In-situ Microstructural Control and Process Monitoring in Laser Powder Bed Fusion of Ti-6Al-4V Alloy

Laser Powder Bed Fusion (LPBF) is a leading Additive Manufacturing (AM) technique for creating metallic alloys. However, its rapid heating and cooling rates can lead to residual stresses, defects, and undesirable microstructures. The main goal in LPBF is to minimize defects, which can often only be achieved within a narrow processing window. This limited range hampers effective microstructural control and frequently necessitates post-heat treatments. In the case of Ti-6Al-4V alloy, various phase transformation mechanisms are influenced by thermal history. Slow cooling allows the transformation from ß to a + ß phases, resulting in a favorable combination of strength and ductility. Conversely, the rapid cooling typical of LPBF produces non-equilibrium a'-martensite, which lacks the ductility needed for engineering applications. Consequently, post-heat treatments are required to convert this phase into the a + ß structure, which is time and energy consuming. This PhD thesis explores new methods of in-situ microstructural control during LPBF of Ti64, focusing on controlling the martensitic transformation. Three key approaches were investigated: in-situ alloying, in-situ Selective Laser Heat Treatment (SLHT), and advanced laser beam shaping. Acoustic Emission (AE) was also employed as a secondary tool to monitor defects and microstructural changes, aiming to detect signatures of martensitic transformation. Through in-situ alloying with Fe, the formation of the martensitic phase was successfully prevented, stabilizing the ß phase. The AE signals related to these microstructures were analyzed in the frequency domain to identify martensitic transformation signatures. SLHT was used to promote the decomposition of a'-martensite into the a + ß structure, with operando synchrotron X-ray diffraction and numerical simulations employed to study the dynamics and kinetics of decomposition in cuboid and thin wall geometries. The findings were used to create composite layers of a + ß between the martensitic regions at a very localized scale. This demonstrated the effectiveness of the SLHT technique in achieving the a + ß structure on a localized scale. The final method investigated the prevention of martensitic transformation through advanced laser beam shaping using a Liquid Crystal on Silicon-Spatial Light Modulator (LCoS-SLM). By creating a tailored beam profile and comparing it with Gaussian distributions, we achieved a reduced cooling rate that activated the ß to a + ß transformation, corroborated by numerical simulation and thermal imaging. This adaptive beam shaping allowed for the printing of architected microstructures, enabling specific placement of a + ß and a'-martensite. The results of this research offer two main contributions. First, they present novel in-situ microstructural control methods that eliminate the need for post-processing heat treatments. Additionally, these techniques enable the creation of architected microstructures that are not achi