Effect of non-lamellar α precipitate morphology on the mechanical properties of Ti5553 parts made by laser powder-bed fusion at high laser scan speeds

Abstract

The influence of α precipitate morphology on the mechanical properties of additively manufactured Ti-5Al-5V-5Mo-3Cr (Ti-5553) is evaluated. Initially, Archimedes density measurements on cylindrical Ti-5553 samples printed using laser powder-bed fusion process were employed to determine the influence of parameters on the conduction melting band. The lowest and the highest volumetric energy densities (VEDs) identified in the conduction band were then selected for further characterization. X-ray Computed Tomography (XCT) was used to validate the expected high relative densities, the nature and distribution of defects in the samples printed at the two selected VEDs. Uniaxial tensile and impact tests were carried out on the samples printed at the two selected process parameter sets. The as-printed samples reached a notable fracture strain of 30 ± 5%. However, the impact-tested samples exhibited poor impact toughness. The results were compared and correlated with observed microstructural features. Electron backscatter diffraction, X-ray diffraction, and optical microscopy (OM) revealed the presence of non-lamellar α particles along the columnar grains in the building direction. The combination of outstanding ductility coupled with poor impact toughness was attributed to the presence of intragranular non-lamellar α particles. Fractured surfaces examined under scanning electron microscopy indicate a bi-modal fracture mode. Microhardness profiles and the corresponding OM images confirmed a grain size difference at the core-contour interface, which was attributed as a major contributing factor for the bi-modal fracture mode.