DE3808123A1 - Process for producing sintered parts of finely particulate metal or ceramic powders - Google Patents

Abstract

To produce the sintered parts, powders having a particle size of up to 500 mu m and a particle size distribution which approaches the "Fuller distribution" are used. The multicomponent binder added contains at least one component which forms a liquid phase below the melting/softening point of the other component(s) while these solidify to give a sponge-like skeleton having contiguous pores going through (open porosity). The amount of the liquid phase is calculated in such a way that after it is removed by chemical means, there remains in the shaped part a free pore volume matched to the sintering process. The other components are then thermally decomposed, under pulsating superatmospheric pressure above their decomposition temperature and above that temperature at which the powder particles just begin to grow together, in a gas atmosphere whose respective carbon or oxygen activity corresponds to the respective carbon or oxygen activity of the shaped part. Decomposition gases remaining therein are removed by a vacuum treatment, before the shaped part is sintered to closed porosity at temperatures below the flow-point of the material under vacuum and is finally densified isostatically under inert gas.

Description

The invention relates to a method for Production of sintered parts from fine-grained metallic or ceramic powders.

In the known processes for the production of Sintered parts made from fine-grained powders Powder with one, mostly organic Existing binders or lubricants (in further referred to collectively as "Binder") mixed and then under pressure, e.g. in in a mechanical press or in a Injection molding machine, converted into a molded part. After immersing the binder in a solvent and / or thermal decomposition has been removed again, the preformed Part densely sintered at high temperatures. The sintering furnace used for this can either be a Vacuum oven or an almost pressure-free Furnace with an inert or reactive gas atmosphere be. To the quality of the finished sintered The molded part can further improve the molded part then by drop forging or pressing to more than 99% of its theoretical density be finally compressed.

A modern process for final compression (or also recompaction) of sintered parts is hot isostatic pressing (HIP). With this procedure the molding is made under inert gas at pressures of  approx. 1000 to 2000 bar and temperatures of approx. 100 up to 300 ° C below its material pour point pressed. The molded parts are pre-pressed either with one even at high temperatures provide a gastight cover or in one additional sintering process so compacted that the Density of the sintered molding more than 93 to Is 96% of its theoretical density. In one such a molded part are not through pores. This will prevents gas from getting into the Mold penetrates and its compression counteracts.

With low-pressure assisted sintering (PAS), the Final compression at approx. 70 to 200 bar carried out. The process temperature becomes tight below the material flow point so chosen that liquid phases occur to a small extent. So that continuous compression is achieved and gas inclusions must also be avoided the density before this final compression of the molded part to 93 to 96% of theoretical Density can be set.

In the previously known manufacturing processes of sintered parts made of fine-grained powders Binder mixed into the powder usually before Sintering process removed by thermal decomposition will. Especially with powder-binder compositions based on Injection molding machines are molded for the thermal decomposition often annealing times longer than 25 hours, with the result that the Sintering furnaces blocked for a disproportionately long time and thus high costs arise.  

Due to the evaporation of the binder resulting gaseous decomposition products moreover, such a high pressure in the Molding built that this bursts or is unusable or destroyed by cracking. This is always the case when the binder evaporates too quickly.

A major disadvantage of the previously known The procedure also consists in that thermal decomposition of organic, carbonaceous binder the carbon content of the molded body is influenced.

So occurs in the manufacture of metallic Moldings with high carbon contents often an uncontrollable degradation of the Carbon content at the corners and edges of the Molding on. It happens inside the molding on the other hand, especially with long glow times uncontrollable carbon enrichment. For the production of molded metal parts with low carbon content (<0.1% carbon) metal powders are used, the Carbon levels well below that desired target size. During the thermal decomposition process Moldings either by the resulting carbon-containing gases or by the at thermal decomposition occurring in the metal carburized free, soluble carbon.

The production of sintered parts from metallic Powders, which organic binders before Shaping were added, with an over entire cross section even, accurate defined higher carbon content or the Production of corresponding sintered parts with very  low carbon content (e.g. <0.07% to 0.086%) is therefore almost impossible to this day.

The metal powders used often have oxidized surfaces. these can not removed in the course of further processing are hindering this in the sintering process The powder particles grow together and cause Oxide inclusions and oxygen enrichments in the Material. This will improve the quality of the Sintered part reduced.

Ceramic oxides form one Quality reduction of the sintered part often in that the organic binder components the oxides contained in the ceramic at reduce thermal decomposition.

It is another disadvantage of the known Process that only for the production of the molded parts metallic or ceramic powder used can be, their particle diameter is predominantly <10 µm and accordingly are expensive.

The invention is based on the object Process for the production of sintered parts fine-grained metal or ceramic powders to indicate that the use of Powder particles with a grain diameter predominantly <10 µm allowed, one binder used, largely by one removed non-thermal process from the molded part can be a stable molded part with a generates defined free pore volume, where this is adapted to the subsequent sintering process that the remaining binder components  removed quickly but without cracking in the molded part can be. The procedure is also intended to be such be such that anoxidized surfaces in Metal powders reduced as completely as possible are defined carbon contents in metallic moldings can be adjusted and none with oxide-containing, ceramic powder The oxides are reduced.

According to the invention, the task is as follows solved:

For the production of the molded parts, metallic or ceramic powder with a Particle diameter up to 500 microns, preferably up to 200 µm, where the Grain size distribution of the powder largely the "Fuller distribution" corresponds to that later is received.

A binder is added to the powder, which consists of consists of at least two components, one Component below the melting or Softening point of the other components liquid phase forms while the rest Components below their melting point evenly formed sponge-shaped, however form a solid skeleton with continuous pores, the after shaping and cooling from the liquid phase is filled and predominantly can be removed by a solvent. The Amount of liquefied Binder component corresponds to that desired free pore volume.

The freed from the liquid binder portion Molding is then in a first  Sintering stage to remove the residual binder a temperature above the evaporation or Decomposition temperature of the residual binder and above the temperature at which the powder sinters just started, heated up. The pressure set in the sintering furnace so that when it flows out of the gaseous decomposition products from the Molding no cracks or other destruction arise and also an overgrowth of pores in the Course of the progressing from the outside inwards Sintering of the powder does not take place. According to the invention, this decreases set pore volume the pressure in the sintering furnace elevated. The pressure can at least temporarily be pulsating.

The gas phase is adjusted so that the Activity of the leading component of the solid in the gas phase is specified. For metallic and metal carbide materials is the carbon Leading component, it is with oxidic materials the oxygen.

The first sintering stage is accordingly at Metal powders or metal carbide powders in a hydrogen or hydrocarbon atmosphere carried out, whose carbon activity is that of Powder corresponds. For molded parts made of ceramic, oxide-containing powders become oxidizing Atmosphere used whose oxygen activity corresponds to that of the oxidic powder.

After the remaining binder components have been removed in a second Sintering stage a temperature that is about 1-30 ° C, preferably 1-10 ° C, below the Material flow point of the molded part is  set, and the molding under vacuum to 93 up to 96% of its theoretical density densely sintered.

At a temperature just below his The molded part becomes the material flow point last under pressure in an inert atmosphere to more than 99% of its theoretical density isostatic finally compressed.

In a particularly advantageous variant of the The method provides for the molded part according to the Removal of the residual binder and after the Solidification of the molded part by sintering continuous porosity of one Undergo vacuum treatment. This allows those still present in the inner pores residual decomposition gases before the sealing sintering the molded part can be removed.

The method according to the invention is as follows with reference also to the drawing explained. In this is the permissible deviation the grain size distribution of the used fine-grained powder of the ideal Fuller distribution shown.

According to the invention, the grain size distribution the powder used so that the so-called "Fuller distribution" (P. Schmid, Mineral Processing (1964) 7, p. 355- 365).  

With

m _F = quantity distribution d _p / d _max = ratio of a particle to the largest
Particle n _F = Fuller exponent

largely corresponds. The allowable deviation the grain size distribution from the ideal Fuller's distribution results from the drawing. By applying this special Grain size distribution succeeds, including powders spherical particle structure with particle sizes up to 500 µm, preferably up to 200 µm. Due to the deliberately mixed fine grain the number of possible sinter bridges increased so that the molded part despite the high proportion of coarse grain receives a very good stability. This advantage is still sufficient, if the individual groups up to the values from the respective proportions of the ideal fuller distribution differ.

According to the invention, the powder becomes a binder added from at least two, preferably there are three components above theirs Melting or softening points a homogeneous liquid phase with good lubricant properties form. One of the components forms below the Melting or softening point of the Residual components a liquid phase while the remaining components below theirs Melting point an evenly formed spongy solid skeleton with inside form evenly distributed through pores, that after the shaping and cooling of the  liquid phase is filled. The proportion the component that is below the melting point the remaining components a liquid phase forms, is according to the inventive method dimensioned such that after removal of the liquid Phase through pores through a solvent with a free pore volume of approx. 20 to 50 vol .-% arise. This pore volume is up coordinated and has the subsequent sintering process themselves in relation to the method according to the invention proven to be particularly advantageous.

The binder according to the invention can, for example, consist of a mixture of oleic acid decyl ester and one Thermoplastic (e.g. polyethylene or polypropylene) consist. The oleic acid decyl ester is below the Melting point of the other components up to Temperatures of around -10 ° C liquid and thus determines the pore volume of the molded part. He does not contain any volatile components that precede the Removal of the residual binder by a thermal Treatment in the oven (e.g. when kneading and during Shaping) can escape. The thermoplastics do not yet form the supporting skeleton of the sintered molding.

The two basic components of the invention Binders can be a so-called third component Adhesion promoter added in the as other binder component used thermoplastics above their melting point is soluble. It can for example as a corresponding addition Ethylene-acrylic acid copolymer or a modified EVA copolymer with proportions of Vinyl acetate can be used. More usable Adhesion promoters for thermoplastics and metals are from the practice of metal foil coating known where it acts as an intermediate layer between  known where it acts as an intermediate layer between Thermoplastic and metal are used. According to the invention by admixing the Adhesion promoter the liability of the binder to the Solid particles increased proportionally to the added share. With increasing liability of the Binder on the solid becomes the danger that it during the shaping to segregate from Binder and solid comes, reduced.

After shaping, the liquid component this binder in an easily volatilized Solvents (e.g. chlorinated and / or fluorinated hydrocarbons and / or alcohols) dissolved. The one adhering to the molded part The solvent is then evaporated. By the application of the solvent bath with Ultrasound can speed up the solution can be increased to more than double.

Another embodiment of the invention Binders is a mix of different ones Methyl celluloses with water and / or alcohol (preferably benzyl alcohol). In this case the scaffolding methyl cellulose from that added water / alcohol dissolved and liquefied. After shaping, water and / or alcohol evaporates while doing the desired free Pore volume released. The methyl cellulose forms a porous, stabilizing part sponge-shaped grid.

Then the sintering process is done in two stages carried out. First, the remaining binder from the Removed molding and then densely sintered.

The molded part is used to remove the residual binder at a temperature above that  Decomposition temperature of the residual binder and above the temperature at which touching Powder particles just connected by sintering continuous porosity is preserved, sintered. This temperature (the is below the actual sintering temperature) is material dependent and is used for every powder determined empirically and / or experimentally; the Sintering process is preferred at pressures between 1 and 100 bar.

With increasing pressure and constant temperature becomes the volume of the resulting decomposition gases thus reducing the flow rate of the gas diminished through the pores. By a Increasing pressure will reduce the decomposition temperature of the Binder moved to higher temperatures. print and temperature can be chosen so that a high decomposition rate of the binder given is.

By specifically adjusting the temperature and Pressure on the selected free pore volume of the Molding can the thermal decomposition of Restbinders can thus be controlled so that of the evaporation or decomposition of the Residual binder in the interior of the molded part Pressure is released so quickly that between the Inside and outside of the molded part none Differences in pressure can arise that the Could destroy molding. At high Pore volume, e.g. 50%, a little pressure is enough of about 1 bar. With low pore volume, e.g. 20%, is a high pressure of e.g. about 100 bar required. A possible crack in the molded part is excluded.  

As a result of the rapid removal of the gases can the decomposition products only briefly on the Act on molding. As a result, metallic Shaped bodies a change in the carbon content the metal phase and for oxide ceramic molded parts the reduction of the oxides largely suppressed.

Uncontrolled changes in the carbon content of metallic molded parts are prevented by the process according to the invention, in particular, by the sintering of the molded parts being carried out in a gas mixture of hydrogen and gaseous hydrocarbons (for example H ₂ / CH ₄ ), the carbon activity of the gas being the carbon activity of the Molding of dissolved carbon corresponds. As a result, the desired carbon content can be set with high accuracy in all areas of the molded part.

For molded parts with a low carbon content sintering in a pure hydrogen atmosphere carried out. Through the hydrogen carbon-containing decomposition products in the molded part hydrogenated and in particular with decomposition gases higher carbon contents / mol in those with low carbon / mol transferred. in the Molding carbon deposits are converted into gaseous carbons and dissipated with the decomposition products. This is for sintered parts with carbon contents <0.1% of particular advantage. At the same time, the oxidized surfaces of the used Metal powder reduced by the hydrogen.

The hydrating, deoxidizing and carbon-removing effect of hydrogen can  can be increased to a considerable extent by the fact that to improve the flushing of the porous trained molding the pressure in the sintering furnace in Range ± 1 to 40% of the currently set Pressure (e.g. 70 ± 30 bar) changed periodically is, the pressure fluctuations proportional to set porosity of the molding can be trained stronger and more intensely, the larger the porosity. This measure has intensive flushing of the molded part with the reactive gas phase and accelerates the Sintering process.

With oxide ceramic powders, the sintering is carried out in carried out in an oxidizing gas atmosphere, where the oxygen activity of the gas Oxygen activity of the oxide in the ceramic corresponds.

After the residual binder has been removed from the molded part, the remaining decomposition gases which are still present are removed by a vacuum treatment while the porosity of the molded part is still continuous. For this purpose, the sintering furnace is evacuated to 1 × 10 ^-4 mbar, preferably 1 × 10 ^-2 mbar, without further increasing the temperature.

The molded part is then in a second sintering stage at a pressure of 1 × 10 ^-4 mbar, preferably 1 × 10 ^-2 mbar and at a temperature which is about 1 to 30 ° C, preferably 1 to 10 ° C, below the material pour point The molded part is densely sintered to 93 to 96% of its theoretical density.

Finally, the molding is pressed at 60 to 200 bar, preferably up to 100 bar, and at a temperature just below his Material pour point under inert gas isostatic to a density of more than 99% of its theoretical density.

It is a particular advantage of the present Invention that the removal of the molding stabilizing residual binder and finished sintering economical in a single sintering furnace can be carried out. According to the Residual binder removed from the molded part so quickly, that this process section within the anyway required heating phase of the sintering furnace can expire. The sintered parts can manufactured particularly quickly and inexpensively will.

Another significant advantage of the invention is that by using Gas atmospheres whose carbon or Oxygen activity of the carbon or Oxygen activity of the sintered part is adapted, Molded parts with a cross-section uniform, precisely defined carbon or Oxygen content can be produced.

An embodiment of the Described method according to the invention:

80 kg of metal powder of a nickel-based alloy with a grain size of up to 150 µm, the grain spectrum of which corresponded to the grain size distribution shown in the drawing, 5% of a binder mixture was added in a kneader, 39% from oil, 55% from polyethylene and 6 % consisted of an ethylene-acrylic acid copolymer. The mass was processed in a kneader at 160 ° C into a homogeneous paste and granulated. The granules were processed in a known manner at a temperature of 180 ° C into molded parts using a conventional injection molding machine. After the moldings had cooled, the oil contained in the binder separated out as a continuous liquid phase in the moldings. After the oil had been removed in an ultrasonic freon bath and the solvent had dried, the molded parts were held together by the polyethylene skeleton. They were then used in a vacuum-pressure sintering furnace, which was heated to a temperature of 750 ° C. in about 15 minutes while setting an H ₂ pressure of 100 bar. The removal of the residual binder was complete after reaching an average oven temperature of about 750 ° C. The molded parts used were stabilized by the formation of sintered bridges at the contact points of the powder particles. The hydrogen penetrating into the molded parts completely reduced the oxidic surfaces of the powder particles. The moldings were then treated in the same sintering furnace under vacuum at about 10 ^-2 mbar at the same temperature until the vacuum stabilized. The temperature was then set to 15 ° C. below the material flow point of the molding under vacuum; after 3 hours the moldings had reached a density which corresponded to approximately 95% of their theoretical density. The sintering furnace was then flooded with argon and raised to a pressure of about 100 bar. The temperature was set at 5 ° C to 1 ° C below the material flow point of the molded part. The density of the molded parts increased to about 99% of their theoretical density in a short time.

For all percentages that relate to the Obtain composition of materials mass percentages.

Claims (20)

1. Process for the production of sintered parts from fine-grained metallic or ceramic, with a binder-mixed powders, which are plasticized and converted under pressure into a molded part, from which before the dense sintering at high temperatures and before the subsequent final compression to more than 99% of it theoretical density of the binder is removed, characterized in that

• a) the grain size of the powder with a grain size distribution which is at least approximate to the “Fuller distribution” is up to 500 μm, preferably up to 200 μm;
• b) the binder is composed of components which form a homogeneous phase with good sliding properties above their melting or softening point, at least one component below the melting or softening point of the remaining component (s) being a liquid phase, while the remaining component (s) below their melting or softening point in the unsintered molded part forms a spongelike skeleton with continuous pores, which are filled with the liquid phase;
• c) the amount of the at least one liquid component filling the skeleton is dimensioned such that when it is removed from the molded part by a chemical solution process, a free pore volume adapted to the sintering process is created therein;
• d) the molded part freed from the liquid phase is treated in a first sintering stage under positive pressure above the decomposition temperature of the residual component (s) and above the temperature at which the sintering of the powder is just beginning in a gas atmosphere, the carbon or oxygen activity of which Carbon or oxygen activity of the material from which the molded part is made corresponds, and the pressure is set such that the decomposition gases of the residual component (s) are rapidly removed through the pores without the formation of a critical internal pressure in the molded part;
• e) the molded part is densely sintered to 93 to 96% of its theoretical density in a second sintering stage under vacuum at temperatures which are below the material pour point of the molded part and
• f) at pressures between 60 and 200 bar, preferably 60 and 100 bar, and at a temperature just below the material flow point of the molded part, its final compression is isostatic.

2. The method according to claim 1, characterized characterized in that the binder from a Mixture of oleic acid decyl ester and one Thermoplastic exists.   3. The method according to claim 2, characterized characterized that the binder as third Component an adhesion promoter is added. 4. The method according to at least one of claims 1 to 3, characterized in that the liquid Binder component by chlorinated and / or fluorinated hydrocarbons is dissolved. 5. The method according to at least one of claims 1 to 4, characterized in that the liquid Binder component dissolved by alcohols becomes. 6. The method according to at least one of claims 4 or 5, characterized in that the Solution process accelerated by ultrasound becomes. 7. The method according to claim 1, characterized characterized in that the binder from a Mixture of methyl cellulose and water consists. 8. The method according to claim 1, characterized characterized in that the binder from a Mixture of methyl cellulose and alcohol consists. 9. The method according to at least one of claims 1 to 8, characterized in that the Proportion of the component below the Melting or softening point of the Residual component (s) forms a liquid phase, is dimensioned so that after removal of the free pore volume present in the liquid phase between 20 and 50% by volume of the molded part matters.   10. The method according to at least one of claims 1 to 9, characterized in that the molded part in the first sintering stage depending on the set pore volume at pressures is sintered between 1 and 100 bar. 11. The method according to at least one of claims 1 to 10, characterized in that the pressure in the first sintering stage in the range between ± 1 and ± 40% of the currently set pressure pulsates. 12. The method according to at least one of claims 1 to 11, characterized in that the pressure in the first sintering stage at least temporarily pulsates. 13. The method according to at least one of claims 1 to 12, characterized in that the pressure in the first sintering stage pulsates regularly. 14. The method according to at least one of claims 1 to 13, characterized in that the im Molding remaining Decomposition gases through a vacuum treatment be eliminated. 15. The method according to at least one of claims 1 to 14, characterized in that the vacuum treatment is carried out at pressures between 1 × 10 ^-4 mbar and 1 × 10 ^-2 mbar. 16. The method according to at least one of claims 1 to 15, characterized in that the Vacuum treatment at the temperature at which the Molding treated in the first sintering stage is carried out.   17. The method according to at least one of claims 1 to 16, characterized in that the Molded part in the second sintering stage Temperatures that are 1 to 30 ° C, preferably 1 to 10 ° C below its material pour point lie, is sintered. 18. The method according to at least one of claims 1 to 17, characterized in that the pressure in the final isostatic compression of the Molding is about 100 bar. 19. The method according to at least one of claims 1 to 18, characterized in that the isostatic final compression of the molded part Temperatures between 1 to 5 ° C below the Material flow point of the molded part carried out becomes. 20. The method according to at least one of claims 1 to 19, characterized in that the Grain size of the metallic or ceramic Powder in one of the "fuller distribution" at least approximate grain size distribution up to 150 µm.