Microstructural control in Ti-6AI-4V additively manufactured by laser powder bed fusion

As a premier additive manufacturing (AM) process creating a 3D object by adding materials layer by layer with a high-energy laser beam, laser powder bed fusion (LPBF) continues to make deep inroads into the manufacturing sector. Of a wide variety of metallic materials processed using LPBF, Ti-6Al-4V (wt.%) has received exceptional attention as the benchmark titanium alloy because of its broad applications in industry and the associated high cost of manufacturing and long lead time. Typically, the Ti-6Al-4V alloy produced by LPBF is characterized by metastable ?? martensites within a columnar prior-? grain structure, resulting in high strength but anisotropic mechanical behavior, inferior ductility and low fracture toughness in the as-built state. To address these issues, recent development in the optimization of the LPBF process aims to replace the quasi-brittle ?? martensite with its equilibrium counterpart ? phase and transform the columnar prior-? grain structure into equiaxed in the as-built state. However, the intricate thermal profile developed in the LPBF process makes it challenging to ascertain the underlying mechanisms responsible for microstructural evolution in LPBF Ti-6Al-4V. This PhD thesis thus focuses on quantitative evaluation of the microstructural evolution and transformation pathways in LPBF Ti-6Al-4V through postmortem examination. With the aid of cyclic and isothermal heat treatment conducted using a DIL 805A dilatometer, the thermal conditions required for the development of a variety of microstructures such as ultrafine lamellar ?+B, dynamic ? globularization and equiaxed prior-? grain structure, can be ascertained.