Near-net Shape Casting of Rare-earth Iron Magnet from Undercooled Melt

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Neodymium-iron-boron (Nd-Fe-B) magnets with excellent magnetic properties have been widely used in various applications such as computer devices and acoustic equipment. The superiority of the magnets originates from the Nd_2Fe_<14>B intermetallicphase, which has a large saturation magnetization and a high magnetocrystalline anisotropy field. Because the ferromagnetic Nd_2Fe_<14>B phase is formed via the peritectic reaction between the pro-peritectic γ-Fe phase and the residual liquid, in the casting alloy, the α-Fe phase inevitably remains as a soft-magnetic phase, deteriorating the hard magnetic properties of magnets. In the present experiment, as one of novel techniques to suppress the peritectic reaction, containerless processing has been tried, in which the rapid quenching of the undercooled melt is expected to form the Nd_Fe_<14>B phase free from the α-Fe. The objective of the present study is to explore the possibility of the near-net shape casting of a magnet from the undercoole … More d melt.In 2003, a splat quenching system was built up and the know-how to perform the near-net shape casting was established. In 2004, in order to elucidate the relationship between the microstructure and the undercooling, the drop-tube experiment was carried out. The experimental results show that, in the as-dropped sample, a large amount of the Nd_2Fe_<17>B_x was contained as a metastable phase. In 2005, the phase transformation from the metastable Nd_2Fe_<17>B_x into stable Nd_2Fe_<14>B was precisely investigated. The results show that the diffusive phase transformation from Nd_2Fe_<17>B_x into Nd_2Fe_<14>B was rapid, because the Nd-rich phase retained at the intergranular region enhanced the diffusivity of the solute atom due to its low melting temperature, implying that the combination with the splat quenching system enables the near-net shape casting of a magnet from the undercooled melt. In the sample with relatively large diameter, the Nd_2Fe_<14>B and γ-Fe dual-phase pearlite-like microstructure that was decomposed from the metastable Nd_2Fe_<17>B_x can be expected to be one of the prominent candidates for a magnet with nano-composite microstructures. Less