选区激光熔化成形极小曲面点阵的结构和性能优化

Optimization of Structure and Performance of Minimal Surface Lattice Formed by Selective Laser Melting

随着激光增材制造技术的快速发展,具有轻质高强和性能可控的点阵结构成为航空航天、骨科医疗等领域的研究热点。三周期极小曲面(TPMS)点阵结构的平均曲率为零,具有消除应力集中和提高结构强度的优点,是轻量化多功能结构材料的理想候选者。本团队采用一种新的曲面偏移方法设计了金刚石(Diamond)、初始晶格(Primitive)、螺旋二十四面体(Gyroid)和体心立方(I-WP)4种TPMS点阵结构,并采用选区激光熔化(SLM)完成了Ti-6Al-4V样件的制备,同时建立了基于Johnson-Cook的有限元仿真模型。点阵结构的仿真结果在受载进程的线性增长、应力降、应力平台各阶段均还原了实验过程,证明了有限元仿真模型的良好预测性,揭示了优化点阵结构在压缩过程中表现出的逐层坍塌的变形行为和连续塑性断裂模式。得益于曲面偏移设计产生的截面系数增加,4种TPMS点阵结构的压缩性能和能量吸收都获得了较大提升,其中I-WP点阵结构的强度提升了244.9%,能量吸收提升了312.7%。

Objective Because of their excellent performance with lightweight and multifunctional integration,lattice structures have been widely used in aerospace,heat exchangers,and bone tissue engineering.Triply periodic minimal surface(TPMS)lattice structures with smooth surface morphology reduce the stress concentration under load,exhibiting higher specific strength,specific stiffness,and energy absorption capacity.Therefore,TPMS has potential applications in lightweight and energy-absorbing buffer devices in the aerospace industry.Sheet and network lattices have been proposed to utilize their advantages,which require further performance improvements with an optimal design.Thus,there is an urgent need to develop a reliable simulation analysis method to reveal the mechanism of structural strengthening and determine optimization direction.Methods In this study,a new surface offset method was developed to design a TPMS lattice structure(Fig.2)to improve mechanical properties and energy absorption.Diamond,Primitive,Gyroid,and I-WP TPMS lattice structures(Fig.3)were optimized using this method and fabricated via selective laser melting(SLM).The compression tests of the lattice structures were repeated three times to reveal the mechanical properties.In comparison,finite element models with the Johnson-Cook model were established to reflect the deformation behaviors of the lattices and predict their mechanical strength,as confirmed by the experimental results.In this study,the influence of surface offset design on the mechanical properties and energy absorption capacity under quasistatic compression was investigated,which provided insight into the optimization strategies and analysis methods of lattice structures.Results and Discussions The experimental and simulated compression stress-strain curves show that the finite element analysis method based on the Johnson-Cook model can precisely replicate the experimental results,including similar linear growth,stress drop,and stress plateau stages.The deviations in the mechanical strength of the lattice structures obtained via the experiment and simulation are all less than 14%,particularly for sheet structures,whose ultimate strength error is within 2%.This indicates that the finite element method can accurately predict the mechanical properties and deformation behavior of lattice structures.The mechanical properties of the four lattice types were improved significantly using the proposed design method,as can be seen from Table 4 showing the critical mechanical properties of all the samples.With the continuous increase in the surface offset,the mechanical strength of Diamond,Gyroid and I-WP lattices increase by 101.5%‒244.9%owing to the increase in the second moment of area.Among them,the I-WP sheet 45-30 exhibits the most outstanding performance,demonstrating an increased mechanical strength(111.64 MPa)compared with that of the rod lattice(32.37 MPa).However,Primitive lattices significantly differ from the other three types.The surface offset helps to improve the stability of the Primitive lattices,avoiding the sudden collapse of the entire structure.The mechanical strength was increased by 47.1%,but continuous growth of the shell offset reduced the mechanical properties owing to the weakening effect of the plastic hinges.The cumulative energy absorption(Figs.15 and 16)reveals that the surface offset design effectively improves the energy absorption capacity of the lattice structure.Specifically,Diamond,Gyroid,and I-WP continuously improve the cumulative energy absorption by 139.8%,279.2%,and 312.9%,respectively,compared with the corresponding rod-type lattices.Similar to strength,the most outstanding performances are contributed by the I-WP sheet 45-30,whose cumulative energy absorption increases from 11.32 to 46.72 MJ/m3,and the plateau stress(σpl)increases from 22.68 to 98.81 MPa.These results highlight the optimization effect of the surface offset on the energy absorption capacity.The shear failure mode of rod-shaped lattice structure changes into the deformation behavior of layer-by-layer collapse using this method.The large-scale collective collapse of lattice structures can be prevented to obtain a smooth,continuous stress-strain curve,which increases the plateau stress of the sheet lattices.Conclusions 1.In compression experiments,the rod lattice structure is prone to a 45°shear fracture.A continuous surface offset can effectively improve the deformation behavior of an abrupt collapse,enhance the mechanical strength and plateau stress,and increase the energy absorption capacity.2.The simulation analysis method based on the Johnson-Cook plasticity and damage model can accurately predict the mechanical strength and energy absorption performance of the TPMS lattice structure,revealing the failure process and fracture behavior of the lattices.This provides essential guidance for structural optimization and performance improvement.3.The I-WP sheet exhibits the best performance among the four typical TPMS lattice structures through surface offset.Compared with rod-shaped lattices,the mechanical strength,plateau stress,and energy absorption of sheet-shaped lattices increased by 244.9%,335.7%,and 312.7%,respectively.This is mainly attributed to the transformation of the deformation mode contributed by the surface offset,which minimizes the 45°shear fracture behavior and improves the plateau stress of the lattice structure,accompanied by a layer-by-layer collapse for deformation optimization.In summary,the surface offset design and Johnson-Cook simulation model were adopted for TPMS lattices in this study,which provides a reference for optimization strategies of lattice structures.Further studies on the fatigue performance of TPMS lattice structures should be conducted to facilitate the development of new lightweight structures in laser additive manufacturing.