Abstract
As an emerging coating building technique, cold spraying has many advantages to elaborate Mg alloy workpieces. In this study, AZ91D coatings and AZ91D-based composite coatings were deposited using cold spraying. Coatings were prepared using different gas temperatures to obtain the available main gas temperature. Compressed air was used as the accelerating gas, and although magnesium alloy is oxidation-sensitive, AZ91D coatings with good performance were obtained. The results show that dense coatings can be fabricated until the gas temperature is higher than 500 °C. The deposition efficiency increases greatly with the gas temperature, but it is lower than 10% for all coating specimens. To analyze the effects of compressed air on AZ91D powder particles and the effects of gas temperature on coatings, the phase composition, porosity, cross-sectional microstructure, and microhardness of coatings were characterized. X-ray diffraction and oxygen content analysis clarified that no phase transformation or oxidation occurred on AZ91D powder particles during cold spraying processes with compressed air. The porosity of AZ91D coatings remained between 3.6% and 3.9%. Impact melting was found on deformed AZ91D particles when the gas temperature increased to 550 °C. As-sprayed coatings exhibit much higher microhardness than as-casted bulk magnesium, demonstrating the dense structure of cold-sprayed coatings. To study the effects of ceramic particles on cold-sprayed AZ91D coatings, 15 vol % SiC powder particles were added into the feedstock powder. Lower SiC content in the coating than in the feedstock powder means that the deposition efficiency of the SiC powder particles is lower than the deposition efficiency of AZ91D particles. The addition of SiC particles reduces the porosity and increases the microhardness of cold-sprayed AZ91D coatings. The corrosion behavior of AZ91D coating and SiC reinforced AZ91D composite coating were examined. The SiC-reinforced AZ91D composite coating reveals higher corrosion potential than magnesium substrate; therefore, it serves as a cathode for the magnesium substrate, the same as the AZ91D coating on magnesium substrate. As the SiC powder is semi-conductive, the embedded SiC particles reduce the electrochemical reaction of the AZ91D coating. The addition of SiC particles increases the corrosion potential of the coating, meanwhile increasing the galvanic potential and decreasing the negative galvanic current of the coating-substrate couple.
Highlights
Magnesium (Mg) alloys have potential use in aircraft and automobile industries due to their excellent specific strength [1]
The microstructures of the coatings elaborated at 500 ◦ C, 550 ◦ C, and 600 ◦ C are shown in Figure 2a–c, respectively
Compressed air was used as the main gas, and the effects of gas temperature were studied
Summary
Magnesium (Mg) alloys have potential use in aircraft and automobile industries due to their excellent specific strength [1]. They are used to protect steel structures from corrosion in freshwater and soil environments as sacrificial anodes [2]. AZ91D work pieces are difficult to be repaired using traditional techniques (tungsten inert gas (TIG) welding [4], metal inert gas (MIG) welding [5], and thermal spraying [6], etc.) due to their high-temperature activity. As an emerging coating building and rebuilding technique, cold spraying has advantages to elaborate Mg alloy workpieces [7,8]. In the view of bonding, the jet formation of impact couples is viewed as the necessary condition
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