Effect of α-Al2O3 on the properties of cold sprayed Al/α-Al2O3 composite coatings on AZ91D magnesium alloy
Introduction
Magnesium alloys are promising lightweight materials applied in automotive, aerospace and electronic industries due to their combination of low density and excellent physical and mechanical properties. However, the relatively high corrosion susceptibility and low wear resistance of magnesium alloys limit their potential for wide industry applications [1], [2]. Protective coatings are one of the most effective ways for corrosion control of magnesium or its alloys. Therefore, different surface treatment techniques have been used to protect magnesium alloys, such as CVD, electroless plating and anode oxidation or microarc oxidation etc [3], [4], [5], [6], [7], [8]. But it is known that magnesium alloys has high chemical reactivity and affinity in aqueous solutions, so it is more challenging to apply these techniques to magnesium alloys than to other metals [9].
As a relatively new surface treatment technique, CS has attracted more and more attention, by which coatings can be prepared without involving both feedstock and substrates into high temperature atmosphere [10], [11], [12], [13], [14]. Spray particles can adhere to substrates without melting before they impact on the substrates, but rather, due to their kinetic energy. Therefore, when compared to a conventional thermal spray process, CS is more suitable for metallic spray materials or substrates which are sensitive to heat or oxidation. In addition, CS technique allows the deposition of a wide range of dense and uniform coatings designed for corrosion protection. Cold sprayed Al, Ti, Ta, amorphous alloy and composite coatings have been investigated for corrosion protection [15], [16], [17], [18], [19], [20], [21], [22], [23].
It is known that coatings for magnesium alloy corrosion protection must be uniform, well adhered to the substrate and pore free, thus, acting as an efficient barrier between the substrate and the environment. In principle, to achieve high density coating by CS, the feedstock materials should have relatively high ductility. Aluminum which satisfies this requirement is a good candidate for CS processing. It has been reported that dense aluminum coatings can be deposited by CS on different substrates [15], [16], [17], [24], [25]. To further improve the performance of cold sprayed coatings, reinforcements have been introduced in the feedstock to produce composite coatings by CS recently [25], [26], [27], [28]. It is known that the deposition efficiency can be improved by adding hard particles to a metal powder with a proper ratio [26]. Also the use of a ceramic-metal mixture increases the bond strength of the coating to the substrate [26], [27]. Some works focused on the influence of the inclusion of ceramic or hard metal particles on cold sprayed coating deposition and properties [25], [26]. However, few research works have been reported how the addition of ceramic particles influences the corrosion protection behavior of the coating as compared to pure metal.
In this paper, composite coatings of aluminum with different weight percentage of α-Al2O3 were successfully deposited on AZ91D alloy by CS. The effects of α-Al2O3 contents on the microstructure and mechanical properties of the coatings were evaluated. It was known that the addition of a reinforcing phase could degrade the corrosion resistance of as-cast aluminum matrix composites [29]. For this reason, the effect of α-Al2O3 on the corrosion behavior of the coatings was also investigated in this paper.
Section snippets
Materials and coatings preparation
Commercially pure aluminum powder (99.5 wt.%) produced by gas atomization, was blended with α-Al2O3 particles as feedstock materials for coatings. The morphologies of the powder were examined using scanning electron microscope (SEM, s-2000 Hitachi, Japan)). As seen in Fig. 1, the spherical pure Al particles (Fig. 1a) and the rectangular α-Al2O3 particles (Fig. 1b) were in the size range of 1–30 μm. The weight percentages of α-Al2O3 in the feedstock were 25% and 50%. Correspondingly, the coatings
Surface microstructural features of the as-sprayed coatings
Fig. 2 shows the surface SEM images of the as-sprayed coatings. It is observed that there are some shallow and smooth craters which must be created by rebounded aluminum particles in the pure Al coating (Fig. 2a) and some deep craters with edges and corners which must be created by rebounded hard alumina particles in the composite coatings (Fig. 2b and c). It suggests that hard alumina particles can lead to larger deformation of pre-deposited aluminum particles than aluminum ones in cold
Conclusions
Composite coatings, consisting of aluminum blended with 25 wt.% and 50 wt.% α-Al2O3 particles as feedstock, were successfully deposited on AZ91D magnesium alloy by CS. Due to dramatic tamping effect of α-Al2O3 on pre-deposited Al splats, the composite coatings processed lower porosity than the pure Al coating. The adhesion of composite coatings to substrate is much higher than that of the pure Al coating, which is attributed to the effects of penetrating and erosion of α-Al2O3 particles on
Acknowledgement
The authors would like to appreciate Prof. Junhua Dong for his help in electrochemical experiments.
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