An effective laser in-situ re-scanning strategy in laser powder bed fusion of Nd-Fe-B permanent magnets: Crack reduction and magnetic properties enhancement

Laser powder bed fusion (PBF-LB) additive manufacturing (AM) has shown significant promising potential in the fabrication of complex-shaped Nd-Fe-B magnets. However, a challenge arises when printing Nd-Fe-B magnets due to their inherent hardness and brittleness. The large thermal stresses caused by rapid cooling rates and steep temperature gradients easily lead to cracking, substantially degrading their magnetic properties. In the present work, an effective laser in-situ re-scanning strategy is proposed to manufacture Nd-Fe-B magnets by the PBF-LB process since it can mitigate residual stress within the deposited magnets and alleviate the cracking tendency, and its effect on the microstructure and cracking behavior is comprehensively investigated through experimental characterization and numerical simulation. The results indicate that the predominant cracks in the PBF-LB-processed Nd-Fe-B magnet deposits are transcrystalline cracks. It is observed that the crack density in the magnet deposit is efficiently reduced, and the corresponding magnetic properties are enhanced by adopting an optimized laser in-situ re-scanning strategy. This is attributed to the reduction of both the thermal gradient and related stresses in the magnet deposit. Especially, the proposed strategy also promotes the consistency of the grain size of the hard magnetic Nd₂Fe₁₄B phase, contributing to the enhancement of coercivity, without generating undesirable phases in the magnets. As a result, a higher relative density of 96.73 %, coercivity of 718 kA/m, remanence of 0.85 T, and maximum energy product of 85.9 kJ/m³ are achieved in the PBF-LB-processed Nd-Fe-B magnet deposits from MQP-S powder, which is superior to previously reported results. The present findings provide new insights for the PBF-LB-processed high-performance Nd-Fe-B magnets with crack-free.