Study on the anisotropy and heterogeneity of microstructure and mechanical properties in additive manufactured Ti-6Al-4V parts by selective electron beam melting

• Main Author: Kok, Yihong
• Other Authors: Loh Ngiap Hiang
• Format: Theses and Dissertations
• Language: English
• Published: 2018
• Subjects: DRNTU::Engineering::Materials
• Online Access: http://hdl.handle.net/10356/75831
• Institution: Nanyang Technological University

Selective electron beam melting (SEBM) is a metal additive manufacturing (AM) technique that can produce high-quality metal parts with high strength and ductility. Due to the layer-by-layer approach and selective melting of the powder bed during the AM process, anisotropy and heterogeneity in the as-fabricated microstructure are challenges in the production of metal AM parts. An understanding of the formation mechanism that results in such microstructures in SEBM built Ti-6Al-4V is not yet fully understood. This study thus aims to further the understanding of the anisotropy and heterogeneity within the as-fabricated microstructure of Ti-6Al-4V alloy for microstructural control and the development of new materials. The control of the microstructure and the development of new materials are important to enable more applications for SEBM-built parts. Firstly, the phases within the as-built SEBM Ti-6Al-4V were studied and identified via phase characterisation techniques (e.g. TEM EDX, XRD and APT). Secondly, the influence of build geometry (e.g. thickness, height, and shape) on the microstructure and the mechanical properties were studied quantitatively via microstructural characterisation techniques (e.g. OM, SEM and TEM) and mechanical testing (e.g. Tensile test and Vickers microhardness). Lastly, the columnar grains within the as-built SEBM Ti-6Al-4V were studied as well. Numerical simulations to support the experimental observations and findings were also done via collaborations with other researchers. The results showed that anisotropy and heterogeneity in the microstructure and mechanical properties indeed existed within the SEBM Ti-6Al-4V part. The phase transformation process that occurs during the fabrication process of SEBM-built Ti-6Al-4V was also shown. Differences in the build geometry thus terminated the phase transformation process at various stages led to heterogeneity in its final mechanical properties. The columnar grain growth study showed that there existed strong texture intensity in the part with large cross sectional area that suggested competition between the columnar grains. Additionally, phenomenon of sub-columnar grain formation was also observed. The study contributes to the scientific knowledge with regards to the phase and microstructural evolution that happens during the SEBM fabrication process. It can also be a useful reference guide on both microstructural control and development of new materials for SEBM process.