Aero Engine Components Manufactured

The processes used are many and varied, some fornot quite touch the workpiece, but removes small
the broad forming of a metal part to shape, someparticles by electrochemical reaction. The rotation of
for finishing, others for joining parts together, andthe wheel sweeps away the by-products, which
yet others for changing the condition of an externalwould inhibit the reaction.
surface. Other, totally different, methods are used toAerospace castings and die forgings.
produce parts in composite materials.Among the basic forming methods, casting and
Aerospace pressings.forging are supreme. In heavy engineering, forging
One of the familiar traditional techniques ismeans heating a rough billet or slab of metal almost
presswork, used to form parts to shape from metalto melting point and then squeezing it to shape either
sheet. Especially with the high-strength metals used inin a giant press or by using blows from a
gas turbines, pressing may have to be done with thesteam-powered hammer. In gasturbine manufacture
material at high temperature, yet even then it is veryalmost all forging is dieforging, in which the workpiece
difficult to achieve the desired dimensional tolerance.- in some cases almost white-hot, in others at room
The metal tends to warp, spring back, twist, or intemperature - is squeezed in a press between upper
some other way deform so that lengthy (andand lower dies. Such parts as main drive shafts,
therefore expensive) hand working may be neededcompressor casings or half-casings, combustor rings,
to correct the shape, and even then there may berotor discs, blades, and gearwheels can be forged
locked-in stress. Better isothermal (constantvery close indeed to the finished shape and
temperature) presswork is now being achieved usingdimensions. Die forging is an economical way of
heated dies, of metal or ceramic, which repeatedlyproducing parts which, in the case of blades,
produce accurate stress-free shapes. Good resultsincorporate thin aerofoils with twist and camber that
are also being achieved by hot blow-forming, in whichwould be difficult to make by other methods, apart
the sheet-metal blank is forced into the die under thefrom ECM. In isothermal hot forging the dies are in a
pressure of argon fed under microprocessor controlfurnace, held at a constant temperature. Such
to maintain the correct strain-rate. Argon, one of theprecision forging demands exact control of forging
inert gases, does not react with metals even in theirtemperature and absolute cleanliness of the dies.
molten state, and so plays an important role in theForged parts are also known as wrought parts.
manufacture of aero engines.A particular type of forging, much used to make
Another related process is superplastic forming SPF.blading, is upset forging. Bar stock is fed into a
About 30 years ago it was discovered that somemachine which, at high speed, electrically brings the
metals with suitably fine grain structure can beworking end of the bar to forging temperature and
subjected to tremendous ductile (tensile) deformationthen, by hydraulically controlling the feed of the stock
without tearing. With careful control of temperatureand the withdrawal speed of an anvil forced against
and strain rate, SPF parts can be made in aluminium,the end, leaves the end of the bar with a particular
titanium alloy and particular superplastic steels byirregular profile. This profile distributes the metal
deep drawing. The presses have to be speciallycorrectly to make the root, tip, and any shrouds or
made, but the forming pressures are quite modest.snubbers in subsequent forging.
The finished part can have very small bend radii andA process somewhat akin to forging is extrusion. As
suffer such large changes in shape that the metalthe name indicates, this is forming a linear part of
literally flows. For example, a billet can be squeezedconstant cross-section by squeezing it like toothpaste
into a thin-walled part with integral stiffeners. SPF isthrough a die of the correct shape. It has been used
often combined with diffusion bonding to producefor many engine parts, including rings used to stiffen
complex components which in effect are a singlecasings, which of course require bending to circular
piece of metal, instead of being made by joiningshape and then joining the ends. Most metals are not
perhaps a dozen separate parts.difficult to extrude, but steels were a challenge until a
Sheet metal spinning.French firm discovered 40 years ago that molten
Another ancient craft is sheet metal spinning. Theglass could be used as the lubricant.
modern equivalent is flow-turning, in which aIn casting, the metal is melted and run as a liquid into
workpiece, initially usually a flat disc (a blank), isa mould. Thus, the problem of shaping a refractory
forced by computer-controlled rollers to bend to(heat-resistant) alloy is sidestepped. Very hot parts,
shape around a central rotating die called a mandrel.such as the flaps of an afterburner primary nozzle,
The result is almost any desired conical or evenused to be welded from sheet, but today are more
cylindrical shape, exactly to size with no joint.cheaply cast in one piece. Casting is also used for
Previously, such a part had to be made by wrappingmaking such parts as aluminium gearbox casings. In
and welding sheet, followed by drawing and sizing totraditional casting the mould is produced in sand by a
correct the shape.pattern which is a replica of the part to be made. In
CNC machining.die casting a permanent mould is used. A particular
Another familiar technique is aerospace cnc machining,form of casting much used to make circular parts is
in which hard tools cut away material from thecentrifugal casting. Here the die is usually
workpiece. There are various kinds of cnc machining.water-cooled metal, for faster solidification, and it is
In turning, the part is rotated on a lathe while beingrotated at high speed on a vertical axis to give a
cut by a tool which slowly moves into or along thefinished part of high density devoid of flaws.
work. In cnc milling, it is the work which is slowlyFor turbine blades the most important method is now
moved past a rotating cutter. Jig boring is a kind ofinvestment or 'lost-wax' casting. Used by the Chinese
high-precision vertical milling. Broaching involves pullingaround 2000 BC, it begins by making a multi-piece
or pushing a cutter past the workpiece to machine asteel die containing a highly polished internal cavity
linear slot such as a fir-tree root or a spline along ahaving the exact inverse shape of the finished part.
shaft; the broach is a linear cutter with many teeth,Molten wax is carefully injected to fill the die
each of which approaches a little closer to thecompletely, and allowed to set to produce a replica
finished profile. All machining is today likely to beof the finished part. Several - typically from 2 to 20 -
computer numerically controlled CNC. The machineof these identical patterns are then assembled on a
tool is controlled by a computer, into which is fed a'wax gating tree' in Christmas-tree style. This is then
tape appropriate to the particular part. This greatlydipped in slurry, a liquid ceramic, which quickly dries.
saves time, and makes possible the rapid machiningThe tree is dipped several more times until the
of complex shapes which previously might have hadceramic coat is about 6 mm (0.25 in) thick. The wax
to be forged, cast, or assembled by joining manyis then melted and run out, care being taken to
parts together. It also virtually eliminates human error,ensure that every ceramic shell mould is completely
so `scrap' has almost become a thing of the past.free from wax by firing it at over 1,000°C. The
Aerospace grinding.red-hot mould is then filled with the blade alloy, which
There are many other techniques which can behas been electric-induction melted and brought to an
employed to shape a part. In grinding, the cutting isexact temperature. After cooling, the ceramic shell is
performed by millions of exceedingly hard particlesremoved and the blades are cut away from the cast
projecting microscopically from the surface of agating tree, chemically cleaned, and carefully
wheel or drum. In electrolytic grinding, the wheel isinspected by numerous methods. Investment casting
electrically conductive and, with the workpiece, isis used for many engine hot section components, but
immersed in a bath of electrolyte (conductive liquid,it is especially important for turbine blades.
usually a solution of salts). The rotating wheel does