In fact, sintered rare earth magnets – NdFeB and SmCo production process belongs to powder metallurgy. In this process, a suitable composition of raw materials is pulverized into fine powder, pressed and heated to cause densification via liquid phase sintering, which is the reason that they are often called sintered magnets. Following is the detailed production process:
Compostion: The raw materials including rare earth metals are measured to produce the suitable composition.
Melting (Strip Casting): The materials are melted under vacuum or inert gas in an induction melting furnace. The molten alloy is either poured into a mold, onto a chill plate, or processed in a strip cast furnace that can form a thin, continuous metal strip.
Milling (Jet Milling): These metal alloys or strips are crushed and pulverized to form a fine powder whose particle size is specified to contain material with one magnetic preferred orientation.
Pressing: The powder is placed in a jig and a magnetic field is applied while the power is pressed into shape. In this mechanical pressing, the magnetic anisotropy is achieved. Then the isostatic pressing process is used to press powder equally on all sides. Through this isostatic pressing, very large magnet blocks can be made and the highest possible energy product can be achieved.
Sintering & Aging: The pressed parts are heated to a sintering temperature and allowed to densify in a vacuum sintering furnace. The particular temperatures and presence of vacuum or inert gas is specific to the type and grade of magnet being produced. Aging the magnets after sintering adjusts the properties of magnets. After sintering & aging, the magnets have a rough surface and only approximate dimensions.
Demagnetization Curve Test: Basic magnetic properties are set after the sintering & aging process is completed. Key data including Br, Hcb, Hcj, (BH)max, HK, should be tested and recorded. Only those magnets that pass the test can go to subsequent processes including machining.
Machining: Sintered magnets should receive some machinings such as grinding, slicing, drilling, EDM, chamfering, etc. What kinds of machining are needed depends on many factors including the magnets’ material, grade, dimension, shape, quantity, cost and delivery time. The machining method for the same magnet may be different, when the important factor considered is different. For example, normally the big cylindrical magnet with axial orientation can be pressed to shape and then grinded and sliced, which can save cost but takes long time if there is no cylinder stock. Sometimes, if delivery time is the most important factor, the same cylindrical magnet can be core-drilled or EDM cut from a block stock, which can reduce the delivery time but increases the cost.
Coating: NdFeB is prone to rusting or reacting chemically, so it is almost always coated. SmCo is naturally more corrosion resistant than NdFeB, but does, on occasion benefit from coating.
Magnetization: After the manufacturing is completed, the magnets require magnetization to produce an external magnetic field. This can be accomplished in a solenoid or with fixtures designed to impart unique magnetic patterns.
Final Inspection & Shippment: Prior to shippment, the following final inspections should be conducted: appearance, dimension, magnetic flux density or magnetic flux, coating thickness, PCT, HAST, SST, ect.
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