Abstract
The 2011 price shock in the rare earth (RE) permanent magnet (PM) marketplace precipitated realization of extremely poor RE supply diversity and drove renewed research in RE-free permanent magnets such as “alnico.” Essentially, alnico is an Al-Ni-Co-Fe alloy with high magnetic saturation and TC, but low coercivity. It also was last researched extensively in the 1970’s. Currently alnico “9” magnets with the highest energy product (10MGOe) are manufactured by directional solidification to make highly aligned anisotropic magnets. This work developed novel powder processing techniques to improve on unaligned anisotropic alnico “8H” with elevated coercivity. Gas atomization was used to produce pre-alloyed powder for binder-assisted compression molding of near-final shape magnets that were vacuum sintered to full density (<1% porosity). Biased grain growth with resulting grain alignment was achieved during a second solution annealing step, during which a uni-axial stress was applied along the axis parallel to the magnetization direction. Evaluation of heavily stressed samples (>250g) showed reduced overall loop squareness compared to unaligned (equiaxed) 8H due to grain rotation-induced misalignment, while low stresses improved squareness and greatly improved alignment compared to equiaxed magnets, with squareness approaching 0.30 and remanence ratio as high as 0.79.
Highlights
Permanent magnet (PM) based motor technologies, permanent magnet based synchronous AC motors (PMAC), have established a strong foothold in the quickly evolving electric vehicle market as the preferred technology for electric drive systems
If the higher coercivity of 8H can be maintained in this test of the concept while controlled microstructure alignment is achieved, the energy product of the final magnet will be amplified considerably. This hypothesis was confirmed initially after “accidental” aligned grain growth was observed in a magnet sample after extended sintering produced a superior energy product of 6.5 MGOe, while maintaining good coercivity.[14]. Consistent with these findings, Chikazumi stated that remanence should primarily be considered a function of microstructure and, as such, a decrease in saturation magnetization could be acceptable for improved coercivity, as long as remanence is recovered through microstructural texturing.[12]
To verify the enhanced performance noted in our previous work for the 8h sintered sample that accidentally achieved pronounced grain growth with a beneficial texture, electron backscatter detector (EBSD) was performed on a longitudinal section
Summary
Permanent magnet (PM) based motor technologies, permanent magnet based synchronous AC motors (PMAC), have established a strong foothold in the quickly evolving electric vehicle market as the preferred technology for electric drive systems. This hypothesis was confirmed initially after “accidental” aligned grain growth was observed in a magnet sample after extended sintering produced a superior energy product of 6.5 MGOe, while maintaining good coercivity.[14] Consistent with these findings, Chikazumi stated that remanence should primarily be considered a function of microstructure and, as such, a decrease in saturation magnetization could be acceptable for improved coercivity, as long as remanence is recovered through microstructural texturing.[12] dead-weight loading was devised as a novel solid-state means to produce texture control of an alnico 8H-type microstructure and a wide range of stress levels were tested to explore the most beneficial level in this work.
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