the cavity by AT tends to be disordered by the fast flow of air in AT leading to a distortion of the compact at its top part. Further improvement in tolerance is in progress Green density The green density of the compacts produced by RIP is homogeneous and adjustable. This characteristic can be utilized for the production of oil-impregnated sintered bearings, sintered filter, tantalum condensers, and so on. The maximum pressure that RIP can exert on compacts practically is 4 t cm 2 with sufficient life of rubber molds and sealing rubber rings. Using a commercially available powder for sintered steel parts ŽFe 2% Cu 0.8C with 0.8% zinc stearate, average particle size 100 m., RIP test was conducted. When the pressure is 4 t cm 2, the green density of the compacts of this powder was 7.0 g cm 3. This high level of the green density demonstrates that RIP is also useful for the production of ordinary sintered steel parts Producti ity In the automated rotary-motion-type RIP press, the filling step controls the productivity. The development of the new filling technique, AT GS has enabled us to improve the productivity remarkably. Our fastest RIP press can produce a green compact every 7 s. This is much faster than CIP. The AT GS technique allows one to use multicavity rubber mold as shown in Fig. 10. Before the development of the GS technique, the powder shots to be filled in the cavity had to be weighed. In the case of Fig. 10, seven weighing devices have to be installed. This makes the apparatus too complex. By using the GS technique, the powder is filled in all seven cavities at the same time from a hopper that has a grid that covers all seven cavities. We have confirmed that the scatterings of the weight and the sizes of the compacts can be suppressed to the level for the single cavity rubber mold. Fig. 10 shows the sizes of a multicavity rubber mold used in an experiment of RIP and the green compacts obtained in this

7 ( ) M. Sagawa et al. Materials and Design can be compacted to various shapes including long pipes easily without adding any binder in the powder. RIP creates no friction between the die wall and the powder so that titanium alloy powder and abrasive powders such as those containing diamond powder for machining tools, both of which are known to be difficult to compact by DP, are well suited to RIP compaction Binder and lubricant Fig. 10. A multicavity rubber mold and green compacts produced from it by RIP. experiment. In this experiment, a powder for the Nd- FeB-sintered magnets were used and the pressure in RIP was 0.8 t cm 2. By using the multicavity rubber mold, the productivity of RIP in producing green compacts becomes competitive to the fastest DP presses Press cost If an RIP press and a DP press Ža press for die pressing. with the same press capability are compared, the RIP press requires much less rigid frame than the DP press because the clearance between the die and punches in RIP can be an order of magnitude larger than in DP. Because of this, the RIP press is much smaller and less expensive than the DP press for the same press capability. This differences in size and cost of the presses between the two types tend to expand as the press capability increases. Even 1000 or 2000-t RIP presses can be surprisingly small in size Powder types RIP can use powders with average particle size ranging from nanometers to 100 m. We have experiences of compacting fine powder of alumina with average particle size of less than 0.1 m. It may be surprising that the dry powder of alumina that is not granulated MIM needs addition of much binder in the powder. Because of this, it takes a long time to dewax. DP needs addition of lubricants for reducing friction between the die wall and the powder, and binder for preventing cracks in the compacts. For CIP, the powder has to be granulated to make filling density of powder homogeneous in the rubber mold. For the granulation, binder has to be added to the powder. To sinter the compacts produced from powder containing binder and lubricant, dewaxing is needed. During dewaxing, substantial carbon-contamination can occur. For RIP, neither binder nor lubricant is needed to be added to the powder to achieve good compaction. Therefore, there is no concern of carbon-contamination in the products produced by RIP Mold cost and rapid prototyping In MIM and DP, the mold cost is enormous and the lead times for the mold are very long. On the other hand, as the RIP mold is manufactured by casting liquid rubber with the master mold produced by the simple machining of cheap materials like brass or plastic, the cost and lead times for the mold are very favorable compared with MIM and DP. This makes the technique well suited to rapid prototyping. References 1 Sagawa M, Nagata H. Novel processing technology for permanent magnets. IEEE Trans Magn MAG-29, 1993: Sagawa M, Nagata H. Improvements of manufacturing process for Nd-FeB sintered magnets. Proceedings of the 13th International Workshop on Rare-earth Magnets. Birmingham, UK, 1994 Ž Suppl Sagawa M, Nagata H, Watanabe T, Itatani O. Rubber isostatic pressing Ž RIP. for ferrite magnets. Proceedings of the Seventh International Conference on Ferrites. Bordeaux, France, 1996:C Sagawa M, Nagata H, Watanabe T, Itatani O. Development of RIP Ž Rubber Isostatic Pressing. technology for powder compaction. Proceedings of the 1998 Powder Metallurgy World Congress & Exhibition. Granada, Spain, 1998;2:103.