Study On Vacuum Sintering Of Bimodal Distribution Tungsten And Its Properties

In order to improve the sintering performance of nano metal materials and reduce its brittleness,a two-step method was used to prepare tungsten bulk material with nanocrystalline and microcrystalline bimodal distribution.The nano WO₃ powders has been successfully prepared via a sol-gel method by using ammonium tungstate as the tungsten source and citric acid as the control agent of crystal growth.The effects of pH value and reaction temperature on the phase and morphology of nano-WO₃ were researched.Mixing the pure nano-W powders prepared by a hydrogen reduction process with the industrial micro W powders by planetary ball mill,the bimodal crystalline distribution W bulk material was prepared by a vacuum hot-pressing sintering process.The phase composition,crystal structure,microstructure and microstructure of nano WO₃,nano-W powder and W bulk materials were tested by XRD,SEM and TEM.The density and microhardness of W bulk was tested by electronic balance and microhardness tester.The effects of sintering temperature,sintering time and sintering pressure on the sintering properties of W bulk samples were studied.The grain size and the grain growth law of pure W material with bimodal distribution was investigated.The results indicate that the average particle of the as-prepared nano-WO₃ powders by the sol-gel method of is 40-50 nm,when the pH=1,the reaction temperature is 70?and the calcination is 600?.The as-prepared nano-WO₃ powders was reduced using hydrogen two temperature ranges of 500?and 800?,respectively,and the pure nano-W powders with the spherical particle was obtained,and the average particle size is about 70 nm.The different proportions of nano-W powders and industrial micron W powders were mixed using a planetary ball mill,the ratio of ball to material was 5:1,the speed was 100r/min and the milling time was 30 min,the bimodal distribution W powders was obtained.The sintering time was 120 min,the sintering pressure was 150 MPa,and the proportion between micron and nano W powders was 30%.With the sintering temperature increasing from 1100? to 1400?,the relative density of the bimodal distribution W bulk increases from 80% to 93%,the average grain size increases from 400 nm to 930 nm,and the microhardness of the W bulk increases from 187HV500 to 412HV500;When the sintering temperature is 1300?,the sintering time is 120 min,the mixing ratio of micro/nano W powders is 30%.With the sintering pressure increasing from 100 MPa to 250 MPa,the relative density of W bulk material with bimodal grain size distribution increases from 78% to 95%,the average grain size increased from 400 nm to 1.10?m,the microhardness of the W bulk material increased from 213HV500 to 503HV500;When the sintering temperature is 1300?,the sintering pressure is 200 MPa and the sintering time is 150 min,The average grain size of the bimodal distribution W bulk material is 854 nm,the maximum grain size is 6.10?m,the minimum is 210 nm,the relative density is 95%,and the microhardness of the W bulk material is 445HV500.Hence,this process parameters is optimum.At the same time,for the purpose of comparative analysis,the pure nano-W powders prepared by the sol-gel method was vacuum hot-pressed under the same sintering process parameters,and the prepared W bulk material has a uniform grain size which is 2.5-3?m,which is a micro-grain W bulk material.Therefore,adding some micron W powders into nano-W powders can significantly inhibit the nano-W powder grain growth in the hot-pressing sintering process.Calculated by fitting,the diffusion activation energy of the bimodal distributed W-powders is 145.495kJ/mol.Which is much smaller than the industry micron W powders with the activation energy of 587.9 kJ/ mol,the grain is easy to grow up.At the same time,the diffusion activation energy is slightly larger than the pure nanocrystals W-powders of 134 kJ/mol,due to the distribution of large particles in the bimodal distribution,which helps to improve the atomic diffusion energy barrier and prevent the growth of the nano grain.The microhardness of the bimodal distribution W bulk material and micro-grain W bulk material were measured at different temperatures.The obvious cracks was observed on the surface of the bimodal distributed W bulk at 200?,but there was no obvious crack at 250?,350?and 425?indentation respectively.It indicats that the ductile to brittle transition?DBTT?of bimodal distribution W bulk material occurs between 200?and 250?.The micro-grain W-bulk has a microhardness of 428HV500 at 350?,a small crack was found around the microhardness indentation impression.At 425?,the microhardness is 405HV500,and the crack disappears.The micro-grain W bulk material undergoes a brittle transition in this temperature range.Therefore,the bimodal grain distribution can reduce the pure W plastic brittle transition temperature compared with the micro-grain W bulk material.