Abstract:
The present invention relates to a metal powder compression tool for forming compacts for sintering, including a first punch ( 15 ) adapted to compress powder located in a die ( 10 ). The clearance between the punch and the die is greater than the radial expansion of the punch under the desired compression effort, and lower than the mean size of the powder grain.

Description:
The present invention relates generally to the manufacture of articles by sintering techniques and more specifically to a powder compression tool for forming a work piece herein termed a compact, which is then placed in a sintering furnace. 
     BACKGROUND OF THE INVENTION 
     In general terms, sintering consists of compressing metal powder, generally a steel powder, to obtain a compact of definitive shape. This compact whose shape is maintained only by cohesion of the powder, is then passed through a furnace at a sintering temperature below the melting temperature, but sufficient for the powder particles to join. 
     After sintering, the product will typically exhibit a final density which approaches, but does not equal the density of the metal in question. In the case of steel powder, it is possible to achieve final densities on the order of 7.4-7.5 g/cc, using the conventional pressing and sintering techniques described below, whereas the density of steel itself is on the order of 7.8-7.9. For ease of reference, this will be referred to as the maximum density. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a modified pressing process and apparatus capable of operation to yield sintered products having a final sintered density which more nearly approaches the maximum density of the material, and in the case of steel, a final sintered density of over 7.5. According to the present invention, this is achieved by a single press, single sinter process in which a metal powder mix containing from about 0.3 to 0.5 weight % of a solid lubricant is pressed in a single step in a die having a working clearance of at least 45 μm under a pressure of at least 800 MPa to form a compact for subsequent sintering. 
     For better understanding of the basic technology, conventional powder compression processes will now be described with the aid of FIGS. 1A,  1 B and  1 C. 
     FIGS. 1A to  1 C illustrate the operation of a powder compression tool. The tool includes a die  10  with a cavity  12  arranged through it. This cavity  12  defines the shape or profile of the desired compact, including features such as a smooth surface, an indentation, or other characteristic. Die  10  co-operates with an upper punch  14  and a lower punch  15  which penetrate through both ends of the cavity  12 . 
     In FIG. 1A, the cavity  12  is filled with metal powder flush with the upper surface of die  10 . Lower punch  15  is at a specific position determined by the volume of powder required to obtain the desired height and density of the final produce Once cavity  12  is filled with powder, upper punch  14  is lowered. 
     In FIG. 1B, upper punch  14  reaches an end position determined by the pressure applied to both punches. A compact  17  of desired shape is then obtained in cavity  12 , formed of powder particles sufficiently cohered together to allow it to be handled and carried to a sintering furnace (not shown). 
     In FIG. 1C, upper punch  14  is withdrawn while lower punch  15  is raised to eject compact  17  from the cavity  12 . The compact is then carried to the sintering furnace. To eject Impact  17 , instead of raising the punch  15 , die  10  could be lowered. It will be appreciated that various options are possible. 
     As illustrated in FIGS. 1A and 1B, the volume of the powder decreases considerably on application of pressure. For conventional pressures, on the order of 700 MPa. the volume decreases by a factor 2.3 to 2.5. This decrease in volume is accompanied by rubbing of the powder against the wails of the cavity  12  over the length of travel of the punch. It is thus essential to lubricate the walls of the cavity  12  to minimise friction. 
     Lubrication of the walls of the cavity  12  being impractical in production, it is preferred to include the lubricant in the metal powder. For the powder to be able to properly flow to fill up the cavity, the lubricant also comes as a powder. 
     Of course, lubrication also facilitates ejection of the compact  17 , without damage. 
     The proportion of lubricant commonly used in the metal powder is from 0.6 to 0.8% in weight. However, the lubricant is about eight times less dense that the metal powder, and occupies an incompressible volume which cannot be replaced by metal during the compression. As a result, especially upon elimination of the lubricant while sintering. the obtained compacts are porous and have a mechanical strength which is substantially lower than that of pure metal. 
     Thus in practical terms, conventional pressing and sintering processes can yield products with a final density (in the case of steel) of up to 7.5. More typically, using a pressure of 700 MPa and 0.8% lubricant, the final density is only around 7.15. In theory, higher pressures would tend to increase the final density, but in practice, pressures exceeding about 800 MPa have been observed to result in rapid tool damage, even though the tool itself is, in isolation, capable of withstanding more than 2000 MPa. 
     It is appropriate to mention that final sintered density is much more significant than the so-called “theoretical maximum density” of the compact, including lubricant, before sintering. Reducing the lubricant quantity may make it possible to achieve a higher percentage of the maximum theoretical density of a particular metal powder/lubricant mixture, but even values such as 96% of maximum theoretical density correspond only to a final sintered density of 7.15 in the case of steel powder containing 0.8% lubricant. 
     A final density, in the case of steel powder, of around 7.15 thus is typical of that obtained through a conventional single press/single sinter process, in which a single powder compression step is performed, at about 700 MPa, followed by sintering to obtain the final product. 
     To obtain sintered compacts with higher densities, a double press/double sinter process can be used, in which, after compression under the above-mentioned conditions, the compact undergoes a pre-sintering treatment to vaporise the lubricant, so as to empty the pores that it occupies. The compact is then submitted, before a final sintering, to a second compression during which the material, not yet generally integral, tends, through plastic deformation, to occupy the empty pores. With such a process, however, final densities above 7.5 cannot be achieved. Further, such a two-stage process is more expensive to implement than a single press/single sinter process. 
     There is also a warm forming process in which the die and powder are heated to about 100-150° C. to liquefy the lubricant which then escapes by draining from the pores. The maximum densities obtained are on the order of 7.4 (in the case of steel) and the process is also expensive to implement. 
     A further object of the present invention is to provide a compression tool which can more successfully withstand operation at higher than normal pressures. 
     Yet another object of the present invention is to provide such a tool which enable compacts of particularly high final density to be obtained through a single pressing process. 
     In conventional compression tools, the clearance between punches and dies has always been made as small as possible. This is to avoid or at least minimise extension of powder through the clearance, as well as the formation of moulding flash, generally referred to as “beards”. The clearance commonly found in typical tools ranges from 10 to 20 μm. 
     FIG. 2 illustrates on an enlarged scale the clearance in the tool and the deformation which take place during a compression operation. The nominal diameters of moving punch  14  and of cavity  12  of the die are indicated in dotted lines. During compression, punch  14  tends to undergo barrel deformation. At a certain pressure level, the punch comes into contact with the die along As entire circumference while still moving. The resulting friction increases as punch  14  comes closer to its final position and the deformation also increases. 
     If the friction were uniform over the punch circumference, the tool would be able to better withstand high pressure. However, in practice, punch  14  always rubs more against one side than against the other, which causes a high bending stress in the punch and even in the die. The compression tool, which is designed primarily for hardness, poorly withstands bending stress and prematurely deteriorates if the pressure exceeds 800 MPa. 
     Of course. the friction of punch  14  against die  10  may also damage the surface finish of the cavity  12 , making the subsequent ejection of the compact  17  more difficult and affecting in turn its surface finish and that of components subsequently pressed in the die. 
     As shown in FIG. 2, the compact  17  itself also tends to undergo barrel deformation when under compression. pushing against the side walls of the cavity  12 . When the punch is withdrawn, the compact  17  and the walls of cavity  12  may, if excessive force has been applied, undergo permanent deformation, making the ejection of the compact more difficult. This ejection is normally facilitated by the presence of a sufficient amount of lubricant, of course. 
     To avoid or at least minimise the above-mentioned problems, the present invention provides an increased clearance between the elements of the tool, especially between the moving punch and the die, so that this clearance is not affected by any deformation of the elements during the compression operation. The presence of a clearance may tend to accentuate the generation of beards on the edges of the produced compacts, but such beards only affects, for the most part the aesthetic appearance of the compacts. The increased clearance is preferably not greater than the mean grain size of the powder, or else the powder grains will tend to jam together in the gap, thereby increasing friction as well as causing excessive loss of powder, in an extreme case. 
     The article by G Bockstiegel et al: “The influence of lubrication, die material and tool design upon die wear in the compacting of iron powders”, Modem Developments in Powder Metallurgy, Proceedings of the 1970 International Powder Metallurgy Conference, vol. 4, 1971, New York, London, describes experiments made with punch/die clearances of 5, 10, 25 and 45 μm, and concludes that the use of large clearances is detrimental in terms of tool wear. 
     In contrast the present invention uses large clearances, in particular greater than 45 μm. 
     According to one embodiment of the present invention, the elements of the tool are arranged to form a compact having one face flush with a surface of the die. 
     According to another embodiment of the present invention, the tool includes a second punch ( 15 ,  14 ) co-operating with the cavity ( 12 ) from the side opposite to the point of entry of the first punch, the second punch, during compression, being arranged to seal the cavity at or in the vicinity of the die surface, the first punch being used to eject the compact at the end of compression. 
     According to another embodiment of the present invention, the first punch ( 15 ) includes axially protruding edge portions which serve to form recessed edge regions on the compact these edge regions serving to accommodate to a significant extent any beards formed. 
     According to a further embodiment of the present invention, the walls of the cavity are coated with a material having a low friction coefficient relative to the powder and which is able to withstand repeated use. 
     According to a preferred embodiment of the present invention, the coating is of a diamond-like carbon material. 
     According to a preferred embodiment of the present invention, less than 0.5, or more preferably, less than 0.4 weight % of lubricant is included in the powder to be moulded into a compact. 
     According to a particularly preferred embodiment of the present invention, the powder includes about 0.3% weight of lubricant when the die cavity walls are coated as mentioned above. 
     It is preferred that the green density of the compact prior to sintering is at least 7.4 g/cc, 
     It has been found that the present invention can in the case of steel powder, achieve, by a cold, single pressing/sintering of a mixture of metal powders and significantly reduced amount of lubricant, a final density of at least 7.5. 
    
    
     THE DRAWINGS 
     In order than the invention be better understood, particularly preferred embodiments of it will now be described, by way of example only, with reference to the accompanying Figures, which are as follows: 
     FIGS. 1A to  1 C, (previously described) show a conventional tool for metal powder compression, at three steps of a compression process; 
     FIG. 2, (also previously described) illustrates on an enlarged scale the deformations of the tool during a compression operation; 
     FIG. 3 further illustrates on an enlarged scale the deformation of a tool according to the present invention during a compression operation; 
     FIGS. 4A and 4B show a tool according to the present invention at two stages of a compression operation; and 
     FIG. 5 shows an enlarged view of one edge of a compact obtained using the tool of FIGS.  4 A and  4 B. 
    
    
     DETAILED DESCRIPTION 
     In the interests of clarity the relative deformations have been exaggerated to make them more visible. In practice they are very small but significant. 
     In FIG. 3, the nominal shapes of punch  14  and of stamp  12  are illustrated by dotted lines. According to the present invention, the dimensions of the tool are chosen so that the clearance between moving punch  14  and die  10  is relatively large with respect to the clearance of a conventional tool illustrated in FIG.  2 . More specifically, as illustrated, this clearance is greater than the maximum radial expansion reached by punch  14  at the desired maximum compression pressure. However, this clearance is selected to be not greater than a limit at which the powder escapes from the die, This limit reaches 100 μm for commonly used metal powders, and it is greater than the mean grain size of the powder, because the grains tend to jam together in the clearance, as mentioned earlier. 
     In practice, the clearance is chosen according to the diameter of the compact. For example, good results are obtained by choosing a clearance of 50 μm for diameters reaching 50 mm, a clearance of 60 μm for diameters between 50 and 80 mm, and a clearance of 80 μm for clearances above 80 mm. 
     By choosing such a clearance the punch and die will undergo much less distortion as compared to the conventional tool of FIG.  2  and will successfully operate at higher pressures. A tool according to the present invention has been successfully tested at more than 1050 MPa. Further, since the contact areas are of smaller extent and the effects of friction are lower, the wall of cavity  12  maintains an acceptable surface finish for a longer period of time in service. 
     Preferably, the largest practicable clearance is chosen for all the tool elements. Indeed, these elements are generally designed to be movable one with respect to, another during use so as to promote homogenisation within the compact. Further, assembly of the tool is thus facilitated by having the largest practicable clearance. 
     It will be noted that the existence of the preferred relatively large clearance between punch and die inevitably causes the forming of beards. One might expect that the beard produced by a tool according to the present invention would be bigger, and thus more unacceptable, than that produced by conventional low or tight clearance tools. In fact the beard produced by a tool according to the present invention is wider than that produced by a conventional tool, but it is not taller. It is mostly the height of beards which in unacceptable. The beards obtained on the compacts produced by means of a tool according to the present invention may be removed or otherwise processed conventionally. 
     If a tool according to the present invention is used under above normal pressure, in the way described in relation with FIGS. 1A to  1 C, compact  17  will exhibit greater barrel deformation than in a conventional case. As a result, compact  17  would be more difficult to eject and more lubricant would accordingly be required, which militates; directly against the desired increase in density. 
     FIGS. 4A and 4B illustrate a particularly preferred form of tool according to the present invention, minimising this problem. The moving punch is in this case the lower punch  15  which is provided with an upwardly extending broach portion  15 - 1 . This co-operates with a corresponding recess in the upper punch  14  to make a recess or opening in a compact  17  to be compressed. 
     In FIG. 4A, lower punch  15  is, as in FIG. 1A, set at a specific position which determines the volume of powder contained by cavity  12 . This cavity  12  is filled flush with the upper surface of die  10 . Then, upper punch  14  is lowered to seal the cavity  12 , if necessary by slightly penetrating into the latter. The compression operation is then performed at the top of the die by appropriately combining relative motions of the punches and of the die. 
     In FIG. 4B, punch  15  has reached its final position, as determined by the pressure applied to it. 
     As previously, the compression of compact  17  generates radial force which deform die  10 . However, since compact  27  is then positioned towards one face of the die, the walls of cavity  12  do not deform as a barrel but, as illustrated, as an upwardly opening cone. This conical shape is partially retained when punch  14  is raised, which considerably helps the ejection of compact  17  by lower punch  15 . 
     Using the concepts of this invention, the proportion of lubricant may be smaller than 0.5 weight %. The combination of this reduction in the proportion of lubricant and of the increase in the compression pressure, up to approximately 1050 MPa, produced final product densities in excess of 7.5 (based on steel powder). 
     Even less lubricant, about 0.3 weight %, may be used when the walls of the die cavity  12  are coated with a material having a low friction coefficient with the powder. This material should, as previously mentioned, withstand repeated forces caused by successive compression operations. A material which meets these requirements is DLC (Diamond Like Carton). 
     The punch edges, as is shown in FIGS. 4A and 4B for lower punch  15 , preferably slightly protrude axially because this has been found to attenuate beard formation. 
     FIG. 5 shows an enlarged and deliberately exaggerated view of an edge of compact  17  obtained with such an arrangement. The edge of compact  17  is indented with respect to the lower surface, so that beard  17 - 1  resulting from the clearance is entirely included within this indentation. Thus, beard  17 - 1  does not affect the technical function of the corresponding surface of the compact, if this surface is not subsequently machined. 
     Various alterations and modifications of the present invention will readily occur to those skilled in the art. For example, it is not required to compress a compact right at or closely adjacent one surface of the die cavity, as shown in FIGS. 4A and 4B, if this compact already has a tapered shape facilitating its ejection. The compact may then be formed at the middle of the die cavity, as shown in FIG. 1B, while applying the higher pressures useable according to the present invention, with a reduced amount of lubricant