Patent Application
20050036899
The present invention relates to sintered components and method for producing sintered components from a sinterable material.
The usual sintering process comprises the steps of filling a sinterable material/composition into a press mold, pressing it to a so-called green compact, and sintering this green compact at sintering temperatures, followed if necessary by a homogenizing annealing, and also a subsequent sizing and possibly a hardening. In particular, the step of sizing is time-consuming and costly, since the component taken from the sintering step proper often does not have sufficient dimensional stability. Typically, a sizing is correspondingly indispensable.
Moreover usually during sizing the already sintered component is acted on with pressure and thereby further compressed, in order to attain a higher density and also hardness. Sizing thus represents a step in sinter processing, which because of its many-layered nature is not only determinative for the quality of the final sintered component to be delivered, but also is negative from the economic viewpoint.
There is therefore a need for a method by means of which the sizing step is either omitted or simplified such that the method becomes more economical. At the same time it must be ensured that the delivered sintered components have high strength and/or high densities and hardnesses, which are sufficient for the corresponding uses.
The present invention relates to sintered components and methods for preparing sintered components, which include the steps of introducing a sinterable material into a first mold, pressing the sinterable material to form a green compact; at least partially post-compressing the green compact in a second press mold, and, sintering the green compact. The density of the post-compressed green compact is greater than the density prior to compression.
The sinterable materials includes conventionally known sinterable materials, such as for example, iron-containing powders, aluminum-containing powders, ceramic-containing powders, metal-containing powders, or combinations thereof.
The present invention relates to sintered components and methods for preparing sintered components, which include the steps of introducing a sinterable material into a first mold, pressing the sinterable material to form a green compact; at least partially post-compressing the green compact in a second press mold, and, sintering the green compact. The density of the post-compressed green compact is greater than the density prior to compression.
Methods for producing sintered components and composite parts from a sinterable material, include steps wherein, the sinterable material, comprising 0.6-1.8 wt. % of a binder or lubricant, with respect to the total amount of the powder mixture, is filled into a first press mold; in a second step, the sinterable material is pressed to a green compact; in a third step, the green compact is at least partially post-compressed in a second press mold by uniaxial compression; in a fourth step, the post-compressed green compact is sintered, a density being attained in the post-compression performed in the third step which is about 2 to 40%, preferably 5-30%, more preferably 15-25%, above that obtained before post-compression.
Preferably, in the second step of the method according to the invention, green compacts with an initial density in a range of 2.1-2.5 g/cm3, preferably 2.2-2.4 g/cm3, more preferably 2.25-2.38 g/cm3, measured according to DIN ISO 2738, are pressed.
The method according to the invention has the great advantage that components can be produced by means of the high density already attained in the third step before the actual sintering and on the one hand have outstanding strength values, and on the other hand also have extremely high densities and hardnesses. In particular, by the post-compression according to the method according to the invention, the usual post-treatment steps following the sintering step, such as sizing and/or hardening by hot storage, can be considerably shortened, or else possibly the usual reheating or else the sizing can be omitted. By this shortening of the whole process, a productivity increase and thereby an economic advantage is attained.
By the post-compression in the third step of the method according to the invention, the advantageous result is attained that the oxide layers present on the surface of the material used are mechanically broken, so that a better cold welding between the individual material particles is attained in the pressing process. Furthermore by this means the diffusion of the individual material particles during the sintering process proper is also improved. Components with increased strength values and in particular higher hardness can hereby be attained.
The pressing process taking place in the second and third steps of the method according to the invention can take place both at elevated temperatures, particularly with addition of the abovementioned media, particularly polyethylene glycol (hot pressing); however also at room temperature (cold pressing), and likewise by vibrational compression. By “vibrational compression” is to be understood here a method in which a vibration is at least intermittently superimposed on the pressing process; the vibration can for example be introduced via at least one of the press plungers. A combination of the abovementioned press methods is also possible. Sinterable materials are, in particular, powder or powder mixtures, particularly metal powder and/or ceramic powder, for example of steels such as chromium-nickel steel, bronzes, nickel-base alloys such as Hastelloy, Inconel, metal oxides, metal nitrides, metal silicides and the like, and in particular aluminum-containing powders or powder mixtures; the powder mixtures can also contain high-melting components, such as for example platinum or the like. The powder used and its particle size are dependent on the respective intended use. Preferred iron-containing powders are the alloys 316 L, 304 L, Inconel 600, Inconel 625, Monel and Hastalloy B, X and C. Furthermore, the sinterable material can be partially or completely composed of short fibers or fibers, preferably fibers with diameters between about 0.1 and 250 μm and a length of a few μm up to millimeter size and up to 50 mm, e.g. bonded metal fibers.
By sintered components are to be understood to be components which are produced completely from a sinterable material; on the other hand, there are hereby understood also composite parts, where the base member of such a composite part can for example be produced from an aluminum-containing powder mixture and the member further connected to the base member from a further material, for example iron or cast steel, sintered or solid, or of solid cast aluminum. On the other hand, the composite part can for example have a sintered layer of an aluminum-containing powder mixture only on the end face or its surface, whereas the base member is for example of steel or cast iron, sintered or solid. The sintered components can be sized and/or heat hardened.
If it is desired to produce composite parts which for example are to consist of a sintered layer of the sinterable material on the end face of a member consisting of steel or cast iron, in a first step of the method according to the invention the sinterable material is applied to the base member, for example by conventional methods; it can also be provided, for example, to spray on the material in powder form (wet powder spraying, WPS). It is necessary to prepare a suspension of the sinterable material for this purpose. The suspension necessary for this purpose preferably includes solvents, binders, stabilizers and/or dispersing agents. Particularly preferred solvents are chosen from a group comprising water, methanol, ethanol, isopropanol, terpenes, C2-C5 alkenes, toluene, trichloroethylene, diethyl ether and/or C1-C6 aldehydes and/or ketones. Preferred solvents are those evaporable at temperatures below 100° C. The amount of the solvent used lies in a range of about 40 through 70 wt. % with respect to the sinterable powder mixture used, preferably in a range of about 50-65 wt. %.
The post-compression (which may also be termed intermediate compression) taking place in the third step can be performed by means of the usual and known method for pressing a green compact. Thus for example the green compact pressed in the second step can be again introduced into a conventional mold and at least partially post-compressed in this by corresponding press plunger. The post-compression tools can be completely or partially of conical design, so that particularly high compressions can be attained at given predetermined places of the green compact.
In a preferred embodiment, the green compact is dewaxed in a further step before the third step. The dewaxing preferably takes place under nitrogen, hydrogen, air and/or mixtures of the said gases, particularly also with a specific supply of air. Furthermore this dewaxing can take place with endogenous and/or exogenous gas, however also in vacuum. The dewaxing can preferably be performed by means of applied microwaves and/or ultrasound, or else with only microwaves to control the temperature. Finally, the dewaxing can also be performed with solvents such as alcohol and the like, or by supercritical carbon dioxide with or without the effect of temperature, microwaves or ultrasound or combinations of the said methods.
In a further embodiment, advantageously a mold in which the possibly dewaxed green compact is introduced is sprayed with a lubricant before introduction of the green compact. The dewaxed green compact can also be soaked in lubricant. Furthermore it is particularly advantageous for the sintering process to be performed under nitrogen with a dew point below −40° C., preferably below −50° C. Here the sintering preferably takes place under pure nitrogen. Furthermore the sintering can also be performed under air, mixtures of nitrogen and hydrogen with or without specific air supply, endogenous gas or exogenous gas, or in vacuum; sintering can take place by superposed microwaves or else with microwaves for temperature control.
A heat treatment, particularly a homogenizing annealing, which may possibly be necessary, can follow the sintering step, preferably directly. Here the heat treatment can be performed in dependence on the chemical composition of the obtained component. Alternatively or additionally to the heat treatment, the sintered component can be quenched from the sintering or homogenizing annealing temperature, preferably in water or else by means of a steep gas cooling.
Before or after sintering, an additional surface compression, in general an introduction of internal pressure stresses in the surface region, is possible by sandblasting or shot peening. Likewise, a sizing may be performed before or after the homogenizing annealing. The sizing is performed at room temperature or an elevated temperature, up to the forging temperature, also with the use of pressures up to 900 N/mm2. If necessary, sizing can be performed even above the solidus line, it then being possible also to remove the component directly from the sintering heat.
The sizing and/or forging tools used for sizing can be wholly or partially of conical shape, whereby particularly high compressions can be attained at given regions of the component. The temperature of the sizing and/or forging tools can differ according to the component to be processed, and can possibly be kept in the isothermal range. A surface compression or application of internal pressures to the component is also possible before or after sizing.
Finally, coatings may be applied to the sintered component. Processes are here preferred with which the components are hard coated and/or anodized, for example, thermal spray processes such as plasma spraying, flame spraying, or else physical and/or chemical processes such as PVD, CVD and the like. However, coatings may also be applied in purely chemical ways such as for example by lubricant lacquers which may contain Teflon, or nanocomposite materials, can be applied. The surface of the composites can be modified by a coating according to the hardness, roughness and coefficient of friction in a predetermined manner according to the use purpose.
Preferably used as the sinterable material is a powder and/or powder mixture containing iron and/or aluminum, more preferably aluminum-containing powder mixtures. By the use of powder-form materials, high densities of the as yet uncompressed green compacts can already be obtained before the actual sintering step.
In one embodiment, a sinterable powder mixture is preferably used comprising 60-98.5%, with respect to the total amount of the powder mixture, preferably 85-98.5 wt. %, of an Al-based powder of metals and/or their alloys, comprising Al, 0.2-30 wt. % Mg, 0.2-40 wt. % Si, 0.2-15 wt. % Cu, 0.2-15 wt. % Zn, 0.2-15 wt. % Ti, 0.2-10 wt. % Sn, 0.2-5 wt. % Mn, 0.2-10 wt. % Ni and/or less than 1 wt. % of As, Sb, Co, Be, Pb and/or B, the weight percent fractions being respectively based on the total amount of Al-based powder; and 0.8-40 wt. %, based on the total amount of the powder mixture, preferably 1.5-20 wt. %, of a metallic powder, chosen from a first group of metals and/or their alloys, consisting of Mo, W, Cr, V, Zr and/or Yt.
By means of the addition of the first group of metals and/or their alloys consisting of Mo, W, Cr, V, Zr and/or Yt, powder-metallurgical components having a very high hardness can be produced with this alloy mixture. The values for the hardness of the components produced with a powder chosen from the first group of metals and/or their alloys, in comparison with those without addition of this first group of metals and/or their alloys, are increased by 5-35%, preferably 10-25%. By the addition of the first group of metals and/or their alloys to an Al-based powder, the cold welding of the particles to one another brought about by the pressing process, particularly the post-compression, is improved. Hereby, finally, the diffusion of the individual particles during the sintering process is also improved, so that components with higher strength values and higher hardnesses are obtained.
The sinterable powder mixture furthermore advantageously includes a second group of metals and/or their alloys, consisting of Cu, Sn, Zn, Li and/or Mg. The addition of the said second group of metals and/or their alloys presumably has the effect that in particular still during the pressing process, in particular during the post-compression, an alloy and/or intermetallic phase is formed with the Al-based powder. Hereby the formation of oxide layers on the surface of the Al-based powder used is prevented. In addition, at least partially in the sintering process proper, the second group of metals and/or their alloys transform into an at least partially liquid state at the sintering temperature, whereby the binding of the first group of metals in particular and of their alloys, and/or their alloying to the aluminum-based powder, is improved.
The ratio of the amount of the first group of metals and/or their alloys to that of the second group in the powder mixture is preferably in a range of 1:8 to 15:1 parts by weight. The ratio preferably lies in a range of 2:1 to 6:1 parts by weight. With such mixing ratios, a maximum binding of the metals and/or alloys of the first group to the Al-based powders is attained. Components with high hardness can hereby be obtained with the powder mixture.
In a further advantageous embodiment of the invention, the Al-based powder has, besides Al, 0.2-15 wt. % Mg, 0.2-16 wt. % Si, 0.2-10 wt. % Cu, and/or 0.2-15 wt. % Zn, respectively with respect to the total amount of Al-based powder. Furthermore the second group of metals and/of their alloys preferably has Cu, Zn and/or Sn.
Preferably the sinterable powder mixture includes lubricant in an amount of 0.2-5 wt. % based on the total amount of the powder mixture. As lubricants there can be provided on the one hand self-lubricating means such as, for example, MoS2, WS2, BN, MnS as well as graphite and/or other carbon modifications such as coke, polarized graphite, and the like. Preferably 1-3 wt. % of lubricant is added to the sinterable powder mixture. By the use of the said lubricant, self-lubricating properties are conferred on the components produced from the sinterable powder mixture.
The sinterable powder mixture can furthermore comprise binders and/or lubricants. These are preferably chosen from a group comprising polyvinyl acetate, waxes, in particular amide waxes such as ethylene-bisstearoylamide, shellac, polyalkylene oxides and/or polyglycols. Polyalkylene oxides and/or polyalkylene glycols are preferably used as polymers and/or copolymers with average molecular weights in a range of 100-500,000 g/mol, preferably 1,000-3,500 g/mol, more preferably 3,000-6,500 g/mol. The media are preferably used in an amount in a range of 0.01-12 wt., preferably in a range of 0.5-5 wt. %, more preferably in a range of 0.6-1.8 wt. %, respectively with respect to the total amount of the powder mixture. The binder and/or lubricant also facilitate the removal from the press mold of the components made from the sinterable powder mixture.
The powder mixture can be produced by mixing the individual components in conventional devices such as tumble mixers, both when hot (hot mixing) and also at room temperature (cold mixing); hot mixing is preferred.
Sintered components produced by the disclosed methods have strength values and hardnesses which clearly exceed those alloys which are produced by conventional methods. The sintered components according to the invention preferably have a tensile strength of at least 140 N/mm2, measured according to DIN EN 10002-1. More preferably the tensile strength is more than 200 N/mm2, yet more preferably more than 300 N/mm2. Advantageously, the components sintered according to the invention have an elasticity modulus of at least 70 kN/mm2, measured according to DIN EN 10002-1, and more preferably is greater than 80 kN/mm2.
In a further preferred embodiment, the sintered components have a hardness (HB 2.5 mm/62.5 kg) of at least 100, measured according to DIN EN 24498-1. The hardness is more preferably greater than 110, and yet more preferably greater than 125.
In a further preferred embodiment, the sintered component is formed as a gearwheel, pump wheel, particularly oil pump wheel, and/or rotor set.
This and further advantages of the invention are explained using the following Examples. It will be appreciated by one skilled in the art that the descriptions given herein are for exemplary purposes only and is not intended in any way to limit the scope of the invention.
An Al-based powder of the composition Al4CulMg0.5Si (corresponds to the designation AC2014 of a conventional aluminum alloy, the basic powder having 4 wt. % Cu, 1 wt. % Mg, 0.5 wt. % Si and 94.5 wt. % Al, with respect to the total amount of powder) of the Company ECKA Granulate GmbH & Co. KG, Velden, Germany, with the company designation ECKA Alumix 123 (92.5% Al), and 1.5 wt. % of an amide wax as binder, of the Hoechst Company with designation Mikrowachs C, were mixed with molybdenum or tungsten powder according to the following Table 1. Mixing took place in a tumble mixer by addition of the molybdenum or tungsten powder to the already present Al-based powder at room temperature during 5 minutes.
The Al-based powder had a grain size distribution between 45 and 200 μm, the average particle diameter D50 being 75-95 μm. The admixed molybdenum or tungsten powder was from the Company H.C. Starck Gmbh & Co. KG, Goslar, Germany and had an average particle diameter D50 of 25 μm with a grain size distribution in a range of 5-50 μm.
The powder mixture was then placed in a die mold and pressed under a pressure of about 175 N/mm2 (calculated for a wheel end surface of 20 cm2) for about 0.2-0.5 sec. at room temperature to a green compact in the form of a pump wheel. The density of the green compact was about 2.35-2.38 g/cm3. The thus produced green compact was then dewaxed for about 30 min at about 430° C., and was then sintered at a sintering temperature of 610° C. under a pure nitrogen atmosphere with a dew point of −50° C. in a belt furnace set to a speed of 3.4 m/h for 30 min. Here the green compacts were on Al2O3 plates. A homogenizing annealing was then performed for 1.5 hours at a temperature of 515° C. The sintered pump wheel was then shock cooled by quenching with water with a temperature of about 40° C. for 10 sec.
A sizing was then performed to a theoretical density of 97-98% using a pressure of about 810 N/mm2 at 200° C.
After the sizing, yet another hardening of the sintered pump wheels was carried out with heating at 160° C. for 16 h. Then tensile strength Rm, elasticity modulus, and extension according to DIN EN 10002-1 were determined on standardized samples.
Furthermore the hardness according to DIN EN 24498-1 (Brinell hardness) was determined with a hardened steel ball as the penetrating member with a diameter of 2.5 mm and with a load of 62.5 kg. The values determined are reproduced in Table 1.
Rm* = Tensile strength
A** = Extension
Elast. = elasticity
The above trial under numeral 1 was repeated, a copper powder however being admixed, sold by the Company Eckhart Granules under the trademark ECKA KUPFER CH-S. The admixture took place such that the molybdenum powder or the tungsten powder was first mixed with the copper powder in a tumble mixer at room temperature for 5 min and this was then mixed with the Al-based powder in a tumble mixer. The copper powder had an average particle diameter D50 of 25 μm and a grain size distribution in a range of about 5 to about 50 μm. The copper powder was produced electrolytically; the individual particles had a dendritic form.
Different mixtures were produced, these being described under numeral 1, and were sintered to give pump wheels, with and without post-compression. For the post-compression, the green compact was dewaxed at 430° C. for 30 min under a nitrogen atmosphere and then placed in a die mold identical to the first mold and sprayed with the lubricant GLEITMO 300, Fuchs Lubritech GmbH, Weilerbach, Germany, and was post-compressed at a pressure of 760 N/mm2 for about 0.2-0.5 sec. at room temperature, such that the density of the post-compressed green compact was about 2.8-2.9 g/cm3 and thus about 19-23% greater than that of the non-post-compressed pump wheels and thus having about 95% of the theoretical density.
The green compacts thus produced were sintered as described hereinabove, sized to a theoretical density of 97-98% at a pressure of 810 N/mm2, however at room temperature, and hardened. The mixing ratio of molybdenum or tungsten powder to the copper powder was 5:1 parts by weight. The mixing ratios and the physical values determined are shown in Table 2.
Rm* = tensile strength
A** = extension
As can be seen from Table 2, the physical properties are affected positively by a post-compression. In particular, a further increase of the hardness of the produced pump wheels can be attained.
It is possible with the present invention to produce sintered components, particularly based on an Al-based powder, which not only have excellent strength values, but also in particular have high hardness. Such articles can hereby advantageously be used at strongly stressed places, particularly in the motor or else in gears. In addition, sintered components can be more favorably and quickly produced by the possible omission of sizing and of hardening by hot storage.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102 03 283.1 | Jan 2002 | DE | national |
This application is a continuation of International Application No. PCT/EP03/00529 filed Jan. 21, 2003, which claims priority to German Application No. 102 03 283.1 filed Jan. 29, 2002, each of which is hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP03/00529 | Jan 2003 | US |
| Child | 10909202 | Jul 2004 | US |
