High-coercivity copper-rich Nd-Fe-B magnets by powder bed fusion using laser beam method

Abstract

Additive manufacturing (AM) is an attractive processing route to make efficient use of rare-earth elements (REE) in systems containing complex-shaped rare-earth (RE) based magnets. Powder bed fusion using laser beam (PBF-LB) is one of the most promising technologies to obtain fully dense AM parts and has seen significant recent research efforts. However, most works use commercial Nd-Fe-B powders with a composition more suited for binder based AM methods, which reduces the parameter window and does not allow property enhancement by the application of annealing cycles. In this work, a close-to-industrial process route was developed in order to produce a narrow-distributed 40-µm Nd-Fe-B powder, derived from strip casting, hydrogen decrepitation and milling, with a composition close to the usual sintered magnet grades having around 30 wt% REE content. The composition was adjusted by preliminary small-scale experiments focused on the reduction of cracking and the promotion of fine-grained equiaxed microstructures. This powder was then used to build magnets by the PBF-LB method. The best magnetic properties could be achieved with building conditions providing just enough energy to completely melt the material, yielding nano-grained microstructures almost deprived of α-Fe phase. After laser parameter optimization and post-process annealing, properties of Br = 0.62 T, Hcj = 1790 kA.m−1 and BHmax = 65 kJ.m−3 were obtained.