04-07-2012, 03:56 PM
Aluminum alloys used for spacecraft shuttle and its properties
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INTRODUCTION
Any brief history of spacecraft materials must mention timber as one of the first materials used to make a powered, manned flight. The Wright Flyer consisted mainly of Sitka spruce and bamboo glued and screwed together to form a canvas-covered frame. Wooden aircraft were h very successful in the early years of flight but by the end of World War I their days were numbered. Today, timber is only suitable for comparatively small aircraft. As aircraft became larger, materials with better specific strength (strength to weight ratios) became necessary. Today aircraft consist largely of aluminium alloys with steel, titanium alloys and polymer composites forming the smaller proportion. The balance of materials does depend on the type of aircraft as military fighter planes have much higher proportions of composites and titanium alloys.
Aluminium alloy
Aluminium alloys are alloys in which aluminium (Al) is the predominant metal. The typical alloying elements are copper, magnesium, manganese, silicon and zinc. There are two principal classifications, namely casting alloys and wrought alloys, both of which are further subdivided into the categories heat-treatable and non-heat-treatable. About 85% of aluminium is used for wrought products, for example rolled plate, foils and extrusions. Cast aluminium alloys yield cost effective products due to the low melting point, although they generally have lower tensile strengths than wrought alloys. The most important cast aluminium alloy system is Al-Si, where the high levels of silicon (4.0% to 13%) contribute to give good casting characteristics. Aluminium alloys are widely used in engineering structures and components where light weight.
Alloy designations
Wrought and cast aluminium alloys use different identification systems. Wrought aluminium is identified with a four digit number which identifies the alloying elements.
Cast aluminium alloys use a four to five digit number with a decimal point. The digit in the hundreds place indicates the alloying elements, while the digit after the decimal point indicates the form (cast shape or ingot).
Aerospace alloys
The addition of scandium to aluminium creates nanoscale Al3Sc precipitates which limit the excessive grain growth that occurs in the heat-affected zone of welded aluminium components. This has two beneficial effects: the precipitated Al3Sc forms smaller crystals
Heat treatment of precipitation hardening alloys
Aluminium alloys are strengthened in a number of ways including: solid solution hardening, cold working, dispersion hardening and precipitation hardening.
Precipitation hardening (otherwise known as age hardening) is a process whereby a fine precipitate structure is formed in the alloy matrix following a heat treatment process.