Formation Mechanism Of Microscopic Defects And Mechanical Properties Of Laser Additive-micro-polishing Co-manufacturing IN718 Alloy

Additive Manufacturing(AM)is an advanced manufacturing technology with advantages such as short processing cycle time,no molds,high material utilization,and good at processing complex parts.SLM is one of the most commonly used AM techniques,and the parts prepared by SLM usually have excellent dimensional accuracy and feature resolution,so it is often used to process small and complex metal parts.SLM technology is widely used in aerospace,automotive,biomedical,mold field.However,SLM parts still suffer from surface quality and internal defects,which seriously affect the assembly accuracy and mechanical properties of the parts and limit the further application of the technology.Laser polishing technology is an effective means to improve the surface quality and density of SLM parts.Combining SLM with in-situ laser micro-polishing(i.e.,co-manufacturing)can effectively improve the surface quality and density of parts while ensuring processing efficiency.In this thesis,the SLM additive-micropolishing co-manufacturing process was adopted.The formation mechanism of microscopic defects in SLM-ed IN718 alloy and the influence of in-situ laser micro-polishing on internal defects and surface morphology were explored by a combination of simulation and experiment.The effects of the main process parameters of co-manufacturing on the microstructure,pore defects,surface morphology and mechanical properties of the samples were obtained.In order to reduce the microscopic defects and improve the surface quality of the co-fabricated IN718 alloy.In an attempt to reduce the microscopic defects and improve the surface quality of the IN718 alloy samples by co-manufacturing.First,a mesoscale numerical model of SLM additive-micro-polishing co-manufacturing was established.The SLM powder spreading process was simulated based on the discrete element method.The mutual collision,motion and deformation of powders in the powder spreading process were considered,and a high-fidelity SLM powder bed model was obtained.The powder bed model was imported into the computational fluid dynamics code,and boundary conditions such as Gaussian heat source,convection heat dissipation,radiation,evaporative heat dissipation,and recoil pressure were added.The free surface was solved using the volume of fluid method.The mesoscopic-scale SLM additive-micro-polished co-manufacturing numerical model with multiphase and multi-physical fields was established.The surface morphology and track profile obtained in the numerical model are compared with those obtained experimentally,and the results show the accuracy of the numerical model.Second,based on the SLM mesoscale numerical model,the characteristics of temperature field,melt pool dynamics and phase transition of the SLM additive-micro-polishing comanufacturing process were obtained,and the formation mechanism of lack of fusion defects and gas pore defects during SLM forming and the evolution mechanism of defect in the subsequent polishing stage were investigated.The results showed that at lower energy densities,lack of fusion defects could be formed due to insufficient melting,and these defects could be divided into inter-track lack of fusion and inter-layer lack of fusion.At higher energy densities,gas pore defects were formed.The formation of keyhole pores was divided into three main stages: the formation of keyhole caused by high-energy laser;the keyhole collapsed to form a pore;the pore moved to the front of the molten pool and shrined,and finally,the pore was captured by the solidified metal to form a pore defect.Both defects may be eliminated in the subsequent micro-polishing stage.Lack of fusion defects would be filled with molten material and pore defects would spill out of the melt pool.Third,based on the SLM mesoscale numerical model,the laws of temperature field,melt pool flow and surface morphology evolution in the SLM additive-micro-polishing comanufacturing process were obtained.The mechanism of surface morphology evolution in the polishing stage and the mechanism of new surface defects generation in the polishing stage are investigated.The results showed that the mechanism of micro-polishing to improve the surface quality of SLM parts is that capillary and thermal capillary forces drive the melt pool flow to reduce the curvature of the surface bulges,capillary forces drive the molten material to the back and bottom of the melt pool,and thermal capillary forces drive the molten material to the back of the melt pool.Capillary forces during the micro-polishing process caused the creation of depressions in the melt pool,and the exchange of material between the center and both sides of the laser track caused by the surface tension gradient resulted in the formation of bulges,and these depressions and bulges can form new surface defects if they are not eliminated in time.Fourth,SLM additive-micro-polishing co-fabrication tests with different micro-polishing power were conducted.The microstructure,pore defects and surface morphology of the specimens were characterized,and the pore defect characteristics were analyzed,and the tensile properties of the specimens were also tested.The results showed that the microstructure of the specimens was mainly columnar dendrites.The co-manufacturing caused the orientation of dendrites to be more complex and changed the size of dendrites.With the increase of micropolishing power,the porosity and the average area of pores showed a trend of decreasing and then increasing,and the smallest at the power of 170W;the effect of micro-polishing power on the average roundness coefficient of pore defects was not significant;the number of pores gradually decreased with the increase of micro-polishing power.The surface roughness decreased and then increased with the increase of micro-polishing power,and was the smallest at a power of 110 W.The tensile strength of the specimens was significantly enhanced by micropolishing,and the tensile strength increased by 313 MPa compared with that of SLM specimens when the micro-polishing power was 140 W.The elongation at break of the specimens increased and then decreased with the increase of micro-polishing power,and reached the maximum at the power of 170 W.The tensile properties of the specimens were linearly fitted to their defect characteristics such as porosity,average roundness coefficient,average area and maximum area.The results showed that there was a stronger linear correlation between the elongation at break and the defect characteristics of the specimens compared to the tensile strength.The linear correlation coefficients between elongation at break and porosity,average area,maximum area,and average roundness coefficient were-0.93,-0.94,-0.71,and 0.86,respectively.