On the role of volumetric energy density in the microstructure and mechanical properties of laser powder bed fusion Ti-6Al-4V alloy

Abstract

The volumetric energy density (EV) is considered as a comprehensive index to evaluate the complex parameters in laser powder bed fusion (L-PBF) of metallic parts. This work aims to investigate the influence of varying laser power and scanning velocity coupling under a specific EV value on the microstructure, texture, and tensile properties of L-PBF Ti-6Al-4 V (Ti64) alloy to illustrate the limitation of EV. Three sets of laser parameters under a specific EV of 58.8 J/mm3 were employed to fabricate dense parts. Analyses by electron backscatter diffraction (EBSD) and X-ray diffraction (XRD) revealed that the same EV in the L-PBF process can result in distinct thickness of α′ lath, crystallographic texture and tensile behavior. The α′-lath was refined from 0.89 µm to 0.66 µm while laser power increased from 180 W to 240 W. Meanwhile, the crystallographic orientation of prior-β grains was oriented < 111 >β direction to parallel building direction (BD) from < 001 >β//BD, which determined the orientations of α′ lath. Tensile test results exhibit that the yield strength, ultimate tensile strength and elongation to fracture all increase with the linearly increases in laser power and speed. An excellent balance between yield strength of ~1042.2 ± 37.1 MPa and total elongation of ~ 10.8 ± 1.1% was obtained after the variant optimization. The finite element analysis shows that even under a constant EV value, varying laser power and scanning velocity coupling will carry out distinct thermal histories, leading to the grain refinement and higher dislocation density. This work shows the limited significance of EV in the guidance of the L-PBF process with complex physics. More efforts should be made on microstructural control via the combination of specific laser parameters.