DE19535444A1 - Prodn. of metal (alloy) powder from corroding material - Google Patents

Abstract

Prodn. of metal (alloy) powders from corroding Fe-contg. material having a m.pt. of more than 1000 deg C comprises sputtering a metal (alloy) melt using pressurised water contg. pickling and passivating agent. Also claimed is a powder metallurgical process for the mfr. of objects or semi-finished produces using the metal (alloy) powders.

Description

The invention is concerned with powder metallurgy Chemical manufacture of objects or semi-finished products of wet powder die casting, which is a variant of the known Represents slip casting process and in more detail below is explained.

In powder metallurgy, which is close to the final dimensions economic production of serial parts are made possible Materials and workpieces made of powdered metal poses. There are essentially three Steps. First, the metal powder is made in brought a certain shape and eventually increased Sintered temperatures to compress the material.

The above wet powder die casting is from various the known shaping techniques have been developed:

During die casting, liquid non-ferrous alloys (tin, Lead, aluminum and magnesium alloys) under high Shed pressure in permanent molds. The solidification of the The melt takes place quickly. Due to the high pressure too the finest details are molded. The high cost of dhow Erform enable economical die casting in the Usually only if the number of items is high enough.

The metal powder injection molding process is based on plastic processing for shaping more complicated Small parts used with large numbers. It is one Combination of 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 process (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 induction 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 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 pressure-free Schickergießverfahren was 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 casting compound used are adjusted can.

On the one hand, the invention relates to a method for manufacturing exhibit metal powders or metal alloy powders a corrosive, ferrous material with a Melting point above 1000 ° C by atomizing a metal melt or a metal alloy melt by means of pressure water atomization. It is the first Step in powder metallurgy manufacturing counter stood according to the invention.

In pressurized water atomization for the production of metal powder, with this term also always metal alloy powder are understood, a beam hits the Molten metal on a high pressure from a nozzle escaping water jet and is of the finest Particles disassembled. The one obtained is advantageous rough, jagged (scratchy) surface of the powder batches kel, the strength of the subsequently manufactured Grünlings increased. In addition to the fragmentation of the molten metal however, it also finds an oxidation of the freshly produced fine metallic particles due to a reaction of 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 pulp ver would become components with an insufficient Lead strength. To solve this problem it is knows to feed inert gas into the atomization chamber.

With this measure, the oxygen content of the raw pulp ver are significantly reduced, but it is still too high to directly transfer the raw powder to the powder metallurgy to be able to use chemical production. Therefore, the Raw powder 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 relevant in connection with the invention The iron-phosphorus alloys are brief here called. The two-substance system Fe-P shows that phosphorus γ region severely constricted. In the temperature range of the applied sintering from 1000 ° C to about 1250 ° C exists for iron-phosphorus alloys with 0.2 to 0.5 wt .-% Phos phor the α-γ mixed crystal region. In the range of 0.5 to 2.8% by weight of phosphorus is found in the pure α phase. (Hansen and Anderko, Constitution of binary Alloys, New York, Toronto, London 1958, McGraw-Hill). In the α phase is the rate of diffusion by a power of ten larger than in the γ phase, so that the sintering in the u region runs much faster than in the γ area of the usual nonferritic iron alloys (Böhm, introduction to Metallkunde p. 88, BI university pocketbooks no. 196 / 196a). The one in the polymerization of phosphoric acid  or phosphates on the iron particle surface below reducing conditions released phosphorus reacts with the iron base material to intermediate Fe₃P-, Fe₂P- FeP and FeP₂ alloys. The partially occurring temporary liquid phase also causes a very high one Sinter activity and thereby rounds the existing pores clear away. This effect reduces that from the pores notch effect and improves the strength of the sin terwerkstoff (Schatt, powder metallurgy, sintered and Composite materials, p. 150 ff, Hühig Verlag, Heidelberg). In addition, the material hardening of the α-Fe by Phosphorus is particularly effective (horn bow, high strength 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 H₃BO₃ 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 (HBO₂) _n , which forms boron trioxide B₂O₃ on further heating, which easily dissolves metal oxides under glass formation (Trzebiatowiski, textbook on inorganic chemistry; VEB German Publishing House of Sciences, Berlin 1970).

The invention is based, to the immit the task Can be used in powder metallurgical manufacturing process suitable metal powder, in particular from a to provide corrosive ferrous material, which is no longer treated after the powder production are needed, the metal powder by pressurized water atomization of the corresponding molten metal becomes.

To this end, the inventor suggests that the atomization water contains a stripping and passivating agent.  

Stripping and passivating agent for removing thin oxide layers and application of thin conversion layers on iron and steel parts are known as such has already been stated above.

According to the invention, however, these agents are already during the production of the metal powder used so that one low-oxygen powder particles with thin conversions layers, i.e. receives corrosion protection layers. The Powders can be used immediately without further treatment the powder metallurgical manufacture of objects and Semi-finished products are used.

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 polyphosphates.

It is also proposed that the atomizing water 0.05 to 5% by weight, in particular 0.05 to 0.1% by weight Phosphoric acid and / or its water-soluble compounds contains.

It is also advantageous if the Ver jet water in the circuit.

The object of the present invention is also a sufficient strength of the green body throughout to ensure ten heating process, so that in contrast to the state of the art with wet powder die casting too large components with undercuts, inner contours and other complicated shapes can be produced.

This object is achieved in that Binders are used that are associated with the metal or Metal alloy powder and / or with its oxide layer react to form a glass.  

In contrast to the prior art, there is no mention the binder almost strikes during the heating process completely removed from the green body. Rather is the presence of the binder even at higher ones Temperatures desired to achieve a sufficiently high strength speed of the green body in every phase of the heating process ensure.

Be preferred, especially in the case of corrody iron-containing powders, phosphoric acid and / or Phosphoric acid compounds used as binders. On such a binder has the additional function of a Corrosion protection layer.

In a further embodiment of the invention is pre suggest that you use boric acid and / or boric acid compounds used as a binder, especially in the case of corro sion-resistant iron-containing powders. In this case it is also advantageous if the binder additional Lich phosphoric acid and / or phosphoric acid compounds contains. The formed, disperse glass phase as the desired one "Alloy component" of the metallic material leaves In this case, modes change to a particularly large extent fection. So there are more degrees of freedom when off choice of composition available.

Part of the binder can be used in the manufacture of the metal powder are added. For this is pre suggest that the metal or metal alloy powder by atomizing a molten metal or metal alloy production melt using pressurized water atomization, wherein the atomizing water is phosphoric acid, in particular Metaphosphoric acid, and / or water-soluble compounds this contains acid. In this case, the addition serves as has already been carried out above, at the same time as the Ent far from oxide layers on the powder and the pass four of the bare metallic powder surfaces.  

In a further preferred embodiment of the invention it is suggested that you turn the slurry into one existing porous plastic shape. In contrast to the state of the art in which the shape is generally made up of If there is gypsum, the plastic mold can be used several times will. In particular, it is proposed that the form in consists essentially of polymethyl methacrylate (PMMA). In this plastic containing styrene, which during removed from the mold during manufacture, enable the porosity.

In summary, there are the following advantages the mold made of open-pore plastic compared to casting with lost mold, for example one Plaster mold. The ongoing effort for molding and for that No core production. Waste-free production with very high metallic output is possible. also in addition to the absence of cavities, good dimensional accuracy as well as surface quality achieved because an exact Filling the mold is guaranteed. A careful one Filling the mold is possible because cold pouring the mold can be loaded relatively slowly. In the opposite The set must be when casting from the molten metal beginning to solidify.

Finally, the invention also includes powder metallurgy manufactured objects or semi-finished products, in particular from an iron-containing material with a melting point above 1000 ° C, which by phosphate and / or borate glasses at the Powder particle surface and between the powder particles Marked are.

Particularly suitable for the production of highly stressed components such as connecting rods or diesel pistons according to the invention Processes are ferritic materials, preferably based on the alloy base silicon and phosphorus. The one here Targetable high sintering activity enables a very high Density and homogeneity of the workpiece. The excellent  Suitability of these alloys for processing in the invent The method according to the invention is briefly mentioned in the following ten special material properties.

The alloying elements silicon and phosphorus also form the α-iron substitution mixed crystals and effect one linear increase in yield strength and tensile strength. Phos phor shows the highest effectiveness on strength, has but a very brittle influence. In addition to the festig properties play the toughness properties an important role. With increasing mixed crystal hardness the toughness decreases. That despite increasing the Alloy content of some mixed crystal elements toughness is improved due to the formation of a fine-grained structure. A fine-grained sintered material add z. B. by an alloy content of 0.2 to 0.3% vanadium achieved without heat treatment.

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

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

• 1 raw material extraction
• 1.1 atomizing the molten metal with high pressure water and Generate a metal powder with a particle size spec range from 1 to 63 µm and low oxygen content
• 1.2 Separate the metal powder from the atomizing water in a pressure nutsche
• 1.3 Mix the damp metal powder with water soluble, environmentally friendly binder, e.g. B. phosphoric acid, to a pourable slurry with a solids content from 60 to 85 vol .-%  
• 2 shaping
• 2.1 Building a mold, namely a fully porous plastic tool with net-like, open channels that are used for ent watering the injected into the mold under pressure Serve slip
• 2.2 Die casting with the metal powder slurry, the Tool durability is about 20,000 shots
• 3 compaction
• 3.1 Drying the green compacts
• 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 iron, 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 liquid metal jet emerging from the funnel 1 hits at a predetermined angle, for. B. 20 °, on the water jet and is divided by the sem into a fine metal powder with grain sizes of less than 63 microns ger. The pressurized water atomization leads here to powder particles with an irregular upper 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 wt .-% phosphoric acid.

The with an oxygen content of less than 0.5% he kept fine metal powder is fed together with the Ver jetting water via a line 4 to a pressure filter 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 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 , a homogeneous, pourable slip of 60 to 85% by weight of metal powder, 35 to 12% by weight of water and 5 to 1% by weight of 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 abge stained with friction in the presence of the aqueous phosphoric acid, further corrosion of the metal powder is prevented and the shiny metallic powder surfaces are formed by the very thin iron phosphate layer or other trivalent, insoluble metal phosphates passivated the 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 what server-smeared metal powder build up on the fully porous tool wall, the open pores with a diameter of about 20 microns or less and a pore volume of 20 vol .-%, the aqueous carrier liquid is separated evenly, so that no segregation (segregation, sedimentation ) or compression (dilation). The separated carrier liquid 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 body is dried to a residual moisture of less than 5%. Phosphoric acid bridges or bridges of metaphosphates or polyphosphates are formed between the powder particles. The drying process of the green body runs non-destructively since the moisture can escape through the pores of the plastic mold 8 unhindered.

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

In summary, the required strength arises the green parts for the subsequent production steps of the De-molding, drying and internal sealing by three effects on. On the one hand, the irregularly shaped ones cling to one another Metal powder particles in the layer structure in the cavity of the Die casting tool. Second, the strength is through the capillary negative pressure and the interfacial forces of the Residual moisture in the pores of the green body increased. Finally form phosphoric acid and phosphate bridges (or boron acid and borate bridges in the following execution game) between the touching powder particles at Drying and heating the moldings.  

Example 2

In the second embodiment, one is made from corrosion constant metal existing sintered workpiece poses. The procedure is essentially the same as in the first Embodiment from. The stripping and passivation Medium in the atomizing water reacts with the corrosion resistant materials (stainless steels) only imperceptibly the water temperature in the process of about 40 ° C. Because changing the atomizing water for different materials is uneconomical, is endowed Atomizing water even with a corrosion-resistant plant preferred substances.

Instead of mixing and homogenizing the Schlickers added phosphoric acid or phosphate content however, boric acid with a share of about 2 % By weight used. When the resulting green body is sintered metal oxides separate with the remaining boron trioxide as finely dispersed glass phases in the base material of the Metal matrix.

Example 3

In a modification of the embodiment 2 is used as a binder an aqueous mixture of 0.2 to 0.5 wt .-% Na₂HPO₄, about 2 wt .-% boric acid and small amounts of additives, for Example ammonium alginate used. During the drying process The casting of the casting evaporates most of the Boric acid together with the residual moisture. When sintering rea greed the metal still present in small proportions oxide on the powder particle surfaces with the remaining extremely small amount of boron trioxide and sodium phosphate to a finely dispersed glass phase, which for Solidification of the metal matrix contributes.  

In conclusion, the advantages of the invention Procedure summarized.

The shaping technology consists of the optimal ver linking the raw material extraction of fine, water-atomized, d. H. irregularly shaped, metallic powder with the Shaping process die casting for economical Manufacture of complex, near-dimensional metal lical components analogous to the production of ceramic parts with the help of phosphoric and / or boric acid or their water-soluble salts or compounds.

The following advantages of the manufacturer according to the invention driving in the case of ferrous metal materials enough.

Water atomized metal powder can be used directly after the ver spraying without working up as an aqueous slip under With the help of phosphoric acid and / or its salts as Stripping and passivating agent after the die casting process be processed into molded parts. The remains very high sintering activity of the fine, ruggedly cooled Raw powder for the production of very dense, close to final dimensions Components (<95% of theoretical density) received.

At the same time, water-soluble phosphoric acid compounds serve gene (e.g. phosphates, metaphosphates, esters of phosphorus acid, phosphorus protein) as a binder additive, so that no softening when drying and heating the castings hole until the sintering reaction of the iron begins metallic materials occurs. Polymer phosphates sieren, and the evaporating residual moisture escapes without destructive effect through the open pores between the powder particles.

A short sintering time is due to the small proportion volatile components achieved only from residual moisture and possible reduction products of the phosphates with the  Carbon content from the alloy of the base material or the phosphorus proteins.

The one created on the surface of the metal powder particles Thin layer of iron polyphosphate or iron metaphosphate and / or the alkali phosphate formed on heating Glasses or phosphorus-iron alloys have a sintering effect activating. The sinter bridges that form are preserved solidification through the diffusion of phosphorus, and the pores are well rounded.

Due to the very thin and extremely even on the Layers of phosphate deposited on metal particle surfaces the sintering shrinkage is isotropic. The dimensional accuracy is therefore guaranteed.

The very high sintering activity of the ferritic Si and / or P alloys are used. The relatively small Sintering temperatures and times compared to conventional ones len powder metal materials improve the economy and at the same time condense those to be sintered Components very well, so that the dynamic material properties can be increased. By using it melt-alloyed powder becomes the homogeneity of the work ensured.

The oxide coating of the metal particle surfaces forms with the Phosphate salts fine dispersing glass and contributes to Solidification of the material at.

The procedure of the entire manufacturing process is through the easy and safe to grasp manufacturing para meters process capable. Within the process chain link between raw material extraction, slurry processing, molding Practice and consolidation exist for the optimizable tax an interaction. It can be done easily intervene within the process chain in order to achieve a desired effect.  

In the case of corrosion-resistant, ferrous metal work The following advantages are achieved.

Water atomized fine metal powder can be used directly after the Spraying without working up as an aqueous slip under With the help of boric acid and disodium hydrogen phosphate as a cleaning and adhesive agent after the die casting process processed into molded parts.

The boric acid-phosphate mixture and its polymerized pro Products are used in conjunction with NH₃-alginate or phosphorus Proteins as binders, the boric acid being zen largely evaporated with the water vapors.

The sintering time is short due to the low proportion components that only consist of residual moisture, proportions of Boric acid and additives exist, and due to the high Sin interactivity of the fine, ruggedly cooled raw powder with Grain sizes below 63 µm.

The one with the oxide impurities on the particles Finely dispersed boron or phosphate surfaces glasphase contributes to the Maßhal through its contact points activity and also has a dispersion-strengthening effect on the metal matrix.

Further advantages result from the fact that requirements right component properties by the combination of Phosphoric acid in the atomizing water with boric acid or with other peptizers in the aqueous casting slip can be. Concentration, exposure time and temperature of acids and inhibitors affect the "Layer thickness" on the powder particle surfaces in the casting ling, whereby the desired material properties at Sintering can be generated in a controlled manner.

Reference list

1 funnel
2 water tank
3 high pressure pump
4 line
5 pressure filters
6 mixers
7 line
8 plastic mold
9 ion exchangers
10 continuous furnace

Claims (12)

1. A method for producing metal powders or metal alloy powders from 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, characterized in that the atomization water contains a stripping and passivating agent. 2. The method according to claim 1, characterized, that the stripping and passivating agent phosphorus acid, especially metaphosphoric acid, and / or what Contains soluble compounds of this acid. 3. The method according to the preceding claim, characterized, that the atomizing water 0.05 to 5 wt .-%, in particular the other 0.05 to 0.1 wt .-%, of phosphoric acid and / or contains their water-soluble compounds. 4. The method according to any one of the preceding claims, characterized, that the atomizing water is circulated. 5. Process for the powder metallurgical production of Objects or semi-finished products, taking a slurry tion from a metal powder or metal alloy powder ver with a melting point above 1000 ° C and a Binder under pressure in a mold and he holding molded body sinters, characterized,  that binders are used that with the Metal or metal alloy powder and / or with the react oxide layer to form a glass. 6. The method according to the preceding claim, characterized, that especially in the case of corrosive, iron containing powders phosphoric acid and / or phosphoric acid compounds are used as binders. 7. The method according to claim 5 or 6, characterized, that especially in the case of corrosion resistant ferrous powders boric acid and / or boric acid ver bindings are used as binders. 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 claims 5 to 8, characterized, that by the metal or metal alloy powder Atomizing a molten metal or metal alloy melt using pressurized water atomization where in the atomizing water phosphoric acid, in particular Metaphosphoric acid, and / or water soluble compounds contains this acid. 10. The method according to any one of claims 5 to 9, characterized, that you put the slurry in an open pore Plastic existing form there. 11. The method according to the preceding claim, characterized,  that the form essentially of polymethyl meth acrylate (PMMA) exists. 12. Powder metallurgically manufactured objects or Semi-finished products, in particular from an iron-containing plant substance with a melting point above 1000 ° C, marked by Phosphate and / or borate glasses on the powder particles surface and between the powder particles.