rnace, 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 1,000°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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