Optimal Design And Mechanical Properties Of Implicit Surface Gradient Porous Structure Manufactured By Selective Laser Melting

Gradient materials are common biological material structures in nature.Researchers have designed bionic porous structures by imitating the structure distribution of gradient materials.Owing to the excellent properties,such as lightweight,high strength,noise reduction,energy absorption and biocompatibility,the bionic porous structures have been widely applied for aerospace,architecture designs,transportation,and biomedical fields.At present,the complex modeling process and low modeling efficiency are the main problems of bionic gradient porous structure.How to achieve rapid design of the gradient porous structure and the optimal distribution of the gradient structures is the important issue for the design and application of gradient porous structures.Triply periodic minimal surface(TPMS)cellular structure has an average zero curvature and high specific surface area,and they can be quickly designed through implicit function-driven parameterization.Therefore,TPMS cellular structures are considered as ideal type porous structures.Imitating three types of graded materials,including gradient porosity,gradient orientation,and gradient material,the gradient TPMS cellular structures were optimized for design.Combining selective laser melting(SLM),finite element analysis(FEA)simulation,topology optimization,MATLAB program and mechanical test,the mechanical properties of gradient TPMS were systemically researched.The main research contents and results are as follows:(1)There is a pinch-off discontinuous problem with the generation of TPMS cellular structures with high porosity by typical functions,including Gyroid,Diamond,Primitive,and IWP.By optimizing the TPMS functions and FEA simulation,an optimal design method of TPMS is proposed.The compressive and bending properties of the optimal TPMS have been researched,and the Gibson-Ashby modified prediction models corresponding to the four kinds of TPMS were established.Both the compressive elastic modulus and the compressive yield strength increase as the porosity decreases.The result shows that the elastic modulus of the four kinds of TPMS(porosity equals to 85%)is ranked in order: Primitive ? Diamond ? IWP ? Gyroid,and the effective energy absorption of TPMS is ranked in order: Primitive ? Diamond ? Gyroid ? IWP.This result provides a reference for selecting the TPMS structure type.(2)Based on the pinch-off optimization design criteria,the linear and conic functions were applied to the design of the TPMS cellular structure,and continuous gradient Primitive and IWP structures were generated by MATLAB parametritis programming.Compression and bending behaviors of gradient TPMS cellular structures were studied.The compression process of linear-gradient IWP and Primitive both show a layer-by-layer behavior and their compression curves are characterized by step rise and multi-stress peak rise.The compressive behavior of axis-centered cellular structure is similar to uniform cellular structure.Bionic design of gradient cellular structure imitated the gradient structure of bamboo.It is demonstrated that the bending elastic modulus of the entire structure can be enhanced when bending force is applied onto the side of low porosity and high strength,but the bending deflection of the gradient structure will be reduced.Besides,Axis-centered gradient porous sandwich structures can greatly improve the bending mechanical properties of porous structures.(3)Based on computer graphics,the quaternion is applied to design rotation models of TPMS cellular structures.By rotating different angles around the [100],[110],and [111] crystal directions,the anisotropic behavior of four types TPMS has been studied,and the pole figures of the relationship between relative elastic modulus and rotation angle were established.A method of designing gradient oriented structures is proposed to imitate gradient oriented structures of biomaterials,and its compression mechanical behavior shows a trend of rising stress peak gradient or stress plateau gradient.Combining the filter pressure drop test and fluid simulation,the hydrodynamic behavior of the disc and conical porous filters was analyzed,and the result shows that the Primitive structure has the best pressure drop performance.(4)Combining FEA,topology optimization,and MATLAB parametric programming,this paper proposes three optimization design methods of gradient porous structure,including stress cloud,topology optimization density cloud,and topology optimization + stress cloud composite gradient porosity.These optimized design models can be printed with little or no support.Combining SLM manufacturing and simulation analysis,it was demonstrated that the mechanical properties of the gradient optimized structure are superior to the uniform porous structure.Based on the parametric programming design method,a femoral stem implant with a topologically optimized density cloud gradient porous structure has been designed and manufactured by SLM.This section provided a solution for the optimal design of the gradient porous implant.(5)The SLM manufacturing method of the gradient material porous structure is proposed,and the mechanical behavior and anisotropy of the gradient material porous structures were studied.The experimental results show that when the bending force was applied onto the highstrength brittle material sides,both the strength and deflection of the bending structure of Cu Sn-18Ni300 can be improved.Compression behavior of a bimetallic porous structure is similar to that of a four-material porous structure: When the compression direction was parallel to the material gradient direction,the structure was compressed into a step-by-step way;When the compression direction was perpendicular to the material gradient direction,the compression curve featured a nearly periodic sawtooth wave.This result demonstrates that mechanical behavior of parts can be adjusted by combining the mechanical properties of different materials.