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

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.

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

“…1a; this places the magnetic field in an orientation along the magnetic axis in the final magnet with the punches acting as pole pieces. 6 The specimens were pressed, sintered, and uniaxially loaded as described in (Table I).…”

“…4 After secondary heat treatment, samples were centerless ground to 3mm in diameter as needed to ensure uniformity of diameter for measurement in the hysteresisgraph. After centerless grinding, solutionizing at 1250 • C followed by magnetic field annealing, and low-temperature drawing was performed, as is typical for alnico and described in Anderson et al 6 Sample properties were then measured on a Laboratorio Elettrofisico AMH-500 hysteresisgraph, and microstructural observations including grain size and orientation made using an FEI Teneo scanning electron microscope (SEM) outfitted with an electron backscatter detector (EBSD) for orientation data.…”

“…5 Even without Tesla, growth is estimated around 40% per year in the electric vehicle market according to the International Energy Agency (IEA), and with this growth, comes a very real critical materials challenge. 1,4,6 Critical PMAC design criteria for the permanent magnets include sufficient levels of remanent magnetization, operating temperature, mechanical strength, and coercivity. These requirements have been a significant design barrier to PM innovation, and they explain why Nd-based 2-14-1 magnets have remained the choice of the PMAC world.…”

“…However, when motor operational conditions increase to 180 • C, they result in a strong reduction of magnetic properties, e.g., energy product (BHmax), in Nd-based 2-14-1, making non-RE PM materials like alnico attractive. [6][7][8] Alnico is an established magnet alloy family consisting primarily of Al, Ni, Co, and Fe, along with Cu and Ti, with a unique spinodal nanostructure that can yield significant shape anisotropy-based coercivity. 9 Alnico also is notable for high saturation magnetization, and high-temperature performance with small magnetic remanence temperature coefficient (-0.025%/ • C) and positive coercivity temperature coefficient (+0.01%/ • C), as well as resistance to corrosion effects.…”

“…12 However, alnico 9 would still require increased mechanical properties, beyond those found in sintered alnico 8H with an increased coercivity. 6,13 Fortunately, initial developments are promising for improved alnico alloys and processing by a powder metallurgical approach that includes the production of gas atomized powder with final-shape compression molding. Samples undergo debinding and sintering to create high-density magnets coupled with secondary sintering and textured grain growth under uni-axial stress, where misorientation of the [001] direction in underlying grains is kept to less than 15-18 • of the magnetic axis.…”