| Superalloy blade is one of the most important parts of power equipments in aviation,aerospace,shipbuilding,energy and other fields.The microstructure and properties of the forged billet are crucial to ensure the quality of the blades.Currently,the precise manufacturing process adopted to superalloy blade is complicated.The manufacturing process of forged billet usually including,hot extrusion and hot forging,and mechining processing after the forging process,which poses low efficiency and precision,and the difficulty to control the microstructure,properties and forming accuracy in coordination.With the continuous development of aviation industry,the requirements of dimensional accuracy,surface quality and properties of blade parts with complex profiles are constantly improved.Therefore,as alternative to hot extrusion,the development of forming methods to make the high preformace blank with high efficiency and high precision has become the key for the manufacturing of blade parts.As an efficient near net forming technology,cross wedge rolling is widely used as the promsing technology of ferrous shaft parts.Whereas,for nickel base superalloy as a typical aeronautical difficult deformation material part,limited studies have been performed on cross wedge rolling,especially on the control of defects in the center and microstructure evolution.In this work,combined experiment and numerical simulation method are adopted to clarify the formation mechanism for defects and microstructure evolution of GH4169 alloy during cross wedge rolling,which provides theoretical fundation for the cross wedge rolling billet of GH4169 alloy blades.Firstly,the effect of deformation process on microstructure and ? phase evolution of GH4169 alloy was quantified by isothermal compression test.Meanwhile,the microstructural evolution of GH4169 alloy under hot deformation and the dissolution fracture process of lamellar 8 phase were clarified.By characterizing the microstructure,strain and dislocation distribution of GH4169 alloy after deformation,the spheroidization mechanism of lamellar ? phase during hot deformation is revealed,and correspondingly a model for spheroidization of lamellar ? phase is established.Through the thermal mechanical coupled numerical simulation of GH4169 alloy cross during wedge rolling process,the influences of rolling process and die structure were analyzed.The flow law of material,stress and damage in rolling process is systematically investigated.The effects of the ? phase and dynamic recrystallization on the defects at core region are also discussed.The formation mechanism of core defects in GH4169 alloy during cross wedge rolling was revealed,and the process model of defect formation was established.Results indicate that the axial material flow is the main contributor to the centre voids.A critical radius was discovered in the irregular material flow region.The critical radius could be minimized through controlling the rolling speed,broadening angle and area reduction.? phase lath has results in crack in the centre of the sample.During cross wedge rolling,the ? phase deformed and transfer into short rods or particles,and promoted the dynamic recrystallisation.Therefore,the voids diminished after the rolling process.under different rolling conditions,the existence of axial tensile stress and circumferential shear stress on the core metal of rolled piece is the inducement of producing center defect.In addition,the existence of the second phase promotes the generation of void defects.The larger the size of the second phase,the easier it is to produce crack defects.The occurrence of dynamic recrystallization in rolling process can heal the micro-defects and reduce rolling defects.By controlling the morphology and content of the second phase and parameters of rolling process,the fully dynamic recrystallization structure of the rolled piece can be obtained in the alloy,which can effectively release the formation of central defects in the cross wedge rolling process of GH4169 alloy.The microstructure evolution of GH4169 alloy cross wedge rolled pieces under different heat treatment conditions was observed by optical microscope(OM),electron backscatter diffraction(EBSD),scan electron microscopy(SEM)and X-Ray Diffraction(XRD).The grain orientation,twinning,misorientation distribution and texture type in GH4169 alloy during cross wedge rolling were studied.The microstructure evolution and dynamic recrystallization mechanism of GH4169 alloy during cross wedge rolling were discussed,which provides a theoretical basis for precisely control of the microstructure and ? phase evolution of rolled pieces.The results show that different types of twins exist in the dynamic recrystallization grains of cross wedge rolling GH4169.The mechanism of dynamic recrystallization is discontinuous dynamic recrystallization.Under the same rolling conditions,precipitation of ? phase is distributed on the grain boundary in granular form,which provides more nucleation sites for dynamic recrystallization,promoting the dynamic recrystallization,and is beneficial to obtain fine recrystallized grains.The precipitation of ? phase increases the deformation activation energy of GH4169 alloy.the deformation activation energy increased due to the present of ? phase,therefore restricts to the center the deformation process.The evolution of ? phase in GH4169 alloy during cross wedge rolling was studied,and the influence of cross wedge rolling on the morphology,content and distribution of ? phase was analyzed.The interaction mechanism between ? phase and dynamic recrystallization during cross wedge rolling was discussed.The local strain evolution law and ? phase precipitation rule of GH4169 alloy cross wedge rolled piece were described.In cross wedge rolling process,the ? phase is mainly torsional fractured.Through the control of the area reduction in the rolling process,the morphology,fraction and distribution of the ? phase could be varied.Based on the above studies,cross wedge rolling billet tests were carried out for a certain type of GH4169 alloy blade.The microstructure,? phase morphology and mechanical properties of the rolled piece were observed.The room temperature tensile properties and hardness distribution of GH4169 alloy cross wedge rolled pieces under different rolling processes were compared and analyzed,and the effect of cross wedge rolling on the yield strength of GH4169 alloy was investigated.The fracture morphology of the tensile parts was observed by SEM,and the tensile fracture types were analyzed.The experimental results show that:after rolling,the elongation decreases,while the hardness and the tensile strength at room temperature of the rolled piece increases significantly.The fracture morphology of the rolled piece shows uniform equiaxed dimple,which is ductile fracture. |
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