Mechanical Property Relationship Of Metallic TPMS Structures Under Additive Manufacturing

Designing a triply periodic minimum surface structure(hereinafter referred to as TPMS structure)foam metal is a new method applied to lightweight,multi-functional structures.Unlike previous cellular and grid structure foam metals,the TPMS structure consists of a continuous and smooth shell that allows for large surface areas and continuous internal passages to avoid stress concentrations.Additive Manufacturing(AM)technology is a technique for designing model files by CAD or SolidWorks and then manufacturing the physical parts in a layer-by-layer manner,also known as 3D printing technology.Recent developments in additive manufacturing have facilitated the fabrication of components with large geometric complexity and relatively small dimensions,enabling rapid prototyping of topologies that are not possible with conventional manufacturing techniques.In this thesis,we mainly carried out research on the mechanical properties of 316 L stainless steel foam metal for two TPMS structures(Primitive and Gyroid surfaces,hereinafter referred to as P and G surfaces,respectively)through the combination of finite element simulation and experimental study.The mechanical simulation of this thesis was carried out by ABAQUS6.14 finite element software.The dynamics algorithm was used to simulate the quasi-static compression mechanics of the full-scale models of P and G surfaces respectively.The simulation results and the test results were mutually verified.The P and G type TPMS structures with different relative densities were fabricated by the SLM additive manufacturing technology.The quality of printed samples was evaluated by optical microscope,scanning electron microscope(SEM),universal testing machine and DIC strain measurement system.Through the mechanical simulation and experimental research,the main findings of this thesis are as follows:(1)The 316 L stainless steel TPMS structure samples fabricated by SLM technology showed good end product quality,with the relative density of the matrix materials being over 98%.Under the SEM observation,there are no severe macroscopic holes and cracking defects on the surface and inside of the sample.(2)The as-fabricated standard tensile specimens exhibited typical austenitic steel mechanical behavior with a high yield strength(530 MPa)and elongation(48%).These properties were also predicted by finite element model.The results are very consistent,and the observation at the SEM fracture indicates that the fracture mode was a ductile failure mode.(3)The elastic behavior and post-yield mechanical behavior of P and G structures predicted by the finite element model were consistent with the experimental results.The simulation results showed that the G-curved structure showed a relatively uniform stress distribution on all unit cells under compression,resulting in a stable collapse mechanism and required energy absorption performance.In contrast,the P-surface showed a rapid diagonal shear band development followed by a partial wall bend.This indicates that deformation mechanism of the TPMS structures largely depend on the geometry of the unit cell.(4)It is also found that the elastic modulus,yield strength,platform stress and toughness of the P surface are 115%,94%,64%,66%,being higher than those of the G surface at lower relative density(? =20%).At higher relative densities(? =40%),the elastic modulus and yield strength of the P-surface are 15% and 2% higher than those of the G-curved surface,while the platform stress and toughness are 13% and 10% lower than those of the G-surface.(5)Following different heat treatments,it was found that the microstructure of 316 L stainless steel laser rapid prototyping showed a complete austenitic phase structure.In the heat treatment temperature range of 650~1050 °C for 2h,the lowest hardness value was found in the sample that was heat treated at 750 °C.This sample showed an ideal coarse grain microstructure,indicating of the partial elimination of the high internal stress caused by rapid laser prototyping.For the TPMS structural compression test after heat treatment,the heat treatment reduced the tensile yield strength of 316 L stainless steel by 19% and the elongation at break by 4%.