| With high magnetic remanence,intrinsic coercivity and maximum magnetic energy product,the sintered Nd-Fe-B permanent alloy based on Nd2Fe14B compound is known as the third generation of rare earth permanent magnet materials.The cost of the sintered Nd-Fe-B permanent alloy is low because it is mainly composed of iron.There aren’t precious elements such as samarium and cobalt in the material.And the content of the rare earth element neodymium in the nature is more than 10 times of samarium.Sintered Nd-Fe-B permanent alloy is one kind of high energy density energy storage material.The efficient transformation between energy and information can be achieved via the alloy.Therefore,sintered Nd-Fe-B permanent magnet is particularly suitable for the production of high-performance,compact and lightweight permanent magnet products.In recent years,it has been widely used in motor,communication,information,wind power,new energy vehicles and many other fields.This paper aims to improve the comprehensive performance of the sintered Nd-Fe-B permanent alloy from grain boundary modification and surface modification.Firstly,the magnetic performance of the alloy was raised by optimizing the aging treatment process.Then,the corrosion resistance was enhanced through the preparation of phosphate chemical conversion coatings on the surface of the magnet.In order to determine the optimal parameters,aging process optimizing experiment was conducted on the N40HCE type of sintered Nd-Fe-B permanent alloy produced by Yantai Shougang Magnetic Materials Co.,Ltd.On this basis,the effect of aging treatment on the mechanical properties and corrosion resistance of the sintered Nd-Fe-B permanent alloy was discussed.Phosphate chemical conversion coatings were prepared on the surface of the sintered Nd-Fe-B permanent alloy by chemical conversion method and the appropriate process parameters were obtained through process tests.The microstructure,composition,functional group,phase structure and electrochemical performance of the coatings were studied by scanning electron microscope,energy disperse spectroscopy,Fourier transform infrared spectrometric analyzer,X-ray diffraction,high resolution transmission electron microscopy and electrochemical workstation,respectively.In order to determine the appropriate pretreatment pickling technology,the corrosion behavior of the sintered Nd-Fe-B alloy in hydrochloric acid solution,nitric acid solution and phosphoric acid solution was studied before the chemical conversion treatment.The results showed that the optimal aging treatment process parameters of the N40HCE sintered Nd-Fe-B permanent alloy were:850 ℃ × 2 h + 530 ℃ x 2 h.After the optimal aging treatment,the morphology of the intergranular Nd-rich phase in the sintered Nd-Fe-B permanent alloy changed from block distribution at the corner of the Nd2Fe14B phase to continuous thin layered structure along the Nd2Fe14B grain boundary,which eliminated the magnetic coupling interactions.So the intrinsic coercivity of the alloy was obviously improved.However,the tensile strength and compressive strength of the sintered Nd-Fe-B permanent alloy decreased,and the brittleness increased after the aging treatment.This was caused by morphology and distribution change of the intergranular Nd-rich phase,too.The change reduced the sliding resistance between the Nd2Fe14B grains and weakened the bonding strength of the grain boundary.Aging treatment also reduced the corrosion resistance of the sintered Nd-Fe-B permanent alloy.Because after the aging treatment,the intergranular Nd-rich phase changed into a three-dimensional network structure and formed a quick corrosion channel,which accelerated the corrosion of the sintered Nd-Fe-B permanent alloy.Vickers hardness indentation combined with acoustic emission technique was employed to test the brittleness of the sintered Nd-Fe-B permanent alloy.The results indicated that the acoustic emission energy accumulated count value En and Vickers hardness indentation load P was in a linear relationship.And the Vickers hardness indentation apparent crack length L and hardness indentation load P was in a linear relationship,too.So it is reasonable and feasible for the slope value K of the En-P line to be used as the quantitative index of the brittleness for the sintered Nd-Fe-B permanent alloy.Based on studying the corrosion behavior of the sintered Nd-Fe-B permanent alloy in hydrochloric acid solution,nitric acid solution and phosphoric acid solution,it was found that the corrosion rate of the alloy was maximum in the hydrochloric acid solution and minimum In the phosphoric acid solution when the hydrogen ion concentration was similar.Both the hydrochloric acid and the nitric acid solution erosion could effectively remove the surface oxide layer of the alloy.The hydrochloric acid solution had an obvious corrosive effect on the intergranular Nd-rich phase.However,the corrosive attack of the nitric acid solution mainly occurred on the Nd2Fe14B phase.And the corrosion effect on the grain boundary Nd-rich phase was not obvious.A layer of block-like phosphate products would be formed when the sintered Nd-Fe-B permanent alloy was in the phosphoric acid solution.Therefore,the oxide layer on the surface of the alloy cannot be removed by phosphoric acid pickling.The nitric acid solution had the minimal effect on the comprehensive magnetic performance of the alloy.So the nitric acid solution was more suitable to be used as the pickling solution for the preparation of protective coatings on the sintered Nd-Fe-B permanent alloy.The optimum PH value range of the chemical conversion solution used for the sintered Nd-Fe-B permanent alloy was 1.00 to 1.50.When the PH value of the conversion solution increased to 2.00,almost no phosphate coatings could be formed.If the PH value increased to 2.50,the coating forming ions in the solution would separate out as columnar precipitates.When the PH value of the conversion solution was 0.52,the grain of the phosphate chemical conversion coatings was coarse.And the coatings exhibited a power corrosion resistance.What’s more,the magnetic performance of the alloy would reduce a lot.The coating mass was gradually decreased with the increasing PH value of the solution.The conversion coating was mainly composed of praseodymium phosphate,neodymium phosphate and small amounts of iron phosphate.With the increasing of the solution PH value,the content of praseodymium phosphate and neodymium phosphate declined,while the content of iron phosphate increased.The corrosion resistance of the phosphate chemical conversion coatings was best when the PH value of the solution was 1.00,1.36 and 1.50.The magnetic properties of the sintered Nd-Fe-B permanent alloy showed a decreasing trend after the phosphate chemical conversion treatment.The decline of the magnetic properties was maximum when the PH value of the conversion solution was 0.52.In other PH value condition,the magnetic properties of the alloy reduced to a less extent.When the PH value of the conversion solution was 1.00,the phosphate chemical conversion coatings could be formed on the surface of the sintered Nd-Fe-B permanent alloy at 50 ℃,60 ℃,70 ℃,80 ℃ and 90 ℃,respectively.The coating mass slightly increased with the rising of the conversion temperature.The phosphate chemical conversion coatings prepared at different temperatures could effectively improve the corrosion resistance of the sintered Nd-Fe-B permanent alloy.Among them,the corrosion resistance of the phosphate coatings obtained at 70 ℃ was relatively better.And conducting phosphate treatment at 70 ℃ had less effect on the magnetic properties of the alloy.Therefore,when the solution PH value was 1.00,70℃ was the optimum conversion temperature to fabricate phosphate chemical conversion coating on the surface of the sintered Nd-Fe-B permanent alloy. |
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