Oxidation behavior of Cr-AlSi12 composite coatings on Ti-6Al-4V alloy substrate fabricated via high-energy mechanical alloying method
Introduction
Titanium and its alloys have significant advantages in terms of specific strength, fatigue resistance, corrosion resistance and biocompatibility, and have been widely used in aerospace, aerospace, naval, chemical and other manufacturing fields [1]. Among them, Ti-6Al-4V has become the most widely used α + β dual phase titanium alloys due to its excellent performance, but the poor oxidation resistance of Ti-6Al-4V alloy limited its application in some harsh environments, especially above 600 °C [[2], [3], [4], [5], [6]]. Therefore, in order to prolong the service life of Ti-6Al-4V under high temperature conditions, there are two main approaches. One is minor element addition in Ti-based alloys [7], the other is to modify the surface. There are many surface modification methods in industrial application [8]. The coating fabricated on the substrate, including low-pressure plasma spraying (LPPS) [9], plasma immersion ion implantation and deposition (PIII&D) [10], laser cladding (LC) [11], physical vapor deposition (PVD) [12] and chemical vapor deposition (CVD) [13], is an effective method for surface modification, which have been extensively used in engine parts and components of automotive. However, there are some drawbacks in these methods such as time-consuming, high cost and complex technology. The preparation of coatings via mechanical alloying (MA) method is high efficiency, short period and energy conservation.
Mechanical alloying (MA) method is a solid state and non-equilibrium powder processing technology, consisting of repeated cold welding and fracture of powder particles and atom diffusion in powder particles, and the method have superiority in the preparation of refractory powder system [14]. Recently, there have been many researches on the preparation of composite coatings by mechanical alloying [[15], [16], [17], [18], [19], [20]]. Moreover, the coating fabricated using MA method exhibits favourable metallurgical bonding property at the appropriate milling parameters covering milling duration, powder weight ratio and ball to powder weight ratio.
Aluminium-silicon alloys, with excellent corrosion and wear resistance as well as attractive strength to weight ratio, are increasingly being used in space and automotive industries [21]. AlSi alloys were considered as one of the best piston materials due to their low coefficient of thermal expansion. They also could be used in automotive engines to replace cast iron cylinders and significantly reduce weight [22]. AlSi12, also called silumins, is a eutectic alloy. According to the phase diagram of AlSi alloy, AlSi12 has the lowest melting temperature of the AlSi alloys, thus, there is almost no microcrack in the AlSi12 coating owing to great filling performance. However, AlSi12 coating will severely oxidize at relatively high temperatures, which is disastrous for the oxidation resistance [23]. As for Cr coating, due to high corrosion resistance, it is widely used as metal protective coating in modern industry [24]. Besides, Cr plays an important role in thermal barrier coatings of NiCrAlCoY [25], NiCrAlY [26], NiCrAl [27]. However, Cr and its alloys are brittle and have poor formability. Microcracks are always unavoidable in Cr-based coatings [28]. In order to solve these problems, the Cr-AlSi12 composite coating was studied in the present work.
In this research, the Cr-AlSi12 composite coatings were respectively fabricated on Ti-6Al-4V alloy substrates using different powder weight ratio and milling duration via high-energy mechanical alloying method. On the one hand, the addition of AlSi12 alloy powder, as the bonding agent, was applied to the improvement of bonding property between coating and substrate due to its good ductility and low melting temperature. On the other hand, the Cr particles of Cr-AlSi12 powder system was used to enhance mechanical property and oxidation resistance, which was attributed to the high hardness and thermal stability of Cr. The surface morphologies, cross-section microstructures, phases and chemical compositions of as-prepared coatings before and after oxidation were characterized. The high-temperature oxidation resistance of the coating was studied, for further discussed, a feasible oxidation mechanism of oxidized coating was proposed.
Section snippets
Sample preparation
A Fritsch Pulverisette six planetary mono-mill was used in the experiments, the schematic illustration of the experimental apparatus is shown in Fig. 1. The Ti-6Al-4V alloy was used in this study as substrate material with the size of 12 mm × 12 mm × 3 mm, which was cut from rectangular TC4 plate, and its chemical composition is shown in Table 1. Before mechanical alloying treatment, in order to remove the oxides and other contaminants, several Ti-6Al-4V plates should be pretreated, including
The effects of Cr-AlSi12 ratio on the coating
After the MA treatment, the Ti-6Al-4V alloy substrates were successfully and fully covered with as-prepared coatings. In this section, all of the coated samples were milled for 5 h, in order to describe the as-prepared coatings simply, samples will be referred to as MA-xAlSi12, where x refers to the AlSi12 fraction in the Cr-AlSi12 powder system. The effects of Cr-AlSi12 weight ratio on the coating would be mainly investigated by means of the comparison of surface morphologies, cross-section
Conclusions
Continuous Cr-AlSi12 composite coatings were successfully fabricated on the Ti-6Al-4V substrate by using MA treatment. In order to select the optimal parameters to prepare the coating, the effects of Cr-AlSi12 weight ratio and milling duration were investigated. The formation process was studied. The high-temperature oxidation process of the as-synthesized coating was discussed through different testing methods. The detailed conclusions were drawn from this research as followed.
- (1)
The Ti-6Al-4V
Acknowledgments
The present work of this paper is supported by the National Natural Science Foundation of China (Grant No. 51475232). This is a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institution.
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