AU2003206280A1 - A process for producing a high density by high velocity compacting - Google Patents

Description

A process for producing a high density body by high velocity compacting

The invention concerns a method of producing a body by coalescence or compaction to higher density.

STATE OF THE ART

In WO-Al-9700751, an impact machine and a method of cutting rods with the machine is described. The document also describes a method of deforming a metal body. The method utilises the machine described in the document and is characterised in that preferably metallic material either in solid form or in the form of powder such as grains, pellets and the like, is fixed preferably at the end of a mould, holder or the like and that the material is subjected to adiabatic coalescence by a striking unit such as an impact ram, the motion of the ram being effected by a liquid. The machine is thoroughly described in the WO document.

In WO-Al-9700751, shaping of components, such as spheres, is described. A metal powder is supplied to a tool divided in two parts, and the powder is supplied through a connecting tube. The metal powder has preferably been gas-atomized. A rod passing through the connecting tube is subjected to impact from the percussion machine in order to influence the material enclosed in the spherical mould. However, it is not shown in any embodiment specifying parameters for how a body is produced according to this method.

The compacting according to this document is performed in several steps, e.g. three. These steps are performed very quickly and the three strokes are performed as described below. Stroke 1 : an extremely light sfroke, which forces out most of the air from the powder and orients the powder particles to ensure that there are no great iπegularities. Stroke 2: a stroke with very high energy density and high impact velocity, for local adiabatic coalescence of the powder particles so that they are compressed against each other to extremely high density. The local temperature increase of each particle is dependent on the degree of deformation during the stroke.

Stroke 3: a stroke with medium-high energy and with high contact energy for final shaping of the substantially compact material body. The compacted body can thereafter be sintered.

In SE 9803956-3 a method and a device for deformation of a material body are described. This is substantially a development of the invention described in WO-Al-9700751. In the method according to SE 9803956-3, the striking unit is brought to the material by such a velocity that at least one rebounding motion of the striking unit is generated, the rebounding being counteracted whereby at least one further stroke of the striking unit is generated. The strokes according to the method described in WO-Al-9700751, give a locally very high temperature increase in the material, which can lead to phase changes in the material during the heating or cooling. When using according to SE 9803956-3 a counteracting of the rebounding motion generating at least one further stroke, this stroke contributes to the wave going back and forth and being generated by the kinetic energy of the first stroke, continuing during a longer period. This leads to further deformation of the material and with a lower impulse than would have been necessary without the counteracting.

It has now been found that the machine according to these documents does not work so well. For example, the time intervals between the strokes, which they mention, are not possible to attain. Further, the documents do not comprise any embodiments showing that a body can be formed. Also, the rebounding strokes have proved to result in cracking of the material.

OBJECT OF THE INVENTION

The object of the present invention is to achieve a low cost process for efficient production of products from a particulate material or a solid material by coalescence or compaction to a higher density. These products may be both medical devices such as medical implants or bone cement in orthopaedic surgery, instruments, such as surgical knives, or diagnostic equipment, or non medical devices such as ball bearings, cutting tools, sinks, baths, displays, glazing (especially aircraft), lenses and light covers. Another object is to achieve a product of the described type.

The object to achieve a higher density is based on the fact that high density is a condition for high mechanical properties.

The process should not be limited to using the above described machine.

SHORT DESCRIPTION OF THE INVENTION

It has surprisingly been found that it is possible to compress different materials according to the new methods defined in claims 1 and 5. 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. The machine to use in the method may be the one described in WO- Al-9700751 and SE 9803956-3 with a vibration device added to achieve vibration of the tool or mould. The material may also be in solid form and be inserted into a mould and subjected to at least one stroke, from two or more sides simultaneously, using two or more striking units emitting enough kinetic energy to form the body when striking the material, causing coalescence or higher density of the material. In this case the machine used comprises at least two opposite striking units.

The method according to the invention may utilise hydraulics in the percussion machine, which may be constructed on the same principle as the machine utilised in WO-A1-9700751 and SE 9803956-3. When using pure hydraulic means in the machine, the striking unit or units can be given such movement as to, upon impact with the material to be compressed, emit sufficient energy at sufficient speed for coalescence to be achieved. This coalescence may be adiabatic. A stroke is carried out quickly and for some materials the wave in the material decay in between 5 and 15 milliseconds. The use of hydraulic may also give a better sequence control and lower running costs compared to the use of compressed air.

However, the invention is not limited to using a hydraulic machine. It may also be possible to use a spring-actuated or electrically actuated percussion machine or a machine using compressed air. Neither is it necessary to always achieve coalescence. In some instances it is sufficient to perform compaction to a higher density.

The optimal machine has a large press for pre-compacting and post-compacting and at least one small striking unit that can strike with variable speed. Machines according to such a construction are therefore probably more interesting to use. Different machines could also be used, one for the pre-compacting and post-compacting and one for the compression.

SHORT DESCRIPTION OF THE DRAWINGS

On the enclosed drawings

Figures la and lb show schematic cross sectional views of two embodiments of a machine for deformation of a material in the form of a powder, pellets, grains and the like, Figures 2-5 show flow diagrams illustrating the process according to the invention and Figures 6- are diagrams showing results obtained in comparative tests described in the following examples. DETAILED DESCRIPTION OF THE INVENTION

The invention concerns a method of producing a body from particulate material. The method comprises the steps of a) filling a pre-compacting mould with the material in the form of powder, pellets, grains or the like, b) vibrating the mould, c) pre-compacting the material at least once with a pre-compacting means and d) compressing the material in a compression mould by at least one stroke, where a striking unit emits enough kinetic energy to form the body when striking the material inserted in the compression mould with a striking means, causing coalescence or higher density of the material.

By the use of vibration the particles in the pre-compacting mould will move closer together, forcing out air or gas from between the particles and they will orient themselves so as to more easily be compacted. Thereby, already before the pre-compaction starts, a higher density is achieved. The pre-compaction will therefore not start from a loosely packed powder, but from a more densely packed powder. Therefor, fewer pre-compaction strokes may be necessary. Should the vibration continue during the pre-compacting step, a higher density will be achieved using the same pre-compaction pressure.

The higher density achieved during pre-compaction will facilitate the compression step.

A further advantage is obtained if the material is compressed from two or more sides simultaneously using two or more striking units.

The pre-compacting mould may be the same as the compression mould, which means that the material does not have to be moved between the steps c) and d). It is also possible to use different moulds and move the material between the steps c) and d) from the pre-compacting mould to the compression mould. This could only be done if a body is formed of the particulate material in the pre-compacting step. The invention also concerns a method comprising the steps of a) inserting the solid material in a mould, b) possibly pre-compacting the material at least once with a pre-compacting means and d) compressing the material in the mould by at least one stroke,, from two or more sides simultaneously, using two or more striking units emitting enough kinetic energy to form the body when striking the material, causing coalescence or higher density of the material.

It is preferable that the pre-compacting means is continuously applied against the material, with the same or a higher pressure, during the compression d) thereof by the striking unit or units.

By using a machine which can strike the material in the mould from two opposite directions and also compact from these directions the particles in the powder will be better oriented, a better contact between the particles will be achieved and a more efficient welding of the particles results. The body obtained is more homogenous than a body produced at the same energy and pressure applied from only one direction. Advantages arise from the double-sided treatment both during pre-compaction and during compression.

The preferred method of producing a body from particulate material according to the invention could be described in the following way.

1) Powder is pressed to a green body with vibration, the body is compressed by impact to a (semi)solid body and thereafter an energy retention may be achieved in the body by a post- compacting. The process, which could be described as Dynamic Forging Impact Energy Retention (DFIER) involves three mains steps. a) Pressuring

The pressing step is very much like cold and hot pressing. The intention is to get a green body from powder. It has proved most beneficial to perform two compactions of the powder. One compaction alone gives about 2-3% lower density than two consecutive compactions of the powder. This step is the preparation of the powder by evacuation of the air and orientation of the powder particles in a beneficial way. The density values of the green body is more or less the same as for normal cold and hot pressuring, b) Impact

The impact step is the actual high-speed step, where a striking unit strikes the powder with a defined area. A material wave starts off in the powder and interparticular melting takes place between the powder particles. Velocity of the striking unit seems to have an important role only during a very short time initially. The mass of the powder and the properties of the material decides the extent of the interparticular melting taking place, c) Energy retention

The energy retention step aims at keeping the delivered energy inside the solid body produced. It is physically a compaction with at least the same pressure as the pre- compaction of the powder. The result is an increase of the density of the produced body by about 1-2%. It is performed by for instance letting the striking unit stay in place on the solid body after the impact and press with at least the same pressure as at pre- compaction, or release after the impact step. The idea is that more transformations of the powder will take place in the produced body.

Figure la shows a machine that compacts and compresses a material from both an upper and a lower side. Figure lb shows a corresponding machine that only can compact and strike from one side. The reference numbers for similar parts are the same in both Figure la and Figure lb.

The machine shown in Figure la comprises an upper and a lower striking device la and lb. Each striking device has an impact ram 2a,b containing weights 10 and arranged inside an impact ram housing 11 a,b. The mass of the impact rams can be changed by adjusting the number of impact ram weights 10 inside the impact ram 2 a,b. Between the striking devices 1 a,b there is a central part including support rods 7 connecting upper and lower static press tables 12 a, b through which upper and lower static pre-compaction press rams 5 a,b pass. On the figure two side support rods 7 are shown. The machine comprises two further front and back rods 7 (not shown on the figure). To the pre-compaction rams 5 a,b an upper and a lower punch 9 a,b are connected. To the support rods a moulding die table 13 is connected holding a mould 8 arranged between the upper and lower punches 9a,b. A vibration device 6 is connected to the moulding die table 13. There is also a hopper 3 from which particles are fed into the mould 8.

The machine in Figure lb comprises the same parts as the machine in Figure la with the following exceptions. It does not comprise any lower impact device lb, nor the lower static pre-compaction press ram 5b or the lower static press table 12b. The lower punch 9b is connected to a rig fundament 4.

The process steps are schematically illustrated on Figures 2-5. Figures 2 and 3 show the process without energy retention (with non-DFIER machines)and Figures 3 and 5 with energy retention (with DFIER machines). Figures 2 and 4 show uni-cycle processes and Figures 3 and 5 multi-cycle processes where several strokes are used. The upper part of Figures 2-5 shows a time base and above this the different steps performed. The lower part of the figures comprises a diagram showing the change of some parameters during the process. The pressure parameter is the atmospheric pressure in the mould.

In the process shown in Figures 2 and 3 particulate material is first filled in the mould. A sub- atmospheric pressure is achieved and the material is compressed with a ram during the pre- compaction step. The ram is removed during the delay and thereafter the pre-compacted material is dynamically forged by being struck one or several times with an impact ram. The material is in this case resistance heated during the strokes, the applied electrical cuπent being synchronized with the triggering of the impacts. The vacuum is released and the component obtained is pressed out, optionally after being compressed further with the pre- compaction ram.

In the process shown in Figures 4 and 5 a sub-atmospheric pressure is achieved and the material is compressed with a ram during the pre-compaction step. The ram is maintained pressing together the particles during the following steps. Vibration of the particulate material is used during pre-compaction. As in the process of Figures 2 and 3 the material is heated with an electric current during shocking. The pressure from the pre-compaction ram is maintained after shocking for energy retention, the vacuum is released and the component obtained pressed out.

In the above embodiments the pre-compaction mould is the same as the compression mould. Further, the material from which a body is formed is in particulate form. However, it is also possible to use a solid material. In this case it may not be necessary to use pre-compaction. Depending on the density of the solid material a pre-compaction step may be of advantage. After the possible pre-compaction the solid material is shocked by two opposite impact rams simultaneously. The same steps as when forming a particulate material may be used.

Both when starting with a particulate material and a solid material the forming may be performed by using two opposite impact rams. It is also possible to use more than two impact rams such as when a deformation sidewise should be obtained.

The pre-compaction step may comprise one or more compactions. As the particulate material is vibrated, only one compaction may be necessary. However, several compactions may still give a somewhat higher density. By using the preferred features of the invention it may be possible to achieve material properties of the same level as those achieved by forging of by using HIP or HIP + forging.

The features which may be modified within the definition of the process of the invention are for instance:

1) the direction of striking, may be in one two or more directions,

2) the vibration, may be during the pre-compaction step and/or the compression step and/or the energy retention stage,

3) the number of pre-compactions, 4) interval between pre-compaction stokes,

5) temperature during pre-compaction

6) pre-compaction pressure

7) the same parameters may be modified for the impact step,

8) impact stroke pressure and energy, may be the same or be different for different strokes, 9) post-compaction in one or more steps may be used or not,

10) atmospheric pressure in the mould, may be decreased or not,

11) use of other gases than air, for instance inert gas or a reactive gas,

12) the temperature of the mould and material, may be increased and in some instances decreased or may be ambient temperature, 13) the material to be formed may be particulate, such as powder, pellets or grains, or solid,

14) electrical current may be used or not,

15) the vibration may be modified as to amplitude, frequency or direction, may be vertical and/or horisontal,

16) the pre-compaction mould and compression mould may be the same or different, 17) the number of steps may be modified, some steps may be repeated several times after repeating an earlier step, more material may be filled in the mould after pre-compaction or compression and thereafter pre-compaction and/or compression may be repeated, 18) the relation between the mass of the impact ram or rams, the mass of the punch or punches and the mass of the material to be formed may be modified, 19) energy retention may be used or not,.

The mass m of the striking unit is preferably essentially larger than the mass of the material. By that, the need of a high impact velocity of the striking unit can be somewhat reduced. When the striking unit hits the material 1, this may cause a local coalescence and thereby a consequent deformation of the material. In any case an increase in density is obtained. Waves or vibrations are generated in the material in the direction of the impact direction and these waves or vibrations have high kinetic energy and will activate slip planes in the material and also cause relative displacement of the grains of the. powder. It is possible that the coalescence may be an adiabatic coalescence. The local increase in temperature develops spot welding (inter-particular melting) in the material increasing the density.

The pre-compaction is a very important step. This is done in order to drive out air and orient the particles in the material. The pre-compaction step is much slower than the compression step, and therefore it is easier to drive out the air. This is further facilitated by the use of vibration before or during the pre-compaction. The compression step, which is done very quickly, does not have the same possibility to drive out air. Therefore, the air remaining after the pre-compaction may be enclosed in the produced body, which is a disadvantage. The pre- compaction is performed at a minimum pressure enough to obtain a maximum degree of packing of the particles which results in a maximum contact area between the particles. This is material dependent and depends on the softness and melting point of the material.

The pre-compacting step in the Examples was performed by compacting with an axial load of about 117680 N. This is done was the pre-compacting mould or the final mould. In all Examples where nothing else is stated the mould used was a cylindrical mould, part of the tool and having a circular cross section with a diameter of 30 mm. The area of this cross section is about 7 cm². This means that a pressure of about 1.7 x 10⁸ N/m² was used. The material may be pre-compacted with a pressure of at least about 0.25 x 10⁸ N/m², and preferably with a pressure of at least about 0.6 x 10^s N/m². The necessary or preferred pre- compaction pressure to be used is material dependent and for some materials it could be enough to compact at a pressure of about 2000 N/m². Other possible values are 1.0 x 10⁸ N/m², 1.5 x 10⁸ N/m². It may be possible to use lower pressures if vacuum or heated material is used. The height of the cylindrical mould is 60 mm.

The compression strokes may emit a total energy corresponding to at least 100 Nm in the described cylindrical tool, in air and at room temperature. Other total energy levels may be at least 300, 600, 1000, 1500, 2000, 2500, 3000 and 3500 Nm. Energy levels of at least 10 000, 20 000 Nm may also be used. There is a new machine, which has the capacity to strike with 60 000 Nm in one stroke. Of course such high values may also be used. And if several such strikes are used, the total amount of energy may reach several 100 000 Nm. The energy levels depend on the material used, and in which application the body produced will be used. Different energy levels for one material will give different relative densities of the material body. The higher energy level, the more dense material will be obtained. Different material will need different energy levels to get the same density. This depends on for example the hardness of the material and the melting point of the material. The energy level needs to be amended and adapted to the form and construction of the mould. If for example, the mould is spherical, another energy level will be needed. A person skilled in the art will be able to test what energy level is needed with a special mould. The energy level depends on what the body will be used for, i.e. which relative density is desired, the geometry of the mould and the properties of the material. The striking unit must emit enough kinetic energy to form a body when striking the material inserted in the compression mould. With a higher velocity of the stroke, more vibrations, increased friction between particles, increased local heat, and increased interparticular melting of the material will be achieved. The bigger the stroke area is, the more vibrations are achieved. There is a limit where more energy will be delivered to the tool than to the material. Therefore, there is also an optimum for the height of the material.

The more a material is compressed by the coalescence technique, the smoother the surface obtained. The porosity of the material and the surface is also affected by the method. If a porous surface or body is desired, the material should not be compressed as much as if a less porous surface or body is desired.

What has been described above about the energy transformation and wave generation also refer to a solid body. In the present invention a solid body is a body where the target density for specific applications has been achieved.

The striking unit preferably has a velocity of at least 0.1 m/s or at least 1.5 m/s during the stroke in order to give the impact the required energy level. Much lower velocities may be used than according to the technique in the prior art. The velocity depends on the weight of the striking unit and what energy is desired. The total energy level in the compression step is at least about 100 to 4000 Nm. But much higher energy levels may be used. By total energy is meant the energy level for all strokes added together. The striking unit makes at least one stroke or a number of consecutive strokes. The interval between the strokes according to the Examples was 0.4 and 0.8 seconds. For example at least two strikes may be used.

The method also comprises pre-compacting the material at least twice. It has been shown in the Examples that this could be advantageous in order to get a high relative density compared to strokes used with the same total energy and only one pre-compacting. Two compactions give about 1-5 % higher density than one pre-compaction depending on the material used. The increase may be even higher for some materials. When pre-compacting twice, the compacting steps are performed with a small interval between, such as about 5 seconds. About the same pressure may be used in the second pre-compacting. Further, the method may also comprise a step of compacting the material at least once after the compression step. This has also been shown to give very good results. The post- compacting should be carried out at at least the same pressure as the pre-compacting pressure, i.e. 2000 N/m². Other possible values are 1.0 x 10⁸ N/m². Higher post-compacting pressures may also be desired, such as a pressure which is twice the pressure of the pre- compacting pressure. The pre-compacting value has to be tested out for every material. A post-compacting effects the sample differently than a pre-compacting. The transmitted energy, which increases the local temperature between the powder particles from the stroke, is conserved for a longer time and can effect the sample to consolidate for a longer period after the stroke. The energy is kept inside the solid body produced. Probably the "lifetime" for the material wave in the sample is increased and can affect the sample for a longer period and more particles can melt together. The result is an increase of the density of the produced body by about 1-4 % and is also material dependent.

When using pre-compacting and/or after compacting, it could be possible to use lighter strokes and higher pre- and/or after compacting, which would lead to saving of the tools, since lower energy levels could be used. This depends on the intended use and what material is used. It could also be a way to get a higher relative density.

To get improved relative density it is also possible to pre-process the material before the process. The powder could be pre-heated to e.g. -50-300 °C or higher depending on what material type to pre-heat. The powder could be pre-heated to a temperature which is close to the melting temperature of the material. Suitable ways of pre-heating may be used, such as normal heating of the powder in an oven. In order to get a more dense material during the pre-compacting step vacuum or inert gas could be used. This would have the effect that air is not enclosed in the material to the same extent during the process.

Before processing the polymer could be homogenously mixed with additives.

The body may according to another embodiment of the invention be heated and/or sintered any time after compression or post-compacting.

Common post-processing steps are following:

Further, the body produced may be a green body and the method may also comprise a further step of sintering the green body. The green body of the invention gives a coherent integral body even without use of any additives. Thus, the green body may be stored and handled and also worked, for instance polished or cut. It may also be possible to use the green body as a finished product, without any intervening sintering. This is the case when the body is a bone implant or replacement where the implant is to be resorbed in the bone.

It has been shown earlier that better results have been obtained with particles having irregular particle morphology. The particle size distribution should probably be wide. Small particles could fill up the empty space between big particles.

The particulate material may comprise a lubricant and/or a sintering aid. A lubricant may be useful to mix with the material. Sometimes the material needs a lubricant in the mould, in order to easily remove the body. In certain cases this could be a choice if a lubricant is used in the material, since this also makes it easier to remove the body from the mould.

A lubricant cools, takes up space and lubricates the material particles. This is both negative and positive.

Interior lubrication is good, because the particles will then slip in place more easily and thereby compact the body to a higher degree. It is good for pure compaction. Interior lubrication decreases the friction between the particles, thereby emitting less energy, and the result is less inter-particular melting. It is not good for compression to achieve a high density, and the lubricant must be removed for example with sintering.

Exterior lubrication increases the amount of energy delivered to the material and thereby indirectly diminishes the load on the tool. The result is more vibrations in the material, increased energy and a greater degree of inter-particular melting. Less material sticks to the mould and the body is easier to extrude. It is good for both compaction and compression.

An example of a lubricant is Acrawax C, but other conventional lubricants may be used. If the material will be used in a medical body, the lubricant need to be medically acceptable, or it should be removed in some way during the process.

Polishing and cleaning of the tool may be avoided if the tool is lubricated and if the powder is preheated. In some cases it may be necessary to use a lubricant in the mould in order to remove the body easily. It is also possible to use a coating in the mould. The coating may be made of for example TiNAl or Balinit Hardlube. If the tool has an optimal coating no material will stick to the tool parts and consume part of the delivered energy, which increase the energy delivered to the powder. No time-consuming lubricating would be necessary in cases where it is difficult to remove the formed body.

For example, one to about six strokes may be used. The energy level could be the same for all strokes, the energy could be increasing or decreasing. Stroke series may start with at least two strokes with the same level and the last stroke has the double energy. The opposite could also be used.

The highest density is obtained by delivering a total energy with one sole stroke. If the same total energy instead is delivered with several strokes, a lower relative density is obtained, but the tool is saved. Multi-strokes can therefore be used for applications where a maximum relative density is not necessary. However, the use of several strokes may give as high density as one sole stroke, provided the time interval between the strokes is extremely short.

Through a series of quick impacts a material body is supplied continually with kinetic energy which contributes to keep the back and forth going wave alive. This supports generation of further deformation of the material at the same time as a new impact generates a further plastic, permanent deformation of the material.

According to another embodiment of the invention, the impulse, with which the striking unit hits the material body, decreases for each stroke in a series of strokes. Preferably the difference is large between the first and second stroke. It will also be easier to achieve a second stroke with smaller impulse than the first impulse during such a short period (preferably approximately 1 ms), for example by an effective reduction of the rebounding blow. It is however possible to apply a larger impulse than the first or preceding stroke, if required.

When the material inserted in the mould is exposed to the coalescence, a hard, smooth and dense surface is achieved on the body formed. This is an important feature of the body. A hard surface gives the body excellent mechanical properties such as high abrasion resistance and scratch resistance. The smooth and dense surface makes the material resistant to for example corrosion. The less pores, the larger strength is obtained in the product. This refers to both open pores and the total amount of pores. In conventional methods, a goal is to reduce the amount of open pores, since open pores are not possible to get reduced by sintering. It is important to admix powder mixtures until they are as homogeneous as possible in order to obtain a body having optimum properties.

A coating may also be manufactured according to the method of the invention. When manufacturing a coated element, the element is placed in the mould and may be fixed therein in a conventional way. The coating material is inserted in the mould around the element to be coated, by for example gas-atomizing, and thereafter the coating is formed by coalescence. The element to be coated may be any material formed according to this application, or it may be any conventionally formed element. Such a coating may be very advantageously, since the coating can give the element specific properties.

A coating may also be applied on a body produced in accordance with the invention in a conventional way, such as by dip coating and spray coating.

It is also possible to first compress a material in a first mould by at least one stroke. Thereafter the material may be moved to another, larger mould and a further polymer material be inserted in the mould, which material is thereafter compressed on top of or on the sides of the first compressed material, by at least one stroke. Many different combinations are possible, in the choice of the energy of the strokes and in the choice of materials.

The invention also concerns the product obtained by the methods described above.

By the use of the present process it is possible to produce large bodies in one piece. In presently used processes involving casting it is often necessary to produce the intended body in several pieces to be joined together before use. The pieces may for example be joined using screws or adhesives or a combination thereof.

A further advantage is that the method of the invention may be used on powder carrying a charge repelling the particles without treating the powder to neutralize the charge. The process may be performed independent of the electrical charges or surface tensions of the powder particles. However, this does not exclude a possible use of a further powder or additive carrying an opposite charge. By the use of the present method it is possible to control the surface tension of the body produced. In some instances a low surface tension may be desired, such as for a wearing surface requiring a liquid film, in other instances a high surface tension is desired.

Here follow some Examples to illustrate the invention. The Examples will present and illustrate how different parameters can be varied to increase the relative density of metal, ceramic and polymer samples processed with the present process. Stainless steel is the material tested in all studies except in the powder height-, theoretical density-, powder hardness- and melting temperature studies. See table 1 for technical data of the stainless steel used.

The sample dimensions are the same for all studies except in the collision area study where two sample dimensions are used. Table 1: Technical data of stainless steel.

The samples produced are in the form of a disc with a diameter of ~30.0 mm and a height between 5-10 mm. The height depends on the obtained relative density. If a relative density of 100 % should be obtained the thickness is 5.00 mm.

In the moulding die (part of the tool) a hole with a diameter of 30.00 mm is drilled. The height is 60 mm. Two stamps are used (also parts of the tool). The lower stamp is placed in the lower part of the moulding die. Powder is filled in the cavity that is created between the moulding die and the lower stamp. Thereafter the impact stamp is placed in the upper part of the moulding die and strokes can be performed.

EXAMPLES

1. Powder height study Metal, ceramic and polymer powders were tested in a powder height study. The powder used were stainless steel, hydroxyapatite and UHMWPE. See table 2 for properties for the powders tested.

Table 2: Technical data for the powders tested in the powder height study.

Metal

Figures 6 and 7 show relative density as a function of impact energy per mass and total impact energy, respectively, for samples processed with different powder masses. All samples were tested in the same cylindrical mould but with different powder heights and thus different masses. The reference mass was 28 g.

Table 3

Result

The results show that to reach the same density, less energy per mass is required for a body with a greater powder height and thus a higher mass compared to a body with a smaller powder height. Approximately the same total energy is required to obtain the same density, irrespective of the powder mass or height.

Ceramic

Figures 8 and 9 show relative density as a function of impact energy per mass and total impact energy, respectively, for ceramic samples processed with different powder masses. All samples were tested in the same cylindrical mould but with different powder heights. The reference mass was 11.1 g.

Table 4

Conditions

Pressure Atmosphere

Temperature Room temperature

Energy retention No

Result

Figure 8 shows that the three curves follow each other, which means that a certain density is obtained no matter of the specimen shape with respect to impact energy per weight. This is also shown in figure 9 where density is plotted as a function of total impact energy. The curve is shifted to the left in the diagram for a lower sample mass. It could also be noted that higher density for the 1 l.l.g sample never reached the plateau density as indicated for the 2.8 and 5.5 g samples. The results show that the sample mass and powder height in the mould influences the density with respect to total impact energy, i.e. a larger sample mass needs more energy in order to obtain a certain density. The results also show that there is a linear relation between mass and density with respect to impact energy per mass up till at least 271 Nm/g, see figure 8.

Polymer

Figures 10 and 11 show relative density as a function of impact energy per mass and total impact energy, respectively, for polymer samples processed with different powder masses. All samples were tested in the same cylindrical mould but with different powder heights. The reference mass was 4.2 g.

Table 5

Result

The curves of smaller masses are shifted to the right or to higher energy in the density energy graph. Also shift towards lower densities could be observed for smaller sample masses.

Figure 10 shows that a higher density is obtained when the powder height is increased for a given impact energy per mass. Hence, the maximum density is reached at a lower impact energy per mass for a heavier sample. Studying the individual density-energy graph, it could be divided into three phases. Phase 1 could be characterised as the compacting phase, phase 2 would be characterised as the plateau phase and phase 3 characterised as the reaction phase. In the compaction phase, the density-energy curve follows a logarithmic relation with an initial high compaction rate. The sloop decreases as the energy is increased and eventually the curve reaches the plateau phase. The plateau phase is characterised with an almost constant inclination and constant density. At a certain energy level the density starts to increase again. This part of the curve is non linear with an initial positive and increasing derivative. The curve derivative is eventually decreasing and the curve is approaching the 100 % relative density asymptotically. Phase 1 and phase 2 could also be seen in the metal counterparts. The samples of phases 1 and 2 are characterised by opaque and brittle properties. Entering phase 3, the samples gradually change in properties. A new material phase occurs, first at the outer edges and at the top and bottom end surfaces. This material phase is characterised as a harder, transparent and with a plastic and fat surface feeling. For the smaller mass samples the reaction does not occur gradually but rather direct. The process in phase 3 was also somewhat dramatic and could be described as a small explosion. Directly after the impact stroke, white smoke was observed coming from the sample, and material had extruded out between the stamps and the moulding die. Further, the pressure occurring at the reaction phase proved to be very high when during one test the moulding die was cracked open. A larger weight sample was found to density faster at lower energy per mass levels and the reaction shift of material phase is occurring gradually rather than direct as for the small samples. The limited test series of the 12.6 g was due to the limited powder pillar height of the tool. The insertion distance was less than the recommended distance of the 30 mm (diameter of stamp). The test was therefore stopped at the impact energy of 2100 Nm to eliminate a tool failure. The two large dips in density for the 8.4 g sample depend on the sample not holding together and coming out as a powder.

Conclusions

For ceramic powders processed according to the invention the same density was obtained independent of the powder height or mass with the same impact energy per mass. On the contrary, for metal and polymer powders processed according to the invention, the same density was obtained independent of the powder height or mass with the same total impact energy.

2. Collision area Figures 12 and 13 show relative density as a function of impact energy and total impact energy, respectively, for samples with different collision surface areas. Table 6

Result

The curves show a linear relation between collision surface area, shock energy and pressure. Samples with different diameters will reach the same density if they are processed with same impact energy per mass.

3. Shrinking

Figure 14 shows relative density as a function of shrinking in volume % for samples processed according to the invention in comparison with samples processed with conventional powder metallurgy (PM). All samples were sintered after the shocking and pressing step, respectively. Figure 15 shows the comparison between achieved density after sintering for a sample processed conventionally and a sample processed according to the invention (DFIER), respectively.

Table 7

Result

The samples processed with Method X reaches a higher density compared to conventional PM processed samples. When samples are sintered after the compacting step, the material shrink because of internal porosity in the material. Shrinking of the material can have negative effects on the structure and the mechanical properties of the final product.

The curves in Figure 14 show that the shrinking volume decreases with increased relative density for all samples. The samples processed with DFIER shrinks more compared with the samples processed conventionally. The reason is probably that the samples processed with DFIER has a better orientation of particles, and that the energy transmitted during the shock phase is stored in the grain boundaries and will be set free during sintering. The free energy will increase the driving force to collapse the porosity and solidify the material during sintering. Samples pressed conventionally have less driving force during sintering compared with samples processed with DFIER.

The samples processed with DFIER have high green density before sintering, which means that the material shrinks less and better mechanical properties are therefore achieved, compared to a sample compacted by using conventional pressing. Samples with a low green density require a sophisticated and expensive sintering process to remove all porosity in the material. The high green density of samples processed with DFIER make it possible to use a cheaper and simpler sintering process to reach full density.

4. Velocity study

Figure 16 shows relative density as a function of impact energy for samples shocked with different impact velocities of the impact ram. The impact velocities for the impact ram and punch, respectively, are showed in figures 17, 18 and 19. Figure 20 shows obtained impact velocity of the punch for different impact ram masses for the maximum used shock energy, 3000 Nm, for all velocity studies.

Table 8

Result

The curves in Figure 16 show diat with a low impact velocity of the impact ram the highest density for a specific shock energy is reached quickest.

The differences between the maximum densities for the seven series performed are up to 10 percent. The results indicate that a higher density is obtained when the impact ram mass is increased or equivalent a decreased impact velocity for a given energy level per mass. The effect is decreased as the energy is increased. The relative density at pre-compacting is to a great extent dependent on the static pressure.

The mass of the impact ram decides the impact velocity of the punch which is the velocity that accelerates the powder. A high impact ram mass will accelerate a light punch to higher velocities compared with a impact ram with a lower mass. Figure 18 and 19 show that the highest impact velocity is achieved for the punch accelerated by the greatest and slowest impact ram.

The diagrams show that if the mass of the impact ram increases to infinity the impact velocity of the ram will reach 0 m/s, which means that there is a limit to how great impact ram can be used to obtain a high punch velocity.

The relation between the mass of the impact ram and the punch to reach the highest density was in this study 1:3846.

The material properties of the processed powder and the configuration of the tool as well as the material used in the tool have to be considered to find the optimal mass relation between the impact ram and punch.

5. Multiple shock study 1

Figure 21 shows relative density as a function of the number of shocks for samples processed with a total shock energy of 3000 Nm and 4000 Nm, respectively.

Table 9 Conditions

Pressure Atmosphere

Temperature Room temperature

Energy retention No

Material Stainless steel

Total shock energy Constant for each study

Result

The curves in the diagram show that the highest density is reached for samples processed with one single shock, compared with the samples processed with the same total energy performed in a multiple shock series.

We can notice a tendency that the distance between the curves increases with increased number of stokes.

6. Multiple shock-study 2

Figure 22 shows the relative density as a function of number of shocks. Four studies were performed. In each study the samples were processed with one single shock or multiple shocks with a constant energy per shock.

Table 10

Result

The curve for the samples processed with the highest energy per shock increases fastest to a high density compared with the samples processed with lower energies per shock. The curves show that high densities are reached for shock energies over 50 Nm per shock, which means that there is a lower limit for the energy per shock when the total energy is divided into multiple shocks.

7. Heating study

Figure 23 shows relative density as a function of impact energy per mass for samples processed in increased temperature. Table 11

Result

The samples processed in a temperature above room temperature reaches nearly 100 % of density. The curve increases faster to a higher density compared with the curve showing the density result for the samples processed in room temperature.

Heating of the powder before and during the DFIER process increases the initial energy state of the powder. The powder compacting starts therefore from a higher temperature level and the result is a higher final density. This means that less energy is required to reach a high enough temperature in the material to achieve spot welding between the powder particles during the shock phase.

A heating to 150°C of stainless steel powder results in a relative density improvement of ~ 2 %.

The material properties of the processed powder have to be considered to find optimal parameter values for the heating

Electric current can be used to heat the powder during DFIER.

8. Vacuum study

Figure 24 shows relative density as a function of impact energy per mass for samples processed in vacuum.

Table 12

Result

The curve for the samples processed in vacuum increases faster and the samples reach a higher density than the samples processed in atmospheric pressure. When the pressure is decreased between the powder particles the reactivity is increased in the material and spot welding is achieved at a lower process energy, compared with a powder processed in atmospheric pressure.

Samples compacted to high densities in air have pores filled with air. If these samples are sintered after DFIER, the heat during sintering will expand the air in the closed pores expanding the material. If the pores have a lower pressure i.e. vacuum or near vacuum, they will not expand instead they will collapse during sintering and 100 % density can be achieved.

The material properties of the processed powder have to be considered to find optimal parameter values for processing in vacuum.

9. Impact direction

Figure 25 shows relative density as a function of impact energy per mass for samples processed in one or two impact directions.

Table 13

Result

The samples processed by impacting in two directions increases faster in density and reaches a higher density compared with samples processed in one impact direction for the same energy. The reason is that the powder processed from two directions obtains a better orientation of the powder particles during DFIER compared with the powder processed from one direction. A good orientation of the powder particles facilitates the solidification process.

The material properties of the processed powder have to be considered to find optimal parameter values for processing powders with two impact directions. 10. Time interval study

Figure 26 shows relative density as a function of time interval between two consecutive shocks. All samples were shocked two times with different time delays between the shocks.

Table 14

Result

The curve shows that the time delay between two shocks should be very short to effect the material properly.

The optimal time delay between two shocks depends on the mechanical properties of the material processed. Important material properties to consider are thermal conductivity and acoustic velocity.

11. Energy retention study 1

Figure 27 shows relative density as a function of impact energy per mass for shocked samples in comparison with samples shocked and post-compacted by energy retention.

Table 15

Result

The curve showing the result for shocked and post-compacted samples increases faster and reaches a higher density compared with the curve for only shocked samples.

The result of using an energy retention step is that the energy transmitted during the shock phase is retained in the material, and can effect the sample and increase the metallic bonding and spot welding among the powder particles. It is therefore possible to increase the relative density to near 100 % by using the post-compacting step. The material properties of the processed powder have to be considered to find optimal parameter values for the energy retention.

12. Energy retention study 2 Figure 28 shows relative density as a function of impact energy per mass for samples processed with different time delays between the shock and the start of the energy retention.

Table 16

Result

The curve for samples processed with a directly start of the energy retention after the shock step increases fastest and reaches the highest relative density. The curves show that the time between the shock process and the energy retention should be less then 1 s to obtain maximum effect. 1 second is not an optimum because the optimal time varies between different material types. The energy retention has less effect if the time delay between shock and energy retention increases.

The time duration of the local increase in temperature between the powder particles after the shock phase is very short. The condition for achieving an effect of the energy retention is that the sample still is in an elevated energy state, which is not the case if the sample already has cooled down to room temperature. The material properties of the processed powder have to be considered to find optimal parameter values for the energy retention.

13. Energy retention study 3

Figure 29 shows relative density as a function of energy retention time for shocked and post- compacted samples. The curve shows the effect of the duration of the energy retention.

Table 17

Result The curve shows that the effect of energy retention decreases after a few seconds.

The optimal time for energy retention is depending on the properties of the processed material in combination with the sample size. A greater mass will retain the heat for a longer period and it is therefore possible to increase the energy retention time and still obtain an effect.

14. Pre-compacting study 1

Figure 30 shows relative density as a function of pre-compacting pressure.

Table 18

Result

The curve shows that an increase in pre-compacting pressure increases the density of the pre- compacted sample.

The pre-compacting pressure on the sample before the shock phase is an important parameter to consider, because the green density of the sample will influence the final material properties achieved after a complete DFIER process.

15. Pre-compacting study 2

Figure 31 shows relative density as a function of pre-compact pressure. The curves show the effect of pre-compacting in different surrounding pressures.

Table 19

Result

The curve for samples pre-compacted in vacuum reaches the highest density. The energy necessary to overcome the atmospheric pressure and removing the air out of the powder at pre-compaction at normal pressure can instead directly affect the powder at pre- compaction in vacuum.

16. Pre-compacting study 3

Figure 32 shows relative density as a function of impact energy per mass. The samples have been pre-compacted differently by varying the time duration of the pre-compaction phase.

Table 20

Result

The curves show that a higher density is reached if the samples are pre-compacted longer than 1 s.

The material properties of the processed powder have to be considered to find optimal parameter values for the pre-compacting step.

Property studies

Three property studies were performed: Theoretical density study, powder hardness study and melting temperature study. Powder properties for the powders are described in tables 21 and 22.

Table 21: Powder properties for the metals used in the property studies.

Tabell 21 forts.

Table 22: Powder properties for the ceramics used in the property studies.

17. Theoretical density study

Figure 33 shows maximum obtained relative density as a function of theoretical density for different metal powders.

Table 23

Result The diagram shows a tendency that a metal powder with a high theoretical density is more difficult to process with DFIER to a high density, compared with a metal powder with a low theoretical density. In table 1 the materials used in the study are listed together with theoretical densities and obtained relative density for each material. It is clear that many parameters are involved to decide if a material is easy to process with DFIER to high densities. Other important powder properties are powder type, alloying elements, powder hardness, melting temperature, particle size and particle morphology.

18. Powder hardness study

Figure 34 shows maximum obtained relative density as a function of powder hardness for different ceramic and metal powders, respectively.

Table 24

Result

The diagram shows that it is more difficult to process a hard metal powder to a high density compared with a soft powder using DFIER. The plotted test values for metal powder has a smaller inclination compared with the values for ceramic materials. The diverging value (400 HV, 70.6%) for the ceramic material is because it is a water based ceramic. This explains the low powder hardness.

A hard metal or ceramic powder can be processed in vacuum and increased surrounding temperature to reach higher densities.

19. Melting temperature study

Figure 35 shows maximum obtained relative density as a function of melting temperature for different metal and ceramic powders, respectively.

Table 25

Result

The metal curve shows that there is no clear relation between the melting temperature for a metal and how easy it is to process with DFIER to high densities. Table 26 shows two steels with the same melting temperature but different powder hardness, which explains that only one material property cannot decide if the material is easy to process to high densities with DFIER.

Table 26

Material type Powder hardness (HV) Melting temperature (°Q

Stainless steel 160-190 1427 Martensitic steel 180-330 1427

Ceramic powders have higher melting temperature and are also more difficult to process to high densities compared with metal powders, which is showen in Figure 35.

20. Vibratory compacting study Figure 36 shows relative density as a function of applied pressure for samples processed with conventional static pressing compared with samples processed with vibratory compacting (VC) in combination With other processes.

Table 27

Result

The curves show that relative density of powders vibrated under controlled conditions during a static pressure or/and in combination with shock energy reach much higher densities compared with samples processed with only static pressure.

Samples compacted with vibratory compacting, shocked from two directions and with double axial static pressure reaches the highest density for the lowest total pressure.

Claims (19)

Claims

1. A method of producing a body from particulate material by coalescence or compaction to higher density, characterised in that the method comprises the steps of a) filling a pre-compacting mould with the material in the form of powder, pellets, grains or the like, b) vibrating the mould, c) pre-compacting the material at least once with a pre-compacting means and d) compressing the material in a compression mould by at least one stroke, where a striking unit emits enough kinetic energy to form the body when striking the material inserted in the compression mould with a striking means, causing coalescence or higher density of the material.

2. A method according to claim 1, characterised in that the pre-compacting mould and the compressing mould are the same mould.

3. A method according to claim 1 or 2, characterised in that the pre-compacting c) is performed while vibrating the mould.

4. A method according to any of claims 1-3, characterised in that d) the material is compressed from two opposite sides simultaneously using two striking units.

5. A method of producing a body from solid material by coalescence or compaction to higher density, characterised in that the method comprises the steps of a) inserting the solid material in a mould, c) possibly pre-compacting the material at least once with a pre-compacting means and d) compressing the material in the mould by at least one stroke, from two sides simultaneously, using two striking units emitting enough kinetic energy to form the body when striking the material, causing coalescence or higher density of the material.

6. A method according to any of claims 1-5, characterised in that the material is compressed from two opposite sides and at least one further side simultaneously using at least three striking units.

7. A method according to any of claims 2-6, characterised in that the pre-compacting means is continuously applied against the material with the same or a higher pressure during the compression d) thereof with the striking unit or units.

8. A method according to any of claims 1-7, characterised in that the energy of the compression d) is retained within the compressed material by e) maintaining or reapplying the striking means to press against the compressed material after the stroke or strokes.

9. A method according to any of claims 1-8, characterised in that the temperature of the material in the mould is increased or decreased during one or more steps.

10. A method according to claim 9, characterised in that the material in the mould is heated before and/or during the pre-compaction c).

11. A method according to any of claims 9-10, characterised in that the material in the mould is heated before and or during the compression d).

12. A method according to any of claims 10-11, characterised in that the material in the mould is heated by the use of electrical current.

13. A method according to claim 12, characterised in that the electrical cuπent flow is syncronized with the pre-compaction c) and/or the compression stroke or strokes d).

14. A method according to any of claims 1-13, characterised in that the material in the mould is subjected to a sub-atmospheric pressure before the pre-compaction c).

15. A method according to any of claims 1-14, characterised in that the material in the mould or moulds is maintained under another gas than air.

16. A method according to claim 15, characterised in that the gas is an inert gas.

17. A method according to claim 15, characterised in that the gas is a reactive gas.

18. A method according to any of claims 1-17, characterised in that the vibration b) is maintained during compressing d) and/or during energy retention e).

19. A body produced by the method according to any of claims 1-18.