Research On Fabrication And Forming Mechanism Of Controllable Porous Structure Of Ti6Al4V Based On Selective Laser Melting

Controllable porous structure, possessing merits of light weight, high corrosion resistance, high melting point, high fatigue strength and plasticity, etc., has been widely applied in the fields of aerospace and biomedicine. This article uses Selective Laser Melting (SLM) technology to form the Ti6A14V controlled porous structure which is difficult to form with the conventional method, and then studies its forming mechanism. The mechanism analysis in this paper, which builds the temperature field model of Ti6A14V controlled porous structure processed by SLM, systematic studies the forming parameters optimization of Ti6A14V controlled porous structure, mainly focuses on the surface roughness calculation model of the Ti6A14V controllable porous structure as a functional material, and the bearing capability calculation model of it as a structural material.The main content and the results of the paper are as follows:1. The experiment in this paper establishes the temperature field model of Ti6A14V controlled porous structure processed by SLM; obtains the expression of laser energy absorption by molten pool with consideration of the influence of the plasma inverse bremsstrahlung on temperature field; analyzes the influence of different phases of powder on the laser absorption rate with laser heating, and so as to put forwards the dynamic absorption rate equation that the powder changes with the time; adjusts the Von Carmen solution of liquid equation to fit the metal liquid calculation by researching the influence of the flow of molten pool on temperature; obtains the formula of migration of gas liquid-solid interface while expounding the influence of powder sublimation on the migration of gas-liquid-solid interface; moreover, obtains the corresponding relationship between the parameters of thermal properties and the temperature through revealing the influence of temperature on the powder thermal properties. To conclude, the temperature field model, close to actual situation, meets actual requirements.2. The technological parameters of SLM Ti6A14V controllable porous structure are optimized through Design-Expert software by building the regression model of forming parameters and density on the basis of Taguchi experiment. It proved, through tests of the microstructure and function of parts manufactured with the optimum technological parameters, that the forming parameters affecting the density are linear energy density ((laser power/scanning speed), thickness, scanning strategy and hatching space, in which the hatching space and scanning strategy are inversely proportional to the density; while the linear energy density is direct proportional to the density; moreover, the thickness has the most significant effect on the density among all these parameters.3. This paper also studies the influencing factors of surface roughness of SLM Ti6A14V controllable porous structure strut; establishes the calculating formula of it on the basis of single track; then revises this formula with the surface roughness of actual forming parts as criterion so as to analyzes the main factors which affects the strut’s roughness; expounds the methods to improve the strut’s surface roughness. And the results show that the morphology of single track in SLM processing approximates circular curve; and the calculated results obtained through least square midline method is closest to the actual situation; the width and lap width of single track are direct proportional to Ra、Rz and Rsm; and the post-processing methods such as electrochemical polishing can greatly improve the surface roughness of the parts formed by SLM.4. The controllable porous structures of hexahedron and octahedron are manufactured. The relationships between maximum bearing capacity and deformation displacement of hexahedral units, octahedral units and controllable porous structures are researched; the analytic formulas of bearing capacity and deformation displacement of these structures are acquired with the building of their simplified models, with which realizes theoretical calculation of Ti6A14V materials. The actual fracturing load and deformation displacement of units and controllable porous structures are obtained via compression test, which are proved that are close to theoretical ones. Therefore it is safe to draw the conclusion that the fracturing load and deformation displacement formula given in this paper has practical application value.5. The studies show that the optimum parameters are:laser power-80W; scanning speed200mm/s; the thickness-0.02mm; scanning strategy-X-Y inter-layer stagger scanning; and hatching space-0.08mm. The microstructure of formed part is mainly composed of acicular martensite, a phase and β phase, in which the grains of primary a phase are fine with the size of0.5-1.5μm. The preferred orientation of primary phase a is along the maximum heat flux direction and the direction closest to<100>. The microhardness is492.56HV0.2; and maximum tensile strength, which is significantly affected by the processing direction, is987MP. The surface roughness of the formed parts are Ra=7.54μm;Rz=52.26μm;RSm=187.39μm. As for octahedral unit, its maximum bearing capacity is218.ION and maximum deformation displacement is0.1570mm; maximum bearing capacity of controllable porous structure composed by it is21990.7N. However, as for hexahedral unit, its maximum bearing capacity is612.6N and maximum of deformation displacement0.6895mm; and the maximum bearing capacity and maximum deformation displacement of controllable porous structure composed by it are1789.9N and0.3917mm respectively.