A Study On The Mechanical Properties Of Metallic Porous Structures Fabricated Using Selective Laser Melting And Its Variable-Density Design Method

In order to improve the research level on Additive Manufacturing(AM),China’s State Council has listed additive manufacturing in “National Outline for Medium and Long Term S&T Development(2006-2020)” and “Made in Chine 2025”.Aiming to the urgent demand for superior lightweight components which are widely used in sectors such as aviation and aerospace,automobile industry,this dissertation investigates metallic porous structures manufactured using selective laser melting(SLM).In particular,in order to study the mechanical properties of metallic porous structures and achieve the optimization design of variable density porous structures,this dissertation uses various theories,such as material mechanics,structural mechanics,static and dynamic finite element method,numerical analysis,data mining and optimization design etc.,and conducts micro detection and mechanical experiments on SLM-fabricated samples in order to study the static and dynamical mechanical properties of SLM-fabricated porous structures.Then based on the previous studies on mechanical analysis on homogeneous metallic porous structures,a novel design method based on data mining and optimization design theory is proposed for variable-density metallic porous structures.The main contents of this dissertation are shown as follows:(1)Based on Euler-Bernoulli beam theory and taking the body-centroid cubic(BCC)lattice structures as an example,this paper proposes a theoretical method for calculating the static mechanical properties of “bending-dominant” porous structures under selective laser melting.Calculation results reveals that the equivalent elastic moduli and initial yield stresses of BCC lattice structures are proportion to the second power and 1.5th power of their relative densities,while the mechanical properties of BCC structures are independent of the sizes and the number of unit cells.The validity of the proposed theoretical method is verified via multi-cells finite element analysis based on beam element method and quasi-static uniaxial compression tests on Titanium Ti6Al4 V lattice structures samples manufactured using selective laser melting.(2)Based on the dynamic numerical analysis theory,this paper proposes an analytical modelling approach – dynamic analytical method for predicting the mechanical properties of metallic porous structures with variable topologies and shapes manufactured using selective laser melting.Taking rigid frame-like lattice structure and 3D spatial minimal curved surface(Gyroid)structure as examples,the mechanical responses of the structures sustaining quasi-static compression load and impact load are calculated analytically using dynamic analytical method.The results reveals that,under loading conditions,the maximum von Mises stress is located in the vicinity of the joints for rigid frame-like lattice structures and located where curvature of the surface is minimum for Gyroid porous structures.(3)This paper conducts micro detection on appearance accuracy and interior morphology of metallic porous structures with micro overhangs and spatial minimal curved surface structures.Using microscopy observation,obvious balling phenomena and powder adhering are observed,which reveals that overhangs should be avoided in the design state of porous structures for selective laser melting in order to improve manufacturing accuracy and avoid obvious defects.In order to evaluate the SLM’s capability on manufacturing Gyroid porous structures,using optical microscopy and micro computed tomography(micro-CT)observe exterior and interior morphology of Gyroid structures,no obvious warping and manufacturing defects are found.(4)This paper also conducts quasi-static and impact experiments on bending-dominant lattice structures samples,uniaxial reinforcement lattice structures samples and Gyroid porous structures samples.Results shows that the static and dynamic mechanical performance of metallic porous structures improves when their relative densities increases.Comparing the results obtained from theoretical analysis,beam element method,dyamic analytical method,and experiment study,using dynami analytical method can predict correctly the change of stress-strain curves of all porous structures under different loading conditions.However,the accuracy of the results on equivalent elastic moduli predicted by quasi-static analysis is decreased when the structures’ relative densites increases or the structures are more complicated.,while the errors between the predictions on the initial yield stresses of the structures by quasi-static analysis and the corresponding experiment data are under 30%(5)Finally,this paper proposes a data mining-based method for achieving stress-matching design and conducting sizes optimization for variable-density metallic porous tructures.This method conducts K-means clustering on equivalent stress conditions of all designing units,and compares mean silhouette values when using different clustering numbers,then opmizing culstring number is obtained.Taking three point bending beam as an example,the numbers of designing units decreases from 625 to 8,and the modelling and optimization speed is rapidly increased.The volume of the variable-density porous beam is only 1/3 of that of the corresponding solid beam,hence the lightweigh performance of the beam is improved.