Research On Mechanical Properties Of Additive Manufactured Triply Periodic Minimal Surface Lattice Structures

Triply Periodic Minimal Surface(TPMS)lattice structure is formed by periodically arranged surfaces with the smallest area and zero mean curvature under certain constraints.Compared with the conventional truss-like lattice structure,the smooth transition of the surface endows TPMS lattice structure uniform stress distribution,balanced load capacity in all directions,as well as long fatigue life.Therefore,the TPMS lattice structure has become one of the most promising structures.However,due to the high complexity,it is difficult to fabricate TPMS lattice structures via traditional processes.Additive manufacturing technology can theoretically form any complex shape and has obvious advantages in forming TPMS lattice structures.The current research on additive manufacturing of TPMS lattice structures faces the following scientific and technical problems:(1)the mechanism of the structural geometry and loading direction on the mechanical properties of TPMS lattice structures is not clear;(2)the influence of graded design on the manufacturing accuracy and mechanical properties of TPMS lattice structures is unknown;(3)The fatigue failure and strengthening mechanisms of TPMS lattice structures need to be studied.In response to the above issues,the following research contents have been carried out:(1)The selective laser melting(SLM)process was used to prepare the uniform-pore Ti-6Al-4V Gyroid lattice structure,which is the typical structure of TPMS lattices.Based on the Johnson-Cook model,a material constitutive model based on the SLM lattice structure was proposed for the finite element simulation,the simulated results showed a deviation less than 25%compared with the experimental results.The simulation reveals that the root cause of the uniform stress distribution and anisotropy of the TPMS lattice structure lies in the surface characteristics of the structure.The experimental and simulation results are used to fit the Gibson-Ashby formula,revealing the relationship between mechanical properties and volume fraction.The pole figure obtained from the finite element,analytical calculation,and experimental results reveals that the mechanical response of the Gyroid lattice structure is anisotropic,and the degree of anisotropy is lower than that of truss-like lattice structures.(2)Gradient TPMS lattice structures with different gradient directions were designed and prepared.The additive manufacturing accuracy and static compression mechanics of the gradient Gyroid lattice structure were systematically analyzed.A mathematical model based on a multi-layer composite structure was established to predict the manufacturing accuracy and mechanical properties of the graded TPMS lattice structure.It is revealed that the manufacturing accuracy of the TPMS lattice structure depends on the unit cell topology and geometry.Negative density gradients along the building direction can improve the manufacturing accuracy.It is clarified that the 316L TPMS lattice structure with gradient change along the loading direction exhibits the deformation behavior of layer-by-layer collapse,and has the largest energy absorption capacity(16.60±0.48 MJ/m~3),while the TPMS lattice structure with gradient change perpendicular to the loading direction has the largest Young's modulus(1165.49±11.55 MPa)and the plateau stress(27.83±0.01 MPa).(3)Fatigue testing and analysis of uniform and gradient TPMS lattice structures were performed.The mechanism of crack nucleation and propagation is revealed and attributed to partially melted powder particles and tensile stress on the surface of TPMS lattice structures during fatigue.The surface treatment such as sandblasting can remove most of the crack nucleation starting point and strengthen the surface matrix material,thereby enhancing the fatigue performance of TPMS lattice structures.In this paper,the fatigue limit of 316L Gyroid lattice structure before and after sandblasting was 0.35 and 0.45,respectively,which were higher than those of the conventional truss lattice structure.The mechanism of restraining crack growth by stress redistribution during the fatigue process of the graded structure is elaborated.The main load-bearing side of the graded structure has stronger toughness and larger crack growth space,and therefore,the fatigue life is higher than that of uniform structure.This paper has systematically studied the additive manufacturing accuracy,static compression mechanics,and dynamic fatigue mechanics of Gyroid-type TPMS lattice structures with uniform and gradient pores.The work has laid an important theoretical foundation for the engineering application of TPMS lattice structures.