This document is downloaded from DR-NTU, Nanyang Technological University Library, Singapore.

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1 This document is downloaded from DR-NTU, Nanyang Technological University Library, Singapore. Title Characterisation of micro-lattices fabricated by selective laser melting Author(s) Citation Sing, Swee Leong; Wiria, Florencia Edith; Yeong, Wai Yee Sing, S. L., Wiria, F. E., & Yeong, W. Y. (2016). Characterisation of micro-lattices fabricated by selective laser melting. Proceedings of the 2nd International Conference on Progress in Additive Manufacturing (Pro- AM 2016), Date 2016 URL Rights 2016 by Pro-AM 2016 Organizers. Published by Research Publishing, Singapore

2 CHARACTERISATION OF MICRO-LATTICES FABRICATED BY SELECTIVE LASER MELTING SWEE LEONG SING SIMTech-NTU Joint Lab (3D Additive Manufacturing), Nanyang Technological University, HW , 65A Nanyang Drive, Singapore , Singapore Singapore Centre for 3D Printing, School of Mechanical & Aerospace Engineering, Nanyang Technological University, HW , 2A Nanyang Link, Singapore , Singapore FLORENCIA EDITH WIRIA SIMTech-NTU Joint Lab (3D Additive Manufacturing), Nanyang Technological University, HW , 65A Nanyang Drive, Singapore , Singapore Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore , Singapore WAI YEE YEONG SIMTech-NTU Joint Lab (3D Additive Manufacturing), Nanyang Technological University, HW , 65A Nanyang Drive, Singapore , Singapore Singapore Centre for 3D Printing, School of Mechanical & Aerospace Engineering, Nanyang Technological University, HW , 2A Nanyang Link, Singapore , Singapore ABSTRACT: β-titanium alloys have been touted as the new titanium alloys in biomedical applications due to its lower elastic modulus as compared to the titanium alloys of other phases. In particular, titanium-tantalum has been explored for such applications due to the high biocompatibility of both titanium and tantalum. The alloying and fabrication of titanium-tantalum using selective laser melting have been proven in previous study. In this study, the effect of SLM processing parameters on the porosity and compression behaviour of titanium-tantalum microlattice structures is investigated. The as-fabricated micro-lattices have elastic constants ranging from 1.36 ± 0.11 GPa to 6.82 ± 0.15 GPa and yield strength of between ± 3.79 MPa and ± MPa. The range of mechanical properties exhibited by the lattice structures shows the versatility of SLM in producing titanium-tantalum lattice structures for orthopaedic applications. KEYWORDS: Selective Laser Melting, Titanium, Tantalum, Cellular, Lattice Structures INTRODUCTION With the advancement in additive manufacturing (AM) techniques, the application of AM has been extended from tissue engineering (Chua et al. 2005, Yeong et al. 2005, Tan & Yeong 2014, Proc. of the 2nd Intl. Conf. on Progress in Additive Manufacturing Edited by Chee Kai Chua, Wai Yee Yeong, Ming Jen Tan, Erjia Liu and Shu Beng Tor Copyright 2016 by Pro-AM 2016 Organizers. Published by Research Publishing, Singapore ISSN:

3 Proc. of the 2nd Intl. Conf. on Progress in Additive Manufacturing Lee et al. 2016) to orthopaedic field. Selective laser melting (SLM) is a powder bed fusion additive manufacturing technology that is capable of forming fully dense metallic functional parts directly (Chua & Leong 2014, Sing et al. 2015). Details of SLM process have been described in various studies (Loh et al. 2014, Loh et al. 2015, Yap et al. 2015). With SLM, it is now possible to fabricate micro-lattices based on computer aided design (CAD) for various applications. In particular, this capability has provided opportunities in creating porous surfaces for orthopaedic implants to improve osteo-integration and reduce stress-shielding (Van der Stok et al. 2013). Stress shielding induces an unfavourable stress distribution at the bone-implant interface, resulting in slower bone healing (Heinl et al. 2008, Traini et al. 2008) and clinical investigations indicate that the mismatch will result in insufficient load transfer from artificial implants to neighbouring bones, resulting in bone resorption and potential loosening of the implant (Laheurte et al. 2010). Various studies have been conducted in the fabrication of lattice structures using SLM. This process has shown great potential in this area. The manufacturability of porous Ti6Al4V structures by comparing the fabricated parts to the design, in terms of pore size, strut thickness, porosity, surface area and structure volume has been evaluated (Van Bael et al. 2011). The mechanical properties of a topologically optimized lumbar interbody fusion cage made of Ti6Al4V by SLM were studied (Lin et al. 2007). The effect of unit cell size on the manufacturability, density and compression properties of the manufactured structures was investigated using a repeating unit called gyroid (Yan et al. 2012, Yan et al. 2014). There is limited information on the effect of processing parameters of SLM on the quality and mechanical properties of lattice structures. In this study, the effect of SLM processing parameters on the porosity and compressive properties of micro-lattices fabricated using SLM will be studied. METHODS AND MATERIALS Selective laser melting Fabrication of the micro-lattices was carried out on a SLM 250HL machine (SLM Solutions Group AG, Germany). The SLM machine is equipped with a Gaussian beam fiber laser with maximum power of 400 W and a focal diameter of 80 μm. The processing parameters used in this study are shown in Table 1. Table 1 SLM process parameters, and mechanical properties for micro-lattices Layer thickness (mm) Laser power (W) Scanning speed (mm/s) Porosity (%) Elastic constant (GPa) Yield strength (MPa) ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

4 Chee Kai Chua, Wai Yee Yeong, Ming Jen Tan, Erjia Liu and Shu Beng Tor (Eds.) The micro-lattices were produced using a mixture of commercially pure titanium (ASTM B348 grade 2) powder from LPW Technology Ltd, United Kingdom and tantalum powder from Singapore Demand Planner Ltd, Singapore. Details of the mixing and characterisation of the powder have been described previously (Sing et al. 2016a). All processing occurred in an argon environment with less than 0.05 % oxygen to prevent oxidation and degradation of the material during the process (Loh et al. 2014). SLM of the powder mixture will results in titanium-tantalum (TiTa) alloy. Design of micro-lattices The cellular lattice structures used is specially designed in order to study the effect of processing parameters on the quality and mechanical properties of these structures. The unit cell designed consist of vertical, horizontal and diagonal square struts of 80 μm sides which corresponds to the laser spot size in the SLM 250 HL. Vertical, horizontal and diagonal struts are chosen to investigate the different building direction capabilities of SLM. The dimensions of the repeating unit cell were 1 mm by 1 mm by 1 mm. The generated CAD diagram is shown in Figure 1(a). The overall dimension of the lattice structures is 10 mm by 10 mm by 10.5 mm, allowance is given in the height to allow for erosion of materials from electrical discharge wire cutting of the samples from the substrate plate. Figure 1 (a) CAD file (b) as-fabricated micro-lattices used for characterisation. Characterisation of micro-lattices Dry weighing occurred under normal atmosphere conditions using a XSE Analytical Balance with sensitivity of g and repeatability of g (Model XS 204, Mettler Toledo), and the density of the samples ρ abs was calculated by dividing the actual weight by the macro volume. The porosity of the samples is obtained using the formula as follows: (1) where ρ theoretical is taken to be 7.10 g/cm 3 (Sing et al. 2016a). The fabricated cubic samples have designed dimensions of 10 mm by 10 mm by 10 mm, which are used as test coupons for compression tests based on ISO (Mechanical testing of metals - Ductility testing - Compression test for porous and cellular metals). Uni-axial compression tests were carried out, at room temperature (25 o C), to assess the compressive 196

5 Proc. of the 2nd Intl. Conf. on Progress in Additive Manufacturing properties of the lattice structures, each with three replicates (n = 3), by using Instron Static Tester Series 5569 (Instron, United States) equipped with a 50 kn load cell. The loading speed was set at a constant of 0.6 mm/min, so as to maintain a constant strain rate for all tests as recommended (Yap et al. 2014). The compressive deformation rate has to be set such that the strain rate experienced by the samples are constant throughout (Sing et al. 2016b). The compression tests were carried out until axial deformation of the samples was equal to 100 % or when the maximum loading of 50 kn was reached, whichever came first. The stress-strain curves, yield strengths and elastic constants in compression of the as-fabricated samples were then obtained through the compression tests. The as-fabricated samples replicate the shapes of the CAD files designed for this experiment, as shown in Figure 1(b). The samples show that the designed CAD models can be fabricated successfully using SLM. RESULTS AND DISCUSSION The resulting porosity, elastic constant in compression and yield strength of the as-fabricated micro-lattices are shown in Table 1. Due to the effect of the SLM process parameters, the elastic constant of the micro-lattices can range from 1.53 ± 0.65 GPa to 6.82 ± 0.15 GPa. The microlattices yield strength also varies from ± 0.76 MPa to ± MPa. These show that there is a need for careful control of the process parameters during the micro-lattice fabrication so as to obtain the desired mechanical properties. It also shows the versatility of TiTa micro-lattices in orthopaedic applications where bones have wide range of elastic constants (Zysset et al. 1999). Using the Gibson Ashby model (Gibson & Ashby 1997), the elastic modulus and yield strength at different porosities can be estimated. The experimental values of elastic constant, yield strength and their corresponding theoretical values based on the Gibson-Ashby model is plotted in Figure 2. Figure 2 Theoretical and experimental values of (a) elastic constant (b) yield strength. Using the Gibson-Ashby model, the empirical formulae for elastic constants and yield strength are obtained. (2) (3) 197

6 Chee Kai Chua, Wai Yee Yeong, Ming Jen Tan, Erjia Liu and Shu Beng Tor (Eds.) where E, Y s and ρ are the apparent elastic constant, yield strength and density of the lattice structures, E 0, Y 0 and ρ 0 are the elastic constant, yield strength and density of fully dense material respectively. P r is the porosity of the lattice structures. The elastic constant and yield strength is taken to be ± 4.04 GPa and ± MPa respectively from the fully dense TiTa (Sing et al. 2016a). It is observed that the elastic modulus and yield strength of the as-fabricated micro-lattices decreases with increase in porosity, which is consistent with the prediction from the Gibson-Ashby model. The differences between the theoretical and experimental values may be attributed to the residual stress inherent due the SLM process, waviness and roughness of the strut surfaces (Yan et al. 2014). It can also be due to the disregard of the SLM process parameters which affects the powder adhesions onto the struts which will affect the compressive properties of the lattice structures. The model also failed to take into account the type of failure underwent by the lattice structures and geometrical information of the micro-lattices. CONCLUSIONS AND FUTURE WORK In this study, micro-lattices were fabricated using SLM using different processing parameters and it is found that micro-lattices can be fabricated successfully using mixture of powder by SLM. In this study, a mixture of commercially pure titanium and tantalum powders is used to produce TiTa micro-lattices. Different SLM processing parameters can results in different porosity of the microlattices fabricated even with the same CAD design. Hence, there is a need for careful control of the parameters to obtain the desired properties of the micro-lattices. The different micro-lattices porosities result in different mechanical properties and these properties can be predicted using Gibson-Ashby model. The suggested future work for this study includes in-depth analysis of the effect of different processing parameters in affecting the manufacturability of micro-lattices using SLM and finite element analysis (FEA) can be carried out to study the facture mechanisms of the micro-lattices and analyse the effect of process parameters on the fracture mechanisms can be carried out. REFERENCES Chua, C. K. and K. F. Leong (2014). 3D Printing and Additive Manufacturing: Principles and Applications. Singapore, World Scientific Publishing Co. Pte. Ltd. Chua, C. K., W. Y. Yeong and K. F. Leong (2005). "Rapid prototyping in tissue engineering: A state-of-the-art report." Virtual Modeling and Rapid Manufacturing, Gibson, L. J. and M. F. Ashby (1997). Cellular solids: structures and properties. New York, Cambridge University Press. Heinl, P., L. Muller, C. Korner, R. F. Singer and F. A. Muller (2008). "Cellular Ti-6Al-4V structures with interconnected macro porosity for bone implants fabricated by selective electron beam melting." Acta Biomaterialia 4(5), Laheurte, P., F. Prima, A. Eberhardt, T. Gloriant, M. Wary and E. Patoor (2010). "Mechanical properties of low modulus β titanium alloys designed from the electronic approach." Journal of the Mechanical Behavior of Biomedical Materials 3, Lee, J. M., M. Zhang and W. Y. Yeong (2016). "Characterization and evaluation of 3D printed microfluidic chip for cell processing." Microfluidics and Nanofluidics 20(1), Lin, C. Y., T. Wirtz, F. LaMarca and S. J. Hollister (2007). "Structural and mechanical evaluations of a topology optimized titanium interbody fusion cage fabricated by selective laser melting process." Journal of Biomedical Materials Research Part A 83(2),

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