The Short-process Preparation Of Ultra-fine-grained WC-Co Composite Powder And The Corrosion Behavior Of Cemented Carbide

WC-Co cemented carbides are widely used in aerospace,engineering machinery,transportation,and other industries owing to the combination of excellent hardness and toughness.The manufacturing of precision instruments and equipment are directly affected by the quality and stability of cemented carbides.There are some shortcomings in the traditional preparation process of WC-Co composite powder and cemented carbide,such as long process and accumulation of defects.In addition,with the continuous expansion of the application field,the corrosion resistance of the alloy is required to be higher.According to the situation mentioned above,the theis starts with the design idea of"shortening the preparation process"and"enhancing the corrosion resistance performance".A short process with only three steps including spray conversion,calcination and low temperature in-situ synthesis has been explored to prepare ultrafine grained WC-Co composite powder with uniform microstructure.The new method can effectively avoid the issues of uneven distribution caused by solid/solid mixing,and of grain growth caused by high carbonization temperature.Then,the effects of different additives and content on microstructure and corrosion resistance of the alloy were investigated using composite powder as the key material.The effects of process parameters on the morphology,composition,particle size distribution,grain size and crystal form of WC of composite powders were investigated.The reaction process and mechanism of low temperature synthesis were analyzed through thermodynamic calculation combined with experiments.The results show that ultrafine WC-Co composite powder with uniform grain and stable composition can be successfully prepared by the short process.Firstly,the morphology of amorphous precursor powder prepared by spray transformation of W,Co,C mixed solution is hollow spherical structure.Secondly,WO₃ and Co₃O₄ oxides could be formed by calcination.The particle size decreases,and many pores are formed on the surface.Moreover,the spherical powder can be broken into dispersed oxide powder by adding a ball milling process for 4 h.Thirdly,WC-Co composite powder could be prepared through the low temperature in situ synthesis.Pure ultrafine WC-Co composite powder can be obtained when the temperature is 900°C,which is much lower than the traditional carbonization temperature（about 1300°C）.Abnormal growth of WC could be inhibited effectively with the lower temperature.In addition,the grain size of WC increases with the increase of reaction temperature,holding time and the decrease of H₂ flow rate.The shape of WC evolves from regular circle to regular polygonal shape gradually with the increase of temperature and the extension of holding time.A new way of carbonization proces“C+W+Co→C+Co₃W→intermediate phases（Co₆W₆C and so on）→WC-Co”has been deduced by combining the thermodynamic and experimental results.WC-6Co cemented carbide was prepared by Spark Plasma Sintering（SPS）using WC-6Co composite powders as raw materials.The effects of type,content of additives,and SPS parameters on the microstructure,mechanical properties and corrosion resistance of the alloys were investigated.The results show that there are many pores in the sintered cemented carbides prepared from hollow spherical composite powder,and the density is only 14.21 g/cm³.Using WC-6Co composite powders generated by ball milling of oxides and low temperature in situ synthesis process as raw materials for the preparation.The alloy has greater density（14.77 g/cm³）than that of the alloys mentioned above.Subsequently,compared with the alloy prepared by traditional WC+Co mixture,the density is almost the same but the microstructure of former is more uniform,the hardness is higher too.The addition of Y₂O₃,Mo and Mo₂C can refine the WC grain size,improve the Hv of alloys.The enhanced effects of Mo are much better than that of other additives in grain refinement and lifting performance.Although Cu can refine WC grain,the density decreases due to the poor wettability of Cu to WC grain.Moreover,the grain size of WC decrease with the increase of Mo content（0⁴ wt.%）,and the crystal of WC grains gradually changes from triangular and quadrilateral to nearly round.The Hv and wear resistance of alloy increase too.However,a large of number of pores appear in the alloy when Mo content is more than 2%.The relative density of the alloy is reduced to 97.74%,and Hv and fracture toughness decrease obviously.According to the comprehensive analysis,when the content of Mo is 1%,the grain size of WC is relatively fine.Hardness and wear resistance are improved.Meanwhile,fracture toughness and relative density are close to those of the non added alloy.The addition of Y₂O₃,Mo and Mo₂C can enhance the corrosion resistance of alloy in HCl solution,then Cu plays the opposite role.The addition of Y₂O₃,Mo,Mo₂C and Cu can enhance the corrosion resistance of alloy in NaOH solution.The enhanced effects of Mo are much better than that of other additives both in HCl and NaOH solutions.With the increase of Mo content,the corrosion resistance is enhanced.However,the corrosion resistance decreases sharply when the content of Mo exceeds 2%owing to the decrease of the relative density.The action process and mechanism of Mo on the corrosion resistance of the reinforced alloys in different solutions are different.In HCl solution,firstly,Co binder phase is enhanced owing to the dissolution of Mo.Secondly,Mo is oxidized to form stable MoO₃,which adheres to the surface to segregate the alloy and solution,and inhibits the conduction of electrons.However,MoO₃ will be transformed into HMoO₄^-and MoO₄^2-,which can dissolve in NaOH solution easily.Therefore,in NaOH solution,the number of WC/Co grain boundaries increases evidently due to the refinement of Mo.In the early stage of corrosion process,WC/Co grain boundaries tend to corrode and form stable Co（OH）₂,which adheres to the surface to segregate the alloy and solution,and inhibits the conduction of electrons.Thus the corrosion resistance of the alloy is enhanced.The proposed method for synthesising WC-Co composite powders has the advantages of less process and low synthesis temperature.The grain size of composite powder is ultrafine and uniform.It has a series of integrated innovation,which can replace the traditional production process to achieve high efficiency and high quality preparation of ultrafine WC-Co composite powder.The microstructure of fine grained WC-6Co cemented carbide prepared by somposite powder is homogeneous.The microstructure and mechanical properties of the alloy can be effectively improved by adding 1%Mo,and the corrosion resistance of alloy could be enhanced both in acidic and alkaline solutions.