Correlations in LPBF-fabricated Ti6Al4V: Influence of laser power and scanning speed on microstructure and mechanical performance

In this study, the coupled effects of laser power and scan speed on densification, microstructure, and mechanical properties of Ti6Al4V fabricated by laser powder bed fusion (LPBF) were systematically investigated. Samples were produced using nine parameter combinations comprising three laser power levels (180, 250, and 320 W) and three scan speeds (500, 1000, and 1500 mm/s), while layer thickness and hatch spacing were kept constant. The results indicate that both insufficient and excessive energy input lead to surface quality degradation and reduced relative density through the formation of lack-of-fusion (LoF) porosity and keyhole-induced instabilities, respectively. The highest relative density, exceeding 99.4%, was achieved within an optimal processing window corresponding to a nominal volumetric energy density (VED) of approximately 45–90 J/mm³, where an appropriate balance between melt pool penetration and process stability is maintained. Within this window, near-full density components with refined α′ martensitic microstructures were obtained, together with a favorable combination of strength and ductility (UTS ≈ 1.02–1.08 GPa and elongation ranging from ≈6% up to ≈12.7%). The microstructure of all samples predominantly consisted of α′ martensite within prior columnar β grains; however, increasing laser power and decreasing scan speed altered the effective cooling rate and phase transformation kinetics, leading to partial coarsening and reduced lattice strain in the martensitic structure at higher energy inputs. X-ray diffraction (XRD) analysis revealed corresponding variations in diffraction features associated with changes in lattice strain and transformation behavior as a function of thermal history. Residual stress measurements based on the sin²ψ XRD method demonstrated a strong dependence of both the magnitude and sign of near-surface residual stresses on laser power–scan speed combinations, with a transition from highly compressive to weakly compressive or tensile stress states observed as scanning speed increased. The evolution of residual stress was closely coupled with thermal gradients, cooling rate, and martensitic transformation behavior, providing an essential link between processing conditions, microstructural refinement, and mechanical performance. Variations in hardness and fracture behavior were governed by the interaction between microstructure, grain anisotropy, and porosity content. Specifically, conditions promoting high density and refined martensitic laths enabled stable strain hardening and relatively high ductility, whereas excessive or insufficient energy input led to premature strain localization. Overall, the results demonstrate that while VED serves as a useful first-order parameter for rationalizing densification trends, the specific laser power–scanning speed combinations play a decisive role in governing melt pool dynamics, microstructural evolution, anisotropy, and mechanical performance.