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Engineering materials

The materials engineering of a gas turbine engine blade

The gas turbine engine is widely used in vehicular scenarios, such as submarines, aircraft, and trains. This report will analyse a particular component of the gas turbine engine, the gas turbine engine blade. The gas turbine engine blade will be referred to as "GTB" from now on.

The GTB must have very specific properties to fulfil its role in the extreme conditions of a gas turbine engine. One of the biggest problems with designing effective components for a gas turbine engine, is that the components must be able to cope with very high temperatures (turbine blades operate at temperatures ranging from 600oc to above 1000oc)(1). Because the blades are rotating constantly at a high temperature, creep-rupture is also a big issue. So the material must have a good creep resistance at high temperatures, so that the gas turbine blade will not fail unexpectedly and damage other parts of the engine. The third main reason for GTB failure is corrosion (2). The turbine blades are exposed to strongly oxidising conditions and the gaseous combustion products contain elements such as sulphur (2). These gases readily attack the GTB as it is made from nickel alloy (2). This needs to be considered when designing and producing GTB, by either adding a protective layer or sacrificial metal.

GTB are usually made from nickel base superalloys because these superalloys offer outstanding strength at elevated temperatures (3). Turbine blades are normally protected with sophisticated coatings, usually based on chromium and aluminium, but often containing exotic elements such as yttrium and platinum group metals to provide resistance to corrosion and oxidation of the GTB(2).

GTB are produced using a forging process (1). There are many different problems encountered when forging GTB as they are made from Nickel base alloys. According to G.W. Meetham, "three major problems include limited hot ductility in a narrow forging temperature range, the need to avoid differential metal flow, which can lead to shear cracking, and the need to apply a specific amount of work to all parts of the forging during the last heat, to produce a uniform grain structure."(1) These have been overcome by carefully designed forging routes which involve many steps and utmost precision.

There are numerous ways to test the properties of the alloys and coatings which are used to manufacture the GTB. To test metals for creep properties, an experiment is conducted where the chosen metal is moulded into a dumbbell-like shape, and put under temperature and pressure with a constant force applied to simulate service conditions.

For corrosion testing, the metal is put in the same atmospheric conditions as what it would be in service, but often the temperature would be elevated to increase the rate of corrosion to speed up the time taken to produce results.

The materials engineering of aluminium window frames

Aluminium window frames are often used in commercial buildings like shop fronts and schools. This report will concentrate on the manufacturing process and properties of aluminium window frames. From now on aluminium window frames will be referred to as AWF.

The problem with manufacturing good window frames is that they need to be thermally insulating, while exerting good impact strength and corrosion resistance. This is because being part of the exterior - interior boundary of a building, insulation is a high priority as energy saving is crucial in the modern environmental and economic climate. A window is difficult and expensive to replace, so high impact strength and durability is crucial to the design of the AWF. A good corrosion resistance is necessary because the window will be exposed to atmospheric conditions such as acid rain, general pollution and even salt in the air if near the sea. This means that corrosion of window frames can be a limiting factor of which materials can be used when designing and producing them.

Aluminium is produced from an ore known as bauxite (4). The ore is refined, and then the product is smelted, then the aluminium is electrolysed to make it pure, and then put through a casting process where it is made into a window profile (4).

There are a number of tests the aluminium has been put through to confirm that it is fit for the role as a window frame. To test for impact resistance, the Charpy impact test is used, where a weighted hammer is dropped and hits the aluminium sample which has been notched in the middle.(5) The carefully calibrated dial produces a number readout. With this number and weight of the hammer, the exact force needed to break or bend the sample can be calculated. For corrosion resistance, an accelerated corrosion test is used to simulate atmospheric corrosion at an accelerated rate to speed up the experiment. This is because atmospheric corrosion can take up to decades to take effect.

References

The Development of Gas Turbine Materials, Meetham, G.W, Applied Science Publishers LTD, London, 1981. (1)

Common Failures in Gas Turbine Engines, Tim J Carter, Johannesburg, 2004, http://www.sciencedirect.com/science?_ob=MiamiImageURL&_imagekey=B6V2X-4DXBBB4-1-F&_cdi=5714&_user=122878&_check=y&_orig=search&_coverDate=04%2F01%2F2005&view=c&wchp=dGLbVtb-zSkzV&md5=8ee774bbaec8925aafd41906ac8b9796&ie=/sdarticle.pdf (2)

Failure analysis of a second stage blade in a gas turbine engine, E. Poursaeidi, M. Aieneravaie and M.R. Mohammadi, Zanjan, Iran, 2007, http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V2X-4RBYD5J-1&_user=122878&_rdoc=1&_fmt=&_orig=search&_sort=d&_docanchor=&view=c&_acct=C000010119&_version=1&_urlVersion=0&_userid=122878&md5=6d878afffdf941cb3bc42c76e5076462 (3)

LIFE CYCLE OF WINDOW MATERIALS - A COMPARATIVE ASSESSMENT, M. Asif BSc MSc, A. Davidson BSc and T.Muneer PhD DSc CEng MlmechE

FICBSE Millennium Fellow, Napier university,

http://www.cibse.org/pdfs/Masif.pdf (4)

Materials science and engineering, an introduction, William D.Callister, Jr, John Wiley and Sons Inc publishers, 2007. (5)

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