Metallographic preparation of aluminium and aluminium alloys Unlike other long established metals such as iron and copper, aluminium is a relatively young metal and has been known only since the beginning of the 19th century. In 1886 a production method for aluminium was developed which is still used today: the Hall-Heroult-Process. Application Notes Aluminium has a silvery white appearance, is very light and is used as pure metal or alloyed for a multitude of different applications. Only very small amounts of alloying elements can increase the strength, and due to their low density, aluminium alloys are especially suited for applications in the aircraft and aerospace industry. Also aluminium alloys are used frequently in the automobile industry to promote weight reductions. The high corrosion resistance of aluminium is based on a passive film of aluminium oxide that is intimately connected to the surface. This oxide film is capable of renewing itself spontaneously when the surface is damaged. Therefore aluminium is suited for polished and brushed surfaces and for anodising in various colours, which makes it an interesting material for the building industry. Additional properties of aluminium are its high heat conductivity and its easy formability, either by casting, hot or cold working or machining. Aluminium is non-toxic and neutral regarding taste and is therefore the preferred material for the food and packaging industry, for instance for cans and foils or machines for bakeries and beverage pumps. Aluminium-silicon cast, colour etched with molybdic acid, 200x. New alloys such as Al-Li and Ti-Al are manufactured by powder metallurgical process and aluminium is also used as matrix metal for composites. Being a very multifaceted material, aluminium will in future open up more fields for new applications. Metallography of aluminium is used in quality control for grain size determination and for micro structure defects on the polished and etched specimen. In addition, the sample is checked for impurities such as oxides or zirconium aluminides (Fig. 2). Cast material is evaluated for shape and distribution of phases and possible porosity. In wrought material, defects from the rolling and extrusion process are investigated and plating thicknesses measured. Difficulties during metallographic preparation - Pure aluminium is very soft and prone to mechanical deformation and scratching. - Silicon carbide and diamond particles can be pressed into the specimen surface (Fig.1) - Severely worked and deformed wrought alloys are difficult to contrast Solution - Plane grinding with the finest possible SiC-Foil/Paper - Diamond polishing and/or final polishing need to be long enough to remove all embedded particles - Final polishing with Collodial Silica suspension - Anodising with Barker’s reagent Fig.1: Embedded diamond particles in pure aluminium after polishing with 3 µm, 200x. Fig. 2: Oxide in the surface of an aluminium pressure die casting, 50x.
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