RU2127923C1 - Permanent magnet manufacturing process - Google Patents

Abstract

FIELD: powder metallurgy; manufacturing alloy base magnets. SUBSTANCE: Nd-Fe-B alloy base permanent magnet is manufactured by molding, sintering, heat treatment in vacuum or in inert medium forming magnetic structure of alloy, grinding, coating with corrosion-preventing oxide layer concurrently with heat treatment to form magnetic structure; procedures are made in inorganic melt containing ion-passivators Cr₂O $\genfrac{}{}{0ex}{}{\text{2-}}{\text{7}}$ ,NO $\genfrac{}{}{0ex}{}{\text{-}}{\text{3}}$ ,NO $\genfrac{}{}{0ex}{}{\text{-}}{\text{2}}$ at 540-600 C followed by fast cooling in water. EFFECT: simplified procedure, improved corrosion resistance of magnets and, hence, their magnetic properties. 1 tbl

Description

 The invention relates to powder metallurgy, in particular to the manufacture of magnets based on an alloy of Nd-Fe-B, and can be applied in the electrical, electronic industry and instrumentation.

 A known method of manufacturing permanent magnets based on an Nd-Fe-B alloy, including pressing, sintering, heat treatment in a vacuum or inert medium, forming the magnetic structure of the alloy, grinding and applying an anticorrosive oxide coating.

 To increase the corrosion resistance of magnets, electrical and chemical coatings are recommended - cadmium plating, chromium plating, nickel plating, phosphoric acid passivation.

 The disadvantage of these methods is the increased complexity, insufficient mechanical strength of phosphate coatings, as well as the penetration of electrolyte into the pores, which subsequently leads to latent corrosion of the magnets.

The closest in technical essence and the achieved result to the invention is a method of manufacturing magnets based on Nd-Fe-B alloy, namely, that magnetic compositions of the formula R ₂ T ₁₄ B (R is the main rare-earth element) are pressed, sintered, subjected to heat treatment in a vacuum or inert medium forming a magnetic structure, and grind. The surface of the magnets is coated with a metal alcoholate and, upon subsequent thermal decomposition, it is provided with an anticorrosive oxide coating.

 The disadvantage of this method is that two additional operations are introduced: the application of the alcoholate and its thermal decomposition. In addition, such coatings, as a rule, have weak adhesion to the base, do not penetrate into the pores, do not passivate them - the activity of the alloy under the coating remains.

 The invention is aimed at simplifying the manufacturing technology of magnets, increasing corrosion resistance while increasing magnetic properties.

The essence of this invention lies in the fact that in the method of manufacturing permanent magnets based on the Nd-Fe-B alloy, comprising pressing, sintering, heat treatment in a vacuum or inert medium, forming the magnetic structure of the alloy, grinding and applying an anti-corrosion coating, heat treatment is combined with the operation of applying an anticorrosive oxide coating and is carried out in an inorganic melt containing ions - passivators Cr ₂ O $\genfrac{}{}{0ex}{}{\text{2-}}{\text{7}}$ NO $\genfrac{}{}{0ex}{}{\text{-}}{\text{3}}$ NO $\genfrac{}{}{0ex}{}{\text{-}}{\text{2}}$ at a temperature of 540 - 600 ^o C followed by rapid cooling of the magnets in water.

 In an anhydrous molten salt or hydroxide in the presence of passivators, the surface of the magnets goes into a passive state, as a result of which the corrosion process is suppressed. In open pores and on the surface of the magnet, a dense and sufficiently strong oxide coating is formed with a thickness of 5-10 microns, which protects the surface from corrosion damage. At the same time, the necessary crystalline structure is formed, which is fixed upon rapid cooling. Due to the low temperature coefficient of expansion of the Nd-Fe-B alloy, cracking upon rapid cooling is not observed. The presence of a dense oxide film on the surface prevents the interaction of the active components of the alloy with water.

Example. After sintering and finishing mechanical operations, Nd-Fe-B magnets were loaded into a container with a potassium dichromate (K ₂ Cr ₂ O ₇ ) melt at a temperature of 560 ^o C. After being held in the melt for 45 minutes, the magnets were removed from the melt and immersed in cold water. After that, the magnets were washed in warm water, controlled by magnetic properties.

Comparative corrosion tests were performed according to the following procedure. The magnets were kept in a thermal moisture chamber at 90 ± 5 ^o C and a relative humidity of 98 - 100% for 7 hours. Then the magnets were cooled together with the chamber, with condensation of moisture to room temperature, and kept under these conditions until the end of the day. Subsequently, the cycle was repeated. Corrosion resistance was assessed by the time before the appearance of the first point corrosion centers visible on the surface of the magnets with the naked eye. Magnetic properties - residual induction, coercive force and coercive force by magnetization were checked according to existing production methods. The test results are summarized in table.

 The implementation of the method allows you to combine the operation of heat treatment of magnets with the operation of applying an oxide corrosion-resistant coating, to increase the corrosion resistance of magnets and their magnetic properties.

Claims (1)

A method of manufacturing permanent magnets based on an Nd-Fe-B alloy, including pressing, sintering, heat treatment in vacuum or an inert medium, forming the magnetic structure of the alloy, grinding, applying an anticorrosive oxide coating, characterized in that the heat treatment is combined with the operation of applying an anticorrosive oxide coatings and carried out in an inorganic melt containing ion-passivators Cr ₂ O $\genfrac{}{}{0ex}{}{\text{2-}}{\text{7}}$ NO $\genfrac{}{}{0ex}{}{\text{-}}{\text{3}}$ NO $\genfrac{}{}{0ex}{}{\text{-}}{\text{2}}$ , at 540-600 ^o C followed by rapid cooling of the magnets in water.

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