Abstract:
The invention refers to a method and an apparatus for producing multi-level components with conform target density from powdered material. The powdered material is filled into a mould die, which includes a multiple of lower and upper relatively movable punches, and the filling height of the column over each punch is associated to the geometrical levels of the final component. The material is optionally pre-compacted by individual static pressure acting on each punch and is compressed by at least one shock or impact device from at least one direction. Compensating adjustments for powder flow between columns and for density gradients are made during the pre-compression and shock compression.

Description:
TECHNICAL FIELD  
       [0001]     The invention concerns a method and an apparatus for producing multi-level or stepped components compressed to higher densities through a shock (impact) compression, or a combination of a static compaction and shock compression.  
         [0002]     At compaction of powdered material to higher densities of final component shapes, including steps or multi-levels with axial pressing techniques, consideration must be taken into account that the fill volume for each level or step is relatively different. Because of powder material&#39;s tendency to compact in vertical columns and generate little hydrodynamic flow, the column for each of the component levels must be compensated for a filling volume corresponding to the final level height and the target density of the component. Excessive density variations may result in crack initiation of the final component and distortion of the component during sintering.  
         [0003]     Patent SE-0200230-1 discloses an invention for producing a material body from powdered material. The material is for example in the form of powder, pellets, grains and the like and is filled in a mould, pre-compacted and compressed by at least one stroke, from one, two or more sides simultaneously, using one, two or more striking units emitting enough kinetic energy to form the body when striking the material, causing coalescence of the material.  
         [0004]     Patents PCT/SE01/01670, PCT/SE01/01671, PCT/SE01/01672, PCT/SE01/01673 and PCT/SE01/01674 disclose a method of producing metal, polymer, multi-layer, ceramic and composite bodies by coalescence, wherein the method comprises the steps according to the invention described in patent SE-0200230-1 above.  
         [0005]     Techniques compensating for the relative difference in compaction volume for each level of the component are well known and used in the art of compacting multi-level (stepped) components of conform density using conventional axial PM presses. To produce components with as close to homogeneous density as possible and step height as large as possible, the filling height of the powder is individually adjusted by using a multiple of punches, preferably one punch for each step. Each of the punches are displaced relatively to each other using individually controlled press rams. These methods for compaction of powder to higher densities of multi-level components, concern axial pressing techniques compacting the powder in a slow motion sequence. These methods of correction of the filling height for each step in combination with a shock (impact) compression process will not be applicable for compensation of the density of each step, due to that the punches during the shock compression operation, with respect to fundamentals of impact mechanics, preferably can be managed with a relatively equidistant displacement of each of the lower punches, and/or a relative equidistant displacement of each of the upper punches. Using the known technologies would render a varying target density of the material above each of the lower punch surfaces and initiated cracking.  
         [0006]     Patent SE-0202324-0 discloses a DFIER-machine for compression and compaction of a working material into a desired shape using a combination of shock compression and static compaction of material. The working material is for example in the form of powder, pellets, grains and the like and is filled in a moulding die cavity, compacted to a body of higher density. The machine comprises an outer system and an inner system. The outer system comprises at least one or more impact units, an upper and/or lower, each comprising an impact ram. The inner system comprises at least one or more static press units, each comprising a static press ram, and a tool unit. The lower units of the inner and outer systems, can be exchanged for a common stationary anvil and a lower punch. The tool unit comprises a moulding die mounted in a moulding die table or carrier, one, two or more upper punches individually controlled in position and load, and one, two or more lower punches individually controlled in position and load. The central system comprises a movable moulding die carrier, which holds a moulding die. The upper punch of the tool unit is removably connected to the inner system&#39;s upper static ram and the lower punch of the tool unit is removably connected to the inner system&#39;s lower static ram.  
         [0007]     The process cycle of the DFIER-machine involves two main operations in processing the material to higher densities. The first operation is a pre-compaction operation where the material is densified to a pre-compressing gross shape. The second operation is the shock compressing operation. The said compressing operation is performed by retaining the static pressure on the inner system and hence the working material simultaneously with a generation of an impact by accelerating the impact ram(s) at least one or more times. The shock wave is created and transferred to the working material through the press ram(s) and the punches while the pressure from the press ram(s) is retained. The impact unit(s) delivers enough kinetic energy to form the material into a body when striking the material, causing coalescence or higher density of the material to the predetermined final body of higher density according to a method described in patent SE-0200230-1.  
       STATE OF THE ART  
       [0008]     Patent GB 2265567A discloses a process for producing stepped pressed components from powder based material where powder material is filled in a cavity of a moulding die, compressing the material between relatively movable lower punches and at least one movable upper punch relative the lower punches, where each punch surface corresponds to each level of the stepped component, monitoring deviations of the filled amount of the material, from a desired value such that the steps have approximately the same density, by altering the filling volume of the moulding die cavity, by individually adjusting the filling levels of the bottom rams, being the distance of punch surfaces of the bottom rams from the moulding die top edge, such that the ratios of the adjusted filling levels correspond, in a first approximation, to the ratios of desired heights of the respective associated steps of the pressed article. The method to adjust the filling level of each step is performed by correcting each of the bottom punches individually. The individual filling levels in at least a first approximation being in a relationship to each other, which corresponds to the relationship of the desired heights of the levels of the pressed component, implying that the greater the desired height of the desired associated step, the greater the individual amount of correction.  
         [0009]     The described patent discloses a method for producing stepped components, where after pressing the material performed according to the invention has reached its final state of processing an approximate homogeneous density is obtained in the component. However, this method refers to conventional powder press machines with a low press displacement velocity (static pressing).  
         [0010]     Powder densification with methods and machines disclosed in the patents WO 9700751 and WO 0222289, machine solutions without apparatus and methods for individual punch control in press force and position for tool systems including two or more punches on either upper or lower side of the moulding die, where each punch corresponds to respective associated steps of the final material body produced, may therefore not be possible.  
         [0011]     In the two patents, WO 9700751 and WO 0222289, no descriptions or claims are mentioned or examples given on how to pre-compact the material in the moulding die cavity for producing multi-level components. Neither are there a solution or claims mentioned or examples given on how to compensate the filling volume in the moulding die cavity, to produce a stepped component by means of shock (impact) compressing the powder. The patents do not include methods and apparatus to compensate the powder densities of the powder columns for each corresponding and associated steps of the shock compressed material body, which is necessary in order to achieve a homogeneous density in all the stepped sections of the final body produced.  
         [0012]     The said machines can therefore not produce multi-level components of a homogeneous target density throughout the stepped sections of the component, due to the lack of individual punch control and therefore compensation.  
         [0000]     Multi-Level Component Approaches  
         [0013]     There are three different approaches for manufacturing of multi-level components. They have different degree of success because of technical limitations due to difficulty in compensation for density gradients.  
         [0014]     One approach of producing multi-level components may be performed by stepped dies, slotted punches or fixed stepped punches, but they all result in heterogeneous densities and cracking. These ways will limit the step height between each level or result in a heterogeneous density distribution in the final body produced, due to the equidistant absolute compression displacement for each of the component steps, resulting in a higher density in a shorter powder column above the stepped or slotted lower punch relative to a higher powder column above the same lower punch.  
         [0015]     A second approach is to perform shock compressing directly by shock compression without a pre-compaction operation. This may be performed by shock compressing each individual punch one by one. A lower step of the final component requires a higher compression ratio and a correspondingly higher punch. The impact ram may compact one punch at a time beginning with the highest, i.e. with the punch base closest to the impact device. The compression at the impact may result in a displacement such that the highest ram base will end flush the second highest punch base. A second or the same shock stroke on both the highest and second highest punches will displace these two punches so that their punch bases will end flush the third highest punch, and the shock sequence is repeated until all powder columns between the associated punches have been compressed to a higher density.  
         [0016]     A third approach to produce stepped components may be performed according to the following:  
         [0017]     Powder is filled in a moulding die cavity. The moulding die cavity comprises a moulding die and one, two or more lower punches inserted into it from the lower side. The lower punches may be stepped or positioned so that the press surfaces have a relative offset to each other. The moulding die is closed by inserting one, two or more top dies. The top punches may be of different length so that when pressed into the powder, different ratios of compaction is obtained under the individual punches. The heights of the punches are such that the relative offset distance between the punch press surfaces corresponds to the step heights of the associated steps of the final component, and with all the punch bases in flush.  
         [0018]     The procedure of shock compressing the powder material to a final material body, including for stepped surfaces, is performed by an alternative pre-compaction wherein the punches are pressed from at least one side. The upper punches, of different lengths, are positioned on the powder surface. The press ram will first meet contact with the highest punch, press it into the powder until the said punch base is in flush with the base of the second highest punch. The press device will then continue pressing the two punches into the powder in a parallel motion. The pre-compaction will end with all punch bases ending flush with each other.  
         [0019]     The shock compression is performed with the shock (impact ram) device impacting on the punches. The punches will be displaced in a parallel and equidistant motion until the geometry of the final body is met. The shock compression may also be performed according to the setup described without the pre-compaction operation, for machines without means or devices to perform a static pre-compaction operation. The initial positions of the punches are in this case retracted from the powder surface so that all the punch bases end flush. The shock compression is performed with the shock compression device impacting on the punch bases. The punches will be displaced into the powder, with an impact energy of a magnitude such as the final shape of the component is met.  
         [0020]     However, the described approaches will give a heterogeneous density distribution in the different stepped sections of the final component and also render undesired powder flow between the powder columns above each of the lower punches. Furthermore, it will be suffering from technical difficulties such as positioning the punches during filling, fixation of the punches before the shock compression stroke and retracting the punches after ejection. Furthermore, these methods may also suffer from uncontrolled effects as a result from internal shock wave such as crack initiation.  
       DISCLOSURE OF THE INVENTION  
       [0021]     The disclosure of the present invention includes a method and an apparatus to produce multi-level or stepped components with conform or predetermined target density from powdered material, produced with means of shock (impact) compression or static compaction in combination with shock compression.  
         [0022]     The method compiles the main steps of: 
        a) filling a moulding die cavity, comprising a moulding die and at least two relatively movable lower punches closing the lower section of the moulding die, with powdered material. The filling volume constitutes of the volumes above each of the lower punches pressing surfaces and of the top surface of the moulding die.     b) compensation of the fill volume of each powder column above each of the lower punch press surfaces by adjusting the column height, by individual relative displacement of the lower punches such that the filling volume of each powder column corresponds to respective associated steps and target density of the final pressed component as well as for possible powder gradient in each powder column, any powder gradients, and any possible powder flow that may occur between the the powder columns.     c) pre-compaction (static compaction) of the powder columns between each of the lower punch press surfaces by individual and relative displacement of the lower punches, and at least one relatively movable upper punch to a predetermined and individual pre-compacted density of each of the powder columns, where the pre-compacted density is compensated for, such that each powder column are related to each other with respect to the target density of the final body produced (the component), possible powder flow and density gradients between each of the powder columns that may occur during the pre-compaction and the shock compression operations and the equidistant displacement of all lower punches and equidistant displacement of all upper punches during the shock compression.     d) shock compressing the pre-compacted body by an equidistant displacement of all the lower punches relative to an equidistant displacement, not necessarily the same as for the lower punch displacement, of one, two or more upper punches, for which the predetermined target density is obtained for all the powder columns above each of the lower punches press surfaces rendering in a conform and predetermined target density throughout the material body produced. The shock compression may be performed on a single side or a multiple of sides.     e) Compensation of the powder density, necessary to obtain homogeneous density in the stepped sections of the final component, may also be performed during shock compression. This is performed by controlling the punch motion individually so that the energy transmitted to the powder is controlled. This could be performed by three different approaches: 
            i) Each punch may be subjected to an additional static pressure applied during the shock compression. This will enhance the acceleration displacement of the punch into the powder column below or above the punch press surface. Each punch may be individually controlled such that each punch is subjected to an individual pressure, i.e. a higher static pressure will enhance the ratio of densification of the powder column below or above the punch press surface. The static pressure may also be applied prior to and/or retained after the shock compression.     ii)The static pressure on any of the punches may act in a counter-acting direction to the direction of shock compressing, meaning braking the punch motion relative any other punch so that the braked punch transfers less compression energy to the powder column below or above the punch press surface. The means of braking any of the punches included in the tool may not necessarily be performed by a counter-acting static pressure.     iii)The mass of the punches may be adjusted relatively to each other. A punch with a relatively higher mass will have a lower velocity and hence reduced energy compressing the powder column facing the punch press surface, resulting in a decreased density of the same powder column relative the powder column compressed with a punch of lower mass.    
               
 
         [0031]     The invention also includes an additional static press operation including an equidistant displacement of all the lower rams controlling the lower punches, and a counter-acting equidistant displacement, not necessary the same as for the lower ram displacement, of all the upper rams controlling the upper punches, during the final stage of the pre-compaction operation. The relative displacement of the punches during pre-compaction will, in the case of including the additional static press operation, compensate for the possible density changes the said operation will inflict on each powder column between each of the corresponding upper and lower punches. The said additional static press operation will ensure that each punch has solid mechanical contact with the shock compressing device during the shock compressing operation, transmitting the shock impact energy to all of the punches.  
         [0032]     The lower punches will have specific lengths in relation to each other, with respect to the step heights that corresponds to respective associated steps of the lower side of the final body produced, so that the bases of each of the punches end exactly at the same plane, or are stacked onto each other in a manner that allows for solid mechanical contact between the punch bases. The upper punches will have specific lengths in relation to each other, with respect to the step heights that corresponds to respective associated steps of the upper side of the final body produced, so that the bases of each of the punches ends exactly at the same plane, and are stacked onto each other in a manner that allows for solid mechanical contact between the punch bases. This arrangement will allow for the exact equidistant displacement of the punches during the shock compression operation and the possible additional static press operation.  
         [0033]     Correction of the individual powder columns&#39; density may be performed in a feedback operation based on position and press force measurements on the corresponding powder columns&#39; lower and upper punch of the previous process cycle. The reference press force and position measurement may preferably be performed at the end of the pre-compaction operation.  
         [0034]     The disclosure of the apparatus includes a press machine for producing stepped components from powdered material, including a press device for individual punch position during the pre-compaction operation and device for shock compression of the pre-compacted material. The pre-compaction may be omitted so that only the shock compression is performed.  
         [0035]     The press device comprises one or a multiple of static press rams connected to one, two or more punches. The press ram controls the position of the punch(es) connected to it, and also applies the press force on the punch in order to compact the powder. The press device may be single sided or multiple sided. The press device allows for individual position, velocity, acceleration, and force drive adjustments of any or all the lower and upper punches. The press device may position the punches so that all punches on one side of the powder, will have solid mechanical contact with the shock compression device during the compression operation.  
         [0036]     The shock or impact compressing device creates kinetic energy through an impact stroke onto the press device, such that the shock pulse is transferred through the press device and punches to the material in the moulding die. The impact stroke may also be performed directly to the punches without the involvement of the press device. The shock compression device includes an upper and a lower shock ram, positioned axially with the punches on either side of the same. The lower side of the shock compressing device may be replaced with a static anvil, and consequently the shock compression is performed from a single side. When shock compression is performed from a single side, pre-compaction, including filling and pre-compaction corrections, are performed in the same manner as previously described, with the exception that the whole package of the inner system in a last operation is positioned with the lower press device resting on the anvil or in the case of omitted press device with the punches resting on the anvil. This means that the press device including all upper and lower punches and the moulding die performs a downward axial motion, without any relative alteration in punch or moulding die motions nor in press force between the upper and lower punches.  
         [0037]     The apparatus&#39; system that monitors the press force and the positions of each punch separately and passing the monitored force and position data to the control system, which compares the data with correspondingly allocated desired values, and wherein the control system is arranged such that, when the monitoring data deviates with the allocated desired values, the filling volume of the individual powder column above corresponding lower punch is corrected appropriately. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0038]     In the following detailed description of the invention, two preferred embodiments will be described with reference to the accompanying drawings:  
         [0039]      FIG. 1A  shows a cross-section view of a tool system, including a moulding die (1), three lower punches ( 2 , 3 , 4 ), a core rod ( 5 ), an upper punch ( 6 ), and an uncompacted powder ( 7 ), when in filling position  
         [0040]      FIG. 1B  shows the same tool system ( 1 , 2 , 3 , 4 , 5 , 6 ) in position after pre-compaction  FIG. 1C  shows the same tool system ( 1 , 2 , 3 , 4 , 5 , 6 ) in position after final pre-compaction adjustment  
         [0041]      FIG. 1D  shows the same tool system ( 1 , 2 , 3 , 4 , 5 , 6 ) after the shock compression operation  FIG. 2A  shows a cross-section view of an tool system including a moulding die ( 1 ), and three lower punches ( 2 , 3 , 4 ), core rod ( 5 ), upper punch ( 6 ), and un-compacted powder ( 7 ) when in filling position  
         [0042]      FIG. 2B  shows the same tool system ( 1 , 2 , 3 , 4 , 5 , 6 ) in position after pre-compaction  
         [0043]      FIG. 2C  shows the same tool system ( 1 , 2 , 3 , 4 , 5 , 6 ) including an intermediate punch adjustment device ( 10 ), in position after final pre-compaction adjustment  
         [0044]      FIG. 2D  shows the same tool system ( 1 , 2 , 3 , 4 , 5 , 6 ), including an intermediate punch adjustment device ( 10 ), after the shock compression operation  
         [0045]      FIG. 3  shows the first examples embodiments tool member motion versus time and an illustration of the corresponding density for each of the powder columns above each of the lower punch press surfaces 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
     Embodiment According to FIG.  1   
       [0046]     The first embodiment of the invention will be described as an example with the reference to  FIG. 1  and  FIG. 3 . The figure shows a tool for powder compression, which includes a moulding die ( 1 ), three lower punches ( 2 , 3 , 4 ), a core rod ( 5 ), and an upper punch ( 6 ), in four different process stations A, B, C and D, in the process of producing a multi level component with a combination of a static compacting and a shock compressing according to the invention.  
         [0047]     The moulding die ( 1 ), the core rod ( 5 ) and the said lower three punches together constitute a moulding die cavity. The inner lower punch ( 4 ) has a bore through which the said core rod is guided coaxial with the said inner lower punch ending flush with the upper surface of the moulding die ( 1 ) and creates a through hole in the final body produced ( 9 ).  
         [0048]     The three lower punches are arranged coaxially in one another relatively divided in the axial direction.  FIG. 1A  shows the tool in its filling position. The lower punches ( 2 , 3 , 4 ) are positioned in predetermined positions, such that the filling volume of uncompressed powder ( 7 ) above each of the punch press surfaces and the top moulding die surface correspond to the associated final body step height and the target density of the final body produced. The filling height of the powder column above each of the lower punch press surfaces in relation to the respective associated steps of the final body produced can be described as:  
         h     filling   ⁢           ⁢   height       =         ρ   target     ⁢     h   target         ρ   apparent           
 
 where index target refers to final body produced and index apparent refers to the filling powder density. The filling height may also be corrected for density gradients occurring in the powder column or powder flow that may occur between the powder columns during any of the proceedings involved in forming the final body ( 1 ). The moulding die ( 1 ) is closed by inserting the upper punch ( 6 ) into it. 
 
         [0049]     The pressing process begins with a pre-compacting operation moving all the lower punches in an upward relative motion towards the said moulding die upper surface and at the same time moving the upper punch ( 6 ) in a counter-acting motion (d). The relative motions of the lower punches (a), (b) and (c) during pre-compaction are performed so that the punch bases at the end of the pre-compaction are in parallel and will have a solid mechanical contact with the shock compressing device during compressing.  
         [0050]     The relative motions of each of the lower punches relative to the upper punch and the punch heights of each lower punches are such that a pre-determined density is obtained in one of the powder columns. The two other powder columns&#39; heights are functions of the first powder column density, the final component associated step height, target density of the final body produced ( 9 ), and the equidistant punch motion (e). The punch displacements during the pre-compaction operation for each of the said lower punch surfaces are illustrated as the displacement steps (a to c) in  FIG. 1  correspondingly.  FIG. 1B  shows the positions of the lower and upper punches after the pre-compaction operation is completed. The pre-compacted powder columns are shown as indexed  8 - 2 ,  8 - 3  and  8 - 4 . Since the shortest powder column ( 8 - 3 ) will encounter the largest compression ratio during the shock compression, the pre-compacted column ( 8 - 3 ) must therefore be compensated with a lower density than for a higher powder column ( 8 - 4 ) and ( 8 - 2 ). The relative movement of the lower punch ( 4 ) below the highest powder column ( 8 - 4 ) must therefore perform the longest pre-compaction displacement.  
         [0051]     The height of any of the powder columns for a predetermined pre-compacted density, e.g. ( 8 - 4 ), is a function of the target density of the final body produced and the height of the component level to the corresponding associated step of the final body ( 9 ) according to:  
         h     8   -   4       =         ρ   target     ⁢     h     target   ⁡     (     8   -   4     )             ρ     8   -   4             
 
 where h 8-4  and ρ 8-4  refers to the pre-compacted height and density of the powder column above the lower punch ( 4 ) and h target(8-4)  is the final component height above the punch ( 4 ) press surface in the final body produced. The equidistant punch motion (e) of the punches is given by the relation: 
 
 e=h   8-4   −h   target(8-4)  
 
         [0052]     The height of the two other powder columns are given by adding the equidistant displacement (e) to the corresponding associated step of the final body ( 9 ).  
         [0053]     The pre-compacting operation is performed in a slow static press motion, what could be described corresponding to conventional PM press techniques such as disclosed in patent GB 2265567A. Throughout the described embodiment the moulding die ( 1 ) is fixed and all punch movements are performed relative to the moulding die in controlled motions. The movements of the punches are performed with different accelerations and velocities relative to each other such that the compressed powder, with respect to the fill volume and pre-compacted volume, is compacted as uniformly as possible.  
         [0054]     During the shock compression operation the punch side facing the impact and static press rams must be arranged so that they all will be exposed to the same impact energy density by surface, transferring the energy to the powder column in the form of an equidistant displacement and consequently resulting in a densification of the individual powder pillars to the same target density throughout the component. The shock compression energy is such that the lower punches are displaced equally the distance (e) and such that the upper punch is displaced the distance (f). The distances e and f may be equal.  
         [0055]      FIG. 1C  shows the tool and pre-compacted powder in the pre-compaction adjustment operation. The upper punch ( 6 ) is displaced a distance f′ downwards and the lower punches ( 2 , 3 , 4 ) are displaced equidistantly e′. This operation will ensure that all punch bases&#39; surfaces are positioned flush prior to perform the shock compression. If the pre-compaction adjustment operation is performed, the compressing distances e and f are reduced with the distances e′ and f′ correspondingly. The density corrections are illustrated in  FIG. 3  through the process operation of filling, pre-compacting and shock compressing.  
       Embodiment According to FIG.  2   
       [0056]     The second embodiment of the invention is a variant of the first embodiment by means of arranging the base sides of the punches so that a parallel motion of all punches could be achieved. In order to transfer the kinetic impact energy to the powder, generated by the shock compressing device, a solid mechanical connection must be present between the shock compressing device and all of punches associated to respectively upper or lower sides of the moulding die. In the first embodiment this is performed by adapting the punch lengths to the step height of the final body produced, such that when the punches are in their final positions of the process cycle, the bases of the punches end flush. A static compacting ram or a shock compression ram can in this position of the punches ( 2 , 3 , 4 ), generate a parallel and equidistant movement of all the said punches.  
         [0057]     The arrangement according to the second embodiment for arranging a solid connection between the punches and the shock compressing is achieved by inserting an intermediate punch adjustment device ( 10 ) at the base of the punches. The intermediate punch adjustment device ( 10 ) has an interface surface facing the punches including steps on which each of the punches ( 2 , 3 , 4 ) are positioned. The opposite side of the said intermediate punch adjustment device surface is planar and parallel to the shock compression ram. The relative height between each step of the intermediate punch adjustment device ( 10 ) corresponds to the relative step height of the final body produced. This means that when all punches ( 2  to  4 ) are of equal length and positioned on the steps in the intermediate punch device ( 10 ), the relative position of the press surfaces of the said punches coincide with the steps of the final body produced. All process operations and apparatus means are the same as for the first embodiment.