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Improvement of Soft Magnetic Properties of FeCo Alloys by Vacuum Heat Treatment

FeCo soft magnetic alloys can reduce the weight and size of magnetic cores in various devices due to their high saturation magnetic induction.

Promoting the miniaturization and weight reduction of devices has long received great attention in defense industries such as aerospace and nuclear industries. The alloy is not only a key material for generator rotors in aero-engines, but also used in receiver coils, switches and storage magnetic cores, high-temperature components, vibration membranes for telephone listening devices, and can also be used as sonar within a certain range. Magnetostrictive sensor in the system.

It is well known that the soft magnetic properties of FeCo alloys determine the actual performance of the above devices to a large extent, especially when the alloy is used as the material of the rotor core of the generator, the loss generated by the generator under alternating frequency operating conditions is a hindrance to power generation. One of the main factors for the normal operation of the machine.

Previous research results show that the magnetic properties of soft magnetic alloys are greatly restricted by their microstructure, and changing the microstructure can effectively improve the soft magnetic properties of the material. For example, for high-silicon electromagnetic silicon steel sheets, by controlling the order-disorder transitions and their mutual proportions, the coercive force can be effectively reduced, thereby greatly reducing its loss. The coercivity of 1J122 alloy is generally around 2.4 x103~3.2 xI03 (30~40Oe), which is far from meeting the actual requirements. Changing its microstructure by subsequent vacuum heat treatment is an effective way to improve the soft magnetic properties of FeCo alloys.

1.Materials

1J22 FeCo alloy is used, and its chemical composition is shown in Table 1.

The as-rolled sheet with a thickness of 0.3 mm was cut into strip-shaped samples, and the samples with a size of 10 mm x 2 mm were subjected to vacuum and magnetic

Before the field heat treatment, ultrasonic cleaning was carried out in acetone for 10 min and then blown, and the surface of the sample crystal was clean and bright, which was used for the experiment. In order to obtain the reproducibility of the test results, three samples were used in each experiment. The vacuum degree of the vacuum heat treatment furnace is set to 2 × 10-3Pa. After reaching the predetermined degree of vacuum, the sample is heated with the furnace. The temperature range of vacuum heat treatment is 500-850 ℃, and the constant temperature time is 1-4 h. During magnetic field heat treatment, the intensity of the applied magnetic field is in the range of 1.84 x 104A/m (230Oe).

In order to really understand the effect of magnetic field heat treatment, the effects of vacuum heat treatment temperature and holding time on the microstructure and magnetic properties of FeCo alloy without additional magnetic field were firstly investigated. When the holding time is constant, the magnetic properties obtained by vacuum heat treatment at different temperatures are quite different. The He of the alloy firstly decreases slowly with the increase of T. When T rises above 650°O, H decreases rapidly. Above 760°O, as T continues to rise, the decreasing trend of H becomes gentle. According to the change of the microstructure of the above alloys with the temperature of vacuum heat treatment, H was obtained. The temperature range where the significant change occurs is near the recrystallization temperature point of the alloy. For different holding time, the change of H with T shows a similar relationship curve. In order to clarify this change rule, structural analysis and microstructure observation are carried out.

The figure below is the X-ray diffraction pattern of FeCo alloy after different vacuum heat treatment temperature and holding time of 2h. For comparison, the figure also shows the diffraction pattern before vacuum heat treatment. The results show that the alloys before and after vacuum heat treatment are all -Fe structure.

When the alloy is in the as-rolled state, peaks 110, 200 and 211 appear. After vacuum heat treatment, the 110 peak disappeared, and with the increase of the vacuum heat treatment temperature, the intensities of the 200 and 211 peaks increased, but when the vacuum heat treatment temperature was 850 °C, the peak intensity decreased rapidly. According to the change rule, the vacuum heat treatment temperature was set to 760 °C and 850 °C, and the microstructure of different holding time was observed.

It can be seen that when the temperature is 760 °C and the holding time is 1 h, the 110 peak still exists, and it can be inferred that the as-rolled structure still remains in the alloy at this time. With the extension of vacuum heat treatment time, the 200 and 211 peaks show the same trend as the temperature change, that is, their peaks increase and then decrease. The microstructure photos after vacuum heat treatment at three different temperatures and times are shown respectively. It can be observed that the alloy has undergone recrystallization after annealing at 700°C for 2h, and the grain size is 3~54m. After being kept at 800°C for 2h. The grains have grown to 20~25um, and after extending the holding time to 4h at 850 °C, the grains are 40~50um. Combined with the results of X-ray diffraction analysis, the 200 texture is formed when the alloy begins to recrystallize and continues to strengthen, which may be related to the size and distribution of stress inside the rolled state. The grains become irregular from a certain orientation, resulting in the gradual disappearance of the texture and recrystallization of the alloys combined with the above-mentioned changes in the magnetic properties.

The rapid decrease of H after vacuum heat treatment around the temperature is mainly attributed to two reasons. One is that the stress caused by rolling inside the alloy can be completely eliminated after recrystallization, and the elimination of stress will reduce the internal microstructure caused by stress. The inhomogeneity favors the movement of the magnetic domain walls, thereby reducing the H of the alloy. The second is that the alloy changes from the rolled structure to the crystalline structure, which greatly improves the inhomogeneity of the microstructure. In addition, it can be seen that the grain size continues to increase with the continuous increase of temperature, so the number of grain boundaries hindering the movement of magnetic domains also decreases, which is the cause of H. The main reason for the continued slight decrease in vacuum heat treatment above the recrystallization temperature.

The figure below shows the variation of H with the strength of the additional magnetic field for the alloy at three different vacuum heat treatment temperatures at the recrystallization temperature (around 700 °C).

When the vacuum heat treatment temperature is 650°C, that is, slightly lower than the recrystallization temperature, the Hc of the alloy hardly changes and remains at about 1.56 × 103A/m (19.5Oe), and there is no obvious magnetic field vacuum heat treatment effect. It can be inferred that the magnetic field heat treatment cannot improve the soft magnetic properties of FeC alloys when the microstructure still retains the as-rolled structure. , the effect is slightly improved. When the additional magnetic field strength is about 1.6 xI04 A/m(200Oe), Hc drops to 97.6A/m(1.22Oe) after treatment at 760 C, and drops to 72 A/m(O. 9Oe) after treatment at 850 C. That is, the increase of vacuum heat treatment temperature is beneficial to improve the effect of magnetic field vacuum heat treatment. In order to reveal the influence mechanism of vacuum heat treatment temperature, the change law of magnetostriction coefficient after different vacuum heat treatment conditions was studied.

4 Summary

In this paper, the microstructure and magnetic properties of FeCo alloy (1J22) used in the rotor of the generator are studied. The following research results are obtained: (1) When vacuum heat treatment is performed without a magnetic field, when the temperature rises to the vicinity of the recrystallization temperature, the coercivity of the alloy increases. The force decreased rapidly, and the soft magnetic properties improved significantly. With further increase in temperature, the coercivity continues to show a downward trend due to grain growth. Vacuum heat treatment with and without additional magnetic field. ②When the microstructure is still mainly in the rolled state, the magnetic field vacuum heat treatment cannot improve the soft magnetic properties of FeCo alloy. When the temperature rises above 760°C, it begins to show a more obvious magnetic field heat treatment effect. After 850 C magnetic field heat treatment , the coercivity of the alloy decreased to 72 A/m(0.9Oe), and it was found that the magnetic field heat treatment effect of FeCo alloy mainly came from the magnetostrictive properties of the alloy.