EP1963038A1

EP1963038A1 - Method and apparatus for consolidation in layers

Info

Publication number: EP1963038A1
Application number: EP06835732A
Authority: EP
Inventors: Roald Karlsen; Olav ÅSEBØ
Original assignee: Sinvent AS
Current assignee: Sinvent AS
Priority date: 2005-12-20
Filing date: 2006-12-20
Publication date: 2008-09-03
Also published as: WO2007073206A1; EP1963038A4; WO2007073205A1

Abstract

The invention relates to a method and apparatus for building an object by the layer-wise deposition of powder layers, the powder layers including building powder being transformed into an object, wherein the building powder is consolidated and made of metal or ceramic mixed with metal. The powder layer is arranged on a printing face of a piston and deposited in a pressure pulse onto a building table or a work piece, wherein the work piece includes previously deposited powder layers, and wherein the work piece is surrounded by a mould being at the sintering temperature of the building powder throughout the construction of the object.

Description

METHOD AND APPARATUS FOR CONSOLIDATION IN LAYERS

INTRODUCTION

The present invention relates to a method and apparatus for the layer-wise deposi- tion and consolidation of powder layers, in particular for use in conjunction with a method for producing objects in layers. The powder layer includes a building powder of metal or ceramic mixed with metal, and/or a support powder of ceramic or a mixture of ceramic materials.

BACKGROUND

For generations, people have been developing methods for producing metallic objects. To facilitate overview of the methods, they are divided into groups. It is distinguished between formative, subtractive, and additive methods. |n modern industrial processes, the products may be subject to formative (casting, forging, rolling, bending, compression molding, extrusion, pulling), subtractive (turnery, milling, grinding, drilling, reaming, sparking), and additive operations in the same production process. Common to all subtractive processes is that the shape of the object is obtained by removing material in the form of filings from a base material. Some of the formative processes, such as forging and rolling, also have the pur- pose of increasing the material quality.

Even though the established production methods have become very sophisticated and are continuously developing, they have their limitations. The requirements of improved product characteristics and automatic production are continuously inten- sifying, driving the emergence of new technology. Since the late 80's, additive methods have been suggested as a solution to many of the challenges that are encountered. Additive processes produce metal objects by combining small amounts of material. The objects are hence built by adding material instead of removing material. Additive processes allows for the automatic manufacturing of components having geometrical shapes, microstructures, and material compositions that are not possible to obtain using the established formative and subtractive manufacturing methods. Additive production is the construction of arbitrarily shaped objects by adding small amounts of material. Mechanically, this is accomplished through in layer construction, and the technology is often referred to as in layer production or layer-wise production.

The building powder within the same powder layer may be comprised of several metallic materials. The powder materials may be mixed, or may be kept apart. Subsequent to consolidation, the layer may be comprised of one or more pure metals, or alloys thereof. Controlling the layer manufacturing process makes poss- ible the formation of gradual transitions between materials of the same layer (grading in two dimensions, the X-Y plane).

The building powder layers are consolidated, becoming part of the object being built. By increasing, or reducing, the portion of a material for each layer being consolidated, it is also possible to form gradual transitions between materials in the building direction (the Z axis). Gradual transition materials are called functionally graded materials (FGM).

The powder particles of the building powder may have different characteristics (size, shape, structure). This makes it possible to control the after-consolidation porosity of the layer. For example, a given surface porosity of an implant may allow blood vessels to grow into the implant. Blood vessels are transformed to bone structure and assist in retaining the implant. The density of the object is determined by the consolidation parameters, powder material, and powder characteristics.

The material properties, in particular wear resistance and hardness, may be improved by adding ceramic particles to the building powder. The object, or parts of the object, then will be comprised of ceramic particles bound together by metallic material (cermet).

None of the prior art methods for manufacturing powder layers is able operate in temperatures above 200 ⁰C. The incographic method has the inherent limitation that the wax evaporates from the surface if the temperature is too high. The xerographic method has the inherent limitation that all known photo receptors operate in room temperature. The ionographic method has the inherent limitation that the binding of the ions to the surface weakens with the temperature. The problem with the capacitive method is that no high temperature application has been described or created. The piston or any other powder transferring mechanism being used must have a low temperature (below 200 °C) during the powder manufacturing/transferring process. The temperature within the consolidation zone when sintering iron must be higher than 800 °C. Hence, we are faced with the problem that the temperature at a given moment must be higher than 800 °C, and in the next moment lower than 200 °C.

Karlsen [Karlsen, Bakkelund 2002, NO 317085] solves this problem by using a non-recurring integration mechanism, a plastic band that is consumed and that does not bring the heat from the consolidation zone back to the powder transfer zone. However, the solution provided by Karlsen is problematic as the plastic band easily wrinkles when it is fed into the consolidation zone. If the plastic band wrinkles, the geometry to the powder layer will be deteriorated. The plastic band then becomes a material of consumption that causes undesired fouling of the object.

Karlsen [Karlsen, Bakkelund, 2004, WO 2004/037469 A1] discloses a recurring process, and solves the above problem by cooling the piston end. However, the solution provided by Karlsen is both energy-intensive and time consuming, as the piston end must be heated to the sintering temperature and then cooled at each cycle. Moreover, in order to obtain an acceptable building time for industrial applications it is necessary that new layers can be added before the previous layer has been fully consolidated. This is not possible with the solution provided by Karlsen as the heat is supplied from the piston. The method subjects the piston end to large thermal stresses. Also, the object is exposed to varying temperatures as heat is only supplied through the piston. The lack of temperature control is disadvantageous for the sintering process. SUMMARY OF THE INVENTION

The present invention relates to a method and apparatus for the layer-wise consolidation of powder layers to create functional objects. By functional is meant that the object satisfies the requirements that are set for the finished product in terms of tolerances and mechanical properties. Consolidation is the transformation from powder to objects having a coherent microstructure, and is comprised of to processes: compaction and sintering. In particular, the invention relates to a concept of in layer consolidation for materials having a high melting point, especially metals and ceramic intermixed with metal. The temperature and pressure necessary to transform metallic powders into a solid material impose high requirements to the machine components.

In a first aspect, the invention provides a method for constructing an object by the layer-wise deposition of powder layers, the powder layers including building pow- der being transformed to an object, wherein the building powder is consolidated and comprised of metal or ceramic mixed with metal. The method is characterized in that the powder layer is arranged onto a printing face of a piston and deposited in a pressure pulse onto a building table or workpiece, wherein the workpiece includes previously deposited powder layers, and wherein the workpiece is surroun- ded by a mould being at the sintering temperature of the building powder throughout the construction of the object.

A mould being at the sintering temperature throughout the building process is necessary for obtaining a coherent microstructure of the object being built. The mould may operate in temperatures of up to 1200⁰C, 400 to 1200 ⁰C being preferred, and in particular 600 to 1000 ⁰C. For example, the mould may be heated by way of induction, resistance heating, microwaves, or by the combustion of gas. Heat is transferred from the mould to the workpiece.

In one embodiment, the powder layer may further include a support powder for supporting the object during the building process, the support powder being compacted at said sintering temperature. The piston operates at temperatures below 200⁰C during the application of powder layers, preferably below 100 ⁰C, and most preferably in the range of 30 to 45 ⁰C. The piston may be cooled using liquid, and, in one embodiment, may be continuously cooled. The duration of the pressure pulse may be less than 0.5 seconds, 0.05 to 0.3 seconds, and in particular 0.07 to 0.2 seconds, being preferred. The pressure applied on the powder layer by the piston may be less than 250 MPa, and is preferably between 50 and 250 MPa.

In a further embodiment, the powder layer adheres to the printing face onto a wax or thermopolymer coating. The building powder may also contain at least one material having a significantly lower melting temperature than the support powder.

In a second aspect, the invention provides an apparatus for constructing an object by the layer-wise deposition of powder layers, the powder layers including building powder being transformed to an object, wherein the building powder is consolidated and comprised of metal or ceramic mixed with metal. The apparatus is characterized in that it includes a mould (10) having a cavity comprising a bottom (12), the bottom constituting a building table for the object, a piston (1) having a piston end (3) of a tool material, the piston being adapted to the cavity of the mould, and the piston end acting to deposit a layer of the object onto the building table or previously deposited layers and applying a pressure pulse on the deposi- ted layer, and a device for causing the mould to maintain the sintering temperature throughout the construction of the object.

The piston may be provided with internal channels for liquid cooling. The mould may further comprise walls and a bottom of a hard metal, the inside wall surfaces and bottom being coated with a ceramic coating. The ceramic coating may be aluminum oxide (AI₂O₃). The piston end may be of a hard metal. The piston is preferably maintained at a low temperature, and operates at temperatures below 200 ⁰C, preferably below 100 ⁰C, and most preferably in the range of 30 to 45 ⁰C. The mould, which is maintained at the sintering temperature during the construc- tion of the object, may operate at temperatures of up to 1200⁰C, preferably at 400 to 1200 ⁰C, and in particular at 600 to 1000⁰C. The apparatus may also include a consolidation unit comprising the mould, the consolidation unit being provided with channels for liquid cooling of the outside of the consolidation unit. The piston is electrically conductive.

In a further embodiment, the apparatus may comprise a slide for positioning the piston relative to the mould. The slide may be shaped so as to act as a lid for the mould when the piston is not positioned to cover the cavity of the mould. As an alternative, the apparatus may include a lid for preventing the intrusion of air into the cavity during the building process. The apparatus may further be provided with a device for supplying a blanket gas into the cavity of the mould. The heating of the mould to the sintering temperature may be accomplished by way of induction, and, in that case, the apparatus includes a coil arranged in proximity of the mould.

The present invention combines additive and formative manufacturing in an automatic production process.

The powder layer is comprised of building powder and optionally a support powder. The building powder may be comprised of one or more powder types that are consolidated in the building process. By consolidation is meant the compaction and sintering of powder to a solid material by exposing the powder to pressure and heat. The support powder has a significantly higher melting temperature than the building powder, and is used for imparting a desired geometric shape to the object. The support powder is subject to the same stresses as the building powder. However, the support powder is compacted but not transformed into a solid material, and is removed when the object is finished.

The present invention resembles a forging process; a piston having a powder layer at its end hits a workpiece (previously deposited powder layers) having a high temperature. The powder layer peels off from the piston and is deposited onto the workpiece. The position of the workpiece is retained by a mould. This means that the only deformation possible is a density increase of the deposited powder layers. Within powder metallurgy, the transformation of a powder into a solid material is referred to as sintering. Sintering differs from melting in that the working temperature is kept below the melting temperature of the material, inter alia. The present invention combines the additive manufacturing process with elements from the forging process. The workpiece is maintained at a high temperature during the transformation of the powder in order to reduce the resistance to plastic de- formation. The hammer absorbs only a little heat energy in the deposition of the powder layer as the powder transfer occurs through an impact (pressure pulse). In the present invention, the hammer is replaced with a liquid cooled piston. The piston end is made of a tool material. The invention differs from a regular forging process in that the hammer carries with it a new powder layer at each blow. As the workpiece is maintained at the sintering temperature of the building powder, the powder also sinters in the period between each blow. The concept of having a cold piston and a hot workpiece is necessary in order for the apparatus to work.

Sintering is the increase of the material density in that atoms diffuse through the lattice structure. Several parameters influence the sintering process. The rate of diffusion is determined mainly by the temperature. The processing time and pressure are essential factors determining the resulting density. The pressure pulse increases the density by causing a plastic deformation of the object at each impact. To obtain an acceptable building time, new layers must be supplied before the consolidation of the previous layer has completed. An object may be comprised of several thousand layers.

The support powder peels off from the printing face of the piston on deposition by interlocking with previously deposited support powder (interparticular bonding). If support powder is pressed against the object, it will penetrate into and attach to the object. It is important that the printing face of the piston is smooth and made of a tool material. In addition to having a high hardness, the piston end must be able to resist the impact stresses that occurs each time a new powder layer is deposited. A hard metal (wolfram carbide in a cobalt matrix) is used that provides an ad- vantageous combination of hardness and roughness.

The manufacture of powder layers on the hammer may be accomplished in several manners. An incographic method is preferred. In incographic manufacturing, the powder materials are transferred one by one to the printing face in sequence. Powder is transferred to the printing face by means of electrostatic forces and adheres to a wax pattern. One such manufacturing method is described in PCT/NO06/00492, having the same priority as the present application.

Prefabricated powder layers may also be transferred to the printing face by applying a uniform wax layer onto the entire printing face. The powder layers have been manufactured at an earlier time and are transferred by electrostatic forces from a powder receptor to the printing face. Methods that provide entire powder layers on a powder receptor are described in [Bynum, 1989, US 5,088,047], [Kumar, 1998, US 6,066,285], [Grenda, 1994, US 6,206,672], [Bjørke, 1992, WO92/22430], [Karlsen, Bakkelund, 2002, NO 317085], and [Karlsen, Bakkelund, 2004, WO 2004/037469 A1].

The method described in WO 2004/037469 subjects the piston end to large thermal stresses. The object is exposed to varying temperatures as heat is only supplied through the piston. The lack of temperature control is disadvantageous for the sintering process.

The present invention solves this problem by avoiding any significant heating of the piston in the consolidation zone. Such heating is avoided by the use of a pressure pulse, which leaves little time for the heat transfer to take place. The heat being absorbed by the piston is continuously removed. The wax being used does not cause fouling of the object. The same wax is also used for manufacturing pro- ducts in conventional powder metallurgy. The sintering temperature of the object is controlled during the entire building process.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, embodiments of the invention will be explained with reference to the accompanying drawings, in which:

Figure 1 is a schematic view showing the value chain from the very idea to the finished product. A method and apparatus for producing objects through the con- solidation of powder layers is described. The manufacture of the individual layers is not described in this document. Layers may be manufactured using electro- graphic methods, or using incography. Electrographic methods are described in [Dessauer, Clark, 1965, Xerography and Related Processes], [Schaffert, 1975,

5 Electrophotography], and [Schein, 1996, Electrophotography and Development Physics];

Figure 2 shows a consolidation unit according to an embodiment of the invention in which the mould is designed for temperatures up to 1200 °C, pressures up to 250 MPa, and a pressure application time of less than 0.5 seconds. The mould is o maintained at the sintering temperature throughout the building process, and the heat is provided by way of induction heating;

Figure 3 shows the consolidation unit of figure 2 in the case of using resistance heating of the mould. An alternative for supplying sintering energy; and Figure 4 shows the principle of heating the mould by way of microwaves. Another 5 alternative for supplying sintering energy.

DETAILED DESCRIPTION

The invention relates to a concept for producing objects by the in layer consolidation of powder layers. The value chain for the layer-wise production of an object, o from the very idea to the finished product, is shown in figure 1.

In the layer-wise production of an object, initially a computer model is created by drawing or scanning the physical object. Then the computer model is divided into thin layers, providing a data file containing information on each layer (thickness, 5 shape, materials, etc.) and the relative location of the layers. The fabrication of the object is initiated by sending information on the first layer to a manufacturing unit. In this unit, a physical powder layer is constructed based on the digital information on the layer. When the powder layer is complete (it may consist of several materials), it is transported to the compaction unit and transformed into a solid material. o While the compaction process is proceeding, the manufacturing unit receives information on the next layer and starts to recreate this layer with powder. The manufacture and consolidation of the powder layers are repeated until the object is finished. Examples of optional post-processing could be the removal of support powder, heat treatment, or processing using subtractive techniques. As such, the process of manufacturing the layer before the consolidation process is not a part of this description. A description of the formation of the powder layer is found in PCT/NO06/00492, having the same priority as the present application.

Two main groups of powder are used for the manufacture of powder layers; building powder and support powder. The building powder of each layer forms a thin slice of the product being constructed (transformed into a solid material). In order to be able to build objects having an arbitrary shape (overhang, inner geometries, etc.), it is necessary to support the object during the building process. To this end, a support powder is used that does not sinter in the consolidation process, but acts as a support during the building process. In the construction of metallic objects, the support powder is typically a ceramic material, or a mixture of ceramic materials. The sintering temperature of the support powder must be substantially higher than the sintering temperature of the building powder, so that the support powder is not sintered but may be easily removed when the object is finished. The powder particles of the support powder typically have an irregular shape.

A powder layer may contain several types of building powder, in terms of both material and particle structure. This allows for the production of objects with custom properties for given applications. An exemplary application could be hip bone (based on a sequence of x-ray pictures of the patient) having a titan base structure, cobalt-chromium friction surfaces, and areas having a porosity that allows the body tissue to grow into and retain the new hip bone. Gradual transi- tions between materials (graded materials) can be obtained by increasing the portion of a new material for each new layer being consolidated. In this manner, the problems associated with differing heat expansion of the different materials are reduced.

In the following, a method and apparatus for the layer-wise consolidation of powder layers for producing functional objects, as shown in the value chain of figure 1 , is described. By functional is meant that the object satisfies the requirements that are set for the finished product in terms of tolerances and mechanical properties. Consolidation is the transformation from powder into objects having a continuous microstructure, and is comprised of to processes: compaction and sintering. In particular, the invention relates to a concept of in layer consolidation for metals having a sintering temperature in the range of 400 to 1200⁰C. The temperature and pressure necessary for transforming metallic powders into a solid material puts high requirements to the machine components of the consolidation apparatus.

In the following, the machine components and materials involved in the process are described, with reference to figure 2.

Piston (1)

The piston 1 is comprised of a piston rod 2 and an end 3 of a hard metal. The piston rod 2 is made of a material having an adequate electrical and thermal con- ductivity (e.g. copper) and is provided with internal channels for liquid cooling 4. The hard metal end may be of the type H10F from Sandvik Hard Materials. The hard metal used is able to resist large impact stresses. The hard metal is also an excellent thermal conductor. Avoiding that powder material is sintered into the piston end is a problem in all hot isostatic pressing. In the present solution, this problem is avoided by impacting (pressure pulse) instead of pressing. The transfer of heat takes time, and by hitting instead of pressing, the piston end is maintained at a low temperature. Powder material in the desired pattern may then quickly be applied directly onto the piston end after the deposition has been completed. The piston operates in temperatures below 200⁰C during the application/deposition of powder layers, preferably below 100 ⁰C, and most preferably in the range of 30 to 45 ⁰C. Sintering between the powder material and the piston is prevented by keeping the temperature at the piston end below the sintering temperature. The piston end 3 forms a printing face and does not need to be an integral part of the piston. For example, the piston end may be able to move independently of the piston and connect to the piston rod 2 immediately before the piston is rammed into the mould for depositing a powder layer. A ceramic coating 5 may be applied to the piston end 3 in order to weaken the binding between the powder material and the piston end. The coating 5 may be of the same type as used in conventional cutting tools for the processing of chips, such as Balinit Alcrona (AICrN). The coating must be electrically conductive.

5

The piston rod 2 may be provided with a coating 6. This coating may be yttrium stabilized zirconium or another material acting as a two-way temperature barrier. The coatings on the piston end resist large temperature variations, and are chemically inactive relative to the powder materials being used. The powder layer 7 to o be deposited is adhering to the underside (printing face) of the piston. The piston 1 has a guide 8 allowing the piston to be rapidly connected to the pressure cylinder 9. The pressure cylinder runs the piston 1 in and out of the mould 10, and applies pressure when the powder layer is to be deposited. The pressure is transferred as a pressure impulse that preferably lasts for less than 0.5 seconds. In particular, the s duration of the pressure impulse is from 0.05 to 0.3 seconds, preferably from 0,07 to 0,2 seconds, depending on the building material being used.

Mould (10)

The mould consists of a pipe 11 of a hard metal and a bottom 12 of a hard metal. o The hard metal is comprised of wolfram carbide powder bound into a cobalt matrix. The hard metal of the bottom contains a higher portion of cobalt than the mould. A higher portion of cobalt increases the impact resistance of the hard metal. The hard metal of the mould may be of the type SGC10C, and that of the bottom of may be of type H10F, both available from Sandvik Hard Materials. The 5 mould and the bottom are coated with a ceramic coating 13. The coating prevents the object from being sintered into the hard metal cobalt matrix. The coating may consist of vacuum evaporated alumina (AI₂O₃). The coating thickness is typically between 2 and 3 μm. The cobalt content of the hard metal of the mould is kept low in order to reduce the likelihood of it being sintered into the object in case the coat- 0 ing should crack open. The mould is not subject to the same level of impact stresses as the bottom, allowing the hard metal thereof to have a lower content of cobalt. The mould is surrounded by a copper induction coil 14. The coil 14 has internal liquid cooling and is thermally isolated from the mould by aluminum oxide.

The coil 14 is connected to an induction generator sending high frequency electri- cal power through the coil. The rapidly alternating field around the coil results in a quick heating of the mould [Barber, 1983, Electroheat, chapter 6]. Temperature sensors measure the temperature of the mould, and are connected to the induction generator. The induction generator adjusts the effect applied to the mould based on the desired temperature and the measured temperature. The induction generator applies the effect necessary to maintain the mould at the sintering temperature during the building process. Temperature sensors are also used for monitoring and controlling the temperature of the mould, and the control system automatically stops the building process if an unacceptable temperature arises due to a system failure.

The mould is isolated from the adjacent equipment by an isolating material 15 (e.g. ^®Macor). The top plate 16 and the four outside walls 17 of the consolidation unit are liquid cooled 18.

The cavity of the mould may be supplied with a blanket gas (protective environment) through a connection point 19. This is done in order to prevent oxidation at the high temperatures. A quick release connector 20 for supplying cooling liquid to the mould wall 11 is provided to allow for a more rapid cooling of the mould. Due to the high temperatures, special oils are used as coolant.

Slide (21)

A slide 21 is operated by a linear actuator 22 and is able to slide forward and backwards along guides 23 at the top plate 16. To the slide 21 is mounted an automatic chuck 24. When the piston 1 arrives with a powder layer to be deposi- ted, the piston is locked to the chuck 24. The slide 21 is then located in the position as shown in figure 2. A clearing is provided between the underside of the piston and the top plate 16 to avoid interference with the powder layer 7. The linear actuator 22 then pulls the slide 21 to position the piston directly above the cavity of the mould. The pressure cylinder 9 is connected to the guide 8 of the piston and interlocked with the connector 25. Chuck 24 releases the piston, and pressure cylinder 9 runs the piston down into the mould. The movement of the compression cylinder is controlled by the pressure. When a predetermined pres- sure has been reached, the piston returns. The pressure cylinder applies a pressure pulse to the powder layer. The duration of the pressure pulse depends on the velocity of the piston and on how much time the system needs to build up the pressure when the piston encounters resistance. The system applies a pressure pulse lasting less than 0.5 seconds. The mould is at the sintering temperature, so the sintering process (the increase of the density of the object) continues after the piston has trusted the powder layer against the object.

Following consolidation, the piston is retracted from the mould and locked to the chuck 24. The pressure cylinder 9 is disconnected and the slide 21 carries the piston to the supply and receiving position (se figure 2). The fork 26 engages the piston, and the chuck 24 is released. When the slide 21 is positioned as shown in figure 2, it acts as a lid on the mould. The shape of the slide makes sure that the materials holding the sintering temperature are not contacted with air each time a new powder layer is to be consolidated.

Building powder

The building material is a metallic powder. The metals used are metals commonly used in the metallurgy for building of metallic products, such as Iron, Copper, Nickel, Aluminum, Cobalt, Chromium, Magnesium, Manganese; Molybdenum; Silicon; Sink, Titan. The building material may be any metal having a sintering temperature in the range of 400 to 1200⁰C. The powder material does not need to be a pure element, but may be an alloy. The metal powder may be spherical, irregular, or spongy, and have a particle size ranging from 6 to 200 μm, preferably from 10 to 50 μm, and most preferably from 15 to 30 μm.

The building powder within the same powder layer may be comprised of several metallic materials. The powder materials may be mixed, or may be kept apart. Subsequent to the consolidation, the layer might be comprised of one or several pure metals, or alloys of such.

The powder particles of the building powder may have different characteristics (size, shape, structure). This makes it possible to control the after-consolidation porosity of the layer.

The material properties, in particular wear resistance and hardness, may be improved by adding ceramic particles to the building powder. In that case, the object, or parts of the object, would be comprised of ceramic particles bound together by metallic material (cermet).

Support powder

The support powder has a significantly higher sintering temperature than the build- ing material, and is used for imparting a desired geometric shape to the object. The support powder is subject to the same stresses as the building powder. The support powder is compacted but not transformed into a solid material, and is removed when the object has been finished.

The support powder may consist of a mixture of aluminum oxide, ^®Wollastonite, and ^®Molocite. The particle size and packability of the support powder must be adapted so as to make the support powder support the building powder during both compaction and sintering. The support powder is typically comprised of aluminum oxide of a given size (diameter 100 μm, for example), Wollastonite being 1/7 of this size (diameter approx. 14 μm), and Molocite being 1/7 of the

Wollastonite size (diameter approx. 2.5 μm). The support powder is an agglomerate kept together by a solid paraffin wax, for example.

Sintering temperature The apparatus is designed for being able to operate in the temperature interval from room temperature to 1200 °C. The building materials to be processed have a sintering temperature in the range of 400 to 1200 ⁰C. The sintering temperature depends on the building material. In general, the sintering temperature of a metal is in the range of 60 to 80 percent of the melting temperature of the material, as measured on the Celsius scale [German, 1994, Powder Metallurgy Science].

In cases in which the building materials used have greatly differing melting tempe- ratures, the consolidation temperature may be made higher than the melting temperature of the material having the lowest melting point. In this case, one or more materials will be in a molten phase, and one or more materials will be in a solid phase. This method is called liquid phase sintering.

Sintering time

An important principle is that the mould is at the sintering temperature throughout the building process. Solid phase sintering occurs in that atoms migrate through vacancies in the atom lattice. However, all the atoms may not relocate at the same time. The degree of sintering depends, inter alia, on for how long time the atoms are allowed to travel (the supplied energy is equal to or larger than the activation energy for atomic migration). The pressure pulse applied by the piston increases the area of contact between the powder particles. This is necessary to ensure a quick initiation of the sintering and to obtain a high-density object. As the mould is at the sintering temperature, the sintering of the powder layer continues when the piston retracts. The atoms are given time to migrate, so that a homogenous micro- structure is formed between the new layer and the previously consolidated layers as well. On each deposition of a new powder layer the object is subject to a pressure pulse that accelerates the sintering process by collapsing any cavities (pores) between the powder particles. Heat, and optionally one or more pressure pulses, may be applied after the last layer has been deposited in order to achieve the desired density throughout the entire object.

Pressure/ Pressure application time

The apparatus is designed for being able to operate at a pressure of up to 250 MPa, and a pressure application time of less than 0.5 seconds. The pressure necessary for compressing the powder layer depends on the material used, the shape of the powder particles, and the distribution of the powder particle size. Irregular particles require a higher pressure to provide the same packing density as spherical particles. Hard particles need a higher pressure to deform so that the intraparticle voids are minimized. A powder having a broad particle size distribution is easier to compact than a powder having a narrow particle size distribution. In order to build objects having the desired porosity, powders may be used in which all particles are of the same size (monodisperse powders).

Description of a consolidation embodiment

The consolidation of a powder layer using the machine configuration of figure 2 will be discussed in the following. The linear guide 27 positions the piston 1 in the chuck 24. The powder layer 7 has been fabricated in an earlier layer manufacturing process (see the value chain of figure 1) and is arranged on the underside of the piston. The powder layer 7 may be retained by electrical forces, or be adhered to a wax coating.

The linear actuator 22 positions the slide 21 so that the piston 1 is located immediately above the cavity of the mould 10. The piston is attached to the pressure cylinder 9 and run into the mould. The powder layer 7 is hit against the object, which is at the sintering temperature. Subsequent to deposition, piston 1 is retracted from the mould and engaged with the chuck 24. The pressure cylinder 9 is dis- connected and the slide 21 carries the piston to a location for being picked up by the fork 26. The consolidation unit is then ready to receive the next powder layer.

In the embodiment of figure 2, induction is used as the means for heating the mould. The advantage of this method is the rapid heating. The field surrounding the coil makes sure the heat generation occurs inside the forming material itself.

Various alternative heating methods exist, such as resistance heating by way of resistance wire (figure 3), direct resistance heating of the tool, microwave heating of the mould (figure 4), and heating of the mould by gas combustion.

5. Alternative solutions

Figure 3 shows the consolidation unit for the case in which resistance wire 28 is being used for heating the mould 10 (indirect heating). The resistance wire is made of a material having a given electric resistance (e.g. isolated ^®Kanthal wire), so that heat is generated when electric power flows through the wire. The wire and mould walls 11 are in contact, so that the mould is heated by conduction. The inside of the outside walls 17 is covered with a heat-reflecting material 29 (e.g. ^®Sigraflex) in order to minimize the heat loss. In all other aspects, the consolidation unit is as described in the case of induction heating. Resistance heating is slower than induction heating, but the associated loss of energy is lower.

In the case of direct resistance heating, the mould 10 forms a part of a closed electric circuit. The material of the mould (e.g. a hard metal) has an electrical resistance that causes the generation of heat. In all other aspects, the consolidation unit is as described in the case of resistance wire heating.

Figure 4 shows a principle of heating using microwaves. In this case, the hard metal mould wall 11 is replaced with a pipe 30 made of a microwave absorbing material (a material having a high coefficient of dielectric loss, such as silicon carbide, for example). A microwave source 31 (magnetron) emits microwaves that travel through the waveguide 32 and hits the mould wall 30. The waves are absorbed by the mould, which is heated in the same manner as in the case of induction [Copson, 1962, Microwave Heating]. As indicated in the figure, several microwave sources may be used in order to improve the heat distribution as well as to further speed up the heating. The bottom 33 of the mould could also be heated directly using this method by selecting a microwave absorbing material. Microwave heating is efficient, as the heat is generated in the mould itself.

In the preceding sections, embodiments of the invention have been described. However, other embodiments of the invention will be apparent to a man skilled in the art. The scope of the invention is defined by the accompanying claims.

Claims

1. A method for constructing an object by layer-wise deposition of powder layers, the powder layers including building powder being transformed into an object,

5 wherein the building powder is consolidated and made of metal or ceramics mixed with metal, c h a ra cte ri z e d i n that the powder layer is arranged on a printing face of a piston and is deposited in a pressure pulse onto a building table or workpiece, wherein the workpiece includes previously deposited powder layers, and wherein the workpiece is surrounded by a mould being at the sintering tempe- o rature of the building powder throughout the construction of the object.

2. The method of claim 1 , characte rized i n that the powder layer further includes a support powder for supporting the object in the building process, wherein the support powder is compacted at said sintering temperature. 5

3. The method of claim 1 , cha racte rized in that the piston operates at temperatures below 200⁰C during the deposition of powder layers, preferably below 100⁰C, and most preferably in the range of 30 to 45 ⁰C.

o 4. The method of claim 1 , characterized in that the duration of the pressure pulse is less than 0.5 seconds, preferably from 0.005 to 0.3 seconds, and even more preferably from 0.07 to 0.2 seconds.

5. The method of claim 4, cha racte rized in that the pressure being app- 5 lied by the piston on the powder layer is less than 250 MPa, and preferably between 50 and 250 MPa.

6. The method of claim 1 , cha racterize d in that the powder layer is adhered to the pressure face onto a wax or termopolymer coating. 0

7. The method of claim 1 , ch a racte rized i n that the mould operates at temperatures up to 1200⁰C, preferably between 400 and 1200⁰C, and more preferably between 600 and 1000⁰C.

8. The method of claim ^characterized in that the mould is heated by way of induction, resistance heating, microwaves, or the combustion of gas.

5 9. The method of claim 1, characterized i n that the piston is cooled using liquid.

10. The method of claim ^characterized in that the piston is continuously cooled. 0

11. The method of claim 2,characterized in that the building powder contains at least a material having an appreciably lower melting temperature than the support powder.

5 12. The method of claim ^characterized in that heat is transferred from the mould to the workpiece.

13. An apparatus for constructing an object by layer-wise deposition of powder layers, the powder layers including building powder being transformed into an o object, wherein the building powder is consolidated and made of metal or ceramics mixed with metal, characterized in that the apparatus includes:

- a mould (10) having a cavity comprising a bottom (12), the bottom forming a building table for the object, 5 - a piston (1) having a piston end (3) of a tool material, the piston being adapted to the cavity of the mould, and the piston end causing deposition of a layer of the object onto the building table, or previously deposited layers, and applying a pressure pulse to the deposited layer, and

- a device for ensuring that the mould is maintained at the sintering temperature 0 throughout the construction of the object.

14. The apparatus of claim 13, characterized in that the piston is provided with internal channels for liquid cooling.

15. The apparatus of claim 13, characterized in that the consolidation unit, comprising the mould, is provided with channels for liquid cooling on the outside.

16. The apparatus of claim 13, characterized in that the mould has walls and a bottom made of a hard metal, the inside wall surfaces and bottom be- ing coated with a ceramic coating.

17. The apparatus of claim 13, characterized in that the ceramic coating is aluminum oxide (AI2O3).

18. The apparatus of claim 13, characterized in that the piston end is made of a hard metal.

19. The apparatus of claim 13, characterized in that the piston operates at temperatures below 200⁰C, preferably below 100⁰C, and most preferably in the range of 30 to 45⁰C.

20. The apparatus of claim 13, characterized in that the mould operates at temperatures up to 1200⁰C, the preferred temperature range being 400 to

1200⁰C, and in particular 600 to 1000⁰C.

21. The apparatus of claim 13, characterized in that the apparatus includes a slide for positioning the piston relative to the mould.

22. The apparatus of claim 13, characterized in that the slide is shaped so as to act as a lid for the mould when the piston is not positioned to cover the cavity of the mould.

23. The apparatus of claim 13, ch aracte rized in that the apparatus includes a lid for preventing the intrusion of air into the cavity during the building process.

24. The apparatus of claim 13, characterized in that the apparatus is provided with a device for supplying a blanket gas to the cavity of the mould.

25. The apparatus of claim 13, characterized in that the apparatus includes a coil arranged in connection with the mould for heating the mould to the sintering temperature by induction.

26. The apparatus of claim 13, characterized in that the piston is electrically conductive.