Patent Application
20160222310
This application claims the benefit of German Patent Application No. 102014226094.9 filed Dec. 16, 2014, which is hereby incorporated by reference for all purposes.
The present invention concerns a compacting auxiliary for powder metallurgy and a sinterable mixture which contains the compacting auxiliary.
The production of products in powder metallurgy essentially comprises three sub-areas. First, powders from elemental metals or alloys are required. These metallic powders are then placed alone or in mixtures in a corresponding mould (pressing tool). The subsequent component shape is formed in this mould under the effect of pressure. This may take place either at room temperature (cold pressing) or at elevated temperature (hot pressing). The compact thus obtained is referred to as a green compact. This green compact is then sent on for actual sintering. This is a type of heat treatment in which the powder particles of the metal used are incorporated into a compound at their contact surfaces by fusion of the metal atoms. In powder metallurgy, it is possible to use the most widely-varying metal powders as starting materials and thus to adjust the properties of the products obtained.
It is a challenge to produce moulded parts with the highest possible density, and also to fill the metals used as needed either evenly or in particular areas at a particularly high concentration into the pressing tool in which the green compact is produced.
The use of a compacting auxiliary is common in powder metallurgy. On the one hand, this ensures that the flowability of the metal or metal alloy powder used is retained and thus distributed in smaller cavities in the pressing tool depending on the geometry of the product to be produced. At the same time, it must be ensured that segregation does not take place when various powders are used as educts. However, as the compacting auxiliary must not influence the properties of the final product, it is removed while heating from the mixture prior to the actual sintering process. This must be carried out in such a way that the form of the component is not changed. As the compacts are subjected to minor dimensional changes in sintering, it may be necessary, depending on the requirements for the finished component, to take these into consideration. For this purpose, components having extremely narrow dimensional tolerances are calibrated in separate tools after sintering. In order to manufacture components for extremely high stresses, components may also be subjected after sintering to a high-temperature forging process. In contrast to conventional forging, however, there is no subsequent need to remove burrs.
The density of the finished moulded part essentially depends on the density of the green compact achieved, referred to as green density, and in this case, in contrast to compaction of ceramic powders, the metal powder particles undergo plastic deformation because of their differing geometric structure and the associated number of mobile lattice defects. Because of their particle geometry, in metallic powders, unlike ceramic powders, the sliding capacity of the individual powder particles with respect to one another must be reduced so that loose fill in the mould already has a pore volume that can only be almost completely eliminated in compacting by using extremely high compaction pressures. However, high compaction pressures result in greater abrasion of the pressing tool in the compaction process and also cause increased ejection sliding friction of the finished green compact in the pressing die, so that in this case as well, higher ejection forces must be applied, with a corresponding increase in abrasion.
In order to avoid these drawbacks, WO 2010/1105740 A1 discloses a lubricant for powder metallurgy which comprises carnauba wax and at least one vegetable or animal-based fat. Carnauba wax as a component of a compacting auxiliary is also described in WO 2008/028589 A1. The compacting auxiliary described therein also comprises an amide.
DE 102 44 486 A1 discloses a compacting auxiliary which contains 20 to 60 wt % based on the total amount of the compacting auxiliary of a polyglycol and 40 to 75 wt % of a montan wax, also based on the total amount of the compacting auxiliary.
In order to obtain a green compact of the highest density, it is therefore preferred to keep the amount of compacting auxiliaries as low as possible. Reduction of the amount of compacting auxiliaries is accompanied in conventional compacting auxiliaries by increased wear in the compacting process and thus greater wear of the pressing tools or excessively high friction losses, so that the desired compaction is not achieved. The compacting auxiliary should therefore also favourably transfer the pressure applied into the inside of the green compact, so that ultimately, lower pressure is required, which means less wear on the pressing tool.
Surprisingly, it has been found that a compacting auxiliary which comprises at least one amide of a carboxylic acid and montan wax prevents the drawbacks known from the prior art. The object of the present invention is therefore achieved by means of a compacting auxiliary for powder metallurgy which comprises at least one amide of a carboxylic acid with 18 to 22 C atoms.
The compacting auxiliary according to the invention comprises at least one amide of a carboxylic acid (carboxylic acid amide). According to the invention, the compacting auxiliary can also comprise mixtures of different amides of the same carboxylic acid or amides of different carboxylic acids. Preferably, an amide of a carboxylic acid is used. According to the invention, the amide may be a primary, secondary or tertiary amide. Preferably, the amide is a primary amide.
According to the invention, the suitable carboxylic acid is a fatty acid with 18 to 22 carbon atoms. The carboxylic acid amide (fatty acid amide) shows, according to the invention, an acid number in the range of 0 to 10, preferably 0 to 5, and particularly preferably 0 to 1. The acid number (AN) is a chemical value for characterizing acidic constituents in fats. It denotes the weight of potassium hydroxide (in mg) required to neutralize the free fatty acids contained in 1 g of fat. The value is therefore given in mg KOH/g of fatty acid.
The melting point of the carboxylic acid amide according to the invention is preferably less than 110° C., more preferably in the range of 50° C. to 100° C. and particularly preferably in the range of 65° C. to 90° C.
Preferably, the iodine number of the carboxylic acid amide is in the range of 50 to 100, and more preferably between 95 or less and 60 or more. The iodine number (IN) is a fat parameter for characterizing fats and oils. It is the amount of iodine in grams that can formally be added to 100 g of fat (fatty acids, fatty acid amides, and comparable compounds).
The amide of a carboxylic acid ensures favourable lubrication in the core of the green compact. If the carboxylic acid has fewer than 18 carbon atoms, the compacting auxiliary becomes extremely soft. This results in the formation of powder agglomerates, so that homogeneous filling into the pressing tool is no longer possible. On the other hand, if the carboxylic acid has more than 22 carbon atoms, the compacting auxiliary will be too hard. In this case, homogeneous application to the metal powder is no longer possible.
Particularly preferably, the carboxylic acid comprises an ethylenic double bond. In particular, the carboxylic acid has 22 carbon atoms. Particularly preferably, erucic acid is used as a carboxylic acid.
In particular, the compacting auxiliary according to the invention comprises more than 50 wt % and less than 15 wt %, preferably 7.5 wt % to 13 wt %, and more preferably 10 wt % of the amide of a carboxylic acid. The weights given are based respectively on a total weight of the compacting auxiliary of 100 wt %. An amount of 5 wt % or less results in poor internal lubrication. If more than 15 wt % of the amide of the carboxylic acid is added to the compacting auxiliary, the compacting auxiliary becomes too soft in this case as well, making homogeneous filling of the powder provided with the compacting auxiliary into the mould possible only with difficulty. Compacting auxiliaries which are highly suitable for powder metallurgy with respect to both internal lubrication and strength contain between 7.5 wt % and 13 wt % of the amide of a carboxylic acid according to the invention.
In the present document, the terms amide of a carboxylic acid, carboxylic acid amide and fatty acid amide are used as synonyms. When “powders” or “metal powders” are described in the present application as starting products of the sintering process, this includes powders of elemental metals as well as powders of metal alloys. When values are given in “%”, this is understood to mean “wt %” unless otherwise indicated. When ranges or numerical ranges are given in the present application, it should be noted that these are not absolute values, particularly in the case of upper and lower range limits. Rather, it is obvious to the person having ordinary skill in the art that the result of the present invention can still be achieved in the case of deviations from the numerically defined values. In such cases, the range of deviation from the indicated numerical values or upper and/or lower limits can be to 5%. In indication of numerical ranges, the numbers lying within the ranges are also to be considered as disclosed.
According to the invention, the compacting auxiliary comprises a montan wax. Montan wax is a natural wax that can be extracted from certain types of lignite. It consists of a mixture of long-chain carboxylic acid esters such as esters of montanic acid. It also contains further constituents such as e.g. montan alcohol, resins, unsaponifiable constituents and trace minerals. The purified product is virtually colourless. As for the compacting auxiliary of the present invention, a correspondingly purified montan wax is preferably used. The montan wax preferably has an acid number in the range of 15 to 20 mg KOH/g. The drop point is preferably in the range of 80° C. to 90° C.
The compacting auxiliary according to the invention preferably comprises montan wax in an amount of 20 wt % to 95 wt %, preferably 22.5 wt % to 87 wt %, and particularly preferably 25 wt % to 60 wt %. The montan wax provides favourable green strength. Green strength means that the green compact retains its form even after removal from the pressing tool. Compared to the carnauba wax described in the prior art, montan wax shows improved green strength and improved lubricating properties. These in turn make it possible to work at a lower compaction pressure, which allows the wear on the pressing tool to be reduced. In addition, the frictional forces generated on the stamp (die) and the mandrel of the pressing tools are lower, which allows lower wear to be achieved.
Surprisingly, it has been found that a part of the montan wax can be replaced by an amide wax without having a negative effect on the green strength or lubricating properties of the compacting auxiliary which are achieved by means of the montan wax. In a preferred embodiment, the compacting auxiliary according to the invention therefore comprises an amide wax in addition to the amide of a carboxylic acid and a montan wax.
Waxes are substances that are defined by their mechanical and physical properties. According to this definition, a substance is designated a wax if it is kneadable, solid to stiff or brittle, shows a coarse or fine crystalline structure, and is translucent to opaque in colour but not glassy at 20° C., melts without decomposing at above 40° C., is slightly liquid (shows little viscosity) above the melting point, has a strongly temperature-dependent consistency and solubility, and is polishable under slight pressure. If more than one of the required properties listed above is not met, the substance is not a wax according to this definition of the German Society for Fat Science [DGF Unified Method M-I 1 (75)].
The amide wax of the present invention is a corresponding wax which is based on an amide of a long-chain carboxylic acid. According to the invention, the amide may comprise a primary and/or a secondary and/or a tertiary amide. Various amides can therefore be used according to the invention. The amide preferably has an acid number of 0 to 10 mg KOH/g, and more preferably 0 to 7 mg KOH/g. The melting point is preferably more than 100° C. and 160° C. or less. More preferably, the melting point is in the range of 110° C. to 155° C., and most preferably in the range of 130° C. to 155° C.
The amide wax is preferably a secondary amide. Particularly preferably, it is an amide, in particular a secondary amide, of a carboxylic acid with 28 to 45 carbon atoms. The carboxylic acid can be linear or branched, saturated, monounsaturated or polyunsaturated; preferably, it is a linear branched carboxylic acid. In particular, a carboxylic acid is preferred which comprises 35 to 40 carbon atoms. Particularly preferably, the amide wax is distearyl ethylene diamide.
The amount of the amide wax in the compacting auxiliary according to the invention is preferably 75 wt % or less based on the total weight of the compacting auxiliary, which is to be taken as 100 wt %. Preferably, the compacting auxiliary contains 20 wt % to 70 wt %, and more preferably 40 wt % to 65 wt % of the amide wax.
By replacing the montan wax with an amide wax which is more economical than the montan wax, it is possible to provide an economical compacting auxiliary which nevertheless has the same properties with respect to green strength and lubricating properties, such as a compacting auxiliary comprising a carboxylic acid amide and montan wax.
In a preferred embodiment, the compacting auxiliary according to the invention is composed of a carboxylic acid amide, a montan wax and an amide wax.
In a further embodiment, the present invention concerns a method for manufacturing a compacting auxiliary. According to the invention, the carboxylic acid amide and montan wax are preferably melted together. Even more preferably, the cooled melt obtained is ground. The product thus obtained can thus be used in powder metallurgy.
If the compacting auxiliary according to the invention also contains an amide wax in addition to the carboxylic acid amide and the montan wax, according to the invention, it is preferable to first melt the carboxylic acid amide and the montan wax together, cool off the melt obtained, and after cooling, grind it. The particles obtained in this manner are ground in a suitable mill together with the fine particulate amide wax.
Alternatively, it is also possible to melt carboxylic acid amide, montan wax, and optionally the amide wax together and then to atomize the melt obtained. However, it is preferable to melt only carboxylic acid amide and montan wax together and then grind them in solid form together with the amide wax, as this requires less energy than melting of the amide wax.
If a melt comprising the carboxylic acid amide and the montan wax, and optionally the amide wax, is atomized or ground, the product obtained preferably has a particle size distribution of x105 to 15 μm, x5020 to 35 82 m, x90 35 to 50 μm, and x99 <70 μm. Testing is conducted with a QICPIC from the firm SympaTec GmbH. Here, x10 means that 10% of the particles have a size in the range of 5 to 15 μm, x50 that 50% of the particles are in the size range of 20 to 35 μm, x90 that 90% of the particles are in the size range of 35 to 50 μm, and accordingly, x99 that 99% of the particles have a size of less than 70 μm.
In a further embodiment, the present invention concerns the use of the compacting auxiliary of the invention in powder metallurgy. The compacting auxiliary for powder metallurgy according to the invention is preferably used for the production of sinterable moulded parts. Sinterable moulded parts within the meaning of the present invention are understood to refer to moulded parts produced completely from a sinterable material, and this term is also understood to include composite parts in which the basic body of such a composite part can be produced for example from an aluminium or iron-containing mixture, and the body further bonded to the basic body can be produced from a further material, for example cast steel, sintered or solid, or from a solid aluminium cast. Conversely, the composite part may also have, for example, a sintered layer only on its end faces or its surface, while the basic body is produced for example from steel or cast iron, sintered or solid. In this case, the sintered moulded parts can preferably be calibrated and/or further hardened by heat using the lubricant according to the invention. Sintered moulded parts within the meaning of the present invention are in particular moulded parts which show a density after sintering of 7.2 g/cm3 or more, such as e.g. pump parts or gear parts of such synchronizer bodies. These moulded parts must be of high density because of the high stress to which they are subjected in further use. The production thereof is therefore a major challenge which is met by means of the compacting auxiliary according to the invention.
In this case, the compacting auxiliary according to the invention can be used in both cold pressing and hot pressing. In cold pressing, the compaction process takes place at room temperature. In hot pressing, the die is heated. Surprisingly, it has been found that the compacting auxiliary according to the invention shows particularly favourable properties in hot pressing with respect to green strength and lubricating properties, and thus protection against wear as well. Particularly preferably, the compacting auxiliary according to the invention is therefore used in hot pressing at a die temperature in the range of 25° C. to 100° C., in particular 35° C. to 90° C., more preferably 40° C. to 75° C., and most particularly preferably 50° C. to 65° C.
The sinterable moulded parts are preferably composed of a mixture comprising at least one metallic material and/or plastic material and at least one lubricant for powder metallurgy according to the invention. Sinterable metallic and/or plastic materials within the meaning of the present invention are in particular powder or powder mixtures of metallic, ceramic and/or plastic components, for example low-alloy steels, chromium nickel steels, bronzes, nickel-based alloys such as Hastalloy and Inconel, metal oxides, metal nitrides, metal silicides and the like, as well as aluminium-containing powders or mixtures, with it also being possible for these mixtures to contain high-melting-point components such as platinum or the like. The powders used and the particle sizes thereof depend on the respective purpose of use. Exemplary iron-containing powders are the alloys 316L, 304L, Inconel 600, Inconel 625, Monel and Hastalloy B, X and C, as well as 17-4PH. Particularly preferably, low-alloy steel powders such as iron-carbon steels, Distaloy AB, AE, DE and HP (Höganäs AB, Sweden) and Ancorsteel 4300 (Hoeganaes Corp., USA) are used. Titanium and/or titanium alloys are also suitable as materials, i.e. when mixed with other materials, in particular iron-containing powders. Moreover, the metallic material and/or plastic material may be completely or partly composed of plastic fibres or fibres, preferably fibres having diameters of about 0.1 μm to about 2 μm and a length of a few microns to about 50 millimetres. In addition, one may also add carbon in particular for the formation of desired alloys in the corresponding amount and other additives such as binders, etc., to the metallic materials and/or plastic materials.
The object of the present invention is further achieved by means of a mixture for the production of sintered moulded parts. These mixtures according to the invention comprise at least one powdery metal and/or powdery alloy and a compacting auxiliary according to the present invention. Surprisingly, it has been found that an amount of 0.1 wt % to 2.0 wt % of the compacting auxiliary, based on the total weight of the mixture, is sufficient to obtain a sinterable mixture in which sufficient green strength and sufficient pressure are achieved inside the green compact. It has also surprisingly been found that the concentration of the compacting auxiliary can be reduced by 0.2 wt % compared to pure amide waxes described in the prior art without negatively affecting the method or the product obtained. Alternatively, with the same amount of the compacting auxiliary used, a higher pressure and thus a higher density of the green compact can be achieved.
According to the invention, the compacting auxiliary can be applied to the metal powder or the alloy powder in elemental form or by spraying. In particular, application by spraying is advantageous because this makes it possible to prevent the occurrence of any dust.
According to the invention, the mixture for the production of sintered moulded parts may also comprise further additives known in the prior art, such as e.g. aerosils, graphite, self-lubricating materials or binders. In particular, carbon for the formation of desired alloys may also be added to the mixture in a suitable amount, as well as further additives known to the person skilled in the art such as e.g. binders or the like. Moreover, the sinterable mixture may also comprise at least one stabiliser and/or at least one anti-agglomerant. The sinterable mixture may also comprise self-lubricating agents, for example, in addition to graphite, alternatively or additionally MoS2, WS2, BN and/or other carbon modifications such as coke, polarised graphite and the like. Moreover, the sinterable mixture may also comprise aerosils and other additives known to the person skilled in the art, selected according to the purpose of use.
The present invention also concerns a method for producing the mixture according to the invention, wherein
in a first step, the at least one carboxylic acid amide and montan wax contained in the compacting auxiliary are melted together; and
in a second step, the compacting auxiliary produced according to the first step is added to the metallic material.
More preferably, it is provided that after the first step of the method according to the invention, the melt obtained is atomized or, after cooling, the cooled melt is ground. Surprisingly, it has been found that in the method according to the invention, compaction densities of the green compacts produced using the mixture of the present invention, whether by uniaxial, biaxial or isostatic compacting, are obtained that are significantly greater than those which can ordinarily be obtained with the compacting auxiliaries known from the prior art.
Alternatively, a method for producing the mixture according to the invention is provided according to which
in a first step, the at least one carboxylic acid amide and montan wax contained in the compacting auxiliary are melted together, then mixed or ground together with the amide wax; and
in a second step, the compacting auxiliary produced according to the first step is added to the metallic material.
Using this alternative method, it is also possible to obtain green compacts by the metallurgical compaction process with high compaction densities and correspondingly high green strength.
The object of the present invention is also achieved by means of a sintering method for manufacturing a component specifically having a density of 7.2 g/cm3 or more using the above-described compacting auxiliary, and by means of a component obtained using this method. The component should preferably be pump parts and/or gear parts, such as e.g. synchronizer bodies, with a density of 7.2 g/cm3 or more.
The advantages of the present invention will be explained in further detail by means of the following example.
These and still other advantages of the invention will be apparent from the detailed description and drawings. What follows is merely a description of some preferred embodiments of the present invention. To assess the full scope of the invention the claims should be looked to as these preferred embodiments are not intended to be the only embodiments within the scope of the claims.
A compacting auxiliary according to the invention comprising 10 wt % of an amide of erucic acid (Crodamide® ER from the firm Croda Chemicals Europe Ltd, UK), 30 wt % of montan wax (Waradur® from the firm Völpke Special Products GmbH, Völpke, Germany) and 60 wt % of an amide wax (distearyl ethylene diamide, marketed under the name Crodamide® EBS by the firm Croda Chemicals Europe Ltd, UK), was used as a compacting auxiliary. The mixture for the production of sintered moulded parts comprised iron as a metal powder, as well as 2 wt % of copper, 0.6 wt % of graphite and 0.6 wt % of the compacting auxiliary of the invention. The total weight was 100 wt %.
The compacting auxiliary according to the invention was composed of 10 wt % of erucic acid amide, 30 wt % of montan wax and 60 wt % of amide wax. The carboxylic acid amide and the montan wax were melted together, reduced to flakes after cooling, and these flakes were then ground with the amide wax. Flakes within the meaning of the present application are small particles primarily showing 2-dimensional extension. Their thickness ranges from only a few nanometres to 1 μm. The flat extension may have any desired regular or irregular shape. Based on an approximately round shape, the diameter is less than 70 μm. Based on an oval shape, the length of the longest axis is less than 70 μm.
For comparison purposes, a mixture was produced that also contained iron as a metal powder, 2 wt % of copper, 0.6 wt % of graphite and 0.6 wt % of a compacting auxiliary known from the prior art, specifically an amide wax marketed under the brand name “Licowachs® C” by the firm Clariant, Germany. In this case as well, the total weight was 100 wt %.
The powder mixtures were homogenously mixed with the respective auxiliary. The sinterable powder mixtures produced in the manner were filled into a conventional pressing tool and compacted at varying pressures. The densities of the green compacts produced using both the compacting auxiliary of the invention and an amide wax known from the prior art were determined at different compaction pressures according to DIN ISO 3369.
Sinterable mixtures such as those described above were compressed at a compaction pressure of 700 MPa. The values obtained in this process are shown in Table 1 below. As can be seen from the Table, at the same compaction pressure, specifically 700 MPa, the pressure inside the green compact is markedly higher with the compacting auxiliary according to the invention than in use of a compacting auxiliary known from the prior art. While the pressure inside the green compact measured in use of a known compacting auxiliary is only 276 MPa, it is 483 MPa in use of a compacting auxiliary according to the invention.
By using the compacting auxiliary of the invention, markedly less pressure is required in order to achieve the same compaction, thus making it possible to achieve reduced wear on the tools. This can also be seen from the fact that the frictional forces acting on the die (stamp) and the mandrel are lower than in the use of a conventional compacting auxiliary. In addition, the ejection pressure required to remove the green compact from the suitable mould can be markedly less in using the compacting auxiliary of the invention than in use of a compacting auxiliary known in the prior art. At the same pressure, using the compacting auxiliary of the invention makes it possible to obtain a moulded part having a density markedly greater than that of parts manufactured using a compacting auxiliary known from the prior art.
This can again be seen from the diagram attached as
It should be appreciated that various other modifications and variations to the preferred embodiments can be made within the spirit and scope of the invention. Therefore, the invention should not be limited to the described embodiments. To ascertain the full scope of the invention, the following claims should be referenced.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102014226094.9 | Dec 2014 | DE | national |
