Abstract
In this study, the phase transition of secondary phase particles in a composite coating is used to estimate the temperature field of the molten pool on a Ti6Al4V alloy in the micro-arc oxidation (MAO) process. The behavior of the sparks and the molten pool during the MAO process was observed in real-time by a long-distance microscope. The microstructures and compositions of the composite coatings were studied by scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). The results revealed that, for the temperature field distribution range of the molten pool in the active period, the lower limit is 2123 K and the upper limit is not lower than 3683 K. The reason for the multiphase coexistence is that the high-temperature phase is retained by the rapid cooling effect of the electrolyte, and the low-temperature phase is formed due to secondary phase transformation during the long active time of the molten pool temperature field. The strengthening mechanism of the composite coating prepared by adding the secondary phase particles is elemental doping rather than particle enhancement. The secondary phase particles are able to enter the composite coating by adhering to the surface during the cooling process. The secondary phase particles will then be wrapped into the coating in the next active period.
Highlights
Micro-arc oxidation (MAO) is an emerging technique used to prepare oxidized ceramic coatings in situ on valve metals such as titanium, magnesium, aluminum, and their alloys
Through applying a high potential, which is much larger than that of the Faraday region of conventional anodization, an oxidized ceramic coating is obtained by the effect of thermochemical, electrochemical, and plasma chemistry in a specific electrolyte [1,2,3,4,5]
Molten materials are ejected from the channels at higher discharge temperatures and pressures, and they rapidly cool due to their direct contact with the electrolyte, resulting in an overall increase in coating thickness [6,7,8]
Summary
Micro-arc oxidation (MAO) is an emerging technique used to prepare oxidized ceramic coatings in situ on valve metals such as titanium, magnesium, aluminum, and their alloys. Molten materials are ejected from the channels at higher discharge temperatures and pressures, and they rapidly cool due to their direct contact with the electrolyte, resulting in an overall increase in coating thickness [6,7,8]. The MAO technique is commonly used to increase the wear resistance on the surface of aluminum, magnesium, and titanium alloys [9,10,11]. MAO coatings are multiphase, even without any additional external particles.
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