Effect of oxidation on spectral and integrated emissivity of Ti-6Al-4V alloy at high temperatures
Résumé
Thermomechanical simulation is challenging for the optimization and virtual monitoring of high temperature shape processes, such as SuperPlastic Forming (SPF) of Titanium alloys alike Ti-6Al-4V (TA6V). Part of this challenge is the accurate and process-representative knowledge of emissivity, which controls radiative heat transfer during the process. In this paper, the characterization of TA6V emissivity, with regards to its oxidation stage, is performed within the [600–1000 °C] thermal range. The main contribution of this work is the use of an in-house characterization bench, enabling the fast radiative heating of samples, and the control of oxide layers thicknesses ranging from 250 nm to 120 µm. An oxidation law is established thanks to post-mortem mass gain and oxide thicknesses measurement, and the various oxide phases are identified. Oxide thicknesses versus heating time were then calculated and implemented in the emissivity analysis. Two patterns are observed for oxide thickness versus temperature: below 900 °C, emissivity values are quasi-continuously increasing with oxide thickness, exhibiting an oxide layer mainly composed by TiO2, above 900 °C, emissivity values are also continuously increasing but with a different trend, due to the formation of alumina, confirmed by X-Ray Diffraction (XRD) measurements. Finally, the dependence of emissivity to temperature and oxide thickness emphasized in this paper constitutes a non-trivial mandatory input to increase the radiative heat transfer simulations codes accuracy.
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