Microstructural characterization of the γ-TiAl alloy samples fabricated by direct laser fabrication rapid prototype technique

Abstract

A direct laser fabrication technique (DLF) has been used to fabricate near net shape samples of a γ γ-TiAl alloy using gas atomized Ti 48 A1 48 Mn 2 Nb 2 alloy powder as a feed stock material. The microstructures of these Ti 48 Al 48 Mn 2 Nb 2 laser treated samples have been characterized using optical, scanning (SEM) and transmission electron microscopy (TEM), both immediately after laser fabrication and after heat treatments. The microstructural studies have shown that the microstructure is heterogeneous in nature and extremely fine in comparison with the conventionally processed material. The process parameters such as laser power and laser scanning speed greatly influence the morphology and the microstructure of the laser treated samples. Heat treatments for a number of process conditions have been carried out to examine the stability of the microstructure which remains stable up to 973 K and rapid grain coarsening occurs at 1273 K. A fully recrystallized and uniform microstructure is obtained after annealing at 1073 K for 24 h and compositional heterogeneity present in the laser-fabricated samples is eliminated. Annealing in the α α phase field followed by air cooling and annealing in (α α 2 + γ γ) phase region gives rise to a homogeneous and uniform microstructure. However, the microstructure is much coarser than the microstructure of the DLF samples.

Additive manufacturing of TiAl-based alloys

“…The literature reveals that a host of other researchers [14,27,47,51] uses TNM powder with Al composition (>43 wt %) could not obtain crack-free components, hence it is evident that the low Al contents and the high content of the b stabilizing elements used by Löber et al [47] enable the production of the crack-free TiAl-based components using the LPBF process. The resultant duplex microstructure obtained by Löber et al [47] is not the ideal microstructure for high-temperature applications [20,21]. The duplex microstructure is because of substituting a high amount of the b stabilizing elements for Al content.…”

“…The X 2 elements are very effective in improving the oxidation resistance and solid-solution strengthening of the two-phase TiAl alloys and the X 3 elements improve creep resistance, oxidation resistance, and ductility of the alloy [8][9][10][11]. Depending on the process route, thermal treatment, and composition the TiAl-based alloys can exhibit three different microstructures: gamma, duplex, fully lamellar microstructures [20,21]. Gamma and duplex microstructures give rise to high strength and some ductility but poor creep resistance, low fatigue strength, and low fracture toughness.…”

“…The fully lamellar microstructure, consisting of the TiAl (g-phase) and a small volume fraction of Ti 3 Al (a 2 -phase) microstructures demonstrates high creep resistance, higher fracture toughness and crack propagation resistance which makes such phases suitable for high-temperature applications [20]. Besides the good mechanical properties offered by TiAl (g-phase) and Ti 3 Al (∝ 2 -phase), they also offered superior oxidation resistance compared to the other phases for high-temperature applications [21].…”

Microstructure and Mechanical Properties of Ti‐48Al‐2Cr‐2Nb Manufactured Via Electron Beam Melting

“…However, the casting of -TiAl alloys has some processing complications in addition to machining difficulties that arise from the low ductility inherent with many intermetallic phases. The limited number of slip systems in -TiAl can also produce relatively low fracture toughness [10].…”

“…A number of different freeform fabrication techniques have been recently used to produce TiAl structures to near net shape. One such approach involves using laser-based techniques to melt pre-alloyed powders to produce TiAl components [10] while more recent approaches use electron beam melting (EBM) via techniques developed by Arcam AB [11]- [13]. EBM allows the production of complex shaped metallic components from a precursor powder [12] and the deposition rate is higher in comparison to laser-based techniques.…”