WO2020104334A1

WO2020104334A1 - Method for producing a component from metal or technical ceramics materials

Info

Publication number: WO2020104334A1
Application number: PCT/EP2019/081530
Authority: WO
Inventors: Thomas Meissner
Original assignee: Samson Ag
Priority date: 2018-11-20
Filing date: 2019-11-15
Publication date: 2020-05-28
Also published as: DE102018129162A1; EP3883709A1

Abstract

The invention relates to a method for producing a component from technical ceramics materials or sintering materials, comprising the following steps: First, a mixture of a binder and a ceramic or sintering granulate is produced (100). This mixture must be suitable for the additive manufacturing of a green body from the mixture. Once the mixture is available, a green body is produced by means of additive manufacturing (110). Said body is then isostatically compressed in the green state or in a fully or partially debound state or in a lightly sintered condition (150). The compressed component is then sintered (180). The increase in density before the sintering process (180) itself and the homogenization of the density distribution additionally resulting from the isostatic compression (150) allow higher sintering densities and significantly improved characteristics in respect of strength and other parameters to be achieved in the subsequent sintering process.

Description

Process for producing a component from metal or materials from technical ceramics

description

Field of the Invention

Nowadays, products made of technical ceramics can achieve very good and reproducible strength values and other properties that enable the use of ceramic components in mechanical engineering, medical technology or other areas of application.

In the case of ceramic components, component properties at the upper limit of the values that are possible today can usually be achieved by producing and using ceramic powders with high chemical purity and with a particularly small crystallite size. These powders are processed into a green body with the lowest possible binder content under high pressures. One goal is to maintain a high green density and, due to the low binder content (<5%), to create as few or small pores as possible during the subsequent debinding.

Despite a higher content of organic binders, high-quality components can also be produced using the injection molding process if the pressure during injection molding is correspondingly high.

To produce such high-quality ceramic components, processes such as uniaxial pressing or cold isostatic pressing are preferably used.

With uniaxial pressing, the pressing pressure is only exerted on the body in one direction. Uniaxial pressing requires a lot of tools.

Isostatic pressing is a pressing process in which the pressing pressure acting on the component is the same in all directions. The isostatic process uses an elastic sleeve around the green body. Depending on the geometry, further processing of this compact by turning or milling is usually necessary. In the case of small batch sizes, mainly cylindrical molds are used as molds for the pressing process, since geometrically optimized molds would require additional mold construction. Through this mostly only a rough preform, depending on the geometry of the desired component, requires a relatively high “machining effort”, which leads to corresponding losses of the ceramic powder material used.

Additive manufacturing processes, in particular various 3D printing processes, have been developed for the material classes plastic and metals in recent years until they are ready for series production. With these processes, both customer-specific individual parts (e.g. crowns and bridges made of CrCo alloys in the dental sector) and sample parts ("fast prototyping") can be manufactured in one-off production.

For some time now, attempts have been made to establish the 3D printing process for the ceramic material class with a large number of different manufacturing processes. Examples include the lithography process (LCM), the suspension-based process DLP, the thermoplastic printing T3DP, the powder-based process (binder jetting), the selective laser sintering (SLS), the fused filament fabrication process (FFF).

With additive 3D processes for ceramic materials, as with metallic or thermoplastic materials, highly complex structures can be produced that cannot be realized with other production processes.

The bodies made of ceramic materials, which are produced using the 3D printing process, have to be processed further by sintering at suitable higher temperatures with a corresponding volume shrinkage. An exception to this are components made of silicon in filtered silicon carbide (SiSiC), also called reaction-bonded silicon carbide (RbSiC). These parts no longer experience any significant shrinkage during the actual sintering process, since the pores present after debinding are filled with molten silicon.

The 3D printing of components leads to problems during the subsequent sintering, since on the one hand the high proportion of binder (from 20 - 40% by volume) can only be expelled very slowly, and the raw body remaining after debinding has a correspondingly high pore content. Debinding which is too fast can lead to crack formation during this process if the resulting gases from the debinding process cannot be discharged sufficiently quickly from the inside of the component through the molded body to the outside. Due to the high pore content after debinding, the component cannot be sintered during the subsequent sintering or only to a very limited extent to a homogeneous body with a very high density. To achieve the best possible property values, however, sintered densities of> 99% are required. Typical defects are the formation of cracks due to rapid debinding or low mechanical characteristics in the sintered state due to residual porosity. This effect occurs with 3-D printed ceramic components, the greater the wall thickness of the manufactured component.

The mechanical properties in the sintered ceramic final state of the components produced by means of 3D printing processes are lower than those for a technical application Mechanical components required and usual values of a high-performance ceramic, which can be achieved with the above-mentioned conventional manufacturing processes.

These considerations apply equally to components made of oxide-ceramic materials and to components made from non-oxide materials. Typical representatives of oxide ceramics include aluminum oxide AI ₂ O ₃ in various degrees of purity, zirconium oxide ZrÜ2 with various stabilizing additives and the mixed materials ATZ (alumina toughened zirconia) or ZTA (zirconia toughened alumina). SSiC (Sintered Silicon Nitride) and S13N4 (Sintered Silicon Nitride) are examples of non-oxide ceramics, each with different degrees of purity and phase modifications as well as with different sintering additives.

In the 3D printing process of ceramic materials, the powdery raw materials must be mixed with various additives in order to enable the respective printing process. The thermoplastic processes T3DP and FFF require particularly high proportions of additives. In the lithography process, due to the low binder content according to the literature, green densities of up to 55% and thus sintered densities of 99.4% are achieved. With other methods, these values are significantly lower.

In all known additive manufacturing processes, the ceramic powder is always brought into the desired shape without pressure or at low pressure. In combination with the high proportion of (mostly organic) additives, the production without external pressure after debinding of the manufactured components leads to a structure with a relatively high pore content and a low green density. These green bodies with a relatively low green density have a relatively high proportion of pores and can subsequently not or only to a very limited extent be processed into a ceramic body with a very high sintered density and very good mechanical properties.

Most other ceramic manufacturing processes (uniaxial pressing, isostatic pressing, high pressure casting, injection molding, etc.) work with high pressures during shaping and the lowest possible proportion of organic binders in order to achieve high sintered densities, a homogeneous and fine crystalline structure and good mechanical properties after the sintering process to reach.

Nowadays, filigree and thin-walled components are mostly produced using 3D printing with ceramic materials. The components often also have internal structures that cannot be produced by other processes. The wall thicknesses range from 0.5 mm to a maximum of 6 mm.

In contrast to the ceramic components that can already be produced using 3D printing processes, relatively solid components with wall thicknesses over 6 mm are required for many applications. These components are usually relatively solid and are in the size range of 20 x 20 x 20 mm to 300 x 300 x 300 mm. There are no complex internal structures, but the wall thicknesses are in the range of 6 to 35 mm.

State of the art

Such solid ceramic parts are produced in the prior art by isostatic pressing of the raw powder and subsequent processing of the green body on lathes and milling machines. This is followed by debinding and sintering with a linear combustion shrinkage of about 20%. In the hard-fired state, grinding is usually still required to achieve the required tolerances and surface qualities. Due to the machining of an isostatically pressed full body required in such a conventional manufacture of a ceramic machine component, depending on the geometry, up to 80% of the ceramic material has to be removed and disposed of.

In US 2018/104743 A1 the sintered powder is compressed by controlled pressure waves that are generated by actuators.

In EP 1 534 461 B1, a 3D object is used to build a metallic object without the addition of a binder. Compaction is achieved when the component is built up in layers by compressing each layer with a pressure roller.

EP 1 292 413 B1 shows an alternative solution for sintered parts with 98-99% of the theoretical maximum density by adding sugar to the sintered powder in connection with a particular process sequence.

task

The object of the invention is to provide a method which enables the production of solid components made of metal or materials of technical ceramics with sintered densities above 99%.

solution

This object is solved by the subject matter of the independent claim. Advantageous further developments of the subject matter of the independent claim are characterized in the dependent claims. The wording of all claims is hereby incorporated by reference into the content of this description.

The use of the singular is not intended to exclude the majority, which must also apply in the opposite sense, unless otherwise stated.

Individual process steps are described in more detail below. The steps do not necessarily have to be carried out in the order given, and the method to be described can also have further steps not mentioned.

To solve the problem, a method with the following steps is proposed: First, a mixture of a binder and a ceramic or sintered metallurgical powder is produced. This mixture must be suitable for the additive manufacturing of a green body from the mixture. If the mixture is available, a green body is created using additive manufacturing. This is then isostatically compressed to achieve a more homogeneous density distribution and a higher green density. The redensification can take place in the green state or in partially or completely debindered or in a slightly sintered state. Then the densified component is sintered.

I.e. In order to achieve the goal of increasing the green density and reducing the porosity after the sintering process, green bodies produced according to the invention are post-compressed using an isostatic pressing process before the sintering process.

The process offers a number of advantages:

- By increasing the density before the actual sintering process and the homogenization of the density distribution, which is additionally achieved by isostatic pressing, higher sintered densities and significantly improved property values with regard to strength and other parameters can be achieved in the subsequent sintering process. This applies in particular to the (bending strength) strength and resulting values such as the temperature shock resistance.

- The additive manufacturing enables a very precise and material-saving production of the green body. The additive manufacturing enables a near-net-shape production of the raw body and accordingly less material is used.

- The material properties of the components are improved by the additional densification before the sintering process.

- The machining removal of up to 80% of the material necessary in conventional production is avoided.

- In the prior art, the mechanical characteristics of ceramic components produced by means of 3D printing processes are worse than the values of conventionally manufactured ceramic components. This is due, among other things, to greater porosity. This disadvantage is remedied by the further treatment according to the invention of the green bodies produced in additive manufacturing.

- The use of the isostatic pressing process eliminates the time-consuming production of specific pressing tools for uniaxial pressing.

With the method according to the invention, components can be produced in an efficient manner from materials of technical ceramics or sintered metals. These include e.g. Valve body, valve cone seat rings, valve balls, wear protection sleeves or similar for process control.

According to the invention, the isostatic pressing is carried out in the form of Nassostatic pressing. To the body produced in an additive process during the recompression before the penetration of the liquid pressure medium during the nasostatic pressing protect, it is preferably covered with an elastic sleeve before the nasostatic pressing.

This is done in a particularly simple manner by means of a plastic sleeve that is evacuated. Commercial vacuuming methods are available for this purpose, by means of which the green body can be welded in a film in a watertight manner. The vacuum process is widely used in food technology, but can also be used for technical objects.

If the component to be manufactured is ring-shaped or tubular, the elastic sleeve must be designed as a double-walled hose.

In addition, the green body produced in an additive process can be coated with an expandable lacquer prior to isostatic pressing. Stretchable plastic-based paints (e.g. polyurethane-based paints) are particularly suitable for this. Such painting can be done in a simple immersion process.

To prevent the varnish from carbonizing or burning during sintering and producing soot and smoke, which could impair the component, the varnish is preferably removed before sintering. This is preferably done by chemical dissolving.

To improve the homogeneity and density of the component, the green body is preferably at least partially removed after the additive manufacturing and before the isostatic post-compression. The binder is preferably removed almost completely.

In order to further increase the homogeneity and density of the component, the component can be deblocked a second time and further after compression, but before sintering.

Before sintering, this lacquer will be removed again by pyrolysis or by means of a solvent. Depending on the type of paint used, water or organic solvents are suitable as solvents. The varnish is preferably removed by thermal treatment between 20 ° C and 650 ° C. In principle, this process corresponds to the "debinding" of the plasticizer / binder of the ceramic green body that has already been carried out. Debinding must, however, be carried out much more slowly, since the resulting gases have to escape from the component through very small pore channels, while the coating is only on the surface.

The following measures can be taken to prevent the welded film or lacquer from tearing open at the edges of the component during isostatic pressing:

a) the outer edges of the component can have radii;

b) the foils are thick enough and stretchable enough to close on edges to stay;

c) the stretchable varnish is applied sufficiently thick;

d) the paint is applied thicker at the edges; or

e) a second layer of lacquer is applied or two elastic covers are used one on top of the other

It is also conceivable to optimize the position of the component in the vacuum-sealed and welded film in such a way that as far as possible no corners protrude into the film.

When the ceramic green body is heated, the organic binders are expelled or oxidized and expelled in the range between room temperature and about 600 ° C. The component does not change its external shape, but is made easier by the escape of the substances. This reduces the macroscopic density of the body and creates a pore volume. The actual sintering process begins through liquid phase formation or solid diffusion only at a higher temperature (depending on the material) of around 800 - 1,000 ° C. During this sintering process, the entire body becomes smaller (ceramic shrinkage) and the pore volume is reduced. The macroscopically measurable density of the body increases. If one stops this sintering process shortly after the onset of shrinkage, one speaks of sintering.

When debinding and sintering, the ceramic component always has a state in which organic binder components have been completely expelled, but there is still no firm ceramic bond. Nevertheless, the component does not fall apart in the firing process.

The object is also achieved by a component which was produced using the method described and which has a pore fraction of less than 1% of the volume after sintering.

If the coating remains at least partially on the component, the remaining coating can enable or improve a function of the component, in particular through a hydrophilic or oleophilic or electrically conductive or electrically insulating property of the coating.

The method described is suitable, among other things. for the production of wear protection sleeves, valve seat rings, valve bodies or valve housing parts.

Further details and features emerge from the following description of preferred exemplary embodiments in conjunction with the figures. The respective features can be implemented individually or in combination with one another. The Possibilities to solve the task are not limited to the exemplary embodiments. For example, range specifications always include all - not mentioned - intermediate values and all conceivable subintervals.

An embodiment of the method according to the invention is shown schematically in the figures. In detail shows:

1A shows a flowchart of the proposed method;

1B shows a flow chart of the proposed method;

2 shows a wear protection sleeve;

3 shows a seat ring for a ball;

Fig. 4 shows a recirculating ball sleeve; and

Fig. 5 is a wear protection sleeve.

1A and 2B show two preferred processes of the proposed method for producing a component from, for example, a technical ceramic.

In step 100, a mixture of a binder and a ceramic or sintered tallurgical granulate is produced.

Depending on the process used, different forms of preparation of the ceramic raw material and the binder system are required for "3D printing":

Filaments or pellets for extrusion processes

Powder for melting processes

Resin (resin) & wax for printing processes with liquid materials

The following polymers and thermoplastics are used as binders for the production of the respective primary material for the various 3D printing processes:

ABS (acrylonitrile butadiene styrene)

PLA (Polyactide - polyactid acid) - polylactic acid

Nylon (polyamide)

PC (polycarbonate)

PP (polypropylene)

PVA (polyvinyl alcohol)

TPE (thermoplastic elastomers)

- XT copolyester

The list is not exhaustive, other polymers and thermoplastics are also possible.

In step 110, a green body is then produced additively from the mixture. At this stage, the green body often has about 50 +/- 15% of the maximum achievable density of the ceramic end material. In step 120, the binder is wholly or partly removed from the green body by pyrolysis or another method. When the ceramic green body is heated, the organic binders are expelled or oxidized and expelled in the range between room temperature and about 600 ° C. The component does not change its external shape, but is made easier by the escape of the substances. This reduces the macroscopic density of the body and creates a pore volume. The actual sintering process begins through liquid phase formation or solid-state diffusion only at a higher temperature (depending on the material) of around 800 - 1,000 ° C. During this sintering process, the entire body becomes smaller (ceramic shrinkage) and the pore volume is reduced. The macroscopically measurable density of the body increases. If you stop this sintering process shortly after the onset of vibration, one speaks of sintering.

When debinding and sintering, the ceramic component always has a state in which organic binder components have been completely expelled, but there is still no solid ceramic bond. Nevertheless, the component does not fall apart in the firing process.

In step 130, the green body is lightly sintered in order to further stabilize it. Whether a thermal process step is required between the production of the green body by one of the various additive processes and the post-compression of the component by isostatic pressing and how far the body should be debinded or even sintered depends on the type of 3D printing process selected and from the residual porosity still present in the green body and the type of binder and plasticizer system used. You cannot specify a fixed number for a residual volume share of binder here. The redensification takes place in the green state or in partially or completely debindered or in a slightly sintered state.

In steps 140 and 145, the green body is packed or coated in a watertight manner. This can be done by vacuuming 145 in a suitable plastic film (made of PE or PP or other plastics) or by immersing 140 in a suitable lacquer. This coating is intended to prevent the hydraulic fluid from penetrating into the pores. This is typically done using varnishes based on elastic plastic that shows no cracking, e.g. Polyurethane-based paints. The stretchable varnish is applied sufficiently thick. The paint is applied thicker at the edges, which prevents the paint layer from tearing open during subsequent isostatic compaction. The thicker application of paint on unbroken edges is set automatically if the application of paint is carried out in the dipping process. This coating process also enables the isostatic pressing of bodies with finer inner contours or (transverse) bores.

In step 150, a cold nasostatic pressing of the green body follows with a pressure fluid. After pressing, the green body usually has a density of approximately 55 +/- 15% of the maximum achievable density of the ceramic end material.

In step 160, the waterproof cover is removed. This can be in use a coating or a varnish by pyrolysis or by means of a solvent, in the case of vacuumed blanks the covering can be removed mechanically.

In step 170, the green body is further debindered. This is done either by pyrolysis or by means of a solvent.

In step 180, the green body is finally sintered. The component should preferably have a density of more than 98% of the maximum achievable density of the ceramic end material.

In step 190, the surfaces are possibly subsequently finished, for example by grinding, sandblasting or machining.

2 to 7 show examples of components that can be produced with the proposed method. The parts are relatively thick-walled, and their manufacture with conventional machining would result in considerable material losses. In detail shows:

2 shows a wear protection sleeve;

3 shows a seat ring for a ball;

Fig. 4 shows a recirculating ball sleeve; and

Fig. 5 is a wear protection sleeve.

glossary

additive manufacturing

Additive manufacturing, also popularly known as 3D printing, refers to processes for manufacturing components by means of point-by-layer or layer-by-layer construction. The production takes place on the basis of computer-aided models of the components made of formless (liquids, gels / pastes, powder, etc.) or form-neutral (band, wire, sheet) material by means of chemical and / or physical processes.

binder

Binder or binder are substances that adhere added solids with a fine degree of division (e.g. powder). Binders are usually added to the fillers to be joined in liquid or pasty form. Both substances are mixed intensively so that they are evenly distributed and all particles of the filler are evenly wetted with the binder.

FFD (Fused Feedstock Deposition) process

The main difference from the FFF process is that instead of filaments, granules are used as the raw material. This means that commercially available raw materials from the injection molding sector can be used.

Fused filament fabrication

Fused Filament Fabrication, FFF for short, also called Fused Filament Manufacturing, is a 3D printing process that uses an endless filament made of a thermoplastic material. This is guided by a large spool through a movable, heated printer extruder head. Melted material is pressed out of the nozzle of the print head and placed on the growing workpiece. The head is moved under computer control to define the printing form. Typically, the head moves in layers, moving in two dimensions to deposit one horizontal plane at a time, before moving up slightly to start a new disk. In the FFF process, ceramic or sintered metallurgical powder has to be introduced into a filament beforehand. By inserting it into a plasticizing filament, almost all ceramic materials can be processed. Relatively simple and inexpensive printing machines can be used. Green body

In the production of sintered workpieces, a green body or green body is an unsintered blank that is still easy to machine. For example, it is powder bonded with binders. The green bodies are dimensioned in such a way that they shrink almost completely to the final shape when they burn.

Isostatic post-compression or pressing

Isostatic post-compression is a pressing process in which the pressing pressure acting on the component is the same in all directions. This method is well suited for small parts with high isotropy and even compression, and is also inexpensive for demanding prototypes and production in small series.

Nasostatic post-compression or pressing

Nasostatic post-compression is isostatic post-compression, in which the pressure is transmitted through a liquid, preferably through water. The mold enveloping the compact is completely immersed in the pressure medium (e.g. water) and removed from the pressure medium for demolding. 3D extrusion process

In the 3D extrusion process, the plastic ceramic mass is pressed through an extruder through a 3D movable nozzle.

cited literature

cited patent literature

US 2018/104743 A1

EP 1 534 461 B1

EP 1 292 413 B1

Claims

1 . Method for producing a component from metal or materials from technical ceramics with the following steps:

1 .1 producing (100) a mixture of a binder and a ceramic or sintered tallurgical granulate;

1 .2 additive manufacturing (1 10) of a green body from the mixture;

1 .3 isostatic redensification (150) of the green body;

1 .4 sintering (180) of the green body, which is post-compressed isostatically;

1 .5 wherein isostatic post-compression (150) uses naso-static post-compression;

1 .6 the green body with an elastic before the nasostatic post-compression (150)

Cover is waterproof;

1 .7 wherein the elastic cover is a plastic cover; and

1 .8 the plastic casing being evacuated before the nasostatic post-compression (150).

2. The method according to the preceding claim,

characterized,

that the elastic sleeve is designed as a double-walled hose for annular or tubular components.

3. The method according to any one of the preceding claims,

marked by

Painting (140) the green body before the isostatic pressing (150) with an elastic paint.

4. The method according to the preceding claim,

marked by

removing (160) the lacquer prior to sintering.

5. The method according to any one of the preceding claims,

marked by,

at least partial debinding (120) of the green body after the additive manufacturing (110) and before the isostatic post-compaction (150).

6. The method according to any one of the preceding claims, marked by

debinding (170) the green body before sintering.

7. The method according to one of the two preceding claims,

characterized,

that the debinding (120, 170) is carried out by pyrolysis or by means of a solvent.

8. The method according to any one of the preceding claims,

characterized,

8.1 that the component is manufactured additively such that edges of the component have radii; and or

8.2 that the varnish or the elastic cover is thick enough and sufficiently stretchable to remain closed on edges during the naso-static re-compaction (150); and or

8.3 that the paint application (140) is thicker at the edges than on the surfaces of the at least partially debindered green body; and or

8.4 that two layers of lacquer or two elastic covers are used one above the other.

9. The method according to any one of the preceding claims,

characterized,

that the green body is sintered (130) before the isostatic post-compression (150).

10. The method according to any one of the preceding claims,

characterized,

that additive manufacturing (1 10) takes place in the fused filament fabrication process.

1 1. Component manufactured according to one of the preceding claims,

characterized,

that after sintering the proportion of pores in the component is less than 1% of the volume.

12. Component manufactured according to claim 3, 6, 7, 8 or 9,

characterized,

that a coating remains at least partially on the component and the remaining coating enables or improves the function of the component, in particular by means of a hydrophilic or oleophilic or electrically conductive or electrically insulating property of the coating.

13. Component manufactured according to one of the preceding method claims,

characterized,

that the component is a wear protection sleeve or a valve seat ring or a valve body or a valve housing part.