User:A Oerter/sandbox/Impregnating sintered materials

'''Impregnating sintered materials ''' The impregnating of sintered materials is a method used in sintering that makes it possible to reliably seal up porous materials in order to subsequently plate them. Impregnation is a technology necessary for the distribution of powder metallurgy materials in applications that require plating.

Necessity of impregnation
Sintered metallurgical structural components tend to absorb the liquids used during the surface finishing process (e.g. electroplating) and gradually release them in the course of time. The resulting defect is known as "bleed-out". In the case of electroplated surfaces, such as zinc or zinc alloy processes, this phenomenon frequently results in salts being deposited on the surface. In most cases, these salts also attack the plating, leading to local corrosion of the plating material.

Porous structure of sintered materials
The subsequent leakage is due to the porous structure of the sintered materials, which can be likened to a pile of marbles. The grains of the sintered material do not lie completely flush on top of one another, but only touch at certain points to form a cavernous or spongelike network of pores. The size of these pores depends on the shape and size of the powder used in the sintering process and the compressed and/or sintered density of the material.

Impacts of the porous structure
The porous structure of the sintered materials has a direct impact on the results of subsequent plating processes, as capillary forces cause the porous structure to absorb liquids when exposed to them, such as during electroplating. The liquids (e.g. degreasers, electrolytes), which are mostly saline, leak out of the openings in the pores after the process has been completed and deposit themselves visibly on the surface. The partially alkaline materials attack the coating metals, leaving behind localised superficial corrosion damage. Moreover, porosity sealing using synthetic resin makes components easier to process, e.g. in subsequent machining, and tool wear is generally higher than for a sealed material of the same composition.

Basic principle of impregnation
In order to counteract the process of liquid absorption, the network of pores is either closed or sealed off using synthetic resin or similar materials prior to surface treatment. This process is known as impregnation, due to its ability to fill and therefore seal cavities and even micropores in a range of materials. The main reason for impregnation is to produce sealed components. Impregnation is therefore used to prevent leakages, avoid bleed-out, or reduce the incidence of trapped moisture. Impregnation is a pre-treatment required for subsequent surface treatments such as plating and facilitates workability and machining. ; Impregnation therefore enables materials produced by means of powder metallurgy to be used in new types of application for which they would have been unsuitable without a sealing process.

Conventional processes from the casting industry
Various methods are available for impregnating sintered materials. Impregnation processes for these types of material have their origins in the casting industry. Cast components are impregnated in order to seal them against leakage. Without this sealing process, many of the castings would be substandard. A difference is usually made between thermoset impregnation using thermosetting sealants, also known as thermal cure sealants, and anaerobic impregnation. In thermal cure processes, the components are subjected to heat to harden the sealants used, such as resins. In anaerobic processes, the sealant (resin) hardens in the absence of oxygen (anaerobically) and in the presence of metallic ions. Due to increasing demand, the processes used in the casting industry were employed in parallel for impregnating sintered materials. However, the success in impregnating sintered materials that were to be subsequently plated proved to be both limited and inconsistent. An important factor for materials requiring subsequent plating is that the pores are really completely filled right up to the surface and not only somewhere in the depths of the material, as is the case with cast materials. Tests show that when conventional immersion methods for impregnating sintered materials are used, approximately 3–5% of the pores remain unfilled, mainly on the periphery of the component. This unfilled rim is approximately 0.2–0.4 mm in size and explains the poor results achieved when applying surface finishes to conventionally impregnated sintered materials, particularly when alkaline alloying techniques such as zinc-iron or zinc-nickel are used.

Process specially designed for sintered materials
Many of the frequently used impregnation techniques result in mediocre surface finishing due to process-related necessities. In order to achieve satisfactory results when surface finishing, the impregnation process and the resins used need to be coordinated to meet the requirements of the surface that will be subsequently applied. Above all, the curing and the processing technique are crucial to ensure that the pores are all completely filled to the surface of the component without leaving behind residues of resin that disrupt the process. This is achieved by means of a special procedure and an adapted curing process that hardens the resin that has been introduced into the pores. The underlying reaction mechanism or processing technique differs from the currently used conventional systems in that all factors that could lead to the resin being washed out of the pores have been eliminated.

Further methods of impregnation
There are also other methods for sealing porosities, such as wet vacuum and dry vacuum processes.

Application
The impregnation process specially developed for sintered materials makes it possible to impregnate and subsequently plate all types of sintered materials widely used for structural applications. Following impregnation, corrosion-protecting platings such as zinc or zinc alloy systems can be applied, as well as decorative surfaces such as copper-nickel-chrome and other surface finishes. A wide range of alloys based on ferrous or non-ferrous metals can be impregnated. If the shaping of a sintered component can only be completed by a downstream machining process, impregnation improves workability and increases tool life. The impregnation method is relevant wherever liquid corrosive processes are employed and where they could therefore result in subsequent leakage, even in cases where liquids are used in pre-treatment processes. If a component is unintentionally exposed to oil or lubricants, such as cooling lubricants during machining, an upstream impregnation process can also prevent the component from becoming contaminated and improve tool life.

Literature

 * ASM Handbook Volume 7, Powder Metal Technologies and Applications. ASM International, 1990
 * Dunn, D. J.: Sealing of Pores in Powdered-Metal Components. In: 5th European Symposium on Powder Metallurgy, Stockholm 1978.
 * Einführung in die Pulvermetallurgie. Verfahren und Produkte. Hrsg.: FPM und EPMA. 6. Auflage.
 * German, Randall M.: Powder Metallurgy and Particulate Materials Processing. Metal Powder Industries Federation, Princeton, New Jersey, 2005.
 * Thummler, F. und Oberacker, R.: An Introduction to Powder Metallurgy (The Institute of Materials Series on Powder Metallurgy). Maney Publishing, London, 1994.
 * Schatt, Werner: Sintervorgänge – Grundlagen. VDI Verlag, Düsseldorf, 1992.
 * Schatt, Werner; Wieters, Klaus-Peter; Kieback, Bernd: Pulvermetallurgie. Springer Verlag, Berlin Heidelberg, 2007.

Weblinks

 * https://en.wikipedia.org/wiki/Porosity_sealing
 * http://www.mpif.org/
 * http://www.epma.com
 * http://www.pulvermetallurgie.com

Awards and prizes
The "Sinter Surface Solutions" process was awarded the "Surface 2012 in Silver" prize by the Fraunhofer Institute for Manufacturing Engineering and Automation (IPA).