氢爆工艺(HD)对钕铁硼磁粉粒度和形状的影响研究

采用热处理态钕铁硼平板铸锭作为母合金,通过氢爆工艺将平板铸锭破碎成磁粉.研究了氢爆工艺过程中温度、氢压对钕铁硼磁粉的破碎程度、粒度和形状的影响.实验发现,随着温度和氢压的提高,吸氢过程活化时间逐渐缩短,当温度为300℃,氢压为0.1 MPa时,活化过程消失,铸锭一旦暴露在氢气中,吸氢过程直接发生.热处理态平板铸锭吸氢之...

Hydrogen Decrepitation on Particle Size and Shape of Nd-Fe-B Magnetic Powders

Hydrogen decrepitation(HD),as a useful method of grinding metallic powders,had played an important role in produc-ing sintered Nd-Fe-B magnet and anisotropic Nd-Fe-B bonded magnetic powders.The HD process parameters including pressure,tem-perature and time significantly affected the crush effect,particle size and shape of the prepared magnetic powders,which would deter-mine the difficulty of the rotation of the powders,i.e.the degree of alignment of the magnet.Thus,the influence of HD process parame-ters on the crush effect,particle size and shape of the obtained magnetic powders were systematically investigated.The plate ingotswith the thickness of~10 mm were prepared using smelting furnace with nominal composition of Nd_12.5Fe_bal.B_6.4Ga_0.3Nb_0.2(%,atom frac-tion).The ingtos were homogenized for 10~20 h at 1050~1100℃to remove the formedα-Fe phase during alloy smelting.The homoge-nized ingots were mechanically crushed into small blocks with the longest dimension<10 mm,placed in the steel box inside the tubefurnace with the vacuum<1×10^-3 Pa.Then the ingot blocks were HD treated at room temperature,100,200,300℃and hydrogenpressure of 0.01~0.18 MPa,respectively.When the temperature reached the target value,the hydrogen was filled to the target pres-sure.After a period of time,the pressure started to decrease,corresponding to the occurrence of the hydrogen absorption of the alloyblocks.The time was recorded,known as activation time.The hydrogen was continuously charged to maintain a constant pressure.TheHD treatment was conducted for 1 h to make sure that the reaction was complete.Afterwards,the hydrogen was removed and theblocks with cracks inside were obtained as cooling down to the room temperature.The blocks were grinded easily into powders in a mor-tar.The powders were divided into coarse powders and fine powders through 178μm sieve and weighed.The particle size distribution(PSD)of the fine powders were measured by the laser particle size analyzer,the shape was observed by the scanning electron micro-scope(SEM),the phase was measured by X-ray diffractometer(XRD).The activation time of HD process was gradually shortenedwith the increase of the temperature and hydrogen pressure.Under the hydrogen pressure of 0.1 MPa at 300℃,the activation processdisappeared and HD process occurred directly once the ingots were exposed at hydrogen.After absorbing the hydrogen,the homoge-nized plate ingots crushed into fine magnetic powders,more than 90%(%,mass fraction)below 178μm.With the increase of pres-sure,the weight fraction of the obtained fine powders was increasing monotonously,which indicates that the increase of hydrogen pres-sure benefits to the hydrogen absorption,volume expansion and fracture of the Nd-rich phase and the crushing effect from small blocksto powders.The particle size distribution did not change much as changing hydrogen pressure.However,at fixed pressure,from roomtemperature to 300℃,with increasing the temperature,the average particle size of the powders became coarser obviously.For exam-ple,the D50 value is changing from~50 to~100μm,as the temperature is increased from the room temperature to 300℃.The mag-netic powder morphology was mostly granular,corresponding to the inter-granular fracture between Nd-rich phase and Nd₂Fe₁₄B phase.A few powders exhibited irregular shapes such as strips and sharp corners,which might be related to the intra-granular fracture of Nd₂Fe₁₄B phase.With increasing temperature,the diffraction peak position of Nd₂Fe₁₄B phase of HD powders after desorption shifted tohigher angle.This could be attributed to that higher temperature is beneficial for the removal of hydrogen,resulting in the decrease ofresidual hydrogen content.With varied pressure,the peak position almost did not change.The influence of process parameters includ-ing temperature and hydrogen pressure on the crush effect,particle size and shape was systematically investigated.The activation timeof HD process decreased tendentiously with the increase of temperature and hydrogen pressure.After absorbing the hydrogen,the ho-mogenized plate ingots almost crushed into fine magnetic powders below 178μm.With the increase of the pressure,the weight fractionof the obtained fine powders was increasing monotonously.The particle size distribution nearly did not change as changing hydrogenpressure.With increasing the temperature,the average particle size of powders became coarser obviously.The magnetic powder mor-phology was mostly granular and a few powders exhibited irregular shapes such as strips and sharp corners,which might be related tointer-granular and intra-granular fracture of Nd₂Fe₁₄B phase,respectively.With increasing temperature,the diffraction peak positionof Nd₂Fe₁₄B phase of HD powders after desorption shifted to higher angle.