| Owing to its excellent properties such as high melting point,high boiling point and low thermal expansion coefficient,tungsten plays an important role in many technology fields such as national defense,atomic energy industry,electric vacuum,and electric light sources.However,the inherent drawbacks of W,high hardness and room temperature brittleness,hinders its shaping to complex shape.Traditional methods are suffered problems such as high cost,complicated process,poor yield and consistency,restricting the application of tungsten.Powder injection molding is a powder near-net-shaping technology that introduces plastic injection molding technology into powder metallurgy,and it is very suitable for three-dimensional complex shape products and provides a new way for net shape fabrication of difficult-to-machine materials.In this paper,we combined the pretreatment of tungsten powders with injection molding,the invention solves the difficulties in fabrication of tungsten products with complex shapes,and realizes precise control of microstructure and properties.The study mainly includes the following aspects:(1)Complex shape tungsten parts with high density and high performance have been prepared by metal injection molding using the powder treated by jet mill,solving the problem of densification of injection molded tungsten parts.The treatment of tungsten powders was carried out by jet milling process,and particle size and morphology of tungsten powder have been optimized.Jet milling treatment caused deagglomeration of tungsten powder,leading to narrow particle size distribution,large specific surface area,high packing and tap density,and monodisperse tungsten powder with narrow particle size distribution and near spherical shape has been obtained.For tungsten powder with particle size of 3 ?m FSSS,with the disappearance of agglomeration and the improvement of dispersion,the values of d10,d50 and d90 change from 1.89?m,3.29 ?m and 5.58 ?m of the as-received tungsten powder to 1.36?m,2.13?m and 3.19?m for the treated tungsten powder,respectively.The specific surface area increase from 1.56 m2/g to 2.64 m2/g,and the process increases apparent density and tap density from 3.57 g/cm3,4.64 g/cm3 to 5.32 g/cm3,8.73 g/cm3.Thus,the critical powder loading of the feedstock increase from 54 vol%of the as-received powder to 65 vol%of the treated powder.After sintered at 1900 ?,the tungsten samples obtain 97.3%relative density and a micro-hardness of 496 Hvo.i,achieving low-temperature sintering densification of injection molded tungsten parts.(2)Fine-grained spherical tungsten powder with narrow particle size distribution has been fabricated by plasma spheroidization in combination with jet mill treatment,solving the problems of non-uniform particle size distribution and low yield of direct plasma spheroidization of commercially available tungsten powder.Due to the agglomeration of commercially available powder,fine-grained or narrow distribution spherical tungsten powder cannot be obtained by directly spheroidization of tungsten powders with particle size of 0.2 ?m,3 ?m,5 ?m,and 10 ?m FSSS.Complete dispersion,effective classification and favorable shape modification can be achieved by jet milling process.Accordingly,monodisperse tungsten powders with narrow particle size distribution have been obtained.For tungsten powder with particle size of 10 ?m FSSS,the particle size distribution becomes narrow,and the particle size of powder is uniform after jet mill treatment.The improved dispersion and modifided morphology of the particles enhances the flowability of the powder,helping to ensure the stability of the process and yield of the powder during the spheroidizing process.Thus,fine-grained spherical tungsten powder with particle size from 6 ?m to 11 ?m,a good sphericity,and a spheroidization rate of close to 100%has been fabricated by plasma spheroidization.(3)The porous tungsten with complex shape has been fabricated by metal injection molding in combination of jet mill treatment,and the pore size and distribution are uniform.The as-received and jet milled tungsten powders with particle size of 5 ?m FSSS were used as raw materials,and the critical powder loading can be increase from 54 vol%of the as-received powder to 60 vol%of jet milled tungsten powder.After sintered at 1800 ?,the open porosity of porous tungsten with jet milled powder is 33.11%,and the average pore size is 1150.7 nm.Due to the good dispersion and narrow particle size distribution of jet milled powder,the surface and internal pores of porous tungsten are evenly distributed,and the grain size and pore size distribution is uniform.With the increase of temperature to 1900 and 2000 ?,the formation of the oversintering areas affects the uniformity of pore distribution.(4)A process for preparing tungsten powder with smooth surface,good dispersibility,high purity and narrow particle size distribution has been developed,and further fabricating uniform porous tungsten.Tungsten powder with particle size of 5?m FSSS was used as raw materials,through jet mill and multi-step annealing treatment at 800?1200 0C,tungsten powder particles remain dispersed which indicated the activity is reduced,the purity is improved,and the particle shape becomes regular.The open porosity of porous tungsten using the treated powder is 31.02%after sintered at 1900?,and the specific surface is 0.071 m2/g.The sintered porous tungsten matrix shows fully interconnected nearly spherical pores with homogeneous pore distribution,and the pore size distribution is uniform.Microhardness measurements and the concept of homogeneity index(HI)are used to evaluate the uniformity of porous structure,the homogeneity index of the porous tungsten powder using the treated powder is 1.7,which is smaller than 8.6 of the porous tungsten using the as-received powder,indicating a more uniformly distributed porous structure.(5)A design method for porous tungsten with gradient porosity is proposed.Monodisperse tungsten powders with different median particle size and narrow particle size distribution were prepared by jet mill tungsten powder with particle size of 8?m FSSS.Three kinds of W powders with different particle size were stacked layer by layer utilizing hot pressing sintering.After sintering at 1600?and pressure of 30 MPa,the open porosity of the gradient porous tungsten is 30.55%,and the specific surface is 0.038 m2/g.The pore size and distribution are uniform,and all of them are inter-connected pores.Furthermore,the interface between the layers is well combined.Pore size distributions revealed that three different sizes pores exist simultaneously,with diameter of 1.5?m,2.5?m and 4.0?m,respectively. |
PDF Full Text Request