| Ni-based superalloy can perform the excellent resistance to the fatigue,creep and corrosion under the serive condition of high temperature and pressure,which was benifical for its wide application on the preparation and production of high temperature resistant parts of key aeroengine components.In contrast,with the rapid development of material science and the temperature gradual improvement of advanced aero-engine turbine combustion chamber,the traditional polycrystalline Ni-based superalloy were difficult to meet the service requirements.The mechanical performanence weakness always occurred at the grain boundaries,which may induce the crack formation and structural damage.However,based on the traditional casting and directional solidification technology,single crystal superalloy was developed.Due to the complete elimination of the crystal boundaries perpendicular to the principal stress axis as the crack source,this material significantly improves the service performance in ultra-temperature conditions and becomes preferred for turbine blades of high performance aircraft engines.Although the seed crystal and screw selecting methods have been adopted to prepare these parts,however,proper precision machining and semifinishing are also necessary for satisfying the application requirements.In contrast,due to the difficult-to-machine property of this material and ambiguous machining mechanism,during the processing,the efficiency and accuracy were all worsen.Therefore,the mentioned problem above has become one of the bottlenecks to restrict the aerospace manufacturing develpement.DD5 developed by the institute of metal research,chinese academic of sciences is affilicated to the second generation Ni-based single crystal superalloy.Due to the 3%Re and minor grain boundary strengthening elements addition,compared to the first generation,the comprehensive performances were significantly improved.In contrast,from the current retrieved literatures,the reports devoted to the researches of the DD5 milling performance are still blank.In addition,due to the crystal structure difference,the material removal mechanism of the single crystal superalloy is obviously distinguished to the shear and slip along the grain boundary of polycrystalline one.Also,the milling performance along the different crystal directions even on the same crystal plane may be varied because of the strong mechanical anisotropy.Morover,because of the physical and chemical properties and alloying elements composition and content differences,during the milling process,the tool wear mechanism,the milled surface integrity,the chip formation process and thermal and mechanical properties of single crystal material may also be differed from the polycrystalline one.Futhermore,though some grain boundary strengthening elements were added in DD5,but under the service conditions,the material recrystallization phenomenan is still shouldn't be ignored.Therefore,under the support of the national natural science foundation of china,the paper compared the milling performance and surface integrity differences between the polycrystalline and single crystal nickel-basesd superalloy,researched the cutting tool optimization method,chip formation characteristics,mechanical and thermal properties,the machining parameters optimization of water-based mininmum quantity lubrication(MQL)in deep,and discussed the surface damage features induced by milling of DD5,which mainly presented five aspects.(1)In order to compare the milling performance and surface integrity differences between the polycrystalline and single crystal nickel-bases superalloy,selected the Inconel718 as the typical representative of the poly crystalline materials,the slot milling experiments were conducted on TH5650 CNC vertical machining center with PVD-AlTiN coated tools of carbide matrix.Based on the fast fourier transform(FFT)method,the influences of milling parameters,cutting tool overhang length and cooling conditions on the machined surface roughness,micro morphology,working hard degree and plastic deformation were discussed.Moreover,the evolution laws of surface roughness,milling force and machining system stability with the tool wear increasing were researched.(2)Based on the Peierls Nabarro models and molecular dynamics simulation,the single crystal material deformation and removal mechanisms were discussed.Combined with the dislocation slip theory and Schmidt coefficient method,the milling process anisotropic model was established.In order to avoid the influence of single crystal material mechanical anisotropy on the miiling performance,considered the DD5 metallographic structure characteristics,the paper proposed the directional cutting method.For improving the applicacity of milling tools,based on the tool wear experiments of PVD-TiAlN and PVD-AlTiN coated carbide tools under dry and water-based MQL conditions,the evolution laws of the tool minor flank wear width,the wear morphology,the machined surface quality and the milling accuracy with accumative cutting length increasing were evaluated and all experiments were conducted on the(001)crystal plane along the[110]crystal direction.In addition,adopted component analysis method,the failed tools wear and failure mechanisms can be discussed.Moreover,based on the aforementioned analysis,the tool optimization was evaluated.(3)Comibined with the finite elements simulation and slot milling experiments methods,based on the chip formation mechanism of difficult-to-machine materials during high speed cutting,the chip morphology of DD5 and the influence factors on chip edge formation under different cutting speed were researched.Then,the critical cutting speed of high speed milling was indentfied.In addition,the stress,the starin and the temperature fields indueced by the chip and chip edge formation were analyzed adopted Deform 2D and 3D software.Moreover,adopted the embedded artificial thermocouple and piezoelectric sensor and FEM,the DD5 mechanical and thermal properties during the milling process were discussed,and the influence laws of the milling parameters,the cooling mode and the crystal characteristics on the mechanical and thermal properties,the machinied surface quality and the machining accuracy were revealed.(4)During the milling process,under the effects of thermal-mechanical coupling and alternating load,the machined material surface may generate some damage and microstructural changes.Under the different milling parameters,cooling modes and crystal characteristics conditions,some slot milling experiments were conducted on DD5(001)crystal planes,the micro morphology,hardness and microstructural changes under machined surface and subsurface were detected and observed.Moreover,the influences of surface defects,slot top burr characteristics,work hardening,microstructural changes and recrystallization features indueced by milling were researched in deep.Then,the effective surface damage controlling methods were propsed.(5)In order to improve the cutting performances,surface integirties and enviormental sustainability during the DD5 Ni-based single crystal superalloy macro milling process,based on the water-based MQL milling experiments results on DD5(001)crystal plane along the directions of slip systems,selected the milling tangential force,surface roughness and the concentrations of particulate less than 2.5?m(PM2.5)as the evaluation indicators,the cutting fluids jetting direcitons and distance to the cutting zones were optimized.Based on the response surface methodology and the variance analysis methods,the interaction effects between the milling and cooling parameters were evaluated,and the mechanisms were discussed.Moreover,based on the GA-BP-ANN multi-objective optimization algorithm and the homogenization experimental design methods,the DD5 surface roughness and milling tangential force were predicted,optimized and experimentally verified respectively.In this thesis,the milling process of DD5 was deeply explored and studied through the experiments mainly.The findings could be of interest to the readers from traditional manufacturing because the results can rich the processing theories and provide the feasibility and some experiments basis in milling the single crystal superalloy parts such as DD5 with the traditional machine tool and cutting tool. |
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