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