Experimental Study On Stress Relaxation Forming Of Nanocrystalline Ni Foil Assisted By Pulsed Current

Nanocrystalline metal materials exhibit many unique properties in thermal,mechanical and electrical aspects compared to traditional coarse crystalline metal materials.However,the poor room temperature plasticity and low forming limit of nanocrystalline metal materials make it very difficult to form and prone to processing defects.Although the materials exhibit superplasticity at high temperatures and can obtain large deformation,with the increase of temperature,the materials will undergo grain growth and surface oxidation,causing them lose their original excellent properties,which greatly limits the application of the materials.To solve the above problems,this paper introduces pulsed current into the forming process of nanocrystalline metal materials.Two kinds of stress relaxation forming experiments were carried out on the nanocrystalline Ni foil prepared by electrodeposition at a lower temperature,and the forming performance under different process conditions was studied.First,according to the characteristics of stress relaxation forming and metal foil forming of small parts,a new method for forming Ni foil solid particles under the action of pulse current was proposed.The corresponding process equipment was designed and the process flow was established.Through uniaxial tensile stress relaxation tests at different temperatures and general solid granules medium forming tests,the optimal forming temperature of Ni foil was determined to be 300?,and the pulse current assisted solid granules medium forming process test was carried out at this temperature.The results show that the application of pulse current can significantly improve the forming performance of Ni foil,and the ratio of high to diameter of the bulging part obtained when the peak current density was 150 A/mm~2reached 0.34.Secondly,in order to verify the universality of the current-assisted stress relaxation forming process,a stress relaxation forming experiment on the complex curved surface of a rigid mold was also carried out.The shape of the intermediate mold necessary for the deformation process is designed by the electric field simulation method.The forming test was carried out at a temperature of 300°C and a peak current density of 150 A/mm~2,and the surface quality of the obtained curved parts was good.In order to study the plastic deformation mechanism of Ni foil under the action of pulse current,TEM was used to analyze the stress relaxation forming parts.The evolution of the microstructure of Ni foil material,such as grain morphology,grain boundary movement,dislocation configuration,etc.,was revealed.Studies have found that pulsed current can promote dislocation movement and grain boundary slippage.In addition,stacking faults and deformation twins have been observed inside the deformed foil,all of which contribute to the improvement of the plastic deformation ability of the material.