MECHANICAL PROPERTIES AND MICROSTRUCTURAL CHARACTERIZATION OF TI-6AL-4V FABRICATED BY SELECTIVE LASER MELTING: EFFECT OF BUILD ORIENTATION AND POST-PROCESS HEAT TREATMENT

Abstract

Selective laser melting (SLM) of Ti-6Al-4V produces components with complex geometry unachievable by conventional subtractive methods, yet strong anisotropy in mechanical properties arising from columnar prior-β grain growth and non-equilibrium α′ martensite formation remains a fundamental concern for load-bearing aerospace structures. This dissertation systematically investigates the effects of build orientation (0°, 45°, 90° relative to the build platform) and two post-process heat treatments (stress relief at 650°C/3 h and HIP at 920°C/100 MPa/2 h) on quasi-static tensile, high-cycle fatigue (R = 0.1, 10⁷-cycle runout), and fracture toughness (Kᴵᶜ) of SLM Ti-6Al-4V. As-built vertical specimens exhibited lowest UTS (1020 MPa) and highest anisotropy ratio (vertical/horizontal UTS = 0.88) due to inter-layer fusion defects. HIP treatment transformed acicular α′ to a coarser (α+β) lamellar microstructure, eliminated sub-surface lack-of-fusion pores, and raised Kᴵᶜ from 44 to 71 MPa·m¹/² (±61%), approaching wrought-annealed values. Fatigue strength at 10⁷ cycles improved from 390 MPa (as-built) to 580 MPa (HIP), attributed to pore closure eliminating crack initiation sites. These results provide a processing–property roadmap for qualification of SLM Ti-6Al-4V in aerospace certification pathways.