Modeling the Oxidation Kinetics of Ti-6Al-4 V Spherical Powder at High Temperatures

Abstract

The aim of the present work is to establish a kinetic law for the oxidation of Ti-6Al-4V (Ti64) spherical powder at high temperatures miming the possible oxidation of such powder within a laser powder bed fusion process. The oxidation experiments were followed by isothermal and isobaric thermogravimetry between 700 and 750 °C, under a controlled partial pressure of O₂ in the range of 0.1 to 0.75 atm. Beside the duplex structure of the oxide layers formed, namely an inner layer composed mainly of TiO₂ and an outer one composed of Al₂O₃, it was found that the oxidation rate is limited by one rate-determining step occurring in a single reaction zone: the Al₂O₃ layer. The study of how the growth rate varies with the partial pressure of O₂ highlighted that the rate-determining step is the diffusion of interstitial oxygen as a dumbbell in this Al₂O₃ layer. Based on physico-geometrical description of the reaction, a complete reaction rate equation is then proposed by taking into account the spherical geometry and the dimensions of the Ti64 particles as well as a dependence of the reaction rate with temperature and partial pressure of O₂. The rate law is very satisfactorily confronted to the experimental data.