Spark Plasma Sintering as a powder processing route for pre-alloyed Sm2(Co, Fe, Cu, Zr)17 permanent magnets

The ’rare earth crisis’ that occurred in 2011 ignited research into mitigating and replacing the dependence on these elements in Rare Earth Permanent Magnets (REPM) and their applications. It is the former that this body of work is based upon. A method of increasing the bulk resistivity of a REPM, Sm(Co, Fe, Cu, Zr)8.2 (henceforth referred to as Sm2(Co, Fe, Cu, Zr)17), through the addition of an electrically insulating material, CaF2, to form a rare earth composite permanent magnet is studied. The aim of increasing the bulk resistivity is to minimise the irreversible demagnetisation and performance losses that occur within REPMs due to circulating eddy currents during operation of high frequency applications, such as electric motors and drives. The project utilises an emerging sintering technology platform, Spark Plasma Sintering (SPS), to rapidly consolidate powders. The processing methodology for Sm2(Co, Fe, Cu, Zr)17 powders by the SPS technique has therefore been studied with the optimal processing parameters found for producing full dense specimens for study: 1050°C to 1100°C hold temperature, 100°C= min to 200°C= min heating rate, 51MPa hold pressure and 5 minute hold time. Sm2(Co, Fe, Cu, Zr)17 permanent magnets require a characteristic multi-stage heat treatment to precipitate a cellular nanostructure which enhances the coercivity of the permanent magnets. The optimal heat treatment for the full density SPS processed Sm2(Co, Fe, Cu, Zr)17 magnets has also been studied, with the following achieving the largest coercivities and energy products: 1. Homogenisation - 1170°C for 2 hours in an argon atmosphere. Cooled slowly in air to room temperature. 2. Ageing - 850°C for 8 hours (for largest coercivity) or 16 hours (for largest (BH)max) in argon atmosphere. 3. Slow cool (1°C=min) from 850°C to 400°C in argon atmosphere. Quench in oil to room temperature. This established processing route was then used to prototype isotropic Sm2(Co, Fe, Cu, Zr)17 and CaF2 composite magnets and study the effect of the electrically insulating phase on the microstructure, and material and magnetic properties before and after heat treatment.