EP0269612B1 - Process for manufacturing articles - Google Patents

Description

The invention relates to a method for the powder-metallurgical production of moldings for tools, machine parts, articles of daily use with metals or metal alloys, powder-fine particles of the alloy components and / or master alloys and / or the alloy itself being mixed with at least one additive containing a metal and at least with heating to be compressed into a desired shaped body.

Powder metallurgy is particularly well suited for the production of tools, machine parts and articles of daily use for economic reasons. Their advantage is that, starting from metal powders, a porous body is produced by mechanical pressing, which is then compressed to the final shape during a so-called sintering.

Sintering is generally understood to mean the caking of powder particles under the action of heat in an oven. In many cases, the air-filled pores originally present in the powder compact are removed. This process is very badly affected by the oxygen content of the powder. The oxygen content on the surface of the powder particles in particular has a sinter-inhibiting effect and leads to porosities and inadequate mechanical properties.

It was recognized at an early stage that oxide coverings reduce the tendency to sinter due to the poor wettability compared to the metal matrix.

It was therefore obvious to sinter metal powder in reducing atmospheres, especially under hydrogen.

With low-alloyed iron materials, sinterings of this type can be used to achieve useful results; in the case of high-alloy steels - especially those containing vanadium, chromium, niobium, aluminum - it was not possible to reduce the thermodynamically particularly stable oxides via the gas phase.

The alternate sintering under a reducing atmosphere and vacuum has not brought about any significant remedy for high-alloy steels. This is probably due to the fact that the oxicarbides that often form on the surface cannot be reduced under the specified conditions.

For high-quality alloys and tool steels, attempts have therefore been made to produce powders with the lowest possible oxygen content.

It is part of the state of the art to produce low-oxygen powders by so-called inert gas atomization. In this process, the molten metal is in a protective gas atmosphere, e.g. Ar, N₂ or He, atomized into fine metal droplets that cool very quickly in the atmosphere and thereby form powder. However, such powders have the disadvantage that they cannot be compressed by mechanical pressing, since the individual powder particles are extremely hard.

To produce a compact body, it is therefore necessary to pour these powders into gas-tight metal or glass capsules, to remove the air from them, to seal the capsules gas-tight and to compress them at high temperatures and high pressure in a so-called hot-isostatic press. The bodies that one obtained in this way must first be freed of the capsule material and then subjected to mechanical shaping and processing in order to obtain a tool, for example. The disadvantage of this process lies in the complex process steps, which are very expensive. There has been no shortage of attempts to shorten the process and make it more economical. The so-called encapsulation, i.e. the filling of the atomized powder into a metal or glass capsule, was the starting point for many developments.

GB-PS 2 084 612 describes e.g. a method for producing a workpiece from gas-atomized powder with low oxygen content, which is characterized in that the powder is ground. The ground powder is then mixed with a pressing aid, pressed to a shaped body, sintered and hot isostatically compressed. The disadvantage of this process is still the use of the expensive gas-atomized powder, which can only be ground with great effort into a uniformly compressible fine powder.

Water atomized powders have the advantage that they have irregular shapes and can therefore be pressed well into preform bodies. However, they have the disadvantage that they contain a relatively large amount of oxide (700 to 1500 ppm) or are surrounded by an oxide layer which hinders the sintering process and causes porosity.

Furthermore, it is known from AT-PS 351 278 to produce a molded body from two metal components, one component being in the form of a metal powder and the other in the form of a metal salt. With the addition of soot, the components are mixed. After conversion of the metal salt into metal oxide, the oxide is reduced to metal in the course of the heating other metal component dissolves. This process also requires several stages, in particular two mixing steps and two reaction steps, which ultimately make it more difficult to form a sintered body that is completely non-porous and true to shape. Furthermore, the oxide is only reduced at very high temperatures, so that sintering must take place at at least 1800 ° C. and is not possible at low temperatures.

A process for the production of sintered blanks for rolling or forging by a powder metallurgical process, in particular the evaporation of alloy components during a tempering and sintering process of the metal powder is known from DE-A-2 461 736. In this case, an additive containing a metal from the group of alkali metals and alkaline earth metals is added to a mechanically produced powder and the mixture is left at a temperature which is higher than the decomposition temperature of the additive to soften the powder grain material. The mixture is then compacted and sintered. The alkali metal-containing additive should be broken down under the influence of heat to form a uniform solid solution in the form of the alkali metal or in the form of an oxide on the surface of the metal particles. On the one hand, this surface layer improves the sliding properties of the powder particles to one another and leads to a dense green blank, on the other hand an azeotropic mixture is formed, for example with zinc, which prevents evaporation losses and accelerates the sintering process.

The object of the invention is to design a method of the type mentioned in such a way that a shaped body is obtained in a labor and energy-saving manner, the sintering properties of which are optimal and whose porosity is minimal.

In a method of the type mentioned at the outset, this is achieved in that surface oxygen-containing particles or powders are brought into contact with at least one compound of at least one metal as an additive, which, at temperatures of 50 to 800 ° C., has at least one alloy-specific or alloy-compatible metal and / or releases a substoichiometric oxygen compound of such a metal with respect to oxygen, after which the final compression of the particles into the shaped body takes place at least with heating or with sintering with at least partial dissolution (breaking up) of the surface oxygen layer of the powder grains. This makes it possible to produce moldings that have practically no porosity and excellent mechanical properties. Sintering at low temperatures is also possible, i.e. it does not have to be sintered at temperatures at or just below the melting temperature, so that local overheating of the shaped bodies e.g. due to insufficient sintering intervals and fluctuating furnace temperatures. In addition, the additives used according to the invention have the effect that the ease of sintering of the shaped body components is increased; the additives bring about an improvement in diffusion at the particle boundaries, so that the achievable sintered composite is improved.

When heated as a result of decomposition, the additives used according to the invention release metal atoms or metal oxygen compounds whose oxygen content is substoichiometric, that is to say which contain oxygen in deficit, and which combine with the oxygen of oxides or oxide layers present on the particles or tear these layers apart. This creates metallic contact between the particles so that they can optimally sinter together. At the same time, the oxygen torn from the oxide compound reacts more easily to CO or CO₂ with the carbon contained in the structure. The resulting metallic contact leaves a decrease in the sintering temperature. Obtained from preforms with 700 to 1500 ppm O₂ moldings with max. 50 to 100 ppm O₂. The densities of sintered bodies achieved to date from about 99.5% to 99.8% based on the The maximum achievable density is increased and the densities of moldings produced by the process according to the invention are more than 99.8%. The temperatures for the compression of the preforms are somewhat below the temperatures currently customary for the respective metals of the alloys. In the method according to the invention, the use of temperatures about 5 to 10 ° lower is possible, which makes the sintering process much more manageable compared to temperature fluctuations in the sintering furnace; sintering is currently taking place just below the melting point of the respective eutectic compounds.

It is preferred if particles obtained with water atomization, optionally with an addition of up to max. 50% of gas atomized and / or ground particles, a metal, an alloy or pre-alloy with an irregular shape and size are used. Together with the irregularly shaped water-atomized particles, other particles can also be used, and a preform can be produced well and precisely due to the possible plastic deformation of the particles. Due to the addition of the additive, it is possible that particles or powders containing surface oxygen are also used. To form the preform body, it is expedient if the particles are subjected to a preform body formation after the addition of the additive with the application of pressure and / or temperature. The additive contributes to the cohesion of the particles.

A durable preform and a structure with a good structure are obtained when the particles are combined with the metal by mixing, in particular Stir, be brought into contact, preferably the entire surface of the particles being contacted, in particular wetted, with the compound of the metal. As a result, the particles can be intimately bonded over their entire surface during sintering. Mixing can take place in known mixing devices.

Depending on the nature of the particles and the compounds used of at least one metal, it can be provided that the compound of the metal is brought into contact with the particles in a solid or fluid state at room temperature or in a state dissolved in a solvent. An addition of a gaseous compound of a metal to the particles can be carried out as well as an admixture of powdery particles of a compound of a metal to the particles to be sintered. The solvent of the compound of the metal is expediently evaporated off before the preform is formed.

It is particularly advantageous with regard to the process control if the heating directly adjoins the preform formation or the evaporation of the solvent for the final compression. The sintering process or the compression to form the shaped body follows the production of the preform directly following the admixing of the connection of a metal or the evaporation of the solvent that may be necessary without any further intermediate step; further reactive intermediate steps are also not required, which makes the procedure simple.

In the method according to the invention, particles of tool steels, in particular high-speed steels, are advantageously used ledeburitic cold work steels, high temperature or sintered alloys based on Ni and / or Co or the like. used. After sintering, the particles form the corresponding steels or alloys with a standardized composition.

According to the invention, it is particularly advantageous that the released metal (s) or the oxygen compound (s) which is substoichiometric with respect to oxygen comprises metals or oxygen compounds of metals from the group Fe, Co, Ni, Cr , Mo, W, Ta, Nb, V, Ti Zr, Hf, Al, Si.

The process becomes particularly simple with good results if, preferably in an organic solvent, dissolved complex, addition and / or coordination compounds of at least one alloy-specific or alloy-compatible metal, preferably with zero valence, are used. Such compounds of metals release the metal or the oxygen-sub-stoichiometric metal compound at a relatively low temperature, and the connection residues leave the heated preform body relatively unhindered even before the compression process begins. It is expedient if the residues of the compound of the metal after release of the metal or the oxygen compound which is substoichiometric with respect to oxygen in the course of the temperature increase before reaching the compression temperature from the preform body, for example be removed by applying a vacuum.

It is also possible that, if appropriate exclusively, compounds containing carbonyl groups, for example Oxalates, the alloy's own or alloy-compatible metals are used. Another possibility is that, optionally exclusively, (cyclo) alkyls, alkenyls or alkynyls, for example C₁ - C₅ alkyl, allyl, cycloalkadienyl, cyclopentadienyl, cycloheptatrienyl, cyclooactadienyl, of the alloy's own or alloy-compatible metals are used.

Particularly good results are obtained if, preferably in organic solvents, dissolved acetylacetonates of the alloy's own or alloy-compatible metals, in particular of Co, Ni, Cr, Mo and / or V, are used. It is advantageous here if the acetylacetonate of the alloy's own or alloy-compatible metal, in particular co-acetylacetonate, based on the metal contained therein, in amounts of 0.05 to 0.8% by weight, preferably 0.08 to 0 , 2 wt .-%, of the weight of the particles used, is possible, for example also the simultaneous addition of Ni acetylacetonate and Co acetylacetonate.

The procedure according to the invention is carried out as follows:
Particles of at least one metal or a metal alloy are mixed with at least one compound of an alloy-specific or alloy-compatible metal which, when heated, releases a metal or a compound of the metal which is substoichiometric with respect to oxygen. The particles can be water-atomized metal powder, which to a certain extent, preferably 50%, is also gas-atomized or produced in another way or may contain pretreated powders. These particles are intimately mixed with the dissolved compound, for example by stirring, the particles being well wetted; after evaporation of the solvent, a layer of the compound should be deposited in molecular thickness on the particles. The particles are then pressed under pressure to form a preform. The preform is pressed in a press mold, which is also the sintered mold, and has a density of 80%. This preform is then heated to the compression temperature without further treatment, the connection being disassembled when heated. The metals released remain in the structure and the connection residues, in particular organic components, leave the preform body. When the sintering temperature is reached, there is only the alloy structure that is sintered, since the organic components are selected such that they have left the structure before the main shrinkage process and have already released the reactive metal atoms and sub-stoichiometric oxygen compounds.

Evaporation of the solvent is carried out by means of a vacuum or vacuum, if appropriate using protective gas, as is heating of the preform and compression or sintering.

Depending on requirements, one or more compounds of one or more metals can be added to the particles to be compressed, which each give off one or more metal (s) or one or more oxygen substoichiometric oxygen compound (s) of a metal when heated.

It should also be noted that the particles to be compacted also have substances which improve the compression or sintering properties, for example, before, after or simultaneously with the addition of the compound of a metal. Paraffin or others can be added.

The invention is explained in more detail below with the aid of examples:

example 1

A commercially available water-atomized high-speed steel powder with the material number 1.3343 and an oxygen content of 710 ppm was intimately mixed with 0.3% by weight of vanadyl acetylacetonate and with mineral spirits in a planetary mill. Then 1.5 wt .-% paraffin was added as a pressing or sintering aid and distributed homogeneously. After the solvent had been evaporated off, the powder was mechanically pressed into shaped bodies in a die at 5000 bar and then sintered under vacuum. The sintering conditions were 1235 ° C with a holding time of 30 minutes. The sintered density of the shaped body was 8.12 g / cm 3. The flexural strength was over 2000 N / mm². The oxygen content was 20 ppm. The comparative body sintered without the additive according to the invention could only achieve a density of 8.00 g / cm 3 and a 20% lower bending strength.

Example 2

A commercially available water-atomized high-speed steel powder with the material number 1.3215 was added with 0.5% cobalt acetylacetonate, 1% molybdenyl acetylacetonate and mixed with benzene in a planetary mill and then dried in a desiccator. The mechanical pressing was carried out at 5500 bar. The size of the moldings was 5 x 5 x 45 mm. Vacuum sintering of the pressed moldings took place at 1250 ° C., the holding time was 1 hour. The density of the sintered body, to which the additives according to the invention were fed, could be increased by 0.15 g / cm³ compared to the comparison body produced without additives. The flexural strength of the molded article produced according to the invention was 25% higher than that of the comparative article.

Example 3

A commercially available water-atomized high-speed steel powder with the material number 1.3343 was mixed according to the invention with 0.5% by weight molybdenyl acetylacetonate, 0.5% by weight vanadyl acetylacetonate and 0.3% by weight aluminum acetylacetonate and benzene. After the solvent had been evaporated off, the dry powder was pressed in a mechanical press at 5000 bar to form inserts with the ISO designation SPGN 120308. The green compacts were sintered at 1240 ° C. and in vacuo for 30 min. The carbon content of the sintered alloy after sintering was 0.91% by weight and the oxygen content was 20 ppm. The residual porosity was less than 0.1% by volume. The comparison body sintered without additives showed a lower carbon content and a residual porosity of 0.7% by volume. A wear test using the grinding wheel method gave the following results in specific weight loss (%):

The cutting inserts according to the invention showed a significant improvement in wear behavior.

Example 4

A mixture of a high-speed steel powder with the material number 1.3343 consisting of 50% water atomized and 50% gas atomized parts was produced and investigated in accordance with the conditions of Example 3. The following tool life was determined in the machining test at a cutting speed of 40 m / min, a cutting depth of 2 mm and a feed of 0.23 mm / rev on a tempering steel with material number 1.6582 and a hardness of 230 HB 30:

Example 5

Water-atomized powder based on cobalt (stellite 6) was mixed with 1% by weight of vanadyl acetylacetonate, 0.1 % By weight of aluminum acetylacetonate and 0.1% by weight of chromium acetylacetonate dissolved in acetone mixed in a stirrer. The dried powder was pressed into molds in a die at 6000 bar and sintered in a vacuum oven at 1290 ° C. with a holding time of 60 min. The residual porosity of the sintered body provided with the additive according to the invention was less than 0.1% by volume, while the comparison body produced without additive had a residual porosity of 12.1%.

Example 6

Water-atomized powder with the material number 1.2379 was mixed according to the invention with 0.3% by weight molybdenyl acetylacetonate, 1% by weight vanadyl acetylacetonate and benzene in a planetary mill and then dried under vacuum and mechanically pressed to give shaped bodies. The sintering took place at 1210 ° C. and a holding time of 1 hour. The residual porosity was below 0.2 vol%. The comparison body produced without additives showed a residual porosity of 3.8% by volume.

Claims (16)

1. Process for powder-metallurgical preparation of formed bodies for tools, machine components, utensils, with metals and/or metal alloys, in which at least one additive containing a metal is added to powder-fine particles of the alloy components and/or of pre-alloys and/or of the actual alloy and these are at least compacted with heating to a desired formed body, wherein surface-oxygen-containing particles and/or powders are brought into contact with at least one compound of at least one metal as additive which at temperatures of from 50 to 800° C liberates at least one metal contained in the alloy or compatible with the alloy and/or a compound of such a metal in a substoichiometric ratio in respect of oxygen, after which the final compaction of the particles into the formed body takes place at least with heating or sintering with at least partial dissolution (disintegration) of the surface oxygen layer of the powder grains.
2. Process according to Claim 1, characterised in that the particles are subjected to a process forming a pre-form body after addition of the additive with application of pressure and/or temperature.
3. Process according to Claim 1 or 2, characterised in that at least some of the powders used are prepared using the water atomisation process.
4. Process according to one of Claims 1 to 3, characterised in that the particles are brought into contact with the metal compound by mixing, especially stirring, the entire surface of the particles preferably being contacted, especially wetted, with the metal compound.
5. Process according to one of Claims 1 to 4, characterised in that the metal compound is brought into contact with the particles in a solid or fluid state which obtains at room temperature or in a dissolved state in a solvent.
6. Process according to Claim 5, characterised in that the solvent is evaporated from the metal compound prior to formation of the pre-form body.
7. Process according to one of Claims 1 to 6, characterised in that heating for the final compaction immediately follows formation of the pre-form body or evaporation of the solvent.
8. Process according to one of Claims 1 to 7, characterised in that the liberated metal and/or metals and/or the oxygen compound(s) of a metal in a substoichiometric ratio in respect of oxygen are metals or oxygen compounds of metals from the group Fe, Co, Ni, Cr, Mo, W, Ta, Nb, V, Ti, Zr, Hf, Al, Si.
9. Process according to one of Claims 1 to 8, characterised in that particles obtained by water atomisation, possibly with the addition of up to a maximum of 50% gas-atomised and/or ground particles, of a metal, an alloy or a pre-alloy of irregular configuration and size are used.
10. Process according to one of Claims 1 to 9, characterised in that particles of tool steels, especially high-speed steels, ledeburitic cold work steels, high-temperature or sintering alloys based on Ni and/or Co or similar are used.
11. Process according to one of Claims 1 to 10, characterised in that complex, addition and/or coordination compounds of at least one metal contained in the alloy or compatible with the alloy, preferably of zero valency, which are dissolved, preferably in an organic solvent, are used.
12. Process according to one of Claims 1 to 11, characterised in that compounds exhibiting carbonyl groups, eg oxalates, of the metals contained in the alloy or which are compatible with the alloy are used, possibly exclusively.
13. Process according to one of Claims 1 to 12, characterised in that (cyclo-)alkyls, -alkenyls or -alkinyls, for example C₁ - C₅ alkyl, allyl, cycloalkadienyl, cyclopentadienyl, cycloheptatrienyl, cyclooctadienyl, of the metals contained in the alloy or compatible with the alloy are used, possibly exclusively.
14. Process according to one of Claims 1 to 3*, characterised in that acetylacetonates of the metals contained in the alloy or compatible with the alloy, especially of Co, Ni, Cr, Mo and/or V, which are dissolved, preferably in organic solvents, are used.
15. Process according to one of Claims 1 to 14, characterised in that the acetylacetonate of the metal contained in the alloy or compatible with the alloy, especially cobalt acetylacetonate, is used in quantities of from 0.05 to 0.8 wt-%, and preferably from 0.08 to 0.2 wt-%, of the weight of the particles used, calculated on the metal it contains.
16. Process according to one of Claims 1 to 15, characterised in that the residue of the metal compound is removed from the pre-form body, eg by application of a vacuum, after liberation of the metal or of the oxygen compound which is in a substoichiometric ratio in respect of oxygen, during the course of raising the temperature before compaction temperature is reached.