Research On Surface Integrity And Fatigue Resistance Of Laser Shock Peening Strengthened Nickel Aluminum Bronze For Milled Surface

Nickel aluminum bronze(NAB)is widely used in the manufacture of marine dynamic positioning propellers,and its machined surface integrity and fatigue resistance are of significant fundamental research problems.The existing methods generally employ the conventional milling process for manufacturing the NAB propeller blades,but the state of their machined surface layers fails to meet the requirements of fatigue resistance due to multiple constraints,such as the production efficiency,equipment,and process.To address these issues,this dissertation takes the milled NAB specimens as the research subject,and first investigates the influence law and mechanism of laser shock peening(LSP)on its surface integrity.Based on the findings,the response mechanisms among the macro/micro characteristics of surface integrity induced by LSP,fatigue crack initiation and fatigue crack propagation are further established,which provides theoretical support for the regulation of machined surface integrity of NAB for fatigue resistance.The main research contents are summarized as follows,(1)Considering the geometric model,eccentricity of the torus cutter and the milling parameters,the kinematic model of the torus cutter in the workpiece coordinate system and the cutting edge trajectory equation are derived,further,a milling surface morphology prediction model is established.In addition,single factor milling experiments with spindle speed,feed speed,and tool inclination as variables are carried out.According to the experimental test results,the state characteristics of surface roughness,residual stress,microhardness and microstructure of NAB under typical milling process are clarified.(2)Based on the laser shock wave pressure loading and unloading model,combined with the identified Johnson-Cook constitutive model parameters of cutting NAB specimen,the effects of different laser pulse energy and laser spot overlap rate on the residual stress distribution of NAB fatigue specimens are analyzed,and the effectiveness of the numerical prediction model is verified by the experiments.(3)The effect of laser pulse energy on the surface integrity of milled NAB specimen is studied systematically.The influence of plastic deformation induced by LSP on the surface morphology of the NAB specimens is comprehensively described by multi roughness characterization parameters and root radius of surface profile,and the closure effect of surface defects is analyzed.The effect of the curvature of the strengthened surface on the distribution of residual compressive stress is studied.The grain size,dislocation morphology and density,precipitated phase of LSPed specimens are observed,and the influence mechanism of different laser pulse energy on microhardness and microstructure of NAB specimen and the strengthening effect is analyzed in detail.(4)Based on the basic theory of elastic-plastic fracture mechanics,combined with the characteristic parameters of surface roughness and residual stress distribution of LSPed NAB specimens,the surface fatigue stress concentration factor model of strengthened specimens is established.Combined with Basquin formula,a prediction method of fatigue crack initiation life of the NAB specimen is proposed and verified by experiments.Furthermore,based on the method of metal physics,the location and mode of fatigue crack initiation are analyzed,and the macro fracture mechanics law and micro mechanism of fatigue crack initiation are studied.(5)Based on the basic theories of linear elastic fracture mechanics and elastic-plastic fracture mechanics,the amplitude formula of equivalent stress intensity factor of three-point bending specimen under external load is deduced.The fatigue crack growth experiments of NAB three-point bending specimens are carried out,and the macro and micro fatigue crack growth characteristics and fracture morphology of the strengthed NAB specimens under different laser pulse energy are analyzed.The response mechanism between surface integrity of NAB specimen and fatigue crack growth threshold,stress intensity factor,fatigue crack growth rate and fatigue crack growth life is established.