Page 33 - Welding of Aluminium and its Alloys

Welding metallurgy 25

mental mechanism is the same in all of the alloy systems and is a function
of how metal alloy systems solidify. As the name suggests, this is a high-
temperature cracking mechanism which, because of its prevalence, is known
by a number of different names – hot cracking, hot fissuring, hot shortness,
liquation cracking, centre-line cracking or solidification cracking.
The addition of alloying elements to a pure metal will cause a change in
the freezing temperature of the alloy from that of the pure metal and may
result in a number of different phases – a solid solution, a eutectic and an
intermetallic compound, for instance, being produced. These changes of
state and the relative proportions of each phase are represented on phase
diagrams. It is not intended to go into any greater detail than this – for
further information refer to the books listed in the Bibliography.The lowest
melting point composition of the alloy is known as the eutectic composition
which freezes at one specific temperature. The other non-eutectic compo-
sitions freeze over a range.
It is necessary next to look at how a metal solidifies. Figure 2.11 shows
the way in which the lowest melting point constituents are pushed to the
grain boundaries by the solidification fronts as the solid particles grow in
size.
The first solid to form is a unit cell that acts as a nucleus to which atoms
attach themselves, forming what is known as a dendrite. The dendrite
increases in size until such time as it begins to collide with its neighbours
that have been nucleating and growing in a similar manner. The point at
which this collision takes place becomes the boundary between adjacent
dendrites, crystals or grains – the grain boundary. Since almost all alloy
systems, except eutectics, solidify over a range of temperatures, it is common
sense to expect that the first metal to solidify will be the highest
melting/freezing point alloy and the last to be the lowest melting point com-
position, always the eutectic if one has formed. The consequence of this
solidification process is that the lowest melting point alloy composition is
pushed ahead of the solidifying dendrite until it becomes trapped between
the adjacent dendrites, i.e. along the grain boundaries. If the difference in
melting point between the low melting point eutectic and the bulk of the
metal is sufficiently great then the liquid film along the grain boundaries
may part as the metal cools and contracts. The results of this are illustrated
in Fig. 2.12.
In most metals this effect is caused by tramp elements or impurities.
Sulphur in steel and nickel alloys is a good example where low melting point
sulphide eutectics are formed. In the aluminium alloys, however, it is the
deliberately added alloying elements themselves that form a range of eutec-
tics with freezing points substantially lower than the bulk metal.This means
that all aluminium alloys are susceptible to some degree to this form of
cracking, differing only in their degree of susceptibility. Cracking tests have