Microstructure and Mechanical Property of MXene-Added Ti–6Al–4V Alloy Fabricated by Laser Powder Bed Fusion

Abstract

Laser powder bed fusion (L-PBF) has been utilized to prepare a high-strength titanium alloy builds using 0.3 mass% MXene-decorated Ti–6Al–4V alloy particles produced by an electrostatic self-assembly. The MXene/Ti–6Al–4V powder mixture had similar spherical morphology and powder size, while showing higher laser absorptivity as compared with the Ti–6Al–4V powders. Under high-energy laser irradiation, MXene was fully decomposed and uniformly dissolved into the Ti matrix. Microstructure observations illustrated that the MXene/Ti–6Al–4V build was completely consisted of ultrafine carbon-saturated martensite structures. Consequently, the MXene/Ti–6Al–4V build exhibited a high tensile strength of ∼1.4 GPa, attributing to the refinement of martensitic structure and the solid solution strengthening of carbon. These findings of this study may broaden the potential application of MXene and pave up the way towards the fabrication of advanced titanium parts via L-PBF.

(a) FE-SEM image of the MXene/Ti–6Al–4V mixed powder; (b) TEM image of the MXene/Ti–6Al–4V build; (c) tensile stress-plastic strain curves of the Ti–6Al–4V and MXene/Ti–6Al–4V builds; (d) comparisons in the UTS versus plastic elongation for additively manufactured Ti–6Al–4V alloys and TMCs.

1. Introduction

Titanium (Ti) alloys have attracted significantly attention in aerospace, automotive and biomedical industries towing to the excellent properties including low density, superior strength, and high corrosion resistance.¹⁾ However, the low hardness and insufficient wear resistance are the primary issues to broaden their potential applications.^2–4) In this regard, the incorporation of reinforcements or interstitial elements was deemed as an effective alternative. For instance, ceramic particles (e.g., B₄C,^5,6) BN,⁷⁾ TiC,⁸⁾ ZrN,⁹⁾ Y₂O₃,¹⁰⁾ Al₂O₃,¹¹⁾ Si₃N₄¹²⁾) have been extensively used for strengthening Ti alloys. Yet, it is challenging to achieve homogeneous ceramic nanoparticles dispersion or strong ceramic-metal interfacial bonding. The interstitial elements (e.g., carbon, nitrogen, or oxygen) have been dissolved successfully into the Ti matrix using powder metallurgy methods, displaying significant solid-solution strengthening effect.¹³⁾ Unfortunately, coarse TiC particles were usually generated, resulting in the crack formation at the TiC–Ti interfaces under loading.¹⁴⁾

Laser powder bed fusion (L-PBF) is a technique extensively used for three-dimensional metallic parts using computer-aided design files.^15–17) The intrinsic features of L-PBF allow the direct manufacturing of complexly customized parts without the use of expensive tooling or heat treatments.¹⁸⁾ In addition, the fabricated samples exhibit many unique characteristics, such as ultrafine microstructure, nonequilibrium phases, and high internal stresses, owing to a rapid solidification rate (∼10⁴–10⁸ Ks⁻¹) of L-PBF.¹⁹⁾ Thus, the L-PBF builds usually display promising mechanical performance compared to the traditionally fabricated materials.²⁰⁾

The novel two-dimensional material known as MXene has gained much attention since the discovery in 2011. Ti₃C₂T_x, in which T_x is the surface functional groups, is the firstly discovered and extensively researched MXene to date.²¹⁾ Due to its superior Young’s modulus (333 ± 30 GPa) and high breaking strength (17.3 ± 1.6 GPa),²¹⁾ solution-processable features, and excellent electrical/thermal conductivities,²²⁾ MXene has been popularly used in catalysis, energy storage, as well as electromagnetic wave absorption and shielding.^21,23) In this work, MXene is firstly chosen as a filler to develop high-performance Ti alloy builds by L-PBF, based on two following considerations. Firstly, due to the outstanding comprehensive properties, MXene is considered as an ideal reinforcement for composite materials.^24,25) For instance, Zhou et al. fabricated few-layer MXene-reinforced Al matrix composites and found that the addition of the MXene increased the tensile strength of aluminum by approximately 66%.²⁶⁾ Secondly, conventional reinforcements (e.g., TiC) are usually mixed with metallic powders by ball-milling process, which leads to uncontrollable changes in powder size and morphology, and thus interior defects in the L-PBF builds. In contrast, MXene has abundant surface functionalities, it could be easily attached on the metallic particles by an electrostatic self-assembly²⁰⁾ without changing powder properties. Thirdly, the thermal degradation temperature of MXene is approximately 1500 K,²⁷⁾ which is much lower than the melting point of TiC (3430 K). During high-temperature manufacturing processes such as L-PBF, the MXene is possibly decomposed into interstitial carbon or oxygen atoms to strengthen the matrix. In this work, the surface of Ti–6Al–4V alloy powders were covered with individual MXene sheets by hetero-agglomeration process. The influence of MXene addition on the powder properties, the L-PBF processability and mechanical response of Ti–6Al–4V alloy was studied. The microstructural evolution and strengthening mechanism of MXene in additive-manufactured alloys were clarified.

2. Experimental

2.1 Fabrication of MXene/Ti–6Al–4V powder mixture

Ti₃C₂T_x MXene colloid was synthesized by removing the Al atoms of Ti₃AlC₂ through selective etching, as reported elsewhere.²⁶⁾ The MXene/Ti–6Al–4V powder mixture was prepared via a hetero-agglomeration process. Firstly, certain amount of MXene was diluted and dispersed in water using ultrasonication and mechanical stirring for 1.8 ks. Secondly, the gas-atomized Ti–6Al–4V powder was pulled into the MXene colloid to be the ratio of 0.3 mass% Mexene using mechanical stirring for 1.8 ks. Thirdly, the slurry mixture was slowly dropped into the liquid N₂. Finally, the frozen powders were subsequently dried in vacuum atmosphere at 227 K for 259.2 ks.

2.2 Fabrication of MXene/Ti–6Al–4V build via L-PBF

The Ti–6Al–4V and MXene/Ti–6Al–4V builds were prepared using a laser cusing systerm (Concept Laser Mlab R, Germany) having a laser source 1070 nm-in wavelength. The optimized building parameters (i.e., laser power: 95 W; layer thickness: 25 µm; hatch distance: 90 µm; scanning speed: 800 mm·s⁻¹) were utilized.

2.3 Characterization

Transmission electron microscopy (TEM; HF-2000EDX, Hitachi, JPN), atomic force microscopy (AFM; SII Nanocute, Hitachi, JPN), scanning TEM (STEM; JEM-ARM200F, JEOL, JPN) and field emission scanning electron microscopy (FE-SEM; JSM-6500F, JEOL, JPN) were used for evaluating morphological features of powders or builds. A HE-NE laser optical equipment (HELOS & RODOS, Sympatec, GER) was applied for determining the particle size distributions. X-ray computed tomography (X-ray CT; ScanXmate-D225RSS270, Comscantecno, JPN) was utilized to characterize the distribution of internal defects in the bulk specimens. X-ray diffraction (XRD; SmartLab 9 kW, Rigaku, JPN) was used for checking the phase constitutions. The carbon content of the specimens was determined by gas fusion combined with infrared spectrum method on elements analyzer (CS844, LECO, USA). Vickers hardness measurement with a load of 980 mN was performed using a HM-200 microhardness tester (Mitutoyo, JPN). The dog-bone shaped specimens were obtained using an electric discharge machine for the tensile tests. The width, length, and thickness of the measure part of tensile samples were 2, 15 and 1.5 mm, respectively. The initial strain rate of 1.10 × 10⁻³ s⁻¹ was utilized to performed the tensile tests (Instron 5982, USA) at room-temperature.

3. Results and Discussions

3.1 Powder properties and L-PBF processability

The TEM image of MXene on a Cu grid is displayed in Fig. 1(a). MXene was sheet-like, showing an average lateral size of <1 µm. The selected-area electron diffraction (SAED) pattern (inset of Fig. 1(a)) indicates the hexagonal symmetry of MXene. The AFM image of a MXene sheet is displayed on Fig. 1(b). The random black spots on the MXene sheet were the pores generated by selective etching of MXene; the bright sites were the impurity nanoparticles. The corresponding height profile (Fig. 1(c)) showed a thickness of ∼3.5 nm, indicating an ultrathin feature of MXene. In addition, MXene had a negative zeta potential of −42 mV, which was attributed to the surface groups (e.g., -O, -F, -OH) induced by the selective etching process.^28,29)

Fig. 1

(a) TEM image of MXene on a Cu grid; (b) AFM image of the MXene and (c) the height profile along the white line in (b). Inset in (a) shows the SAED pattern of MXene taken from the blue spot.

The morphology of the original Ti–6Al–4V particles is displayed in Fig. 2(a). The Ti–6Al–4V powder was nearly spherical, exhibiting smooth surfaces. Figure 2(b) shows the STEM image and EDS mappings of a cross-sectional Ti–6Al–4V particle near the surface. An oxide layer existed on the particle surface induced by the gas atomization process,³⁰⁾ leading to a positive charge of Ti–6Al–4V particle in water.²⁾

Fig. 2

(a) FE-SEM image of the original Ti–6Al–4V powders; (b) cross-sectional STEM image and EDS mappings of a Ti–6Al–4V powder near the surface.

Figure 3(a) displays the morphology of prepared MXene/Ti–6Al–4V powder mixtures. Thanks to the ultrathin feature of MXene sheets, the MXene/Ti–6Al–4V powder mixture had a similar spherical shape and particle size as the original Ti–6Al–4V particles (Table 1). The high-magnification FE-SEM image (Fig. 3(b)) and EDS analysis (Fig. 3(c)) displayed that the MXene sheets (the arrows in Fig. 3(b)) were uniformly coated on particle surface without apparent agglomeration. It is thought that the positively charged metallic particles and negatively charged MXene were spontaneously attracted to each other via the electrostatic attraction during the mixing process.

Fig. 3

(a), (b) FE-SEM images of the MXene/Ti–6Al–4V mixed powders; (c) EDS analysis taken from the red spot in (b).

Table 1 Particle size distribution of the Ti–6Al–4V and MXene/Ti–6Al–4V powders.

Figure 4 compares the laser absorptivity of the Ti–6Al–4V, MXene and MXene/Ti–6Al–4V powders. Owing to the high laser absorptivity of MXene sheets (82.9%), the laser absorptivity of the Ti–6Al–4V particles showed an increasing from 70.6% to 77.6% at 1070 nm. This was caused by the uniform decoration of MXene on the Ti–6Al–4V particles. Therefore, the excellent powder properties such as spherical morphology, unchanged particle size, or increased laser absorptivity may benefit to the improvement of the L-PBF processability of the MXene/Ti–6Al–4V mixed powders.

Fig. 4

The laser absorptivity of the Ti–6Al–4V, MXene and MXene/Ti–6Al–4V mixed powders as a function of the laser wavelength from 1000 to 1200 nm.

To illustrate the effect of MXene decoration on the L-PBF processability, both Ti–6Al–4V and MXene/Ti–6Al–4V builds were fabricated using the identical building conditions. The cross-sectional appearance of the obtained L-PBF builds is displayed on Fig. 5. As compared with the Ti–6Al–4V build, a few spherical pores <100 µm in size (black arrows in Fig. 5(b)) were observed in the MXene/Ti–6Al–4V build. These pores were key hole pores or gas pores induced from gases trapped within the molten pools.³¹⁾ Wang et al.³²⁾ claimed that the functional groups of MXene such as -F and -OH could be removed during the thermal reduction. Thus, the decomposition of the MXene surface groups in MXene possibly occurred under the high-temperature L-PBF, leading to the formation of CO₂-, F₂-, or H₂O-trapped pores. Even though small amounts of internal defects were formed in the MXene/Ti–6Al–4V build, it still had a high relative density of 99.1% (i.e., porosity of 0.9%) as determined by the Archimedes’ principle.

Fig. 5

FE-SEM images of the cross-sectional (a) Ti–6Al–4V and (b) MXene/Ti–6Al–4V builds.

The porosity information of the both L-PBF builds were visualized in Fig. 6 by reconstructing the X-ray CT images. The colorful spots indicate the pores with different diameter from 1 µm to 250 µm. It is noted that the pores were randomly distributed in the builds. The total porosity of Ti–6Al–4V and MXene/Ti–6Al–4V build was determined to be 0.051 and 0.054% respectively. It is found that the number of large pores >120 µm in size increased by MXene addition (Fig. 6). Under the high temperature during L-PBF, the decomposition of surface groups of MXene may lead to forming large pores. Moreover, the porosity deviation between the used Archimedes’ principle and X-ray CT analysis may be possibly due to the existence of small pores under the CT resolution (1∼250 µm). It needs further investigation using various builds with different parameters to conclude the influence of MXene addition on the L-PBF manufacturability of the Ti–6Al–4V.

Fig. 6

Porosity distributions in (a) Ti–6Al–4V and (b) MXene/Ti–6Al–4V builds by X-ray CT analysis.

3.2 Microstructure of the L-PBF builds

Figure 7 displays the XRD patterns of the Ti–6Al–4V alloy and MXene/Ti–6Al–4V build. Similar to the Ti–6Al–4V build, the phase constitution of MXene/Ti–6Al–4V build was dominated by α/α′-Ti phases.³³⁾ β-Ti or TiC phase was not detected in the MXene/Ti–6Al–4V build. The lattice parameters of the Ti–6Al–4V and MXene/Ti–6Al–4V builds were calculated from the XRD profiles by using Bragg’s law³⁴⁾ and the c-axis lattice constant of MXene/Ti–6Al–4V build (4.663 Å) was larger than that of the Ti–6Al–4V build (4.650 Å). The reason of the lattice expansion induced by MXene addition will be discussed later.

Fig. 7

XRD patterns of the Ti–6Al–4V and MXene/Ti–6Al–4V builds fabricated by L-PBF.

Figure 8 displays the microstructures of the L-PBF builds. Both the Ti–6Al–4V and MXene/Ti–6Al–4V builds were completely consisted of acicular structures, which was corresponding to α′-Ti.³⁵⁾ During L-PBF treatment, prior β was preferentially formed in Ti–6Al–4V alloy. Subsequently, the β phase transformed to α′ phase by shear-type transformation³⁶⁾ due to its high cooling rate (∼10⁴–10⁸ Ks⁻¹) of L-PBF.³⁷⁾ Comparing with the Ti–6Al–4V build, the MXene/Ti–6Al–4V build possessed finer martensitic microstructure, which is due to the grain refinement effect of solid solute carbon^38,39) from MXene. However, no residual MXene or precipitate was found in the MXene/Ti–6Al–4V build, agreeing with the XRD result in Fig. 7.

Fig. 8

FE-SEM images of the longitudinal cross-section of (a) Ti–6Al–4V and (b) MXene/Ti–6Al–4V builds.

The TEM image of the MXene/Ti–6Al–4V build is displayed on Fig. 9(a), which consisted of fine martensite structures. As revealed by the EDS mappings in Fig. 9(b), Ti, Al, V, O or C was homogenously distributed in the matrix without segregations. According to the combustion LECO analysis, the carbon contents of Ti–6Al–4V and MXene/Ti–6Al–4V powders or builds were summarized in Table 2. The L-PBF builds had a similar carbon content as the starting powders. Based on the microstructural evaluations in Figs. 8 and 9, it is considered that MXene was fully decomposed and dissolved into Ti during L-PBF process. In general, interstitial atoms such as carbon causes the increase of the c-axis lattice constant of α-Ti.^8,40) Thus, the c-axis lattice expansion in MXene/Ti–6Al–4V build in Fig. 7 could be caused by the solid solution of carbon atoms.

Fig. 9

(a) TEM image and (b) STEM image and corresponding EDS maps of the longitudinal cross-section of the MXene/Ti–6Al–4V build.

Table 2 Carbon content of Ti–6Al–4V and MXene/Ti–6Al–4V powders and builds.

3.3 Mechanical performances

The Vickers hardness of the Ti–6Al–4V and MXene/Ti–6Al–4V builds was determined to be 391 HV and 418 HV, respectively. The nominal tensile stress-plastic strain curves of the Ti–6Al–4V and MXene/Ti–6Al–4V builds are shown in Fig. 10(a). The obtained ultimate tensile strength (UTS), 0.2% offset yield strength (YS), and plastic elongation were summarized in Table 3. The UTS and YS of Ti–6Al–4V build were determined as 1286 MPa and 1179 MPa, respectively. With the incorporation of MXene, the UTS and YS of the MXene/Ti–6Al–4V build increased to 1389 MPa and 1309 MPa, respectively. As displayed in Fig. 10(b), the MXene/Ti–6Al–4V build exhibited a highest UTS comparing with the additively manufactured Ti–6Al–4V alloys^41–47) and Ti matrix composites (TMCs),^8,48–52) revealing an excellent strengthening effect by trace MXene addition. This strength enhancement is due to the formation of fine α′-Ti microstructures and the solid solution strengthening of interstitial carbon atoms. In order to obtain an excellent balance between strength and ductility, precise heat treatment and control of MXene concentrations will be performed in the near future.

Fig. 10

(a) Tensile stress-plastic strain curves of the Ti–6Al–4V and MXene/Ti–6Al–4V builds; (b) comparisons in the UTS versus plastic elongation for additively manufactured Ti–6Al–4V alloys and TMCs.

Table 3 Mechanical properties of the Ti–6Al–4V and MXene/Ti–6Al–4V builds.

4. Conclusions

A novel MXene/Ti–6Al–4V component with extraordinary strength was developed via L-PBF. Uniform MXene/Ti–6Al–4V powder mixture was fabricated via hetero-agglomeration. Thanks to the ultrathin feature of MXene sheets, the MXene-decorated Ti–6Al–4V powder exhibited higher laser absorption compared to Ti–6Al–4V powder, while maintaining similar spherical morphology and particle size distribution. After the L-PBF process, MXene/Ti–6Al–4V build was obtained, which fully consisted of fine acicular α′-Ti structures. Microstructural analysis illustrated that the MXene sheets were completely decomposed under laser irradiation and dissolved into the Ti–6Al–4V to form the carbon-saturated alloy. The Vickers hardness, YS and UTS of the Ti–6Al–4V alloy were enhanced by the incorporation of MXene, e.g., the YS increased from 1179 MPa to 1309 MPa. This work demonstrated that MXene can be exploited as an effective nanocarbon source for Ti alloys.

Acknowledgements

The present work was funded by Grant-in-Aid for Transformative Research Areas (A) (Publicly Offered Research) (Grant Number: 22H05273). The authors appreciated the technical assistance of Dr. Kosei Kobayashi and Dr. Takamichi Miyazaki at Tohoku University during the TEM observations.

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