Presentation Information
[P1-62]Grain alignment by single track printing using anisotropic sintered Nd-Fe-B magnet as a substrate
Peishu Song1, Zhenyuan Liu1, Yan Li2, Dan Han3, *Lanting Zhang1 (1. Shanghai Jiao Tong University (China), 2. Inner Mongolia University of Science and Technology (China), 3. Inner Mongolia Research Institute of Shanghai Jiao Tong University (China))
Keywords:
Selective laser melting,Nd-Fe-B,<001> orientation,melt pool,dendrite growth
The relatively high cooling rate during SLM is likely to suppress the formation of a-Fe and allow designed composition distribution in the magnet which is beneficial to the magnetic properties. Many works have devoted to the effects of printing parameters on grain size, phase transition and magnetic properties of printed magnets[1]. Wu et al[2] presented an in-depth analysis on grain structure of the SLM magnet for the first time in terms of morphology, size distribution, and texture. The formation and transformation mechanisms of complex phases and the development of nanocrystalline microstructures during the SLM process were investigated. Huber et al[3] reported that it was confirmed that the coercive force increased by infiltrating the low melting point rare earth eutectic alloy into the Nd-Fe-B magnet fabricated by LPBF. However, current works are predominantly focused on optimizing the printing parameters to enhance the coercivity of magnets, with scant attention given to the enhancement of grain orientation in SLM processed Nd-Fe-B magnets. the present study explores the impact of utilizing an anisotropic sintered Nd-Fe-B magnets as a substrate to induce orientated grains and desired microstructure of the magnets after SLM.
Spherical Nd-Fe-B commercial powder (MQP-S-11-9-20001) from Magnequench with an average size of 43 μm and a composition of Nd17.2Pr1.9Fe69.8Co2.8Ti2.1Zr4.3B1.7 (wt%) was used. An N52-grade commercial sintered magnet (Nd29FebalCo1.5B0.99Cu0.17Ga0.1Ti0.18 (wt%)) was employed as the substrate. SLM equipment applied was Truprint 1000 equipped with a 200 W Yb: YAG pulsed fiber laser (spot size: was 30 μm). The laser power P and scanning speed V attempted in the present work varied from 80 W to 200W and from 20 mm/s to 800 mm/s using fixed layer thickness of 30 μm of the powder. The distance between the adjacent single tracks is set to 2 mm and the dimension of the Nd-Fe-B substrate magnet was 25×10×5 mm3, with its easy magnetization axis (c-axis) parallel to the building direction. Optical microscopy (OM) was used to observe the surface morphology of the single tracks after SLM. The melt pools and cross sections of the tracks were observed by a scanning electron microscope (SEM). The orientation of the grains in the melt pool was characterized by electron backscatter diffraction (EBSD) operated at 20 kV.
As shown in Figure 1, The cross section of the melt pool after solidification can be broadly classified into four regions: the dendritic zone, the equiaxed fine grain region, the heat-affected zone and the un-melted substrate region. The area fraction of the dendritic region increases with increasing line energy density (EL). The majority of the grains in the melt pool (140W, 50mm/s, EL=2.8 J/mm) are <001> orientated which aligns with that of the substrate magnet. Only a small portion of the initially grown grains at the edge of the melt pool (140W, 200mm/s, EL=0.7 J/mm) keep the same orientation as the substrate grains. The width and depth of the melt pool under 140 W, 50 mm/s (~500 μm wide and ~400 μm deep) are larger than that those under 140 W, 200 mm/s (~320 μm wide and ~210 μm deep). A sufficient growth of the dendrite zone is crucial to form the orientated grains with the same orientation as the substrate.
In the direct laser remelting process, a slow scanning speed of 20 mm/s (EL=7 J/mm) is favorable to form <001> oriented grains than 200 mm/s (EL=0.7 J/mm) under 140 W laser power. In the SLMed single track study, <001> orientated Nd-Fe-B grains are formed in the dendrite zone under the EL = 2.8 J/mm. The grains in the MP-fine zone are randomly orientated. It confirmed the feasibility of using anisotropic sintered Nd-Fe-B magnet as the substrates to enhance the orientation of SLMed magnet.
References
[1] B. Yao et al. International Journal of Extreme Manufacturing 6(1) (2024) 015002.
[2] J. Wu et al. Acta Materialia 259 (2023) 119239.
[3] C. Huber et al. Acta Materialia 172 (2019) 66-71.
Spherical Nd-Fe-B commercial powder (MQP-S-11-9-20001) from Magnequench with an average size of 43 μm and a composition of Nd17.2Pr1.9Fe69.8Co2.8Ti2.1Zr4.3B1.7 (wt%) was used. An N52-grade commercial sintered magnet (Nd29FebalCo1.5B0.99Cu0.17Ga0.1Ti0.18 (wt%)) was employed as the substrate. SLM equipment applied was Truprint 1000 equipped with a 200 W Yb: YAG pulsed fiber laser (spot size: was 30 μm). The laser power P and scanning speed V attempted in the present work varied from 80 W to 200W and from 20 mm/s to 800 mm/s using fixed layer thickness of 30 μm of the powder. The distance between the adjacent single tracks is set to 2 mm and the dimension of the Nd-Fe-B substrate magnet was 25×10×5 mm3, with its easy magnetization axis (c-axis) parallel to the building direction. Optical microscopy (OM) was used to observe the surface morphology of the single tracks after SLM. The melt pools and cross sections of the tracks were observed by a scanning electron microscope (SEM). The orientation of the grains in the melt pool was characterized by electron backscatter diffraction (EBSD) operated at 20 kV.
As shown in Figure 1, The cross section of the melt pool after solidification can be broadly classified into four regions: the dendritic zone, the equiaxed fine grain region, the heat-affected zone and the un-melted substrate region. The area fraction of the dendritic region increases with increasing line energy density (EL). The majority of the grains in the melt pool (140W, 50mm/s, EL=2.8 J/mm) are <001> orientated which aligns with that of the substrate magnet. Only a small portion of the initially grown grains at the edge of the melt pool (140W, 200mm/s, EL=0.7 J/mm) keep the same orientation as the substrate grains. The width and depth of the melt pool under 140 W, 50 mm/s (~500 μm wide and ~400 μm deep) are larger than that those under 140 W, 200 mm/s (~320 μm wide and ~210 μm deep). A sufficient growth of the dendrite zone is crucial to form the orientated grains with the same orientation as the substrate.
In the direct laser remelting process, a slow scanning speed of 20 mm/s (EL=7 J/mm) is favorable to form <001> oriented grains than 200 mm/s (EL=0.7 J/mm) under 140 W laser power. In the SLMed single track study, <001> orientated Nd-Fe-B grains are formed in the dendrite zone under the EL = 2.8 J/mm. The grains in the MP-fine zone are randomly orientated. It confirmed the feasibility of using anisotropic sintered Nd-Fe-B magnet as the substrates to enhance the orientation of SLMed magnet.
References
[1] B. Yao et al. International Journal of Extreme Manufacturing 6(1) (2024) 015002.
[2] J. Wu et al. Acta Materialia 259 (2023) 119239.
[3] C. Huber et al. Acta Materialia 172 (2019) 66-71.
