Effects of La2O3 on microstructure and wear properties of laser clad γ/Cr7C3/TiC composite coatings on TiAl intermatallic alloy

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Abstract

The effects of La2O3 addition on the microstructure and wear properties of laser clad γ/Cr7C3/TiC composite coatings on γ-TiAl intermetallic alloy substrates with NiCr–Cr3C2 precursor mixed powders have been investigated by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy-dispersive spectrometer (EDS) and block-on-ring wear tests. The responding wear mechanisms are discussed in detail. The results are compared with that for composite coating without La2O3. The comparison indicates that no evident new crystallographic phases are formed except a rapidly solidified microstructure consisting of the primary hard Cr7C3 and TiC carbides and the γ/Cr7C3 eutectics distributed in the tough γ nickel solid solution matrix. Good finishing coatings can be achieved under a proper amount of La2O3-addition and a suitable laser processing parameters. The additions of rare-earth oxide La2O3 can refine and purify the microstructure of coatings, relatively decrease the volume fraction of primary blocky Cr7C3 to Cr7C3/γ eutectics, reduce the dilution of clad material from base alloy and increase the microhardness of the coatings. When the addition of La2O3 is approximately 4 wt.%, the laser clad composite coating possesses the highest hardness and toughness. The composite coating with 4 wt.%La2O3 addition can result the best enhancement of wear resistance of about 30%. However, too less or excessive addition amount of La2O3 have no better influence on wear resistance of the composite coating.

Introduction

Because of superior strength-to-weight ratio, high specific modulus and good creep resistance, γ-TiAl intermetallic alloy (hereafter referred as TiAl alloy) is of growing interests for elevated temperature applications in the aerospace, automotive and power generation industries [1], [2], [3]. Significant progress has been made in the past decade on improving the room-temperature ductility and high-temperature (>850 °C) oxidation resistance by alloy modifications, processing innovation and surface engineering [4], [5]. Some advanced TiAl alloys are now nearly maturing to the stage where it is possible to implement them to the industrial applications. As a new generation of light weight, elevated-temperature candidate structural materials, the tribological properties need to be enhanced, especially when applied as tribological components such as shafts, blades in gas turbines and exhaust valves in internal combustion engines, in which tribological properties are critically important to the components performance. However, only few reports are available in published literature. Plasma carburization [6] and plasma assisted chemical vapor deposition [7] have been utilized to improve the wear resistance of TiAl alloy by producing Ti2AlC and TiN thin films, respectively. But all the above fabricated films are too thin (the thickness is about 3–4 μm) to withstand large loads, let alone severe mechanical stress. Laser surface modification is also employed [8], [9], [10], [11] to enhance the wear resistance by fabricating in situ wear resistant composite thick coatings (coating thickness up to 1.5 mm) reinforced by TiN and TiC phases. However, both single TiN and TiC phase show poor oxidation-resistance at temperatures higher than 600 °C because of the preferential oxidation of the single TiN and TiC reinforcing phases during high-temperature exposure process. In order to meet the service requirements for TiAl alloy applied as the high-temperature structural components, recently, the authors are exploring new laser surface modification techniques that can noticeably improve the wear- and high-temperature oxidation resistance of the TiAl alloy, simultaneously. The experimental results show that in situ laser surface modification layers that contain both hard wear-resistant reinforcing phases, which can transform into continuous and dense hybrid oxidation scale during the long term high-temperature exposures, can be successfully fabricated on substrate of TiAl alloy with NiCr–Cr3C2 mixed precursor alloy powders by laser surface alloying [12] and laser cladding [13]. The addition of rare-earth (RE) in metal materials has multi-functions, such as purification, modification and alloying, thus can improve a series of properties of metal materials to different extent, for example, metallurgy, casting, heat processing, mechanical properties (toughness and low-temperature brittleness). An area of current interest is the modification of RE to surface engineering (wear-, corrosion- and oxidation-resistance). Previous studies have shown the preliminary unique modification effects of RE in surface engineering [14], [15], [16], [17], [18], [19]. China has plenty of RE resources, applying RE to the materials surface engineering is a very important aspect of expanding the application field of RE. However, only limited literatures are concerning the modification effects of adding RE on microstructural and tribological characteristics in laser cladding in detail. In this paper, the effects of rare-earth oxide La2O3 on the microstructure and wear behaviors of laser clad γ/Cr7C3/TiC composite coatings on γ-TiAl intermetallic alloy is studied so as to offer an experimental basis to expand a new promising field of RE.

Section snippets

Experimental procedures

The starting experimental material is a commercial titanium aluminides alloy Ti–48Al–2Cr–2Nb (at.%). The TiAl alloy is melted using high purity charge materials by a vacuum magnetic-suspension induction skull melting furnace (100 kW). After repeated melting for three times, each time for 15 min, ingots 40 mm in diameter and 180 mm in length were cast. The as-cast ingots vacuum-sealed in quartz tube were homogenized at 1360 °C for 2 h to produce a fully lamellar microstructure. Specimens of 8 mm × 10 mm × 40

Microstructure

The results of X-ray diffraction are shown in Fig. 2. As can be seen in Fig. 2(a and b), the addition of 4%La2O3 has no significantly influence on the phase constitutions of the laser clad composite coating. Similar to the situation without La2O3, an in situ wear-resistant composite coating reinforced by very hard chromium carbide (Cr7C3) and titanium carbide (TiC), which distributed uniformly in the tough and corrosion- and oxidation-resistant γ-NiCrAl nickel based solid solution matrix, has

Conclusions

The addition of rare-earth oxide La2O3 in the NiCr–Cr3C2 precursor mixed powders leads to obvious refinement and spheroidization of the primary phase of the laser clad composite coatings and relative decrease in volume fraction of primary blocky Cr7C3 to Cr7C3/γ eutectics. When the addition of La2O3 is approximately 4 wt.%, the laser clad composite coating possesses the highest hardness and toughness. The refinement of the microstructure is beneficial to improve the hardness, strength and

Acknowledgements

One of the authors (X.-B. Liu) wishes to thank Professor Gang Yu, Dr. Hong-Wei Song and PhD student Ming Pang of the Laboratory for Laser Intelligent Manufacturing, Institute of Mechanics, Chinese Academy of Sciences, for their beneficial discussions.

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