Powder Metallurgy Advances Boost Magnet Strength

Have you ever wondered how those powerful magnets that can pick up nails or drive motors transform from unremarkable raw materials into objects with almost magical properties? Today, we'll explore the manufacturing process of magnets and reveal how powder metallurgy plays a crucial role in their creation.

While there are several methods for manufacturing magnets, the most mainstream and widely used is the powder metallurgy process. In simple terms, this involves crushing selected materials into extremely fine powder, pressing it into shape, and then heating it at high temperatures (known as liquid phase sintering) to achieve densification. Magnets produced this way are commonly called "sintered magnets."

From ferrite magnets (a ceramic material) to rare earth magnets like samarium cobalt (SmCo) and neodymium iron boron (NdFeB), all rely on powder metallurgy techniques. Notably, with the exception of ferrite magnets, all rare earth magnets are metal alloys.

Let's examine the step-by-step manufacturing process for rare earth magnets (samarium cobalt and neodymium iron boron):

The process begins by placing the required raw materials into an induction melting furnace under vacuum or inert gas protection. This prevents oxidation at high temperatures, ensuring alloy purity and performance.

The molten alloy is quickly cooled and solidified through one of several methods:

• Casting into ingots: Pouring directly into molds to form metal ingots
• Melt spinning: Pouring onto a rapidly rotating chilled wheel to produce thin flakes
• Strip casting: Using a strip casting machine for continuous production of thin metal strips

These solidified metal "chunks" are broken down and ground into superfine powder with diameters between 3 to 7 microns. This powder is highly reactive and can spontaneously combust in air, requiring strict oxygen-free handling.

Powder pressing is a critical step in magnet manufacturing, aiming to align the magnetic orientation of all powder particles as uniformly as possible. There are several primary pressing methods:

• Axial/transverse pressing: Powder is poured into a mold cavity and compacted under pressure from a punch. Before pressing, an alignment magnetic field is applied to orient the powder particles in a specific direction.
• Isostatic pressing: Powder is loaded into a flexible container, sealed, subjected to an alignment magnetic field, and then placed in an isostatic press where liquid pressure (typically hydraulic oil or water) is applied uniformly from all directions.

The pressed magnet blanks are loaded into "boats" and placed in vacuum sintering furnaces for high-temperature sintering. Specific sintering temperatures, vacuum levels, or inert gas atmospheres are precisely controlled according to the magnet type and grade.

After sintering, magnets are cooled to room temperature and undergo low-temperature annealing to enhance magnetic stability.

Note that during sintering, magnets experience linear shrinkage of 15-20%. The sintered magnets have rough surfaces, poor dimensional accuracy, and initially possess no magnetic properties.

Sintered magnets require several finishing processes before they're ready for practical applications.

The most common finishing process includes:

• Surface grinding for flatness and parallelism
• External/internal diameter grinding for dimensional accuracy
• Slicing larger magnet blocks into smaller sizes

For unique shapes like arcs or bread loaves, formed diamond grinding wheels can create the final dimensions directly. For small batches of complex shapes, electrical discharge machining (EDM) is typically used.

For large-scale production (typically over 5,000 units), custom molds for pressing prove more economical. For smaller batches or special performance requirements, machining from block materials is preferred.

Machined magnets often have sharp edges prone to chipping. The most common solution is vibratory tumbling in abrasive media to remove sharp edges, typically creating a 0.005 to 0.015 inch (0.127 to 0.38 mm) radius.

NdFeB magnets are particularly susceptible to rust and chemical reactions, requiring protective coatings. Common coatings include epoxy resin, electrolytic nickel, aluminum IVD, or various combinations. Conversion coatings like zinc or iron phosphate may also be used as base layers.

After manufacturing, magnets undergo "charging" to develop their external magnetic fields. This can be done in solenoids or with specially designed fixtures that create unique field patterns. Large assemblies may be magnetized as complete units to avoid handling powerful magnets in their charged state.

Some applications require stability treatment or calibration. Stability treatment involves preconditioning magnets to prevent subsequent flux loss during use. Calibration narrows the performance range across a batch of magnets. These processes require careful control in high-temperature ovens or with reverse-pulse magnetic fields below full breakdown power.