NdFeB surface treatment

- Nov 22, 2017 -

1, definition:

Sintered NdFeB permanent magnet material is a chemically active powder material, its characteristics hard and brittle, easily oxidized. Currently the most effective way is to take the surface of the protective layer, but the NdFeB surface due to the existence of the grinding process of deterioration layer and density of incomplete holes, oxidation equal. The surface treatment must take the necessary pre-treatment and the appropriate plating process.

2, the way

NdFeB surface rust treatment with direct de-oiling passivation film completed one;

NdFeB surface prone to corrosion, such as: NdFeB permanent magnet material Curie temperature is low, poor temperature characteristics, chemical activity, hard and brittle, easily be chalking, oxidation corrosion; NdFeB sintered permanent magnet surface in Grinding processing of the deterioration of the formation and density of incomplete holes, oxidation equal;

NdFeB surface to take appropriate and suitable surface treatment, NdFeB is a multi-phase structure, the potential difference between the different phases, resulting in the potential of the battery itself become a battery, the acidic environment is to accelerate the corrosion; NdFeB surface passivation, blunt Chemical technology is simple, good control, the surface of the formation of dense passive film can enhance the anti-corrosion ability;

The traditional surface anti-rust treatment steps are generally: degreasing degreasing → washing → pickling activation → washing → phosphating, electroplating, etc. → washing → dry drying, complex process;

Refer to MJ3 series of degreasing passivation solution processing steps: degreasing degreasing passivation simultaneously → washing → drying dry; its surface degreasing passivation does not affect its performance, the surface of non-corrosive, anti-corrosion Antioxidant effect is good.


3, HD hydrogenation method and advantages and disadvantages

NdFeB hydrogen hydrogenation grinding method is only applicable to the hydrogenation of metal or alloy coarse crushing and grinding, feeding size 100-0.1MM, particle size: 10-1000UM, for hydrogen storage alloy Ni-HM battery anode material Need powder, this size has to meet the practical requirements. NdFeB permanent magnet powder size should be 3-5UM, to be jet mill grinding treatment. Hydrogenation crushing (HD) method is the lattice expansion of hydrogen absorption and dehydration reduction refinement of particle size, and hydrogen or dehydrogenation is a reversible chemical reaction process, physical and chemical reactions exist its chemical composition and magnetic changes.

NdFeB hydrogen absorption, the formation of hydride lattice expansion, and the formation of heat, chemical processes; expansion of internal stress to NdFeB crystal cracks into loose physical phenomenon, both at the same time; heating dehydrogenation, most of the main Phase hydride changed back to the original Nd2Fe14B powder, part of the remaining Nd-rich hydride to be deep treatment. NdFeB hydrogen absorption process, the first hydrogen is exposed on the surface of Nd-rich phase, followed by the main phase Nd2Fe14B and H2 reacts with the formation of the main phase of the hydride with the exothermic reaction, the total heat can make the reactant temperature Raise to 300 degrees. As the lattice constant becomes larger and the thermal expansion process produces the powder burst, the HD powder of NdFeB has undergone the qualitative change.

Dehydrogenation turns Nd2Fe14Bhy into Nd2Fe14B, which decomposes the hydride. Temperature and pressure effects, 650. C, the Nd-rich phase becomes soft and melts, and the HDDE reaction continues to heat up and the optimum dehydrogenation temperature is now 500. C, in this condition, the hydrogen of the main phase hydride all released, neodymium-rich phase NdH3 in 500. After C, part of the hydrogen off into NdH2, 1040. C can be completely discharged from the main phase Nd2Fe14B hydrogen.


Advantages and disadvantages:

1, the pulverizing efficiency of HD + JM (jet mill) increased by 2-3 times to reach 90-100KG / HR (originally only 30KG / HR); in terms of mechanical properties close to the main phase composition of NDFEB (RE = 11.76at%) , Hard and tough (Hv = 530, bending strength 24-26KG / mm2);

2, effectively reducing the oxygen content of the magnetic powder, HD hydrogenation grinding method effectively reduces the degree of oxidation of the milling section ((200-800) × 10-O2), there is hydrogen in the sintering process, you can restore the neodymium oxide, Purification of the grain boundary to promote densification, to achieve part of the activation sintering.

3, HD powder along the grain boundary phase cracking: HD + JM powder mostly single crystal particles, close to the best particle size 2-3UM, the grain edge are neodymium-rich phase, effectively improve the IHC, in fact, can increase 500- 5000Oe.

HD magnetic powder performance slightly variation: HD + JM powder as part of the powder in the form of oxide (Nd2Fe14BHxNdHy), the magnetic variation: 4πJs high, Br and IHC low phenomenon, weak magnetic, easy stripping, the appearance of neat, Easy to stack

HD powder has a good oxidation resistance: HD + JM powder has good oxidation resistance, save a long time, the practice shows that the powder is not easy to burn in the air ≥ 4UM;

6, HD oxidation crushing the negative effects and effects:

(1) HD hydrogenation process safety issues, in the chemical industry, metallurgical production in some occasions to use hydrogen, hydrogen can spontaneous combustion, can explode,

Therefore, the relevant process equipment must be considered to eliminate the conditions of the explosion, such as hydrogen and oxygen mixed up to a certain concentration and open flame explosion. Another need to prevent hydrogen embrittlement, hydrogen storage windows and pipe materials must be anti-hydrogen material (with hydrogen hair chemical reaction material) composition, such as stainless steel 1Cr18NI9Ti is a good anti-hydrogen material.

(2) Too much fine powder, HD + JM powder is easy to grind, there is a need to improve the jet mill to produce all the useful powder.

(3) The surface of HD + HM powder has a large number of flute angles, and the friction coefficient increases during the orientation and bloom of the magnetic field. The decrease of the orientation leads to the decrease of Br and the decrease of the residual magnetic induction Br. Compared with the NDFEB alloy powder + HM powder reduces oxidation, Nd (R) composition is relatively higher than the same alloy, the increase of non-magnetic phase also leads to the decrease of Br;

(4) Dehydrogenation and abnormal grain growth (AGG) during sintering, HD + JM powder containing different amounts of hydrogen, the final in the sintering process can be exhausted. In the heating process there will be cracking, the vacuum is not high and the emergence of oxidation, nitriding and other phenomena, the reason is that the mixture sintering, equipment does not meet the temperature is not correct. HD + JM powder containing hydrogen and fine powder (~ 3μm) in the general system is prone to abnormal grain growth (coercivity HC substantially reduced), the solution: to reduce the sintering temperature from 1060 degrees to 960 Degree, add grain resist.

(5) If HD + JM powder is used to make magnet compared with mechanical powder magnet of the same composition, the result is that Br, Hk / Hci are all low with mechanically pulverized magnet. The reason is that the easy axis of magnetization of the PrFeBHx of Pr-Fe-B alloy is changed from the C axis to the A surface and the magnetic orientation is shifted. When the sintering is completed, the easy axis becomes C axis from A surface Br, Hk / Hci are low, as long as the Pr-free Nd-Fe-B magnets do not appear this phenomenon.

Hydrogen Crushing (HD) Equipment: The preparation of high-performance magnets is primarily to assure the low-oxygen process. It is the requirement of low-oxygen environment that promotes the early practical use of hydrogenated milling. The HD technology is the core technology for ensuring oxygen-free milling.

Fluidized Bed Jet Mill: The presence of a high performance magnet (50MGOe) has an average grain size in the milling organization of 4.6 to 5.0 μm and advances the requirement for fine and uniform grain refinement to the milling stage The average particle size of magnetic powder should be 3 ~ 5um, the particle size distribution curve should be very concentrated, sharp, the frequency distribution of traditional magnetic technology, just separated from the new magnetic particle was significantly dispersed, wide and blunt, the largest particles up to 40um. The new technology of magnetic powder, manufactured using SC + HD + JM combination technology. Long-term test of magnetic particle size is the average particle size (FSSS), without the use of magnetic powder frequency distribution curve. The average particle size should be 3um, the maximum about 7um. The flow of fluidized bed jet mill in our country is difficult to meet this requirement.


Fluidized bed

Fluidized bed collision Airflow mill, closed-loop airflow Mill Advantages:

(1) low energy consumption, due to multi-jet nozzle jet jet force on the point of large, group wheel so that qualified fine powder discharged in time, unqualified powder was returned to the crushing chamber for secondary crushing, jet kinetic energy is best utilized than the round Disk airflow mill to reduce energy consumption by 30-40%;

(2) wear and adhesion is small, due to jet air stream material into the recognition chamber, thus avoiding the particles on the way impact, friction and adhesion deposition, but also to avoid the nozzle and pipe wear.

(3) grading machine can be adjusted independently, the particle size distribution is more concentrated.

(4) automation, compact structure, small wear and tear, washing more convenient.

(5) Closed-loop can prevent oxidation and pollution.


Closed-loop airflow mill shortcomings:

(1) in the crushing chamber, if the raw material particles too, density is too large to be fluidized state can not be crushed;

(2) grouting machine blade wear serious;

There are several problems with the collision jet mill:

(1) Tail powder (feed spit) problem (There is no tail powder in crushing a single variety, single ingredient material, long-term continuous production. NdFeB permanent magnet production always involves different ingredients, different batches, and not Allow mixed with each other, such as 100KG raw materials into the mill, a powder 88KG, there 12KG detention machine, this 12KG powder is the tail powder, only continue to join the raw materials to guide these tailings 12KG. Raw material particles are too large, too high density can not present the fluidized state can not be crushed.But it may be the opposite reason, the raw material is small, but in the classification wheel requirements, the density is too small, can not show the normal fluidization state, the crushing efficiency Very low, unable to powder .Tail powder was hold in the crusher room smashing indoor spinning, can not get out, that is the cause of tail powder).

(2) tailing powder is the reason for the emergence of large particles, the only way to export tailings, that is, stop the rotation of the classification wheel,

So that the crushing chamber to abandon the tail powder blowing out from the cyclone powder collector tailings particle size collected in different bulk is not good to deal with, there is a deposit to be centralized after treatment. Tailings are not fully collected and many of the large particles are trapped in the pipe they are passing through until the next run of the mill, the new strong powder flow may have trapped large particles of tailings previously trapped in the pipe Into the cyclone powder set, the new powder mixed with large particles, which is the problem of large particles of pollution.

(3) ultrafine powder is not recovered, the fluidized bed collide by the flow mill has two discharge openings, the cyclone powder collector is the powder outlet, the filter is the export of ultrafine powder, for the neodymium This arrangement is extremely unsuitable for boron permanent magnet production because the entire powder needs to be used, not the fine powder. Already have a senior wheeled grading machine, and then set off a whirlwind separation of word of mouth horse ultrafine powder separated, it is unnecessary, ultrafine powder is difficult to collect, in fact, ultrafine powder has become limited oxidation of waste powder.

(4) Design of closed-loop system, anti-oxidation of NdFeB permanent magnet production line is an important issue, milling nitrogen in the system closed-loop,

However, the inlet and outlet are not fully sealed design, through the oxygen into a large amount of nitrogen to dilute, both slow and not economical;

(5) The normal powder production difficulties, below the cyclone until the export design is unreasonable, resulting in powder clogging, sticking, retention and other phenomena, due to powder difficulties, the operator had to beat the pipe with iron,

(6) Oxygen meter (zirconia probe) Earlier zirconia probe, the case of hydrogen, methane and other failure, the reading of zero.

Impact air jet mill to improve the indicators to achieve low oxidation, miniaturization and homogenization of magnetic powder, such as

(1) The system is totally sealed. The raw material tank and discharge tank with uniform caliber are sealed and sealed with the machine. The air in the machine is eliminated by the vacuum system and then rinsed with high purity nitrogen to ensure the oxidation of the working air of the machine.

(2) Cancel the cyclone separator and collect the full-size powder from a large trap.

(3) reroute the tailing powder, install a butterfly valve and powder tank below the crushing chamber, unload the tail powder and then use the disk jet grinder to grind the powder, then add the original batch number to the tail powder to achieve all recovery.

(4) convenient powder, due to the use of pinch valves, pneumatic tapping hammer and the best cone angle, eliminating the clogging when the powder, sticky phenomenon.

Article from NdFeB Industry Network

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