| The emerging solid-state friction stir welding technology, and its many derivative new processing methods, can not only realize the bonding of the materials, but also can be used on the surface modification of materials. The research on the surface modification mechanisms of the ductile cast iron via friction-stir processing(FSP) technology was performed in the present dissertation. The modified surface layers with new-typed structures or different chemical compositions were prepared on ductile cast iron, through the different tools(tool with pin and tool without pin), aiming to improve its properties. The appearance of friction-stir processing surfaces in friction-stir zone and its influences were verified. The interaction mechanisms between microstructures evolutions(phase evolution and graphite morphology) and properties were investigated during friction-stir behaviors.The experiment results showed that the formation of friction-stir processed surface of ductile cast iron is determined by heat input. By controlling the processing parameters(rotational speed or traveling speed) to adjust the heat input, can obtain the defect-free modified surface layers of ductile cast iron. During the FSP, the friction-stir behaviors resulted in significant changes of morphology and distribution of graphites. And the pin had important effect on the morphology and distribution of graphites. FSP resulted in the significant breakup of the large size spheroidal graphite and a redistribution of fine graphite particle in the matrix by using the tool with pin. The size of the fine graphite particles in the stirred zone(SZ) was estimated to be on the order of about 0.5 ~ 1 μm. The graphites in the thermo-mechanically affected zone suffered from plastic deformation and flake-shaped or tadpole-shaped graphites distributed along flow lines because of the plasticized material flow. When the fine graphite particles as a tracer material, A visible shear zone was formed in the edge of the SZ just beside the TMAZ due to the results of soften material shear flow. The shear zone gradually increases from the advancing side(AS) to the retreating side(RS). During FSP by tool without pin, the graphites were mainly happened deformation. The flake-shaped or tadpole-shaped graphites distributed in the SZ, while elliptic ball-shaped graphites distributed in the TMAZ. The graphites morphology had a significant relationship with the processing parameters. The variation in the microstructures through the modified surface layers could be attributed to the thermal history and plastic deformation. During FSP, the ferrite and pearlite organization changed into martensite, pearlite and austenitic organization. The amount of martensite, pearlite and austenitic had a significant relationship with the processing parameters. The faster rotational speed or travelling speed could promote the occurrence of the martensitic transformation, so the amount of martensite was high in the SZ, because a large amount of austenite changed into martensite. The mechanical properties of the modified surface layers had a significant relationship with microstructures. The microstructure characterization of the martensite phase was benefited to the improvement of hardness and wear resistance, in comparison with that of the base material(BM). The maximum microhardness of the processed zone was more than 800 HV. By analyzing the experimental results, the mechanisms of the deformation, broken, distribution of graphites were investigated. The friction-stir behaviors of the tool resulted deformation and broken of graphites during FSP. The required conditions for microstructure evolutions were assessed by thermodynamic analyses. |
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