1. Prepared by Dr Diane Aston, IOM3

2. MODULE TWO
Materials around us
Prepared by Dr Diane Aston, IOM3

3. Materials around us
The aim of this module is to introduce you to three areas in which discoveries and developments in materials have helped to improve our technology in three key areas:
Turbofan engines for civil aircraft
Biomedical materials
Sports equipment
This builds on Module 1 in that these advances have been made by understanding the links between processing, structure and properties.
Prepared by Dr Diane Aston, IOM3

4. Session 1 Materials in aerospace
Prepared by Dr Diane Aston, IOM3

5. Aims and objectives
The session aims to introduce you to the technology and materials behind modern high by-pass turbofan jet engines.
At the end of this session you should be able to:
Explain in simple terms how a turbofan jet engine works;
Describe the key characteristics of materials used in the fan, compressor and turbine stages of a turbofan engine;
Describe the materials and processing used in wide chord fan blades;
Describe how changing microstructure has helped to improved the performance of turbine blades;
Describe in simple terms how turbine blades are manufactured.
Prepared by Dr Diane Aston, IOM3

6. How does a turbofan engine work?
A turbofan jet engine is a type of internal combustion engine. It works on the same four stage, suck, squeeze, bang, blow process as the engine in a car, it is just a bit bigger and arranged in a slightly different way.
The cold air sucked into the front of the engine by the fan can go one of two ways. The majority of the air (80-90%) is directed around the core of the engine through bypass channels. This air exits the back of the engine much faster than it went in through the front so it creates a force which pushes the aircraft forward.
A small amount of air (10-20%) is directed through the core of the engine and it is this which drives it.
As air enters the core it is squeezed by the compressor. The fast moving, hot, compressed air is mixed with aircraft fuel in the combustion chamber and this mixture is ignited with a glorified spark plug. As the fuel burns very hot and fast moving exhaust gases are produced and these are forced through a turbine before they exit through the nozzle at the back of the engine. The stream of exhaust gas causes the turbine to rotate. The turbine is at the hot end of a shaft which is connected to the compressor and fan so as the turbine is forced to rotate by the exhaust gas it causes the compressor and fan to rotate to suck more air in and continue the process.
Modern engines operate on a multi-shaft basis. In this diagram the high pressure turbine drives the high pressure compressor and the low pressure turbine is connected to the fan and low pressure compressor. This is a two-shaft set up, however, some engines have three shafts, with an additional intermediate pressure turbine driving the intermediate pressure compressor.
Prepared by Dr Diane Aston, IOM3

7. Turbofan key facts
On a modern engine the fan can be up to 3m in diameter.
Between 80 and 90% of the air entering the engine is directed through the bypass. This fast moving air produces most of the thrust.
A large engine can take in the same amount of air as there is in a squash court every second.
Having a high bypass ratio reduces noise and increases fuel efficiency.
A RR Trent 800 jet engine generates 100,000lbs of thrust.
A typical large turbofan costs in excess of £10 million!
Prepared by Dr Diane Aston, IOM3

8. Materials for the fan
Blade design was limiting factor in jet engine size.
Blades used to be solid forgings that were very heavy.
Blades need to be lightweight, strong and stiff.
Titanium blades made by super-plastic forming and diffusion bonding.
Blades have a hollow, corrugated cross section.
Traditionally fan blades were solid metal forgings. These were very heavy and limited the size of the fan.
Fan blades need to be lightweight, strong and stiff as they rotate at high speed. A fan blade will typically carry a load equivalent to hanging a locomotive engine from it when it is a cruising speed and each blade costs around the same as an average sized luxury family car!
The fan blades are made from a titanium alloy called Titanium 6/4. This is comprised mainly titanium with 6% by weight of aluminium and 4% by weight of vanadium. Titanium has the highest strength-to-weight ratio of all the metals so strong, lightweight blades can be manufactured.
Modern wide chord fan blades are made by a process call superplastic forming and diffusion bonding. The first stage in this process is making a titanium sandwich with the outer layers slightly thicker than the middle layer. These layers are masked out where they do not want to stick together and then the sandwich is welded round three sides. This whole assembly is placed into a mould and heated to a critical temperature where the titanium becomes superplastic. This means that it can be stretched to far greater extensions than are possible at room temperature. In this state the blade is inflated with an inert gas so that it stretches and takes on the shape of the mould in which it is sitting. Internally, where the layers are not masked out they stick together. The blades are then machined to their final manufacturing tolerance. The result is a blade that varies in width and thickness from root to tip and twists from root to tip to allow the most efficient passage of air into the engine.
This photograph is reproduced with the permission of Rolls-Royce plc, copyright © Rolls-Royce plc 2010
Prepared by Dr Diane Aston, IOM3

9. Materials for the compressor
The compressor is made up of a series of rotating blades separated by stationary vanes.
These compress the air so that when it exits the high pressure compressor it is at around 600C.
Compressor blades made from titanium as it is strong, light and able to operate at 600C.
Blades are made by hot forging and then machining.
Blades sit on a series of rings.
As the air enters the compressor stage it travels through a series of rotating blades and fixed vanes. As it is squashed its temperature increases until it is at a temperature of about 600ºC before it enters the combustion chamber.
The compressor blades must be light and strong and like the fan they are usually made from Titanium 6/4. The fan blades are hot forged and machined to a tolerance of about 2 microns. Like the fan blades they are shaped to allow the most efficient passage of air. The joint where the blades slot in to the ring is a weak point in the system a number of alternatives are being explored. Blisks and blings (bladed disks and bladed rings) can both give significant weight savings on traditional systems.
This hottest part of the engine where it is safe to use titanium. Titanium is actually a very reactive metal, however, it oxides rapidly to produce an invisible passive oxide layer on the surface which is very stable. The inert oxide layer gives the impression that the titanium is unreactive. At temperatures approaching 700ºC it is possible for this oxide layer to breakdown, particularly in a high friction environment. If this were to happen the exposed titanium below would react so vigorously it would become explosive.
Moving forward it may be possible to introduce a titanium metal matrix composite reinforced with silicon carbide fibres to produce a stronger, stiffer and lighter bling. Nickel alloys are also under investigation.
Prepared by Dr Diane Aston, IOM3

10. Materials for the turbine
Forced rotation of turbine drives rest of engine.
Operate under extremes of temperature and pressure.
Made from nickel-based superalloy.
Blades attach to disc with fir tree root.
Blades have in-built cooling system to prevent them overheating and melting during operation.
Over the years there has been a huge effort to try and get engines working more efficiently and the best way to do this is to get them operating at higher temperatures. Unfortunately, as the combustion and exhaust gas temperature increases the number materials that the very hot parts of the engine can be made out of decreases.
At present the turbine entry temperature is in the region of 1600ºC. At this temperature most common engineering materials have melted!
Turbine blades are made from a nickel-based super alloy and are designed to operate under extreme conditions of temperature and stress.
The disc in a high pressure turbine will typically rotate at 10,000rpm and this rotation puts a huge force on each of the individual 80-ish blades on the disc. If the force were to be described in every day terms it would be roughly the same as hanging a fully loaded articulated lorry from the tip of every blade. All that on a piece of metal about 10cm long, 3cm wide and a 0.2-2cm thick!
One of the biggest considerations in a material operating at high temperature with a load on it is creep; a slow gradual extension and failure of the material. In metals creep is caused by the presence of defects in the structure, particularly at crystal boundaries.
Over the years the microstructure of turbine blades has evolved to make them more creep resistant. Turbine blades are now grown as single crystals, which eliminates crystal boundaries in the microstructure.
This might overcome creep but it does not address the melting point of the alloy which is some 200ºC or so below the temperature of the exhaust gas. In order to prevent the blades from melting they have a complex cooling system built into them. There are a series of hollow channels running along the length of the blade which lead to tiny holes on the surface. Cool air from the back of the compressor at 600ºC (cool is very much a relative term here!) is blown through the hollow channels and out through the tiny holes, providing a protective layer of cool air that stops the hot exhaust gas from getting through to metal. Blades are coated in a thermal barrier layer of ceramic too to improve their high temperature behaviour.
Prepared by Dr Diane Aston, IOM3

11. Turbine blade technology
Materials
Manufacturing
Superalloys retain their properties up to over half of their melting point
Nickel-based superalloys are mainly Ni with small amounts of Ti, Cr, Mo, Mn, Al, Fe and B added; third and forth generation alloys also contain Ru and Re too.
Each element is added to control the microstructure and properties.
First a core is made from ceramic in the shape of the internal channels.
A wax pattern in the shape of the blade is built around the core
A shell mould is made around the wax patterns.
The blade is cast and cooled under critical conditions
The core is dissolved out
Final machining of root and cooling holes.
Materials
The composition of nickel alloys for turbine applications has evolved over the years to give the optimum microstructure for making the strongest, lightest blades. Some of the alloying additions are soluble, thus providing strengthening by introducing strain into the crystal lattice. Others are insoluble and form tiny particles which provide precipitation strengthening. Each of the alloying additions contributes towards the amazing properties of the alloy.
Manufacturing
Single crystal turbine blades for jet engines are actually made using a process that was invented thousands of years ago: the lost wax process for investment casting. This technique allows blades to be produced precisely and accurately with very fine control over microstructural evolution.
The blades are essentially made from the inside out so the starting point is the manufacture of ceramic cores. These are made from silica and are the size and shape of the hollow channels inside the blades (allowing for shrinkage). A wax pattern is built up around the core to be the shape of the blade. A number of waxes are put together and a filling system added so that multiple blades can be cast at the same time.
A ceramic shell mould is built up around the wax pattern and once this has hardened the wax is flash melted out using steam. It has to be melted out quickly as the coefficient of thermal expansion of the wax is greater than that of the mould and if it were done slowly the ceramic would crack. This leaves the cores suspended in the mould cavities on tiny platinum pins.
The finished mould is put into the furnace and heated before hot metal is poured into the running system allowing the blades to fill in a controlled way from the bottom to avoid splashing. Once the mould is full it is removed through a door in the bottom of the furnace. As it cools slowly from the bottom upwards the crystals start to grow vertically. When these columnar grains hit a special spiral in the bottom of the mould called a ‘pig tail’ only one grain can continue to fill the remainder of the mould, thus giving the single crystal structure.
Once the blades have solidified and cooled they are removed from the mould and the ceramic core dissolved out. The small cooling holes across the surface are cut using electrical discharge machining.
Prepared by Dr Diane Aston, IOM3

12. Engine testing
The engines have to be thoroughly tested to ensure that they can cope with all possible operating conditions. There are two key tests:
Blade-off test
Bird strike test
These are two short video clips that show how engines costing millions of pounds are tested to destruction to show that they will be safe in service.
The front part of the engine casing is lined with a layer of Kevlar about 5cm thick to catch any debris associated with a blade-off event and stop it exiting the engine where it could cause potentially catastrophic damage to the aircraft.
Kevlar is a type of polymer called an ‘aramid’. It was developed by DuPont in the mid-1960s as an alternative to steel wire for reinforcing racing car tyres. It is now used in many applications including lining jet engines, fuel tanks and pressure vessels and for making abrasion resistant clothing.
Prepared by Dr Diane Aston, IOM3

13. Session 1 Summary
Jet engines are incredible pieces of technology operating under extremes of temperature and pressure.
Advances in materials and manufacturing technology have allowed engineers to create quieter and more efficient engines.
Prepared by Dr Diane Aston, IOM3

14. Activity time! Calculating fuel efficiency
Journey through a jet engine?
Prepared by Dr Diane Aston, IOM3

15. Prepared by Dr Diane Aston, IOM3

16. Session 1 activities Calculating fuel efficiency
Prepared by Dr Diane Aston, IOM3

17. Calculating fuel efficiency
This is a quick and easy calculation to put into perspective how much fuel a jet engine uses.
Calculate the fuel efficiency of two different aircraft and compare this to the fuel efficiency of two different cars.
Compare the relative fuel efficiency per person for each vehicle.
Discuss which mode of transport is most fuel efficient.
Useful figures:
Airbus A380
Maximum fuel capacity: 320,000 litres
Maximum range: 15,700 kilometres
Number of engines: 4 RR Trent 900
Typical capacity: 525 passengers
Airbus A
Maximum fuel capacity: 156,000 litres
Maximum range: 15,600 kilometres
Number of engines: 2 RR Trent XWB
Typical capacity: 350 passengers
As of 07 October 2013 aircraft kerosene cost $2.90/gallon. This equates to about 50p/l.
As a comparison:
Citroen Saxo
Maximum fuel capacity: 45 litres
Maximum range: 500 miles
Number of engines: 1 x 1.1l, 4 cylinder petrol engine
Typical capacity: 5 passengers (but only if they are little in the back!)
Rolls Royce Phantom
Maximum fuel capacity: 82.3litres
Maximum range: 700 miles
Number of engines: 1 x 6.7l, V12 petrol engine
Typical capacity: 5 passengers
As of 21 October 2013 unleaded petrol cost £1.287/l (at Asda)
Prepared by Dr Diane Aston, IOM3

18. Journey through a jet engine?
The Rolls Royce website has an excellent resources that encourages the user to go through the stages in a jet engine and then test their understanding.
Can you get all 16 questions right?
Prepared by Dr Diane Aston, IOM3

19. Session 1 useful links
The Wikipedia article on turbofans explains how they work in a relatively easy to understand yet detailed way.
The Rolls Royce website has some excellent written information, diagrams and video clips on all aspects of jet engine design. It is worth having a look at the materials and gas turbine sections on the technology page.
The same site also has downloadable PDF factsheets on all of the large Rolls- Royce engines.
Rolls Royce produced a book called The Jet Engine. It is a fabulous read with some beautiful photographs. It is quite expensive at £35 and is available from here
The BBC did a documentary a couple of years ago called How to build a jumbo jet engine. Someone has uploaded it to YouTube and it has great sections on each area of the engine.
Prepared by Dr Diane Aston, IOM3