| Ultra-high strength steel AerMet100 has excellent comprehensive mechanical properties,and is an advanced material for manufacturing important structural parts such as carrier aircraft landing gear,jet engine shaft and rocket shell.AerMet100 steel is a typical difficult-to-cut material.There exist some prominent problems such as low tool life and difficulty in chip breaking during machining of AerMet100 steel.Laser-assisted milling is a kind of compound machining technology applied in processing difficult-to-cut materials.During laser-assisted milling,the material is heated and softened by the thermal effect of laser beam,thus its cutting performances can be improved before machining.Compared with conventional milling,laser-assisted milling adds a laser heat source in front of the tool feed,resulting in a more complicated material removal mechanism.At present,the achievements in theoretical modeling of laser-assisted milling forces,heat,tool wear and surface integrity(heat-affected zone and residual stresses)are still quite limited.In order to improve the processing efficiency and quality of laser-assisted milling,it is necessary to explore the relevant mechanism and provide theoretical guidance for process optimization.The common problems in cutting field mentioned above are studied in this paper,the main contents are as follows:An analytical prediction model for the heat-affected zone in laser-assisted milling of AerMet100 steel is presented.Based on the moving continuous point heat source model and the heat source temperature field superposition method,temperature field caused by laser heating is predicted analytically.By analyzing the phase transformation mechanism of AerMet100 steel,the austenization temperature is defined as the critical value for the formation of heat-affected zone.The steady-state temperature field caused by laser heating is compared with the critical temperature.Then the boundary of the heat-affected zone is determined,the width and depth of the heat-affected zone are obtained.XRD technique is employed to analyze the phase composition in the heated and machined surface layers,and the validity of the heat-affected zone model is verified.The influences of laser power,feed speed,spot size and laser incident angle on the width and depth of the heat-affected zone are analyzed.An analytical prediction model for cutting forces in laser-assisted milling considering material softening is established.A method for calculating shear plane temperature and shear flow stress considering the combined effects of laser heating and plastic deformation is proposed,and the influence of material softening on cutting force coefficients is taken into account.Based on the classical oblique cutting theory and slip line field theory,shearing force and ploughing force are calculated,and the analytical prediction of cutting forces in laser-assisted milling is obtained.The accuracy of the model is verified by cutting force measurement experiments under different laser power and feed per tooth.The influences of laser power,feed per tooth,laser-tool distance,edge radius and tool nose radius on laserassisted milling cutting forces are analyzed.An analytical prediction model for workpiece temperature field in laser-assisted milling considering the combined effects of multiple heat sources is proposed.The cutting heat source is discretized and simplified as instantaneous rectangular heat sources.The influence of material softening on the heat flux of the cutting heat source is taken into consideration.Based on the instantaneous rectangular heat source model and the heat source temperature field superposition method,workpiece temperature rise caused by cutting heat source is predicted analytically.Adding up the temperature rise contributed by cutting heat source,the temperature rise contributed by laser heat source,and the ambient temperature,the workpiece temperature in laser-assisted milling is obtained.The accuracy of the model is verified by temperature measurement experiments under different laser power,spindle speed and feed per tooth.An analytical prediction model for residual stresses in laser-assisted milling considering material softening is presented.In order to deal with the challenge of tool rotation and interrupted cutting,the milling process is decomposed into a number of rotation cycles.The influence of material softening on the distributed loads in the cutting region is taken into account.Based on the rolling/sliding contact theory,a workpiece stress field model in laser-assisted milling is established.Then the analytical prediction of residual stresses in laser-assisted milling is realized by using the classical S-J algorithm.To improve the computational efficiency of this model,a methodology which supposes the cutter makes reverse movement is developed to determine the tool feed range.The validity of the model is verified by residual stress measurement experiments under different laser power conditions.The influences of laser power,spindle speed and feed per tooth on residual stresses in laser-assisted milling are analyzed.Tool wear mechanism in milling of AerMet100 steel and the impact mechanism of laser-assisted process on tool wear are investigated.Based on the experimental phenomenon of tool wear in conventional milling,the formation mechanism of notch wear on bull-nose tool is revealed from the view point of local stress concentration.A stress concentration factor is proposed,and a notch wear prediction model considering the influence of stress concentration is established.The influences of axial depth of cut,radial depth of cut and feed speed on notch wear are analyzed.The improvement effect of laser-assisted process on notch wear is theoretically discussed,and is confirmed by laser-assisted milling experiments.Tool life tests are carried out,and the impact mechanism of laser-assisted process on flank wear and tool life in milling of AerMet100 steel is analyzed. |
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