Laser Additive Manufacturing(LAM)is a comprehensive Manufacturing technology including new materials,computer,numerical control,laser and other advanced technologies.Laser Melting Deposition(LMD)is one of LAM technology characterized by powder feeding,which has attracted much attention due to its unique advantages such as being capable of manufacturing and repairing of large high-performance parts.316 L austenitic stainless steel is widely used in pressure equipment of petroleum,chemical and other industries,but the fatigue properties of 316 L stainless steel LMD parts are less studied,the mechanism of fatigue crack propagation is not clear.Therefore,aiming at how to control and improve the fatigue properties of 316 L austenitic stainless steel LMD parts,this paper proposed a research method combining experiments and finite element method(FEM),and explored the evolution law of LMD parts density under different LMD process parameters.The influence mechanism of residual stress field on fatigue crack propagation rate in LMD parts with high density was studied.The main research contents and conclusions of this paper are as follows:(1)The effect of laser power and scanning speed on the geometrical characteristics of LMD single-track deposition was investigated by depositing316 L austenitic stainless steel powder with laser melting deposition equipment.A regression model was established by regression analysis of process parameters and the geometric characteristics of LMD single-track deposition.It was found that the geometric characteristics of LMD single-track deposition were positively correlated with laser power,negatively correlated with scanning speed,and energy density could not accurately predict the geometric characteristics of LMD single-track deposition.(2)The influence of dilution rate of LMD single-track deposition on remelting behavior and density of LMD parts was analyzed by combining FEM and microscopic observation.Firstly,the density of LMD parts under different process parameters were measured,and the fitting curve of the surface energy density of LMD single-track deposition and the density of LMD parts was established.Then,the influence of process parameters on the geometrical characteristics of LMD single-track deposition and the influence mechanism of dilution rate of LMD single-track deposition on remelting behavior of LMD parts during LMD process were studied.Finally,the fitting curve between the dilution rate of LMD single-track deposition and the density of LMD parts was established,and the fitting degree is higher,which is more suitable for predicting the density of LMD parts.(3)The fatigue crack growth rate of 316 L austenitic stainless steel LMD parts after different heat treatment process were studied.By comparing with the traditional 316 L hot-rolled plates,it was found that the fatigue crack initiation cycles of LMD as-built CT samples and LMD stress relief CT samples,under the condition of high density,were 44.3% and 57.0% of those of hot-rolled plate CT samples respectively,and the final fatigue crack initiation cycles were 63.9%and 69.0% respectively.The numerical values of residual stress intensity factor under different processes were calculated by finite element method.It was revealed that the residual stress intensity factor accelerated the fatigue crack growth rate by increasing the stress ratio of cyclic load.The effective stress ratio of LMD as-built CT samples and LMD stress relief CT samples were 0.119 and0.122,respectively.Fatigue crack growth rate increased 43.51% and 52.62%respectively,compared with CT sample of hot-rolled sheet.The nonlinear fitting of fatigue crack growth rate,stress intensity factor range and effective stress ratio was obtained based on Walker formula,which would provide data support for the subsequent study and prediction of fatigue properties of 316 L austenitic stainless steel LMD parts. |