PhD Thesis Colloquium: Mr. Saurabh Kumar Gupta (07/05/24)
Thesis title:
Additive Manufacturing of Titanium Alloys for Orthopaedic applications: Processing-Microstructure-Property-Function Relationship
Faculty advisor(s):
Prof. Satyam Suwas (Co-Supervisor - Kaushik Chatterjee)
When?
07th May, 2024 (Monday), 04:00 PM (India Standard Time)
Where
KPA Auditorium, Department of Materials Engineering
Abstract:
Metallic implants are crucial for osteosynthesis, aiming for effective healing outcomes. These situations often demand patient-specific treatments, as standard implants may not deliver expected functional outcomes. Additive manufacturing (AM) offers a promising solution by enabling the fabrication of near-net shape parts. However, the properties of AM-fabricated alloys differ significantly from conventionally processed ones due to inherent high cooling rates and thermal gradients during AM, thus altering the microstructural features.
In this thesis, two different Ti alloys, namely Ti-6Al-4V ELI (α+β) and Ti-35Nb-5Ta-7Zr (β) were investigated for biomedical applications, particularly for load-bearing orthopedic implants. The first part of the thesis is focused on applying cyclic heat treatment to modify the microstructure of bone plates (prepared by selective laser melting, SLM) from acicular martensite to bimodal, significantly enhancing biomechanical performance, especially in bending. A comparison between additively manufactured, heat-treated (HT), and wrought (WR) Ti-6Al-4V ELI (Ti64) has been brought out, considering microstructural (including texture), mechanical, electrochemical, and biological aspects. The study revealed comparable biomechanical performance between additively manufactured materials after heat treatment and wrought materials. Subsequent fracture toughness evaluation indicated poorer values for AM samples, showing strong anisotropy in crack initiation along the build direction. However, heat treatment effectively improved fracture toughness in both orientations. It has been observed that hard orientations impede crack initiation, while twinning activity aids in blunting crack tips and limiting propagation. Tribocorrosion measurements revealed much more wear volume loss due to the synergistic effect of wear and corrosion. The mechanism of tribocorrosion varied depending on the testing conditions; wear was dominant under open circuit potential (OCP) conditions, whereas wear-induced corrosion prevailed under Potentiostatic conditions. The performance of HT samples was significantly improved compared to wrought Ti64, as reported in the literature.
Building on these findings, patient-specific implants, biomechanically improved post AM through heat treatment, were evaluated in human subjects with upper limb complications, resulting in safe and effective treatment for malunions and deformities, with reduced surgical time. This constitutes the second part of the thesis.
The last part is focused on developing an alloy with the composition Ti-35Nb-5Ta-7Zr using directed energy deposition (DED) technique. Nearly dense components exhibited complete β phase stability, lower elastic modulus, strong crystallographic texture, high strength, good ductility, excellent corrosion resistance, and cytocompatibility.
Overall, the work presented in the thesis has important implications for development and translation of additively manufactured Ti alloys for orthopedic implants.