Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Powder pretreatment
Reexamination Certificate
1999-07-27
2001-05-01
Jenkins, Daniel (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Powder pretreatment
C419S037000, C419S038000, C419S054000
Reexamination Certificate
active
06224823
ABSTRACT:
DESCRIPTION
The problem encountered when producing metallic shaped parts with a powder-metallurgical process is that the shaped parts must be produced with the highest possible density since the metallic powders are initially filled into a mold cavity and are then compacted at high pressure with the aid of single-axle or multi-axle hydraulic or mechanical presses. A shaped part obtained in this way, which is generally referred to as a green compact, is subsequently sintered in a thermal process, mostly in a protective atmosphere, so as to result in a solid, accurately dimensioned metal shaped part.
The density of the finished, sintered shaped part in this case depends essentially on the green compact density that can be achieved. In contrast to the compacting of ceramic powders, the metal powder particles experience a plastic deformation, owing to their different crystalline structure and the number of movable lattice defects connected with this. With metallic powders, the sliding ability of the individual particles relative to each other is reduced as a result of the particle geometry—also in contrast to ceramic powders—so that the loose bulk material in the mold already has a pore volume, which can be removed completely only if extremely high forces are used for the compacting operation. However, high compacting forces result in high wear of the compacting tool during the compacting operation and also lead to increased sliding friction in the mold cavity during the ejection of the completed green compact, so that higher ejection forces with correspondingly increased wear must be generated in this case as well. On the other hand, high ejection forces carry the danger of an undesirable local secondary compacting and the formation of cracks in the green compact.
In order to avoid these disadvantages, a process was suggested in the EP-A-0 375 627, whereby a lubricant that is liquefied with a liquid solvent is added to the metal powder to be compressed. The lubricants suggested for this include metal stearates, particularly lithium stearate or zinc stearate, as well as paraffin products, waxes, natural or synthetic fat derivatives, which are first liquefied, e.g. with organic paraffin solvents as liquid solvent. The disadvantage of this process is that the dry metal powder must initially be mixed with a two-component lubricant system, namely the stearates and the solvents, wherein this preliminary mixture for the most part must be homogeneous. Another disadvantage is that prior to filling the powder mixture into the pressing mold, it must first be preheated to a relatively high heat, up to the range of the softening point for the lubricant used. This entails the danger of baking on while moving through the feeding devices for the mold. Following the completion of the compacting operation and the ejection of the green compact, the lubricant must be vaporized in a separate operation before the green compact can be heated to the actual sintering temperature. In the process, it cannot be avoided that lubricant residues remain in the sintered body, which can also result in disadvantages, depending on the application and the type of pure or alloyed metal powder used.
An iron-based metallurgical powder composition is known from the EP 0 559 987, which contains an organic binder for the iron-based powder components and the alloy powder components. In order to improve the compacting behavior, the organic binder contains a share of polyalkylene oxide, which must have a molecular weight of at least 7000 g/mol. However, considerably higher molecular weights are preferred.
It is the object of the invention to improve the above-described process.
This object is solved with a process for producing sinterable metallic shaped parts from a metal powder, mixed with an auxiliary compacting agent, which contains at least in part components from the polyalkylene oxide family, is filled into a pressing mold and, following the compacting under pressure, is ejected as compressed shaped part from the pressing mold. The use of auxiliary compacting agents containing at least components from the polyalkylene oxide family, particularly polyalkylene glycols and preferably polyethylene oxides, especially in the form of polyethylene glycols, surprisingly showed that the compacting forces required to achieve higher densities and higher green compact strengths are much lower than for other auxiliary compacting agents. The forces needed for ejecting the compacted shaped part from the mold are also clearly reduced, so that the aforementioned disadvantages of the known processes are avoided. Owing to the “lubrication” of the powder particles moving relative to each other during the compacting operation, the powder mixture does not require a special binder since it is possible to achieve a high green compact strength during the compacting operation in addition to the high density, owing to a much higher “packing density” of the powder particles and thus an increase in the direct contact between the metal particles in the powder. A high green compact strength is always desirable if the green compact must be reworked further prior to the sintering. “Metal powder” within the meaning of the invention refers to the powder mixture intended for the production of the shaped part, including all alloying agents and other admixtures, with the exception of the auxiliary compacting agent.
A special advantage of auxiliary compacting agents selected from the family of polyethylene oxides, particularly if these are used in the form of polyethylene glycols, is that the compacting parameters can be influenced through a corresponding selection of the molecular weight, that is to say with respect to the flow properties during the mixing and filling of the mold, as well as with respect to the softening point and thus the temperature control and the material flow during the compacting operation. It is particularly advantageous in this connection if the softening point for the auxiliary compacting agent suggested according to the invention is between 40° C. and 80° C., so that the temperature adjusting at the tool during a continuous compacting operation for the series production as a rule is sufficient to effect a trouble-free “flow” of the powder mixture during the filling of the mold as well as during the compacting. Accordingly, the metal powder with added auxiliary compacting agent can be filled into the mold at room temperature. Particularly for the series production, it may be useful if the compacting tool is heated accordingly to prevent possible interruptions in the series run. A controlled heating of the compacting tools to about 55° C. makes sense, so that the heating caused by frictional heat as well as the cooling caused by interruptions in the operation are taken into account and constant compacting conditions can be specified. The handling of the metal powder is simplified considerably by this, particularly the filling operation because it is possible to work with “cold” powder, meaning powder at room temperature. A baking on, lump formation and the like cannot occur since the metal powder with mixed-in auxiliary compacting agent is heated only in the mold. An additional preheating of the powder may be advisable for extremely large volume particles.
The low softening temperature additionally has the advantage that immediately after the filling operation, the shares of auxiliary compacting agent in the metal powder, which make contact with the heated mold walls, are initially warmed to the softening temperature. Thus, during the subsequent compacting operation, the relative movements occurring at the tool wall between powder filling and compacting tool are already “lubricated” and the friction in these regions is reduced. During the following operation where total compacting pressure is applied, the complete powder filling is subsequently heated past the softening point as a result of the compacting pressure. Thus, even the internal and relatively high relative movements in the metal powder filling, which result from the part
Dollmeier Klaus
Kynast Wieland
Lindenau René
Wahnschaffe Jens
GKN Sinter Metals GmbH & Co. KG
Jenkins Daniel
Kelemen Gabor J.
Schneller Marina V.
Venable
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