IPMD 14th Edition 2010-2011, 4 pages, 2218 words

Author: Bernard Williams, PM Consultant, Shrewsbury, United Kingdom

Powder metallurgy (PM) is the preferred method for manufacturing more than 80% of permanent (hard) magnets. Permanent magnets include the hard ferrites, Alnico alloys and the rare earth magnets based on samarium-cobalt (Sm-Co) and iron-neodymium-boron (FeNdB) alloys. These magnets have properties opposite to those of soft magnetic materials described in a separate feature in this edition and retain their magnetism after being magnetised.

As detailed in the soft magnets review, there are two kinds of materials in a magnetic circuit - designated as ‘soft’ and as ‘hard’ (permanent) magnetic materials. They are separated by the property that one kind, the hard or permanent magnets - once charged - retain their magnetism. The soft magnets, on the other hand, lose their magnetic charge once the electrical field or the permanent magnet is removed or reduced. Modern motors today are designed with both kinds of magnetic materials within their components.

All magnetic materials have characteristic hysteresis loops. Typical hysteresis loops for both soft and hard (permanent) magnets are shown in Fig. 1. Permanent magnets require a high remnance and a high coercive field to prevent the magnetic domains from changing once they have been aligned. Therefore, the hysteresis curve of these materials is broad or wide as opposed to thin or narrow (blue in Fig. 1). The property of interest is known as the energy product (BHMax) and is expressed in mega gauss oersteds. The higher the energy product, the higher will be the applied field. Of these, the FeNdB magnets introduced in the mid-1980s have the highest energy products. Fig. 2 shows the evolvement of permanent magnets and their maximum energy products and Table 1 and Table 2 give some of the properties of the various permanent magnets which are summarised below.

Alnico magnets

The group of alloys called Alnico was developed in the 1930s to provide the first improvement over the then used steels. These alloys are based mainly on the elements Ni, Co and Fe, with smaller amounts of Al, Cu and Ti (Typical weight%: Fe-35, Co-35, Ni-15, Al-7, Cu-4, Ti-4). The properties of Alnico permanent magnets reached a maximum in 1956 with the introduction of anisotropic columnar Alnico 9, with an energy product of ~80kJm-3.

These alloys are still used today as they have a high Curie temperature (~850°C) and as a result can operate at higher temperatures as well as having more stable properties around room temperature than some of the more modern alloys. However, their main disadvantage is that they have low intrinsic coercivity (~50kAm-1) and as a consequence must be made in the form of horseshoes or......

Further sections of this article include:

• Alnico magnets

• Hard ferrites

• Rare earth SmCo

• NdFeB magnets
  - Press and sinter route
  - Melt-spinning route

• Injection moulding opens up new opportunities

• Hard magnet applications and future trends

• Further reading

Figures and Tables:

Fig. 1 Hysteresis loops for soft magnetic material and hard magnetic material (Source: William Smith, Principles of Materials Science and Engineering, McGraw-Hill, Inc. 1986; CD Rom: ‘Powder Metallurgy: Materials, Processes and Applications’. EPMA, Shrewsbury, UK, 2001)

Fig. 2 Rare earth and Nd-Fe-B materials have dramatically increased the maximum energy product of permanent magnets (Source: ‘Hard Magnetic Materials’, ; CD Rom: ‘Powder Metallurgy: Materials, Processes and Applications’. EPMA, Shrewsbury, UK, 2001)

Fig. 3 Production steps used to produce NdFeB magnets (Courtesy University of Birmingham (UK), Magnetic Materials Group)

Fig. 4 Breakdown of applications for Magnequench’s MQTJ NdFeB powders (Courtesy Magnequench – Division of Neo Materials Technology)

Fig. 5 Examples of complex shape NdFeB magnets produced by injection moulding (courtesy Magnet Applications Ltd, UK)

Fig. 6 Map of the magnetic properties of NdFeB sintered magnets and corresponding applications (courtesy Masato Sagawa (Intermetallics Ltd, Japan). Presented by O. Grinder: ‘Key Areas for PM Development’, EuroPM2009, Copenhagen, October 2009)

Fig. 7 Examples of NdFeB magnets (courtesy Anfeng Magnets Co Ltd, China)

Fig. 8 NdFeB magnet used in computer hard disk drive

Table 1 Typical magnetic properties of permanent (hard) magnets

Table 2 Physical and thermal properties of rare earth permanent magnets (Courtesy Magnet Sales & Manufacturing Inc., USA)

Table 3 Examples of applications for permanent magnetic materials