Mechanical Processing in Magnetic Materials

Abstract

The classification of magnetic materials as soft and hard is based on the magnetic properties. Whereas the soft magnetic materials exhibit a high saturation induction, a high remanence, a high permea-bility and a low coercivity, the hard magnets posses a high saturation induction, a high remanence and a very high coercivity. The properties such as remanence, permeability and coercivity are struc-ture-sensitive and hence are affected to a great extent by their microstructures and textures, which in turn depend on the processing methods[1-4]. Soft magnetic alloys which are in great commercial use are the silicon steels and the Ni-Fe alloys. In majority of the applications, these alloys are used in the form of thin sheets or strips. The processing of these alloys includes hot forging, hot rolling, pickling, cold rolling/drawing and suitable heat-treatment. Through judicious selection of the processing parameters including heat treatment, one can evolve the most desirable microstructure or texture. In the case of magnetic materials, cold work (plastic deforma-tion) makes magnetisation more difficult. The M-H or the B-H curve is lower than that for the fully annealed condition. The hysteresis loop becomes wider (a larger coercivity) and hence a higher hysteresis loss for the same maximum value of M or B. The mechanical strength and hardness also increase. These changes are associated with the increased numbers of dislocations and other lattice defects. The resulting microstress impedes both domain wall motion and domainrotation, increasing the magnetic hardness. Also, the maximum (magnetic) permeability decreases sharply even with a small amount of cold work.