| Tungsten copper alloy is one of suitable packaging materials for high-power integrated circuits because of its high thermal conductivi ty and low coefficient of thermal expansion.With the high integration of integrated circuits,the shell is designed smaller and more complex.The traditional preparation method of such W-Cu parts is machining,which has the disadvantages of high processing cost,low material utilization,and thus advanced manufacturing techniques are urgently required.The fabrication of complex W-Cu parts by Powder Injection Molding（PIM）using ultrafined W-Cu composite powders as raw material is expected achieve the purpose of good performance and low cost.However,the ultra-fined W-Cu composite powders have the characteristics of small particle size,serious agglomeration and low tap density,leading to low powder loading of feedstock and poor rheological property,and thus the dimensional accuracy of PIM products is too low to meet the demands of practic al application.Under the premise of high sintering activity of powders,how to enhance the tap density of ultra-fine W-Cu powders,powder loading and rheological properties of feedstock has become a vital issue,and further improvement in the dimensional accuracy of PIM products is significantly important for the practical application of W-Cu parts prepared by PIM technology.In order to address the issues as mentioned above,four aspects including the powder preparation,powder modification,binder formula and PIM process were investigated in present work.The W-10Cu,W-20Cu and W-30Cu tungsten copper composite powders were prepared by mixing method,electroless plating method and spray drying method respectively,and then the tap density and sintering density of powders were selected as experimental indexes,the Response Surface Methodology（RSM）was used to carry out the experimental design and analysis,the function model between the key influencing factors and experimental indexes in the process of powder preparation was established,and the reliability of the model was verified by variance analysis and significance test,and the optimal solution of the regression model is obtained.The relative tap densities of the powders prepared by mixing method are 37.5%,37.1%and 43.0%respectively,and the relative sintered densities are 94.5%,98.1%and 98.5%respectively;The relative tap densities of the powders prepared by electroless plating method are25.8%,32.0%and 37.8%respectively,and the relative sintered densities are98.2%,98.0%and 98.9%respectively;The relative tap densities of the powders prepared by spray drying method are 27.8%,25.8%and 22.5%respectively,and the relative sintered densities are 99.0%,98.1%and 99.2%respectively.In order to further improve the tap density of the powders,the W-Cu powders prepared by electroless plating and spray drying were modified by Jet Milling（JM）method.Meanwhile,a new powder modification method of"Bonding Treatment and Ball Milling"（BTBM）was proposed,which was used to modify tungsten copper powder s prepared by mixing method and spray drying method.The optimum process parameters of powders modification were determined by single factor experiment.The influence mechanisms of the modification on the characteristics of powders were revealed through the analysis of the micro morphology.JM was used to eliminate the aggregates in the powder by strong impact force through h igh-speed collision and friction between powder particles,and smooth the sharp edges and corners on the particle surface,which plays a role in eliminating aggregates and modifying the powder morphology;BTBM was used to change the micro morphology of the powder from seriously agglomerated loose small particles to closely arranged pebble like larger particles through high-temperature bonding,crushing and ball milling steps,thus eliminating the gap between the powders and changing the particle size and distribution of the powder.JM and BTBM modification retained the sintering activity of the original powder and improved the tap density of the powders.Compared with the original powders,the tap density of JM powders were increased by 23～70%,and that of BTBM powders were increased by 25～91%.In order to improve the rheological properties of W-Cu feedstock,six wax based binder formulations were designed.The optimized binder formula containing 60%PW+20%PP+10%HDPE+5%PS+5%SA was obtained by calculating solubility parameters,thermodynamic compatibility parameters and feed composite index.Feedstock with 56 vol.%was obtained by mixing the W-20Cu powders（prepared by mixing method and modified by BTBM）and the optimized binder,the flow index n is 0.319,the viscous flow activation energy E is 50.68 k J·mol-1,and the comprehensive index is 4.2 m2/NSK under the test conditions of 160℃and shear rate of 1000s-1.Solvent debinding combined with thermal debinding techniques were used,and the reasonable process parameters were designed according to the solvent debinding rate curve and the thermogravimetric curve of binder.After debinding,the brown parts with good appearance and less residual carbon was obtained,and its carbon content was about 0.005～0.008 wt.%,and then the brown parts were sintered in hydrogen to obtain PIM parts with good appearance,excellent physical properties and high dimensional accuracy.The influences of powders preparation method on PIM product dimension accuracy,microstructure and physical properties were revealed.It was found that the single weight fluctuation of green parts,dimension fluctuation of sintered parts,microstructure uniformity,sintered density,thermal conductivity and conductivity of the samples prepared by spray drying method were the best,followed by electroless plating and the last is mixing method.After JM and BTBM modification,the critical powder loading of the feedstock was improved,the single weight of the green parts were more stable,the sintering shrinkage of the product was reduced,and the dimensional accuracy of sintered parts were improved.For the W-10Cu,W-20Cu and W-30Cu samples prepared by the BTBM modified spray drying powders,the best physical properties and dimensional stability were obtained,and the corresponding relative densities were 99.5%,98.6%and 99.3%respectively,the conductivity were 36.8%IACS,45.13%IACS and 53.48%IACS respectively,the thermal conductivity are 213 W/m·K,233 W/m·K and 248 W/m·K respectively,the thermal expansion coefficients were 6.80×10-6/K,8.43×10-6/K,and 9.18×10-6/K respectively,the variation coefficients of the length direction size were 0.22%,0.21%and 0.22%.In summary,the effects of powders preparation techniques and modification methods on powders characteristics,critical powders loading of feedstock,single weight stability of green parts,dimensional stability of sintered samples,and physical properties of materials were studied in this paper.A new wax base binder was designed and optimized.The W-Cu parts with excellent physical properties,complex shape and high dimension accuracy were successfully prepared by PIM technology,which provides t heoretical and technical support for the production of W-Cu parts with complex shape by PIM. |
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