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

doi:10.1016/j.jmst.2019.05.008

Abstract

Abstract:

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 Widmanst?tten, α laths, gradually converted to elongated acicular α and a large fraction of Widmanst?tten 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 $\widetilde{1}$3.3% 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.

Key words: Directed energy deposition, Ti-6Al-4V, Microstructure, Thermal cycle history, Mechanical properties

Hua Tan, Mengle Guo, Adam T. Clare, Xin Lin, Jing Chen, Weidong Huang. Microstructure and properties of Ti-6Al-4V fabricated by low-power pulsed laser directed energy deposition[J]. J. Mater. Sci. Technol., 2019, 35(9): 2027-2037.

Figures/Tables 14

Fig. 1. Illustration of pulsed laser deposition experiment, (a) schematic diagram of system, (b) schematic diagram of thin-wall laser deposition process, (c) Ti-6Al-4V thin-wall sample deposited.

Fig. 2. Preparation of the samples for metallographic observation and tensile test, (a) schematic illustration of specimen extractions, (b) dimensions of the test specimen, (c) tensile test samples arranged as built.

Fig. 3. Morphologies of prior β grains of Ti-6Al-4V thin wall, (a) the bottom, (b) the middle, (c) the top and (d) local enlargement of typical morphology.

Fig. 4. Solidification analysis of the melt pool, (a) schematic relationship among scanning velocity Vb, solidification velocity Vs and melt pool boundary, (b) the calculated CET curve (calculated from Lin’s model [28]) and schematic solidification condition at the melt pool tail.

Fig. 5. Mechanism of the wavy epitaxial growth of β grain, (a) temperature fields simulation by FEM with laser scanning back and forth, (b) schematic of the wavy growth.

Fig. 6. Schematics of melt pools with different depth-to-width ratios.

Fig. 7. Microstructures in β columnar grain, (a) “bright” grain, (b) “dark” grain.

Fig. 8. Analysis of the top of the sample, (a) schematic of deposited layer formation at the top of the sample, (b) microstructure observed at the top of the sample and diagram of deposition layer marking.

Fig. 9. Microstructures in the same β grain and final five layers of the sample, (a), (b) the microstructures of the last layer, (c), (d) the microstructures of the last second layer, (e), (f) the microstructures of the last third layer, (g), (h) the microstructures of the last fourth layer, and (i), (j) the microstructures of the last fifth layer.

Fig. 10. Schematic diagram of the thermal cycle history and microstructure formation.

Fig. 11. Micro-hardness test in different β grains, (a) the schematic of test position, (b) hardness results.

Fig. 12. Tensile properties of specimens taken from the thin wall.

Fig. 13. Schematic of tensile process analysis, (a) transverse directoin, (b) build direction.

Fig. 14. SEM micrographs of the typical fracture surfaces of two direction tensile samples, (a), (b) height direction, (c), (d) transverse direction.

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