Compacting of Nd-Fe-B powders by shock consolidation

Shock pressing takes advantage of the shock wave produced by explosions, which isostatically compacts the canned powder with a pressure of several GPa. The density of shock pressed magnets reaches 98% of theoretical value. Some small pores and cracks are observed in the microstructure. These defects can be minimised by the application of multistage pressing and optimization of its parameters. The magnetic properties are affected by green density and powder particle size. Large pores and coarse particles lead to higher local temperature increase, which results in better density but lowers the magnetic properties. It is, however, difficult to calculate the real temperature. SEM analysis shows transcrystalline cracks in the low green density samples whereas the specimens with higher green density the cracks develop through the particle boundaries. Shock pressing results in some decrease of coercivity. These decrease is greater for the alloys having higher Nd contents (over 12%). Structural analysis for these alloys revealed in the microstructure of shock pressed magnets a presence of droplet-like inclusions of Nd-rich phase, which evidences the effect of local temperature increases. This leads to local melting and squeezing of the Nd-rich melt. The shock pressing method gives good results especially for the low Nd alloys (nanocomposites). (author)