Soft magnetic alloy

Brief History

The production and application of soft magnetic alloys has a history of more than 100 years. In 1890, the production of thermomagnetic pure iron (or low carbon steel) was used to manufacture motor and transformer cores. Fe-silicon alloy (silicon steel sheet) came out in 1900, which soon replaced pure iron, and has been the largest soft magnetic material in production. With the development and needs of the telephone system, in 1913 the American GWElmen invented a nickel-iron alloy (called permalloy) that had better magnetic properties than silicon steel under weak and medium magnetic fields. From 1929 to 1931, they continued. Fe-cobalt alloys and iron-silicon aluminum alloys with different magnetic properties for special purposes have appeared. In the early 1970s, an amorphous soft magnetic alloy with a structure and production method completely different from that of traditional soft magnetic alloys came out. The development of the company has entered a new stage. China has been producing hot-rolled silicon steel sheets since the 1950s, various soft magnetic alloys based on Fe-Ni and Fe-Co alloys have been produced in the 1960s, cold-rolled oriented silicon steel sheets have been produced in the 1970s, and large quantities have begun in the mid to late 1980s. Trial production of amorphous soft magnetic alloys. Microcrystalline and nanocrystalline soft magnetic alloys were developed in the 1990s. Since the 1980s, China has successively formulated and promulgated national standards for the production technical conditions of various soft magnetic alloys.

Classification

There are many types of soft magnetic alloys. According to the different elements of the alloy, they can be divided into electromagnetic pure iron (industrial pure iron), iron-silicon alloy, iron-nickel alloy, iron-aluminum alloy , Iron-silicon aluminum alloy and iron-cobalt alloy, etc. If the magnetic properties of the alloy are different, it can be divided into high magnetic saturation alloy, high permeability alloy, high squareness ratio alloy, electrical steel, constant permeability alloy, high △B alloy, corrosion-resistant soft magnetic alloy and thermomagnetic compensation alloy. Etc. According to the internal structure of the alloy, it can be divided into crystalline soft magnetic alloy, amorphous soft magnetic alloy and nanocrystalline soft magnetic alloy. Table 1 lists the properties of several major soft magnetic alloys.

Table 1 Magnetic properties of several main soft magnetic alloys

Electromagnetic pure iron Iron and mild steel with carbon content less than 0.04% , Including electrical pure iron, dielectric iron and carbonyl iron. The characteristic is that the saturation magnetization is up to 2.15T, the price is low, and the processing performance is good; but the resistivity is low, and the eddy current loss is large when used under an alternating magnetic field, which is only suitable for use under static conditions. It is mainly used as the magnetic conductor of electromagnet core, pole shoe, relay and loudspeaker, and magnetic shield.

Fe-Si alloy Fe-silicon alloy with a silicon content of 0.5% to 4.8%, generally produced in the form of thin plates, including hot-rolled silicon steel, non-oriented cold-rolled silicon steel and oriented cold Rolled silicon steel, etc. After adding silicon to iron, the serious magnetic aging phenomenon of pure iron can be eliminated. With the increase of silicon content, although magnetic saturation (saturation magnetization) decreases, its resistivity and permeability increase, coercive force and eddy current loss Reduced, which can expand its application in the field of communication. Due to its low price, it has become the leading product of soft magnetic alloys and is widely used in the power industry, mechanical electronics and instrument industries as transformers, power amplifiers, inductors, motor stators and rotors, etc. Generally, electromagnetic pure iron and silicon steel sheets are collectively referred to as electrical steel.

Iron-aluminum alloy The iron-aluminum alloy with an aluminum content of 6%-16% has good soft magnetic properties, not only high permeability and resistivity, but also high hardness , Good wear resistance. However, it is relatively brittle in nature, difficult to roll and stamp, and its use is affected. The alloy is mainly used for the cores of magnetic heads and small transformers, magnetic amplifiers, and relays.

Fe-Si-Al alloy Adding silicon to the binary iron-aluminum alloy can make the magnetocrystalline anisotropy K₁ and magnetostrictive λs at the same time Tend to zero to obtain a sendust soft magnetic alloy with high magnetic properties, named Sendust alloy. Its typical composition is 9.6% silicon, 5.4% aluminum, and the rest is iron. It is characterized by extremely high hardness, high magnetic saturation (approximately 1T), high permeability and high resistivity and other characteristics; the disadvantage is that the magnetism is very sensitive to the fluctuation of the composition, is brittle, and has poor processing performance. This alloy is mainly used for audio and video heads.

Iron-nickel alloy An iron-nickel alloy with a nickel content of 30% to 90%, often called Permalloy. Within this composition range, by adding appropriate alloying elements and adopting appropriate processes, soft magnetic alloys with different magnetic properties such as high permeability, constant permeability, and constant moment magnetism can be obtained. Permalloy has high plasticity and can be cold-rolled into 1μm ultra-thin strips. It is the most widely used soft magnetic alloy. It can be used as an iron core and magnetic shield in a weak magnetic field, as a pulse transformer and inductor core with low remanence and constant permeability, as well as a high squareness ratio alloy, thermomagnetic compensation alloy and magnetostrictive alloy, etc. . The disadvantage is that it is expensive, and the loss is relatively large when used in a UHF magnetic field.

Fe-Co alloyFe-Co alloy with a cobalt content of 27%～51% has the highest saturation magnetization. For example, the saturation magnetic induction of 35Co-Fe alloy reaches 2.43T. 13% higher than electromagnetic pure iron. The binary iron-cobalt alloy has poor mechanical properties and low resistivity. Adding appropriate amounts of chromium and vanadium can improve processing performance. It is most suitable for high saturation magnetic induction materials and high-performance soft magnetic materials. It is mainly used for pole shoes, motor rotors and Stator, transformer core, etc.

Amorphous soft magnetic alloy An alloy without long process and no crystal grains, also known as amorphous metal or metallic glass. Amorphous alloys with soft magnetic properties have high magnetic permeability, low coercivity, insensitive to stress, and have the characteristics of corrosion resistance and high strength. In addition, its resistivity is high and can be used for high frequency. The disadvantage is that it will crystallize at a lower temperature, and the structure will relax at a lower temperature, which will change the magnetism. Therefore, the working temperature should not be too high and should not exceed 100-150°C. There are two main types of amorphous soft magnetic alloys: metal-metalloid type and metal-metal type. The former is widely used, and it is divided into three categories: cobalt-based, iron-based and iron-nickel-based. Amorphous soft magnetic alloys have different types of magnetic properties, which can replace other soft magnetic materials.

Ultramicrocrystalline soft magnetic alloy A soft magnetic alloy newly discovered since the 1980s. Composed of a crystalline phase less than 50nm and an amorphous grain boundary phase, it has better comprehensive magnetic properties than crystalline and amorphous alloys. Not only does it have high permeability, low coercivity, low loss, and saturation magnetism High sense and good stability. For example, iron-based ultra-microcrystalline alloys mainly include FeCuNbSiB and FeZrB. Like high-nickel permalloys, different heat treatment processes can be used to obtain different magnetic properties.

Influencing factors

Factors affecting soft magnetic properties

Soft magnetic properties parameters can be divided into two categories: (1) Structure Sensitive, such as initial permeability μ_i, maximum permeability μ_m, remanence B_r, coercivity H_c, hysteresis loss P_h and eddy current loss Pe, etc. (2) Structure insensitive type, such as saturation magnetic induction B_s, Curie temperature T_c, magnetostriction coefficient λ_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. It has been found in practice that many metallurgical and physical factors have significant effects on the magnetic properties of soft magnetic alloys. The main influencing factors are:

Composition The chemical composition determines the performance of soft magnetic alloys. One of the main factors, for example, in iron-nickel alloys, good soft magnetic properties appear in the range of 36% to 83% nickel. Adding some alloying elements such as molybdenum can increase the resistivity, reduce the sensitivity to stress, and increase the initial permeability at the same time, but the saturation magnetization and Curie point will decrease. Another example is iron-silicon-aluminum alloys. The magnetic properties are more closely related to the composition. If the composition is slightly deviated, the magnetic properties will drop sharply. In iron-cobalt alloys, as the content of cobalt increases, the saturation magnetic induction increases, and the Curie point also increases. At 35% Co, B_s can reach more than 2.4T.

Impurities Some impurity elements exist in the soft magnetic alloy as interstitial or substituted solid solutions, causing lattice distortion and causing microscopic stress to hinder the free movement of domain walls. Certain elements cannot be dissolved in a solid solution to form compounds of carbon, nitrogen, and oxygen. These non-magnetic inclusions can pin the domain wall, thereby increasing the coercive force and reducing the magnetic permeability. For high-quality soft magnetic alloys, in addition to requiring pure alloy raw materials and low impurities, vacuum smelting and high-temperature heat treatment in pure dry hydrogen or high vacuum are often used to further remove impurities.

Stress The magnetic properties of soft magnetic alloys are very sensitive to stress. The internal stress in the manufacturing process can reduce the magnetic permeability of the alloy and increase the loss. External force also affects the magnetic properties of soft magnetic alloys to a certain extent, especially for alloys with higher performance, its harmful effects are greater. Therefore, the iron core must be put into the protective box. The coupling between external stress and magnetostriction will change the direction of magnetization, causing stress anisotropy. Depending on the force application method, the magnetic permeability of the alloy can be improved or deteriorated.

Grain orientation There are easy magnetization directions and difficult magnetization directions in the crystal. When magnetized along the easy magnetization direction, the magnetic properties are better. Soft magnetic alloys are mainly made by cold and hot processing. The magnetic properties of hot-worked materials are basically isotropic, but after cold-worked, cold-rolled textures or crystalline textures are produced, causing the materials to form grain orientations. Magnetized in different directions, the magnetic properties are different, and iron-nickel and iron-silicon alloys have better magnetic properties along the rolling direction. Therefore, it should be magnetized along the rolling direction in use.

Temperature Changes in ambient temperature and changes in core temperature caused by loss will affect magnetic properties. As the temperature increases, the arrangement of atoms tends to be chaotic, the spontaneous magnetization becomes smaller, and the changes in magnetic permeability and coercivity are related to the change of magnetocrystalline anisotropy and magnetostriction coefficient with temperature.

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

Heat treatment

A key process for heat treatment of soft magnetic alloys to achieve excellent performance. It changes the internal structure and magnetic structure of the alloy in a certain heating medium by controlling the heating temperature, holding time, cooling speed and cooling method to obtain the required magnetic properties. The heat treatment of soft magnetic alloys includes high temperature annealing and magnetic field heat treatment.

(1) High temperature annealing. The purpose is to eliminate processing stress, remove non-metallic inclusions harmful to soft magnetic properties, recrystallize the crystal grains and grow fully uniformly, and form crystal grain orientation. There are two types of high temperature annealing: hydrogen heat treatment and vacuum heat treatment. The latter mainly deals with non-vacuum melting alloys and alloys containing easily oxidizable elements. In order to change the degree of order of the soft magnetic alloy during the order-disorder transition, thereby adjusting the magnetocrystalline anisotropy constant K₁ and the magnetostriction coefficient λ_s, In order to obtain better magnetism, it is usually achieved by controlling the cooling rate or changing the holding time below T_c.

(2) Magnetic field heat treatment. Some soft magnetic alloys can form uniaxially induced magnetic anisotropy and magnetic texture after magnetic field heat treatment, thereby changing the magnetization behavior of the alloy. Especially for alloys with relatively small magnetocrystalline anisotropy, this induced magnetic anisotropy plays a major role. For example, 65% Ni-Fe alloy, K₁≈0 under slow cooling conditions, so after the magnetic field heat treatment, when the magnetic field is measured along the direction of the applied magnetic field, the hysteresis loop is rectangular; When measured perpendicular to the direction of the applied magnetic field, the hysteresis loop is flat. There are two process systems for magnetic field heat treatment: one is to heat the alloy to a temperature slightly higher than the Curie temperature and then cool it in a magnetic field; the other is to heat the alloy at a temperature slightly lower than the Curie temperature and apply a magnetic field for heat treatment. There are also two ways of applying the magnetic field during the treatment: longitudinal and transverse. The magnetic field direction of the former is consistent with the magnetization direction of the material in the working process, and the latter is perpendicular to each other.

Development prospects

After entering the 1990s, people have paid more attention to energy, electronics and environmental protection, and the research and development of magnetic alloys for power engineering and electronic devices are also tight It is developing around these centers. For example, in terms of electric power, oriented silicon steel sheets with extremely thin tertiary recrystallization with iron loss less than iron-based amorphous alloy ribbons are being developed. In order to overcome the difficulty in processing 0.5% Si silicon steel sheets, research on the production process is also ongoing. In terms of electronics, it is increasingly developing in the direction of high frequency and miniaturization, and extremely thin cobalt-based amorphous alloy ribbons and wires, ultra-microcrystalline alloys, etc. have been developed. High-density magnetic recording media and integrated component magnetic media made of soft magnetic films are also eye-catching. In the future, the development of soft magnetic alloys will continue to control their microstructure in order to continuously improve their magnetic properties. The soft magnetic alloy will be used at a higher frequency with lower loss, thereby greatly improving the function of the product.