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Soft Magnetic Materials
The processing of soft magnetic materials is another area of expertise for MPP. As with high temperature sintering, MPP's Vice President of Technology, Dr. Chaman Lall, is an internationally recognized expert in the field, having authored several papers on this subject. Some Magnetism Basics Magnetism as a subject has two distinctly separate fields. One is soft (or temporary) magnetism and the second is hard (or permanent) magnetism. The soft magnetic materials respond easily to the smallest external magnetic field, while hard magnetic materials resist the influences of even the largest external fields. At the microstructural level, the magnetic response of a material is determined by the size, shape and alignment of magnetic domains, plus the ability with which they can be altered by external magnetic fields. Each magnetic domain can be regarded as a magnet, fully aligned and magnetized in a given direction. In an "unmagnetized" part, the magnetic domains are orientated in all directions, but they cancel the effect of each other so that the external observer senses no magnetism. As an external magnetic field is applied to the part, the magnetic domains that are aligned with the external field start to grow and upset the balance. Increasing the external magnetic field causes orientation and growth of those magnetic domains in the direction of that field. When all the domains are aligned in one direction, that part is said to be saturated or fully magnetized. The important point is that the ease of magnetic domain wall motion and domain rotation determines if the material is soft magnetic or hard magnetic. While both types of products can be made by the powder metallurgy technology, the goals or objectives of the processing method are entirely opposite. For soft magnetic parts the goal is to make the microstructure as clean as possible, while the aim for hard magnetic parts is to make the microstructure as "dirty" as possible, with as many hindrances as possible to inhibit magnetic domain wall motion. Soft Magnetics at MPP The most common soft magnetic powder metal materials used by MPP are, in order of decreasing coercive field (width of a magnetic hysteresis curve): pure Fe, Fe-0.45% P, Fe-3% Si, and Fe-50% Ni. In addition, 410L and 434L plus similar grades of stainless are also good candidates for soft magnetic applications where corrosion resistance is needed. Examples of these are anti-lock brake systems (ABS) such as "sensor" or "tone" wheels. These are often located at the vehicle wheels and are exposed to water, dirt and salt from the road. Other soft magnetic materials covered by a protective coating compete in these types of applications. In order to enhance the soft magnetic properties of these powder metal materials, MPP keeps contamination to a minimum in both the powder preparation and molding stages, as well as during sintering. We take special care to minimize pickup of carbon, oxygen, and/or nitrogen, as even minor amounts of these interstitial elements can decrease the coercive field of a given material. The form precipitates that "pin" the magnetic domain wall motion and, therefore, soft magnetic response. In a similar way, deformation damage in the form of dislocations at the microstructural level will also inhibit domain wall movement. A component that is subjected to a secondary operation such as machining or grinding should be magnetically annealed for optimum properties. Higher temperatures during sintering and the use of pure hydrogen for oxide reduction will improve soft magnetic performance. The absence of nitrogen eliminates the deleterious effects of the element, while the higher diffusion rates increase density, dissolve precipitates, and promote recrystallization and grain growth, In the case of silicon steel and phosphorus steel, the increased densification occurs with the aid of transient liquid phases. For more information on P/M soft magnetics, go to Technical Articles.
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