Laser Shock Peening Method for Improving the Corrosion Resistance of Sintered Nd-Fe-B Magnet

Abstract

Disclosed is a surface modification technique for permanent magnetic materials. First, a sintered Nd—Fe—B magnet is immersed in a chlorine-containing solution to corrode its surface after the sintered Nd—Fe—B magnet is ground, polished and cleaned, so that atomic vacancies or gaps are produced at the grain boundaries in the surface layer of the corroded sintered Nd—Fe—B magnet; then, compound nanopowders coated on the surface of the sintered Nd—Fe—B magnet are implanted into the grain boundaries by laser shock peening to obtain a gradient nanostructure layer along the depth direction; at the same time, the surface nanocrystallization of the sintered Nd—Fe—B magnet and a residual compressive stress layer are induced by laser shock peening which remarkably improves the corrosion resistance of the sintered Nd—Fe—B magnet.

Claims

1: A laser shock peening method for improving the corrosion resistance of a sintered Nd—Fe—B magnet, wherein: first, a sintered Nd—Fe—B magnet is immersed in a chlorine-containing solution to corrode its surface slightly after the sintered Nd—Fe—B magnet is ground, polished and cleaned, so that atomic vacancies or gaps are produced at the original grain boundaries of the corroded sintered Nd—Fe—B magnet; then, compound nanopowders coated on the surface of the sintered Nd—Fe—B magnet are implanted into the grain boundaries by laser shock peening, to form a gradient nanostructure layer along the depth direction; at the same time, the surface nanocrystallization of the sintered Nd—Fe—B magnet and a residual compressive stress layer are induced by laser shock peening; whereupon the compositions and structures of the grain boundary phases are modified, and the physicochemical properties of the grain boundary phases are improved, an effect of inhibiting grain boundary corrosion in the surface of the magnet is achieved, and corrosion resistance of the sintered Nd—Fe—B magnet is improved remarkably, wherein, the chlorine-containing solution is NaCl solution with a mass fraction of 3.5% or MgCl.sub.2 solution with a mass fraction of 14%, and the immersion time is 30-120 minutes; the compound nanopowder is AlN nanometer powder with a high melting point, which belongs to covalent compounds, has an excellent thermal stability, and can stably exist in grain boundaries; the atomic percentage of the sintered Nd—Fe—B magnet is Nd.sub.aR.sub.bFe.sub.100-a-b-c-dB.sub.cM.sub.d, wherein, 8≤a≤18, 0.5≤b≤5, 3.5≤c≤8, 0.1≤d≤5, R is one or more of Pr, Dy, Tb, Ho, Gd, Ce, Co, Ni, Al, Cu, and Ga elements, and M is one or more of Al, Cu, Ga, Mg, Zn, Sn, Si, Co, Ni, Nb, Zr, Ti, W, and V elements; the laser is a single-pulse Nd:YAG laser, and the working parameters are as follows: the wavelength is 1,064 nm, the pulse width is 8-16 ns, the energy per pulse is 5-7.6 J, and the laser spot radius is 2-3 mm.

2: The laser shock peening method for improving the corrosion resistance of a sintered Nd—Fe—B magnet according to claim 1 comprising the steps of: (1) grinding and polishing the surface of a sintered Nd—Fe—B magnet, and then immersing the sintered Nd—Fe—B magnet in an alcoholic solution and removing the dust and oil stains from the surface of the sintered Nd—Fe—B magnet with an ultrasonic cleaner; (2) immersing the sintered Nd—Fe—B magnet into a chlorine-containing solution to corrode its surface so that atomic vacancies or gaps are produced at the grain boundaries of the corroded sintered Nd—Fe—B magnet; (3) removing the pretreated sintered Nd—Fe—B magnet from the chlorine-containing solution and drying it in air, and then mounting the sintered Nd—Fe—B magnet on a fixture controlled by a manipulator; (4) setting laser output power and laser spot parameters by a laser control device; at the same time, superposing the spot center of the laser beam on the top left corner of the magnet surface to be treated and taking the position as an initial position of laser shock peening, and making the X-direction and Y-direction of the area to be treated in line with the X-direction and Y-direction of the loading platform; (5) uniformly coating compound nanopowders on the surface of the sintered Nd—Fe—B magnet sample, and turning on the laser at the same time; controlling the sintered Nd—Fe—B magnet sample with a manipulator to move to the focus of the laser beam, and carrying out laser shock peening on the corroded surface of the sintered Nd—Fe—B magnet using a line-by-line processing method; implanting the compound nanopowders into the surface layer of the sintered Nd—Fe—B magnet sample under the mechanical effect of the ultra-strong shock wave produced by laser shock peening; and inducing a residual compressive stress layer by laser shock peening so as to obtain a high-performance gradient nanostructure layer along the depth direction.

3: The laser shock peening method for improving the corrosion resistance of a sintered Nd—Fe—B magnet according to claim 2, wherein, in step (4), the overlapping rate between two neighboring laser spots in both transverse and longitudinal directions is set to be 50%.

4: The laser shock peening method for improving the corrosion resistance of a sintered Nd—Fe—B magnet according to claim 2, wherein, in the step (5), the thickness of the compound nanopowder layer coated in step (5) is 0.5-1 mm, and the average particle size of the compound nanopowders is 30-150 nm.

Description

IV. DESCRIPTION OF DRAWINGS

[0017] FIG. 1 shows the surface corrosion morphology of a sintered Nd—Fe—B magnet;

[0018] FIG. 2 is a schematic diagram illustrating the comparison between the potentiodynamic polarization curve of a sintered Nd—Fe—B magnet Nd.sub.8Pr.sub.4Fes.sub.81Co.sub.2B.sub.3.5Cu.sub.1.5 with AlN nanometer powder and the potentiodynamic polarization curve of a sintered Nd—Fe—B magnet Nd.sub.8Pr.sub.4Fe.sub.81Co.sub.2B.sub.3.5Cu.sub.4.5 without AlN nanometer powder in NaCl solution with a mass fraction of 14%;

[0019] FIG. 3 is a schematic diagram illustrating the comparison between the potentiodynamic polarization curve of a sintered Nd—Fe—B magnet Nd.sub.10Dy.sub.2Fe.sub.79B.sub.8Al.sub.0.5Mg.sub.0.5 with AlN nanometer powder and the potentiodynamic polarization curve of a sintered Nd—Fe—B magnet Nd.sub.10Dy.sub.2Fe.sub.79B.sub.8Al.sub.0.5Mg.sub.0.5 without AlN nanometer powder in NaCl solution with a mass fraction of 3.5%;

[0020] FIG. 4 is a schematic diagram illustrating the comparison between the potentiodynamic polarization curve of a sintered Nd—Fe—B magnet Nd.sub.15Gd.sub.0.5Fe.sub.80B.sub.4Ni.sub.0.5 with AlN nanometer powder and the potentiodynamic polarization curve of a sintered Nd—Fe—B magnet Nd.sub.15Gd.sub.0.5Fe.sub.80B.sub.4Ni.sub.0.5 without AlN nanometer powder in NaCl solution with a mass fraction of 3.5%.

V. EMBODIMENTS

[0021] Hereunder the technical scheme of the present invention will be further detailed in some embodiments with reference to the accompanying drawings.

[0022] In the following examples using the above-mentioned strengthening method to process sintered Nd—Fe—B magnets, the steps include:

Embodiment 1

[0023] (1) The surface of a sintered Nd—Fe—B magnet Nd.sub.8Pr.sub.4Fe.sub.81Co.sub.2B.sub.3.5Cu.sub.1.5 is ground and polished with 500 #-2400 #SiC abrasive paper, and then the sintered Nd—Fe—B magnet is immersed in an alcoholic solution to remove the dust and oil stains from the surface of the sintered Nd—Fe—B magnet with an ultrasonic cleaner; [0024] (2) The sintered Nd—Fe—B magnet is immersed into NaCl solution with a mass fraction of 14% and held for 30 minutes so that atomic vacancies or gaps are produced at the grain boundaries when the surface of the sintered Nd—Fe—B magnet is corroded; [0025] (3) The pretreated sintered Nd—Fe—B magnet is taken out and dried by cold air, and then the sintered Nd—Fe—B magnet is mounted on a special fixture controlled by a manipulator; [0026] (4) The laser output power and laser spot parameters are set by means of a laser control device; specifically, a single-pulse Nd:YAG laser is used, and the working parameters are as follows: the wavelength is 1,064 nm, the pulse width is 16 ns, the energy per pulse is 5.6 J, the radius of the laser spot is 3 mm, and the overlapping rate between two neighboring laser spots in both transverse and longitudinal directions is set to be 50%; at the same time, the spot center of the laser beam is superposed on the top left corner of the magnet surface to be treated and the position is taken as an initial position of laser shock peening, and the X-direction and Y-direction of the area to be treated is kept in line with the X-direction and Y-direction of the loading platform; [0027] (5) AlN compound nanopowders with an average particle size of 50 nm are uniformly coated on the surface of the sintered Nd—Fe—B magnet sample, wherein, the thickness of the coating is 0.5 mm; after turning on the laser, the sintered Nd—Fe—B magnet sample is controlled with a manipulator to move to the focus of the laser beam, and massive laser shock peening is carried out on the surface of the magnet using a line-by-line processing method; the AlN compound nanopowders are implanted into the surface layer of the sintered Nd—Fe—B magnet under the mechanical effect of the ultra-strong shock wave produced by laser shock peening, and a residual compressive stress layer is induced by laser shock peening at the same time so as to obtain a high-performance gradient nanostructure layer along the depth direction. [0028] In this embodiment, an electrochemical corrosion test is carried out for the sintered Nd—Fe—B magnet Nd.sub.8Tb.sub.3Fe.sub.83Co.sub.2B.sub.3.5Cu.sub.1.5, and the test result is compared with that of the sintered Nd—Fe—B magnet before treatment. It is seen from FIG. 2: after adding the AlN compound nanopowders, the corrosion potential of the sample increases, and the corrosion current density decreases. The test result indicates: adding AlN nanometer powders at the grain boundaries decreases the quantity of Nd-rich phases in the grain boundary area, the corrosion potential of the grain boundary phases increases, and the stability of the grain boundaries is improved. According to the mechanism of electrode reaction, the increase of potential of the grain boundary phases leads to the increase of corrosion potential of the entire sintered Nd—Fe—B magnet. The result further demonstrates that adding AlN nanometer powder at the grain boundaries can remarkably improve the corrosion resistance of the sintered Nd—Fe—B magnet Nd.sub.8Pr.sub.4Fe.sub.83Co.sub.2B.sub.3.5Cu.sub.1.5.

Embodiment 2

[0029] (1) The surface of a sintered Nd—Fe—B magnet Nd.sub.10Dy.sub.2Fe.sub.79B.sub.8Al.sub.0.5Mg.sub.0.5 is ground and polished with 500 #-2400 #SiC abrasive paper, and then the sintered Nd—Fe—B magnet is immersed in an alcoholic solution to remove the dust and oil stains from the surface of the sintered Nd—Fe—B magnet with an ultrasonic cleaner; [0030] (2) The sintered Nd—Fe—B magnet is immersed into NaCl solution with a mass fraction of 3.5% and held for 60 minutes so that atomic vacancies or gaps are produced at the grain boundaries when the surface of the sintered Nd—Fe—B magnet is corroded; [0031] (3) The pretreated sintered Nd—Fe—B magnet is taken out and dried by cold air, and then the sintered Nd—Fe—B magnet is mounted on a special fixture controlled by a manipulator, [0032] (4) The laser output power and laser spot parameters are set by means of a laser control device; specifically, a single-pulse Nd:YAG laser is used, and the working parameters are as follows: the wavelength is 1,064 nm, the pulse width is 8 ns, the energy per pulse is 7.6 J, the radius of the light spot is 3 mm, and the overlapping rate between two neighboring laser spots in both transverse and longitudinal directions is set to be 50%; at the same time, the spot center of the laser beam is superposed on the top left corner of the magnet surface to be treated and the position is taken as an initial position of laser shock peening, and the X-direction and Y-direction of the area to be treated is kept in line with the X-direction and Y-direction of the loading platform; [0033] (5) AlN compound nanopowders with an average particle size of 150 nm are uniformly coated on the surface of the sintered Nd—Fe—B magnet sample, wherein, the thickness of the coating is 1 mm; after turning on the laser, the sintered Nd—Fe—B magnet sample is controlled with a manipulator to move to the focus of the laser beam, and massive laser shock peening is carried out on the surface of the magnet using a line-by-line processing method; the AlN compound nanopowders are implanted into the surface layer of the sintered Nd—Fe—B magnet under the mechanical effect of the ultra-strong shock wave produced by laser shock peening, and a residual compressive stress layer is induced by laser shock peening at the same time so as to obtain a high-performance gradient nanostructure layer along the depth direction. [0034] In this embodiment, an electrochemical corrosion test is carried out for the sintered Nd—Fe—B magnet Nd.sub.8Pr.sub.4Fe.sub.83Co.sub.2B.sub.3.5Cu.sub.1.5, and the test result is compared with that of the sintered Nd—Fe—B magnet before treatment. Likewise, it is seen from FIG. 3: after adding the AlN compound nanopowders, the corrosion potential of the sample increases, and the corrosion current density decreases.

Embodiment 3

[0035] (1) The surface of a sintered Nd—Fe—B magnet Nd.sub.15Gd.sub.0.5Fe.sub.80B.sub.4Ni.sub.0.5 is ground and polished with 500 #-2400 #SiC abrasive paper, and then the sintered Nd—Fe—B magnet is immersed in an alcoholic solution to remove the dust and oil stains from the surface of the sintered Nd—Fe—B magnet with an ultrasonic cleaner; [0036] (2) The sintered Nd—Fe—B magnet is immersed into NaCl solution with a mass fraction of 3.5% and held for 90 minutes so that atomic vacancies or gaps are produced at the grain boundaries when the surface of the sintered Nd—Fe—B magnet is corroded; [0037] (3) The pretreated sintered Nd—Fe—B magnet is taken out and dried by cold air, and then the sintered Nd—Fe—B magnet is mounted on a special fixture controlled by a manipulator; [0038] (4) The laser output power and laser spot parameters are set by means of a laser control device; specifically, a single-pulse Nd:YAG laser is used, and the working parameters are as follows: the wavelength is 1,064 un, the pulse width is 10 ns, the energy per pulse is 6 J, the radius of the laser spot is 3 mm, and the overlapping rate between two neighboring laser spots in both transverse and longitudinal directions is set to be 50%; at the same time, the spot center of the laser beam is superposed on the top left corner of the magnet surface to be treated and the position is taken as an initial position of laser shock peening, and the X-direction and Y-direction of the area to be treated is kept in line with the X-direction and Y-direction of the loading platform; [0039] (5) AlN compound nanopowders with an average particle size of 100 nm are uniformly coated on the surface of the sintered Nd—Fe—B magnet sample, wherein, the thickness of the coating is 0.7 mm; after turning on the laser, the sintered Nd—Fe—B magnet sample is controlled with a manipulator to move to the focus of the laser beam, and massive laser shock peening is carried out on the surface of the magnet using a line-by-line processing method, the AlN compound nanopowders are implanted into the surface layer of the sintered Nd—Fe—B magnet under the mechanical effect of the super strong shock wave produced by laser shock peening, and a residual compressive stress layer is induced by laser shock peening at the same time so as to obtain a high-performance gradient nanostructure layer along the depth direction. [0040] In this embodiment, an electrochemical corrosion test is carried out for the sintered Nd—Fe—B magnet Nd.sub.8Pr.sub.4Fe.sub.83Co.sub.2B.sub.3.5Cu.sub.1.5, and the test result is compared with that of the sintered Nd—Fe—B magnet before treatment. Likewise, it is seen from FIG. 4: after adding the AlN compound nanopowders, the corrosion potential of the sample increases, and the corrosion current density decreases.