Preparation And Micro-forming Of Ni-based Nanomaterials Reinforced By Graphene Oxide

Array micro parts with high-precision manufactured by plastic micro-forming technology have great application in the biomedical field.However,the stability of forming process is decreased which attributes to the special plastic deformation law in the micro/nano-size.The nano-materials with a large amount of grains can effectively reduce the undesirable influence of raw material on forming process.The graphene oxide with excellent mechanical properties was used as the enhanced phase to prepare the nano-metal matrix composites with great superplasticity,which can effectively enhance the stability of forming process and significantly promote the development of micro-forming technology.In this dissertation,Ni/GO and Ni-Co/GO nanocomposites were prepared by a pulse electrodeposition.The microstructure and mechanical properties of Ni-based nanocomposites were studied by combined methods of experimental study,performance test,microstructure analysis and mechanical property characterization.The influences of pulse electrodeposition parameters on the plasticity and superplasticity of the nanocomposites were studied.The enhancement mechanism of ultra-thin flexible two-dimensional enhanced phase was discussed.The relationship of microstructure and macroscopic mechanical properties for cold rolling of Ni-based composites was revealed.The rupture regularities of different types of punches for array micro-forming were established,which reveals the forming mechanism of array micro-forming and influences of the size effect.GO was successfully prepared by an ultrasound-assisted Hummers method with a transverse size of 3~5?m,the lamellae of 2~3 layers,accompanying with a number of oxygen-containing functional groups.It was added to nickel alloy matrix as an enhanced phase to synthesize Ni-based nanocomposites.The elements in the composites were well distributed and the crystallinity was well.The grain size of the matrix was 30~50 nm,and the hardness and elastic modulus of the composites were greatly improved.Compared to the ordinary coarse-grained material,these mechanical properties increased more than one time.The fracture strength and tensile strain of Ni-Co/GO nanocomposite increased first and then decrease with the increase of current density and duty cycle,and increased with the increased p H value of the electrodeposition solution.The maximum fracture strength was 997 MPa and the maximum elongation at room temperature was 7.53%.The effects of different amounts of GO on the mechanical properties such as tensile strength and elongation for nano Ni/GO and Ni-Co/GO composites were systematically studied.It was found that the fracture strength and elongation increased with the addition of GO,and the high temperature tensile elongation was higher than nano Ni with the addition of GO more than 0.03 g/L.The effect of GO morphology on the mechanical properties of composites was discussed.It was found that cyclic or long fibrous GO can make more pores in the material and reduce the mechanical properties.Combined with the research of other scholars about the mechanism of composite deposition,the deposition process of GO on composite and the interaction between free-form substances in electrodeposition bath were analyzed in detail.The enhancement method of GO was explored and discovered,which means refining the matrix metal grains,changing the texture and covering the grain boundaries of the matrix metal inhibiting the growth of the matrix grains at high temperature.The experiment of the multi-pass unidirectional rolling and cross-rolling of Ni-based nanocomposites was successfully conducted.It found that the hardness of the Ni-based nanocomposites decreased first and then increased during cold rolling.The hardness of the cold-rolled foil was smaller than the original foil's while the hardness of cross-rolled foil was slightly higher than the unidirectional rolled foil.The reason of the decrease of hardness for the foil after cold rolling was that the grain size of the matrix grew.The reason for the increase of the foil's hardness after multi-pass cold rolling was that the matrix grain size reduced and high-density nano twins generated.The reason for the higher hardness of the foil was that there were more twins in the material.The GO in the multi-pass unidirectional cold-rolled nanocomposites showed a strip-like morphology and the grains around GO did not enlarge significantly.When the temperature was 500?,the maximum strain was obtained for the cold-rolled foil by tensile tests at the high temperature and the strain in the extending direction was greater than the strain in spread direction at different temperatures.However,the strain of the foil after cold rolling was lower than the original foil.The main sources of crack propagation in Ni/GO nanocomposites during high temperature plastic deformation were researched.It was found that the fracture location and method of the foil were depended by the numerical size between the binding force of matrix metals and GO with the matrix metals' the binding force.The micro arrayed forming experiment of the Ni based nanocomposite was successfully conducted,and the forming part with good shape was obtained.It was found that the depth-to-diameter ratio of the micro arrayed forming parts of the cold-rolled foil was smaller than that of the original foil.It was also found that forming parts have different shapes and rupture positions with different male die types.The optimum forming temperature of micro forming of Ni/GO foils was 550?,and the pressing speed of male die was 0.04 mm/min,and the wall thickness of foils was 60 ?m.The optimal forming temperature of Ni-Co/GO foils with flexible aluminum micro arrayed die was 500?,and the pressing speed of punch was 0.05 mm/min,and the wall thickness of foil was 60 ?m.The size effect results in a smaller depth-to-diameter ratio for the smaller die diameter,and there was also the size effect of the foil wall thickness variations during the micro arrayed forming.