An ancient form of metalworking is being used by Rolls-Royce to create a single-crystal turbine blade for jet engines. Stuart Nathan reports.
Casting is one of the oldest and most basic methods of metalworking. If you can make a fire hot enough to melt a metal, and manufacture a crucible to melt it in and a mould that can withstand the heat, you can cast complex metal forms; and we’ve been doing it for millennia. The oldest-known casting is a copper frog made 6,000 years ago in Mesopotamia. Many of the gleaming marble sculptures of Ancient Greece are in fact more recent Roman copies of originals that had been cast in bronze: the few surviving originals, such as the Riace Bronzes of Greek warriors found in the sea off Sicily, show the incredible sophistication and level of detail achieved by these long-dead masters of metals.
Yet this most ancient of skills is still in use today, and indeed is still being developed. Its most recent incarnation is arguably the most advanced procedure that has ever been undertaken in metals, and is vital for one of the emblematic activities of the modern world: routine air travel. It is to be found in the UK’s historic centre of metalworking, Sheffield, at the Rolls-Royce Advanced Blade Casting Facility (ABCF), a facility purpose built near Sheffield University’s Advanced Manufacturing Research Centre in Rotherham.
The components the ABCF is producing are not ones that most people ever see: they are the turbine blades that are hidden away in the hottest part of jet engines. Far from the decorative brilliance of Greek bronzes, they combine a utilitarian appearance with complexity of form and function and a jewel-like internal perfection: weighing only about 300g and small enough to fit in the palm of a hand, they are in fact perfect single crystals of a metal alloy whose composition has been fine-tuned over many years to operate in the hellish conditions of the fastest-moving part of a jet engine.
“Back at the birth of the jet engine, Sir Frank Whittle’s prototypes were made entirely of steel,” said Rolls-Royce chief of materials Neil Glover. “Steel is great for strength and surface hardness, but if you need high-temperature performance it isn’t actually very good; 450–500°C is about its limit.”
Its unsuitability led to a search for a more temperature-resistant material, and jet makers turned to nickel alloys. Relatively abundant, with large deposits in Australia, and low in price, nickel melts at 1,728K (1,455°C) and is resistant to corrosion – both valuable properties for components that function inside a jet engine. Even more important is its ability to form alloys, and the particular property of one of those alloys, a compound known as gamma-prime in which nickel combines with aluminium, to retain its strength at high temperatures. “In steel or even titanium, the strength rapidly drops off as you reach 40–50 per cent of the melting point,” Glover said. “Nickel alloys retain their strength up to 85 per cent of the melting point.
And engine manufacturers make full use of this property. Jet engines work by positioning turbine blades, which spin in the current of hot gases expanding out of the combustion chamber, on the same shaft as the compressor blades that force air into the engine at high pressure. So at the back of the engine, the low-pressure turbine blades, which operate in a gas stream that has cooled down somewhat, are on the same shaft as the large fan blades at the front of the engine, which accelerate air to generate the engine’s thrust. This shaft runs through the middle of the shorter, wider intermediate pressure (IP) shaft, which again has turbine blades at the back and compressor blades at the front. Outside this is the high-pressure shaft, which runs the compressor that forces air into the combustion chamber itself. The combustion chamber is annular, with an exit ring at the back controlling the flow of exhaust gases, and it’s here where the single-crystal blades are found. The gases, fresh from combustion, are at around 1,700°C; and the shaft spins at speeds in excess of 12,000rpm.
This means the blades operate in an environment several hundreds of degrees hotter than the melting point of the nickel alloy. To stop them melting, the metal must be cooled. This is done via two mechanisms: the blades are coated with a low-conductivity ceramic; and they are riddled with a complex, branching structure of internal channels. “Air is drawn from the HP compressor, routed through the core of the engine and into the root of the blades,” explained Glover.
“It passes through the cooling channels and exits through a myriad of holes in the surface of the blade, to create an envelope of cool air around the blade. So the metal is never above its melting point, even though the environment is. The cooling air isn’t actually that cool; it’s at about 600–650°C, but we have to take it from the hot core of the engine so it has enough pressure to get through the channels and out of the holes. It’s still enough to keep the blade temperature down to about 1,150°C.”
Heat is vital to jets; the hotter they can operate, the more energy they can extract from their fuel. This is the major point of competition between engine makers, so over the six decades jets have been in operation, forcing the temperature higher, and developing turbine blades that can withstand the heat, has been one of the most important technology races in the sector. It’s been a gradual process, Glover said, culminating in the development of single-crystal blades in the late 1980s.
The blades operate in an environment several hundreds of degrees hotter than the melting point of the nickel alloy, but because of the cooling mechanisms, the metal is never above its melting point, even though the environment is.
Neil Glover, chief of materials, Rolls-Royce
The single-crystal structure isn’t intended to cope with temperature, however; it’s to make the blades resistant to the huge mechanical loads that result from their rotational speed. “Every single blade extracts power from the gas stream equivalent to a Formula One car engine,” Glover said. “And the centrifugal force on them is equivalent to the weight of a double-decker bus.”
Normally, metals are composed of many crystals – ordered structures of atoms arranged in a regular lattice, which form naturally as the metal cools from a molten state. These crystals are typically of the order of tens of microns in size, positioned in many orientations. At high temperatures and under strain, the crystals can slide against each other, and impurities can diffuse along the boundaries between the grains. This is known as creep, and it badly affected early turbine blades, which were forged from steel and later nickel bars.
The first stage in development was to get rid of any grain boundaries at right angles to the centrifugal loading, which led to the development of blades that were cast so the metal crystals all ran from top to bottom. Later, this was optimised further by casting single crystals, with no grain boundaries at all. It’s a highly complex process: not only must the blades be cast with the internal cooling channels already in place, but the crystals are not homogeneous. Rather, zones of different composition and crystallographic structure exist within the blade.
“You can think of nickel superalloys like these as being like composites,” said Rolls-Royce aerofoil turbine materials technologist Neil D’Souza. “It’s a mixture of two phases, one of which – gamma-prime – gives rise to the sustained increase in strength at high temperature.”
When it crystallises, nickel forms a structure known as face-centred cubic (fcc); each cube has a face with five atoms, one at each corner and one in the middle. When alloys are made, generally the atoms just swap in and out of the fcc lattice. But under the right conditions, aluminium and nickel combine in such a way that nickel goes to the centre of the faces and aluminium to the corners. This is known as a precipitate; it forms islands of greater order within the bulk of the alloy, about half a micron in dimension, packed closely together in a rectilinear formation. Because the size of the lattices of the precipitate and the less ordered bulk alloy are almost identical, they are all part of the same crystal.
“You could imagine building a ball and stick lattice model,” said Glover. “In the bulk alloy, you’d place the balls representing the components of the alloy, about 10 different elements including nickel, aluminium, chromium, tantalum and titanium, pretty randomly, and when you got to the gamma-prime precipitate you’d put in this ordered arrangement of aluminium at the corners and nickel in the middle. It’s all on the same regular lattice, oriented the same way, so it’s all the same crystal, but you have these much stronger regions where there’s the array of gamma-prime precipitate.”
But this doesn’t just happen naturally. To make the blades, the first stage is a ceramic ‘core’, of the form of the tortuous internal cooling channels. Wax is injected around this to form the shape of the aerodynamic blade, plus several other features that assist in the casting process. Platinum pins are inserted to support the core inside the wax; then the form is ‘shelled’ by coating it in an slurry of alumina-silicate material to form a ceramic coat. Several more coats of different compositions are applied and then the wax is melted out to leave a void in the shape of the blade. This is investment or ‘lost-wax’ casting, the same technique those Ancient Greek sculptors used to make the Riace Bronzes.
Our people are fantastically skilled, but they’re human, and no human is going to produce the same quality of work at the end of a shift as they do at the beginning.
Steve Pykett, Manufacuring manager, Rolls-Royce Advanced Blade Casting Facility
Molten metal is then poured into the mould, which is placed inside a furnace to keep the metal molten. At the base of the mould is one of the additional casting features: a helical structure about the same shape as three turns of a standard corkscrew. Known as the pigtail, this is attached to a plate that is cooled by water. Once filled, the mould is slowly withdrawn from the furnace into a cooler chamber. The metal starts to solidify at the chilled plate, and crystals begin to grow into the pigtail. The crystals grow in a straight line in the direction that the mould is being withdrawn, but because of the pigtail’s twisted shape, all but the fastest-growing crystals are eliminated. Only a crystal with the correct orientation emerges into the blade mould proper, and the gradual withdrawal of the mould ensures the crystal continues growing through the melt into the rest of the space.
The formation of the vital precipitates results from careful control of the external temperature and from the design of the mould; those multiple layers of ceramic determine how fast the heat from the molten metal can dissipate, and this provides the extra finesse to achieve the required internal structure. The platinum pins holding the core in place diffuse into the alloy without affecting its properties.
Once solidified, the casting is removed from the mould and the first of some 20 processes begins to prepare it for assembly into an engine. First, the ceramic cores are dissolved away with caustic alkalis. Then the extra features for casting are machined away. The holes for the cooling air to escape are drilled using electrical discharge machining, which forms the required hole geometry to direct the air to the points where it is needed. Finally, the blade receives its insulating ceramic coating by electron-beam plasma deposition.
The ABCF in Rotherham concentrates on components for large civil airliner engines because, with the advent of aircraft such as the Airbus A350 XWB, for which Rolls-Royce has developed the Trent XWB engine, this is where the company sees its main growth coming from.
Costing some £110m, the ABCF was built to automate as much of the production process as possible. “Single-crystal casting is expensive, and many parts of the process have traditionally been very hands-on,” said ABCF manufacturing manager Steve Pykett. “Our people are fantastically skilled, but they’re human, and no human is going to produce the same quality of work at the end of a shift as they do at the beginning.”
The production of the wax assembly is a good example of this philosophy. “You’ll always find a wax room at an investment casting foundry,” Pykett said. “It requires hand-eye co-ordination and dexterity to make the wax form, but that doesn’t deliver consistency.”
Working with the Manufacturing Technology Centre near Coventry, Rolls-Royce developed an automated system to hold the ceramic core, inject wax, pin the core in place and conduct the assembly process. “It used to take a whole shift to make an assembly; now it takes an hour,” Pykett said. “But time was not the main driver here. We now know that we have consistent product coming out of the wax process, whatever the time of day, and that gives us a solid platform from which we can reduce cost.”
We’ve transferred some of the skills in making these components from the manufacturing engineers on the line to the process developers. And that doesn’t mean we’ve deskilled
Mark Hulands, casting manufacturing executive
Some other processes have also been automated, including the dressing operation to remove the sacrificial features of the casting. The blades then go into inspection, where Rolls-Royce has replaced five processes with two. The castings are then shipped to another plant at Crosspointe, Virginia, for further machining of the features that will allow them to be attached to their discs in the engine, and for drilling of the cooling holes; they come back to a plant in Annesley, Nottinghamshire, for coating.
“This process is so complex, with precise control of temperatures and materials handling to manage, virtually atom by atom, how the blades are formed,” said casting manufacturing executive Mark Hulands. “What we’ve done is to transfer some of the skills in making these components from the manufacturing engineers on the line to the process developers,” Hulands said. “And that doesn’t mean we’ve de-skilled. Our engineers still need to be highly skilled to keep the processes running smoothly, but they’re different skills and we’ve improved the consistency so we can drive costs down.”