Sandwiched structure of hot-deformed Nd-Fe-B permanent magnets processed by Nd-Cu eutectic alloys diffusion

A series of sandwiched structures with different near-surface mass fractions x(x=3 wt%,4 wt%,5 wt%)was employed to develop high-coercivity hot-deformed Nd-Fe-B magnets by the addition of 2 wt%Nd-Cu eutectic alloys via adjusting the middle thickness and near-surface thickness.The designed magnet with a pronounced composite structure shows a 23% increase in coercivity with a 6% loss of remanence by adjusting the sandwiched structure at 4 wt% Nd-Cu eutectic alloys in the near-surface regions.The results indicate that the near-surface Nd-Cu-rich "shell" structure can effectively suppress the magnetization reversal of overall magnets,enhancing the coercivity.With the help of loading stress,Nd-Cu liquid enriched at the near-surface regions of the sample is infiltrated into the Nd-Cu-lean middle region,resulting in a concentration gradient.Microstructure characterizations further demonstrate that the infiltrated Nd-Cu eutectic plays a critical role in inhibiting grain growth and intergranular magnetic interaction.The optimized microstructure features suppress the reversed magnetization process,which makes a positive contribution to coercivity.     

Densification and anticorrosion performances of vacuum evaporated aluminium coatings on NdFeB magnets

Al coated NdFeB magnets obtained by vacuum evaporation technique were densified by high energy ball milling method.The surface morphology,metal composition and micro structure of the coatings were characterized by scanning electron microscopy,X-ray diffraction and X-ray photoelectron spectroscopy,respectively.The anticorrosive properties were investigated by potentiodynamic polarization curves and neutral salt spray test.The pores in the Al coatings of columnar crystals(Al) induced by the evaporation technique,were apparently filled in the following ball milling process,leading to the densification of Al coatings and the evident improvement of the anticorrosive performances.When treated with ball milling for 30 min,the sample achieves the best anticorrosive performances with the self-corrosion potential of-0.87 V,self-corrosion current density of 1.65 μA/cm~2 and the neutral salt spray(NSS) time of 144 h(red rust).The improvement of the anticorro sive performances of vacuum evaporated Al coating mainly lies in the densification effect of the coating,which depends on different loading conditions of ball milling process.     

真空热处理提高烧结钕铁硼镀Cu层结合力的研究

目的 提高烧结钕铁硼表面镀Cu膜层结合力,改善可焊性,进一步制备生物友好的强耐蚀性防护薄膜。方法 采用磁控溅射技术在钕铁硼表面制备约7 μm厚的Cu膜,研究热处理温度和时间对Cu/NdFeB界面组织、膜基结合力和样品磁性能的影响,选取最优化热处理样品电镀约2 μm厚的Sn膜,再于280 ℃在其表面焊接Au片,评估其可焊性。结果 500 ℃热处理样品的Cu膜与基体间发生了明显扩散,扩散深度及结合力随时间延长而增加。热处理2 h样品的膜基结合力由处理前的11.0 MPa提高至31.5 MPa,膜基分离位置发生在磁体亚表面层,矫顽力、剩磁和最大磁能积等磁性能无显著下降。进一步镀Sn后,在其表面焊接的Au层与Cu膜层基体冶金结合良好,耐腐蚀性能优异。700 ℃热处理样品的Cu膜与基体间扩散过快,易造成Cu膜消失及钕铁硼基体表面损伤。结论 真空热处理温度和时间对Cu/钕铁硼界面组织有根本性影响,通过适宜的热处理可大幅提高磁控溅射的Cu膜与烧结钕铁硼之间的膜基结合力,同时不明显降低磁性能,可采用焊接方法在热处理后的Cu膜表面制备结合力高、长效耐蚀且生物友好的防护薄膜。     

湿式烧结金属摩擦片缩比模拟试验

湿式烧结金属摩擦片的摩擦性能测试,一般用实物在试验台架上进行,但国内大多厂家不具备建立试验台架的条件。本文介绍以一定比例缩小摩擦片实物尺寸,应用MM-15型多片式摩擦片离合器试验箱,模拟工况条件,进行摩擦片的摩擦系数、磨损率、滑磨功、滑磨功率及能量负荷许用值测试和计算,可以取得台架试验同样满意的试验结果。     

Zn基钎料钎焊烧结NdFeB与DP1180钢接头微观组织及性能

采用Zn-5Sn-2Cu-1.5Bi(ZSCB)钎料实现了烧结NdFeB永磁材料(NdFeB)与DP1180钢的钎焊连接。在惰性气氛控制的高频感应炉中进行钎焊,采用OM、SEM、EDS、微区XRD和NIM-2000H磁性测试仪等手段分析了接头界面的微观组织结构、NdFeB的磁性能和接头剪切强度。结果表明,NdFeB与ZSCB钎料形成Nd-Fe-Zn和Fe-Zn冶金结合,FeZn13和Fe3Zn10相在DP1180钢侧的界面处形成。焊接温度对NdFeB的磁性能影响较小。与传统方法的粘接相比,接头的剪切强度从32.50MPa提高到44.00MPa,提高了35.38%。由于NdFeB和ZSCB钎料之间的热膨胀系数差异很大,在接近NdFeB的反应层处产生较高的残余应力,导致接头从NdFeB界面处断裂。     

合金元素对(NdPr)_6Fe_(79)B_(15)非晶厚带的晶化行为及磁性能影响

利用熔体快淬法制备了(Nd Pr)6Fe79B15和(Nd Pr,Dy)6Fe74.5Co3Cu0.5Zr1B15非晶带。通过X射线衍射(XRD)和差热分析(DSC),并借助Kempen模型和Kissinger方程,研究了合金的非晶晶化过程及非等温晶化动力学。结果表明,与(Nd Pr)6Fe79B15合金相比,(Nd Pr,Dy)6Fe74.5Co3Cu0.5Zr1B15合金的非晶形成能力明显提高,在9 m/s的辊速下获得了厚度为100μm以上的非晶厚带。2种合金的非晶厚带具有不同的晶化过程及晶化动力学机制。(Nd Pr)6Fe79B15合金的晶化分4步完成:非晶相(A)→Nd2Fe23B3+A’→α-Fe+Fe3B+Nd2Fe23B3’→α-Fe+Fe3B+Nd2Fe14B→α-Fe+Fe3B+Nd2Fe14B+Nd1Fe4B4;而(Nd Pr,Dy)6Fe74.5Co3-Cu0.5Zr1B15合金的晶化分两步完成:非晶相(A)→Fe3B+A’→α-Fe+Fe3B+Nd2Fe14B。与(Nd Pr)6Fe79B15合金由界面控制的多晶型晶化不同,(Nd Pr,Dy)6Fe74.5Co3Cu0.5Zr1B15合金第1步为界面控制的多晶型晶化,第2步则以扩散控制的共晶型晶化为主。由于退火后组织结构的细化和改善,(Nd Pr,Dy)6Fe74.5Co3Cu0.5Zr1B15合金带的磁性能明显优于(Nd Pr)6Fe79B15合金带。     

Stress analysis of nano porous material using computed tomography images

Sintered silver materials possess an advantage that allows excellent bonding between chips and their substrate. Stress manifests as the materials resistance against external and internal forces, however during stress analysis the complex physical behavior caused by the forces and the nature of the material is often disregarded by researchers. This paper analyses the stress behavior of nano porous sintered silver in X and Y directions using finite element modelling. This study also focuses on observing the growth of the crack in samples in different directions. It is found that the occurrence of the cracks in the specimens starts at the peak stress point and the failure is observed at the largest void. It is also detected that the stress across the whole area of the model is regularly distributed in those samples which have relatively smaller voids. Thus, these samples have a better material integrity. In addition, this fact indicates that the material in these sample could hold higher levels of stress. Finally, the level of the stress and the possibility of failure in those samples that have smaller voids is found to be less than others.     

Effect of average grain size and sintered relative density on the imaginary part – µ″ of the complex magnetic permeability of (Cu0.12Ni0.23Zn0.65)Fe2O4 system

The influence of the sintered microstructure on the electromagnetic properties of Cu-doped NiZn ferrites were investigated. Two of the main variables of the thermal cycle have been modified: the maximum sintering temperature and the dwell time at that temperature, or sintering time. The evolution of the imaginary part – µ″ of the complex magnetic permeability was studied as a function of relative density, grain size and amplitude of the grain size distribution of the sintered pieces. The results show that µ″ depends on the sintered microstructure and that there is a limiting value of the average grain size (~20–25 µm) from which the electromagnetic properties of these kinds of materials worsened significantly.     

Improving pore morphology of PM 316L stainless steels by prealloyed powder prepassivation in 20% nitric acid

Abstract

The effect of prepassivation of prealloyed powder of 316L stainless steels on pore morphology and powder particle shape was investigated. Image analysis technique was used to study the effect of prepassivation, compaction and sintering temperature on the pore morphology of powder metallurgy 316L stainless steels. Porosity, dimension and morphology of the pores were characterised by means of four basic parameters: fraction of surface porosity, equivalent circle diameter, shape factor and elongation factor. In addition, SEM macrographs of powder particles were also investigated by applying the image analysis technique. The Feret’s average diameter and elongation factor were employed to describe the size and roundness of powder particles respectively. Annealing treatment reduced the equivalent circle diameter of pores and simultaneously improved f_shape and f_elong towards higher values. It was proposed that the prepassivation treatment reduced irregularity of powder particles through elimination of sharp corners of powder particles by exposure to acid environment.     

Effect of niobium on thermal stability and impact toughness of Nd-Fe-B magnets with ultra-high intrinsic coercivity

The effects of Nb on the thermal stability and impact toughness of ultra-high intrinsic coercivity of Nd-Fe-B magnets were investigated.The results showed that the addition of Nb could improve the thermal stability,and obviously increased the impact toughness of sintered Nd-Fe-B magnets.The optimum thermal stability of sintered Nd-Fe-B magnets was obtained when the content of Nb was 1.0 at.%.The maximum impact toughness of sintered Nd-Fe-B magnets was obtained when the content of Nb was 1.5 at.%,but the magnetic properties of sintered Nd-Fe-B magnets drastically deteriorated when the content of Nb increased from 1.0 at.% to 1.5 at.%.The microstructure showed that overfull Nb addition made many ultra-fine grains get together,which led to the density of sintered Nd-Fe-B magnets decline and drastically deteriorated the magnetic properties of sintered Nd-Fe-B magnets.