Study On Strengthening And Toughening Mechanism And Stress Control Of Laser Additive Manufacturing 316L Stainless Steel

Due to its superior corrosion resistance,toughness,and weldability,austenitic stainless steel is widely employed in the aerospace,nuclear,chemical,and biomedical sectors.Laser Directed Energy Deposition（L-DED）additive manufacturing technology applies a high-energy laser beam as the heat source and is based on the principle of“layer-by-layer manufacturing and stacking”,which can realize the mold-free and rapid manufacturing of complex structural parts of stainless steel.However,due to the non-equilibrium rapid solidification of the melt pool under the L-DED process with the influence of thermal cycling,which typically results in complex multi-scale microstructures,the quantification mechanism of strengthening and toughening mechanisms described multi-scale microstructures remains unclear.Furthermore,the melt pool has a high temperature gradient and cooling rate under the L-DED process,inevitably inducing residual stress and deformation,the evolution of the residual stress and the regulatory mechanism remains unclear,limiting the engineering application of L-DED stainless steel.In light of the aforementioned issues,this thesis employs L-DED additive manufacturing of 316L stainless steel as the research object and conducts research on the quantification mechanism the strengthening and stress regulation of additive manufactured stainless steel utilizes a combination of process experiments and numerical simulations,and the specific research contents and results are as follows:First,the unique multi-scale heterostructures of austenitic stainless steel during laser direct energy deposition were investigated using electron backscatter diffraction,transmission electron microscopy,and other characterizations.In conjunction with dislocation and dynamic recrystallization formation mechanisms,the post-solidification microstructure characteristics related to residual stress during laser direct energy deposition of austenitic stainless steel were analyzed,and the main formation mechanisms of discontinuous recrystallization and continuous dynamic recrystallization were explored.The results reveal that the unique multi-scale heterostructure of 316L stainless steel is generated by the microstructure features of solidified cell,nano oxide,high dislocation density,and heterogeneous grain.The main mechanisms for the formation of discontinuous dynamic recrystallization（DDRX）and continuous dynamic recrystallization（CDRX）are caused by initial grain boundary bulging and progressive sub-crystalline rotation（PSR）,respectively.In addition,quantify the effect of laser direct energy deposition of 316L stainless steel with multi-scale heterogeneous organization（dislocations,solidification cells,nano-oxides,grains）on yield strength were investigated by systematic annealing experiments,microstructure characterization,and tensile property testing.The results reveal that the yield strengths of the experimentally deposited and annealed samples are comparable to those calculated using the traditional Taylor hardening formula,etc.The results show that the maximum contribution of dislocation substructure to the strengthening effect(△σ_DIS,200MPa)>micro-segregation strengthening effect(△σ_SEG,135 MPa)>grain boundary strengthening effect(△σ_GB,108 MPa)>strengthening effect of nano-oxide（△σ_P,19 MPa）,with GND-type dislocations accounting for more than 60%of the total dislocations.Despite the presence of nano-oxide inclusions in laser direct energy deposited steels,the synergistic effect of deformation twinning and pre-existing GND-induced back stresses leads to the high plastic properties of laser direct energy deposited stainless steels.Further,a global sensitivity analysis method（D-MORPH-HDMR）was proposed in conjunction with a finite element model to develop an innovative residual stress optimization framework capable of analyzing the response of the sum of the independent contributions of single parameter and the coupling effects of two parameters to the output results with a small sample size.This approach is designed to investigate the effect of contentious processing parameters on residual stress（thickness of deposition layer L,laser power P,scanning speed v）.The results of research reveal that the amplified effect of the influence of the three input variables on the transverse residual stress and thickness direction residual stress is L>P>v,on the contrary,the influence of longitudinal residual stress is P>L>v.Simultaneous thermodynamic coupled 3D modeling results demonstrate good agreement with experimental monitoring data of temperature fields.Furthermore,multi-objective optimization was carried out forresidual stress of forming parts based on MOPSO optimization algorithm.For the forming of austenitic stainless steel thin-walled parts by laser directenergy deposition,the residual stress corresponding to the non-dominated solution with laser power of 400W,scanning speed of 9.38mm/s and scanning layer thickness of 200 um is compared with the unoptimized results,the maximum residual stress in X direction is 406.21mpa（r educed by 28.5%）,and the maximum VON residual stress is 973.11mpa（reduced by 9.6%）.Finally,a three-dimensional transient thermal model based on sequential coupling is established to investigate the melt pool properties,temperature gradients and residual stress distribution of 316L produced by the laser direct energy deposited method.In both CW and PW modes,the simulated single-track transient temperatures matched the measured values.In PW,the melt pool reduces during off-laser and increases during on-laser,forming a sawtooth pattern.A zigzag residual stress distribution in the laser melting direction mimics a heartbeat because the PW mode generates a regularly fluctuating melt pool.Periodic heat input also reduces stress buildup by reducing the temperature gradient and releasing stress continually.PW mode reduces longitudinal residual stresses by 45%in low-power samples in the heat-affected zone and places prone to microcracking or delamination.The maximum stress value of tensile stress along laser scanning direction may be reduced by decreasing the duty cycle and laser pulse frequency.