Presentation Information
[P1-23]Nd2Fe14B MAGNETS SINTERED BY THE HDDR PROCESS: the first results
*Renhui Liu1,2,3, Ihor I. Bulyk1,2,3, Heping Zhu1,2,3, Munan Yang1,2,3,4 (1. Jiangxi University of Science and Technology (China), 2. Jiangxi Province Key Laboratory of Magnetic Metallic Materials and Devices (China), 3. National Rare Earth Functional Materials Innovation Center (China), 4. Ganjiang Innovation Academy, Chinese Academy of Science (China))
Keywords:
Nd-Fe-B permanent sintered magnets,Hydrogenation,disproportionation,desorption,recombination,Low temperature sintering,Fine microstructure
The production volume of the Nd-Fe-B based permanent magnets is increasing rapidly and will continue to do so with the rapidly growing demand for environmentally friendly devices such as wind turbines and electric cars. Such magnets need to have a high coercivity due to high operating temperatures. One of the ways to increase the coercivity of magnets is to grind their microstructure [1]. Conventional technologies, namely sintering ones, are not acceptable due to the high temperatures involved. It is well known that the hydrogenation, disproportionation, desorption, recombination (HDDR) process is capable of grinding the grain size of the microstructure down to the sub-micron level [2]. In addition, this approach can be used to sinter Nd-Fe-B-type materials [3].
Efforts have been made to study in detail the application of the HDDR process to the sintering of Nd2Fe14B-type materials and the first results will be presented here.
The Nd-Fe-B-type alloy powder was prepared by melt spinning. The hydrogenated alloy was milled by jet milling, and the resulting powder was used as the starting material. The peculiarities of the HDDR process in the powder were investigated at the hydrogen pressure in the range of 2 to 80 kPa. Differential thermal analysis and X-ray diffraction (XRD) measurements were used, and based on these data, the phase composition-pressure-temperature nonequilibrium diagram for the Nd-Fe-B powder-hydrogen system was constructed. The microstructure of the powder particles after different HDDR stages, HD and DR, and their parameters, hydrogen pressure, temperatures, and interaction times, were investigated. The influence of the HDDR parameters on the magnetic properties and texture of the powder particles was determined.
The sintering parameters of the Nd-Fe-B-type alloy powder were selected from the data on the HDDR process in this powder. Thus, during the HD stage the hydrogen pressure was equal and below 30 kPa and the temperature was 630-750 °C. During the DR stage, the pressure was close to 5 kPa, the temperature was 800-850 °C and the holding time was up to 60 min.
Sintering was carried out as a continuous process when the DR stage followed immediately after the HD stage without cooling to room temperature. The grain boundary diffusion process (GBDP) followed the HDDR treatment. The phase composition of the alloy was analysed by the XRD method after each sintering stage and GBDP treatment. The XRD measurements were also used to evaluate of the degree of texture of the green compact, of the samples after HD, DR and GBDP treatments. The microstructure of the materials was also studied after each of the sintering and treatment process stages.
The dependence of the magnetic properties of the sintered magnets on the sintering parameters of the HDDR route and the subsequent GBDP treatment was evaluated. The results show that the HDDR technique is a promising new technology for the sintering of Nd-Fe-B-type magnets. The sintering temperature is lower than the temperatures used in the conventional technology. This makes it possible to obtain the sintered magnets with a fine microstructure. This feature also opens up the possibility of using less expensive sintering furnaces. Further optimisation of the sintering and treatment parameters will make it possible to obtain heavy rare earth-free magnets with fine microstructure and consequently high coercivity.
1. K. Hono et al., Scr. Mater. 67 (2012) 530–535. https://dx.doi.org/10.1016/j.scriptamat.2012.06.038.
2. O. Gutfleisch and I. R. Harris, J. Phys. D: Appl. Phys. 29 (1996) 2255-2265.. https://doi.org/10.1088/0022-3727/29/9/006
3. I.I. Bulyk and I.V. Borukh, Powder Metall. Met. Ceram. 61 (2023), 657-669. https://doi.org/10.1007/s11106-023-00354-9.
Efforts have been made to study in detail the application of the HDDR process to the sintering of Nd2Fe14B-type materials and the first results will be presented here.
The Nd-Fe-B-type alloy powder was prepared by melt spinning. The hydrogenated alloy was milled by jet milling, and the resulting powder was used as the starting material. The peculiarities of the HDDR process in the powder were investigated at the hydrogen pressure in the range of 2 to 80 kPa. Differential thermal analysis and X-ray diffraction (XRD) measurements were used, and based on these data, the phase composition-pressure-temperature nonequilibrium diagram for the Nd-Fe-B powder-hydrogen system was constructed. The microstructure of the powder particles after different HDDR stages, HD and DR, and their parameters, hydrogen pressure, temperatures, and interaction times, were investigated. The influence of the HDDR parameters on the magnetic properties and texture of the powder particles was determined.
The sintering parameters of the Nd-Fe-B-type alloy powder were selected from the data on the HDDR process in this powder. Thus, during the HD stage the hydrogen pressure was equal and below 30 kPa and the temperature was 630-750 °C. During the DR stage, the pressure was close to 5 kPa, the temperature was 800-850 °C and the holding time was up to 60 min.
Sintering was carried out as a continuous process when the DR stage followed immediately after the HD stage without cooling to room temperature. The grain boundary diffusion process (GBDP) followed the HDDR treatment. The phase composition of the alloy was analysed by the XRD method after each sintering stage and GBDP treatment. The XRD measurements were also used to evaluate of the degree of texture of the green compact, of the samples after HD, DR and GBDP treatments. The microstructure of the materials was also studied after each of the sintering and treatment process stages.
The dependence of the magnetic properties of the sintered magnets on the sintering parameters of the HDDR route and the subsequent GBDP treatment was evaluated. The results show that the HDDR technique is a promising new technology for the sintering of Nd-Fe-B-type magnets. The sintering temperature is lower than the temperatures used in the conventional technology. This makes it possible to obtain the sintered magnets with a fine microstructure. This feature also opens up the possibility of using less expensive sintering furnaces. Further optimisation of the sintering and treatment parameters will make it possible to obtain heavy rare earth-free magnets with fine microstructure and consequently high coercivity.
1. K. Hono et al., Scr. Mater. 67 (2012) 530–535. https://dx.doi.org/10.1016/j.scriptamat.2012.06.038.
2. O. Gutfleisch and I. R. Harris, J. Phys. D: Appl. Phys. 29 (1996) 2255-2265.. https://doi.org/10.1088/0022-3727/29/9/006
3. I.I. Bulyk and I.V. Borukh, Powder Metall. Met. Ceram. 61 (2023), 657-669. https://doi.org/10.1007/s11106-023-00354-9.