Effect of low melting point powder doping on the properties and microstructure of sintered NdFeB magnets

•Systematic comparative studies on the same types of low-melting-point elements.•A low-melting-point phase formed results in the uniform and continuous RE phase.•The increased RE-rich phase impedes magnetic exchange and improves coercivity.•A phase with better fluidity and wettability can increase the density and remanence.•Some element increased the RE phase potential and improved the corrosion resistance. The effects of Al, Sn and Zn nano-powders doping on the magnetic properties and microstructure of sintered NdFeB was investigated. The separate doping of Al and Zn can improve the coercivity (Hcj). Al and Zn powder co-doping at 0.6 wt% resulted in an increase in magnetic remanence (Br) and coercivity in 0.68 kGs and 1.04 kOe, respectively. Al-Zn co-doping can form a low-melting-point phase in the magnet, which enhanced the wettability between the grain boundary phase and the main phase and improved the rare-earth(RE)-rich phase ratio significantly. The structure and distribution of the RE-rich phase changed from clustering at the triangular grain boundary to thin and continuous distribution along with the crystal, which suppressed the demagnetization coupling between the hard magnetic phase and enhanced the coercivity. Although low-melting-point Sn doping can form a low-melting-point phase, the poor wettability between the main phase lead to RE-rich phase agglomerate formation at the triangular grain boundary, a deterioration in continuity of the grain boundary and a decrease in the remanence and coercivity.