Aviation Technology: Understanding Jet Engine Metallurgy
Created on: December 22, 2009
Last Updated: January 19, 2010
To
understand the capabilities of modern jet engines, it is not sufficient to
explain how jet engines work. The issue is rather how the most sophisticated
materials of the day allow operation in the extreme conditions of temperature,
mechanical loading, and corrosive resistance that is required for a global
economy dependent on affordable long-range jet transportation. The design of
jet engines is fundamentally limited by the state of metallurgical engineering
available at the time the engine was developed. The status of metallurgy is
what we will attempt to expose, at least to some extent.
From the
Heinkel HeS3, used in the world’s first jet plane in 1939 to the newest GE
90115B which provides the motive force for the Boeing 777, jet engines and
their materials have evolved to provide vastly improved performance and
reliability. These remarkable advances can primarily be attributed to advances
in high temperature metallurgy.
During
operation, the components in a jet engine are exposed to one of the harshest environments
encountered in any engineered product. High temperatures, corrosive gases,
vibrations and high mechanical loads that would humble any conventional
materials are everyday toil for jet engine materials. This extreme environment
can lead to failure of the most robust materials in much the same way that a
paperclip that is bent back and forth breaks because of metal fatigue.
What
metallurgical engineered materials make a jet engine a practical driver of
transportation, rather than a showcase for modern technology? Most of the high performance parts of a modern jet engine are made of super-alloys.
Discovered
in the 1950s, super-alloys quickly became the industry standard owing to their
excellent mechanical and thermal properties combined with outstanding corrosion
resistance. Nickel based super-alloys are precipitation-hardened, giving the
super-alloy excellent creep resistance[ii] (long operating times) and strength
which increases with temperature. Due to these unique material properties, Ni based
super-alloys are still the primary material choice for high-temperature jet
engine components.
Modern Ni based
super-alloys contain many additives such as tungsten, tantalum, molybdenum, or
rhenium to increase their maximum operating temperature. A typical nickel-base
super-alloy (Inconel-100) has a density of 7.8 g/cc and is composed of (by
weight) 62% nickel, 15% chromium, 10% cobalt, 5% each of titanium and aluminum,
3% molybdenum, and trace amounts of carbon, boron, zirconium, and vanadium.
Inconel-100 melts at 1135 °C, and can be used as temperatures as high as 1000
°C. In contrast, conventional carbon steels lose much of their strength by 500
°C, and have (by comparison) very little corrosion resistance.
Unlike most
of the engine components, the fan, which pulls large quantities of air into the
engine, does not experience high temperatures. It does, however, face the
special challenge of surviving encounters with rain, snow, hail, and the
occasional bird strike. Thus, when selecting materials for the fan impact
toughness as important as strength. By comparison, low carbon steels are hardly
corrosion resistant, and lose most of their strength once an operating
temperature of[iii].
Fans in most
modern turbojet engines are made of titanium alloy fan blades mounted to a
super-alloy fan disk. Originally solid fan blades were used, but a recent
approach to reducing fan weight is being championed by Rolls Royce, who are
making fan blades of hollow titanium, often filled with a titanium honeycomb,
which greatly reduces its parasitic weight. The two halves of a hollow fan
blade are cast of titanium alloy (often Ti-4Al-6V), welded together, then
machined to the desired dimensions and polished. The fan disk, which includes
mounting notches for the blades, is usually machined from a single titanium
alloy forging.
A new
approach to fan design appears in the GE 90 series of commercial turbofan
engines. Here the fan is made of advanced composites, with titanium added to
the leading and trailing edges of the blades to even out internal stresses.
This fan is 10 ½ feet in diameter, and has a maximum rated rotational speed of
2550 rpm, so that the blade tips are supersonic. Centripetal force pulls the fan
blades apart with over 10000 g’s of acceleration[iv].
When a fan fails under this enormous stress, it falls into high-velocity
pieces, so measures must be taken to contain shrapnel. With composite
materials, however, the main failure mechanisms result in production of a
tangled mess of fibers and composite, which is much easier to safely confine,
leading to weight savings of as much as 700 pounds for the largest engines.
The
compressor accepts the air from the fan (over a ton per second[v] in the GE 90115B at
take-off), and compresses the air to a pressure 40 or more times the inlet
pressure, which also increases the temperature to as high as 700 °C. This
compressed air is then injected into the combustion chamber, where it is mixed
with jet fuel and ignited. The turbine then spins in the exhaust of the
combustion chamber, generating the power needed to drive the fan and the
compressor. Modern turbines operate at inlet temperatures as high as 1600 °C,
and can produce in excess of 100,000 horsepower. The combination of these
conditions requires remarkably strong components. Compressors and turbines
share many design and manufacturing techniques, so these will be described
together.
The most
common manufacturing technique for compressors or turbines is to form a
compressor or turbine disk using super-alloys and hot isostatic[vi]
pressing. Such a disk is rather like a large notched wheel to which the blades
are attached. In hot isostatic pressing, a fine grained metal powder is
injected into a flexible mold which is evacuated of air. The mold is then
compacted at high temperature by surrounding it with a high pressure gas.
Process conditions are about 25000 psi and 1200 °C. Diffusion of atoms between
the particles results in a fine grain polycrystalline compact. This compact is
nearly fully dense, and is near net shape, so that only minor finishing
operations need be carried out. The compressor or turbine blades are then cast,
and the two assembled into the final compressor or turbine assemblies. In some
cases one piece assemblies called blisks[vii]
are made, often using friction welding to affix the blades to the disk.
Blades for
compressors having outlet temperatures below about 350 °C can be made of high
temperature titanium alloys such as Ti6Al4V. Compressor blades for higher
temperature operation are generally made of nickel base super-alloy. As it is
extremely difficult to cool the compressor blades, operating temperatures are
currently limited to about 700 °C, although if necessary some of the methods
used to make turbine blades could expand this envelope to some extent.
Perhaps the
most interesting area of jet engine metallurgy is the manufacture of the
turbine blades. Solid turbine blades cast from a suitable super-alloy can be
used for turbine inlet temperatures up to about 900 °C. Adopting hollow blades
with cooling channels allows excess air from the compressor
to be
circulated within the blade and through small holes onto the blade surfaces.
Further improvement can be obtained by coating the blades with a thermal
barrier. The thermal barrier coating is composed of thermally insulating
materials, often ceramics and refractory metals, which can withstand high
temperature and corrosive atmosphere, and improve the effectiveness of the
blade cooling system. The net improvement in operating temperatures is
substantial, but still not enough to allow operation at modern turbine inlet
temperatures.
The high temperature
mechanical properties of the super-alloy itself had to be improved. One
approach is directional solidification, developed in the 1960s. Directional
solidification produces long columnar grains along the loading direction of the
turbine blades – that is, aligned from the base of the blade to the tip. The
relative lack of grain boundaries transverse to this loading direction results
both in increased strength and retention of that strength to higher
temperatures.
Directionally
solidified components are made in an induction heated mold, with an initial
temperature of about 1500 °C. This mold contains a water-cooled copper plate at
the bottom of the mold. The super-alloy is poured into the mold, where it
begins to solidify near the copper plate. The copper plate has a knurled
surface, so that a large number of evenly spaced crystallites appear in the
initial freezing process. The mold is then slowly cooled from the bottom up, so
that the super-alloy crystals grow in alignment with the thermal gradient,
along the axis of the blade. The speed of cooling is critical to obtaining the
desired microstructure, requiring several hours to produce an average turbine
blade.
Although
directionally solidified super-alloys are a considerable improvement over
super-alloys produced using traditional casting or hot isostatic casting, they
still have defects which limit their performance in gas turbines. Single
crystal turbine blades, in which such defects are reduced to the maximum practical
extent, were developed in the 1970s, largely through the efforts of the
research labs of Pratt & Whitney.
The
fabrication of a single crystal turbine blade is rather similar to the
directional solidification process, where the crystal grows in a carefully
controlled one dimensional thermal gradient. However, in this case the growth
is started in a lower chamber called the starter. The growth process in the
starter is the same as in directional solidification, where columnar crystals
form at a chill plate. The starter chamber narrows at the top to form a helical
tube known as the pigtail. As the super alloy grows, only a few crystallites
can enter the pigtail. Because the pigtail curves, most crystallites will grow
into the pigtail wall and then stop, as there is no space left into which they
can grow. In the end, only one crystallite emerges from the pigtail. The blade
grows from this lone crystallite, forcing the turbine blade into a single
crystal structure. This is essentially the same method used to grow very large
silicon crystals for use in microelectronics.
Because
single crystals have no grain boundaries, grain boundary strengthening elements
are not needed. As a result, the super-alloys used in single crystal
fabrication have a simpler composition than traditional super-alloys.
For example,
CMSX-6 is 70.4% nickel, 10% chromium, 5% cobalt, 4.8% aluminum, 4.7% titanium,
3% molybdenum, 2% tantalum, and 0.1% hafnium. Designing such a super-alloy is
an art form, combining basic concepts of metallurgy with a great deal of
experimentation aimed at optimizing specific metallurgical properties. Recently
developed super-alloys for single crystal blades also have several percent of
rhenium and ruthenium to further increase operating temperatures.
The combustor
has the difficult task of burning large quantities of fuel, supplied through
fuel spray nozzles, with enormous volumes of air supplied by the compressor,
and releasing the resulting heat in such a manner that the air expands to
deliver a smooth stream of uniformly heated gas into the turbine. This must be
accomplished without melting anything important, and with the minimum loss in
pressure and with the maximum heat release possible within the limited space
available.
The
combustor for a high performance jet engine is made of super-alloys, which
often receive special ceramic coatings, such as alumina and zirconia. These
coatings serve to insulate the inner surface of the combustor from the intense
heat of the burning fuel-air mixture, and also provide an important degree of
erosion[viii]
protection.
Jet engine
metallurgy has shown rapid progress over the past 70 years, and the rate of
improvement continues to this day. The sum of small continuous improvements is
an important part of this evolution, but it has also depended on dramatic
moments of insight and discovery. The advent of new classes of high-temperature
structural materials, such as metal matrix composites and boron fiber
reinforced metals and ceramics, should form the basis for a fascinating story
into the future.
[1] The following calculations,
all endnotes and typographical changes were added by Lester C. Payne. Heavy use was made of http://en.wikipedia.org/ .
[ii] In materials science, creep is the tendency of a solid material to
move slowly or deform permanently under the influence of stresses. It occurs
as a result of long term exposure to high levels of stress that are below the yield strength of the material. Creep is more severe in materials that are subjected to heat for long
periods, and near melting point. Creep always increases with temperature.
The rate of this deformation is a function of
the material properties, exposure time, exposure temperature and the applied structural load. Depending on the magnitude of the applied stress and its duration, the
deformation may become so large that a component can no longer perform its
function — for example creep of a turbine blade will cause the blade to contact
the casing, resulting in the failure of the blade. Creep is usually of concern to engineers and metallurgists when evaluating components that operate under high stresses or high
temperatures. Creep is a deformation mechanism that may or may not constitute a
failure mode. Moderate creep in concrete is sometimes welcomed because it relieves
tensile stresses that might otherwise lead to cracking. Unlike brittle
fracture, creep deformation does not occur suddenly upon the application of
stress. Instead, strain accumulates as a result of long-term stress. Creep is a
"time-dependent" deformation. The temperature range in which creep
deformation may occur differs in various materials. For example, tungsten requires a
temperature in the thousands of degrees before creep deformation can occur
while ice will creep near 0 °C (32 °F).[1] As a rule of
thumb, the effects of creep deformation generally become noticeable at
approximately 30% of the melting point (as measured on a thermodynamic
temperature scale such as kelvin or rankine) for metals and 40–50%
of melting point for ceramics. Virtually any material will creep upon approaching its
melting temperature. Since the minimum temperature is relative to melting
point, creep can be seen at relatively low temperatures for some materials.
Plastics and low-melting-temperature metals, including many solders, creep at room temperature as can be seen markedly in old lead hot-water pipes. Glacier flow is an example of creep processes in ice.
Plastics and low-melting-temperature metals, including many solders, creep at room temperature as can be seen markedly in old lead hot-water pipes. Glacier flow is an example of creep processes in ice.
[v] Using
one ton (907 kg) of air (density of about 1.225 kg/m3) per second
corresponds to gulping approximately 740 cubic metres per second of air at the
intake of
a GE90-115B.
[vi] Hot isostatic
pressing (HIP) is a manufacturing process used
to reduce the porosity of metals and increase the density of many ceramic materials. This improves the material's mechanical properties and
workability.
The HIP
process subjects a component to both elevated temperature and isostatic gas pressure
in a high pressure containment vessel. The pressurizing gas most widely used is
argon. An inert gas is used,
so that the material does not chemically react. The chamber is heated, causing
the pressure inside the
vessel to increase. Many systems use associated gas pumping to achieve the
necessary pressure level. Pressure is applied to the material from all
directions (hence the term "isostatic").
For
processing castings, metal powders can also be turned to compact solids by
this method, the inert gas is applied between 7,350 psi (50.7 MPa) and 45,000
psi (310 MPa), with 15,000 psi (100 MPa) being most common. Process soak
temperatures range from 900 °F (482 °C) for aluminum castings to
2,400 °F (1,320 °C) for nickel-based super-alloys. When castings are treated with HIP, the simultaneous
application of heat and pressure eliminates internal voids and microporosity through a
combination of plastic deformation, creep, and diffusion bonding; this
process improves fatigue resistance of component. Primary applications are the
reduction of microshrinkage, the consolidation of powder metals, ceramic composites
and metal cladding. Hot isostatic pressing is also used as part of a sintering (powder
metallurgy) process and for fabrication of metal matrix composites.
[vii] Refer
to http://en.wikipedia.org/wiki/Wikipedia.
“A blisk (or bladed disk) is a
single engine component consisting of a rotor disk and blades, which may be
either integrally cast, machined from a solid piece of material, or made by
welding individual blades to the rotor disk. The term is used mainly in aerospace engine
design. The word is a portmanteau of blade and disk, the two components it
replaces in turbo machinery. Blisks may also be known as integrally bladed
rotors (IBR).”
[viii]Should this be corrosion?
The calculation of centripetal force and tangential velocity for a 101/2
foot diameter fan. Specifically designed for the Boeing 777, the GE90-115B
holds the world record for thrust totalling 127,900 lbs.
| Formula:
a=rω2 (m/s2) |
a (m/s2)
|
# of gees
|
r (feet)
|
ω (rpm)
|
114107
|
11644
|
5.25
|
2550
|
|
| Formula:
v=rω (m/s) |
(m/s)
|
Mach #
|
||
372
|
1.1
|
[1] In materials science, creep is the tendency of a solid material to move slowly or deform permanently under the influence of stresses. It occurs as a result of long term exposure to high levels of stress that are below the yield strength of the material. Creep is more severe in materials that are subjected to heat for long periods, and near melting point. Creep always increases with temperature.
The rate of this deformation is a function of the material properties, exposure time, exposure temperature and the applied structural load. Depending on the magnitude of the applied stress and its duration, the deformation may become so large that a component can no longer perform its function — for example creep of a turbine blade will cause the blade to contact the casing, resulting in the failure of the blade. Creep is usually of concern to engineers and metallurgists when evaluating components that operate under high stresses or high temperatures. Creep is a deformation mechanism that may or may not constitute a failure mode. Moderate creep in concrete is sometimes welcomed because it relieves tensile stresses that might otherwise lead to cracking. Unlike brittle fracture, creep deformation does not occur suddenly upon the application of stress. Instead, strain accumulates as a result of long-term stress. Creep is a "time-dependent" deformation. The temperature range in which creep deformation may occur differs in various materials. For example, tungsten requires a temperature in the thousands of degrees before creep deformation can occur while ice will creep near 0 °C (32 °F).[1] As a rule of thumb, the effects of creep deformation generally become noticeable at approximately 30% of the melting point (as measured on a thermodynamic temperature scale such as kelvin or rankine) for metals and 40–50% of melting point for ceramics. Virtually any material will creep upon approaching its melting temperature. Since the minimum temperature is relative to melting point, creep can be seen at relatively low temperatures for some materials.
Plastics and low-melting-temperature metals, including many solders, creep at room temperature as can be seen markedly in old lead hot-water pipes. Glacier flow is an example of creep processes in ice.
[1] This value is missing in the original document.
[1] See
calculation. This justifies Dodson’s
value.
[1] Using
one ton (907 kg) of air (density of about 1.225 kg/m3) per second
corresponds to gulping approximately 740 cubic metres per second of air at the
intake of
a GE90-115B.
[1] Hot isostatic pressing (HIP) is a manufacturing process used to reduce the porosity of metals and increase the density of many ceramic materials. This improves the material's mechanical properties and workability.
The HIP
process subjects a component to both elevated temperature and isostatic gas pressure
in a high pressure containment vessel. The pressurizing gas most widely used is
argon. An inert gas is used,
so that the material does not chemically react. The chamber is heated, causing
the pressure inside the
vessel to increase. Many systems use associated gas pumping to achieve the
necessary pressure level. Pressure is applied to the material from all
directions (hence the term "isostatic").
For
processing castings, metal powders can also be turned to compact solids by
this method, the inert gas is applied between 7,350 psi (50.7 MPa) and 45,000
psi (310 MPa), with 15,000 psi (100 MPa) being most common. Process soak
temperatures range from 900 °F (482 °C) for aluminum castings to
2,400 °F (1,320 °C) for nickel-based super-alloys. When castings are treated with HIP, the simultaneous
application of heat and pressure eliminates internal voids and microporosity through a
combination of plastic deformation, creep, and diffusion bonding; this
process improves fatigue resistance of component. Primary applications are the
reduction of microshrinkage, the consolidation of powder metals, ceramic composites
and metal cladding. Hot isostatic pressing is also used as part of a sintering (powder
metallurgy) process and for fabrication of metal matrix composites.
[1] Refer
to http://en.wikipedia.org/wiki/Wikipedia.
“A blisk (or bladed disk) is a
single engine component consisting of a rotor disk and blades, which may be
either integrally cast, machined from a solid piece of material, or made by
welding individual blades to the rotor disk. The term is used mainly in aerospace engine
design. The word is a portmanteau of blade and disk, the two components it
replaces in turbo machinery. Blisks may also be known as integrally bladed
rotors (IBR).”
[1]
Should this be corrosion?
