Production of anisotropic MIM NdFeB magnets - A feasibility study

Since their finding in the early 1980s, the interest in NdFeB-based magnets has expanded continuously. NdFeB magnets can be found in voice coil motors (VCM) for computer hard disc drives or automobiles; for example 2 kg of NdFeB are being used in the Toyota Prius for motors and generators. Currently, sintered NdFeB-based magnets possess the highest energy product (BHmax). Responsible for the high coercivity, high remanence and the large energy product is the microstructure of the magnets, consisting of mainly two different phases: the magnetic phase Nd2Fe14B and a non-ferromagnetic Nd-rich phase. The ideal NdFeB magnet would contain small, regular shaped grains of Nd2Fe14B isolated by a thin layer of Nd-rich grain boundary phase. Mostly, NdFeB-magnets are produced either via pressing / cold isostatic pressing (CIP). Here bonded isotropic magnets with limited properties but complicated designs or sintered anisotropic magnets with limited geometries but maximum magnetic properties are being produced. The Metal Injection Moulding (MIM) process would allow a combination of both: high magnetic properties and complicated designs. A production of large numbers at low cost is possible. This study evaluates the possibility of producing isotropic and anisotropic NdFeB-based magnets via MIM process. Here the liquid phase sintering (LPS) plays the most important role in producing a useful microstructure and therefore good magnetic properties. Different sintering parameters were investigated to achieve the highest energy product. Here for anisotropic magnets a (BH)max of about 300 kJ/m3 along with a remanence of around 1.2 T and coercivities of HcB > 900 kA/m and HcJ > 2000 kA/m was accomplished; respectively for the isotropic magnets a (BH)max of 55 kJ/m3 and a iHc of above 1400 kA/m. The influence of the oxygen and carbon content on the magnetic properties caused by MIM was analyzed. The MIM samples were compared to magnets produced via CIP, containing the same initial powder, sintering parameters etc.