EP0525325B1 - Process for preparing dense sintered articles - Google Patents

Abstract

The invention relates to a process for producing dense sintered articles from metal or ceramic materials. A flowable casting composition consisting of the pulverised material, a binder and a solvent is poured into a predetermined mould and subsequently dried. The binder is removed from the green compact thus formed and the latter is sintered. The proportion of binder in the casting composition should be minimised. Expediently, the mould is vibrated during the casting. Heating control during drying with simultaneous application of a vacuum is advantageous. Multi-layered articles can also be produced by suitable application of the process.

Description

The invention relates to a method for producing porous or dense sintered workpieces from metal, a metal alloy or from ceramic materials, in which a green body is first formed from a mixture of the metal present as a powder, the metal alloy or the ceramic material and a binder debindered, sintered and the sintered body is compressed, if necessary, by hot isostatic pressing (HIP process). Such a manufacturing process is known from EP-A-0260 101.

This known method in which the green body e.g. Sintered in a vacuum and then using the HIP script is used to compact workpieces approximately to their theoretically possible material density.

To produce the green body, a viscous mixture of powder and binder (waxes and plastics) has previously been produced, and this mixture has been pressed into the predetermined shape under a pressure of a few hundred bars. Quite apart from the fact that in this procedure narrow cavities of the form to be filled can be inaccessible to the viscous mass, it also shows that during the subsequent binder removal or sintering process to which the viscous mass filled under pressure is subjected to the binder does not completely escape from the mass or the green body, so that undesired residues can remain in the workpiece.

It is therefore an object of the invention to provide a method of the type described at the outset which largely avoids the aforementioned disadvantages.

This object is achieved according to the invention by the method according to the features of claim 1.

As has been shown, by this procedure according to the invention, which can be called "wet powder casting", three-dimensional, near-net-shape, solid bodies of a previously designed shape and / or size can be formed from powders of different materials and with different particle size distributions. The rheological properties of the powder-carrier-binder mixture before casting are used to achieve a free (by gravity), possibly also forced mold filling, whereby after the removal of the solvent (during drying) the powder-binder mixture becomes solid Green body solidified, which reflects the inner contours of the shape. At most, pressures slightly above the atomic sphere pressure are used to force the mold to be filled.

The green body is removed from the mold after drying and is thus ready for further processing.

The casting compound is expediently produced in a separate vessel, after which it is poured into the mold through a sprue or a funnel.

Adequate filling of the form is achieved by gravitation and can be accompanied by targeted vibration of the form. External vibrations and / or rotations of the shape by hand or mechanically are useful for this.

The solvent can be removed from the mass filled into the mold under normal pressure and at room temperature. However, this drying process can also take place at elevated temperature and / or under a slight negative pressure or can be accelerated thereby.

If workpieces are to be produced from different layers (graded materials), the filling can be built up in several steps using different materials. A procedure which is expedient for this is that mixtures formed from different materials are poured into the mold one after the other, so that the poured-in mass and thus also the green body formed thereafter consists of layers of different composition arranged one above the other.

After pouring the first material with a certain composition, a second is poured onto the first. This process is complete Filling of the mold continued, so that a layer system green body composed of different materials with a graduated structure, for example with regard to material, porosity, grain / particle size distribution, arises.

Another variant of the method is that the mixture is poured in via / or around a green body previously introduced into the mold or an already finished workpiece.

In this way, an inner or outer coating (s) or covering of a solid part that has already been produced can be produced, the casting compound being poured into the cavities and the surface to be coated after being introduced into a suitable mold.

As has been shown, the casting compound can also be applied with a brush or sprayed on and then further processed with good success.

Composite materials or graded materials can be produced by introducing the insert component into the premix or by introducing them into the mold before casting.

• a. vom relativen Volumenanteil des Binders im Grünkörpers
  und
• b. von der mittleren Teilchengröße des verwendeten Pulvers.

The strength of the green body generally depends on two parameters:

• a. the relative volume fraction of the binder in the green body
  and
• b. from the average particle size of the powder used.

The general rule is that the proportion of the binder in the mixture should be such that it is sufficient to solidify the mass to form a manageable green body. The amount of binder required for this can easily be determined by means of a few preliminary tests; it is in the range between 2 and 5% by volume.

As has been shown, green bodies with a binder content of 0.02 (2.00% by volume) with particle sizes of approx. 20 µm have considerable strength. However, when using larger particles with the same binder content, the strength is still sufficient to ensure safe handling of the green bodies. The binder content can be varied by controlled addition or evaporation of the liquid, volatile solvent component (carrier). In practice, the maximum volume fraction is limited by the solubility limit of the binder in the solvent or by the relative natural porosity, which is approximately 26% for ideally packed spherical particles.

Since the method according to the invention uses only relatively small amounts of binder (2-5% by volume), only part of the space between the particles of binder is fulfilled. Therefore, the binder condenses as a thin film on the powder particles during the removal (evaporation) of the solvent (carrier) and a rapid, continuous removal of the solvent (carrier) through the free particle spaces takes place.

The final strength of the green body is achieved after the solvent has been completely removed (Carrier) and after the binder has solidified at room temperature (possibly also at elevated temperature) as a solid network with connecting bridges between adjacent powder particles.

The fact that the binding of the powder particles, one with the other, is ensured by the formation of local bridges is not only advantageous for the removal of the solvents, but is also of great importance for the subsequent sintering process.

After the green body has been formed, it is subjected to a thermally activated debinding and sintering treatment. The green body is heated to remove the binder. This debinding process is not limited to a specific time-temperature program (profiles, sequences, cycles), although some sub-steps are required to enable complete binder removal. A typical procedure is for the green body to be heated to a temperature in the range from 280 to 420 ° C. at a rate of 3-10 ° C./min and, depending on the size of the body, to be kept at this temperature until the binder is removed. The body is then heated to sintering temperature up to a rate of> 10 ° C / min.

The best debinding results are usually obtained under fine to high vacuum conditions, even if a considerable amount of dewaxing takes place at atmospheric pressure or a light vacuum. Debinding under a flowing gas atmosphere is also possible.

• a. der geringe Volumenanteil des Binders hat eine offene Struktur zwischen den Teilchen zur Folge. Diese gewährleistet für die Dämpfe, die von dem sich zersetzenden Binder stammen, einen unbehinderten Weg aus dem Grünkörper.
• b. die intrinsichen Eigenschaften des Binders, der, wenn er über den Schmelzpunkt erhitzt wird, in ein hoch-viskoses Produkt polymerisiert, so daß die netzwerkartige Struktur zwischen den Teilchen und die damit verbundene Form des Grünkörpers bestehen bleibt.

The insensitivity of the debinding process to the special thermal cycle as well as the possibility to use relatively high heating rates is primarily due to two factors:

• a. the low volume fraction of the binder results in an open structure between the particles. This guarantees an unobstructed path out of the green body for the vapors that come from the decomposing binder.
• b. the intrinsic properties of the binder, which, when heated above the melting point, polymerizes into a highly viscous product, so that the network-like structure between the particles and the associated shape of the green body remain.

High temperatures promote the rapid splitting of the binder into a vapor that sublimes outside the green body either in the atmosphere or pumped out by a vacuum system. The splitting and removal takes until the binder has burned out.

The fact that the strength of the debindered green body is sufficient to ensure further manageability is due to possible binder cleavage residues that hold the powder particles in their respective positions.

Materials such as superalloys, stainless steel, titanium alloys and aluminum alloys, iron materials, ceramic powders such as zirconium oxide, chromium oxide, lathane oxide, perovskite, aluminum oxide and silicon oxide can be used as the material for the workpieces to be produced be provided. Wax, shellac, PMMA and alcohol, trichlorethylene, toluene (toluene) are mentioned as binders.

The sintering of the green bodies produced by the process according to the invention was carried out using specific, known schemes for each material. Density measurements showed that the sinterability of the materials is not affected by the process itself. The end product can be sintered to a closed porosity. The sintered parts with closed porosity can therefore be compacted containerless up to the theoretical density by HIP.

Chemical analyzes of end products produced by the method according to the invention showed no increase in the concentration of contaminants related to the chemical composition of the binder, such as oxygen, carbon, nitrogen and hydrogen. The overall composition was within the nominal concentrations of the starting products.

Embodiment 1 1. Material

Powder:

Ni-based superalloy (2.4636)
Particle size: ≦ 100 µm
Weight: 100 g
Volume: 12.6 ml (calculated from the theoretical density)

solvent

(Carrier): alcohol (ethanol)
Volume (20 ° C): 50 ml
Weight: 39.5g

Binder:

shellac
Weight: 2g
Volume: 1.8 ml (calculated)

Binder weight fraction:

0.014 (1.4%)

Binder volume fraction:

0.028 (2.8%)

2. Mixing the components

The materials were mixed in a tumble mixer for 2 hours.

3. Pouring (mold filling) and drying

The mixture was poured into a two-part stable Teflon mold, which consisted of two eccentrically arranged cylinders (20 and 10 mm ⌀ and 10 or 15 mm in length). The product reached high strength after 14 hours of drying time in the air or after 4 hours of drying in a desiccator, so that handling is easy.

4. Debinding and sintering

• 1) Aufheizen von Raumtemperatur auf 350°C mit 3°C/min (1,9h).
• 2) Halten bei 350°C für 3 h.
• 3) Aufheizen von 350°C auf 900°C mit 10°C/min (0,9 h).
• 4) Halten bei 900°C für 3 h.
• 5) Aufheizen von 900°C auf 1265°C mit 10°C/min (0,6 h).
• 6) Halten bei 1265°C für 3 h.
  Der Druck im Ofen wurde dabei zwischen 10⁻⁵ und 10⁻⁴ mbar gehalten.

Debinding and sintering were carried out in the same furnace in a continuous cycle as follows:

• 1) Heating from room temperature to 350 ° C at 3 ° C / min (1.9h).
• 2) Hold at 350 ° C for 3 h.
• 3) Heating from 350 ° C to 900 ° C at 10 ° C / min (0.9 h).
• 4) Hold at 900 ° C for 3 h.
• 5) Heating from 900 ° C to 1265 ° C at 10 ° C / min (0.6 h).
• 6) Hold at 1265 ° C for 3 h.
  The pressure in the furnace was kept between 10⁻⁵ and 10⁻⁴ mbar.

The total process time was 12.4 hours.

5. Density and chem. analysis

The density of the sintered part was 96% of the theoretical density (Archimedean method) of the material and the chem. Analysis showed no deviation from the nominal composition of this material.

Embodiment No. 2 1. Material

Powder:

Ti6A14V
Particle size: +53-180 µm
Weight: 275 g
Volume: 62.2 ml (calculated from the theoretical density)

Solvent (carrier):

Alcohol (ethanol)
Volume (20 ° C): 50 ml
Weight: 39.5 g

Binder:

Shellac
Weight: 3g
Volume: 2.75 ml (calculated)

Binder weight fraction:

0.009 (0.9%)

Binder volume fraction:

0.027 (2.7%)

2. Mixing the components

As 2nd in embodiment number 1

3. Pouring (mold filling) and drying

The mixture was poured into a four-part stable Teflon mold, with the dimensions 80x20x1 mm. After 2 hours of drying in air, the product reached high strength, so that handling is possible.

4. Debinding and sintering

• 1) Heating from room temperature to 350 ° C at 25 ° C / min (0.22 h).
• 2) Hold at 350 ° C for 1 h.
• 3) Heating from 350 ° C to 1100 ° C at 100 ° C / min (0.12 h).
• 4) Hold at 1100 ° C for 20 minutes (0.33 h).
• 5) Argon filling at 400 mbar pressure.
• 6) Heating from 1100 ° C to 1600 ° C at 100 ° C / min (0.1 h).
• 7) Hold at 1600 ° C for 2.5 h.

The total process time was 4.27 h.

5. Density and chem. analysis

The density of the sintered part was 92.5% of the theoretical density of Ti6A14V and the chem. Analysis showed within the measurement accuracy the same composition as that of the starting powder.

Embodiment 3 1. Material

Powder:

Perovskite La _0.84 Sr _0.16 MnO₃
Particle size: +45 -90 µm
Weight: 25 g

Solvent:

Alcohol (ethanol)
Volume 25 ml

Binder.

shellac
Weight: 0.5 g
Substrate density: ZrO₂-8Y film
40 mm φ.

2. Mixing the components

Like 2 in working example No. 1 and No. 2

3. Application of the mixture and drying

The mixture was applied to the substrate with a brush. The 50 µm thick layer produced in this way was then air-dried for half an hour.

4. Debinding and sintering

• 1) Aufheizen von Raumtemperatur auf 350°C mit 1°C/min (5.7 h)
• 2) Halten bei 350°C für 2 h.
• 3) Aufheizen vo 350°C auf 1500°C mit 1°C/min (19.2 h)
• 4) Halten bei 350°C für 5 h.

Binding and sintering were carried out in the same furnace in a continuous cycle as follows:

• 1) Heating from room temperature to 350 ° C at 1 ° C / min (5.7 h)
• 2) Hold at 350 ° C for 2 h.
• 3) Heating from 350 ° C to 1500 ° C at 1 ° C / min (19.2 h)
• 4) Hold at 350 ° C for 5 h.

5. Result

Metallopraphic analysis of the perovskite layer produced showed clearly visible adhesion to the zirconium oxide substrate. In addition, a uniform porous structure with a pronounced particle connection was found.

Claims (10)

1. A process for producing porous or dense sintered workpieces made of metal, a metal alloy or of ceramic materials, wherein a green body is first formed from a mixture of the metal, the metal alloy or the ceramic material which is present as a powder, and a binder, and the green body is subsequently freed from binder and sintered,
   characterised in that
   the mixture is first formed comprising the powder, the binder, the proportion of which in the mixture is calculated so that is sufficient to consolidate the powder-binder mixture in order to form the green body and in this respect amounts to 2 to 5 % by volume in the mixture, and a solvent for the binder, wherein the relative proportions of binder and solvent are calculated so that the binder is completely dissolved in the solvent, and in addition the proportions of powder, binder and solvent are calculated so that the material formed in this manner (the casting material) for casting into the mould permits free filling of the mould (by gravitation) on account of its rheological properties or if need be permits forced filling of the mould by a pressure slightly above atmospheric pressure, after which this material is cast into the predetermined mould made of non-porous material or is deposited or sprayed on and is subsequently dried, wherein the solvent escapes by evaporation and the remaining binder consolidates the powder-binder mixture so that the green body is formed.
2. A process according to claim 1,
   characterised in that
   the material is introduced into the mould with simultaneous jarring of the latter
3. A process according to claim 1 or 2,
   characterised in that
   the casting material situated in the mould is subjected to a reduced pressure during drying.
4. A process according to any one of claims 1 to 3,
   characterised in that
   the casting material situated in the mould is heated during drying.
5. A process according to any one of claims 1 to 4,
   characterised in that
   mixtures formed from different materials are cast successively into the mould, so that the casting material, and thus the green body which is subsequently formed also, consists of layers of different compositions lying one on top of another.
6. A process according to any one of claims 1 to 5,
   characterised in that
   the mixture is introduced over or around a green body or a ready-made workpiece which is introduced previously into the mould.
7. A process according to any one of claims 1 to 6,
   characterised in that
   the green body is heated to a temperature within the range from 280 to 420°C at a rate of 3 to 10°C/min and, depending on the size of the body formed, is held at this temperature until the binder is removed.
8. A process according to claim 7,
   characterised in that
   the green body is subsequently heated to a sintering temperature at a rate of > 10°C/min.
9. A process according to any one of claims 1 to 8,
   characterised in that
   bake-out is effected under medium to high vacuum conditions.
10. A process according to any one of claims 1 to 9,
    characterised in that
    the sintered body is densified by hot isostatic pressing (HIP process).