High-speed Machinability Evaluation Method Of Difficult-to-machine Material For Fatigue And Corrosion Resistance

The difficult-to-machine materials,such as the stainless steel,titanium alloy and superalloy,were widely used in machinery parts,while higher machining accuracy and surface quality of the machined parts were required.In order to promote the application of high-speed cutting technology in actual machining processes,evaluating the machinability of difficult-to-machine materials accurately by considering the processing quality and cost was necessary.The method of constructing Johnson-Cook(JC)constitutive equation based on the orthogonal cutting was improved by considering the radius of tool cutting edge,and a new constitutive model of 17-4PH stainless steel,which was suitable for high-speed machining simulation,was established,The effect of the machined surface integrity on fatigue and corrosion resistance in face-milling the 17-4PH stainless steel was studied,and the effect of cutting parameters on the machined surface integrity,fatigue and corrosion resistance were revealed.Tool failure process and mechanism in face-milling the 17-4PH stainless steel were experimentally studied,and the effects of tool wear mode on machined surface integrity,fatigue and corrosion resistance were investigated.A new method to evaluate the high-speed machinability of difficult-to-machine materials by considering the fatigue and corrosion resistance was built.The method of constructing Johnson-Cook(JC)constitutive equation based on the orthogonal cutting was improved by considering the radius of tool cutting edge,and a new constitutive model of 17-4PH stainless steel,which was suitable for high-speed machining simulation,was established.The Johnson-Cook constitutive equation construction method based on the orthogonal cutting was improved by using Waldorfs slip-line field model,which considered the rounded cutting edge,and simulating iteration operation.The dynamic mechanics performance of 17-4PH stainless steel at the temperature range of 20?-356?,strain rate range of 0.001s-1?3.5×105s-1 and strain range of 0.1?0.65 was obtained by orthogonal milling experiments.Then,the constitutive model of 17-4PH stainless steel was built by combining the stress-strain relation obtained in normal temperature quasi-static compression test.The orthogonal milling and symmetry face milling processes of 17-4PH stainless steel were simulated,and the new constitutive model's availability was verified.The effect of the machined surface integrity on fatigue and corrosion resistance in face-milling the 17-4PH stainless steel was studied,and the effect of cutting parameters on the machined surface integrity,fatigue and corrosion resistance were revealed.The results indicated that,when the surface roughness was increased,the fatigue and corrosion resistance of the machined surfaces was reduced.The work-hardening reduced the fatigue resistance of the machined surfaces.The plastic deformation and grain refinement of the machined surfaces can improve their corrosion resistance.The compressive residual stress on the machined surface can improve the fatigue and corrosion resistance,and the thick oxidation film can inprove the corrosion resistance.Tool failure process and mechanism in face-milling the 17-4PH stainless steel were experimentally studied,and the effects of tool wear mode on machined surface integrity,fatigue and corrosion resistance were investigated.The results indicated that,when the cutting parameters were minor,the main tool flank wear mechanism was oxidation and difusion wear.Under the increased cutting parameters,the main tool flank wear mechanism was oxidation wear,diffusion wear and adhesion wear.When the flank wear occurred,the machined surface roughness decreased and the work hardening remained stable or decreased slightly,while the compressive residual stress increased,leading to the increased fatigue resistance of machined workpiece.When the tipping occurred,on the one hand,the machined surface roughness and work hardening both increased significantly;on the other hand,the tool tipping increased the compressive residual stress,reduced the fatigue resistance slightly.When the compressive residual stress and oxide film thickness increased,the machined surface corrosion resistance improved.A new method to evaluate the high-speed machinability of difficult-to-machine materials by considering the fatigue and corrosion resistance was built.When using the new method,the tool failure standard should be determined by considering the surface roughness,fatigue and corrosion resistance of machined workpieces under various tool failure modes.The machinability could be evaluated by the material removal rate,which should be optimized with the constraints of fatigue and corrosion resistance improvement of machined surface,roughness and tool life.According to the new method,the machined surface roughness Ra no larger than 0.8?m,the machined workpiece's fatigue and corrosion resistance no lower than the material itself and the tool life no shorter than 10min were adopted as the constraints to evaluate the high-speed machinability of 17-4PH stainless steel,TC17 titanium alloy and GH2132 super alloy.It was found that their machinability could be ranked as 17-4PH stainless steel>GH2132 super alloy>TC17 titanium alloy.