Study On Preparation Of Nd-Fe-B Powders For Sintered Magnet By HDDR-G Technology

Sintered Nd-Fe-B magnets are widely used in the fields of hybrid/electric vehicles,wind turbines and consumer electronics.The improvement of performance,especially the coercivity,is still the research emphasis of Nd-Fe-B magnets.The Nd-Fe-B powders for sintered Nd-Fe-B magnets are generally prepared by strip casting,hydrogen decrepitation and jet milling processes.However,the powders exhibit characteristics of large particle size,wide size distribution,irregular particle shape and poor Nd-rich phase coating on particle surface,which result in grain coarsening and uneven distribution of Nd-rich phases in sintered magnets,and thereby restrict the improvement of magnet performance.In this study,a new HDDR-G technology(HDDR and Growth)is designed and utilized to prepare Nd-Fe-B single-crystal powders with 2.0-3.0 ?m,narrow size distribution,regular particle shape and good Nd-rich phase coating on particle surface.The HDDR reactions and microstructure evolution of ternary Nd-Fe-B and multi-element Nd-Fe-B-M(M=Cu,Al,Ga,Co,Zr)strip cast alloys were studied.During hydrogen absorption process,hydrogen firstly reacts with the Nd-rich phases and enters into alloy interior along Nd-rich phase channels.Then disproportionation reaction starts from the surface of Nd-Fe-B strip cast alloy and the internal Nd-rich phase channels,finally propagates the entire Nd-Fe-B strip cast alloy.In HDDR process,not only the columnar grains are refined into Nd2Fe14B equiaxed grains,the Nd-rich phases are also refined and redistributed at triple junctions or along the grain boundaries.The growth behavior of HDDR Nd2Fe14B grains and the influence factors for grain growth were investigated.After recombination reaction,Nd2Fe14B grains grow up normally and then abnormal grain growth occurs.The increase of Nd content in HDDR Nd-Fe-B alloy is conducive to the formation of Nd-rich wetting grain boundaries,which promotes the grain growth due to the change of grain boundary migration from solid atom transition to liquid atom diffusion.The increase of temperature can greatly improve the grain growth rate,but it also shortens the incubation period for abnormal grain growth and make abnormal grains appear earlier.The Nd2Fe14B equiaxed grains with 2.0-3.0 ?m,narrow size distribution,regular shape and uniform distribution of Nd-rich phases were obtained by prolonging the incubation period for abnormal grain growth and increasing the grain growth rate.The growth mechanism of HDDR Nd2Fe14B grains was revealed and the control mechanism for abnormal grain growth was also clarified.In HDDR Nd-Fe-B alloy,the Nd2Fe14B grain that directly contacts with the large liquid triple junction forms smooth facet interface and develops into abnormal grain.The faceted Nd2Fe14B grain presents the maximum driving force ?gmax.When ?gmax is less than the critical driving force for abnormal grain growth ?gc,the faceted Nd2Fe14B grain will not grow abnormally.As ?gmax is greater than ?gc,the faceted Nd2Fe14B grain will grow into abnormal grain.The strategy for suppressing abnormal grain growth is adjusting Agmax and ?gc to make ?gmax lower than ?gc.Increasing temperature will reduce ?gc and shorten the incubation period for abnormal grain growth.The faceted Nd2Fe14B grains at higher temperature exhibits lower ?gc,when ?gmax is about to increase to ?gc,the rapid cooling treatment reduces ?gmax to below ?gc again,thereby prolongs the incubation period for abnormal grain growth and suppresses the abnormal grain growth.The HDDR-G Nd-Fe-B powders were prepared by hydrogen decrepitation and ball milling treatments.The Nd-Fe-B powders exhibit narrow size distribution,regular particle shape and good Nd-rich phase coating on particle surface.The d50 size,size distribution of d90/d10 for ternary Nd-Fe-B powders are 2.26?m,4.07 while the d50 size,size distribution of d90/d10 for multi-element Nd-Fe-B-M powders are 1.66 ?m,3.52.