Grain Refinement Processes and Magnetic Performance Enhancements in Cast Alnico Magnets
Abstract
Alnico magnets, as one of the earliest developed permanent magnetic materials, have unique advantages in high-temperature and high-stability magnetic applications. Grain refinement is an important means to improve the magnetic properties of Alnico magnets. This paper provides an in-depth analysis of the grain refinement processes of cast Alnico magnets, including chemical treatment, mechanical vibration and stirring, and external physical field treatment. It also explores the impact of grain refinement on key magnetic performance indicators such as coercivity, remanence, and maximum magnetic energy product, and looks forward to future research directions in this field.
Keywords
Cast Alnico magnets; Grain refinement; Magnetic performance; Chemical treatment; External physical field treatment
1. Introduction
Alnico magnets, developed by Japanese metallurgist Mishima Tokushichi in 1932, were once the dominant force in the permanent magnet industry before the emergence of rare-earth permanent magnet materials. Alnico magnets are known for their high Curie temperature of up to 890 °C, which endows them with excellent high-temperature resistance and stability. They also have good corrosion resistance, ensuring long-term reliability in harsh environments. Although the market share of Alnico magnets has been compressed by low-cost sintered ferrites and high-performance rare-earth permanent magnets, they still have unique advantages in high-temperature applications above 500 °C and are widely used in scenarios that require high stability and durability, such as loudspeakers, watt-hour meters, electric motors, generators, and alternators.
The magnetic properties of Alnico magnets are closely related to their microstructure, and grain refinement is an effective way to improve their magnetic properties. By reducing the grain size, the number of grain boundaries increases, which can hinder the movement of magnetic domain walls, thereby improving coercivity. At the same time, a more uniform microstructure can also enhance the remanence and maximum magnetic energy product of the magnet. Therefore, studying the grain refinement processes of cast Alnico magnets is of great significance for improving their magnetic performance and expanding their application range.
2. Grain Refinement Processes of Cast Alnico Magnets
2.1 Chemical Treatment
Chemical treatment is a common method for grain refinement in metal materials, and it is also widely used in the production of cast Alnico magnets. This method involves adding a small amount of chemical substances, known as inoculants or modifiers, to the metal melt. These substances can promote heterogeneous nucleation in the melt, increase the number of nuclei, and thus refine the grains.
For Alnico magnets, the selection of inoculants is crucial. According to the lattice mismatch degree theory and empirical electron theory, different inoculants have different effects on the heterogeneous nucleation of δ-Fe and γ-Fe. For example, CaS, La₂O₃, TiN, Ce₂O₃, TiC, CeO₂, Ti₂O₃, TiO₂, and MgO have significant effects on the heterogeneous nucleation of δ-Fe, while ZrO₂, Ti₂O₃, MnS, SiO₂, CaO, Al₂O₃, and CeO₂ are more effective for γ-Fe.
When adding inoculants, it is necessary to ensure that they are fine and well-dispersed in the melt. Otherwise, if the inoculants aggregate, they may not only fail to refine the grains but also affect the performance of the Alnico magnets. In addition, the amount of inoculant added also needs to be precisely controlled. Generally, an appropriate amount of inoculant can achieve a good grain refinement effect, but excessive addition may lead to an increase in non-metallic inclusions in the melt, which is detrimental to the magnetic properties of the magnets.
2.2 Mechanical Vibration and Stirring
Mechanical vibration and stirring are physical methods that can achieve grain refinement by causing relative motion between the liquid and solid phases in the metal melt, promoting the breakage and proliferation of dendrite arms.
2.2.1 Mechanical Stirring
Mechanical stirring can create different degrees of relative motion between the liquid and solid phases in the metal melt, that is, the convective motion of the liquid metal. This convective motion can cause the breakage of dendrite arms, and the broken dendrite fragments can act as new nuclei for crystal growth, thereby increasing the number of nuclei and refining the grains.
However, mechanical stirring also has some drawbacks. On the one hand, when stirring the melt, it is easy to introduce gas, and if the gas cannot be timely supplemented by the molten metal, defects such as pores and shrinkage porosity may form. On the other hand, when stirring high-melting-point metal melts, the stirrer is prone to wear and tear, which may contaminate the metal melt and cause new quality problems.
2.2.2 Mechanical Vibration
Mechanical vibration also relies on the convective motion of the metal melt to break the dendrites and cause nucleation proliferation to achieve grain refinement. However, in practical operation, when the mechanical vibration frequency increases, the grain refinement effect of the metal凝固 (solidification) system may decrease, and problems such as carbide segregation and looseness in the steel ingot may become more serious.
2.3 External Physical Field Treatment
External physical field treatment is a promising grain refinement technology, which has the advantages of being environmentally friendly and easy to operate. It mainly includes current treatment, magnetic field treatment, and ultrasonic treatment.
2.3.1 Current Treatment
When a rapidly changing strong pulsed current passes through the metal melt, a rapidly changing strong pulsed magnetic field will be generated in the melt. The interaction between the strong pulsed current and the strong pulsed magnetic field will produce a strong contraction force in the metal melt, causing the melt to be repeatedly compressed and move back and forth in the direction perpendicular to the current. This back-and-forth motion can not only break the dendritic crystals but also make the melt quickly lose its superheat and increase the nucleation rate. Therefore, the stronger the pulsed current, the more significant the grain refinement effect.
2.3.2 Magnetic Field Treatment
When the metal melt solidifies in a alternating magnetic field, an induced current will be generated in the solidification system. The interaction between the magnetic field and the induced current will produce an electromagnetic force, which will press the metal toward or pull it away from the axis along the radial direction, causing regular fluctuations in the solidification system. This fluctuation has a similar effect to the enhanced convection usually used, so the alternating magnetic field has a grain refinement effect.
From the perspective of the fluctuation effect brought by the magnetic field, the stronger the magnetic induction intensity, the greater the electromagnetic pressure, and thus the more intense the fluctuation, and the better the grain refinement effect. However, when the magnetic induction intensity increases, the induced current also increases proportionally, which will correspondingly increase the thermal effect in the solidification system, reduce the supercooling degree, and thus decrease the nucleation rate. Therefore, the relationship curve between the magnetic field intensity and the grain refinement effect is a curve with an extreme value.
In addition, pulsed magnetic fields can also generate pulsed eddy currents in the melt. The interaction between the eddy currents and the magnetic field produces Lorentz forces and magnetic pressures, which are intensely changing and much stronger than the dynamic pressure of the metal melt. This causes intense vibration of the metal melt, increasing the supercooling degree during solidification, improving the nucleation rate, and causing forced convection in the melt, preventing the dendrites from growing or breaking and crushing them. The broken dendrite particles float in the liquid at the crystallization front and become new growth cores. Therefore, the stronger the pulsed magnetic induction intensity, the more significant the grain refinement effect.
2.3.3 Ultrasonic Treatment
Ultrasonic treatment utilizes the acoustic cavitation and acoustic streaming effects generated when ultrasonic waves propagate in a liquid to achieve grain refinement. When ultrasonic waves act on the metal melt, the liquid molecules are subjected to the action of a periodic alternating sound field, generating acoustic cavitation and acoustic streaming effects. These effects can cause changes in the flow field, pressure field, and temperature field in the melt, generating local high-temperature and high-pressure effects. The vibration of the liquid causes the dendrite arms to detach from the solidification front and act as heterogeneous nucleation cores in the melt, and the dispersing effect of ultrasonic waves on the melt makes the particles distribute more uniformly. In addition, ultrasonic metallurgy can also remove gas and slag, which is a melt purification technology.
3. Impact of Grain Refinement on Magnetic Performance Indicators of Cast Alnico Magnets
3.1 Coercivity
Coercivity is an important indicator to measure the ability of a permanent magnet to resist demagnetization. Grain refinement can significantly improve the coercivity of Alnico magnets. In a coarse-grained structure, magnetic domain walls can easily move across grain boundaries, making the magnet more susceptible to demagnetization. After grain refinement, the number of grain boundaries increases, and the grain boundaries can act as pinning centers for magnetic domain walls, hindering their movement. Therefore, a greater external magnetic field is required to move the magnetic domain walls, that is, the coercivity of the magnet increases.
For example, in Alnico 5 magnets, through appropriate grain refinement processes, the coercivity can be increased from the original value to a higher level, enhancing the magnet's ability to maintain its magnetic properties in the presence of reverse magnetic fields or external disturbances.
3.2 Remanence
Remanence refers to the magnetic induction intensity remaining in the magnet after the external magnetic field is removed to zero. Grain refinement can also have a positive impact on the remanence of Alnico magnets. A more uniform microstructure obtained through grain refinement can reduce the scattering of magnetic moments and make the magnetic moments more aligned in the same direction, thereby increasing the remanence of the magnet.
In addition, grain refinement can also reduce the number of defects such as pores and inclusions in the magnet. These defects can disrupt the alignment of magnetic domains and reduce the remanence. By eliminating or reducing these defects, the remanence of the magnet can be further improved.
3.3 Maximum Magnetic Energy Product
The maximum magnetic energy product is a comprehensive indicator that reflects the energy storage capacity of a permanent magnet. It is proportional to the product of the remanence and the square of the coercivity. Since grain refinement can improve both the coercivity and the remanence of Alnico magnets, it will inevitably lead to an increase in the maximum magnetic energy product.
A higher maximum magnetic energy product means that the magnet can store and output more magnetic energy under the same volume, which is very important for applications that require high magnetic energy output, such as electric motors and generators. For example, in the design of high-efficiency electric motors, using Alnico magnets with a higher maximum magnetic energy product can reduce the size and weight of the motor while improving its performance.
4. Case Analysis
4.1 Case 1: Grain Refinement of Alnico 5 Magnets by Chemical Treatment
In a certain Alnico magnet production enterprise, in order to improve the magnetic properties of Alnico 5 magnets, chemical treatment was adopted for grain refinement. The selected inoculant was a compound containing Ti and B elements. During the production process, an appropriate amount of the inoculant was added to the Alnico melt according to the melt weight.
After solidification and subsequent heat treatment, the microstructure of the Alnico 5 magnets was observed using a metallographic microscope. It was found that the grain size of the magnets treated with the inoculant was significantly smaller than that of the untreated magnets. The average grain size decreased from about 100 μm to about 30 μm.
Magnetic property tests showed that the coercivity of the grain-refined Alnico 5 magnets increased from 52 kA/m to 60 kA/m, the remanence increased from 1.2 T to 1.25 T, and the maximum magnetic energy product increased from 40 kJ/m³ to 48 kJ/m³. This indicates that chemical treatment with an appropriate inoculant can effectively refine the grains of Alnico 5 magnets and significantly improve their magnetic properties.
4.2 Case 2: Grain Refinement of Alnico 8 Magnets by Ultrasonic Treatment
In another research project, ultrasonic treatment was applied to the grain refinement of Alnico 8 magnets. During the solidification process of the Alnico 8 melt, an ultrasonic probe was inserted into the melt, and ultrasonic waves with a certain power and frequency were applied for a certain period of time.
Metallographic analysis showed that the grains of the Alnico 8 magnets treated with ultrasound were much finer than those of the non-treated magnets. The ultrasonic treatment broke the dendrites in the melt, increased the number of nuclei, and achieved grain refinement.
Magnetic property measurements revealed that the coercivity of the ultrasonic-treated Alnico 8 magnets increased from 140 kA/m to 160 kA/m, the remanence increased from 1.0 T to 1.05 T, and the maximum magnetic energy product increased from 60 kJ/m³ to 70 kJ/m³. This demonstrates that ultrasonic treatment is an effective grain refinement method for Alnico 8 magnets and can significantly enhance their magnetic performance.
5. Conclusion and Outlook
Grain refinement is an important means to improve the magnetic properties of cast Alnico magnets. Chemical treatment, mechanical vibration and stirring, and external physical field treatment are all effective grain refinement processes. Among them, chemical treatment is simple to operate and has a significant refinement effect, but the selection and addition amount of inoculants need to be strictly controlled. Mechanical vibration and stirring can achieve grain refinement through physical means, but they may introduce some defects. External physical field treatment, such as current treatment, magnetic field treatment, and ultrasonic treatment, has the advantages of being environmentally friendly and easy to operate, and has great potential for development.
Grain refinement can improve the coercivity, remanence, and maximum magnetic energy product of Alnico magnets, making them more suitable for high-performance magnetic applications. In future research, the following aspects can be further explored:
- Optimize the grain refinement processes, such as studying the optimal parameters of chemical treatment, mechanical vibration and stirring, and external physical field treatment to achieve the best grain refinement effect.
- Develop new and more effective inoculants or grain refinement agents to improve the refinement efficiency and reduce costs.
- Combine different grain refinement methods to give full play to their respective advantages and further improve the magnetic properties of Alnico magnets.
- Study the relationship between grain refinement and other factors such as heat treatment, alloy composition, and processing technology to comprehensively improve the performance of Alnico magnets.
In conclusion, through continuous research and innovation in grain refinement processes, the magnetic properties of cast Alnico magnets can be continuously improved, expanding their application range and promoting the development of the permanent magnet industry.
