Why is Titanium Used in Aerospace Applications?
Advantages of Titanium
The short answer is to save weight. Saving weight is like finding gold to aircraft designers and operators. Commercial aircraft may accumulate 100,000 hours of flight time over a 30-year lifespan. As a rough rule of thumb, a 1% decrease in weight will yield a 0.75% savings in fuel usage. Three-quarters of a percent may not sound like much, it adds up quickly. It costs around $5,000 per hour to fly a Boeing 737. This means reducing weight one percent would reduce fuel costs by $37.50 per hour. If the aircraft is operated 12 hours a day for 300 days a year, that adds up to $135,000 per year, or about 4 million dollars over the life of the aircraft. Imagine now that you are the person at Southwest Airlines in charge of reducing costs, and you can now see the importance of aircraft weight.
Here are approximate densities and yield strengths (the stress at which the material shows significant permanent deformation) of three major structural materials:
- Steel weighs 0.3 pounds per cubic inch and has a yield strength of 100,000 psi (pounds per square inch).
- Aluminum weighs 0.1 pounds per cubic inch and has a yield strength of 35,000 psi. It is roughly one-third the weight of steel and one-third the strength.
- Titanium weighs 0.15 pound per cubic inch and has a yield strength of 100,000 psi. It has the strength of steel at half the weight.
Based on these properties, it is pretty clear that in highly stressed areas titanium should be used to keep weight down, and aluminum should be used where stresses are low.
Another significant advantage that titanium has over aluminum is that it can with stand much higher temperatures than aluminum. The temperature limit for aluminum is around 300F, while some titanium alloys can be used to up to around 1000F. For commercial airliners this is not an issue, but the outer skin of the SR-71 Blackbird traveling at Mach 3 could reach 500F, with leading edges and areas around the engines around 1000F.
Titanium in Gas Turbine Engines
Titanium is also used in the gas turbine engines which power today’s commercial and military aircraft. Titanium is generally used in the compressor stages of turbine engines. The purpose of the compressor section of a gas turbine is to increase the pressure and density of the air prior to combustion. As the air is compressed, its temperature increases, and each stage gets hotter. Because of this, different alloys of titanium are often used in the early and later stages of the compressor. Early stages typically use Ti-6Al-4V titanium, which is the most common alloy, and is good up to about 660F. Later stages may use Ti-6Al-2Sn-4Zr-2Mo, which has good strength up to around 950F.
Drawbacks of Titanium
Titanium is a great material where high strength and low weight is required, but it does have some drawbacks:
- It is more expensive than materials like steel and aluminum.
- It is difficult to machine.
- It has a low fracture toughness, which means smaller cracks can result in catastrophic failure with titanium than with other metals.
Despite these negatives, titanium is an excellent material for aircraft and aircraft engines when the demanding environment justifies the extra expense.