EP1523390B1 - Method for producing highly porous metallic moulded bodies close to the desired final contours - Google Patents

Abstract

The starting material, metal powder, is mixed with a place holder. The mixture is pressed to a green molding. It is subjected to conventional mechanical processing. The place holder material is removed thermally in air, vacuum or protective gas. The molding is then sintered.

Description

The invention relates to a method with which a near-net shape production of porous, in particular of highly porous components can be achieved.

State of the art

The pressing of metal powders for the production of porous metal bodies is known. In order to produce the desired porosity, so-called spacer materials can be added to the metal powders, which make it possible to stabilize the desired porosity. After pressing green bodies from the powder mixture, the spacer material is then removed from the green bodies in such a way that the green body is held solely by the remaining metal powder framework, which has cavities between its framework structure. The green body thus already has the later porous structure of the shaped body. When expelling the placeholder material, care must be taken to ensure that the metal powder framework remains intact. Subsequent sintering of the foundations results in a highly porous shaped body, wherein the contact surfaces of the powder particles are diffusion-bonded during sintering.

As a spacer materials for the formation of porous metallic moldings, relatively high-melting organic compounds are known, which by evaporation or pyrolysis (cracking) and dissolving the resulting cracking products are removed by means of suitable solvents from the green bodies. The problem here is the considerable amount of time in the removal of the placeholder material and cracking products that react with almost all metals to be processed powder metallurgy, such as Ti, Al, Fe, Cr, Ni, etc., and leave high concentrations of impurities. A disadvantage also affects the use of thermoplastics, which are removed by heating the green body, the expansion at the glass transition point, the necessary stability of the green body is thereby impaired.

On the other hand, high-melting inorganic compounds such as alkali metal salts and low-melting metals such as Mg, Sn, Pb etc. are used as placeholder materials. Such blank materials are removed under vacuum or under inert gas at temperatures between about 600 to 1000 ° C with high energy and time expenditure from the green bodies. Not to be prevented in these placeholder materials in the green body remaining impurities, which are particularly harmful in moldings of reactive metal powders, such as Ti, Al, Fe, Cr, Ni.

Out DE 196 38 927 C2 is a method for the production of highly porous, metallic moldings known in which first metal powder and a placeholder are mixed and then pressed into a greens. Both uniaxial and isostatic pressing can be used. The placeholder is thermally expelled and the green body subsequently sintered. If the powder-spacer mixture is stabilized by a binder, it is in principle possible to realize relatively complex component geometries directly by means of multi-axial pressing. However, the preparation of a suitable pressing tool is complicated and expensive. Especially for small series, it is therefore advantageous first to produce semi-finished products with a universal geometry (eg cylinders or plates) and bring them to the desired final contour by subsequent mechanical processing.

According to the current state of the art, the final shaping of highly porous shaped bodies takes place only after sintering by conventional mechanical methods such as, for example, turning, milling, drilling or grinding. The disadvantage of this subsequent processing of the already sintered semi-finished product is associated with a local material deformation. The plastic deformation regularly causes smearing of the pores. As a result, the desired open porosity of the molding is lost regularly, especially in the surface region. This adversely affects the functional properties of the molding. Furthermore, due to its high porosity, the workpiece should be clamped and machined with care, since it is not very pressure-stable. The uneven surface of the porous molding also causes a relatively high tool wear.

Task and solution

The object of the invention is to provide a simple method for producing a highly porous, metallic shaped body, in particular a highly complicated geometry and not the aforementioned disadvantages z. B. has deterioration of the porosity at the surface.

Subject of the invention

The invention relates to a process for the preparation of highly porous metallic moldings. The method comprises the following method steps. A metal powder used as a starting material is mixed with a placeholder. The metal powder may be, for example, titanium and its alloys, iron and its alloys, nickel and its alloys, copper, bronze, molybdenum, niobium, tantalum and tungsten.
Suitable materials as placeholders are, for example, carbamide CH ₄ N ₂ O (H ₂ N-CO-NH ₂ ), biuret C ₂ H ₅ N ₃ O ₂ , melamine C ₃ H ₆ N ₆ , melamine resin, ammonium carbonate (NH ₄ ) CO ₃ H ₂ O and ammonium bicarbonate NH ₄ HCO ₃ , the residue-free at temperatures up to max. 300 ° C can be removed from the green body. Ammonium bicarbonate has proven particularly advantageous as a placeholder material, which can be expelled already at about 65 ° C in air. The grain size, ie the particle size and the particle shape of the placeholder material, determine the porosity that forms in the shaped body. Typical particles - diameter of the placeholder material are 50 microns to 2 mm. By suitable choice of the placeholder and the amount of placeholder in relation to the metal powder, a high, homogeneous and open porosity can be achieved in the final molded part. Porosities up to 90% are readily achievable.

From the mixture, a green body, in particular a green body with a simple geometry, pressed. This can be, for example, a cylinder or a plate. As a pressing method, the multi-axial pressing and the cold isostatic pressing can be used. The multi-axial pressing leads to dimensionally stable semi-finished products with defined outer contours. The wall friction during demolding causes the formation of a so-called. Press skin, which is formed from plastically deformed, metallic powder particles. This can be removed before sintering by mechanical processing, provided no further green processing takes place. The wall friction limits the length to diameter ratio to 2 to 1. Above this value occur to large differences in density in the compact. The cold isostatic pressing takes place, for example, in rubber molds. The pressure transfer medium is an oil-containing emulsion in which the powder-filled rubber mold is located. Since the wall friction eliminates the demolding, it is possible to produce semi-finished products with a length to diameter ratio greater than 2 to 1 with a sufficiently homogeneous density distribution. A disadvantage is the low dimensional accuracy of the outer contour, which, however, hardly affects the subsequent green processing.

The green body is then subjected to a conventional mechanical processing, in which the workpiece is given its final shape, wherein the shrinkage is taken into account during the sintering process. The processing in the stage of the greenery, in which the placeholder still exists, has the advantage that the workpiece is very easy to work, and the porosity is not affected. The tool wear is kept low regularly. Even highly complicated shapes are possible with this method. The still existing placeholder makes the workpiece to be machined sufficiently stable in terms of pressure in order to be able to clamp it for subsequent mechanical processing.

When the final shape is achieved, the blank material is thermally removed from the green body in air or under vacuum or under inert gas. The atmosphere depends on the chosen placeholder material. For example, an air atmosphere above 65 ° C is sufficient to remove ammonium bicarbonate as a wild card. The green body is then sintered to the shaped body.

The mechanical processing prior to sintering advantageously allows a simple, near-net shape production even for complicated geometries of the shaped body to be produced, without affecting the porosity, and without high tool wear.

This method is not limited to the production of moldings with a uniform porosity, but it can also moldings with a different, z. B. produce graded porosity.

When coarse starting powders are used, some particles regularly have a weak connection to the sintered network because the sintering bridges are incomplete. Even with a small load it can regularly lead to a chipping. However, this may be illegal for some applications. In order to avoid this disadvantageous effect, highly porous components made of coarse starting powders are advantageously trovalised or slide-ground prior to actual use. In these processes, the weakly adhering particles are regularly removed from the surface by a grinding process.

Special description part

The subject matter of the invention will be explained in more detail below on the basis of figures and an exemplary embodiment, without the subject matter of the invention being restricted thereby.

Figur 1:

mögliche Ausführungsformen der Halbzeuge, die durch mehraxiales Pressen und durch kaltisostatisches Pressen hergestellt wurden.

Figur 2:

verschiedene Modellgeometrien, die aus rostfreiem Stahl 1.4404 (316L) nach dem erfindungsgemäßen Verfahren hergestellt wurden.

Figur 3:

Darstellung der Makroporosität, die durch den Platzhalterwerkstoff eingestellt wird, und der Mikroporosität, die innerhalb der Sinterstege auftritt.

Show it:

FIG. 1:

possible embodiments of the semi-finished products produced by multi-axial pressing and by cold isostatic pressing.

FIG. 2:

various model geometries made of stainless steel 1.4404 (316L) according to the method of the invention.

FIG. 3:

Illustration of the macroporosity, which is set by the placeholder material, and the microporosity, which occurs within the sintered webs.

1. 1. Zunächst wird ein Halbzeug in Anlehnung an DE 196 38 927 hergestellt. Dazu wird ein Metallpulver, insbesondere der rostfreie Stahl 1.4404 (316L) oder Titan, mit einem Platzhalter, insbesondere Ammoniumbikarbonat, gemischt und uniaxial oder kaltisostatisch verpresst. Je nach Presswerkzeug stehen für die Weiterverarbeitung als Halbzeuge z. B. Zylinder oder Platten zur Verfügung. Figur 1 zeigt mögliche Ausführungsformen der Halbzeuge, die durch mehraxiales Pressen und durch kaltisostatisches Pressen hergestellt wurden.
2. 2. Es folgt die Grünbearbeitung des ungesinterten Halbzeugs durch konventionelle mechanische Bearbeitung (Sägen, Bohren, Drehen, Fräsen, Schleifen...). Der Platzhalter erhöht vorteilhaft die Grünfestigkeit der Halbzeuge und wirkt sich somit günstig auf die Bearbeitbarkeit aus. Ein weiterer Vorteil der Bearbeitung ist die niedrige Schneidkraft und dementsprechend der geringe Werkzeugverschleiß. Eine Verschmierung der Poren wird ebenfalls vermieden.
3. 3. Das Entfernen des Platzhalters und die Sinterung kann konventionell auf einer planaren Sinterunterlage aus Keramik oder alternativ in einer Schüttung aus Keramikkugeln erfolgen. Die Parameter zur Entfernung des Platzhalters können in Anlehnung an DE 196 38 927 C2 gewählt werden. Als Ergänzung zu DE 196 38 927 C2 erfolgt die Entfernung der Platzhalter Ammoniumkarbonat und Ammoniumbikarbonat an Luft. Die Sinterung in einer Kugelschüttung hat den Vorteil, dass die Berührungsflächen zum Bauteil gering sind und so eine Anhaftung des Bauteils an den Keramikkugeln verhindert wird. Zudem kann die Kugelschüttung die Sinterschwindung durch eine Umorientierung der Kugeln leicht ausgleichen, so dass während des gesamten Sinterprozesses ein gleichmäßiger Kontakt zur Sinterlage besteht. Dies vermeidet einen Verzug der Bauteile beim Sintern. Als Option können die Formkörper zur Verbesserung der Oberflächenqualität im Anschluss trovalisiert werden.

The typical procedure of the method according to the invention is divided as follows.

1. 1. First, a semi-finished product is based on DE 196 38 927 produced. For this purpose, a metal powder, in particular the stainless steel 1.4404 (316L) or titanium, mixed with a placeholder, in particular ammonium bicarbonate, and uniaxially or cold isostatically pressed. Depending on the pressing tool are available for further processing as semi-finished z. As cylinders or plates available. FIG. 1 shows possible embodiments of the semi-finished products produced by multi-axial pressing and by cold isostatic pressing.
2. 2. Greening of the unsintered semi-finished product follows by conventional mechanical processing (sawing, drilling, turning, milling, grinding ...). The placeholder advantageously increases the green strength of the semi-finished products and thus has a favorable effect on the machinability. Another advantage of machining is the low cutting force and consequently the low tool wear. Smearing of the pores is also avoided.
3. 3. The removal of the placeholder and the sintering can conventionally on a planar sintered substrate made of ceramic or alternatively in a bed of ceramic balls. The parameters for removing the wildcard can be based on DE 196 38 927 C2 to get voted. As a supplement to DE 196 38 927 C2 the removal of the placeholders ammonium carbonate and ammonium bicarbonate takes place in air. The sintering in a ball bed has the advantage that the contact surfaces to the component are low and thus an adhesion of the component is prevented to the ceramic balls. In addition, the ball bed can easily compensate for the sintering shrinkage by reorienting the balls, so that there is uniform contact with the sintered layer during the entire sintering process. This avoids distortion of the components during sintering. As an option, the moldings can subsequently be trovalised to improve the surface quality.

embodiments

FIG. 2 shows various model geometries, which were made of stainless steel 1.4404 (316L) according to the invention and described in the following procedure. The starting material used was a water-atomized powder (grain fraction <50 μm). The steel powder was mixed with the placeholder ammonium bicarbonate (grain fraction 355 to 500 microns) in the ratio steel powder to ammonium bicarbonate 45 to 55 (in vol.%). This corresponds to a ratio of steel powder to placeholder of 80.5 to 19.5 in wt.%. The mixture was uniaxially pressed with a pressing pressure of 425 MPa to cylinders whose diameter was 30 mm and whose height was 22 mm. The cylinders were machined in the green state by drilling and turning. In addition to drilling, both right-angled and rounded heels could be realized in the model geometries. The placeholder ammonium bicarbonate was removed in air at a temperature of 105 ° C. Although the decomposition of the wildcard already starts at 65 ° C, the higher temperature was chosen to dissipate the decomposition product water in the gaseous state. The sintering was carried out at 1120 ° C for 2 hours under argon atmosphere. The model geometries showed a shrinkage of about 4%. The final porosity of the components was about 60%. It is composed of the macroporosity, which is set by the placeholder material, and the microporosity, which occurs within the sintered webs ( FIG. 3 ). The microporosity results from incomplete sintering of the metal powder particles. To reduce the microporosity, the use of finer starting powders or sintering at higher temperatures is recommended.

Claims (7)

1. Method for producing highly porous metallic moulded bodies with the following steps:

   - a metal powder used as the initial material is mixed with a place marking material in powder form with a particle size of between 50 µm and 2 mm, which can be removed from the main casting with no residue at temperatures of up to a maximum of 300° C,

   - a main casting is pressed out of the mixture,

   - the main casting is subject to conventional mechanical processing,

   - the place marking material is removed from the main casting with air or under a vacuum or thermally with protective gas, so that a treated main casting with open porosity is produced,

   - the main casting is sintered to the moulded body.
2. Method according to the previous claim 1, in which carbamide, biuret, melamine, melamine resin, ammonium carbonate or ammonium bicarbonate are used as place markers.
3. Method according to one of the previous claims 1 to 2, in which the place marker is removed at temperatures of less than 300° C, particularly of less than 105° C, and particularly advantageously of less than 70° C.
4. Method according to one of the previous claims 1 to 4, in which stainless steel 1.4404 (316L) or titanium is used as the initial metal powder.
5. Method according to one of the previous claims 1 to 6, in which the moulded body is produced by sawing, drilling, turning, milling or cutting close to the final contours in the main casting state.
6. Method according to one of previous claims 1 to 5, in which sintering is carried out in a fill of ceramic balls.
7. Method according to one of the previous claims 1 to 6, in which the moulded body is finished by vibratory finishing or barrel finishing after sintering.

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