Study On Selective Laser Melting Process Control And Porous Structure Performance Optimization Of Titanium Alloy

The development of aerospace and national defense equipment and other fields will inevitably pose new challenges to the lightweight and functionalization of components,and also put forward higher requirements for the flexibility of manufacturing processes.Selective laser melting（SLM）can solve the problems of difficult processing,long cycle time and high cost of metal parts such as lightweight lattice sandwich porous structure and complex cavity flow channel structure,and realize the integrated manufacturing of material-structure-performance/function in the process of innovative design.With the aim of designing and manufacturing titanium alloy（Ti-6Al-4V）parts with light-weight and functional integration,this paper systematically carried out the research on the process optimization of SLM manufacturing high-performance Ti-6Al-4V.Through topology optimization and model reconstruction,the design of porous components with built-in topological microstructures that are lightweight and have a large specific surface area is realized,and research on its anti-compression behavior,energy absorption performance and flow heat transfer characteristics is of great value for expanding the application of structure-performance/function integrated Ti-6Al-4V in the aerospace field.The main contents are lasted as follows:（1）Study on the influence mechanism of laser linear energy density on the microstructure evolution and performance of Ti-6Al-4V manufactured by SLM.Based on the requirements of multiple performance goals,the influence of laser linear energy density（LED）on the surface morphology,tissue phase composition,mechanical properties,and friction and corrosion properties of Ti-6Al-4V was explored,and the mechanism of sticking powder and pore formation,microstructure evolution and tribo-corrosion mechanism.The research results clarified the optimal forming process window for SLM manufacturing Ti-6Al-4V,and realized the preparation of Ti-6Al-4V with low surface roughness（10.3μm）and high relative density（>99.8%）.In addition,the microstructure is dominated by“acicular”αand secondaryα’phases,accompanied by a small amount ofβphase grain boundaries.When the LED is 0.18J/mm,the maximum microhardness value of Ti-6Al-4V_0.18 is 390.7 HV_0.2,the ultimate tensile strength（UTS）is as high as 1179.7 MPa,and the friction and corrosion performance is optimal(Volume wear rate:1.098×10^-4 mm³·N^-1·m^-1,corrosion voltage:0.381V).In addition,tribocorrosion consists of three stages:dissolution-passivation,tribo-corrosio to form a primary battery,and repassivation,respectively.Furthermore,corrosion products mainly contain oxides of Ti and Al,including Al₂O₃,Al（OH）₃,and Ti O₂.（2）Research on thermal-mechanical coupling distribution of SLM forming titanium alloy scanning strategy and forming accuracy of thin-walled components.Clarified the basic theory of temperature field-stress field coupling in ANSYS Workbench numerical simulation,designed five different scanning strategies,and focused on analyzing the influence of different scanning strategy paths on the temperature field-stress field coupling in the SLM manufacturing process.The reliability of the numerical simulation is further verified by surface roughness and residual stress experiments.Based on the optimized laser process parameter combination and scanning strategy,the forming accuracy of titanium alloy thin-walled components is explored.Research results show that under the action of a Gaussian heat source,the molten pool is in the shape of“water droplets”,and the temperature in the molten pool diffuses from the center to the surrounding,and the highest point temperature in the central area of the molten pool can reach 2100°C,and among them,the back-shaped scanning strategy has the worst heat dissipation ability to the substrate,and the maximum cooling rate of the cross-scanning strategy exceeds 6.0×107℃/s.The Von Mises equivalent stress in the solidification direction of the molten pool presents a“tension-compression-tension”distribution,and the maximum Von Mises equivalent stress is distributed in the edge area of the model,showing a“tensile”stress state,and among them,the maximum equivalent stress of reciprocating scanning strategy reaches 498 MPa,but the minimum equivalent stress of partition checkerboard scanning strategy is only 277 MPa.In addition,the minimum residual stress of the checkerboard scanning strategy is 313.2 MPa,and the average surface roughness is as low as 8.5μm,which is 35.1%lower than that of the loop scanning strategy with the highest surface roughness.Finally,the forming dimensional accuracy error of thin-walled components（150-TWC）is stable within 2.0%,which verifies the reliability of SLM manufacturing high-precision Ti-6Al-4V thin-walled components.（3）Design lightweight,large specific surface area and manufacturable unit cells and built-in microstructure porous components based on topology optimization.Firstly,based on the ABAQUS software,a topological unit model was established.Using the Topology Optimization module,by setting boundary constraints,etc.,with the minimum strain energy as the objective function,the topology optimization of the designed cubic unit model was carried out,and the density distribution cloud map was obtained.Secondly,using the 3D software Solid Works to reconstruct the model of the topology-optimized unit cell structure,four types of topology-optimized unit cells（TopFC-L,TopV-L,TopLC-L and TopS-L）were obtained.Finally,twelve types of built-in microstructured porous components were prepared by scaling filling.The research results show that the theoretical porosity（PR）of all topology-optimized microstructured porous components exceeds 50%,and the proportional scaling filling does not change the volume of the theoretical model.However,the surface area and specific surface area（SSA）increase significantly with the increase of filling order,and among them,the TopV-L16 model has the largest theoretical volume,surface area,PR and SSA of 457.57 mm³,2956.38 mm²,54.24%and 0.66 mm²/g,respectively.（4）Research on the fabrication,compression behavior,energy absorption performance and fracture mechanism of Ti-6Al-4V built-in microstructure components.Using SLM forming technology and adopting the optimal combination of process parameters,Ti-6Al-4V topologically optimized porous components with built-in microstructure were fabricated.Combined with finite element numerical simulation and compression experiments,the stress distribution,stress concentration,compression line change,energy absorption performance and fracture mechanism during the compression process are systematically analyzed.The research results show that the first-order filled TopFC-L structure exhibits relatively excellent compression performance.However,as the filling order increases,the diameter of the internal filled lattice rods of the TopFC-L structure becomes thinner,and the stress concentration shifts to the connection between the built-in filled lattice structures,and significantly reduces the compression stability of the TopFC-L lattice structure.When filling with TopV-L,TopLC-L and TopS-L lattice structures,as the filling order increases,it means that the lattice size decreases,which will not only increase the integrity of the connection between the lattice and the wall,but also improve the local stress concentration phenomenon.This is the key factor that makes the corresponding multi-stage filled Ti-6Al-4V built-in microstructured porous components have excellent compressive and energy-absorbing properties.Among them,the compressive performance and energy absorption value of the TopLC-L structural sample are the best among all the samples filled with the same stage,and the compressive strength and energy absorption value of the TopLC-L structural member filled with the fourth stage are as high as 486.87 MPa and 85.56MJ/m³ respectively.In addition,a small amount of unmelted powder appeared in the fracture morphology of the compressed cross-section,and the surrounding area showed a smooth surface.The fracture edge of the compression fracture edge of the internal support rod frame was relatively smooth and flat,and a layered slip zone appeared,and the overall fracture is uneven,composed of large dimples,small dimples,cracks,closed cells and cleavage planes,which is a composite mode of ductile fracture and brittle fracture.（5）Research on the overall temperature field,flow heat transfer characteristics and heat dissipation mechanism of Ti-6Al-4V built-in microstructure porous components.By constructing the flow heat transfer simulation model（microstructure solid domain and cooling fluid domain）,using the Fluent fluid dynamics module,the flow and heat transfer characteristics of the temperature field,velocity field,and vorticity cloud diagram inside the topological structure were analyzed.The influence of the configuration and order of the unit cell structure itself on the near-wall flow and heat transfer type in the unit cell is elaborated in detail.The research results show that both the filling order（or series）and the configuration of the unit cell itself will affect the overall temperature field and heat transfer performance.As the filling order increases,the specific surface area（SSA）increases,and the unit cell configuration size decreases,so that the flow and heat transfer performance of Ti-6Al-4V built-in microstructure porous components increases.Furthermore,when the number of stages increases to the fourth stage,the temperature difference of the first-stage filling increases by more than 14%.Among them,the temperature difference of TopFC-L and TopS-L with the fourth-stage filling increases by 22.9%and 22.8%.When the filling order is the same and the unit cell unit is changed,the TopLC-L structure has the largest temperature difference among the same order filling.The temperature difference of TopLC-L₁₆ is as high as 296.1 K,and the temperature difference ratio is as high as 46.1%.In addition,the multi-shear struts of the TopLC-L unit cell can significantly enhance the fluid vortex strength and vortex scale.Moreover,the entrainment effect of the eddy current promotes fluid flow mixing,thins the boundary layer,increases fluid disturbance,has excellent“shear-reattachment”effect,easily forms“S-shaped”flow trajectory,and reduces“straight-flow channel”at the same time.