Evaluation of the size and orientation-dependent mechanical properties of additively manufactured Ti-6Al-4V: The role of microstructure, defects, and surface roughness and its implications for lattice structures

Resumen

Laser powder bed fusion (LPBF) enables the fabrication of complex components beyond conventional methods. However, this complexity introduces variability in the feature size, from large sections to fine struts, leading to differences in thermal and processing history. Even with identical processing conditions and same base alloy, this variability can affect the microstructure, the defects, and the mechanical behaviour of the printed material. This is the case particularly in Ti-6Al-4V, where microstructure depends strongly on thermal history. This work systematically examines the impact of size and LPBF processing conditions on microstructure, defects, and mechanical performance of Ti-6Al-4V. Defect analysis, geometry, and microstructure characterization are conducted to assess the effects of sample size, orientation, and LPBF parameters. Results indicate increased porosity in smaller-sized specimens, while roughness remains largely independent of size. Microstructural analysis reveals α-lath refinement as the size decreases and when high-energy density parameters were used. Then, the mechanical properties of the specimens are extracted accurately based on X-ray tomography and computational modelling. The results show that high roughness causes up to a 5.05% reduction in the load-bearing area of small-size specimens (struts). Once accurately evaluated, the ultimate tensile strength (UTS) and yield strength (YS) are found to increase by 3.87% and 6.65%, respectively, for small-size specimens when compared to large ones, which has been attributed mainly to microstructural changes. The outcomes highlight the significance of size effects and their accurate assessment in the design of LPBF parts.