Study On Formability And Surface Pressing Process Of Nanocrystalline Ni And Ni/ZrO₂ Foils Under Pulse Current

Compared with crystalline or amorphous materials,nanocrystalline materials have attracted the attention of researchers due to their unique properties.However,the room temperature plasticity of nanocrystalline materials is generally poor,which is reflected in high brittleness and low forming limit.As a method to significantly improve the forming ability of materials,superplastic forming is undoubtedly the best choice.However,under the condition of high temperature of 0.5 Tm,the growth of nanocrystalline grains is bound to occur,and the long-term high temperature environment will inevitably lead to the oxidation of the surface of the material,which will have an adverse effect on the forming of nanocrystalline materials.Therefore,this paper proposes to introduce high-density pulse current into the plastic forming process of nanocrystalline materials,and study the plastic forming of nanocrystalline Ni and Ni/Zr O₂ composite foils assisted by pulse current under the condition of less than 0.5 Tm.Nanocrystalline Ni and Ni/Zr O₂ composite foils were prepared by electrodeposition.In order to explore the effect of pulse current on the formability of nanocrystalline foil and provide process parameters for subsequent pressing forming tests,tensile tests were carried out at different temperatures,different strain rates and different peak current densities.Comparing the test results,it was found that the foil had good tensile properties at a temperature of 350°C and a strain rate of 7×10^-4 s^-1.Under this condition,the pulse current assisted tensile test was carried out.It was found that when the peak current density reached 500 A/mm²,the elongation of the Ni/Zr O₂ composite foil reached 12.3%,showing good plastic deformation ability.It is also found that the addition of Zr O₂ can significantly improve the tensile deformation properties of the material.The SEM analysis of the fracture surface of the tensile specimen shows that the fracture of the Ni/Zr O₂ composite has a more obvious dimple structure.During TEM analysis,it was found that the pulse current increased the speed of dislocation movement to a certain extent.Under the action of electron wind,the degree of dislocation accumulation near the grain boundary was reduced.The texture of nanocrystalline Ni and Ni/Zr O₂ composite foils under different conditions was analyzed by X-ray diffraction.Both materials showed strong{110}plane texture and weak{111}plane texture at 350°C.After continuously applying the peak current density of 500 A/mm² under this condition,both materials showed a decrease in{110}plane texture and an increase in{111}plane texture.In order to further verify the process applicability of pulse current in the micro forming of complex curved surface of foil,this paper also carried out the pressing forming test of arch curved surface parts.The intermediate die of multi-pass press forming was designed and processed by equal potential field simulation method.The pressing experiments of nanocrystalline Ni and Ni/Zr O₂ composite foils were carried out at 350°C,strain rate of 7×10^-4 s^-1 and peak current density of 500 A/mm².It was found that the surface quality of the arch was significantly improved,which indicated that the forming ability of nanocrystalline Ni and Ni/Zr O₂ composite foils was effectively improved after continuous pulse current.