Effect of Heat Input on Microstructure Evolution and High-Cycle Fatigue Behavior of Ti-6Al-4V Fabricated by Wire-Laser Directed Energy Deposition

This work examines the response of Ti-6Al-4V fabricated by wire-laser directed energy deposition (DED) to heat inputs between 175 and 350 J/mm. Microstructure, tensile behavior, and rotating-bending fatigue were evaluated. As input increased, the transverse size of columnar prior β grains rose from about 642 to 1079 μm and the mean α-lath width increased from 0.65 to 1.32 μm. The α structure consequently changed from a fine acicular/basketweave form to a coarser lamellar form with clearer colonies. Tensile and yield strengths stayed near 900 and 840 MPa, whereas elongation fell from 14.3% to 10.3%. After a 600 °C, 4 h stress relief, fully reversed fatigue tests were performed at 500 MPa (R = −1). Specimens with 175 J/mm heat input reached about 3 × 107 cycles, while one with 350 J/mm fractured at roughly 1 × 105 cycles. The longer life of H175 coincided with its finer interwoven α structure, closer striations, more secondary cracks, and a more tortuous crack route.