Critical firing conditions for titanium alloys by molten droplet ignition

Abstract

Reliable service of indispensable titanium alloys in aeroengines is limited by titanium fire with unclear critical conditions. In this study, critical conditions in the form of parameter phase diagrams for secondary burning were determined by molten droplet ignition. Finite element analysis (FEA) reveals that the molten droplet size (R₀), relative oxygen content (P_r), and initial temperature (T₀) significantly affect titanium fire following power exponential laws. In addition, P_r and T₀ affect ablation synergically, and a unified empirical relationship for critical ablation of titanium alloy is established. Furthermore, the critical T₀ of a typical TC4 alloy under 20% P_r is higher than 973 K, which explains the reliable service performance of TC4 alloy at ≤ 873 K in compressors in commerical air engines. In addition, the critical empirical conditions are consistent with the FEA results, and the deviation occurs due to oxidation at T₀ ≥ 673 K. Moreover, the ablated TC4 alloy can be divided into five characteristic zones, and loose and porous oxide scales are formed at the front of the titanium fire. The present study points the way towards long-lifetime services of TC4 alloys through sound titanium fire protection strategies.