Introduction
As we continue our detailed exploration of Neodymium Magnet production, this fourth part of the series shifts our focus to the vital processes of sintering, heat treatment, and machining. These stages are instrumental in refining and finalizing the magnets’ properties and form. Sintering and heat treatment are key to developing the magnets’ intrinsic magnetic qualities, while precise machining ensures their utility in diverse applications. This segment of our series will provide an in-depth look at how these processes contribute to the overall excellence of Neodymium Magnets.
Sintering Process
Sintering in Neodymium Magnet production is a finely calibrated process essential for achieving the magnet’s high coercivity and desired grain structure. Operating typically between 1050-1080°C, this process aims to form magnets with near-zero porosity and grain sizes within 5-15μm. The control over temperature is crucial to prevent grain growth, which could otherwise reduce the magnet’s coercivity. This stage not only aligns the magnetic particles but also sets the stage for the two-tier heat treatment crucial for optimizing magnetic properties. The first tier around 900°C focuses on the Nd-rich phase, enhancing the magnet’s surface at the atomic level without excessive grain growth. The second tier, around 500°C, is critical for adjusting the magnet’s phase composition and microstructure, directly impacting its intrinsic coercivity, demagnetization curve squareness, and temperature-induced losses. The integration of these carefully controlled processes ensures the production of magnets with exceptional magnetic strength and stability.
Heat Treatment Techniques
Heat treatment is a critical stage in Neodymium Magnet production, significantly impacting their magnetic characteristics and stability. This process involves various methods, each serving a specific purpose.
Different Methods: Common methods include annealing, tempering, and quenching. Annealing helps in relieving internal stresses and improving magnetic properties, while tempering and quenching are employed to enhance the magnet’s mechanical strength and thermal stability.
Impact on Magnetic Properties: The heat treatment adjusts the microstructure of the magnets, refining their coercivity (resistance to demagnetization) and magnetic remanence (the magnet’s ability to retain magnetism). This fine-tuning is vital for the magnets to perform reliably under different environmental conditions.
Structural Enhancement: Alongside magnetic improvements, heat treatment also contributes to the magnet’s structural integrity. The process strengthens the magnet against physical stresses and thermal fluctuations, ensuring longevity and consistent performance.
Machining
Due to the characteristics and technical limitations of Orientation and Forming process, it is challenging to achieve the desired shape and dimensional precision for sintered magnets in a single step. Therefore, the mechanical processing of sintered blanks becomes necessary. The primary reasons for this are as follows:
- Small Size and Complex Shapes: Many finished magnets are small and intricate in design, necessitating the fabrication of specific-shaped blank magnets.
- Uneven Mold Filling: Even for nearly final-formed blank magnets, the low density and poor flowability of powder compacts lead to uneven filling of the mold, resulting in fluctuations in the shape and dimensions of the sintered magnets.
- Sintering Shrinkage Differences: Significant differences in sintering shrinkage between Nd-Fe-B blank magnets along the parallel and perpendicular orientations, as well as variations in shrinkage between the boundary and central regions of the blank magnets, pose challenges in achieving the required dimensional accuracy for finished magnets.
Rare-earth permanent magnets prepared by powder metallurgy methods are typical examples of metallic ceramic products. Being hard and brittle materials, the machining options are limited to cutting, drilling, grinding, and honing. Based on the fundamental characteristics of the machining surfaces, the following distinctions can be made:
- Blade cutting typically employs diamond or cubic boron nitride (CBN) powder-electroplated blades. Different blade thicknesses and blade edge positions are chosen based on the required depth of the cut and positional tolerances. In the case of inner-circle cutting blades, their support by the blades and outer circle rings ensures good flatness during cutting, allowing for a blade thickness as low as 0.1 mm. However, the depth of the cut and the dimensions of the magnet being cut are limited by the inner and outer diameter differences of the blades. For outer-circle cutting blades, the blade edges protrude outward, and their ability to support the blade is inferior to inner-circle cutting blades. Therefore, to maintain the same tolerance level, slightly thicker blades, generally in the range of 0.2 to 0.5 mm, are required, leading to higher material loss. For large-batch, single-sized products, the efficiency of using circular saws for slicing is very high.
- Electrical discharge machining (EDM) and laser cutting fall under direct thermal processing methods and can be employed for cutting complex shapes. However, these methods are relatively less efficient, costlier, and research has revealed that the processing surface of sintered Nd-Fe-B magnets forms a thickness of approximately 15 μm of Nd-rich layer due to the temperature rise during machining, reducing the material’s chemical stability.
- Drilling of magnets relies on diamond drills or lasers. To increase material utilization, the technology of hollow drilling and hole enlargement has been developed. In larger inner diameter products, the solid cylindrical cores extracted from the center can also be used to make other smaller-sized products. The use of ultrasonic-assisted drilling methods can mitigate brittle damage, which is more favorable for the machining of high-performance or high-thermal-stability magnets with high brittleness.
- Grinding wheels are classified into metal-based and resin-based types. Profile grinding involves creating a grinding wheel base according to the profile of the grinding surface, followed by plating with diamond or BN powder and shaping to meet the final product requirements.
Mechanical processing can introduce defects on the surface of magnets, significantly affecting their performance and corrosion resistance. This is especially critical for small and thin products, necessitating methods for the removal or repair of surface defect layers.