DE19535444C2 - Process for the powder metallurgical manufacture of articles and articles produced in this way - Google Patents

Description

The invention relates to a method for powder metallurgy Manufacture of objects or semi-finished products, taking one on slurry from a metal powder or metal alloy powder with a melting point above 1000 ° C and a binder below Gives pressure in a mold and sinters the molded body obtained. This wet powder die casting represents a variant of the known Slip casting process and is closer below explained.

In powder metallurgy, which is close to the final dimensions economic production of series parts becomes possible fabrics and workpieces made from powdered metal. There are three main steps. First the metal powder is made into a certain shape brought and finally at elevated temperatures Compaction of the material sintered.

The above wet powder die casting is of various types known shaping techniques have been developed:

During die casting, liquid non-ferrous alloys (tin, lead, Aluminum and magnesium alloys) under high pressure in Shed permanent molds. The melt solidifies quickly. Due to the high pressure, even the finest details are molded. The high cost of permanent form enables one economical die casting usually only if enough high quantities.

The metal powder injection molding process is based on the Plast processing for the shaping of complicated small parts used with large quantities. It is a combination of the Sintering and injection molding technology. The  Shaping is done by injection molding, compression by sintering. Processed in general plastic-filled, very fine metal, hard metal or Ceramic powder. The expulsion of the thermoplastic form Giving aid limits this manufacturing technology small and thin parts. When thermally removing the Binders (pyrolysis) on the one hand destroy steam pressures and on the other hand there is a tem temperature range with only low strength of the molded body (Softening hole) below the temperature at which sintering begins.

Wet powder casting modifies the above method (DE 41 20 706 A1). It is assumed that the flow is good Mixture of a carrier liquid (e.g. alcohol), a binder and the metal powder. This suspension will in a form, e.g. B. cast from silicone rubber. At the binder dries after the subsequent drying tiny bridges between the metal particles together and secures the green body hold and shape. After drying the casting is demolded and the binder content from 2 to 5 vol .-% decomposed by heating to 350 ° C. Finally the metallic powder particles become almost compact component sintered.

The problem of the softening hole also arises here. The one added in the slurry of the metal powder Binder should be possible when sintering the green body (castings) as completely as possible from the mass or the green body give way so that unwanted residues are not in the workpiece remain. In the prior art, waxes are used as binders as well as plastics. It is also known from the DE 41 20 706 A1, as a binder wax, shellac or Use polymethacrylic methyl ester (PMMA). With these the green body has known processes during the heating zens after the decomposition of the organic binder and before the onset of the sintering process is only a very small one  Strength used in industrial manufacturing can cause significant problems.

In the process according to EP 0 260 812 A2, an aqueous one is used Slurry from water-soluble ethyl cellulose Binder, carbon and water-atomized, unreduced Metal powder extruded into moldings and under control ten reduction conditions of the binder carbon content sintered that the oxygen content of the metal moldings is broken down.

The slip casting process in which the metal powder a pourable mass (slurry) is slurried, is borrowed from ceramics. The slip is depressurized Poured into porous, absorbent forms (generally made of plaster). The carrier liquid can be accelerated by suction be removed. The molded body is made from the dismantled Form removed and then fed to the sintering process. The procedure Ren is mainly suitable for the production of large filter bodies or hollow bodies in which after reaching a certain wall thickness the liquid slip is poured out again.

The slip was used in the process according to DE 35 25 752 C2 pour moldings modified so that one over the Melting point heated mixture of tertiary butanol and the metal powder is filled into a form in which the Slurry solidified under cooling.

The slip casting process was used in ceramic ver Driving technology for the die casting process of aqueous, meal technically processed powder mixtures further developed. This process, in which the slurry of Presses metal powder into the mold under pressure referred to this patent application as "wet powder die casting" net. Compared to the pressureless slip casting process the productivity and quality of the products significantly improved since the respective casting parameters  (Pore size distribution of the fully porous plastic mold, filling time, Filling pressure, etc.) individually the filtration properties of the used casting compound can be adjusted.

When manufacturing metal powders or metal alloy powders made of a corrosive, ferrous material with a Melting point above 1000 ° C by atomizing a metal melt or a metal alloy melt by means of pressurized water atomization is the first step in powder metallurgy Manufacture of articles according to the invention.

In pressurized water atomization for the production of metal powders, with this term also always metal alloy powder a beam of molten metal hits one water jet emerging from a nozzle at high pressure and will broken down into fine particles. That is advantageous the rough, jagged (scratchy) surface of the Powder particles, the strength of the following manufactured green body increased. In addition to dividing the However, molten metal also finds an oxidation of the fresh generated fine metallic particles due to a reaction the hot melt with the atomizing water instead. The receive Powders tend not to be directly shaped be further processed because the oxide content of the raw powder would become components with insufficient strength to lead. To solve this problem, it is known to use inert gas in feed the atomization room.

With this measure, the oxygen content of the raw powder may be significantly reduced, but it is still too high, around the raw powder directly to the powder metallurgical To be able to use manufacturing. Therefore, the raw powder needs to be on in any case through annealing and reduction treatment  lungs, for example in a hydrogen-containing atmosphere, be treated.

Korro consisting of phosphates are also known sion protection layers for iron and steel. These layers have been diving since the beginning of this century of the workpieces in boiling phosphoric acid and in acid solutions containing iron phosphate applied. With this Corrosion protection process is first the thin oxide layer on the iron-containing workpiece and then pickled a phosphate layer to protect against further corrosion, the so-called passivation layer. The Combination of pickling and passivation is under the Be handle "Bondertechnik" known.

The use of phosphates as a bin is also known of the manufacture of refractory masses. So include some fireproof ramming and patching acidic aluminum minum phosphate as a so-called "mineral glue". Furthermore are Binder for granular refractory mixtures based on acidic zirconium phosphate known.

The properties of the iron-phosphorus alloys relevant in connection with the invention are briefly mentioned here. The two-substance system Fe-P shows that phosphorus severely constricts the γ region. In the temperature range of sintering used from 1000 ° C to about 1250 ° C for iron-phosphorus alloys with 0.2 to 0.5 wt .-% phosphorus the α-γ mixed crystal region. The pure α phase can be found in the range from 0.5 to 2.8% by weight of phosphorus. (Hansen and Anderko, Constitution of binary Alloys, New York, Toronto, London 1958, McGraw-Hill). In the α-phase the diffusion rate is a power of ten greater than in the γ-phase, so that the sintering in the α-region is significantly faster than in the γ-region of the usual non-ferritic iron alloys (Böhm, Introduction to Metallurgy p. 88, BI university pocketbooks No. 196 / 196a). The phosphorus released during the polymerization of phosphoric acid or phosphates on the iron particle surface under reducing conditions reacts with the iron base material to form intermediate Fe ₃ P, Fe ₂ P-FeP and FeP ₂ alloys. The temporary liquid phase that occurs in some cases also causes a very high sintering activity and rounds off the existing pore spaces. This effect reduces the notch effect emanating from the pores and improves the strength of the sintered material (Schatt, powder metallurgy, sintered and composite materials, p. 150 ff, Hühig Verlag, Heidelberg). In addition, the material strengthening of α-Fe by phosphorus is particularly effective (Hornbogen, Hochfeste Werkstoffe, p. 14, Verlag Stahleisen Düsseldorf 1974).

The properties of boric acid and borates also play a role in this invention. The solubility of boric acid H ₃ BO ₃ in water increases with temperature (19.5 g / l at 0 ° C; 291 g / l at 100 ° C). The boric acid is highly volatile in the presence of water vapors. When heated, boric acid converts to metaboric acid (HBO ₂ ) _n , which, when heated further, forms boron trioxide B ₂ O ₃ , which easily dissolves metal oxides under glass formation (Trzebiatowiski, textbook on inorganic chemistry; VEB German Publishing House of Sciences, Berlin 1970).

The object of the present invention is in the method of sufficient strength of the green body during the entire heating process, so that in contrast to the prior art with the wet powder Die casting also large components with undercuts, inside contours and other complicated shapes can be produced.

This object is achieved in that bandage agents are used with the metal or metal alloy Powder and / or with its oxide layer to form a Glases react.

In contrast to the prior art, the aim is not the binder almost completely during the heating process to remove from the green body. Rather is the presence the binder is desired even at higher temperatures, for a sufficiently high strength of the green compact in each Ensure phase of the heating process.

It is also proposed as a metal or metal alloy a powder atomized with water, from a corrosive, iron-containing material existing powder is used, wherein the atomizing water is a pickling and passivating agent contains. This metal powder is for immediate use in powder metallurgical manufacturing process suitable and no longer needs post-treatment after powder production become.

The pickling and passivating agent preferably contains Phosphoric acid, especially metaphosphoric acid, and / or water soluble compounds of this acid, e.g. B. phosphates or poly phosphates.  

It is also proposed that the atomization water 0.05 to 5% by weight, in particular 0.05 to 0.1% by weight, of phosphoric acid and / or contains their water-soluble compounds.

It is also advantageous if you have the atomization circulates water.

Phosphoric acid and / or phosphoric acid are preferred compounds used as binders. Such a bandage medium has the additional function of corrosion protection layer and is particularly suitable for corrosive, iron-containing powders advantageous.

In a further embodiment of the invention is pre suggest that one as boric acid and / or boric acid compounds Binder used, especially in the case of corrosion resistant ferrous powders. In this case it is furthermore advantageous if the binder additionally Contains phosphoric acid and / or phosphoric acid compounds. The formed, disperse glass phase as the desired "Alloy component" of the metallic material can be in this case, modify to a particularly large extent. So there are more degrees of freedom when choosing the Composition available.

Part of the binder can be used in the manufacture of the Metal powder can be added. If you look at the metal or Metal alloy powder by atomizing a molten metal or metal alloy melt using pressurized water atomization produces, the atomizing water phosphoric acid, ins special metaphosphoric acid, and / or water-soluble compound contain this acid. In this case, the addition serves as already explained above, at the same time as the ent removal of oxide layers on the powder and passivation of the bare metallic powder surfaces.  

In a further preferred embodiment of the invention suggested that you open the slurry in one out porous plastic existing form there. In contrast to the stand the technique in which the mold is generally made of gypsum, the plastic mold can be used several times. In particular it is suggested that the shape be essentially Polymethyl methacrylate (abbreviated as PMMA) exists. In this Plastic containing styrene, which during the Making the mold can be removed again the porosity.

In summary, the following advantages of the mold made of open-pore plastic compared to the Casting with a lost mold, for example a plaster mold. The ongoing effort for molding and for core production not applicable. Waste-free production with a very high level metallic application is possible. In addition to the Free of voids, good dimensional stability and surfaces Condition achieved because of an exact filling of the mold is guaranteed. A careful filling of the form is namely possible because the mold is relatively slow when pouring cold can be loaded. In contrast, when pouring out the beginning of solidification of the metal melt.

Finally, the invention also comprises powder metallurgy items or semi-finished goods, in particular from a ferrous material caused by phosphate and / or borate glasses on the powder particle surface and between the powders particles are marked.  

Particularly suitable for the production of highly stressed Components such as connecting rods or diesel pistons according to the inventions processes according to the invention are ferritic materials, preferably on the alloy basis silicon and Phosphorus. The high sintering activity that can be achieved here possible a very high density and homogeneity of the Workpiece. The excellent suitability of this alloy stations for processing in the invention The procedure is special in the following their material properties justified.

The alloying elements silicon and phosphorus bil the with the α-iron substitution mixed crystals and be act a linear increase in the yield strength and Tensile strenght. Phosphorus shows the highest effectiveness on strength, but has a very brittle Influence. In addition to the strength properties, the Toughness properties play an important role. With stei mixed hardness decreases the toughness. That despite the increase in the alloy content, some mixed crystal-forming elements improve toughness is due to the formation of a fine-grained structure total A fine-grained sintered structure is z. B. due to an alloy content of 0.2 to 0.3% vanadium achieved without heat treatment.

The invention is explained below in concrete case and with reference to the single drawing ( Fig. 1) explained in more detail, which shows a process diagram of the wet powder die casting according to the invention.

The method according to the following exemplary embodiments consists of the following process steps:

• 1. Extraction of raw materials
  • 1. 1.1 atomizing the molten metal with high pressure water and producing a metal powder with a particle size range of 1 to 63 microns and low oxygen content
  • 2. 1.2 Separate the metal powder from the atomizing water in a pressure filter
  • 3. 1.3 Mixing the damp metal powder with a water-soluble, environmentally friendly binder, e.g. As phosphoric acid, to a pourable slip with a solids content of 60 to 85 vol.%
• 2. Shape
  • 1. 2.1 Construction of a mold, namely a fully porous plastic tool with mesh-like, open channels, which serve to drain the slurry injected under pressure into the tool
  • 2. 2.2 Die casting with the metal powder slurry, the tool durability is about 20,000 shots
• 3. Compact
  • 1. 3.1 Drying the green compacts
  • 2. 3.2 Sealing of the components under reducing conditions in a continuous furnace

example 1

The liquid molten metal, which in the first embodiment is a corrosive alloy consisting mainly of eggs, is poured into a funnel 1 . The so-called atomizing water is fed from a water tank 2 by means of a high-pressure pump 3 to a flat jet or ring slot nozzle known per se, from which the water emerges in a fine jet under a pressure of 150 to 200 bar. The emerging from the funnel 1 liquid metal jet hits at a predetermined angle, for. B. 20 °, on the water jet and is divided by this into a fine metal powder with grain sizes of less than 63 microns. The pressurized water atomization here leads to powder particles with an irregular surface, which advantageously enables mutual clinging of the powder particles when building up the layer in the cavity of the die casting tool. The atomizing water contains 0.05 to 0.1% by weight phosphoric acid.

The fine metal powder obtained with an oxygen content of less than 0.5% is fed together with the atomizing water via a line 4 to a pressure groove 5 . After filtering off under a pressure of 2 bar, the powder, which has a residual moisture of about 5%, is placed in a mixer 6 . The filtrate is returned to the water tank 2 . The resulting sludge during the current process with grain sizes below 1 micron are withdrawn via a line 7 in order to shorten the drying and sintering time of the green body subsequently produced.

In the mixer 6 is a homogeneous, pourable slurry from 60 to 85 wt .-% metal powder, 35 to 12 wt .-% water and 5 to 1 wt .-% phosphoric acid or metaphosphoric acid or polyphosphoric acid and / or a mixture of these acids or their connections made. During the mixing and homogenization, any existing or newly formed thin oxide layers are pickled under friction in the presence of the aqueous phosphoric acid, further corrosion of the metal powder is prevented and the bright metallic powder surfaces are formed by the very thin iron phosphate layer or other trivalent, insoluble metal passivated phosphates of the corresponding alloys. The viscosity and pH of the casting slip can be adjusted by adding additives.

The homogenized aqueous slip is injected into an open-pore plastic mold 8 made of PMMA under a pressure of up to 50 bar. While the layers of water-atomized metal powder build up on the fully porous tool wall, the open pores with a diameter of about 20 microns or less and a Po volume of 20 vol .-%, the aqueous carrier liquid is separated evenly, so that no separation (segregation , Sedimentation) or compaction (dilation). The separated carrier liquid speed is regenerated in an acidic cation exchanger and returned to the mixer 6 . In the ion exchanger, the iron phosphate contained in the carrier liquid is converted to phosphoric acid. This guarantees environmentally friendly water management. If necessary, as can be seen from FIG. 1, the regeneration can also be omitted.

The green compact is left to a residual moisture of less than 5% dried. This creates between the powder batches phosphoric acid bridges or bridges made of meta phosphates or polyphosphates. The drying the process of the green body runs non-destructively, since the Moisture unhindered by the pores of the plastic form 8 can emerge.

The dried green compact is further dried in a first zone of a continuous furnace 10 , sintered in the middle zone in the presence of hydrogen or endogas (mixture of natural gas and hydrogen) or inert gas and finally cooled in a third zone. During sintering, the oxygen content of the phosphates and of the crystallized phosphoric acid is reduced by carbothermal reduction of the proportion of carbon which is correspondingly higher in the metal matrix. The glass-like iron-phosphor dispersions between the individual powder particles increase the strength of the sintered workpiece.

In summary, the required strength is obtained green parts for the subsequent production steps of demolding, drying and sealing sintering through three effects. For one, they cling to one another irregularly shaped metal powder particles when Layer structure in the cavity of the die casting tool. Second, the strength is due to the capillary Un pressure and the interfacial forces of the residual moisture in the green body's pores increased. Finally form Phosphoric acid and phosphate bridges (or boric acid and borate bridges in the following embodiment game) between the touching powder particles drying and heating the moldings.

Example 2

In the second embodiment, a corro sion-resistant metal Workpiece manufactured. The process runs essentially chen as in the first embodiment. The stripping and passivating agent in the atomizing water reacts with the corrosion-resistant materials (stainless steel len) only imperceptibly with the one occurring in the process Water temperature around 40 ° C. Because a change of Atomizing water for different materials doped atomizing water is also economical in the case of corrosion-resistant materials, preferably ver turns.

Instead of mixing and homogenizing the Schlickers added phosphoric acid or phosphate however, boric acid with a share of about 2 wt .-% used. When sintering the resulting Green bodies separate metal oxides with the ver remained boron trioxide as finely dispersed glass phases in the Base material of the metal matrix.

Example 3

In a modification of embodiment 2, an aqueous mixture of 0.2 to 0.5% by weight of Na ₂ HPO ₄ , about 2% by weight of boric acid and small amounts of additives, for example ammonium alginate, is used as the binder. During the drying process of the casting, most of the boric acid evaporates together with the residual moisture. During sintering, the small amounts of metal oxides still present on the powder particle surfaces react with the remaining, extremely small amount of boron trioxide and sodium phosphate to form a finely dispersed glass phase, which contributes to the solidification of the metal matrix.

Finally, the advantages of the fiction method summarized.

The shaping technology consists of optima len linking of raw material extraction of fine, water gloomy, d. H. irregularly shaped, metallic Powder using the die casting process complicated to manufacture economically, endab measurement-related, metallic components similar to the Her provision of ceramic parts with the help of Phosphoric and / or boric acid or their water-soluble Chen salts or compounds.

The following advantages of the Her invention setting procedures are in the case of ferrous metallic Materials reached.

Water atomized metal powder can be used directly after the Spraying without working up as an aqueous slip with the help of phosphoric acid and / or its Salts as pickling and passivating agents after Die casting processes are processed into molded parts. The very high sintering activity of the fine, ruggedly cooled raw powder for making very dense components close to the final dimension (<95% of the theoretical density).

At the same time, water-soluble phosphoric acid is used compounds (e.g. phosphates, metaphosphates, esters of phosphoric acid, phosphorus proteins) as binders additive, so that when drying and heating the casting do not make a softening hole until the Sin is inserted reaction of the ferrous metallic materials occurs. The phosphates polymerize, and the evaporating Residual moisture escapes without destroying it through the open pores between the powder particles.

A short sintering time is due to the low An partially volatile components, which only from the rest damp and possible reduction products of Phosphates with the carbon content from the alloy of the base material or the phosphorus proteins stand.

He on the surface of the metal powder particles  produced thin layer of iron polyphosphate or iron metaphosphate and / or which form when heated the alkali phosphate glasses or phosphorus iron alloy stanchions have a sinter activating effect. The developing ones Sintered bridges get through the diffusing Phosphorus solidifies and the pores become good rounded.

Due to the very thin and extremely even phosphate deposited on the metal particle surfaces layers, the sintering shrinkage is isotropic. The Dimensional accuracy is therefore guaranteed.

The very high sintering activity of the ferritic Si and / or P alloys is used. The relatively small Sintering temperatures and times compared to the conventional ones tional powder metal materials improve Economic efficiency and at the same time condense to sin terenden components very well, so that the dynamic Material properties can be increased. Through the The use of melt-alloyed powder becomes homogeneous guaranteed of the material.

The oxide coating of the metal particle surfaces forms finely dispersing glass with the phosphate salts and contributes to the solidification of the material.

The procedure of the entire manufacturing process It is easy and safe to grasp process parameters. Within the process chain Linkage between raw material extraction, Schlic core preparation, shaping and hardening a change for the optimizable control parameters interaction. It can be done easily within the process can be intervened to a desired one To achieve effect.

In the case of corrosion-resistant, ferrous metal Materials have the following advantages.

Water atomized fine metal powder can directly after spraying without working up as an aqueous Slips with the help of boric acid and dina trium hydrogen phosphate as a cleaning and adhesive agent processed into molded parts using the die casting process be tested.

The boric acid-phosphate mixture and its polymerized products serve in conjunction with NH ₃ alginate or phosphorus proteids as binders, the boric acid largely evaporating with the water vapors when heated.

The sintering time is short due to the low proportion volatile constituents that only consist of residual moisture, proportions of boric acid and additives, and due to the high Sintering activity of the fine, ruggedly cooled raw powder ver with grain sizes below 63 µm.

The one with the oxidic impurities on the Finely dispersed boron particles or phosphate glass phase contributes through their contact contribute to the dimensional accuracy and also has a disperous effect sion-hardening on the metal matrix.

Further advantages result from the fact that requirements Component properties that meet the requirements of the combination nation of phosphoric acid in the atomizing water Boric acid or with other peptizers in the aqueous Cast slurry can be adjusted. Concentra tion, exposure time and temperature of acids and inhi bitters influence the "layer thickness" on the powder particle surfaces in the casting, whereby the ge desired material properties during sintering controls can be generated.

Reference list

1

funnel

2nd

Water tank

3rd

high pressure pump

4th

management

5

Pressure filter

6

mixer

7

management

8th

Plastic mold

9

Ion exchanger

10th

Continuous furnace

Claims (11)

1. A process for the powder-metallurgical manufacture of objects or semi-finished products, wherein a slurry of a metal powder or metal alloy powder with a melting point above 1000 ° C and a binder is put under pressure into a mold and the molded body obtained is sintered, characterized in that binders are used which react with the metal or metal alloy powder and / or with its oxide layer to form a glass. 2. The method according to claim 1, characterized, that as a metal or metal alloy powder with water atomized, from a corrosive, ferrous material existing powder is used, the atomization water contains a stripping and passivating agent. 3. The method according to claim 2, characterized, that the stripping and passivating agent phosphoric acid, ins special metaphosphoric acid, and / or water-soluble ver contains bonds of this acid. 4. The method according to claim 3, characterized, that the atomizing water 0.05 to 5 wt .-%, in particular 0.05 to 0.1% by weight of phosphoric acid and / or its contains water-soluble compounds. 5. The method according to any one of claims 2 to 4, characterized, that the atomizing water is circulated. 6. The method according to any one of the preceding claims,  characterized, that phosphoric acid and / or phosphoric acid compounds as Binders are used. 7. The method as claimed in one of the preceding claims, characterized, that boric acid and / or boric acid compounds as binders be used. 8. The method according to the preceding claim, characterized, that the binder additionally phosphoric acid and / or Contains phosphoric acid compounds. 9. The method according to any one of the preceding claims, characterized, that the slurry into an open-pored art existing form there. 10. The method according to the preceding claim, characterized, that the form is essentially polymethyl methacrylate consists. 11. Objects or semi-finished products manufactured by powder metallurgy, especially from a ferrous material, marked by Phosphate and / or borate glasses on the powder particles surface and between the powder particles.