Microstructure and mechanical properties of 3D printed beta titanium alloys

• Main Author: Chew, Jian Eng
• Other Authors: Matteo Seita
• Format: Final Year Project
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Materials::Biomaterials
• Online Access: https://hdl.handle.net/10356/147375
• Institution: Nanyang Technological University

The primary goal of this project is to characterize the microstructure and mechanical properties of Ti-45Nb alloy fabricated by the selective laser melting (SLM) targeted for orthopaedic applications. Optimised SLM printing parameters were applied in the fabrication of Ti-45Nb alloys samples to focus on suitability of zigzag and alternating zigzag scanning strategies on obtaining strongly textured microstructures for desired mechanical properties such as modulus, strength, and ductility. The findings of this work demonstrate the feasibility of fabricating the Ti-45Nb alloy parts with well-developed crystallographic texture and grain morphology using SLM from the pre-alloyed mixture of elemental powders of pure titanium and pure niobium. To fabricate the Ti-45Nb alloy, selective laser melting, one of several additive manufacturing (AM) technologies, was chosen as the method of fabrication. Owing to the large disparity in melting points between Ti (1668°C) and Nb (2469°C), SLM was used to address the issue of mixing materials with significantly different melting temperatures which is otherwise not feasible using other conventional or traditional processing techniques. In the SLM technique, use smallersize distribution of niobium powder particles compared to Titanium and higher laser energy and volumetric density to solve the problem of inhomogeneous melting, resulting in a homogeneous microstructure with a fully beta Ti phase with improved mechanical properties. Furthermore, SLM can use a tailored low thermal gradient (G) to growth rate (R) ratio to achieve near equiaxed grain morphology and eliminate unwanted anisotropic mechanical properties. Biomedical implants with a long service life are expected in the orthopaedic industry to repair lost, broken, or weakening bone structure. Unfortunately, the operation lifespan is often disrupted by implant failure attributable to the impact of "stress shielding", which is mostly affected by a Young's modulus mismatch between the bone (10 – 30 GPa) and implant (> 30 GPa). The Ti-45Nb alloy framework was explicitly selected in the report to resolve this prevalent problem. A biocompatible niobium alloying element is used in this Ti-45Nb alloy. Furthermore, the Nb content in Ti allows for total β phase preservation, resulting in a low modulus alloy with strong ductility and ample yield strength. Due to the optimized microstructural properties on mechanical properties via SLM, Ti-45Nb is expected to have a longer service lifespan than most commonly available implant products when manufactured.