AEROSPACE APPLICATIONS OF TITANIUM ALLOYS

 

Introduction

Titanium alloys are grouped into Alpha Alloys, Beta Alloys and Alpha + Beta Alloys. At low temperature, pure titanium has HCP structure and is known as Alpha titanium and at high temperature it has BCC structure and is known as Beta titanium. Pure titanium contains some amount of impurities such as Nitrogen, Hydrogen, Oxygen and Iron. The strength of titanium increases with the amount of oxygen and Iron present.

 Applications of Alpha Alloys

 Apart from the use of CP Ti and Ti-3-2.5 for ducts and tubing, another major application of α-alloys is in the fuel pump impeller of the turbomachinery of liquid hydrogen fuelled rocket engines. These impellers are typically made from Ti-5-2.5 and operate at high stresses at about −250 ◦C.

Applications of Near Alpha Alloys

 The near-α alloys are not as strong at or near room temperature as either the α + β or β-alloys, but have superior creep resistance along with the improved strength retention which is important for high temperature applications such as the later stages of the high pressure compressor in an aircraft turbine engine. At the point where the gas temperature exceeds about 500 °C, the remainder of the compressor rotor is made of Ni-base alloys. In current engines it is common for all of the Ti alloy the compressor rotor stages to be a single piece as shown in Figure.

Properties of Titanium Alloy a Choice Material in Aerospace/Aeronautics Applications

 (a) Weight-to-Strength Ratio

 Compared to steel and aluminium, titanium is much lighter in weight. As such, it provides the much-needed strength and durability required to structure aircraft with high fuel efficiency without adding extra weight. The only lightweight metal that works well with composites is titanium. Because of their lightweight, composite materials in aeronautic engineering are beneficial. They are very important for improving fuel efficiency, as the heavier an aircraft is, the more fuel it will consume. Many composites containing aluminium and other metals are not as lightweight as titanium, making it the number one choice for composite structures in aircraft.

 

(b) Thermal Expansion Rates

 A distinguishing feature of titanium is its high-temperature performance, a property that makes it highly reliable at different temperatures, as well as a great interface material. Its ability to withstand high temperatures also makes it a better replacement for aluminum.

 (c) High Corrosion Resistance

In the aerospace industry, corrosion resistance is of great importance. Titanium is naturally resistant to corrosive elements, which eliminates the need to cover it with special corrosion-resistant paints unless the titanium is used in alloys. In the long run, titanium is a great choice when extending aircraft life becomes a priority.

 Various Grades of Titanium Alloys used in Aircrafts

 (a) Ti-6Al-4V alloy

 Ti-6Al-4V alloy is designed for a good balance of characteristics, including: strength, ductility, fracture toughness, high temperature strength, creep characteristics, weldability, workability, and thermal processability. This alloy is therefore used for many airframe and engine parts. In airframes, it is used for general structural material, bolts, seat rails and the like. In engines, due to the relatively low allowable temperature of about 300˚C, the alloy is used for fan blades, fan case and the like in the intake section where temperatures are relatively low.

 

 (b) Ti-6Al-2Sn-4Zr-2Mo alloy

 Ti-6Al-2Sn-4Zr-2Mo alloy is a heat resistant alloy. Its heat resistant temperature is approximately 450˚C. Ti-6Al-2Sn-4Zr-2Mo0.1Si was developed to improve oxidation resistance and creep property with the addition of Si of 0.06~0.2wt%, and the heat resistant temperature was improved to approximately 500˚C. Therefore, this alloy is commonly used for compressor discs where 500˚C is the upper service temperature limit.

 (c) Ti-8Al-1Mo-1V alloy

 Ti-8Al-1Mo-1V alloy heat resistant temperature is approximately 400˚C. Since its heat resistant temperature is higher than that of Ti-6Al-4V alloy, it is used for compressor blades and the like, rather than fan blades.

 (d) Ti-5Al-2Sn-2Zr-4Cr-4Mo (Ti-17) alloy

 Ti-5Al-2Sn-2Zr-4Cr-4Mo alloy having high strength and excellent fracture toughness. Its heat resistant temperature is approximately 350˚C. In commercial aircraft engines, the fan and shaft are built as one piece to reduce engine weight.

 (e) Ti-6Al-2Sn-4Zr-6Mo alloy

 Ti-6Al-2Sn-4Zr-6Mo is a titanium heat resistant temperature is about 450˚C. This alloy has high strength and excellent creep characteristics.

 (f) Ti-15V-3Cr-3Sn-3Al alloy

 Ti-15V-3Cr-3Sn-3Al alloy heat-treated material has excellent cold workability and, in the form of a thin sheet, a strength higher than that of pure titanium JIS H 4600 (TP550H) can be obtained. For airframes, welded pipes and ducts made by welding thin sheets are used.

 (g) Ti-10V-2Fe-3Al alloy

 Ti-10V-2Fe-3Al alloy has excellent hardenability, high strength, and high fatigue strength. It is mainly used for landing gear

Applications of 3D printed titanium-based alloys

 3D printed titanium based alloys has been used in different applications, and some of these include tool steels used as mould inserts, cobalt-chromium alloy for dental prostheses, and titanium alloy as a choice material for various medical applications as a hip endoprosthesis. Furthermore, the fabrication of aircraft body parts such as fuel nozzles (GE LEAP aero engine) has been of assistance in fabricating such parts with complex designs with powders being used repeatedly without any alteration in the mechanical and physical properties of fabricated parts. Aside from the biomedical applications of titanium-based alloys, they are also used in the aerospace and automotive industries due to their attractive properties. These include improved resistance to oxidation, high stiffness, and low density, making them a perfect fit for lightweight applications. Pictorial representation of applications where 3D printed titanium alloy have been reportedly utilised are shown in Figure

 

Conclusion

 Demand for titanium for airframes and engines is increasing, accompanied by improvements in aircraft fuel consumption. Various titanium materials are used for aircraft, each material selected according to use. Commercially pure titanium is used for airframes where formability is considered important; for engines where heat resistance and strength are considered important, titanium alloys are used. Titanium alloys are used for both low temperature as well as high temperature applications. It is clear that the growth and maturation of the Titanium alloy industry has played a significant role in enabling more durable, quieter and fuel efficient commercial aircraft.

 

                                                                             [Dr. G. MANIKANDAN]

                                                                             HEAD, AERO

                                                                             Sandip University