Effect of LPBF Process Parameters on Porosity Distribution and ¿/ß Phase Transformation in Near-ß Ti 5553 Alloy

Ti-5553 is a metastable ß titanium alloy widely used in aerospace and high-performance engineering applications due to its excellent combination of high strength, low density, good corrosion resistance, and favorable fatigue performance. Ti-5553 is particularly attractive for aircraft structural components, landing gear systems, and heavily loaded aerospace parts where high specific strength and weight reduction are critical requirements. Compared to conventional titanium alloys, Ti-5553 offers superior hardenability, deep section strength, and good fracture toughness, making it highly suitable for advanced manufacturing routes such as Laser Powder Bed Fusion (L- PBF). Additionally, its microstructure can be tailored through solution treatment and aging processes, allowing precise control of ¿ precipitation and mechanical properties for demanding engineering applications. This thesis investigates the influence of Laser Powder Bed Fusion (L-PBF) process parameters and post-process heat treatment on the densification behavior, microstructural evolution, phase transformation, and hardness response of Ti-5553 metastable ß titanium alloy. Different combinations of laser power and scan speed were employed to evaluate their effects on melting mode stability and porosity formation during LPBF processing. Microstructural characterization was performed by using light microscopy, scanning electron microscopy (SEM) with different detectors such as SE, BSE, while the density measurement was performed in two different methods; Archimedes and visually (light microscope). To link obtained microstructure to the mechanical properties of as built and heat-treated samples, microhardness measurement was performed and phases emerging after printing and HT process were confirmed trough XRD measurement. Result of this study shows that process parameters have great influence on defect size, type and distribution. Furthermore, depending on employed process parameters, melt pool dynamic exhibited different behavior such as conduction, stable keyhole and unstable keyhole melting as well as transition between them. After applied HT, microhardness values increased from 300- 310 HV (as built) to the 380- 440 HV range, which confirmed successful precipitation of primary and secondary ¿ particles resulting with strengthening. Result of HT also reviled that defect existence could have a role on precipitated ¿ particles. Summarily, findings of this provide essential information regarding relationship of used processed parameters and porosity behavior as well as ¿ phase formation .