What is the Fabricating Process routine of Sintered Nd-Fe-B Permanent Magnet?

Q: What is the Fabricating Process routine of Sintered Nd-Fe-B Permanent Magnet?

A: The sintered Nd-Fe-B magnets are usually formed by powder metallurgical process. The general fabricating process routine is as follow:

  • Melting—alloying ingot;
  • Milling—powders with suitable particle size;
  • Pressing—magnetic field oriented green compaction;
  • Sintering—condensed magnet ;
  • Annealing—increase coecivity of the magnet;
  • Magnetic properties testing;
  • Grinding for rough magnet profile modification;

Machining processes such as cutting, drilling or milling to final shape;

Plating processes such as Ni, Zn, Al, epoxy coating etc;

Quality inspecting and testing for final products;

Packing and delivering.

 

Q: How about the Melting Process?

A: Melting process consists of melting the pure metals of Nd, Pr, Fe, Dy, Tb, Al, Co, Nb or Nb-Fe, B-Fe, Nd-Pr, alloys in an alumina crucible to obtain a homogeneous liquid, and pouring it into a mold. Such a process is usually achieved by using an induction furnace. Firstly, a rigorous vacuum environment in the chamber of the furnace must be reserved to prevent the rare earth metal from oxidation. Then pure argon gas is filled into the furnace to restrain vaporizing of the rare earth metal. After that, the alloys are melted and mixed by electromagnetic force into a homogeneous liquid. Finally the alloying liquid is poured into the water chilling copper mold and solidified into ingot.

 

Q: What is a-Fe?

A: Shifting the Nd-Fe-B alloy composition to the stoichionmetric phase Nd2Fe14B in alloy design is an effective way to decrease the volume fraction of nonmagnetic phases, thus reaching higher remanence and energy product. Figure 3-1 is a vertical section of the Nd-Fe-B phase diagram. The commercial important neodymium content range is from Nd2 Fe14B main phase (T1) to peristaltic reaction point P5.It clearly illustrates the a composition within this range brings the alloy solidifying through a phase region of liquid + α-Fe (L+γ). By the non-equilibrium solidification, γ-Fe is remained and transforms into α-Fe in the ingot.

The presence of α-Fe in Nd-Fe-B ingot not only causes pulverizing difficulties for the ingot, but also decreases the magnetic properties and the corrosion resistance of the magnet. Therefore, in the sintered Nd-Fe-B magnet process, minimizing or completely removing α-Fe phase in Nd-Fe-B ingot is very important. So far, there are three types of technique for α-Fe control. In addition to annealing the ingot at high temperature for adequate time, an alternative way is by increasing the solidification rate during alloy casting. Moreover, substitution rate during alloy casting. Moreover, substitution of a small amount of refectory elements such as Nb, Mo, Ti and so on for Fe in Nd-Fe-B alloy is also effective for α-Fe suppressing.

 

 

 

 

              

          Fig.3-1: A vertical section of the Nd-Fe-B phase diagram.

 

Figure 3-2 is the microstructure image of conventional Nd-Fe-B alloy ingot taken from electron scanning microscopy (SEM) by using back scattered electron. Figure 3-2a is observed at the end of the ingot where the solidifying rate is rather low. In Fig.3-2a, the main phase is in gray color and the grains are equiaxial. The Nd-rich phase is block shaped and is in white color. α-Fe is in black color and ranges over several main phase grain sizes. Figure 3-2b is observed in the middle of the ingot where the solidifying rate is relatively high. There is no a-Fe, and the main phase grains are in sheet-like. Importantly, the distribution of Nd-rich phase is much more uniform than that of in Fig.3-2a.The homogeneous microstructures of the Nd-Fe-B ingot play an important role in the fabrication of high performance sintered Nd-Fe-B magnets.

         

         

                            (a)

 

 

 

                                  (b)

 

Fig.3-2: The microstructure image of conventional(Nd0.95 Dy0.05)15.5(Fe0.99A0.01)

78.0B6.5 ingot taken by using back scattered SEM.

 

 

 

Fig.3-3: A typical microstructure image of Nd-Fe-B strip cast ingot takenby using back

scattered (SEM).

 

Q: What is Strip Cast Process?

A: Strip cast is a newly developed solidification process for Nd-Fe-B alloy ingot, which is suitable for fabrication of high performance sintered Nd-Fe-B magnets. The process involves casting molten alloys onto a rotating water chilling roller, enable the formation of very thin, typically 0.3mm thick, flake shaped ingots. Owing to the high c00ling rates, the formation of a-Fe is suppressed.

A further advantage of strip cast procedure is that the excess Nd content needed for the liquid phase sintering can be kept at minimum because the Nd-rich phase regions remain small and are distributed homogeneously in the ingot.

Figure 3-3 is a typical microstructure image of Nd-Fe-B strip cast ingot measured by using back scattered SEM.

Compared with Fig.3-2, the microstructure of strip cast ingot is much more uniform, and the main phase grains are very tiny.

The combination of strip cast and hydrogen-decrepitating process is very useful for the production of high performance sintered Nd-Fe-B magnets.

 

Q: How about the Milling process?

A: To achieve a high coercive force, the Nd-Fe-B ingot must be pulverized into powders with average particle size of 3-5μm. Firstly, the ingot needs to be crushed into an average particle size of several hundred microns that is consistent with the afterward-milling process. This is usually done by mechanical pre-mill such as jaw crusher or by chemical reaction with hydrogen (hydrogen decrepitating, HD). Then, the coarse powders are further pulverized by jet milling to get the final average particle size powders. Jet mill employs high-pressure nitrogen flow to make the solid particles intensively collide each other. Compared to mechanical milling such as ball milling and attrition milling, jet milling holds advantages of much sharper particle size distribution, lowed and more stable oxygen content for the fabricated powders which contribute better coercivity and squareness to the demagnetizing curve of the final sintered Nd-Fe-B magnets.

 

3-6Q: What is Hydrogen Decrepitation (HD) Process?

A: HD process employs the fact that most rare earth-3d compounds readily absorb large quantity of hydrogen gas at room temperature. For Nd-Fe-B, the reaction is as follow:

Nd2-Fe14-B + (1/2) x H2=> Nd2-Fe14-BHx                    (3-1)

The volume increase accompanying the absorption of hydrogen gas leads to pulverization of even large lumps of the cast material, simply by exposure the Nd-Fe-B alloy ingot to hydrogen at approximately 1 bar at room temperature. The absorption of a total of about 0.4 wt% hydrogen, firstly by the Nd-rich phase and then by the Nd2-Fe14-B main phase. HD technique is usually completed in a vacuum chamber, involving process of absorption and subsequent absorption of hydrogen.

The HD particles are very friable and very amenable to further reduction in size by jet milling. Moreover, the lower oxidizability of the HD material results in less oxygen pick-up during milling.

 

3-7Q: How about the pressing process?

A: Nd2-Fe14-B inter metallic compound has strong uniaxial anisotropy. To maximize the magnetic performance of Nd-Fe-B material, it is necessary to apply a magnetic field during the pressing operation. Thus the magnetization of every particle in the produced green compaction is optimized to have the same preferred green compaction is optimized to have the same preferred direction. Pressing and aligning techniques can substantially vary the degree of alignment and thus influence the residual induction Br and energy product (BH)m of the finished magnet.

The direction of the magnetic field during die pressing (DP) can be either parallel or perpendicular to the pressing direction. The alignment degree of the magnet fabricated by parallel pressing process is comparably lower than that of perpendicular pressing process. But parallel pressing process is more suitable for the automatic pressing of magnet pieces and is more convenience for the magnets of certain special shapes.

After orientation pressing, isostatic pressing is usually adopted in order to increase the density of the green compacts, so that the green compacts have enough strength for middle-process transformation.

Magnets can also be directly formed by rubber mold isostatic pressing (RIP). Comparing to DP process, RIP process has advantage of higher alignment degree for the fabricated sintered Nd-Fe-B magnet.

 

Q: How about the Sintering Process?

A: Sintering process is to get the full dense magnet by using the vacuum furnace. The magnetic properties of Nd-Fe-B magnets are significantly dependent on this process. The vacuum sintering stage must proceed by outgassing steps at different temperatures in order to remove as many impurities as possible from the green compact, such as hydrogen, oxygen, carbon oxygen, hydrocarbons etc. Optimum sintering temperature and time duration are affected by all the composition parameters and all the previous process variables.

During sintering, the behavior of Nd-rich phase is crucial to the permanent magnetic properties of Nd-Fe-B magnets. First of all, the distribution of Nd-rich phase in conventional ingot is Not uniform, so the densification of compacts and homogenization of microstructures are all determined by the fluidity of Nd-rich phase that is in liquid state during sintering, Secondly, it also contributes to removing micro-defects to ensure reasonable coercive force of the magnet.

 

3-9 Q:  How about the Annealing Process?

A: Generally, the sintered Nd-Fe-B magnets should be annealed to increase coercivity. Annealing helps to modify the microstructures of the sintered magnet where micro-defects usually exist, Annealing is therefore known as to maximum the coercivity and squareness of demagnetization curve of the Nd-Fe-B magnets. Annealing process generally involves two steps. The first step is carried out at about 900oC while the second step is at about 600oC.

The coercivity of sintered Nd-Fe-B magnets is very sensitive to annealing parameters such as temperature duration and cooling rate especially for the step at about 600oC.

 

Q: How about the Machining Process?

A: Machining process involves rough grinding for magnet profile modification, and then cutting, drilling or fine grinding to final shape.

For cylindrical magnet, centerless grinder is usually employed to modify the cylindrical surface. While for rectangle magnet, profile modification is usually carried out by using plane-grinding equipment. The sintered Nd-Fe-B magnets can be readily ground with abrasive wheels if liberal amounts of coolant are used. The coolant serves to minimize heat cracking, chipping and also eliminates the risk of fires caused by sparking, chipping and also eliminates the risk of fires caused by sparks contacting the easily oxidized dust, Cutting process is usually employed for final shape machining by using man-made diamond tools or wire cutting equipment ,For size tolerance modification fine grinding process is usually carried out. Moreover, for plating specified products such as Zn, Ni coated magnet edge chamfering process must be previously taken to prevent point spark during plating so as to improve the uniformity of the coating layer.

 

Q: How about the Machineablity of sintered Nd-Fe-B Permanent Magnet?

A: Nd-Fe-B magnets is a type of intermetallic compound material, it lacks ductility and is inherently brittle. Therefore compared with iron and steel materials there should be special considerations for the machineability. Here we list some design examples as reference for magnet users.

Figure 3-4a is the original design with a dissymmetrical concave hole on the thin wall of a ring magnet. This hole cannot be machined by wire cutting equipment Drilling or fine grinding is also difficult because the thin wall has not enough strength and it is actually brittle, if this hole is designed symmetrically like Fig 3-4b it is easy to be machined by using wire cutting equipment.

 

 

                       (a)                                (b)

Fig.3-4 (a) Machinability difficult (b) machinability easy

 

Figure 3-5a is a rectangle magnet with a caeco-like hole in it The caeco-like hole is machinability difficult because its depth and bottom size tolerances are very difficult to be control by using conventional drilling machine Furthermore coating of the caeco-like hole is also difficult In fig.3-5-b we use two magnets that are bonded together ,  in which the upper magnet has a through hole Thus all size tolerances of the hole can be controlled easily and the coating layer control of the hole is much more reliable.

 

 

              Fig 3-5 (a) machinability difficult, (b) machinability easy

 

Figure 3-6 is four typical machinability difficult cases of the sintered Nd-Fe-B for plating process.

Figure 3-6a is a thin and long rectangle magnet, as there is an effect of point spark on the rectangle edges during Zn or Ni plating, the metal deposition around these edges will be thicker than that in the middle of the magnet. Thus the control of planeness tolerance of this shaped magnet is machinabilty difficult for plating process. Moreover the corrosion resistance of the coating layer is weak because the coating layer on the middle of the magnet is usually thin If the shape and size of the magnet cannot be changed one should design the coat layer with epoxy deposition or phosphatization.

Figure 3-6b is a tube shaped magnet with a small and long internal hole. The plating machinability difficult also happens to the internal wall of the magnet. For Zn or Ni plated layer, the corrosion resistance in the middle of the internal surface is usually poor compared with the other regions of the magnet .If the shape and size of the magnet cannot be changed, it is suggested to take the similar coating technique as above.

Figure 3-6c is a magnet with an extremely small hole drilled in its center, for example, a hole with diameter less than 3.0mm is drilled in a disc-like magnet with out diameter greater than30mm. For Zn or Ni plating, sharp edge radius may result in concentrated plating layer stresses in those regions and induce coating defects such as peeling off. It is suggested that the size of the hole should be designed as large as possible.    

 

 

      Fig 3-6: Four typical machinability difficult sintered magnet cases:

(a) Thin and long magnet, (b) Small and long tube magnet, (c) Magnet with extremely

small hole in the center, (d) Ring magnet with extremely thin wall.

 

Figure 3-6d is a ring magnet with extremely thin wall, taking a thickness less than 1.0mm for example, This shaped magnet is machinability difficult because sintered Nd-Fe-B magnets is brittle. Drilling process for this thin wall ring may result in high ratio of inferior product and thus high cost. Moreover, extremely thin magnet has poor magnetic performance because the coercivity and squareness of the demagnetizing curve of sintered Nd-Fe-B magnets decreases significantly when its size decreases to less than 1.0mm. it is suggested that the thickness of the wall in Figure 3-6d should be designed as possible.

 

Q: How about the Surface Protection?

A: Sintered Nd-Fe-B magnets are relatively are relatively inferior on corrosion resistance comparing with the traditional magnets such as Sm-Co, Alnico and ferrite. For general applications, sintered            Nd-Fe-B magnets may need to be coated or surface treated depending on their intended uses. Coating treatment for the sintered Nd-Fe-B magnets not only improves their corrosion resistance, but also appearance, wear resistance as well as appropriate for applications in clean room conditions.

There are various coatings suitable for the sintered Nd-Fe-B magnets, ranging from plating coating layers such as Ni, Zn, Au to organic electro-deposition (epoxy for example), physical vacuum deposition (PVD) of Al, galvanic tin, chemical vapor deposition (CVD) or any combinations of these.

Besides of coating protection, the sintered Nd-Fe-B magnet can also be surface treated by chromatation, phosphatization, and other chemical passivation treatments to improve their corrosion resistance.

Not all types of coating or passivation treatment will be suitable for every magnet, and the final choice will depend on their applications and environment. For example, Ni coating has good corrosion resistance for a variety of atmospheres, but it shields part of the magnetic flux because nickel is a ferromagnetic material. Furthermore, the affinity of Ni coating glued to other metals is inferior to that of Zn coating. Comparing with Ni coating, Zn coating has advantages of good gluing ability, less magnetic flux shielding so that the magnet has good homogenous apparent flux features from piece to piece, But the corrosion resistance of Zn coating is inferior to Ni coating when the magnet is used in strong corrosive surroundings, it is suggested that the magnet be coated with multiple layers such as Ni /Cu/ Ni/, Ni/epoxy, Zn/epoxy etal. If the magnet is intended to be used in dry atmosphere and weak corrosive environment, Zn coating is recommended.

 

 

 

 

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