Page 13 - Welding of Aluminium and its Alloys

Introduction to the welding of aluminium 5

• The oxide film on aluminium is durable, highly tenacious and self-
healing. 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 distor-
tion 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 electri-
cal 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 improve-
ment 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 pro-
duced 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).