Sand Cast Aluminium

This is an alloy of aluminium made from an unknown mix of alloying elements. It has been cast, a process with ancient roots first developed in the Bronze Age, which involves pouring liquid material into a hollow mould and then allowing it to set. In the case of aluminium, molten metal is poured into a mould and then allowed to cool and solidify. Silicon is usually added to aluminium in small quantities in order to give the alloy good casting characteristics; increasing the fluidity of the melt, lowering the melting temperature of the alloy, and reducing the shrinkage that happens when the material solidifies. Aluminium alloys, like all metals, solidify into crystals; this means their atoms arrange themselves into neat, regular, organised structures that fill space efficiently. Most metal objects contain thousands or even millions of metal crystals, which are all oriented differently from one another, which distinguishes each from their neighbours. Casting metals gives them a unique crystalline texture. This is because the speed at which molten metal is cooled to solidification affects the size of the crystals: when liquid metal is cooled quickly, the crystals are smaller, whereas if it cools slowly they tend to be larger. In a cast aluminium alloy such as this, molten metal which touches the cold sides of the mould will solidify extremely quickly, and so a layer of very tiny crystals will from the outer edges of the solidified object. Next to those will form long, larger crystals which reach into the centre of the piece, on account of the temperature difference between the mould and molten liquid. Last to solidify will be the crystals in the centre of the piece, which will be large and rounded. In applications where the strength and stiffness of the component is less critical, the grain structure will not impact the performance of the cast aluminium object, but the piece can be made stronger and more structurally sound if the grains are made smaller and more uniform in shape, in a process called grain refinement. This can be achieved by the addition of alloying elements like boron or titanium, or by physical means using ultrasonic and mechanical stirrers. Many objects are made from cast aluminium alloys, such as outdoor furniture (which won’t rust), cookware, and lightweight automotive components.