Mechanical Properties Tailoring Of Porous Structure Using Topology Optimization And Selective Laser Melting

The design and fabrication of porous implant have been an important problem in orthopedic medical field.On the one hand,the complex surface and porosity of porous structure are required to provide sufficient space for the growth of bone cells and the transport of nutrients.On the other hand,the mechanical properties of the porous structure should be tailored and match human bones for preventing stress shielding.Traditional methods still can’t solve these problems comprehensively.This paper provides a comprehensive solution for four basic problems in Ti6Al4V porous implants,including structure design,fabricating restrictions,mechanical properties matching and service life,while using topology optimization design and selective laser melting(SLM)technology.Digital control of porous implant performance has been realized.Firstly,based on the SIMP material interpolation theroy,the topology optimization design of human skeleton element was realized by using ANSYS software.SolidWorks and Materialise Magics 21.0 software were used to reconstruct and repair the designed unit porous structure to reduce the amount of data.From the specific surface area measurement results,the specific surface area of topology optimized porous structure whose porosity is above 60%surpassed porous structure constructed by implicit surface method,indicating a better biocompatibility.Secondly,the affecting factors of designed Ti6Al4V porous structure in SLM fabrication process were discussed from three aspects of surface quality,fabricating accuracy and manufacturability.Meanwhile,the optimized parameters were given here:the size error of X-Y axis can be decreased by setting 0.015 mm beam offset;the size error of Z axis can be decreased by setting 60μm compensatory value;the warpage deformation of parts can be eliminated by optimizing placement position and setting support structure.Black smoke and sparks were reduced by controlling the flow rate of protective gas,the gas pressure value was recommended at about 120 Pa.In addition,four tiny structures were abstracted from the designed unit porous lattice structure,and their fabrication limitations were given:the rising angle of the“spring”structure should be greater than 45°;the maximum overhang length of the arc structure should be 1.04mm;the thickness of thin wall and the diameter of small hole structure should be greater than 0.1 mm.Then,the relationship between mechanical properties and structural parameters of topology optimized porous lattice structures were investigated.From compressive test,the compression behavior and fracture failure modes of porous lattice structures were discussed.The compressive strength ranged from 23 to 498 MPa,which is inversely proportional to the structural parameters(porosity and unit cell size).A theoretical Strength-Density-Unit Cell Size model was established for tailoring compressive strength.From Resonance Frequency and Damping Analyzer test,the stacking mode of porous lattice structures was optimized.The dynamic elastic modulus ranged from 3.5to 55.47 GPa,which was inversely proportional to the structural parameters(porosity and unit size).Additionally,the designed unit porous structure with porosity of 60%and 70%could meet the requirements of human bone.Meanwhile,the Modulus-Density-Unit Cell Size model was established for tailoring elastic modulus.Finally,for elongation improvement of horizontally SLM as-built Ti6Al4V,some special SLM conditions were carried out in this work.Three kinds of fabricating parameters were adopted to help transforming~′martensite into(+)microstructure:the energy density(E)=50.62 J/mm~3,hatch spacing(h)=0.12 mm,layer thickness(t)=60μm,area ratios between support structure and part(A_s/A_p)=0.6;E=44.9J/mm~3,h=0.12mm,t=60μm,A_s/A_p=0.6;E=50.62 J/mm~3,h=0.10 mm,t=60μm,A_s/A_p=0.6。Moreover,the tensile test shown high elongation(>8%)was achieved for horizontally SLM as-built Ti6Al4V specimens without sacrificing their strength(the yield strength>1100 MPa and ultimate tensile strength>1200 MPa),which can meet the standard for Ti6Al4V castings for surgical implants.