Novel pre-alloyed powder processing of modified alnico 8: Correlation of microstructure and magnetic properties

Abstract

Progress is reviewed on development of an improved near-final bulk magnet fabrication process for alnico 8, as a non-rare earth permanent magnet with promise for sufficient energy density and coercivity for electric drive motors. This study showed that alnico bulk magnets in near-final shape can be made by simple compression molding from spherical high purity gas atomized pre-alloyed powder. Dwell time at peak sintering temperature (1250 °C) greatly affected grain size of the resulting magnet alloys. This microstructure transformation was demonstrated to be useful for gaining partially aligned magnetic properties and boosting energy product. While a route to increased coercivity was not identified by these experiments, manufacturability of bulk alnico magnet alloys in near-final shapes was demonstrated, permitting further processing and alloy modification experiments that can target higher coercivity and better control of grain anisotropy during grain growth.

Novel mechanisms for solid-state processing and grain growth with microstructure alignment in alnico-8 based permanent magnets

“…17 Samples were then cut from bulk to create experimental samples of 3.25mm x 10mm in size using electro-discharge machining (EDM), and centerless ground to 3mm in diameter for consistency.…”

“…Specifically, the target composition was: 7.3Al-13.0Ni-38Co-32.3Fe-3.0Cu-6.4Ti (wt.%). 16,17 The charge was created using high purity (99.99%) elemental additions and pre-alloyed buttons, and then atomized using high purity argon. The powder was riffled and screened according to ASTM standard screen sizes, yielding multiple size ranges in the sub 106µm range.…”

Development of controlled solid-state alignment for alnico permanent magnets in near-final shape

“…11 The key to improving mass produced alnico for drive motors will be not only the ability to improve coercivity, but also the ability to refine its non-square hysteresis loop shape, without resorting to complex directional solidification methods and extensive machining to final shape, like practiced for alnico 5-7 and 9. 8,[13][14][15] Our recent work (described in another paper of this volume) revealed that coercivity improvements can be approached through alloy changes, and nano-structure refinement during the magneto-thermal heat treating process. However, the loop shape problem must be taken on by radical processing and microstructure changes, especially if this technology is to be brought to mass production scales.…”

Exploration of Alnico permanent magnet microstructure and processing for near final shape magnets with solid-state grain alignment for improved properties

“…The most common modern alnico alloys, alnico 8 and 9, consist of a unique two-phase, B2 type ferromagnetic Fe-Co and weakly magnetic L21 Ni-Al based body-centered cubic (BCC) highly elongated spinodal decomposed nanostructure ( Figure 2), which derives much of its coercivity from the high shape anisotropy derived during a short anneal under an applied magnetic field. (39,40) least, in theory, be achieved in the system. (21,24,26) The last concern that of loop shape for alnico is typically one that can be perceived as a challenge solely of the microstructure.…”

“…(39) Samples of the blended material were compression molded in a 2.54cm (1 in) diameter die at 175 MPa (12.7tsi), to create "green bodies" suitable for de-binding and sintering processes. Subsequent binder burnout is detailed through Anderson et al and a series of vacuum sintering experiments were performed between 1-12h at 1250°C, below the solidus of 1281°C as determined by differential scanning calorimetry, to produce samples with density >99% (cast density = 7.3 g/cm 3 ) (38,39).…”