| Recently,bonded Nd-Fe-B magnets have been widely used in high-tech fields such as electronics,communications,new energy,and medical treatment due to their high scale accuracy,strong design control,and good mechanical properties.Compared with isotropic bonded Nd-Fe-B magnets,anisotropic bonded Nd-Fe-B magnets have higher remanence and larger energy product.In order to achieve the same magnetic performance,the use of isotropic magnetic powder will consume more raw materials,and the quality and volume of the corresponding products will also increase.The use of anisotropic magnetic powder not only saves raw materials,but also reduces costs,and its products are more competitive in the market.Based on this research background,this paper prepares a series of various magnetic powders for bonding Nd-Fe-B magnets by using rapid quenching magnetic powder,low quenching rate alloy strips and thermally deformed magnets as raw materials,through surfactant-assisted ball milling,and the correlation between the preparation process parameters and the magnetic properties of the magnetic powder was studied and analyzed.First,Nd-Fe-B nanoparticles and flakes with a coercivity of 17k Oe are prepared by using fast quenched magnetic powder as the ball milling raw material.As the ball milling time increases,coercivity and remanence of the magnetic powder tend to decrease.The ball milling time was increased from 2h to 8h,and coercivity was reduced from 17k Oe to 7.7k Oe.The ball-to-powder ratio increased from 10:1 to 25:1,and coercivity decreased from 12.4k Oe to 3.2k Oe.By analyzing the initial magnetization curve of the ball milled magnetic powder,it is found that coercivity mechanism of the magnetic powder has the characteristics of a pinned model.Further calculations revealed that there is a large gap between coercivity Hcj of the magnetic powder and the pinning field Hp.This is caused by the defects caused by ball milling weakening the pinning effect of some grains and the defects themselves hindering the improvement of the coercive force.In addition,through comparative experiments,it is known that with the increase of oleic acid content,the performance of magnetic powder after ball milling showed an increasing trend.Second,anisotropic Nd-Fe-B magnetic powder was obtained by ball milling with low quenching speed alloy strips,and the residual magnetic ratio of the magnetic powders obtained by ball milling 2m/s quenching speed strips for 2h could reach 0.92.By comparing the grain size of the alloy strip and the particle size of the ball milled magnetic powder,it is concluded that it is a single crystal magnetic powder.When the ball milling time is increased from 1h to6h,coercivity of magnetic powder made by 2m/s quenching speed strips is on the rise,the maximum coercivity is 3.06k Oe,and coercivity of magnetic powder made by 5m/s quenching speed strips increase first and then decrease,and the maximum coercivity is 3.4k Oe in 4h.Ball milled magnetic powder is a nucleation-type coercivity mechanism,and the defects generated by ball milling can easily become the center of demagnetization nucleation,which hinders the increase of coercivity of magnetic powder.In addition,the magnetic powder ball milled for 6hours was kept at 450?for 30 minutes,coercivity increased from 1.8k Oe to 2.8k Oe,and the remanence ratio decreased to a certain extent.Finally,using thermally deformed magnets as the raw material for ball milling,anisotropic magnetic powder was prepared.As the ball milling time increases,the particle size of the magnetic powder becomes smaller,but coercivity and remanence both decrease to some extent.At 30 and 60 minutes of ball milling,coercivity was 9.2k Oe and 8.1k Oe,and the remanence ratio was 0.63 and 0.61,respectively.The use of low-energy ball milling can reduce the occurrence of magnetic powder defects,thereby reducing the extent of magnetic performance degradation.When using quick quenching magnetic powder to prepare bonded magnets,increasing the molding pressure can increase the remanence and magnetic energy product of the magnet.When the molding pressure is 800MPa,the improvement of magnetic performance is small,and the magnet has many cracks.Excessive molding pressure is not conducive to the improvement of the mechanical strength and magnetic performance of the magnet. |
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