File Name: powder metallurgy science technology and applications .zip
The aim of this article is to give a general idea about powder metallurgy and its employment in the production of permanent magnets with especial specification according to their use. The pressures which are used for this purpose are either of a mechanical or hydraulic type. The strength of the compacts made by pressing should be sufficient to withstand transportation to the next metallurgical operation.
More information about this subject has been reported by Kuhn and Lawley . There are many compacting techniques which have been used for fabricating different shapes; i Die compaction ii Isostatic compaction iii Explosive compaction iv magnetic compaction v Compaction by rolling vi Compaction by extrusion vii Compressing shearing method which was successfully developed by Tetsuji et al.
The use of any compaction technique depends on the shape of the magnet, and which one of these techniques is suitable for the production process to reach the desirable magnet. One of the most useful properties to be specified in the powder metallurgy process, generally the green density increases with; i Increasing compaction pressure according to the type of the starting powder.
If this ratio decreases, the green density becomes more homogeneous. A schematic illustration is given in Fig. In most cases, a green density compact cannot be used as the finished article because of low strength and brittleness. Sometimes a compact is required with a certain amount of porosity, but even in this case mechanical properties need to be improved by means of a heat treatment, which increases the cohesion between the particles of the green compact, or within a loose powder confined to a required shape.
This heat treatment is known as "sintering". Many authors have discussed the phenomena occurring during sintering and it is agreed that sintering can occur in the solid state, normally taking place at a temperature below the melting point of the main component .
The Metals Handbook. Six stages are considered during the sintering process; initial bonding, neck growth, pore channel closure, pore rounding, pore shrinkage and pore coarsening . This process is widely used for consolidating metallic powder, ceramic powder, or both of them together, into final shapes. The advantages of production by this sintering method are a very fast densification rate, and production of microstructures which often provide mechanical and physical properties superior to those produced by solid state sintering.
Three stages in the liquid phase sintering process have been identified ; rearrangement, solution-reprecipitation and solid-phase bonding. One case should be noted; that is the liquid phase dose not wet the solid particles because there is no reaction between them. This condition is often termed "sweating" and is shown by the presence of droplets on the compact surface. It is very clear to recognise the Nd-rich phase at the grain boundaries, surrounding almost all the particles of the hard phase Nd 2 Fe 14 B.
In general, the maximum value of this sintered density is usually considered as a criterion of the quality of the sintering process, unless otherwise is specified.
It seems that the best magnetic properties probably often achieved by employing Powder metallurgy routes rather than casting processes. Although in some cases a combination between them is used in the manufacturing of permanent magnets. 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.
Chemical composition, impurities, and metallic and non-metallic inclusions probably controlled by using powder techniques. Additionally, extremely pure powders and freedom from inclusions are easy to attain by powder metallurgy processing. Composition of the permanent magnets may be limited in precision by the purity with which powder can be produced.
In fact, all these factors mentioned above play a vital role in controlling and enhancing the magnetic properties of permanent magnets and soft magnets as well. The products of powder metallurgy almost semi-finished produced to specifications with different shapes and sizes, so from this point of view the powder metallurgy technique is very advantageous as the machining stage can be dispensed with, thus decreasing costs and saving materials.
These materials are hard to magnetise and demagnetise. Magnetic hardness is obviously related with the microstructure, heat treatment and chemical composition of the starting material. Magnetically hard substances are made into permanent magnets which provide magnetic field in working air, or vacuum gapes. The distinguishing characteristics of hard magnets are large hysteresis loops, high remanence and corercivity with different Curie temperatures.
Permanent magnets are fundamentally energy storage systems when they are magnetised. Development of hard magnetic materials is in the direction of obtaining improved magnetic properties, reducing the cost of production and the volume of material required.
Hard magnets are used to perform a wide variety of magnetic functions, such as in electrical machines, motors, computers, direct drive motors, electrodynamic braking, clamping and holding devices for ferrous materials, loudspeakers, magnetic sealing, head-phones, focusing and steering of charged particles, medical equipment, etc. This was mixed with linseed oil, the paste moulded into shape and baked hard.
The resulting blocks were then magnetised and formed strong magnets for that time, reputedly . The beauty of this story shows that the beginning of the development of the first permanent magnet by powder metallurgy primitive technique. In the 's, the idea of this process of surrounding hard phase particles with non-magnetic material has been strongly re-introduced for producing permanents magnets [14,15].
Some ferrites tend to be dissociated during heating at elevated temperature, so that the conventional route of melting and casting is impractical for ferrites, and ferrites are normally fabricated by Powder metallurgy .
If a magnetic filed is applied during the cooling or tempering while the temperature is just below the Curie point, a crystallographic growth along the direction of the applied filed which is nearest to the easy axis is encouraged. This process called magnetic annealing, the first attempt of this process was carried out at Sheffield UK , and in it was found that heat treatment in a suitable magnetic field could enhance the magnetic properties of Alnicos permanent magnets .
This process was also used in the production of Sm-Co permanent magnets to solidify the ingots from the liquid state in a magnetic field produces oriented polycrystalline materials .
The production of magnets, by employing powder metallurgy technology probably give improved products because of the simplicity of aligning most of the powder particles along the easy direction at room temperature, and hence producing anisotropic compacts. The degree of alignment is influenced by particle shape, particle size distribution, magnitude of aligning field, and pressing pressure.
The remanennce and the energy product are thereby sharply enhanced . Magnetic alignment of Nd-Fe-B hydride powder using a flexible bag is shown in Fig. In , synthesis of magnetite particle-chain microwires by applying magnetic field during the fabrication process has been firstly reported by Fashen et al.
Their conclusions were that Fe 3 O 4 microwires are successfully produced under external magnetic field, and the connection of the particles is very tight with each other. Many of them exhibit a fairly strong tendency to orientate their crystals when they are cold rolled or worked, depending upon the fabricability of the materials. Plastic deformation can produce a grain orientation which gives improved magnetic properties.
An investigation of this phenomenon utilising a single crystal of Ni 3 Fe was made by Chikazumi, et al. They found that the anisotropy is strongly dependent on the crystallographic orientation during the rolling process. Crystallographically orientated or anisotropic magnets can be prepared by this route, which consists of hot pressing followed by hot deformation.
This was discovered in by R. This method could be applied for many magnetic materials to see the results of it to fabricate anisotropic magnets. The great advantages of this technique include the fact that: i Magnetic alignment may be induced with no magnetic field. The representatives of this class of ferrites, which have been given the name of Ferroxdure, have a hexagonal structure the easy axis being along the c-axis.
Ferroxdure is manufactured either as a solid permanent magnet or in the form of ferrite particles dispersed in plastic bonded ferrites , which is the same idea of Gowin Knight. Production of SrFe 12 O 19 powders by direct use of celestite as a source of strontium has been reported by Mortaza and Jamshid .
Their results were compared with those of powders fabricated by normal ceramic route. They showed that their process is more covenant for production of SrFe 12 O 19 powders than the conventional one. Synthesis and orientation of barium hexaferrite ceramics by magnetic alignment was studies by Denis AUTISSIER, who showed that the magnetic properties strongly depends upon the structural quality of the produced ceramic, magnetic alignment, particle size and the density of the sintered magnets .
Starting powder of BaFe 12 O 19 and the production route of these hexaferrite permanent magnets by powder injection molding have been reported by Zlatkov et al. They concluded that these alloys promising candidates for fine particles permanent magnets with high anisotropy. Magnets fabricated by compacting fine powders of SmCo 5 in a magnetic field gave different maximum energy products BH max according to the production factors [14,33].
In the s, the preparation and enhanced properties of SmCo 5 magnets produced by liquid phase sintering were reported by Benz et al. Their conclusions were that this route gives improved magnetic properties and eliminates porosity. After the development of SmCo 5 permanent magnets in the early of 's, alloys with some quantity of copper as well as rare earth and cobalt emerged. The effects of various additive elements on magnetic hardness of the Sm-Co-Fe-Cu system studied Ojimo et al.
It was found that Zr addition to the Sm-Co-Fe-Cu system improves the hard magnetic properties whilst a post-sintering annealing process enhances the coercivity. Production and development of these magnets have been reviewed by Strant and Ormerod [39,40]. These magnets based on Nd-Fe-B alloys could be looked upon as the third generation of Rare-Earth permanent magnets.
At the beginning, and independently, two routes were developed for fabricating these new magnets, and these have continued to be used. The first is a conventional powder metallurgy route which was developed by Sagawa et al.
This route is the well established powder metallurgy technique traditionally employed for the production of ferrite magnets. It is used by most magnets manufactures and was successfully employed by Sagawa and his collogues to produce sintered Nd-Fe-B permanent magnets, starting from an as-cast ingot. After preparing the ingots, the following steps were used for making magnets: i The ingots were crushed to a particle size of about 1mm by using a jaw crusher under an inert gas atmosphere.
The sintered specimens were given a post-sintering to enhance the coercivity. Many fundamental investigations have been reported on this type of rear earth permanent magnets [19,. The second route for the production is rapid solidification to give an isotropic permanent magnet, this method being announced by the General Motors Corp. GM USA. They announced in the fabrication of isotropic Nd-Fe-B hard magnets by employing the rapid solidification technique [48,49].
Large lumps can be crumbled up readily to obtain an extremely friable produced which is consequently very amenable to further reduction in particle size. After the discovery of Nd-Fe-B permanent magnets, the HD process was successfully re-applied to decrepitate as-cast ingots of the Nd 15 Fe 77 B 8 alloy and related compositions .
The production of rare-earth-sintered magnets by a low-cost has been published by Takiishi et a.
Powder Metallurgy discusses the production of metal powders and other materials made from it. It defines the meaning of metal powders with some illustrations. The book also identifies the processes similar between the production of metal powder and ceramic products. The technology involved and the variation in the process of metallurgy are covered in some chapters of the book. The book enumerates certain advantages in using powder metallurgy over other processes. Methods such as the reduction of the oxides of metals, electrolysis, thermal dissociation, and chemical disintegration are explained. The origin and improvement made on the method are discussed in detail.
Save extra with 2 Offers. About The Book Powder Metallurgy Book Summary: This textbook is written primarily for undergraduate and postgraduate students of metallurgical and materials engineering to provide them with an insight into the emerging technology of powder metallurgy as an alternative route to conventional metal processing. It will also be useful to students of materials science, mechanical engineering and production engineering to understand and appreciate the importance of powder metallurgy as an effective and profitable material processing route to produce a variety of products for engineering industries. The book will enable the students as well as practising engineers to understand and practise the science and technology of powder production and processing, as well as to choose the right method to suit the application in hand. The various techniques used for powder production and the versatile nature of these techniques to produce a wide range of powders have been highlighted with suitable examples.
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Written in English. The book also identifies the processes similar between the production of metal powder and ceramic products. The technology involved and the variation in the process of metallurgy are covered in some chapters of the book. The book enumerates certain advantages in using. Powder Metallurgy: Science, Technology, and Materials - CRC Press Book Since the Powder metallurgy science book, modern powder metallurgy has been used to produce a wide range of structural powder metallurgy components, self-lubricating bearings, and cutting tools.
Some relationships between powder processing, microstructure and mechanical properties are discussed, with particular emphasis on the specialty alloys. Finally, an objective look at the interface between powder metallurgy and rapid solidification is presented. This is a preview of subscription content, access via your institution.
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