Microstructure and magnetic properties of CO2 laser surface melted Nd-Fe-B magnets

The near-surface microstructure and magnetic properties of permanent magnets produced from hot-pressed melt-spun ribbons of a Nd0.13Fe0.81B0.06 alloy were modified by CO2 laser beam melting. Self-quenching of the molten surface layer at an average cooling rate of ≈104K/s resulted in a stratified structure consisting of a zone of large columnar grains between thinner layers of equiaxed grains. The resolidified material was comprised mostly of the magnetically hard Nd2Fe14B phase dispersed interdendritically among primary α-Fe dendrites. A small volume fraction of a fcc Nd-rich phase was also present. A solidification texture in the columnar zone resulted in alignment of the c-axis of the Nd2Fe14B phase, the direction of easiest magnetization, parallel to the surface of the sample. The magnetic properties of the resolidified layer were anisotropic. The magnetization was oriented parallel to the surface, in accordance with the preferred orientation of the Nd2Fe14B phase. Low coercivity in all directions (≈140Oe) was attributed to the relatively large grain size of the Nd2Fe14B phase and also to the amount of ferrite present.