Fundamental Study On Application Of High Efficiency Machining Of Nickel-Based Super Alloy Integrated Impeller

As the key component of turbine machinery of aero-engine, integrated impellerhas an extremely severe working environment with high temperature and alternatingshock load, which requires the high performance of impeller materials. Nickel basedsuperalloy is considered as a suitable materials for integrated impeller for its goodoxidation resistance as well as corrosion resistance and creep resistance and excellentfatigue life at high temperature. However, good physical and mechanical propertyoften means poor machinability. Especially, great challenge is encountered inmachining of nickel based superalloy because of big cutting force, high cuttingtemperature, severe tool wear as well as notable machining hardening and residualstresses. Except for difficult-to-cut material, the structure of integrated impeller ismore complicate than before due to the requirement of big thrust-weight ratio andhigh efficiency of aero-engine. This research focuses on the semi-open centrifugalnickel-based superalloy (GH4169) impeller with long-short blade. This kind ofimpeller often has narrow flow passage, thin and twist blade. Machining of this kindof impeller becomes very difficult for cutting deformation and cutting vibration wouldbe often happened, which requires high stability for machining process.Saw tooth chip formation due to the local thermal-plastic instability in the shearzone and the periodically brittle break in the chip free surface appears in the millingprocess of GH4169when the cutting speed increases. It is because the strainhardening effect in the chip free surface far exceeds the thermal softening effect in theshear zone under low cutting speed. Thus material brittleness near the chip freesurface increases and micro-crack propagation happens. In contrast, the thermalsoftening effect in the shear zone far exceeds the strain hardening effect in the chip free surface under high cutting speed. Then the local thermal-plastic instability causesmaterials sliding along the shear surface and saw tooth chip formation. Furthermore, itis also found that the cutting force fluctuates at almost the same frequency of thesaw-tooth, which makes the cutting tool enduring periodic shock load and fast wear.A three-dimensional thermal-mechanical finite element model based on variableundeformed chip is established to predict cutting force. Then the deformation of bladeduring milling process could be obtained. An effective two-dimensional finite elementmodel considering the continuous feed effect is established to predict residual stressand the effect of cutting edge radius on residual stress is also evaluated, and then thedeformation of blade after milling process could be obtained. An experiment is carriedout to validate the finite element model and study the working hardening underdifferent cutting conditions. At last, optimal parameters could be obtained.Because of the weak rigid of plunge milling tool, the cutting force model as wellas the kinetic equation is established for the plunge milling tool with big corner radius.With the help of modal test, cutting force based on tool chatter displacement anddeformation could be obtained so the stable cutting condition could be determined. Aresearch on stability of the dynamic weak rigid blade flank milling is also carried out.A stability model based on regeneration chatter is established to obtain therelationship between critical dept of cut and spindle speed. A modal test is carried outto acquire modal parameter at different blade thickness. At last, stable cuttingconditions could be obtained for flank milling of the dynamic weak rigid blade.Based on the fundamental theory and experiment researches, a complete set ofmachining process for nickel based superalloy has been successfully applied in themanufacturing process of a certain type of integrated impeller, which comes from anational “863” High-Tech Program. The integrated impeller has been applied in benchtest.