| Author: | Hu, Dien |
| Title: | Optimizing mechanical and functional properties of light-weighting stainless steel 316L triply periodic minimal surface structures fabricated using micro laser powder bed fusion |
| Advisors: | Fu, Mingwang (ME) |
| Degree: | Ph.D. |
| Year: | 2026 |
| Department: | Department of Mechanical Engineering |
| Pages: | xxi, 165 pages : color illustrations |
| Language: | English |
| Abstract: | Triply periodic minimal surface (TPMS) structures have distinct geometric features and demonstrate excellent mechanical and multifunctional performance, making them promising for a variety of industrial applications. The development of micro laser powder bed fusion (μLPBF), an additive manufacturing technology, means that low-density metallic TPMS structures with ultra-thin walls can now be produced easily, thereby advancing their use in lightweight design. Stainless steel (SS) 316L, a classic austenitic SS, is renowned for its excellent corrosion resistance, favorable mechanical properties, and biocompatibility. Its extensive use in TPMS structures has unlocked significant potential for lightweight applications, enabling the fabrication of components that integrate structural integrity, corrosion resistance, and additional functional capabilities. However, the mechanical response of SS316L TPMS structures is undesirable owing to the inherent weak material properties, limiting the applications of SS316L TPMS structures in extreme environments. This thesis aims to enhance the μLPBFed SS316L TPMS structures with the trait of lightweight via processing optimization in terms of pre- and post-processing design. Composites TPMS structures offer a promising approach to tailor material properties by combining the advantages of constituent materials. A novel composites SS316L-AlCoCrFeNi2.1 functionally graded (FG) TPMS structure was developed by μLPBF. A dual structural design enables the integration of the inherent high strength of the low relative-density AlCoCrFeNi2.1 part with the high relative density-induced excellent load-bearing capacity of the SS316L section. During compression along building direction, the deformation behavior of the composites FG structures was transformed from layer-by-layer to a coordinated deformation during the stress plateau stage, characterized by a relatively stable stress level as strain increases, undergoing strengthening, stress plateau, and strengthening stages at a high stress level. The participation of the AlCoCrFeNi2.1 section also induces an enhanced mechanical response during the transversal compression. As a result, the developed structure processes enhanced mechanical properties, including plateau stress and energy absorption capacity, compared to the pure SS316L counterparts. Based on the findings of the composites FG TPMS structures. A compositionally graded design strategy for Primitive (P) and Gyroid (G) type TPMS structures with a low relative density was proposed to overcome the strength (Cr-rich SS)-ductility (SS316L) dilemma of TPMS structures, coupled with potential vulnerabilities to shear failure and inhomogeneous deformation. This novel structure is designed to feature a continuous transition in Cr content, ranging from a Cr-rich unit cell layer (30 wt.% Cr), through two transition layers (25.5 wt.% and 21 wt.% Cr), to a standard SS316L unit cell layer. Experimental and simulated results show that the developed structure demonstrates a distinct layer-by-layer deformation mechanism and significantly delays the densification of the P-type structure, and the Cr addition-induced enhanced inherent high strength ensures a desirable overall load-bearing capacity. Thus, the developed compositionally graded design structures demonstrate an evenly matched energy absorption capacity in comparison to the pure 30Cr SS TPMS structures with a predictable and tunable deformation mode. Meanwhile, the addition of Cr content facilitates an enhanced corrosion resistance of the developed TPMS structures in comparison to the SS316L structures. In addition to pre-processing, heat treatment, an effective post-processing method has the potential to enhance the multifunctional performances of SS316L TPMS structures, such as thermal and electrical conductivity, but it usually results in a weakening effect on the strength of the components. An abnormal phenomenon of coarse grains and slender walls-induced mechanical strengthening behavior was observed in the TPMS structures fabricated via μLPBF. The results indicate that a homogenized internal material distribution at the grain level leads to obvious localized strengthening behaviors in the TPMS structures during the localized and densification stage in the compression process. As the grains become coarser or the walls become thinner, the deformation mode of the TPMS structures transforms from the localized collapse deformation to the localized coordinated deformation, in which a homogeneous internal grain distribution is triggered by grain coarsening and wall thinning, promoting a homogeneous stress distribution. Overall, this thesis presents novel optimized processing methods via μLPBF to realize SS316L-based TPMS structures with superior mechanical and multifunctional performances. Meanwhile, the corresponding enhancing mechanisms of the proposed methods were proven. This thesis outlines a guideline for acquiring an excellent synergy of the mechanical properties and multifunctional performances of the SS316L TPMS structures. |
| Rights: | All rights reserved |
| Access: | open access |
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