Preparation And Microstructure Of Directionally Solidified Nd-Fe-B Ingots By Cold Crucible Approach

In this paper, directional solidification experiments of selected Nd-Fe-B alloys are carried out with circle cold crucible in different technical parameters. The preparation technology of Nd-Fe-B alloys are researched. Position parameters and heating power range that applicable to circular cold crucible and Nd-Fe-B alloy are summarized.The surface quality and microstructures of these ingots are observed and analyzed. The main reason of surface crack is shell softening of the heat, and a friction between the ingot and crucible wall. The macro structure is better directional in the heating power to45kW, withdraw rate of0.5mm/min and heating power50kW, withdraw rate0.8mm/min. It is more easy to get more tiny columnar crystal in the high velocity of solidification and low heating power.The microstructures and phase components of Nd-Fe-B ingots are observed and analyzed with optical microscopy(OM), scanning electron microscopy(SEM)and X-Ray diffraction(XRD). And in traditional ceramic tube of legal compared with the Bridgman directional solidification, samples made by cold crucible directional solidification have the more pure component. Nd2Fe14B phase grows with plane front peritectic interfaces at lower velocity and with the dendritic interfaces at higher velocity. It was found that dendrite of primary γ-Fe dendrites were difficult to reduce during peritectic reaction in the samples in higher velocity. Volume fraction of ferromagnetic T1phase was found to first increase and then decrease with increasing velocity. In the sample grown at the velocity of0.8mm/min and hearing power of45kW, the volume fraction of T1phase reached up to98%.Parameters of isothermal heat treatment and grain annealing heat treatment are analyzed in the vacuum environment. With the isothermal heat treatment temperature of1050℃, the volume fraction of a-Fe phase is just1.25%. In the samples after730℃crystal annealing, diffraction peak of Nd2Fe14B phase in XRD analysis results obvious enhanced. Try to measure magnetic properties of the different samples directly, and analysed the cause of the poor magnetic properties in the view of structure differences.