Microstructure and properties of Ti-6Al-4V fabricated by low-power pulsed laser directed energy deposition

Thin-wall structures of Ti-6Al-4V were fabricated by low-power pulsed laser directed energy deposition. During deposition, consistent with prior reports, columnar grains were observed which grew from the bottom toward the top of melt pool tail. This resulted in a microstructure mainly composed of long and thin prior epitaxial β columnar grains (average width ≈200 μm). A periodic pattern in epitaxial growth of grains was observed, which was shown to depend upon laser traverse direction. Utilizing this, a novel means was proposed to determine accurately the fusion boundary of each deposited layer by inspection of the periodic wave patterns. As a result it was applied to investigate the influence of thermal cycling on microstructure evolution. Results showed that acicular martensite, α' phase, and a small amount of Widmanstätten, α laths, gradually converted to elongated acicular α and a large fraction of Widmanstätten α laths under layer-wise thermal cycling. Tensile tests showed that the yield strength, ultimate tensile strength and elongation of Ti-6Al-4V thin wall in the build direction were 9.1%, 17.3% and 42% higher respectively than those typically observed in forged solids of the same alloy. It also showed the yield strength and ultimate tensile strength of the transverse tensile samples both were ˜13.3% higher than those from the build direction due to the strengthening effect of a large number of vertical β grain boundaries, but the elongation was 69.7% lower than that of the build direction due to the uneven grain deformation of β grains.