Laser deposition of graded γ-TiAl/Ti2AlNb alloys: Microstructure and nanomechanical characterization of the transition zone

doi:10.1016/j.jallcom.2021.159946

Journal of Alloys and Compounds

Volume 875, 15 September 2021, 159946

https://doi.org/10.1016/j.jallcom.2021.159946 Get rights and content

Highlights

•
In this study, we have successfully used the LMD technique to fabricate a γ-TiAl/Ti₂AlNb graded alloy, which is fully dense without cracks and pores.
•
We carried out detailed phase and microstructure analysis on the transition zone from the Ti₂AlNb substrate to the γ-TiAl alloy: Three layers are observed in the transition zone.
•
We employed a novel high-speed nanoindentation technique, for the first time, to statistically study the microstructure-property relationship in the transition zone.

Abstract

Additive manufacturing (AM) is a transformative technology to the aerospace industry. As one of the AM techniques, laser metal deposition (LMD) enables the fabrication of engine blades and a disk, as a single component, known as blisk. In this study, we use the LMD technique to fabricate a γ-TiAl/Ti₂AlNb graded metallic alloy by depositing γ-TiAl powder on a Ti₂AlNb alloy substrate. High-resolution scanning electron microscope (SEM) and high-speed nanoindentation are employed to characterize the microstructure and mechanical properties of the transition zone from the Ti₂AlNb substrate (disk) to the γ-TiAl alloy (blade). The results show that the transition zone includes three layers (I, II and III) with gradient compositions and phases: (I) mainly β/B2 matrix with randomly distributed α₂ and γ phases, (II) (α₂ + γ) lamella with γ and β/B2 phases, and (III) similar microstructure with Layer II but finer γ and β/B2 phases. The results of nanoindentation mapping show good correlations between the mechanical properties (nanohardness and elastic modulus) and microstructure in the transition zone. Attributing to the rule of mixtures, the nanohardness and elastic modulus gradually increase from the substrate Ti₂AlNb to Layer I, and gradually decrease from Layer I to γ-TiAl. This work demonstrates that the microstructure and phase analysis in combination with high-speed nanoindentation offers a new opportunity to study graded materials made using LMD.

Graphical Abstract

Introduction

Conventional blades and disks in gas turbines are typically made of different materials due to their distinct working conditions [1]. Turbine disks generally are subjected to higher centrifugal stresses, but lower temperatures, than turbine blades [2]. Titanium aluminide (γ-TiAl) is attractive to be used in low-pressure turbine blades [3], owing to its low density (~4 g/cm³), excellent creep resistance, and oxidation resistance at high temperatures [4], [5], [6]. For turbine disks, Ti₂AlNb alloy is a promising material due to its high strength, excellent fatigue, and fracture toughness [7]. The blades and disk are conventionally joined by pin joints or dovetails [8]. Different from the conventional assembly of a disk and blades, a blisk integrates a disk and blades as a single component [9], benefiting from the high aerodynamic efficiency [10], a low fuel consumption [11], and a high thrust-to-weight ratio [12].

Laser metal deposition (LMD) is a metal additive manufacturing (AM) technique that fabricates parts by the layer-by-layer deposition of powder or wire feedstock [13], [47]. Such a method offers a new opportunity to produce compositionally graded alloys. Graded alloys, for example, Ti6Al4V/TiC, TC4/γ-TiAl, and In718/SS316L, were successfully fabricated using the LMD technique [14], [15], [16]. The microstructures of the transition zones in the graded materials play an essential role in determining the mechanical performance of the LMD-produced components [17]. Defects, such as cracks and voids, can easily be formed in the transition zone during the LMD process [18], [19].

Recently, Wu et al. [7] applied the LMD method to produce a graded γ-TiAl/Ti₂AlNb alloy. They showed that the compositions of the transition zone changed stepwise due to the remelting of previous layers, leading to the microstructure evolution across the layers. However, there is still a lack of detailed phase and microstructure analysis for the transition zone. More importantly, because the size of the phases in the transition zone is not large enough to be tested using the conventional microhardness in their study, the mechanical properties of each phase are still unknown. In this study, we employ a novel high-speed nanoindentation technique to statistically study the microstructure-property relationship in the transition zone.

Section snippets

Sample preparation

Fig. 1 illustrates the fabrication process. Gas atomized γ-TiAl alloy (composition Ti-47Al-2Cr-2V (at%)) powder was deposited layer-by-layer on the casted 40 mm × 40 mm × 60 mm Ti₂AlNb (Ti-22Al-25Nb (at%)) alloy plate during the LMD process. The LMD system used in this study consisted of a GS-TFL-8000 CO₂ laser, a BSF-2 powder feeder equipped with a coaxial powder delivery nozzle, and an HNC-21M CNC multi-axis motion system. We deposited 20 layers using the following processing parameters:

Composition gradient

No crack or pores are observed in the fabricated part (Fig. S1) and the transition zone (Fig. 2a), indicating that the deposited γ-TiAl alloy bonds well with the Ti₂AlNb substrate. Three distinct layers (Layer I, Layer II, and Layer III) in the transition zone are shown in Fig. 2a (the black region on the left of Fig. 2a is mounted resin). The thicknesses of Layer I, Layer II and Layer III are approximately 300 µm, 425 µm, and 1870 µm, respectively. The concentrations of Nb in the substrate,

Microstructure and phase evolution

The microstructure and phase evolution of the graded alloy is controlled by two factors: solidification process and subsequent in situ heat treatment [7], [15], [18], [34], [35]. According to the Nb content (12%) in Layer I, we adopt the available Ti-Al-10Nb (at%) phase diagram (Fig. 10a) [36] to perform a qualitative phase analysis. Light grey region represents a possible solidification path of Layer I. β/B2, α₂, and γ phases are found in Layer I, the types of phases in Layer I are almost the

Conclusions

In this study, we used the LMD technique to successfully fabricate a γ-TiAl/Ti₂AlNb graded alloy, which is fully dense without cracks and pores. We studied the phase evolution, hardness, and elastic modulus in the transition zone, analyzed their relationships and reach the following conclusions.

1.
The transition zone can be divided into three layers: Layer I, Layer II and Layer III. The majority of Layer I is β/B2 phase with randomly distributed α₂ and γ phases. At the substrate-Layer I interface,

CRediT authorship contribution statement

H.X. Chen: Conceptualization, Data curation, Investigation, Writing - original draft, Writing - review & editing. Z.Y. Liu: Resources, Writing - review & editing. X. Cheng: Conceptualization, Investigation, Writing - review & editing. Y. Zou: Conceptualization, Supervision, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors would like to thank Michel Haché for his help in the sample characterization, Changjun Cheng for his help in XRD analysis, and Jiahui Zhang for his help in image preparation. H.C., Z.L., and Y.Z. acknowledge the financial support from Natural Sciences and Engineering Research Council of Canada (NSERC Discovery Grant #RGPIN-2018–05731) and the Dean’s Spark Assistant Professorship in the Faculty of Applied Science & Engineering at the University of Toronto. H.C. acknowledges the China

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Laser deposition of graded γ-TiAl/Ti2AlNb alloys: Microstructure and nanomechanical characterization of the transition zone

Highlights

Abstract

Graphical Abstract

Introduction

Section snippets

Sample preparation

Composition gradient

Microstructure and phase evolution

Conclusions

CRediT authorship contribution statement

Declaration of Competing Interest

Acknowledgements

Eng. Fail. Anal.

Acta Mater.

Intermetallics

Acta Mater.

Mater. Des.

Vacuum

Mater. Des.1980–2015

Addit. Manuf.

Acta Mater.

J. Mater. Process. Technol.

J. Alloy. Compd.

Trans. Nonferrous Met. Soc. China

J. Alloy. Compd.

Mater. Des.

Intermetallics

Intermetallics

Acta Mater.

Mater. Sci. Eng. A

J. Alloy. Compd.

Mater. Sci. Eng. A

J. Alloy. Compd.

J. Alloy. Compd.

Intermetallics

Laser deposition of graded γ-TiAl/Ti₂AlNb alloys: Microstructure and nanomechanical characterization of the transition zone