| With the implementation of the "Made in China 2025"policy,the construction of a modern,secure,reliable,advanced,efficient,and reasonably distributed electrical network is the important guarantee for realizing China’s dream of being a strong industrialized country.As the key component of the high-voltage circuit breaker,the electrical contact plays the role of breaking the current.The electrical contact not only suffers from arc erosion and thermal cycling,but also withstands mechanical friction and wear caused by multiple insertion and removal during service.Once the electrical contact fails,the circuit cannot be disconnected normally,which may pose a serious threat to production safety.The number of breaks of electrical contacts in traditional transmission and transformation grids is low and it is difficult to meet the current demand for use.The trend of the circuit breaker’s development towards extra-high voltage,large capacity and miniaturization poses a more crucial requirement for the design and preparation of electrical contact materials.In this thesis,the starting point is to improve the mechanical and physical properties of tungsten-copper composites.First,based on the theory of electrical field emission,the first-principle calculation was used to select the best modifying alloy elements,followed by solid experimental verification.Then,on the basis of the home-made instrument,the novel powder extrusion formability of tungsten-copper composites was tested.The composition of the binders and feeds,printing parameters,printing strategy,degreasing and sintering procedures of the powder extrusion technology are determined.Also,the relative density,hardness and electrical conductivity,as well as microstructure and strength and so on,were analyzed and evaluated.The following conclusions have been made:(1)The element which effectively balance the arc erosion resistance of both tungsten and copper phases has been determined to be Nb by the first-principle calculation,which has also been verified by traditional powder metallurgy preparation.The obtained W-30 wt%Cu-3 wt%Nb exhibited high compressive strength of 1024 MPa and plasticity of 8.6%,and also excellent arc erosion resistance.EBSD and DFT techniques revealed that the improved mechanical property can be ascribed to the gradient loading transition and strain mediation among various length-scaled grains,and additionally,the homogenously precipitated fcc-Nb with the lowest work function value would breakdown as the first phase,effectively dissipated the arc energy.(2)Based on the home-made Reeyee-Choc Desktop 3D metal printer,the powder extrusion process of W-Cu composites was explored.The best parameters are determined to be 150 degree of the nozzle heating temperature,45 degree of the filling strategy,65 wt.%PW-5 wt.%HDPE-25 wt.%SA-5 wt.%EVA of the binder composition,3 wt.%binders and 97 wt.%powders for feeds composition,with two-step degreasing procedures.The optimal obtained composite exhibited a relative density of 92.01%,hardness of 185 HB,electrical conductivity of 31.03%IACS,as well as horizontal strength of 782.6 MPa and vertical strength of 1100.2MPa.(3)Based on the home-made Reeyee-Choc Desktop 3D metal printer,the powder extrusion process of W-Cu-Nb composites was also explored.The binder composition of 65 wt.%PW-5 wt.%HDPE-25 wt.%SA-5 wt.%EVA,as well as 3 wt.%binders and 97 wt.%powders for feeds composition,contributed the best formability of the composite.The obtained material by powder extrusion exhibited a relative density of 85.39%,hardness of 204HB,as well as horizontal strength of 462.5 MPa and vertical strength of 794.4 MPa.Finally,based on the home-made Reeyee-Choc Desktop 3D metal printer,complex shaped 3D metal products were successfully made,which can be directly utilized as the final products after sintering. |
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