Powder metallurgy, PM, is a route of manufacturing based on the compaction of powders that are afterwards sintered to create a solid product. This technique has been used in the production of permanent magnets since the 18th century and most of high performance permanent magnets have been fabricated this way. Magnetic properties are so dependent on the starting material, microstructure, magnetic alignment and heat treatment process, therefore powder metallurgy represents an ideal route to control most of these factors.
What is powder metallurgy?
Although powder metallurgy encompasses many steps an operation before and after, the basic procedure in the manufacture of PM parts is comprised by the following stages. For deeper explanations and more information about each one, it is worth to take a look to this Introduction to Powder Metallurgy.
Figure 1 â Schematic diagram of the powder metallurgy process. Source: Ames
The object of mixing is to provide a homogeneous mixture and to incorporate the lubricant. The main function of the lubricant is to reduce the friction between the powder mass and the surfaces of the tools along which the powder must slide during compaction, thus assisting the achievement of the desired uniformity of density from top to bottom of the compact. Of equal importance is the fact that the reduction of friction also makes it easier to eject the compact and so minimises the tendency to form cracks.
The mixed powders are pressed to shape using either a press, which could be uniaxial, or isostatic; or an injection system under pressures depending on the characteristics of the starting material. At this stage, the compacts maintain their shape by virtue of cold-welding of the powder grains within the mass. The compacts must be sufficiently strong to be handled safely and avoid its breakage. This is a critical operation in the process, since the final shape and several properties are essentially determined by the level and uniformity of the as-pressed density.
The thermal treatment of a powder or compact at a temperature below the melting point of the main constituent, for the purpose of increasing its strength by bonding together of the particles.
Suffice to say that atomic diffusion takes place and is followed by recrystallization and grain growth. Consequently, the pores tend to become rounded and the total porosity, as a percentage of the whole volume, tends to decrease.
The operation is almost invariably carried out under a protective atmosphere, because of the large surface areas involved, and at temperatures between 60 and 90% of the melting-point of the main constituent. For powder mixtures, however, the sintering temperature may be above the melting-point of the lower-melting constituent, so that sintering in all these cases takes place in the presence of a liquid phase, hence the term liquid phase sintering.
Why powder metallurgy in permanent magnets?
Powder metallurgy techniques have been found to offer advantages in the fabrication of permanent magnets. It seems that the most of the best magnetic properties are obtained by employing powder metallurgy routes rather than casting processes. Careful control of particle size and particle orientation, using a magnetic field to align the starting particles, are among two of the greatest advantages of the powder metallurgy process.
Powder metallurgy is a near net shape manufacturing technique which means that the final product is almost finished due to the compaction was done in a shape as close as possible to the final one. So from this point of view the powder metallurgy technique is very advantageous as the machining stage can be whether dispensed or minimised, thus decreasing costs and saving materials.
Summarizing, all these factors mentioned above play a vital role in controlling and enhancing the magnetic properties of permanent magnets and soft magnets as well.
How this relationship started?
As explained in a previous post (link of my previous post about History of permanent magnets), Gowin Knight, an English entrepreneur, made a fortune by manufacturing magnets but he didnât publish his methods in life. After his death in the 1770âs his method was published and consisted of stirring a slurry of iron fillings to obtain a suspension of finely divided iron oxide. This was mixed with linseed oil to create a paste that was moulded into shape and baked. The resulting block was magnetized with an energy product of 2 kJ/m3, which was really high by that time.
However, it is not the strength of the magnets what we are focusing at today, but the technique used by Gowin Knight. It means that the first time powder metallurgy was used in the production of permanent magnets was more than 250 years ago.
Figure 2 – Portrait of Gowin Knight. Source: Science & Society Picture Library Prints
During the 19th century, casting techniques were developed and magnetic steels were industrially processed thus replacing powder metallurgy as the dominant permanent magnet manufacturing method.
In the 1950s Philips Company discovered ferrites immediately after the Second World War and due it is not a metal but a ceramic, they decided to choose powder metallurgy as the production technique. This decision was helped by the massive development of powder metallurgy as a consequence of the manufacture of tungsten filaments for bulbs as well as the development of cement carbides in 1909 and 1922 respectively.
Dowson, G., Whittaker, D., 2008. Introduction to Powder Metallurgy. The process and its products. European Powder Metallurgy Association, EPMA.
Moosa, I.S., 2014. History and Development of Permanent Magnets. International Journal for Research & Development in Technology, 2, p18-26.
Ramakrishnan, P., 1983. History of powder metallurgy. Indian Journal of History of Science, 18, p109-114.