Research On Forming Mechanism Of Nano/Ultrafine Grained Cylindrical Parts By Power Stagger Spinning

Nano/ultrafine grained materials (NUGM) fabricated via SPD (severe plastic deformation)has aroused a growing interest of specialists in materials science. However in recent years, itis difficult to manufacture the ideal three dimensional block NUGM with high purity, highdensity and distinct grain boundaries for lack of optimal processing technologies.Nevertheless, with the development of nano-technologies and advanced studies, more andmore researchers began to combine the power spinning technology with nano-technologiesfor fabricating the nano/ultrafine grained cylindrical parts. Related research shows thatthrough the repeated power spinning processes, cylindrical parts with nano structured surfacewere prepared, namely surface nano crystallization. However, in order to realizenano/ultrafine crystallization of the whole cross section, studies of nano/ultrafine grainforming mechanism induced by severe shear deformation in process of power staggerspinning and the effect of initial grain sizees, power stagger spinning process parameters andrecrystallization heat treatment conditions on grain refinement were carried out.Research in this dissertation was mainly focused on the mechanism of fabricatingnano/ultrafine grained cylindrical parts by power stagger spinning which has not beeninvestigated till now, and its purposes are to discuss the forming methods and mechanism ofnano/ultrafine grained cylindrical parts by power stagger spinning and the effects of processparameters on grain sizes.Based on crystal plasticity theory and the commercial finite element software ABAQUS,the polycrystalline mesoscopic plastic constitutive model and Poisson-voronoi finite elementmodel (PVFEM) were established, and several kinematical equations of polycrystalline insevere plastic deformation were also built. Further more, a rate independent program wascompiled (by ABAQUS user subroutine UMAT). By importing the program as a UMATsubroutine, study of the variable grain shape factor (the length ratio of longer axes to shortaxes of the equivalent ellipse) in forming area was carried out, and then the relationshipbetween resolved stress of slip system and macroscopic stress was also discussed in detail. Bydoing so, a visual model for accurately forecasting and controlling of forming process fornano/ultrafine polycrystalline was completed.Selecting power stagger spinning as the optimal forming method for fabricatingnano/ultrafine grained cylindrical parts, the tooling and fixture were prepared. Theexperimental study of three different programs were carried out by selecting20steel pipe as its blank and choosing different process parameters including roller profile, spinning passes,wall thickness thinning ratio, roller intervals, roller feed ratio, and so on. The results revealthat the nano/ultrafine grained cylindrical parts were successfully fabricated by a compositeforming process that started using3-pass spinning followed by580℃×0.5h staticrecrystallization,2-pass spinning and580℃×1h static recrystallization annealing insequence, and its size is600nm on average.Based on the theoretical analysis of deformation mechanism, the microstructural evolutionof20steel both in process of power stagger spinning and subsequent annealing wasinvestigated via metallographic analysis technology, SEM and TEM techniques, and then theforming mechanism of nano/ultrafine grains induced by severe shear deformation in theprocess of power stagger spinning was also revealed. Further more, the affecting factors forgrain refinement were discussed. By the investigation of selected area electron diffraction(SAD), the crystal structures, texture and the activated slip system of specimen were obtained,then forming mechanism of large angle grain boundaries (LAGB) was also discussed.Through analysis of the metallographic organizations, SEM/TEM images, tensileexperiment datas and fracture morphology of20steel spun workpieces, effect of layer spacingof pearlite (LSP) and cementite precipitation on mechanical properties during the process ofpower stagger spinning was investigated, and then both the characteristics and its mechanismof the microcrack and the plastic fracture for20steel and quenched20steel were also studied.Further more, the pinning&hinder effect of the second phase particles (granular cementite)induced by power stagger spinning on grain boundary migrations were discussed. Theexperimental results show that the layer spacing of pearlite (LSP) decreases as the thinningratio increases during power stagger spinning, and the tensile strength σbof spun tubes isproportional to the reciprocal of LSP, while the elongation δ is inversely proportional to thereciprocal of LSP. Small tough dimples connect gradually with tensile stress σxaround cracktip, then micro crack is formed later with multi-pass and multi-direction loading. By this way,crack further extends along60°to tangential direction. Finally, ductile fracture of spunworkpiece comes into being.