Research On Formation Condition Of Nano/Ultrafine Grained Structures During Power Spinning Of Cylindrical Parts

With the rapid progress of the manufacturing technology, the excellent mechanical properties have become the most important requirement for structural components. The pats with nano/ultrafine grained structures have attracted significant interests in scientific research and industry application due to its unique mechanical, such as high strength and hardness, good plasticity. The traditional manufacturing methods of the pats with nano/ultrafine grained structures have some disadvantages, such as complex processes, large required strains and limited dimension of the workpiece. Therefore, how to manufacture the parts with nano/ultrafine grained structures and large dimension based on small plastic strains has became the main scientific conundrum for industrial application of the nano/ultrafine materials.The research of this thesis was financially supported by National Natural Science Foundation of China(Subject title: Research on forming method and mechanism of the cylindrical parts with nano/ultrafine grained structures by power spinning; Subject No.: 51075153) and Natural Science Foundation of Guangdong province(Subject title: Research on forming method and mechanism of the cylindrical parts with nano/ultrafine grained structures by stagger spinning; Subject No.: 10151040301000000). A new manufacturing method consisting of quenching, power spinning and recrystallization annealing to achieve nano/ultrafine grained structures in cylindrical parts based on small strains is proposed. It mainly focuses on the research of manufacturing method, microstructural evolution, formation conditions of the nano/ultrafine grained structures, plastic deformation mechanism and mechanical properties. The main contents in this thesis are as follows:(1) Manufacturing method of the cylindrical parts with nano/ultrafine grained structures by power spinningThe metal flow model of counter-roller spinning was built based on upper bound method and was compared with that of the stagger spinning. The rationality of the derived metal flow models both of the stagger spinning and counter-roller spinning were further verified by the numerical simulation and the metallographic analysis results. The equivalent strains distribution and the influence on grain refinement both of the stagger spinning and counter-roller spinning were obtained based on the derived metal flow models, the numerical simulation and metallographic analysis. The influence of spinning methods and the distributions of equivalent strains on grain refinement were studied to obtain the grain refinement rule during power spinning, which lays a theoretic foundation for the manufacturing the parts with nano/ultrafine grained structures.(2) Experimental investigation of manufacturing the cylindrical parts with nano/ultrafine grained structures by power spinningAccording to the analysis of the traditional manufacturing methods of the pats with nano/ultrafine grained structures, the method of manufacturing the parts with nano/ultrafine grained structures based on the small plastic strains are studied. A new manufacturing method consisting of quenching, power spinning and recrystallization annealing to achieve nano/ultrafine grained structures in cylindrical parts is proposed. The power spinning devices are designed. The ASTM 1020 is selected as the materials used in experiment and the process parameters of the three procedures are determined(the quenching temperature Tq, austenizating holding time τ1, quenching medium for quenching; the feed rate of roller f, the thinning ratio per pass Ψi, the axial offset a and radial offset b for power spinning; recrystallization temperature Tre, holding time τ1 for recrystallization annealing). The experimental investigation is carried out, the maro-forming quality(Relative wall thickness deviation Δ, Ovality eo, Straightness es, Roughness Ra) and forming defects are analyzed. The prevention measures of the forming defects are obtained.(3) Microstructural evolution during manufacturing the cylindrical parts with nano/ultrafine grained structuresThe evolution rules of phase composition, grain size, grain morphology and dislocation during the proposed material process procedure are investigated in details based on the optical microscopy(OM) and Transmission Electron Microscope(TEM) observation. The microstructural evolution in manufacturing the cylindrical parts with nano/ultrafine grained structures is obtained. The evolution of the orientation angle, dislocation cells and subgrains during the power spinning is researched to obtain the grain refinement mechanism. The dislocation density during the proposed material process procedure is measured by X-ray diffraction. The influence of the dislocation density and stored energy on grain size during the recrystallization annealing is researched. The formation conditions of the nano/ultrafine grained structures are obtained based on the microstructure and macro-forming processes.(4) Plastic deformation mechanism and mechanical properties of the cylindrical parts with nano/ultrafine grained structuresThe plastic deformation mechanism of the spun parts with nano/ultrafine grained structures, which are prepared by combining quenching with stagger spinning and recrystallization annealing, is researched experimentally based on the power spinning. The results show that the dominative plastic deformation mechanism of the cylindrical parts with nano/ultrafine grained structure is the slipping of the grain-boundary and the rotation of the grain although a small amount of dislocation pile-up existing inside the interior of the grains. The mechanical properties of parts with the nano/ultrafine grained structures are researched by tensile test. The results show that the tensile strength is increased from 425 MPa to 815 MPa, the hardness is increased from 155 HV to 305.3HV, but the elongation is slightly decreased from 24% to 17.5%. The influence of the microstructure on the mechanical properties(strength, hardness and elongation) is researched, which provided theoretical basis for industrial production and large-scale applications of the parts with nano/ultrafine grained structures.