Aero-Engine Titanium from Alloys to Composites

Abstract

The aero-engine has provided the major drive for the development of new improved titanium alloys in recent years. This paper covers these developments from the workhorse alloy Titanium 6/4 and it’s higher temperature stable mates through to the more exotic intermetallic materials and on to their reinforcement with ceramics. The use of Ti6/4 alloy is now widespread throughout the aero space industry providing a good combination of strength at moderate temperatures (~300°C) a relatively low density and a wide range of processing options ranging from castings to forgings to powder HIP and diffusion bonding. Alloy development for the aero-engine essentially concentrated on either increasing the temperature capability and creep resistance or increasing the strength at intermediate temperatures. Alloys such as Ti 6242 and IMI 834 were aimed at compressor disc applications with operation up to around 600°C. Improvements resulted from compositional control and thermal processing to optimize the microstructure for creep and fatigue. High strength intermediate temperature capability (~500°C) alloys were also developed (Ti6246) where higher levels of molybdenum balance the alpha strengthening additions. The drive for lighter weight led to the development of titanium intermetallic systems. Alloys such as 45-2-2XD and Alloy 7 have been the subject of much research and manufacturing development over the last 20 years, demonstrating that they are capable of operating at temperatures well above those of conventional titanium. More recently, alloys with higher additions of Nb and Ta have shown improved mechanical properties and offer promise to extend the application of TiAl above 700°C. In parallel with intermetallic developments combining titanium alloys with the extreme high strength of ceramic fibres has proved irresistible and many ways to produce titanium composites have been developed. The majority of application development has focused on Ti6/4 alloy as the matrix although other matrix alloys have been investigated and tested in U.S. engine demonstrators. Recently a combination of Ti-22Al-26Nb disks reinforced with orthorhombic MMC ran for over 100 hours in an engine test. However, none of these niche composite systems has yet made the transition into large volume production and the fibre reinforced Ti6/4 system probably offers the greatest potential for implementation. The main barrier to the take up of both advanced intermetallics and titanium composites is the cost of raw materials and processing. The challenge still exists to produce net shape components and provide weight savings at an acceptable cost. This will be the key to future exploitation.