Achieving the strength-ductility balance through the control of the prior β-grain size in laser beam powder bed fusion of Ti-6Al-4V

Limited work exists on the potential for microstructural control during Laser Beam Powder Bed Fusion (LB-PBF) in α+ β Ti-alloys, particularly the impact of the process parameters on the prior-β grain size and its subsequent influence on the strength-ductility balance. Through controlling the solidification conditions, this work explores the role of the heat input parameters and laser scanning strategies in controlling the prior-β grain size and consequently the mechanical performance in LB-PBF-processed Ti-6Al-4V. Electron backscattered diffraction (EBSD) was used in combination with β-phase reconstruction to elucidate the effect of the heat input and scanning strategies on the microstructure, combined with coupled thermal microstructural modelling. The study highlights the possibility for controlling the prior-β grain size through manipulating the solidification conditions, ultimately resulting in enhanced ductility in refined structures. Moreover, whilst the majority of LB-PBF literature relies on the energy density concept to express the heat input, its significance when it comes to the microstructural characteristics was found to be minimal. Conditions with similar energy densities yielded different prior-β grain sizes, and to a lesser extent variations in the α′/α-lath size following a 650˚C/2hrs stress-relief treatment. Similarly, the use of large island scanning strategies resulted in finer prior-β grain sizes, and ultimately higher ductility following stress-relief treatment. The study highlights the possibility for controlling the microstructure in LB-PBF of Ti-alloys, both to improve the mechanical performance and design tailored microstructures.