Date on Master's Thesis/Doctoral Dissertation

12-2017

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Industrial Engineering

Degree Program

Industrial Engineering, PhD

Committee Chair

Yang, Li

Committee Co-Chair (if applicable)

Stucker, Brent

Committee Member

DePuy, Gail

Committee Member

Starr, Thomas

Author's Keywords

additive manufacturing (AM); laser powder bed fusion (LPBF); cellular structure; material optimization; geometrical design; integrated material/structure model

Abstract

Powder bed fusion additive manufacturing (PBF-AM) has been broadly utilized to fabricate lightweight cellular structures, which have promising potentials in many engineering applications such as biomedical prosthesis, aerospace, and architectural structures due to their high performance-to-weight ratios and unique property tailorabilities. To date, there is still a lack of adequate understanding of how the cellular materials are influenced by both the geometry designs and process parameters, which significantly hinders the effective design of cellular structures fabricated by PBF-AM for critical applications. This study aims to demonstrate a cellular structure design methodology that integrates geometrical design and process-material property designs. Utilizing both analytical modeling and empirical modeling, this research aims to significantly improve the design flexibility and robustness of the metal PBF-AM cellular structures. Experimental designs were carried out to establish the process-material property knowledge for the Ti6Al4V using the EOS M270 laser powder bed fusion (LPBF) system. Utilizing optical microscopy, scanning electron microscopy (SEM), micro-tensile testing and micro-hardness testing, the characteristics of thin struts with different strut dimensions and orientations under different process conditions were characterized and compared with those of the bulk materials from LPBF. The results clearly indicated significant effects of strut geometries (dimension and orientation angle) on their qualities. Struts with large orientation angle (i.e. more aligned to the build direction) exhibit lower process robustness and are sensitive to process parameters. Due to the resolution limitation of the LPBF process, the geometrical accuracies of the struts increases drastically when the designed dimension is smaller than 0.2mm, with minimum achievable dimensions around 0.2mm for the process parameters investigated in this study. On the other hand, the struts with smaller dimensions tend to exhibit higher mechanical properties, which might be associated with the smaller grain size and lower porosities. There also does not appear to be a single set of process parameters that would result in minimum porosities for struts with various dimensions. Utilizing center-joint based unit cell design, an analytical model for unit cells with designable numbers of struts was established in the attempt to enhance geometry designabilility of the geometries. Timoshenko beam theory was verified to be the most accurate modeling method, although for large strut orientations Euler-Bernoulli beam theory might suffice. The analytical model for elastic modulus was verified by finite element analysis (FEA) and experiments. Additionally, predictable size effect was modeled for the cellular structures with the center join connectivity of 8 (octahedral). Utilizing the material property database for the cellular designs, the integrated material performance/structural geometry model was demonstrated for both single strut sand small cellular patterns. It was shown that the integrated model is able to provide improved prediction to the properties of cellular structures.

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