Study On Intermittent Electrodeposition Preparation Of W-based Core-Shell Powder And Its Densification

W has high melting point,high hardness,high thermal conductivity,low thermal expansion coefficient,low sputtering rate,and no reaction with H,which is widely used in electronics,military,aerospace and other fields.For example,W-Cu alloys are mainly used in electrical materials,heat sink materials in electronic packaging and high thermal load components in nuclear field,etc.W-Ni-Fe heavy alloys are used in counter-weights,armor-piercing projectile cores,and radiation shields.In tungsten matrix composites,the differences of density and melting point between components are very large,which lead to the failure of obtaining uniform and high-performance composite by powder metallurgy.Thus,the powder mixing uniformity is a critical tissue to be solved.This paper proposed a low-cost,simple process,mass production of powder coating technology with controllable coating content and caldding rate for the preparation of core-shell powders.The core-shell powders have been formed by powder metallurgy and cold spraying to obtain W-Cu composite materials with excellent thermal properties,W-Cu composite coatings with high W content and non-deformable gradient W-Ni-Fe heavy alloys.Meanwhile,the sintering and cold spraying densification behaviors of core-shell powders were studied.Taking the preparation of W@Cu core-shell powders as an example,the new coating technology proposed in this paper was described.a sulfate system was selected as the electroplating solution without any additives.The optimal current density of the system was 7 A/dm².Combining with Faraday’s law,the composition design of core-shell powders was carried out by controlling the electrodeposition time.Furthermore,the cladding rate of the core-shell powders was regulated by controlling the pulse width.Finally,a core-shell powders with suitable composition ratio and high cladding rate were obtained.The cladding layer had a compact structure and no impurities.The morphology of W@Cu core-shell powders was rough and "cauliflower-like".The core-shell powders obtained after 40 minutes of electrodeposition of W powders（with the original particle size of 28 μm）had an average thickness of 2.8 μm,cladding rate of 92.7%,oxygen content of 527 ppm and better fluidity of 15s（50g）.Meanwhile,the formation processes of cladding layer of the core-shell powders were clarified.Finally,the relationship between the thickness（content）of the cladding layer and the load capacity,current intensity and electrodeposition time was proposed,which can provide theoretical guidance for the selection of electrodeposition parameters in the electrodeposition amplification process.On this basis,electrodeposition amplification device was designed independently.The experimental values were consistent with the theoretically predicted value of coating thickness.W@Ni,W@Ni@Cu and W@Fe core-shell powders were also prepared by intermittent electrodeposition.The W@Cu core-shell powders obtained after electrodeposition were densified by park plasma sintering and cold spraying.Studies showed that core-shell powder is beneficial to the uniformity of the two phases in the final product.Compared with other studies,the composite materials had excellent thermal conductivity and matching coefficient of thermal expansion.Thermal conductivity value was 241 W/m/k at 25℃,209 W/m/k at 100℃,and 175 W/m/k at 600℃.The excellent thermal conductivity benefits from the formation of copper-network structure in composites.By designing the composition of the core-shell powder coatings and combining with the subsequent heat treatment,the cold sprayed coatings avoided the peeling of W particles,showing the high retention rate of W（the value was 98.4%）and reduced body porosity（the value was 1%）in the final coatings.W@NiFe core-shell powders were prepared by intermittent electrodeposition.Ni:Fe ratio was 7:3 by controlling current density.Cladding rate was controlled by pulse width to avoid block deformation.By optimizing the sintering temperature and holding time,results showed that liquid-phase sintering at a temperature of 1470℃ and holding for 0.5 h can obtain W-Ni-Fe heavy alloys with gradient property and slightly deformation.The bending strength and hardness of the W-Ni-Fe heavy alloys reached their maximum values,about 1245 MPa and 340 HV_0.2.