Remanufacture of sintered NdFeB magnets via recasting high dysprosium content end-of-life magnets

This work examines the remanufacture of NdFeB sintered magnets via recasting of corroded, high dysprosium content end-of-life (EoL) sintered magnets. Researchers at the University of Birmingham have previously successfully developed an efficient short loop recycling process for NdFeB magnets termed “the hydrogen processing of magnet scrap” (HPMS)[1], [2]. HPMS utilises the hydrogen decrepitation (HD) reaction [3] to break down sintered NdFeB magnets at room temperature, into a friable demagnetised powder. The hydride powder than can then be easily liberated from any ferrous scrap, and reprocessed into a new magnet via milling, pressing/aligning, and sintering.
Whilst HPMS is a highly efficient method of recycling, there are limitation to the amount of oxygen that can be present in the as-received EoL material. For primary sintered magnet production, from cast material to final sintered magnet, oxygen content increases during processing [4]. Typically, minimal quantities of oxygen (1100 – 1700 ppm) can be beneficial [5], however, when oxygen content is excessive (>3000 ppm) densification during sintering and final magnet properties can be reduced [4]. Moreover, during operation magnets can become corroded, further increasing the oxygen content.
As HPMS utilises several of the processing steps from primary manufacture [1], it also experiences increased oxygen content in the final recycled magnet. Moreover, as there is no effective way to separate the oxides from the hydride powder formed during HPMS, the oxygen can increase to levels which may significantly impact the final magnet. Thus, highly corroded EoL magnets may not be suitable for HPMS. One solution to reduce the oxygen content could be to melt and recasting highly corroded magnets, and process new magnets the recast alloy, creating a longer loop recycling process.
This work utilises 14 kg of EoL sintered magnets, composition Nd_9.55Dy_2.76Fe_78.08Co_2.95Al_0.24Cu_0.11Ga_0.19Nb_0.28B_5.84, which have significant corrosion, and oxygen contents of ~3500 ppm. Recasting these magnets at 1480 °C into a book mould cast (BMC) condition reduced oxygen to 250 ppm. However, the BMC microstructure had significant quantities of free iron present and heat treatment was required before subsequent processing.
The BMC alloy was reprocessed into a sintered magnet via hydrogen decrepitation and either a ball milling (BM) or jet milling (JM) procedure. Whilst both techniques were successful in producing a magnet with coercivities comparable to the as-received EoL magnets (>2000 kA/m), BM produced ~73% of the remanence of the as-received magnets, whereas JM produced almost identical remanence values.The JM sintered magnets had a final oxygen content of 850 ppm, far lower than that of the as-received magnets. However, the JM processing removed significant quantities of minority elements present in the as-received magnets, leading to a minor decrease in coercivity and altering optimal sintering conditions.