The sintering process plays a crucial role in the production of NdFeB magnets, and its parameters exert a significant influence on the magnets' properties.
The sintering temperature has a direct impact on the degree of densification of NdFeB magnets. When the sintering temperature is relatively low, the diffusion rate of atoms between powder particles is slow. As a result, the magnet fails to achieve full densification, leading to the presence of a greater number of pores within it. These pores interfere with the arrangement of the magnetic domains, thereby reducing the content of the effective magnetic phase and ultimately degrading the magnetic properties of the magnet. For instance, if the sintering temperature is 50 to 100 °C lower than the optimal temperature, the remanence (Br) of the magnet may decrease by 10 to 20%.
On the other hand, an excessively high sintering temperature can cause abnormal grain growth. Larger grains make it easier for domain walls to move, which in turn causes the coercivity (Hcj) to decline. Simultaneously, a very high temperature may also trigger the decomposition or oxidation of the alloy phase, disrupting the chemical structure of the magnet and consequently affecting both its magnetic and mechanical properties.
Sintering time and temperature are interdependent factors. Appropriate sintering time is essential for ensuring the uniformity of the internal structure of the magnet. If the sintering time is too short, the powder particles cannot fully diffuse and fuse together, giving rise to the phenomenon of composition segregation. This results in variations in magnetic properties across different parts of the magnet. For example, in cases where the sintering time is insufficient, the magnetic properties of the edge part of the magnet may be 20% - 30% lower than those of the central part.
However, if the sintering time is overly long, aside from increasing production costs and energy consumption, it will also lead to grain growth and a subsequent reduction in coercivity. Moreover, prolonged exposure to a high-temperature environment during extended sintering will heighten the probability of chemical reactions between the magnet and its surrounding environment, such as oxidation reactions. These reactions will then deteriorate the surface performance of the magnet.
Applying a certain amount of pressure during the sintering process can promote the close packing of powder particles. The appropriate pressure can reduce porosity and increase the density of the magnet, which is beneficial for enhancing the magnetic properties. If the applied pressure is insufficient and the magnet's density remains low, its mechanical strength and magnetic properties will be negatively affected. For example, a lower pressure may lead to a 30% - 50% reduction in the compressive strength of the magnet, along with a decrease in magnetic performance parameters like permeability.
Nevertheless, if the pressure is too high, it may cause excessive deformation of powder particles or even local fractures. Such occurrences will impact the microstructure of the magnet and subsequently have an adverse effect on its magnetic properties.
In conclusion, during the production process of NdFeB magnets, it is imperative to precisely control the sintering process parameters to ensure the production of magnets with outstanding performance. Only by carefully optimizing and regulating these parameters can the desired magnetic, mechanical, and other related properties of the NdFeB magnets be achieved.
