Research On Rapid Solidification Of Atomized SmFeN Alloy Droplets

SmFeN permanent magnet has excellent magnetic properties and good temperature stability.There is still no way to achieve efficient preparation of SmFeN.Gas atomization becomes a new way to produce SmFeN because it has the advantages of high efficiency and high quality.The rapid solidification process of Sm Fe droplets is one of the key steps in gas atomization.In the research we defined,by theoretical analysis,the cooling mechanism of Sm Fe in the quick solidifying process.Hot state tests were carried out to analysis the microstructure and phase structure of Sm Fe before and after nitriding.We made predictions for rapid solidification of Sm Fe droplets to lay the foundation for the control technology of SmFeN powder prepared by gas atomization.The theoretical researches show that high ambient pressure can make the atomized droplet transfer heat with bigger convection heat transfer coefficient in a short period of time and along with the increase of ambient pressure,the cooling rate of droplet increases,undercooling degree also has a tendency to slowly increase.They both increase obviously along with the decrease of droplet size,cooling rate of droplets within 50?m reaches 106K/s,the nucleation undercooling degree reaches above 200?.The melt can be broken into smaller droplets by increasing the ambient pressure in crushing stage,therefore cooling rate and the nucleation undercooling degree are improved significantly.The results of hot tests showed that the cooling rate is 10⁵K/s-10⁶K/s using melt spin quenching method,which is in the same range as atomization.There are Sm₂Fe₁₇,SmFe₃,SmFe₂ and ?-Fe in the alloy when Sm Fe material solidifies under normal cooling condition which contains Sm 30%.Sm Fe melt-spun ribbons do not contain ?-Fe,and as the cooling rate increases to 5×10⁵K/s,the Sm Fe3 phase disappears,and only SmFe₂ and Sm₂Fe₁₇ are left in the alloy,continue to increase the cooling rate to 7 × 10⁵K/s,amorphous is discovered.After nitriding,we found Sm₂Fe₁₇N_x,and there were N atoms into amorphous while partial crystallization occurred in the amorphous phase.The Tg of Sm Fe is about 435?,Tx is 450? and Tp is about 520?.There is a big difference betwee the roller surface and unconfined flow surface of Sm Fe melt-spun ribbons before and after nitriding,and white matters containing N are precipitated after nitriding.