Factors affecting soft magnetic properties

- Aug 01, 2019-

Soft magnetic performance parameters can be divided into two categories:


(1) structurally sensitive, such as initial permeability mui, maximum permeability mum, remanent Br, coercivity Hc, hysteresis loss Ph and eddy current loss Pe, etc.


(2) structurally insensitive type, such as saturation magnetic sensing Bs, Curie temperature Tc, magnetostrictive coefficient lambda s, etc. The former is closely related to the magnetization process, while the latter is related to the chemical composition and microstructure of the alloy. In practice, many metallurgical and physical factors are found to have significant effects on the magnetic properties of soft magnetic alloys. The main influencing factors are:


Chemical composition is one of the main factors that determine the properties of soft magnetic alloys. For example, in fe-ni alloys, good soft magnetic properties occur in the range of 36% ~ 83% nickel content. The addition of some alloyed elements, such as molybdenum, can increase the resistivity, reduce the sensitivity to stress, and increase the initial permeability, but the saturation magnetization and Curie point decrease. Such as iron, silicon and aluminum alloy, magnetic properties and composition of a closer relationship, composition slightly deviation, magnetic properties will drop sharply. In iron-cobalt alloy, with the increase of cobalt content, the saturation magnetic induction intensity increases, and the Curie point also increases. At 35%Co, Bs can reach more than 2.4t.


Impurity some impurity elements exist in soft magnetic alloys as interstitial or substituted solid solution, causing lattice distortion, causing microscopic stress and preventing domain wall from moving freely. Some elements cannot be dissolved in solution and form compounds of carbon, nitrogen and oxygen. These non-magnetic inclusions can nail domain walls, thus increasing coercivity and decreasing magnetic permeability. For high-quality soft magnetic alloys, in addition to requiring pure raw materials and few impurities, vacuum smelting and high-temperature heat treatment in pure dry hydrogen or high vacuum are often adopted to further remove impurities.


The magnetic properties of stress soft magnetic alloys are very sensitive to stress. The internal stress in the manufacturing process can decrease the permeability and increase the loss of the alloy. External force also affects the magnetic properties of soft magnetic alloys to some extent, especially for the alloys with higher performance. Therefore, the core must be placed in a protective box. The coupling of external stress and magnetostriction will change the direction of magnetization and lead to stress anisotropy. The magnetic conductivity of the alloy can be improved or deteriorated with different ways of applying force.


There are two directions of easy magnetization and difficult magnetization in orientation crystals. Soft magnetic alloys are mainly made by cold and hot processing. The magnetic properties of hot-processed materials are basically isotropic, but after cold processing, the materials form grain orientation due to cold rolled texture or crystal texture. The magnetic properties of fe - ni and fe - si alloys are better along the rolling direction. Therefore, magnetization should be carried out along the rolling direction in use.


Changes in ambient temperature and core temperature caused by loss will affect magnetic properties. With the increase of temperature, the atomic arrangement tends to be chaotic, the spontaneous magnetization decreases, and the changes of permeability and coercivity are related to the changes of magnetocrystal anisotropy and magnetostriction coefficient with the change of temperature.


The effect of the thickness of cold rolled strip on the properties of cold rolled soft magnetic alloy strip is that the eddy current loss caused by eddy current effect is proportional to the square of the thickness under alternating magnetic field. At the same time, due to skin effect, the thickness of the alloy should be less than a certain value at a certain frequency. Therefore, it is used to reduce the eddy current loss and improve the material utilization rate. But the thinning of thickness will increase abnormal loss and increase production cost.