| Selective laser melting(SLM)is a layer-by-layer accumulation manufacturing technology that uses a laser beam as an energy source to quickly melt and solidify metal powder.It has the characteristics of high forming accuracy,excellent mechanical properties,etc.,and can fabricate special-shaped parts with complex topological characteristics.The porous structure,represented by the triply periodic minimal surface(TPMS)structure,is a type of replicable unit with regular porous features.It has the characteristics of interconnected pores,adjustable pore size,and stable performance.Through parameterized design,continuous gradient varying of porous unit morphology,unit type,unit wall thickness,unit aperture,etc.can be realized,and then fabricated by selective laser melting,which are the superior candidates for lightweight,energy absorption,and orthopedic implantation applications.This paper aims to solve the problems of easy loosening,stress shielding effect,and difficult tissue growth in the demands of bone replacement.The titanium alloy graded-porous structure is manufactured via SLM,and then through experiments and numerical calculation methods to research the multi-properties of bionic bone scaffolds,including the graded-porous design method,forming quality and characterization,mechanical properties and energy absorption,biocompatibility and osseointegration ability.The main research contents are:(1)The porous structure is divided into two types: truss structure and TPMS structure,and three graded-porous TPMS structure design methods are studied.The characteristics and modeling method of truss structures are introduced in detail,and the model construction of skeletal-based and sheet-based TPMS are realized by parametric modeling methods,and the functional relationship between control parameters and volume fraction has been calibrated.Then by changing the control parameter function,the porous structure design of graded morphological,graded fusion and graded wall thickness are realized,and the application situations of different design methods are analyzed.Among them,the graded wall thickness design is improved based on the principle of surface thickening.It not only has the characteristics of high surface area and smooth surface,but also has the effect of simultaneous adjustment of wall thickness,pore size,and porosity.(2)Three types of TPMS structures with graded wall thickness were designed and fabricated by SLM,and their manufacturing performance was studied.First,the SLM-fabricated solid parts are used to measure the density and mechanical characteristics of the as-built Ti6Al4 V alloy.Then the internal and external morphology of the as-built samples were observed by the optical microscope,scanning electron microscope and Micro-CT,and there were found that both the inside and outside of the TPMS samples showed the characteristics of continuous graded wall thickness,so the manufacturing quality was excellent.The distribution of porosity,surface area and pore size of TPMS samples are proved that they can be applied to filling bionic bone structure.Due to the large amount of overhang structure and surface semi-melted powder at the low overhang angle zones,as well as the residual powder in the large wall thickness area in as-built samples,the volume fraction deviation of the fabricated TPMS parts are between 2.1% and 9.5%,and the deviation of the wall thickness are between24.0 ?m and 95.3 ?m.(3)Compression experiments and FEA calculations are both used to study the mechanical properties and energy absorption capacity of the as-built TPMS samples.Experiments results show that the modulus of the TPMS sample is distributed within the applicable range of bones,the strength is better than that of natural bones,and the deviation of the FEA results from the experimental values are all less than 50%,which indicated that the FEA method can predict mechanical properties of the TPMS samples.By summarizing the mechanical properties of both the constant and gradient thickness samples,there found that the modulus of graded Gyroid and Neovius samples are 94 MPa and 136 MPa higher than that of uniform structure,the cumulative energy absorptions are 15.4% and 24.0% higher,and the stress drop ratios are smaller and the energy absorption efficient are more stable.The first failure position of the TPMS units usually occurs at the maximum local curvature zone.The layer-by-layer failure behavior of the graded-thickness structures are different from the overall shear failure of the uniform structure,which making them show obvious advantages in applications such as energy absorption,impact resistance,and bionic bone structure design.(4)The biological properties and osteogenic ability of SLM-built TPMS scaffolds with gradient thickness were studied by the in-vitro cell culture and animal implantation experiments.Cell culture experiments show that the smaller the porosity,the slower the flow rate in the pores,which is beneficial to increase the amount of cell seedings.The graded wall thickness design effectively improves the cell proliferation ability.With the increase of culture time,the cells gradually spread outwards,continued to divide and proliferate,forming clusters of cell groups,which are easily distributed in areas with large local curvatures,indicating that the SLM-built titanium alloy porous scaffold has good biocompatibility.Animal experiments show that the TPMS scaffold implants are tightly interlaced with the inside and outside surrounding bone tissue,which indicated that they have good structural stability and can solve the problem of easy loosening and difficult growth in bone replacement.This study shows that the bone scaffolds have excellent osseointegration ability,which proved that the graded-thickness TPMS structures can be used to design bionic bone structure,and this paper provided detailed theoretical and experimental references for the future applications. |
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