Characteristics of aluminium
Listed below are the main physical and chemical characteristics of aluminium, contrasted with those of steel, the metal with which the bulk of engineers are more familiar. As can be seen from this list there are a number of important differences between aluminium and steel which influence the welding behaviour:
• The difference in melting points of the two metals and their oxides. The oxides of iron all melt close to or below the melting point of the metal; aluminium oxide melts at 2060 °C, some 1400 °C above the melting point of aluminium. This has important implications for the welding process, as will be discussed later, since it is essential to remove and disperse this oxide film before and during welding in order to achieve the required weld quality.
• The oxide film on aluminium is durable, highly tenacious and selfhealing. This gives the aluminium alloys excellent corrosion resistance, enabling them to be used in exposed applications without additional protection. This corrosion resistance can be improved further by anodising - the formation of an oxide film of a controlled thickness.
• The coefficient of thermal expansion of aluminium is approximately twice that of steel which can mean unacceptable buckling and distortion during welding.
• The coefficient of thermal conductivity of aluminium is six times that of steel. The result of this is that the heat source for welding aluminium needs to be far more intense and concentrated than that for steel. This is particularly so for thick sections, where the fusion welding processes can produce lack of fusion defects if heat is lost too rapidly.
• The specific heat of aluminium - the amount of heat required to raise the temperature of a substance - is twice that of steel.
• Aluminium has high electrical conductivity, only three-quarters that of copper but six times that of steel. This is a disadvantage when resistance spot welding where the heat for welding must be produced by electrical resistance.
• Aluminium does not change colour as its temperature rises, unlike steel. This can make it difficult for the welder to judge when melting is about to occur, making it imperative that adequate retraining of the welder takes place when converting from steel to aluminium welding.
• Aluminium is non-magnetic which means that arc blow is eliminated as a welding problem.
• Aluminium has a modulus of elasticity three times that of steel which means that it deflects three times as much as steel under load but can absorb more energy on impact loading.
• The fact that aluminium has a face-centred cubic crystal structure (see Fig. 2.2) means that it does not suffer from a loss of notch toughness as the temperature is reduced. In fact, some of the alloys show an improvement in tensile strength and ductility as the temperature falls, EW-5083 (Al Mg 4.5Mn) for instance showing a 60% increase in elongation after being in service at -200 °C for a period of time. This crystal structure also means that formability is very good, enabling products to be produced by such means as extrusion, deep drawing and high energy rate forming.
• Aluminium does not change its crystal structure on heating and cooling, unlike steel which undergoes crystal transformations or phase changes at specific temperatures. This makes it possible to harden steel by rapid cooling but changes in the cooling rate have little or no effect on the aluminium alloys (but see precipitation hardening p 16-17).