Patent Application
20090123690
The invention relates to blends of metal, alloy or composite powders having a mean particle diameter D50 of no more than 75, preferably of no more than 25 μm, produced according to a process in which a base powder is first transformed into flake-like particles and these are then crushed with further additives in the presence of milling auxiliary agents and also the use of these powder blends and moulded objects produced from them.
From the patent application PCT/EP/2004/00736, not yet laid open for public inspection, powders are known that can be obtained by a process for the production of metal, alloy and composite powders having a mean particle diameter D50 of no more than 75, preferably of no more than 25 μm, measured with a Microtrac® X 100 particle size analyser according to ASTM C 1070-01, from a base powder with a larger mean particle diameter, the particles of the base powder being processed in a deformation step into flake-like particles having a ratio of particle diameter to particle thickness of 10:1 to 10000:1 and these flake-like particles being subjected in a further process step to pulverisation or to a high-energy load in the presence of a milling auxiliary agent. This process is advantageously followed by a de-agglomeration step. This de-agglomeration step, in which the powder agglomerates are broken down into their primary particles, can be carried out for example in a gas counter-current mill, an ultrasound bath, a kneader or a rotor-stator. In this specification such powders are called PZD powders.
These PZD powders have various advantages over conventional metal, alloy and/or composite powders used for powder metallurgy applications, such as improved green strength, compressibility, sintering behaviour, a wider sintering temperature range and/or a lower sintering temperature, but also better strength, oxidation and corrosion behaviour of the moulded parts produced and lower production costs. A disadvantage of these powders is, for example, poorer flowability. The changed contraction characteristics combined with the lower tap density may cause problems during powder-metallurgic processing as a result of greater sintering contraction. These characteristics of the powders are disclosed in PCT/EP/2004/00736, to which reference is made.
Conventional powders, obtained for example by atomisation of metal melts, also have disadvantages. For certain alloy compositions, known as high-alloy materials, in particular, these are lack of sintering activity, poor compressibility and high production costs. These disadvantages are less significant in particular for metal injection moulding (MIM), slip casting, wet-spraying and thermal spray coating. As a result of the poorer green strength of conventional metal powders (in the sense of metal, alloy and composite powders, abbreviated to MLV) these materials are unsuitable for conventional powder-metallurgic compression, for powder rolling and for cold isostatic pressing (CIP) with subsequent green processing, as the green compacts do not have sufficient strength for this.
The object of the present invention is to provide metal powders for powder metallurgy, which do not have the above-mentioned disadvantages of conventional metal powders (MLV) and the PZD powders, but combine to the greatest possible extent their respective advantages, such as high sintering activity, good pressability, high green strength, good pourability.
A further object of the present invention is to provide powders containing functional additives, which can provide the moulded objects produced from PZD powders with characteristic properties, such as for example additives that increase the impact strength or abrasion resistance, such as superhard powders, or additives that facilitate the working of the green compacts, or additives that function as templates to control the pore structure.
A further object of the present invention is to provide high-alloy powders for the whole spectrum of powder-metallurgic moulding processes, so that applications in fields that are not accessible to conventional metal, alloy or composite powders, are also possible.
This object is achieved by metallic powder blends containing a Component I, a metal, alloy and composite powder having a mean particle diameter D50 of no more than 75, preferably of no more than 25 μm, or 25 μm to 75 μm, measured with a Microtrac® X100 particle size analyser in accordance with ASTM C 1070-01, which can be obtained by a process in which the particles of a base powder with a larger or smaller mean particle diameter are processed in a deformation step to flake-like particles having a ratio of particle diameter to particle thickness of 10:1 to 10000:1 and these flake-like particles are subjected in a further process step to pulverisation in the presence of a milling auxiliary agent, a Component II, which is a conventional metal powder (MLV) for powder metallurgy applications, and/or a Component III, which is a functional additive. The steps of flake production and pulverisation can be combined directly, by carrying out one immediately after the other in the same unit under conditions adapted to the particular objective (flake formation, crushing).
This objective is also achieved by metallic powder blends containing a Component I, a metal, alloy and composite powder, the contraction of which, measured with a dilatometer according to DIN 51045-1, up to the temperature of the first contraction maximum, is at least 1.05 times the contraction of a metal, alloy or composite powder of the same chemical composition and the same mean particle diameter D50, produced by atomisation, the powder to be investigated being compacted to a pressed density of 50% of theoretical density before contraction is measured, a Component II, which is a conventional metal powder (MLV) for powder metallurgy applications and/or a Component III, which is a functional additive. Where a compact that can be handled cannot be produced from conventional powders of the desired density (50%), greater densities are also permissible, for example, by using pressing auxiliary agents. However this should be understood to mean the same ‘metallic density’ of the powder pressed bodies and not the average density of the MLV powder and pressing auxiliary agent.
The use of Component 1 also makes it possible to produce metallic powder blends in which the contents of oxygen, nitrogen, carbon, boron, silicon can be precisely set. If oxygen or nitrogen enter the process, the high energy input can lead to the formation of oxide and/or nitride phases during the production of Component I. Such phases may be desirable for certain applications, as they may have a significant material-strengthening effect. This effect is known as the Oxide Dispersion Strengthening Effect (ODS). However, the incorporation of such phases is often associated with a deterioration in processing properties (for example compressibility, sintering activity). As a result of the generally inert properties of the dispersoids towards the alloy component, the latter may thus inhibit sintering.
Pulverising immediately distributes the phases referred to finely in the powder produced. The phases formed (e.g. oxides, nitrides, carbides, borides) are therefore distributed considerably more finely and homogeneously in Component I than in conventionally-produced powders. This again leads to increased sintering activity in comparison with phases of the same type incorporated discretely. This improves the sinterability of the metallic powder blends according to the invention. Such powders with finely-dispersed intercalations can be obtained in particular by precise introduction of oxygen during the milling process and lead to the formation of very finely-distributed oxides. Specific use of milling auxiliary agents, which are suitable as ODS particles and undergo mechanical homogenisation and dispersal during the milling process, is also possible.
The metallic powder blend according to the present invention is suitable for use in all powder-metallurgic moulding processes. Powder-metallurgic moulding processes according to the invention are pressing, sintering, slip casting, sheet moulding, wet-spraying, powder rolling (either cold, hot or warm rolling), hot pressing and hot isostatic pressing (HIP), sinter-HIP, powder charge sintering, cold isostatic pressing (CIP), in particular with green processing, thermal spraying and deposit welding.
The use of the metallic powder blends in powder-metallurgic moulding processes leads to significant difference in the processing, the physical and material properties and allows the production of moulded objects, which have improved properties, although the chemical composition is comparable or identical to that of conventional metal powders. The presence of Component II allows precise ‘tuning’ of component properties such as high-temperature strength, strength, toughness, wear-resistance, oxidation resistance or porosity.
Pure, thermal spray powders can also be used as a repair solution for components. The use of pure agglomerated/sintered powders according to the patent application PCT/EP/2004/00736, not yet laid open for inspection, as a thermal spray powder allows the characteristic coating of components with a surface layer that has better abrasion and corrosion behaviour than the base material. These properties result from very finely-distributed ceramic intercalations (oxides of elements having an affinity with oxygen) in the alloy matrix resulting from mechanical loading during production of the powders according to PCT/EP/2004/00736.
Component I is a metal, alloy, and composite powder, which can be obtained by a two-stage process, in which a base powder is first transformed into flake-like particles and these are then crushed in the presence of milling auxiliary agents. In particular, Component I is a metal, alloy and composite powder having a mean particle diameter D50 of no more than 75, preferably of no more than 25 μm, measured with the Microtrac® X100 particle size analyser according to ASTM C 1070-01, which can be obtained by a process in which, are obtainable from a base powder with a larger mean particle size, the particles of the base powder being processed in a deformation step to flake-like particles having a ratio of particle diameter to particle thickness of 10:1 to 10000:1 and these flake-like particles being subjected in a further process stage to pulverisation in the presence of a milling auxiliary agent.
The particle size analyser Microtrac® X100 is commercially available from Honeywell, USA.
To measure the ratio of particle diameter to particle thickness, the particle diameter and the particle thickness are measured by photo-optic microscopy. For this purpose, the flake-like powder particles are first mixed with a viscous, transparent epoxy resin in a ratio of 2 parts by volume of resin to 1 part by volume of flakes. The air bubbles incorporated when mixing are then removed by evacuation of the mixture. The now bubble-free mixture is then poured onto a level substrate and rolled out into a wide sheet with a roller. In this way, the flake-Like particles align themselves preferably in the field of flow between the roller and the substrate. The preferred layer is characterised in that the normal line to the surface of the flakes is on average aligned parallel to the normal line to the surface of the level substrate, in other words, the flakes are on average arranged flat on the substrate in layers. After hardening, samples of suitable dimensions are worked out of the epoxy resin sheet lying on the substrate. These samples are studied with a microscope vertically and parallel to the substrate. Using a microscope with a calibrated lens and taking account of adequate particle orientation, at least 50 particles are measured and a mean value is produced from the measured values. This mean value represents the particle diameter of the flake-like particles. The particle thicknesses are measured on a vertical section through the substrate and the sample to be analysed using the microscope with a calibrated lens, which was also used to measure the particle diameter. Care should be taken to ensure that only particles lying as nearly parallel as possible to the substrate are measured. As the particles are coated on all sides in the transparent resin, it is not difficult to select suitably-orientated particles and to assign reliably the limits of the particles to be evaluated. Once again, at least 50 particles are measured and a mean value is produced from the measured values. This mean value represents the particle thickness of the flake-like particle. The ratio of particle diameter to particle thickness can be calculated from the dimensions measured before.
Fine, ductile metal, alloy or composite powders in particular can be produced by this process. Ductile metal, alloy or composite powders are understood to mean those powders that, when mechanically loaded to breaking point, undergo plastic elongation or deformation before significant material damage (embrittlement of the material, breakage of the material) occurs. Such plastic material changes are dependent on the material and can range from 0.1 percent to several hundred percent, in relation to the initial length.
The degree of ductility, i.e. the ability of materials to achieve plastic i.e. lasting deformation under the influence of mechanical strain, can be measured or described by means of mechanical tensile and/or pressure tests.
To measure the degree of ductility by means of a mechanical tensile test, a so-called tensile test specimen is produced from the material to be evaluated. The specimen can be e.g. a cylindrical specimen the diameter of which is reduced by ca 30-50% centrally along its length, over a length of ca 30-50% of the overall length of the specimen. The tensile test specimen is loaded into the clamping device of an electro-mechanical or electro-hydraulic tensile test machine. Before actual mechanical testing, length measurement sensors are placed in the middle of the specimen over a measuring length amounting to ca 10% of the overall length of the specimen. These measuring sensors make it possible to track the increase in length over the selected measurement length whilst applying a mechanical tensile strain. The strain is increased until the specimen breaks, and the plastic portion of the length change is evaluated with the aid of the stress-strain chart. Materials, which in such an arrangement achieve a plastic length change of at least 0.1%, are described as ductile according to this specification.
Similarly, it is also possible to subject a cylindrical material specimen, which has a ratio of diameter to thickness of ca. 3:1, to a mechanical pressure load in a commercially available pressure testing machine. After exerting sufficient mechanical compressive strain, the cylindrical specimen also undergoes permanent deformation. Once the pressure has been released and the specimen removed, it can be seen that the ratio of diameter to thickness has increased. Materials, which achieve a plastic change of at least 0.1% in such a test are also described as ductile according to this specification.
Fine, ductile alloy powders having a degree of ductility of at least 5% are preferably produced according to the process.
The crushability of alloy or metal powders which, per se, cannot be further crushed, is improved by the use of mechanically, mechano-chemically and/or chemically active milling auxiliary agents, which are added precisely or are produced in the milling process. A fundamental aspect of this approach is that the chemical ‘target composition’ of the powder thus produced should not be changed overall, or even should be influenced in such a way that the processing properties, such as e.g. sintering behaviour or flowability are improved.
The process is suitable for the production of a wide variety of fine metal, alloy or composite powders having a mean particle diameter D50 of no more than 75, preferably of no more than 25 μm.
The metal, alloy and composite powders produced are characterised conventionally by a small mean particle diameter D50. The mean particle diameter is preferably no more than 15 μm, measured according to ASTM C 1070-01 (measuring device: Microtrac® X100). For the purpose of improving product properties for which fine alloy powders tend to be unfavourable (porous structures, with which a certain material thickness can better withstand oxidation/corrosion in their sintered state), it is also possible to set significantly higher D50 values (25 to 300 μm) than are mostly attempted, whilst maintaining the improved processing properties (pressing, sintering).
As a base powder, powders can be used for example, that already have the composition of the desired metal, alloy or composite powder. However, it is also possible to use a mixture of several base powders in the process, which produce the desired composition only through the choice of a suitable mix ratio. In addition, the composition of the metal, alloy or composite powder produced can be influenced also by the choice of milling auxiliary agent, where this remains in the product.
Powders with spherical or irregularly shaped particles and a mean particle diameter D50, measured according to ASTM C 1070-01 normally of greater than 75 μm, in particular greater than 25 μm, preferably of 30 to 2000 μm or of 30 to 1000 μm, or of 75 μm to 2000 μm or 75 μm to 1000 μm, or 30 μm to 150 μm, are preferably used as a base powder.
The required base powders can be obtained for example by atomisation of metal melts and, if necessary, subsequent classification or sieving.
The base powder is first subjected to a deformation step. The deformation step can be carried out in known devices, for example in a rolling mill, a Hametag mill, a high-energy mill or an attritor or agitated ball mill. By selecting suitable process parameters, in particular by the effect of mechanical strains that are sufficient to achieve plastic deformation of the material or the powder particle, the individual particles are transformed, so that they finally take the form of flakes, the thickness of which is preferably 1 to 20 μm. This can take place for example by loading once in a roller or hammer mill, by loading several times in ‘small’ deformation steps, for example by impact milling in a Hametag Mill or a SimoIoyer®, or by a combination of impact and abrasive milling, for example in an attritor or a ball mill. The high material loading during this transformation produces structural changes and/or material embrittlement, which can be utilised in the following step to crush the material.
Known melt-metallurgy rapid-setting processes can also be used to produce ribbons or flakes. Like the mechanically-produced flakes, these are then suitable for the crushing process as described below.
The device in which the deformation step is carried out, the milling media and the other milling conditions are preferably selected in such a way that the impurities resulting from abrasion and/or reactions with oxygen or nitrogen are kept at the lowest possible level and lie below the level critical for the application of the product, or within the specification applying to the material.
This can be achieved, for example, by a suitable choice of material for the milling vessel and milling medium, and/or the use of oxidation and nitridation-inhibiting gases and/or the addition of protective solvents during the deformation step.
In a particular embodiment of the process, the flake-like particles are produced directly from the melt in a rapid-setting step, e.g. by so-called melt spinning, by cooling on or between one or more, preferably cooled, rollers so that flakes form immediately.
The flake-like particles formed in the deformation step are subjected to crushing. This changes first the ratio of particle diameter to particle thickness, primary particles (to be obtained by de-agglomeration) being generally obtained with a ratio of particle diameter to particle thickness of 1:1 to 100:1, preferably 1:1 to 10:1. Secondly, the desired mean particle diameter of no more than 75, preferably of no more than 25 μm is set without again producing particle agglomerates that are difficult to crush.
Crushing can be carried out, for example, in a mill, such as an excentric vibrating mill, but also in material bed roller mills, extruders or similar devices, which effect material destruction in the flakes by means of differing movement and loading speeds.
Crushing is carried out in the presence of a milling auxiliary agent. Liquid milling auxiliary agents, waxes and/or brittle powders, for example, can be added as milling auxiliary agents. The milling auxiliary agents may have a mechanical, chemical or mechano-chemical action.
The milling auxiliary agent can be, for example, paraffin oil, paraffin wax, metal powder, alloy powder, metal sulfides, metal salts, salts of organic acids and/or hard material powders.
Brittle powders or phases act as mechanical milling auxiliary agents and can be used for example in the form of alloy, element, hard material, carbide, silicide, oxide, boride, nitride or salt powders. Pre-crushed element and/or alloy powders are used, for example, which, together with the base powder used, which is not readily-crushable, produce the desired compositions in the product powder.
Powders which consist of binary, ternary and/or higher compositions of the elements A, B, C and/or D present in the base alloy to be used, are preferably used as brittle powders, wherein A, B, C and D have the meaning given further below.
Liquid and/or readily-deformed milling auxiliary agents, for example waxes, can also be used. Examples of these are hydrocarbons, such as hexane, alcohols, amines or aqueous media. These are preferably compounds that are required for the subsequent steps of further processing and/or can easily be removed after crushing.
It is also possible to use special organic compounds, which are known from pigment production, and are used there to stabilise non-agglomerating single flakes in a liquid environment.
In a particular embodiment, milling auxiliary agents are used, which enter into a precise chemical reaction with the base powder to promote milling and/or to set a particular chemical composition of the product. These can be, for example, degradable chemical compounds, of which only one or more constituents are needed to set a desired composition, and wherein at least one component or constituent can be largely removed by a thermal process.
Examples are reducible and/or degradable compounds, such as hydrides, oxides, sulfides, salts, sugars, which are at least partially removed from the crushed material in a subsequent processing stage and/or powder-metallurgic processing of the product powder, and which together with the remaining residue chemically supplement the powder composition in the desired manner.
It is also possible, rather than adding the milling auxiliary agent separately, to produce it in-situ during crushing. This can be done, for example by producing the milling auxiliary agent by the addition of a reaction gas, which under the crushing conditions reacts with the base powder to form a brittle phase. Hydrogen is preferred as the reaction gas.
The brittle phases produced during treatment with the reaction gas, for example by formation of hydrides and/or oxides, can generally be removed again by corresponding process steps once crushing is complete or during processing of the fine metal, alloy or composite powder obtained.
If grinding auxiliary agents are used, which cannot be removed, or cannot fully be removed from the metal, alloy or composite powder produced, these are preferably selected in such a way that the remaining constituents have a desirable influence on the properties of the material, such as for example the improvement of the mechanical properties, the reduction of susceptibility to corrosion, an increase in hardness and improvement of the abrasion behaviour or friction and slip properties. An example of this is the use of a hard material, the proportion of which is increased in a subsequent step to such an extent that the hard material together with the alloy component can be further processed into a hard metal or a hard metal-alloy composite material.
After the deformation step and crushing, the primary particles of the metal, alloy or composite powders produced have a mean particle diameter D50, measured to ASTM C 1070-01 (Microtrac® X100) of normally 25 μm, advantageously less than 75 μm, in particular less than or equal to 25 μm.
In spite of the use of milling auxiliary agents, coarser secondary particles (agglomerates) with particle diameters significantly greater than the desired maximum mean particle diameter of 25 μm, may be formed in addition to the desired formation of fine primary particles, as a result of the known interaction between very fine particles.
For this reason, crushing is preferably followed by a de-agglomeration step, where the product to be produced allows or requires no (coarse) agglomerate, in which the agglomerates are broken up and the primary particles are released. The de-agglomeration can be carried out, for example, by applying shearing forces in the form of mechanical and/or thermal stresses and/or by removing interlayers inserted between the primary particles earlier in the process. The particular de-agglomeration method to be used depends on the degree of agglomeration, the intended use and the susceptibility to oxidation of the very fine powder and the admissible impurities in the finished product.
De-agglomeration can take place, for example, by mechanical methods, such as by treatment in a gas counter-current mill, sieving, classification or treatment in an attritor, a kneader or a rotor-stator disperser. A voltage field can also be used, such as that produced in ultrasound treatment, a thermal treatment, for example dissolution or conversion of a previously-incorporated interlayer between the primary particles by cryo- or high-temperature treatments, or a chemical transformation of incorporated or purposely created phases.
De-agglomeration is preferably carried out in the presence of one or more liquids, dispersion auxiliary agents and/or binders. In this way, a slip, a paste, a kneading composition or a suspension with a solid content of 1 to 95 wt. % can be obtained. Solid contents of 30 to 95 wt. % can be processed directly by known powder-technology processes, such as for example, injection moulding, sheet moulding, coating, hot casting, and then converted to an end product in suitable drying, debinding and sintering steps.
For de-agglomeration of particularly oxygen-sensitive powders, a gas counter-current mill is preferably used, which is operated under inert gases, such as for example argon or nitrogen.
The metal, alloy or composite powders produced are characterised by a number of particular properties in comparison with conventional powders with the same mean particle diameter and the same chemical composition, which are produced for example by atomisation.
The metal powders of Component I for example have excellent sintering behaviour. At a low sintering temperature, the same sintering densities can mostly be achieved, as with powders produced by atomisation. At the same sintering temperature, higher sintering densities can be achieved in relation to the metallic portion of the pressed body on the basis of powder compacts of the same pressed density. This increased sintering activity can be seen for example in the fact that the contraction of the powder according to the invention during the sintering process is higher up to the main contraction maximum than that of conventionally-produced powders and/or that the (standardised) temperature, at which the contraction maximum occurs, is lower with the PZD powder, Monoaxially pressed bodies can produce different paths of contraction parallel and vertically to the direction of pressing. In this case, the contraction curve is determined mathematically by addition of the contractions at the relevant temperature. Here, the contraction in the direction of pressing contributes one third and the contraction vertically to the direction of pressing contributes two thirds to the contraction curve.
The metal powders of Component I are metal powders whose contraction, measured by dilatormeter according to DIN 51045-1, up to the temperature of the first contraction maximum, is at least 1.05 times that of a metal, alloy or composite powder of the same chemical composition and the same mean particle diameter D50, produced by atomisation, the powder to be analysed being compacted to a pressed density of 50% of theoretical density before contraction is measured.
The metal powders of Component I are characterised as a result of a particular particle morphology with a rough particle surface, also by comparatively better pressing behaviour and as a result of the comparatively broad particle size distribution, by high pressed density. This manifests itself in the fact that compacts of atomised powder with otherwise identical production conditions, have a lower bending strength (so-called green strength) than compacts of PZD powders of the same chemical composition and the same mean particle size D50.
The sintering behaviour of powders of Component I can also be influenced specifically by the choice of milling auxiliary agents. Thus one or more alloys can be used as milling auxiliary agents, which, as a result of their low melting point in comparison with the base alloy, form liquid phases already during heating, which improve particle rearrangement and material diffusion and thus the sintering and contraction behaviour, and thus allow higher sintering densities to be achieved at the same sintering temperature or the same sintering density at lower sintering temperatures than the reference powder. Chemically degradable compounds can also be used, whose degradation products, together with the base material, produce liquid phases or phases with a raised diffusion coefficient, which are beneficial to compaction.
Conventional metal powders (MLV) for powder metallurgy applications are powders with a substantially spherical particle shape, as shown for example in FIG. 1 of PCT/IEP/2004/00736. These metal powders may be element powders or alloy powders. These powders are known to the person skilled in the art and can be obtained commercially. Numerous chemical and metallurgic processes are known for their production. If fine powders are to be produced, the known processes often begin by melting a metal or an alloy. The mechanical coarse and fine crushing of metals or alloys is also frequently used for the production of ‘conventional powders’, but produces powder particles with a non-spherical morphology. In so far as it functions in principle, this constitutes a very simple and efficient method of powder production. (W. Schatt, K.-P. Wieters in ‘Powder Metallurgy—Processing and Materials’, EPMA European Powder Metallurgy Association, 1997, 5-10). The atomisation method is also decisive for establishing the morphology of the particles.
Where the melt is broken up by atomisation, the powder particles form directly by setting from the melt droplets produced. Depending on the method of cooling (treatment with air, inert gas, water), the process parameters used, such as nozzle geometry, gas speed, gas temperature or nozzle material, and also the material parameters of the melt, such as melting and setting point, setting behaviour, viscosity, chemical composition and reactivity with the process media, a large number of possibilities arise, and also restrictions on the process (W. Schatt, K.-P. Wieters in ‘Powder Metallurgy—Processing and Materials’, EPMA European Powder Metallurgy Association, 1997, 10-23).
As powder production by atomisation is of particular industrial and economic importance, various atomisation concepts have become established. Depending on the powder properties required, such as particle size, particle size distribution, particle morphology, impurities and properties of the melts to be atomised, such as melting point or reactivity, and also the tolerable costs, certain processes are selected. Nevertheless, in an industrial and economic respect, there are often limits to achieving powder with a certain property profile (particle size distribution, impurity contents, yield of ‘target grain’, morphology, sintering activity etc.) at reasonable cost (W. Schatt, K.-P. Wieters in ‘Powder Metallurgy Processing and Materials’, EPMA European Powder Metallurgy Association, 1997, 10-23).
The production of conventional metal powders for powder-metallurgy applications by atomisation has above all the disadvantage that large quantities of energy and atomisation gas must be used, which makes this process very costly. In particular, the production of fine powder from high-melting alloys with a melting point >1400° C. is uneconomical, because on the one hand the high melting point requires a high energy input to produce the melt and on the other tHe gas consumption increases sharply as the desired particle size falls. In addition, there are often difficulties, if at least one alloy element has a high oxygen affinity. Cost advantages can be achieved in the production of fine alloy powders by using specially-developed nozzles.
In addition to the production of conventional metal powders for powder metallurgy applications by atomisation, other single-stage melt-metallurgy processes are often also used, such as ‘melt-spiming’ i.e. pouring a melt onto a cooled roller, producing a thin, generally easily crushable ribbon, or ‘crucible melt extraction’ i.e. immersing a cooled, profiled, rapidly-spinning roller into a metal melt, extracting particles and fibres.
A further important variant for the production of conventional metal powders for powder metallurgy applications is the chemical route, via reduction of metal oxides or metal salts. However, alloy powders cannot be produced in this way (W. Schatt, K.-P. Wieters in ‘Powder Metallurgy—Processing and Materials’, EPMA European Powder Metallurgy Association, 1997, 23-30).
Extremely fine particles, which have particle sizes of below one micrometer, can also be produced by combining evaporation and condensation processes of metals and alloys, and by gas phase reduction (W. Schatt, K.-P. Wieters in ‘Powder Metallurgy—Processing and Materials’, EPMA European Powder Metallurgy Association, 1997, 39-41). However, these processes are very costly on an industrial scale.
If the melt is cooled in a larger volume/block, mechanical process steps for coarse, fine, and very fine crushing are required, to produce metal or alloy powders that can be processed by powder metallurgy. A summary of mechanical powder production is given by W. Schatt, K.-P. Wieters in ‘Powder Metallurgy—Processing and Materials’, EPMA European Powder Metallurgy Association, 1997, 5-47.
Mechanical crushing, particularly in mills, as the oldest method of particle size setting, is very advantageous from an industrial point of view, because it can be applied at little expense to a large number of materials. However, it makes particular demands on the charge material with regard to the size of the pieces and the brittleness of the material for example. In addition, crushing cannot be continued for an indefinite time. Rather, a milling equilibrium forms, which is established even if the milling process is started with finer powders. The conventional milling processes are modified when the physical limits of crushability are reached for the particular milling material, and certain phenomena, such as for example embrittlement at low temperatures, or the effect of milling auxiliary agents, improve the milling behaviour or crushability. The conventional metal powders for powder metallurgy applications can be obtained by these aforementioned processes.
The Components I and IL, independently of each other, can be chemically the same or different and can be element powders, alloy powders or mixtures thereof.
The metal powders of Components I and II may have a composition of Formula I
hA-iB-jC-kD (I)
wherein,
In a further embodiment of the invention
h stands for 50 to 80 wt. % or for 60 to 80 wt. %, i means 15 to 40 wt. % or 18 to 40 wt. %, j means 0 to 15 wt. % or 5 to 10 wt. %, k means 0 to 5 wt. % or 0 to 2 wt. %.
In a further embodiment of the invention, Components I or II are element powders or binary alloy powders, so that a moulded object, which can be obtained from a metallic powder blend according to the invention, has a corresponding, more complex composition. For example, in this embodiment of the invention a moulded object can be obtained, through the use of binary alloys for Components I and II, that consists of a quaternary alloy.
In a further embodiment of the invention, Components I and II are higher alloy powders such as binary or quaternary alloy powders, so that a moulded object, which can be obtained from a metallic powder blend according to the invention, has a corresponding more complex composition. Components I and II, independently of each other, can thus also consist of alloys containing two, three, four or five different metals, so that more complex alloys are possible. For example, in this embodiment of the invention, a moulded object can be obtained through the use of a binary alloy for Component I and a quaternary alloy for Component II, that consists of an alloy containing six metals.
In a further embodiment of the invention, the compositions of Components I and II of the metallic powder blend and also of a moulded object obtained from them are each different from the other.
In a further embodiment of the invention, a moulded object, which can be obtained by subjecting a metallic powder blend according to the invention to a powder-metallurgic moulding process, has a composition of Formula I.
In a further embodiment of the invention, the moulded object, Component I and/or Component II consist substantially of an alloy selected from the group consisting of Fe20Cr10Al0.3Y, Fe22Cr7V0.3Y, FeCrVY, Ni57Mo17Cr16FeWMn, Ni17Mo15Cr6Fe5W1Co, Ni20Cr16Cu2.5Ti1.5Al and Ni53Cr20Co18Ti2.5Al1.5Fe1.5.
In a further embodiment of the invention, Component I and/or II may even be a powder blend of different element powders or alloy powders. For example a moulded object containing six metals as alloy components can be obtained in this case by mixing a Component I, which is a binary alloy with a Component IIa and a Component IIb, which are each binary alloys, and subjecting them to a powder-metallurgic moulding process.
The quantity of Component II in the metallic powder blend depends on the type and extent of the intended effect to be achieved and on the desired chemical composition of the moulded object obtained when the metallic powder blend is subjected to a powder-metallurgic moulding process. If Components I and II are identical, the chemical composition of the moulded object is already established. However, if Components I and II have a different composition, the composition of the resulting moulded object depends on the type, composition and content of Components I and II and these must be adjusted accordingly. According to the invention, moulded objects can be produced from high-alloy metallic materials using processes that were previously not suited to their production. The person skilled in the art is, in principle, familiar with the effects arising, so that the optimum blends for the respective application can be established with a small number of trials. In general, the conventional metal powder is used in proportions of Component I: Component II of a ratio of 1:100 to 100:1 or of 1:10 to 10:1 or of 1:2 to 2:1 or of 1:1.
The present invention can be used for the production of high-alloy materials. Possible procedures are described in more detail here. The production of complex alloy components for the metallic powder blend can in general be described as follows, the sum of the factors a, b and c being made up to 100 percent by weight and the symbols ABMP-bLEM-cDOT dMHM-eFUZ being used as follows:
The indices d and e state the quantity of milling auxiliary agent or functional additive that can be obtained additionally.
In one embodiment of the invention, the alloy composition is retained. The composition of the metallic powder blend is as follows:
Component I: a1BMP-b1LEM-c1IDOT-d1MHM
Component II: a2BMP-b2LEM-c2DOT
Component III: −e3FUZ
In this case, the alloy of which the moulded object consists, which is obtained from the metallic powder blend, is composed as follows:
(a1+a2)BMP−(b1+b2)LEM−(c1+c2)DOT
(without milling auxiliary agents)
In this case a1a2 and b1=b2 and c1=c2, which means that this is a mixture of the same alloys, in which Component I is a PZD powder. The (organic) milling auxiliary agent (MHN) is not mentioned, as it is completely removed during processing and does not change the alloy. The proportions of Components I and II can vary between 100% Comp. I and 0% Comp. II and 1% Comp. I and 99% Comp. II, depending on the requirements of processing or functional properties.
In a further embodiment of the invention, the alloy composition changes according to the proportions of Components I and II. The metallic powder blend is composed as follows:
Component I: a1BMP-b1LEM-d1MHM
Component II: a2BMP-c2DOT
Component III: . . . not present
In this case, the alloy of which the moulded object consists, which is obtained from the metallic powder blend, is composed as follows:
(a1+a2)BMP−(b1)LEM−(c1)DOT
(without milling auxiliary agents)
In this case a1≠a2 and b1≠b2 and c1≠c2, which means that there are two alloys. Component I consists only of base metal powder (BMP) and alloy elements (LEM), Component II contains the dopant in concentrated form as a compound to be added, advantageously with particular metallurgic (e.g. low melting point) and/or mechanical (e.g. brittle, easily crushable) properties. In this way, powder technological advantages (sintering with a liquid phase) can be used, to form the desired end alloys. Here, the dopant is introduced in the form of a masterbatch, which can be advantageous depending on the type and composition of the alloys. The (organic) milling auxiliary agent is not mentioned, as it is completely removed during processing and does not change the alloy. The proportions by volume of Components I and II are selected by the person skilled in the art according to the target composition.
In a further embodiment of the invention, the alloy composition changes according to the proportions of Component I, IIa and IIb. The metallic powder blend is composed as follows:
Component I: a1BMP-b1LEM-d1MHM
Component II: a2BMP-b2LEM-c2DOT
In this case, the alloy of which the moulded object consists, which is obtained from the metallic powder blend, is composed as follows:
(a1+a2+a3)BMP−(b1)LEM−(c1)DOT
(without milling auxiliary agents)
In this case a1≠a2≠a3 and b1≠b2 and c1≠c2, which means that the components are two alloys and a base metal powder. Component I consists only of base metal powder (BMP) and alloy elements, Component II contains as a mixture the dopant in ‘concentrated’ form together with base metal and/or alloy elements in order to advantageously use particular metallurgic and mechanical properties. Component IIb contains base metals that can be produced simply and cost-effectively, which when added to Component I, II and IIb form the whole alloy. In this way, in addition to the powder technological advantages of the embodiment disclosed immediately above, technical and economic advantages can also be utilised. The (organic) milling auxiliary agent is not mentioned, as it is completely removed during processing and does not change the alloy.
In a further embodiment of the invention, the alloy composition changes according to the proportions of Components I and II. A brittle alloy is used advantageously as a milling auxiliary agent. The metallic powder blend is composed as follows:
Component I: a1BMP-b1LEM-d1MHM=(a2BMP-c2DOT)
Component II: a3BMP
Component III: −e3FUZ=paraffin
In this case, the alloy, of which the moulded object consists, which is obtained from the metallic powder blend, is composed as follows:
(a1+a2+a3)BMP(b1)LEM−(c2)DOT
(without milling auxiliary agents)
In this case a1≠a2≠a3, which means that there is an alloy and a base metal. Component I consists only of base metal powder (BMP) and alloy elements (LEM). A particularly brittle composition consisting of BMP and DOT is used as a milling auxiliary agent. Paraffin in powder form is mixed in as Component III. With Component II, in this case a base metal powder, corrections can be made to the composition. In this way the powder technological advantages of the alloy (a2BMP-c2DOT) can be used. The milling auxiliary agent is not listed separately, as it disappears into the alloy of which the moulded object consists.
In a further embodiment of the invention, the composition changes according to the proportions of Component I and II. A brittle alloy a2BMP-c2DOT is used as the milling auxiliary agent, organic constituents and ceramic particles are used as a functional additive (FUZ). The metallic powder blend is composed as follows:
Component I: a1BMP-b1LEM-d1MHM=(a2BMP-c2DOT)
Component II: a2BMP
Component III: −e3FUZ=PVA, ceramic
In this case, the alloy of which the moulded object consists, which is obtained from the metallic powder blend, is composed as follows;
(a1+a2+a3)BMP=−(b1)LEM−(C2)DOT
(without grinding auxiliary agents)
In this case a1≠a2≠a3, which means that there is an alloy and a base metal powder. Component I consists of base metal powder and alloy elements. A brittle composition consisting of base metal and dopant is used as the milling auxiliary agent. Corrections can be made to the composition with the base metal powder. Component III contains PVA (polyvinyl alcohol) and ceramic particles, which are advantageous for farther processing, for example by spray drying. This blend can be processed to a thermal spray powder for example. In this way, the powder technological advantages of the alloy (a2BMP-c2DOT) and the action of functional additives (hardness, resistance to wear) can be utilised, if the powder is processed accordingly, for example by thermal spraying.
The metallic powder blend can contain functional additives as Component III. Functional additives can give characteristic properties to objects moulded from PZD powders, such as for example additives that increase the impact strength or resistance to abrasion, such as superhard powders, or additives that facilitate processing of the green compacts by reducing the brittleness of the green compact and/or increasing the green strength, or additives that act as templates to control the pore structure or surface properties.
Functional additives are understood to mean additives to be incorporated homogeneously, which are either largely or completely retained in the finished product, a moulded object, or which are largely or completely removed from the product.
The first of these are functional additives, which control the mechanical properties, such as for example hardness, strength, damping or impact strength, or the chemical properties such as oxidation/corrosion behaviour or functional properties such as tribology, haptics, electrical and magnetic conductivity, modulus of elasticity, electrical burn off behaviour, magnetostrictive behaviour, electrostrictive behaviour by their proportions and primary properties.
The complex mechanical, chemical and Functional properties can be brought about by the incorporation of various phases/constituents, such as chemical particles or hard materials, for example carbides, borides, nitrides, oxides, silicides, hydrides, diamonds, in particular carbides, borides and nitrides of the elements of groups 4, 5 and 6 of the periodic system, oxides of the elements of groups 4, 5 and 6 of the periodic system and also oxides of aluminium and rare earth metals, silicides of aluminium, boron, cobalt, nickel, iron, molybdenum, tungsten, manganese, zirconium, hydrides of tantalum, niobium, titanium, magnesium and tungsten; slip additives with lubricant properties such as graphite, sulfides, oxides, in particular molybdenum sulfide, zinc sulfide, tin sulfides (SnS, SnS2), copper sulfide and also intermetallic compounds with particular magnetic or electrical properties on a rare earth-cobalt or rare earth-iron base.
By this means, the coating of superhard powders with PZD powders can also be achieved using a metallic powder blend. This is advantageously achieved by fluidised bed granulation.
Coarse (50-100 μm) hard material particles of BN and TiB2 for example, can be used as feedstock for fluidised bed granulation and can be provided with a corrosion—resistant coating. Thus it is possible to serve new applications in the field of wear under high corrosive and mechanical loads. After coating, the agglomerates are debound, sintered in an inert atmosphere and applied by thermal spraying.
In the second case, in other words when using functional additives that are largely or completely removed from the product, the additives used are so-called place-holders which are removed by suitable chemical or thermal processes and thus function as a template. These can be hydrocarbons or plastics. Suitable hydrocarbons are long-chain hydrocarbons such as low-molecular waxy polyolefins, such as low-molecular polyethylene or polypropylene, and also saturated, wholly or partially unsaturated hydrocarbons having 10 to 50 carbon atoms, or having 20 to 40 carbon atoms, waxes and paraffins. Suitable plastics are in particular those with a low ceiling temperature, in particular with a ceiling temperature of less than 400° C., or lower than 300° C. or lower than 200° C. Above the ceiling temperature, plastics are thermodynamically unstable and tend to degrade into monomers (depolymerisation). Suitable plastics are, for example, polyurethanes, polyacetals, polyacrylates, in particular polymethyl methacrylate, or polystyrene. In a further embodiment of the invention, the plastic is used in the form preferably of foamed particles, such as for example foamed polystyrene beads, as used as a preliminary material or intermediate in the production of packaging or thermal insulation materials. Inorganic compounds tending towards sublimation can also act as place-holders, such as for example some oxides of refractory metals, in particular oxides of rhenium and molybdenum, and also partially- or fully-degradable compounds such as hydrides (Ti hydride, Mg hydride, Ta hydride), organic (metal stearate) or inorganic salts.
By adding these functional additives, largely dense components (90 to 100% of theoretical density), low-porosity (70 to 90% of theoretical density) and high-porosity (5 to 70% of theoretical density) components can be produced, by subjecting a metallic powder blend according to the invention containing such a functional additive as a place-holder to a powder-metallurgic moulding process.
The quantity of functional additives depends on the type and extent of the intended effect to be achieved, with which the person skilled in the art is, in principle, familiar, so that the optimum blends can be established with a small number of trials. When using these compounds, care should be taken to ensure that the compounds used as place-holders/templates are present in the metallic powder blends in a structure suitable for their purpose, in other words in the form of particles, as a granulate, powder, spherical particles or similar.
In general, the functional additives are used in proportions of Component I Component III in a ratio 1:100 to 100:1 or of 1:10 to 10:1 or 1:2 to 2:1 or of 1:1. If the functional additives are hard materials, for example tungsten carbide, boron nitride or titanium nitride, these are advantageously used in quantities of 3:1 to 1:100 or of 1:1 to 1:10 or of 1:2 to 1:7 or of 1:3 to 1:6.3.
In a further embodiment of the invention, the functional additives are advantageously used in quantities of 3:1 to 1:100 or of 1:1 to 1:10 or of 1:2 to 1:7 or of 1:3 to 1:6.3. In a further embodiment of the invention the metallic powder blend is a mixture of Component I with Component II and/or Component III, provided that the ratio of Component I to Component III is 3:1 to 1:100 or 1:1 to 1:10 or 1:2 to 1:7 or 1:3 to 1:6.3
In a further embodiment of the invention the metallic powder blend is a mixture of Component I with Component II and/or Component III, provided that, if a hard material is present in Component III, the ratio of Component I to Component III is 3:1 to 1:100 or 1:1 to 1:10 or 1:2 to 1:7 or 1:3 to 1:6.3.
In a further embodiment of the invention, the metallic powder blend is a mixture of Component I with Component II and/or Component III, provided that, if tungsten carbide is present in Component III, the ratio of Component I to Component III is 3:1 to 1:100 or 1:1 to 1:10 or 1:2 to 1:7 or 1:3 to 1:6.3.
Further additives will improve in particular the processing properties such as pressing behaviour, strength of the agglomerates or re-dispersibility. These can be waxes, such as polyethylene waxes or oxidised polyethylene waxes, ester waxes such as montanic acid ester, oleic acid ester, esters of linoleic acid or linolenic acid or mixtures thereof, paraffins, plastics, resins such as for example colophony, salts of long-chain organic acids, such as metal salts of montanic acid, oleic acid, linoleic acid or linolenic acid, metal stearates and metal palmitates, for example zinc stearate, in particular of the alkali and earth alkali metals, for example magnesium stearate, sodium palmitate, calcium stearate, or slip agents. These are substances that are normal in powder processing (pressing, MIM, sheet moulding, slip casting) and are known to the person skilled in the art. The compaction of the powder to be analysed can be carried out with the addition of conventional auxiliary agents which assist pressing, such as for example paraffin waxes, or other waxes or salts of organic acids e.g. zinc stearate. Suitable additives are further described in W. Schatt, K.-P. Wieters, ‘Powder Metallurgy—Processing and Materials’, EPMA European Powder Metallurgy Association, 1997, 49-51’, to which reference is made.
The following examples serve to explain the invention in more detail. The examples are intended to facilitate understanding of the invention, and should not be understood as a restriction thereof.
The mean particle diameters D50 given in the examples were measured with a Microtrac® X100 from Honeywell, US, according to ASTM C 1070-01.
An argon-atomised alloy melt of the type Nimonic® 90, with the composition Ni20Cr16Co2.5Ti1.5Al was used as the base powder. The alloy powder obtained was sieved to between 53 to 25 μm. The density was ca 8.2 g/cm3. The particles of the base powder were largely spherical.
The base powder was subjected to deforming pulverisation in a vertical agitated ball mill (Netzsch Feinmalitechnik; type: PR 1S), so that the originally spherical particles became flake-like. The details of the parameters used are as follows:
This was followed by pulverisation. A so-called excentric vibrating mill (Siebtechnik GmbH, ESM 324) was used for this, with the following process parameters:
After a pulverising time of 2 hours very fine particle agglomerates are obtained. In an REM image of the product obtained at a magnification of 1000, the cauliflower-like structure of the agglomerate (secondary particles) can be seen, the primary particles having particle diameters of far less than 25 μm.
A sample of the primary particles or very fine particle agglomerates was subjected in a third process step to de-agglomeration by a 10 minute-long ultrasound treatment in isopropanol in a TG 400 ultrasound apparatus (Sonic Ultraschallanlagenbau GmbH) at 50% of the maximum output in order to obtain separated primary particles.
The particle size distribution of the de-agglomerated sample was measured by Microtrac® X100 (manufacturer: Honeywell/US) according to ASTM C 1070-01. The D50 value of the base powder amounted to 40 μm and had fallen to ca 15 μm as a result of the treatment.
The residual quantity of primary particles from pulverisation was subjected in an alternative third process step to de-agglomeration by treatment in a gas counter-current mill with subsequent ultrasound treatment in isopropanol in a TG 400 ultrasound apparatus (from Sonic Ultraschallanlagenbau GmbH) at 50% of maximum output. The particle size was again measured by Microtrac® X100. The D50 value was now only 8.4 μm.
The paraffin pulverisation auxiliary agent incorporated can be removed during powder-metallurgic further processing of the alloy powder by thermal degradation and/or evaporation or can serve as a pressing auxiliary agent.
A metallic powder blend according to the invention was produced as follows from the PZD powder obtained as disclosed above:
5 kg Nimonic® 90 PZD powder (d50: 10 μm and d90; 20 μm), produced as disclosed above and 5 kg spherical (gas-atomised) Nimonic® 90 powder (d50: 10 μm and d90: 20 μm) are added to an Eirich mixer together with 233 g of a pressing auxiliary agent in powder form (Licowax C). Over a period of 20 minutes the three constituents are intensively mixed with each other. This powder is called VSP-711.
Analogously to this, 10 kg purely atomised (conventional) powder (Nimonic® 90 powder (d50: 10 μm and d90: 20 μm)) is processed in the same way, however 300 g Licowax is added. This powder is called KON-711.
Both powders are processed by monoaxial pressing at a pressure of 500 MPa to cylinders 10 mm in length with a diameter of 30 min. The pressed density of KON-711 was 75% of theoretical density, however the test specimen had only a low green strength. The specimens obtained from VSP-711 had significantly improved strength, in spite of their lower theoretical density (70%).
For the exact measurement of green strength, square-shaped pressed bodies are produced at a pressing pressure of 500 MPa.
Both powders (VSP-711 and KON-711) are pressed in a metal powder press to a further test specimen, a PM tensile test bar in accordance with DIN ISO 3927 with an area of 6.35 cm2 (parallel to the direction of pressing) and a length of ca 5 mm. The pressure is varied from 300 to 800 NWa. The density of the components increases with the increase in pressure. Table 2 describes this dependency of the influence of the pressing pressure on the green strength of tensile test specimens pressed directly from the powders as (A (area in the direction of pressing): 6.35 cm2; L (length of the specimen in the direction of pressing): 4-5 mm). It should be borne in mind here that the density values given relate to the mixture of metal powder and pressing auxiliary agent (3% Licowax).
The PM tensile test bars are debound in a gas stream under hydrogen at a heating rate of 2 K/min from room temperature to 600° C. and then sintered in a high vacuum at ca 10−3 mbar at a temperature of 1290° C. for 2 h. The specimen of the powder type KON-711 shows damage (cracks, signs of destruction) after debinding and sintering, which was not visible in the pressed state. In contrast to this, the tensile test specimens of VSP-711 show no damage and also have an even specimen surface with little roughness. The specimens are shown in
Pressed bodies were pressed at 500 MPa and sintered in a kiln at 1300 and 1330° C. for two hours in an argon-hydrogen atmosphere (6.5 vol. % H2), after which the organic pressing auxiliary agent has been removed up to 600° C. under hydrogen. The results are presented in Table 2b.
A further peculiarity lies in the pore structure of the specimens produced from KON-711 and VSP-711, which is shown in
Production of a readily-compressible, flowable and readily-sinterable granulate in the following manner:
5 kg Nimonic® 90—PZD powder (d50: 10 μm and d90: 20 μm), produced as in Example 1, and 5 kg spherical (gas-atomised) Nimonic® 90 powder (d50: 10 μm and d90: 20 μm) are added to 2-3 l water together with an organic binder (polyvinyl alcohol, PVA, 3 wt. %) and a surface-active stabiliser. This mixture is dispersed until a stable suspension has formed. This suspension is processed by spray-drying to an agglomerate of largely spherical single particles having a diameter of 1 to 150 μm. Heated nitrogen (gas temperature 30 to 80° C.) in counter-current is used as the working gas to dry the suspension. The gas mixture formed during drying is released into the environment through a filter at the spray dryer outlet.
To improve further processability and to ensure compliance with health criteria, the ‘powdery’ fine content (<10 μm) and the content of grains >150 μm, which are too coarse are separated off by sieving. Such a granulate (−150 μm+10 μm) possesses excellent flow behaviour. The granulate thus obtained is called VSP-712.
In parallel with the production of this granulate, an atomised (conventional) powder (10 kg) (Nimonic® 90—powder (d50: 10 μm and d90: 20 μm)) is processed in the same way to a granulate (−150 μm+10 μm). This powder is called KON-712.
Both powders (VSP-712 and KON-712) are evaluated in the same way—as described in Example 1—for the pressing properties, green compact strength, sintering behaviour and surface quality (roughness) of the sintered parts. The result corresponds with the data determined in the example given above.
In each case, a pressed body was produced by cold isostatic pressing (CIP) using the powder blends VSP-711 and KON-711 produced in Example 1. For this purpose, the granulate is poured into a rubber mould, sealed with a gas-tight seal and then compacted at a hydrostatic pressing pressure of 2000 bar. A compaction of 70% TD is measured on the pressed bodies of KON-711, however VSP-711 achieves a pressed density of ca 65% TD. The CIP pressed bodies were then broken down one after the other by machining (loaded into a lathe and cut into coarse ‘chips’). In the case of VSP-711 a large proportion (>50% with a particle size of d50; >100 μm) can successfully be processed into coarse grains. A primarily powdery product (particles >100 μm (<5%)) is obtained from the pressed bodies of KON-711.
These pre-granulates are then processed further with a sieve granulator plate. This process rounds off the edges of the ‘powder chips’, producing a more flowable granulate. After sieving, a fraction −65 μm+25 μm, that is a fraction with a particle size of less than 65 μm and greater than 25 μm, is obtained. This granulate can be further processed by powder-metallurgic moulding processes. The fractions are called VSP-721 and KON-721. The total yields from the production of a high-density and flowable granulate are 20 to 50% in the case of VSP-721 and <20% in the case of KON-721. The granulate portions not lying within the desired grain band can be recycled in the production process for the CIP bodies.
The investigation of the processing properties of the metallic powder blends VSP-721 and KON-721 from Example 2 (green strength, sintering properties) produces comparable results. VSP-721 has a higher green strength and higher sintering density in comparison with KON-721 at a pre-determined sintering temperature, when using the same initial densities.
The VSP-721 and KON-721 granulates produced previously and a powder, VER-6525, of the same composition and same particle size as the granulate used (−65/+25) produced by protective gas atomisation, are processed in the following way to produce porous moulded bodies:
Each of the three grain types is first placed in each of three identical sintering pans (base area: 6 cm×2 cm; pouring height: 3 cm). These are heated under hydrogen in a kiln at a rate of 2 K/min to a temperature of 600° C. for debinding. This is followed by heating to 1250° C. at a heating rate of 10 K/min. The temperature of 1250° C. is maintained for 2 h, and the kiln containing the sintered bodies is then brought to room temperature at a rate of 10 K/min.
The (contracted) moulded bodies formed are removed and evaluated in the three-point bending test. This shows that the moulded bodies achieve the following, very different, bending strengths: VSP-721: 40-ca 20 MPa, KON-721: ca. 20-5 MPa and VER-6525: <5 MPa. The comparatively high sintering activity of the variant VSP-721 therefore allows production of sufficiently strong moulded bodies, as required for example for use in filter elements. Optimisation of the sintering conditions allows the strength of VSP-721 to be increased to over 50 MPa.
Production of a porous body in the form of a tube, by sintering a powder charge of high-density granulates (VSP-721, KON-721) and a powder produced by atomisation (VER-6525) of the same chemical composition and particle size as the granulate. A correspondingly produced granulate and the roughly-atomised powder are each put into a ceramic mould with a core that allows full burn out. The core is in the form of a thin-wall plastic tube, which is sufficiently stable to withstand the pressure of the powder over its area after filling. It is filled only with a narrow granulate or powder fraction (−65+25 μm) produced by sieving.
In a subsequent step, the organic constituents and the inserted tube are removed by thermal decomposition or expulsion in a kiln and at the same time, pre-sintering is started at a higher temperature (1000° C.). The pre-sintered bodies are then placed, still vertically, into another kiln, which reaches a temperature of 1300° C. at high gas purity (vacuum, pressure of 10−2 mbar). After sintering, a moulded body of the VSP-721 granulate is obtained, which has sufficient contraction and also sufficient strength. The moulded body of KON-721, on the other hand, has less strength. The moulded body of the coarse powder (VER—6525) achieved only a strength of ca 5 MPa under the conditions used, rendering industrial use impossible because of insufficient strength.
The granulates VSP-721 and KON-721 disclosed above are poured into the cavity of a powder pressing mould of a monoaxial press. Moulded bodies are produced under monoaxial pressing pressure of 700 MPa, which have the following densities: VSP-721: 5.3 g/cm3 (65% of theoretical density) and KON-721 ca 6 g/cm3 (73% of theoretical density). The green strengths are 10 to 15 MPa for moulded bodies of VSP-721 and 2 to 5 MPa for moulded bodies of KON-721. After sintering according to the temperature-time programme disclosed in Example 4, the moulded bodies of VSP-721 achieve densities of 7.8 g/cm3 (95% of theoretical density), the moulded bodies sintered from KON-721 achieve densities of 7.7 g/cm3 (94% of theoretical density). A typical structure is shown in
Fluidised bed granulation for the production of good flow- and press-ready powders The processing of PZD powders (NIMONIC® 90 according to Example 1) by fluidised bed granulation (using the ProCell machine, from Glatt) allows the production of agglomerates with particle diameters of 10 to ca 300 μm. An aqueous suspension is produced, which is sprayed into a fluidised bed chamber. When the material jetted in is dried, tiny agglomerates are first formed, which are built up from several primary particles. These act as seeds for fluidised bed granulation. Further separation and drying of droplets produces agglomerates of growing diameter. This growth process is accompanied by impacts between the growing particles, achieving compaction of the surface. As a result of the binder contained in the suspension the primary particles adhere to the surface of the seeds and growing agglomerates. The particle size and agglomeration properties can be influenced by appropriate setting of flow conditions and air quantities. Agglomerates produced in this way have particularly good homogeneity of the components in the single-cell agglomerate grain.
By using pure Nimonic® 90 PZD powder with a d50 of 10 μm and d90 of 20 μm produced in the same way as Example 1, it is possible to carry out an agglomeration, in which the primary properties of the very fine powder (in particular sintering and pressing behaviour) are largely retained.
In detail, 600 g of the PZD powder is added to the measuring container of an excentric vibrating mill. Steel balls of the material 100Cr6 (DIN 1.3505) with a diameter of 15 mm are used. After a milling time of 1 h at a speed of 1500 rpm in argon 4.8 as the medium, a ball fill level of 80% and a milling vessel volume of 51, a clearly ‘coarsened’ powder is removed from the mill. The particle size d50 is ca 40 μm.
Production of a readily-flowable granulate for use as a powder for thermal spraying in the following way:
A spherically atomised Ni17Mo15Cr6Fe5W1Co alloy with a mean particle diameter D50 of 40 μm, which is commercially available under the brand name Hastelloy® C, was subjected to a deformation step as disclosed in Example 1.
The pulverisation of the flake-like particles formed was carried out in an excentric vibrating mill in the presence of tungsten carbide as a pulverisation auxiliary agent under the following conditions:
Pulverisation produced an alloy-hard material composite powder, the alloy component of which was crushed to a mean particle diameter D50 of ca 5 μm and the hard material component to a mean particle diameter D50 of ca 1 μm. The hard material particles were distributed largely homogeneously in the volume of the alloy powder.
1.5 kg of the Hastelloy C® PZD powder thus obtained having a d50 of 5 μm and d90 of 10 μm and 9.5 kg tungsten carbide (d50: 1 μm, d90: 2 μm) are processed together by spray granulation, as described in Example 2 for the production of VSP-712, to form a granulate. The parameters for spray granulation were set in such a way as to produce a minimum proportion of fine particles. In order to remove the portions that were unsuitable for further processing (thermal spraying), the particles with a diameter greater than 65 μm were sieved out and the coarse portion was fed back into the spray-ready suspension (mixed in). The fraction with a diameter of less than 65 μm is debound in a sintering boat with a base area of 15 cm×15 cm filled to a level of 3 cm and then debound under hydrogen (heating at a heating rate of 2 K/min to 600° C.) and sintered at a temperature of 1150° C. The sinter cake is removed after cooling and processed further by lightly crushing in a mortar. The fine portion thus formed is classified with a 50 μm sieve for the ‘top’ and with a 25 μm sieve for the ‘bottom’. The fraction thus formed with a particle size of less than 50 μm and greater than 25 μm is applied by thermal spraying (high-speed flame spraying) as a wear and corrosion-resistant layer to a Hastelloy C material with low wear-resistance. The part image ‘B’ in
The granulate is produced following the method in Example 2. However, a mixture of benzene (ca 10 vol. %) and ethyl alcohol (ca 90 vol. %) is used as the solvent and polymethyl methacrylate (PMMA) is used as the plastic. Spray drying, taking account of the conditions for handling highly flammable solvents, produces a granulate in which the individual particles (Hastelloy C and tungsten carbide) form a largely strong bond. The parameters for spray granulation are set in such a way, that coarse granulate with a low content of fine particles is formed, which has good flowability (d50: 100 μm, d90: 150 μm). By investigating individual narrower fractions by x-ray fluorescence analysis it can be demonstrated quantitatively that the same chemical composition and therefore the same ratio of powder constituents used is present in the different fractions. On this basis, it can be concluded that the granulate produced is homogeneously distributed, and also because separation is unlikely from a chemical point of view, even if individual constituents of the fraction separate. Even after a longer period of movement—for example when determining the capped density to DIN EN ISO 787-11 or ASTM B 527, only marginal changes in the particle size distribution arise, from which it can be concluded that a strong bond between the powder constituents used has been achieved in the granulate.
By stirring the granulate produced in Example 10 into alcohol, the individual particles (Hastelloy C and tungsten carbide) can be released. The addition of waxes, polypropylene and stabilisers and the simultaneous exertion of high shear forces on a shear roller at a sufficiently high processing temperature, achieves a homogeneous distribution of the powdery functional materials in the organic environment. The bubble-free composition is processed via a granulation system into a readily conveyable and homogeneously melting cold granulate. This can then be added to the dispenser system of a metal powder injection moulding machine, heated, and injection moulded under process parameters to be determined (temperature, pressure, pressure change, after pressure, cooling time in the injection mould etc). 80 to 95% of the organic constituents are extracted from these injection-moulded parts by solvent extraction. This is followed by thermal residual debinding by slow heating of the test specimens under hydrogen (heating rate of 1 K/min from room temperature to 600° C.). The parts are pre-sintered at a temperature of 1000° C. under hydrogen in the same kiln. The sintering of these specimens is then completed in a vacuum kiln at a pressure of ca 10−2 to 10−3 mbar (heating at 5 K/min from room temperature to 1250° C., 2 h holding time at 1250° C. and cooling at 10 K/min to room temperature).
The granulates VSP-712 and KON-712 produced in Example 2 are placed one after the other into the nip of a vertical powder rolling machine and compressed. In the case of VSP-712, this pressing produces in an easy-to-handle sheet with a green strength of 2 to 10 MPa. With the granulate KON-712, it is not possible to remove test specimens on which the green strength can reliably be measured.
By thermal post-treating, debinding and sintering as described under Example 11, a sheet of VSP-712 can be produced which, depending on the sintering temperature selected, can be dense (93 to 98% of theoretical density) or porous (60 to ca 90% of theoretical density). In spite of the low density of the porous structure, these sheets still have a high strength of at least 50-100 MPa.
The granulates VSP-712 and KON-712 produced in Example 2 are debound as a loose powder charge and pre-sintered to stabilise (compact) the granulate. This takes place under the conditions described in Example 5 for debinding/pre-sintering to 1000° C. After de-fragmentation, including classification to −50+25 μm as described in Example 9, the granulate thus formed is processed in each case by powder rolling into a green ribbon. The strength of the green ribbon is sufficient in the case of the granulate VSP-712 for further processing by sintering. The fragments of KON-712 are unsuitable for the intended further processing into a sheet. If the VSP-712 green ribbon is sintered at a temperature of 1300° C., as described in Example 5, a density of over 92% of theoretical density can be achieved.
The green ribbon described in Example 13 must not necessarily be compacted by sintering. A simple option for compaction is to heat the green ribbon inductively under an inert protective gas atmosphere (argon) to 1100° C., before running it into a roll nip and to subject it to intensive pressure loading at this temperature. This will very simply produce a sheet-like component in which complete compaction (>98% of theoretical density) or a desired residual porosity (50 to 90% of theoretical density) can be set by varying the roll nip. Here too, the variant KON-712 has lower green strength to obtain a sintered component.
On the basis of and following the method described in Example 10 for producing a readily re-dispersible powder blend, a granulate is produced, which consists only of Hastelloy C powder. The tungsten carbide portion is omitted to allow a sheet to be produced that consists only of an alloy.
In the same way as and following the process described in Example 11, a sheet-mouldable, pore-free composition is produced by intensive pulverisation.
This composition is continuously applied to a smooth surface by blade coating. Drying produces as a green body a metal-powder-filled sheet with organic constituents, which is rubber-like in nature. This green body is now subjected to debinding by heating from room temperature to 600° C. at a heating speed of 0.1 K/min. The part is then subjected to sintering under the conditions described in Example 5, to achieve an increase in strength. Linear contraction typically occurs in this step. This can amount to 10- to 25%, depending on the sintering temperature and time.
A green compact produced as in Example 15 is treated in a stamping tool in the shape of a needle press (stamp formed from needles with a diameter of 0.1 to 0.5 mm) in such a way that tube-like deformations remain vertically to the normal line to the surface.
After debinding and sintering under the conditions described in Example 5, a sheet is formed, which consists of dense material areas and pore channels lying on a normal line to the surface. The flow resistance can easily be set by the number and diameter of the channels, without the particle size of the powder particles directly playing a role, which can be important for the setting of any corrosion and oxidation properties, if very fine powder particles are used.
A bubble-free feedstock of ‘honey-like’ viscosity is produced in a kneader from 3.7 kg PZD powder (VSP-711), 148 g powdery (<30 . . . 50 μm) polymethyl methacrylate (PMMA) and a sufficient quantity of a mixture of benzene (ca 10 vol. %) and ethyl alcohol (ca 90 vol. %). 0.671 foamed polystyrene beads (Ø 1 to 1.5 mm) are added to this feedstock in the kneader. This composition (volume ca 0.9 . . . 1.1 l) is placed into a flat ceramic mould (ca 30×30×1.5 cm3) and dried. The green body thus produced is freed from the organic constituents polystyrene place holders, PMMA, residual solvent) by slowly heating to ca. 400° C. (0.5 K/min) under hydrogen. The mould is then heated in the same kiln at 5 K/min from room temperature to 1000° C. Sintering is completed in a vacuum kiln (10−2-10−3 mbar), the pre-sintered specimens being brought from room temperature to 1300° C. at 10 K/min and maintained at this temperature for 2 h. The volume of the fully-sintered specimens is ca 0.4 l lower than the initial volume (ca 1 l). This is equivalent to a linear contraction of ca 26%. The pores (as a result of the place-holders) have reduced from 1 mm originally to 1.5 mm in the green state, equivalent to a reduction of 0.74 to 1.1 mm and a material density of ca 7.4 g/cm3 is achieved in the metallic area.
The PZD powder is produced as described in Example 1, although unlike in Example 1 an atomised Fe22Cr7V0.3Y alloy is used as the educt (instead of Nimonic® 90 powder).
The processable powder blends summarised in Table 3 were produced in an Eirich mixer from the PZD powder produced accordingly and conventional spherical powders (−25 μm, −53 μm/+25 μm).
Before processing by hot pressing, partial quantities of 18.2, 18.3 and 18.4 are subjected to debinding at a heating rate of 2 K/min from room temperature to 600° C. under hydrogen. The hot pressing takes place under the following conditions: 1150° C./2 h/35 MPa/argon 4.8 in a graphite mould. After hot pressing, the temperature is reduced by ca 5 to 15 K/min, until room temperature is reached. The discs thus produced have a diameter of ca 100 mm. Tensile test specimens are produced from them by water jet cutting as in Example 1 and are ground to the same thickness (ca 3.4 mm). All samples have virtually the same material densities of 7.55 to 7.50 g/cm3. The results of the mechanical tensile test at room temperature are given in Table 4.
Table 4 shows that the strength values Rp0.2 and Rm are better for all PZD powders containing variants (Rp0.2: +5-70%/Rm; +20-50%). 18.1 has the best values for elongation (At-Fmax: elastic and plastic part), the PZD-containing variants achieve At-Fmax values of 95 to 45%. In view also of the fact that variants 18.2, 18.3 and 18.4 are at all processable by pressing and sintering techniques, the basic advantages of metallic powder blends according to the invention result.
By mixing the powder blends 18.1, 18.2, 18.3 and 18.4 listed in Table 3 with Licowax as a pressing auxiliary agent, the powder mixtures 19.1, 19.2, 19.3 and 19.4 are obtained. With these, it is possible to obtain, by monoaxial pressing, moulded bodies in the form of tensile test bars (A (area in direction of pressing): 6.35 cm2 l (length in the direction of pressing): 4-5 mm, p: 700 Mpa). The quantity of Licowax is selected in each case so that the compacts contain a total of 4 wt. % of organic constituents. This high content is necessary only for the PZD-free variant (18.1 and 19.1), to make it at all possible to obtain the compacts with sufficient green strength. To improve comparability, the remaining powders were provided with the same quantities of pressing auxiliary agents.
After production the moulded bodies were subjected to debinding (2 K/min from room temperature to 600° C.) under hydrogen. Sintering then takes place in a cool-wall kiln with a Mo heater (Thermal Technology) at four different temperatures (1290, 1310, 1340 and 1350° C.) under argon 4.8. Heating is carried out at 10 k/min, and the maximum temperature is maintained for 2 h. After sintering, the specimens were cooled to room temperature at a cooling rate of 10 to 15 K/min.
The results are summarised in the tables below. Although the greatest care was taken, it was not possible to produce testable specimens of 19.1 for 1310 and 1340° C. This is not due to the sintering temperature, but to the defects arising after pressing, which are not immediately visible, but frequently result in destruction after debinding. Such problems did not arise with 19.2 to 19.4.
It can be established that (in so far as can be determined) all properties of the specimens according to the invention (19.2, 19.3 and 19.4) were the same as or better than those of the conventional powder 19.1. At optimum temperatures, an improvement in Rm of +40-130% (Table 5.1), in Rp0.2 of 5-45% (Table 5.2), in At-Fmax of +0-270% (Table 5.3) and in of 0-2% (Table 5.4) was achieved. It should be stated, nevertheless, that the sintering process has so far not been optimised. Once this has been done, an improvement in the properties of 19.2 to 19.4 can be expected, as they have considerable advantages in the reproduction of properties as a result of their significantly lower tendency to ‘pressing defects’
The PZD powder is produced in the same way as Example 1. Instead of Nimonic® 90 powder, an atomised Fe20Cr10A10.3Y alloy is used as an educt. The PZD powder produced is called 20.1 (PZD-720) and the reference powder 20.2 (KON-720). Table 6 contains information about the powder blends processed. Licowax was used as the pressing auxiliary agent.
The powders contained in Table 6 are processed to tensile test bars (A:6.35 cm2, 1:4 . . . 5 nm; p:700 MPa). Test specimens were produced for dilatometer measurements by abrasive cutting (vertically to the direction of pressing), which were then measured vertically to the direction of pressing. Measurement comprised, in addition to slow heating at a heating rate of 2 K/min from room temperature to 500° C. for debinding, heating to 1320° C. at 10 K/min (holding time: 10 min) and cooling at a cooling rate of 10 K/min from 1320° C. to room temperature. The result is shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2005 001 198.5 | Jan 2005 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP06/00085 | 1/7/2006 | WO | 00 | 7/26/2007 |
