Heat treatment of sintered alnico magnets: the key path to optimize internal structure and improve magnetic performance

Sintered alnico magnets are mainly composed of iron, aluminum, nickel, cobalt and other elements, and are prepared through powder metallurgy technology. This kind of magnet has high saturation magnetization, good temperature stability and corrosion resistance, as well as high remanence and coercivity, allowing it to maintain good magnetic properties in a variety of extreme environments. However, the internal microstructure of freshly sintered magnets often contains defects, such as pores, inclusions and non-uniformly distributed grains. These defects will directly affect the magnetic and mechanical properties of the magnet.

Heat treatment, as an important technology in the preparation process of magnetic materials, can significantly optimize the microstructure of magnets by accurately controlling the heating, holding and cooling processes, thereby reducing internal defects, improving grain orientation, and thus improving magnetic properties.
Reduce internal defects:
During the sintering process, defects such as pores and inclusions may form inside the magnet due to the metallurgical bonding between powder particles. These defects not only reduce the density of the magnet, but also affect the arrangement of the magnetic domains, resulting in a decrease in magnetic performance. Heat treatment can effectively reduce these defects and improve the density and uniformity of magnets through substance diffusion and rearrangement at high temperatures.
Improve grain orientation:
The orientation of the grains has an important influence on the magnetic properties of the magnet. Ideal grain orientation enables more magnetic domains to be aligned in the same direction, thereby increasing the magnetic energy product and coercive force of the magnet. By adjusting the temperature and time, heat treatment can promote the preferential growth of crystal grains and make the orientation of the crystal grains more consistent, thereby improving the overall magnetic properties of the magnet.
Optimize grain boundary structure:
Grain boundaries are the transition areas between different grains in a magnet. Their structure and properties have an important impact on the magnetic and mechanical properties of the magnet. Heat treatment can change the composition and structure of the grain boundaries, reduce defects and stress at the grain boundaries, thereby improving the magnetic properties and stability of the magnet.

To optimize the performance of sintered Alnico magnets through heat treatment, the following key factors need to be precisely controlled:
Heating temperature:
The choice of heating temperature is crucial. Too high a temperature may cause changes in the internal structure of the magnet, such as abnormal growth of grains, thereby reducing the magnetic performance; while too low a temperature may not be able to fully eliminate internal defects and optimize grain orientation. Therefore, the appropriate heating temperature needs to be selected based on the specific composition and expected performance of the magnet.
Keeping time:
The length of heat preservation time directly affects the effect of heat treatment. If the holding time is too short, the diffusion and rearrangement of substances may not be fully realized; if the holding time is too long, it may lead to excessive growth of grains and degradation of magnetic properties. Therefore, the holding time needs to be reasonably determined based on the heating temperature and the specific conditions of the magnet.
Cooling rate:
The cooling rate has a significant impact on the final performance of the magnet. Rapid cooling can fix the organizational structure at high temperatures and obtain higher hardness and strength; while slow cooling helps reduce internal stress and improve toughness. For sintered Alnico magnets, an appropriate cooling rate is usually used to balance the needs of magnetic and mechanical properties.

After a carefully designed heat treatment process, the magnetic properties of sintered alnico magnets will be significantly improved:
Enhanced magnetic energy product: Magnetic energy product is an important indicator of a magnet's ability to store magnetic energy. Heat treatment improves the orientation of crystal grains and the arrangement efficiency of magnetic domains by optimizing the microstructure of the magnet, thereby significantly enhancing the magnetic energy product of the magnet. This makes sintered Alnico magnets excellent in applications requiring high energy density, such as permanent magnet motors for electric vehicles, rotors for wind turbines, etc.
Improved coercivity: Coercivity is an important indicator of a magnet's ability to resist interference from external magnetic fields. Heat treatment improves the magnet's resistance to magnetic degradation by reducing internal defects and optimizing the grain boundary structure, thereby significantly increasing the coercive force. This gives sintered Alnico magnets significant advantages in applications that require high stability and immunity to interference, such as high-precision sensors, magnetic recording media, etc.