Ignition mechanism of near α high temperature titanium alloy

Abstract

The risk of titanium fire increases significantly with the development of future aero-engine, however, the burning mechanisms of titanium alloys remain uncertain. Therefore, the ignition behaviors and mechanisms of near α high-temperature titanium alloy are studied in this paper by employing an integrated experiment method, including laser-oxygen concentration ignition method, infrared temperature measurement and observation of molten metal by high-speed camera. Based on this, the ignition boudary curve is determined and the ignition temperatures of the alloy are found to decrease from 1610 to 1550 ℃ with the increasing of laser power from 200 to 325 W and of oxygen concentration from 21% to 60%. The ignition microstructure was characterized by FIB and TEM to study the evolution of reaction products. Pores are found to form under the TiO<sub>2</sub> surface layer, which can be attributed to the instablity of TiO. The failure mechanism of protective oxide layer is further analyzed according the oxide layer damage model caused by thermal stress. As temperature approaches the ignition temperature, which locates below the melting point, the high vapor pressure of TiO causes the formation of porous defects under the TiO<sub>2</sub> surface, thus accelerating the fracture and failure of the TiO<sub>2</sub> layer under thermal stress. It is revealed that critical conditions of temperature and instantaneous temperature change rate are both required to fulfill the ignition. Based on this, an ignition model is futher constructed to discuss the relationship among ignition temperature, laser power and oxgyen concentration. According to the experimental data fitting, the reaction activation energy of TA19 alloy during the ignition stage is calculated as approximately 300 kJ/mol, and the function for ignition temperature calculation is concluded as 8.3×10<sup>9</sup><i>e</i>-300000/R<i>T</i><sub>ig</sub>c<sup>1/2</sup>＋0.52<i>P<sub>L</sub></i>-331=0. This provides a theoretical reference for predicting the ignition temperatures of near α high temperature titanium alloy and other types of titanium alloys under complex airflow conditions in aircraft engines.