Integrated Study on Structure and Mechanical Properties of Additive Manufactured Ti-Nb-Zr Alloy by Selective Laser Melting Process

Abstract

Additive manufacturing-based dental and orthopedic implants have gained significant attention globally, owing to advancements in the technology. These implants offer several advantages, such as the ability to manufacture patient-specific implants with complex shapes that are difficult to achieve through conventional manufacturing methods. By using additive manufacturing techniques, such as selective laser melting (SLM), the degree of freedom, time, and cost required for production can be significantly reduced compared to traditional methods involving casting, heat treatment, machining, and more. The utilization of Titanium-based alloys, particularly the Ti-39Nb-6Zr (TNZ) alloy, has potential in the field of implant applications. These alloys are valued for their exceptional characteristics, including their lightweight composition, chemical stability, and remarkable biocompatibility, which enable effective integration with bone tissue during the osseointegration process. Biomedical applications demand materials with high biocompatibility, minimal toxicity, superior strength, low modulus, and excellent wear and corrosion resistance. The TNZ alloy, with its biocompatible composition and low modulus of elasticity resulting from its BCC crystal structure, is an ideal biomaterial for 3D printing-based implantology and osteoarticular surgery. Understanding the relationship between powder characteristics, process parameters, and resulting microstructure is vital for optimizing the SLM process for TNZ alloy in implant materials. Powder characteristics, such as particle size, morphology, and composition, can significantly influence the flowability, packing density, and powder bed quality during the SLM process. Proper powder selection and optimization can improve the uniformity and quality of the printed parts. Process parameters, including laser power, scanning speed, hatch distance, and layer thickness, play a crucial role in determining the thermal history and cooling rates experienced by the material during the SLM process. These parameters directly affect the microstructure and mechanical properties of the printed implants. Finding the optimal process parameters that balance productivity, quality, and material performance is essential. The high cooling rates during the SLM process can result in rapid solidification and fine-grained microstructures, which can affect the mechanical properties of the printed implants. The harsh thermal history experienced during the process can lead to the development of residual stresses within the printed structures. These residual stresses can affect the mechanical behavior of the implants. Therefore, understanding and mitigating residual stresses through post-processing treatments or process optimization is crucial to ensure the reliability and performance of the printed implants. This study aims to systematically investigate the microstructural properties of TNZ alloy produced through additive manufacturing, with a focus on improving its mechanical properties. The entire powder metallurgy process for additive manufacturing is performed, including the development of TNZ alloy powder through vacuum inert gas atomization. To prevent contamination during melting, a cold crucible is employed instead of a ceramic crucible, and the powder production is carefully controlled using a gas atomizer with a cold crucible. Mechanical properties are evaluated through compression tests, nanoindentation, SEM-EDS, and EBSD analysis to understand the microstructure factors formed during the SLM process. Varying the scanning speeds during SLM processing leads to different strength levels in the produced alloys. By comparing and analyzing the contribution of individual microstructural elements to reinforcement, it is observed that different energy inputs play a significant role in creating diverse microstructures with varying reinforcement contributions. The effect of morphological and crystalline organization on the mechanical behavior of SLM-treated TNZ alloys was investigated. Results demonstrate that the anisotropic grain structure formed along the cooling direction acts as a dominant factor in the significant differences observed in compression characteristics according to the deformation load direction. The combined effects of various anisotropic microstructural factors on the tensile properties of SLM-treated TNZ alloys are both qualitatively and quantitatively investigated. Furthermore, the study examines the influence of aging heat treatment after SLM on the microstructure evolution and changes in physical properties. The findings confirm the elimination of segregation between titanium and niobium along the cooling direction during the solid solution-aging heat treatment, resulting in the formation of fine precipitates. The study focuses on the application of selective laser melting (SLM) to titanium-niobium-zirconium (TNZ) alloys in the field of materials engineering. The primary objective is to demonstrate how SLM can be effectively utilized to control microstructural parameters in TNZ alloys. By achieving this control, it becomes possible to tailor the microstructure and enhance the overall performance of the alloys, particularly for structural metal parts in biomaterials applications.