Investigation of HVOF spraying on magnesium alloys
Introduction
Magnesium alloys are promising materials for lightweight constructions such as in aircraft industry and in automobile industry due to their high specific strength and stiffness [1]. However, the use of Mg alloys is limited by their low corrosion and wear resistance. Surface treatments like anodising, painting, electroplating and conversion coatings have been conventionally used until now to improve their surface properties [2]. Among others, the high oxidising nature of Mg alloys usually demands a complex and often expensive process chain to ensure a good coating adhesion in the above mentioned processes. In addition to those conventional methods, recent studies have shown that the thermal spray technology has an enormous potential for the surface modification of Mg alloys [3], [4], [5].
The thermal spray technology offers a variety of techniques, which allow the deposition of a wide range of functional coatings designed for specific environments. Among these techniques, the High Velocity Oxy-Fuel (HVOF) flame spray process is of large significance. Today's HVOF spray systems provide gas velocities up to 2000 m/s. The high velocities and the moderate temperatures of the particles make it possible to produce very dense coatings with outstanding wear and corrosion behaviour and high bond strengths [6], [7].
In this study, the application of wear protective coatings on Mg alloys using the HVOF spray process was investigated. For a better understanding of the coating bond mechanisms on Mg alloy substrates, the flattening behaviour of the splats was investigated. Additionally, the corrosion behaviour of the coated samples was investigated.
Section snippets
Experimental procedure
In this study, the most commonly used Mg alloy AZ91 and the Mg alloy AE42 with an improved creep behaviour were used as the substrate materials. As coating material, the spray powder WC–12Co − 45 + 20 μm was applied. A commercially available HVOF system DJ 2600 by Sulzer Metco was employed for deposition of the coatings. The investigated spray parameters are summarised in Table 1.
The coatings were sprayed onto flat AZ91 samples with 5 mm thickness for metallographic examination, corrosion testing
Particle flattening behaviour and bond strengths of coatings
As seen in Fig. 1, the AZ91 substrate was significantly deformed by the impacting particles due to their high kinetic energy. The large deformation of the Mg substrate absorbed so much energy of the particles so that the spray particles themselves deformed less than in case of spraying them on the steel substrate. As seen in Fig. 2 (under the same spray conditions as for the Mg alloy), the steel substrate almost did not change, while the WC–12Co particles underwent a significant flattening. In
Conclusions
The results in this study show that wear resistant WC–12Co coatings with high bond strengths could be applied on Mg alloy substrates by the HVOF spray process. The high kinetic energy of the WC–12Co particles led to a “self roughening” effect on the substrate, enabling the deposition of WC–12Co coatings on polished Mg alloy substrates with high bond strengths. The real bond strengths could however not be determined because of the failure of the epoxy glue. The corrosion tests showed that
Acknowledgements
The investigations were promoted from funds of the German Federal Ministry of Economic Affairs and Employment via the Federation of Industrial Research Associations (AiF No. 13.769 N) and were supported by the Research Association for Welding and Allied Processes of the DVS. We would like to record our appreciation of this promotion and support.
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