Design And Optimization Of Heterogeneous Lattice Structure For Additive Manufacturing

The lattice structure has the characteristics of low density and high specific stiffness,and it has a very wide application scope in many fields due to its excellent structural properties.However due to its complex geometries,it is difficult for the conventional technique of manufacturing to realize lattice structure directly.Additive manufacturing(AM)is an emerging technique that provides a great flexibility for the fabrication of complex structures.AM can achieve rapid preparation of lattice structures.At the same time,a large number of studies have shown that heterogeneous structures have better performance than homogeneous structures,so it is very meaningful to obtain a heterogeneous lattice structure by optimizing the homogeneous lattice structures.Aiming at the above background,this paper takes the lattice structure as the research object,proposes an architecting method and its parametric description for lattice structures,and proposes several kinds of heterogeneous lattice structure(including heterogeneous lattice filling structure)optimization design method.Specific research content and results include:(1)Finite-Element-Mesh based method for modeling of lattice structures for AM.This paper propose a new modeling method based upon finite element mesh to create complex large-scale lattice structures for additive manufacturing.In the modeling method,meshing technique is employed to obtain the meshes and nodes of lattice structures for a given geometry.Then,a parametric description of lattice unit cells based on the element type,element nodes and their connecting relationships is developed.Once the unit cell design is selected,the initial lattice structure can be assembled by the unit cells in each finite element.Furthermore,modification of lattice structures can be operated by moving mesh nodes and changing cross-sectional areas of bars.The graded and non-uniform lattice structures can be constructed easily based on the proposed modeling method.(2)Optimization design method of heterogeneous lattice structure based on optimization of cross-sectional area of rods for AM.A size optimization algorithm based on MIST(moving iso-surface threshold)method is proposed to optimize the initial lattice structures(constructed by FEM based modeling method)to get heterogeneous lattice structures.The MIST method is recently formulated topology optimization method without requiring sensitivity analysis.In MIST,a physical response of an objective function is used.In this paper,we choose strain energy expression of the bar element as response function and cross-sectional area of the bar as variable values.The overall equivalent stiffness of the given structure was maximized by minimizing the total strain energy in the design domain whilst subject to a volume constraint.(3)Optimization design method for heterogeneous lattice structure based on concurrent optimization method of sparse distribution and cross-sectional area of lattice structures for AM.In this paper,a cluster-based optimization method for lattice structure based on MIST method is proposed.The feature length parameter is used to describe the spares distribution of lattice structures.The sparse distribution of the lattice structure is optimized by choosing the feature length parameter as the design variable.Combined with the optimization method of the cross-sectional area size of the lattice structure,the concurrent optimization design of the cross-sectional area size and sparse distribution of the lattice structure is realized.By optimizing the uniform lattice structure,the graded lattice structure is obtained.(4)Topology optimization design of heterogeneous shell-infill structure with connectable graded microstructures for AM.Shell-infill structure has excellent mechanical properties and wide application prospects,whose features can span several dimensional scales.Both the configurations and layout pattern of the infill(cellular lattices)have a great impact on the overall performance of the shell-infill structure.This paper presents a multi-scale density-based topology optimization method for generating simultaneously optimized shell and infill in the context of minimum compliance topology optimization.In macro-scale,a coating approach is used to obtain an optimized shell.The thickness of the shell is assumed to be uniform,which is guaranteed by two-step filtering process.In micro-scale,the numerical homogenization method is applied to evaluate the effective elasticity matrix of the microstructural infill.A family of connected graded microstructures(GMs)with similar topological features and material distribution patterns at their edges is developed by using physical response function in microstructure of MIST(moving iso-surface threshold)formulation.The method enables an optimized load-bearing solution through optimization of geometries of global shell structures and down scale infill microstructures,as well as global distributions of spatially-varying graded microstructures.