There are many considerations during welding aluminum alloys because of their unique physical and chemical characteristics of aluminium, contrasted with those of more familiar steel. Here are some comprehensive lists excerpted from the wonderful book "The welding of aluminium and its alloys" by G. Mathers:
• 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; aluminum oxide melts at 2060°C, some 1400°C above the melting point of aluminum. It is essential to remove and disperse this oxide film before and during welding in order to achieve the required weld quality.
• The coefficient of thermal expansion of aluminium is approximately twice that of steel which can mean unacceptable buckling and distortion during welding or high residual stress on welds.
• The coefficient of thermal conductivity of aluminum 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 crucial for thick sections, where the fusion welding processes can produce lack of fusion defects if heat is lost too rapidly. Preheating is usually important.
• The specific heat of aluminium – the amount of heat required to raise the temperature of a substance – is twice that of steel.
• Aluminum 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.
• Aluminum 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 aluminum welding.
• Aluminum is non-magnetic which means that arc blow is eliminated as a welding problem.
• Aluminum 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 aluminum has a face-centred cubic crystal (FCC) structure 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 4.5 Mn) for instance showing a 60% increase in elongation after being in service at -200 C for a period of time.
• 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. However, for heat treatable alloys, it is important for the loss of mechanical properties from overaging problem.
3 Comments:
• Aluminum 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.
This is not correct - Young's Modulus E for Aluminium and most of its allows is about one third that of steel not three times that of stee. It does mean that it deflects threee times as much as steel within the elastic limit, but often Al alloys deform plastically at relatively low levels of applied stress. Some heat treated alloys are of high strength and low ductility and they are relatively brittle and have very limited resistance to impact. The solid solution alloys in the annealied condition or are only work hardened to a limited extent are tough - and that means shock resistant, and are capable of resisting impact without fracture although they will suffer permanent plastic deformation.
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Hi all,
Aluminum may be welded with a variety of techniques, such as metal inert gas welding. Welding with aluminum requires some specific adjustments for welders who are accustomed to welding steel. Aluminum is a better conductor of heat and has a lower melting point so it's much more difficult to weld aluminum without burning through the base metal. Thanks a lot.
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