Characterization of titanium aluminide alloy components fabricated by additive manufacturing using electron beam melting

doi:10.1016/j.actamat.2009.11.032

Acta Materialia

Volume 58, Issue 5, March 2010, Pages 1887-1894

https://doi.org/10.1016/j.actamat.2009.11.032 Get rights and content

Abstract

Intermetallic, γ-TiAl, equiaxed, small-grain (∼2 μm) structures with lamellar γ/α₂-Ti₃Al colonies with average spacing of 0.6 μm have been fabricated by additive manufacturing using electron beam melting (EBM) of precursor, atomized powder. The residual microindentation (Vickers) hardness (HV) averaged 4.1 GPa, corresponding to a nominal yield strength of ∼1.4 GPa (∼HV/3), and a specific yield strength of 0.37 GPa cm³ g⁻¹ (for a density of 3.76 g cm⁻³), in contrast to 0.27 GPa cm³ g⁻¹ for EBM-fabricated Ti–6Al–4V components. These results demonstrate the potential to fabricate near net shape and complex titanium aluminide products directly using EBM technology in important aerospace and automotive applications.

Introduction

For at least the past decade, considerable efforts have been made worldwide in the development, technology and applications of intermetallic, γ-TiAl-based alloys: Ti–(34–49)Al–(5–10)Nb–(2–5)Cr (in at.%), at high-temperatures for long-term operation [1], [2], [3], [4]. Applications include propulsion exhaust system components, such as nozzle and divergent flaps fabricated from wrought and cast gamma substructures [5], and aeroengine compressor blades. Other aerospace and automotive applications have also been examined in recent years [4], [6], [7], especially in the context of replacing the heavier, nickel-based superalloys for the next generation of aircraft engines, space vehicles and automotive engine components, including turbine wheels and engine exhaust valves and pistons for improved auto fuel economy [5], [6], [7], [8], [9], [10]. Central to such applications are low density, high specific strength (yield strength/density), high specific stiffness (elastic modulus/density), fracture toughness and ductility, fatigue strength and creep, and corrosion and oxidation resistance (at high temperature). Many of these properties have already been demonstrated for a number of titanium aluminides above 700 °C [3], [6], [11].

The industrial scale processing routes for titanium aluminides have included ingot casting, powder processing and ingot forging and sheet production by hot-rolling [6], [12], [13], [14], [15]. Powder metallurgy (PM) processing and compaction offers an alternative for prematerial production through hot isostatic pressing of γ-TiAl-based alloy powder into billets which can be rolled into large sheets [3], [13], [14]. Prealloyed γ-TiAl alloy powders can be produced in large quantities using high-pressure Ar gas atomization [16]. These powders are normally filled in a Ti-container, evaluated at elevated temperature, sealed and HiPed to full density in a variety of billet dimensions. This paper describes the characterization of γ-Ti–47 at.% Al–2 at.% Nb–2 at.% Cr titanium aluminide alloy components produced by additive (layer-based) manufacturing by electron beam melting (EBM) from precursor powders having the same nominal composition. Cormier et al. [17] have also recently described some preliminary EBM fabrication of Ti–47Al–2Nb–2Cr components using prealloyed and blended precursor powders, while Liu and DuPont [18] used laser-engineered net shaping to study in situ reactive rapid prototyping of intermetallic compounds. In the present study, characterization of the EBM-manufactured titanium aluminide alloy products has included optical metallography and electron microscopy (including both scanning electron microscopy (SEM) and transmission electron microscopy (TEM)). Hardness measurements, including microindentation hardness, were also made for the EBM-produced components.

Section snippets

Experimental and analytical issues

In this investigation we utilized an atomized, rapidly solidified γ-TiAl-based alloy powder with a nominal composition of Ti–47 at.% Al + 2 at.% Nb + 2 at.% Cr [17]. In contrast to the single-phase, stoichiometric γ-Ti–55 at.% Al eutectic, this alloy was slightly lean in Al, which creates a two-phase structure: γ-TiAl with ideally the tetragonal (L1_o) (p4/mmm) structure and the α₂-Ti₃Al hexagonal (D0₁₉) (p63/mmc) structure (a = 0.58 nm, b = 0.46 nm). With a somewhat lower Al-content, as in this study, this

Results and discussion

Fig. 3 shows a typical EBM-built test-block specimen. Fig. 3a shows the build direction (arrow) and the top surface, while Fig. 3b shows a magnified section showing vertical surface particles (at S_V) and the final horizontal surface melt-scan features (S_h). The measured density (ρ) for the test blocks (measured weight/volume) as in Fig. 3 was ∼3.76 g cm⁻³. This compares favorably with the ideal (or theoretical) density for this composition of 3.84 g cm⁻³. The relatively wide melt ridges on the

Summary and conclusions

In this study we have characterized the structures and microstructures for precursor titanium aluminide powder and solid two-phase titanium aluminide components fabricated by EBM. The precursor powder was α₂-phase-rich (hcp Ti₃Al), while the EBM-fabricated test components were largely γ-TiAl (fcc). The EBM-fabricated test specimens exhibited an equiaxed γ-TiAl grain structure with a lamellar γ/α₂ colony structure having an average spacing of 0.6 μm within the γ-grains, which had an average size

Acknowledgements

This research was supported in part by Mr. and Mrs. MacIntosh Murchison endowments at the University of Texas at El Paso. We are grateful for the help of Dr. John McClure in conducting the XRD analyses.

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Characterization of titanium aluminide alloy components fabricated by additive manufacturing using electron beam melting

Abstract

Introduction

Section snippets

Experimental and analytical issues

Results and discussion

Summary and conclusions

Acknowledgements

Intermetallics

Intermetallics

Ultramicroscopy

Intermetallics

Adv Eng Mater

Adv Eng Mater

Intermetallics