Impregnation of sintered materials
The impregnation of sintered materials is a method of sintering to reliably seal porous materials or to subsequently coat them . Vacuum impregnation , or "impregnation" in colloquial terms , is a necessary technology for the distribution of powder metallurgical materials in applications that require a coating .
Necessity of impregnation for the coating
As with most manufacturing processes, porosity is inherent in powder metallurgy. Porosity is only considered a defect if it is interconnected and creates a leakage path that can compromise the structural integrity and performance of the part. The pore structure of the sintered materials directly influences the result of the subsequent coating processes. Because the pore structure absorbs liquids through capillary forces in aqueous media, such as those used in electroplating . These mostly liquids emerge from the pore openings after the end of the process and visibly deposit salts / reaction products on the surface. The substances attack the galvanic metallic layer and leave local corrosion damage there.
Basic principle of impregnation
In order to counteract this process of liquid absorption, the network of pores is closed or sealed prior to coating with synthetic resin or similar substances. This process is called vacuum impregnation / impregnation. The impregnation has the ability to fill and seal the pores of various materials. The main reason for impregnation is to produce dense workpieces. Impregnation is a necessary pre-treatment for subsequent surface treatments and improves the machinability / machinability . Impregnation helps to open up powder metallurgically produced materials for new areas of application for which they would not be usable without sealing. Porosities in sintered parts filled with synthetic resin prevent the diffusion of media into the pores and improve the machinability of the components. With subsequent machining, tool wear is lower than with a material without impregnation.
Impregnation methods
Conventional processes from the casting industry
There are various methods for impregnating sintered materials. The impregnation processes for this type of material have their origins in the casting industry . Cast parts are impregnated in order to seal them against leakage; without this seal, the cast parts would not be usable and would have to be melted down. A distinction is usually made between thermosetting impregnation (thermosetting sealants) and anaerobic impregnation (resins harden in the absence of oxygen, catalyzed by metals). In temperature-reactive processes, heat is introduced to cure the sealant used. In the anaerobic process, the sealant (resin) hardens in the absence of oxygen and in the presence of metal ions. The processes used in the casting industry were used in the course of the increasing demand for the impregnation of sintered materials and had proven to be not optimal for sintered parts due to the edge washing.
Investigations show that when conventional immersion processes are used to impregnate sintered materials, approx. 3–5% of the pores are not filled; these pores are concentrated in the edge area of the workpiece. This unfilled edge is approx. 0.2-0.4 mm and explains the poor results when refining conventionally impregnated sintered materials, especially when using alkaline alloying processes such as zinc-iron and zinc-nickel.
Process especially for sintered materials
Many of the frequently used immersion impregnation processes lead to moderate pore filling due to process-related process requirements. Above all, the curing process and the process technology are decisive in order to achieve a pore filling up to the edge of the component surface without leaving annoying resin residues on the surface. This can be achieved with anaerobic resins in the immersion process or with a special environmentally friendly process for thermosetting resins. The underlying process engineering / resin system differs from the conventional systems currently used in that all factors are eliminated that lead to the resin being washed out of the pores.
Effect of the galvanic process on the impregnation
The quality of the electroplating is not only determined by the pores that are completely filled with impregnating resin, but also by the process control during electroplating; incorrectly selected parameters can partially or completely remove the impregnating resin from the pores.
application
With impregnation processes specially developed for sintered materials, all common sintered materials for structural applications can be impregnated and then coated with adapted galvanic parameters. After impregnation, anti-corrosive surface coatings such as zinc or zinc alloy systems are possible, but decorative surfaces such as z. B. copper-nickel-chromium and other refinements can be implemented. A large number of alloys based on ferrous or non-ferrous metals can be impregnated.
In the event that the shaping of a sintered workpiece can only be completed by subsequent machining , the impregnation that is introduced increases the machinability and the tool life.
The impregnation method is relevant wherever aqueous corrosive processes are used and, as a result, later leakage of liquids can occur. This is also the case where aqueous media are used in pretreatment processes. If oil and lubricants are accidentally absorbed - for example by cooling lubricants during machining - prior impregnation can also prevent absorption and improve tool life.
Honourings and prices
The “Sinter Surface Solutions” process by the industrial company Holzapfel Group was awarded the “Surface 2012” prize in silver by the Fraunhofer Institute for Manufacturing Engineering and Automation IPA.
The environmentally friendly impregnation process (thermosetting, recycled impregnation resins) by VDT Vakuumdichttechnik GmbH was awarded the "Special Prize in the tender for the Efficiency Prize NRW 2005 in the" Production "category in November 2005.
literature
- ASM Handbook Volume 7, Powder Metal Technologies and Applications. ASM International, 1990
- Dunn, DJ: Sealing of Pores in Powdered Metal Components. In: 5th European Symposium on Powder Metallurgy, Stockholm 1978.
- Introduction to powder metallurgy. Process and products. Ed .: FPM and EPMA. 6th edition.
- German, Randall M .: Powder Metallurgy and Particulate Materials Processing. Metal Powder Industries Federation, Princeton, New Jersey, 2005.
- Thummler, F. and Oberacker, R .: An Introduction to Powder Metallurgy (The Institute of Materials Series on Powder Metallurgy). Maney Publishing, London, 1994.
- Schatt, Werner: Sintering Processes - Basics. VDI Verlag, Düsseldorf, 1992.
- Werner Schatt , Klaus-Peter Wieters, Bernd Kieback (eds.): Powder metallurgy . Technologies and materials (= VDI book ). 2., arr. and exp. Edition. Springer-Verlag, Berlin, Heidelberg, New York 2007, ISBN 978-3-540-23652-8 ( limited preview in Google book search).
- factorY magazine for sustainable business - future eV Effinzienz-Agentur NRW 04/2005
Web links
Individual evidence
- ↑ Andy Marin: 4 Reasons to Seal Powder Metallurgy. Retrieved August 5, 2020 (American English).
- ↑ Charles M. Muisener: Resin impregnation of Powder Metal Parts; from: ASM Handbook Volume 7, Powder Metal Technologies and Applications. ASM International, 1990; Pp. 688-692.
- ↑ Introduction to powder metallurgy. Process and products. Ed .: FPM and EPMA. 6th edition, p. 25.
- ↑ Introduction to powder metallurgy. Process and products. Ed .: FPM and EPMA. 6th edition, p. 27.
- ↑ Immel, Michael: Only completely impregnated sintered parts can be electroplated without errors. In: MM Maschinenmarkt, edition 45/2012, pp. 38–41.
- ↑ Prize winner 2012: 2nd place ( Memento of the original from September 19, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. .
