materials Article Effect of Fillets on Mechanical Properties of Lattice Structures Fabricated Using Multi-Jet Fusion Technology Aamer Nazir 1,2 , Ahmad-Bin Arshad 1,2 , Chi-Pin Hsu 2,3 and Jeng-Ywan Jeng 1,2,4, *   Citation: Nazir, A.; Arshad, A.-B.; Hsu, C.-P.; Jeng, J.-Y. Effect of Fillets on Mechanical Properties of Lattice Structures Fabricated Using Multi-Jet Fusion Technology. Materials 2021, 14, 2194. https://doi.org/ 10.3390/ma14092194 Academic Editors: Alexander A. Gromov and Patricia Krawczak Received: 8 March 2021 Accepted: 22 April 2021 Published: 24 April 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Mechanical Engineering, National Taiwan University of Science and Technology, 43 Keelung Road, Section 4, Taipei 10607, Taiwan; aamernazir.an@mail.ntust.edu.tw (A.N.); ahmed9c23@gmail.com (A.-B.A.) 2 High Speed 3D Printing Research Center, National Taiwan University of Science and Technology, No. 43, Section 4, Keelung Road, Taipei 10607, Taiwan; bingohsu@mail.ntust.edu.tw 3 Graduate Institute of Biomedical Engineering, National Taiwan University of Science and Technology, No. 43, Section 4, Keelung Road, Taipei 10607, Taiwan 4 President Office, Lunghwa University of Science and Technology, No.300, Sec.1, Wanshou Rd. Guishan District, Taoyuan City 333326, Taiwan * Correspondence: jeng@mail.ntust.edu.tw Abstract: Cellular structures with tailored topologies can be fabricated using additive manufacturing (AM) processes to obtain the desired global and local mechanical properties, such as stiffness and energy absorption. Lattice structures usually fail from the sharp edges owing to the high stress concentration and residual stress. Therefore, it is crucial to analyze the failure mechanism of lattice structures to improve the mechanical properties. In this study, several lattice topologies with fillets were designed, and the effects of the fillets on the stiffness, energy absorption, energy return, and energy loss of an open-cell lattice structure were investigated at a constant relative density. A recently developed high-speed AM multi-jet fusion technology was employed to fabricate lattice samples with two different unit cell sizes. Nonlinear simulations using ANSYS software were performed to investigate the mechanical properties of the samples. Experimental compression and loading– unloading tests were conducted to validate the simulation results. The results showed that the stiffness and energy absorption of the lattice structures can be improved significantly by the addition of fillets and/or vertical struts, which also influence other properties such as the failure mechanism and compliance. By adding the fillets, the failure location can be shifted from the sharp edges or joints to other regions of the lattice structure, as observed by comparing the failure mechanisms of type B and C structures with that of the type A structure (without fillets). The results of this study suggest that AM software designers should consider filleted corners when developing algorithms for generating various types of lattice structures automatically. Additionally, it was found that the accumulation of unsintered powder in the sharp corners of lattice geometries can also be minimized by the addition of fillets to convert the sharp corners to curved edges. Keywords: additive manufacturing; lattice structure; design for AM; unit cell; fillets; energy absorp- tion; loading–unloading 1. Introduction Cellular structures with multifunctional properties are common in nature and have shown promise for applications in the automotive, biomedical, and aerospace industries. Studies have revealed that mechanical properties, such as the strength-to-weight ratio [1–4], stiffness [5], acoustic [6] and thermal properties [7], energy absorption [8–10], electrical con- ductivity [11], and impact resistance [12], can be simultaneously improved by tailoring the topology of the cellular structures. Recent developments in additive manufacturing (AM) technologies, such as improved accuracy, higher fabrication speed, and the use of advanced materials, make it possible to fabricate cellular structures with intricate architectures, thus Materials 2021, 14, 2194. https://doi.org/10.3390/ma14092194 https://www.mdpi.com/journal/materials