Page 24 - Welding of Aluminium and its Alloys

16 The welding of aluminium and its alloys

If a cold worked metal is heated a temperature is reached where the
internal stresses begin to relax and recovery begins to take place. This
restores most of the physical properties of the unworked metal but without
any observable change in the grain structure of the metal or any major
change in mechanical properties. As the temperature is increased, recrys-
tallisation begins to occur where the cold worked and deformed crystals are
replaced by a new set of strain-free crystals, resulting in a reduction in
strength and an increase in ductility. This process will also result in a fine
grain size, perhaps finer than the grain size of the metal before cold working
took place. It is possible therefore to grain refine a metal by the correct
combination of working and heat treatment. On completion of recrystalli-
sation the metal is said to be annealed with the mechanical properties of
the non-cold-worked metal restored.
At temperatures above the recrystallisation temperature the new grains
begin to grow in size by absorbing each other. This grain growth will result
in the formation of a coarse grained micro-structure with the grain size
depending upon the temperature and the time of exposure. A coarse grain
size is normally regarded as being undesirable from the point of view of
both mechanical properties and weldability.

2.2.5 Precipitation (age) hardening

Microstructures with two or more phases present possess a number of ways
in which the phases can form.The geometry of the phases depends on their
relative amounts, whether the minor phase is dispersed within the grains or
is present on the grain boundaries and the size and shape of the phases.The
phases form by a process known as precipitation, which is both time and
temperature controlled and which requires a reduction in solid solubility as
the temperature falls, i.e. more of the solute can dissolve in the solvent at
a high temperature than at a low temperature.A simple analogy here is salt
in water – more salt can be dissolved in hot water than in cold. As the tem-
perature is allowed to fall, the solution becomes saturated and crystals of
salt begin to precipitate.
A similar effect in metals enables the microstructure of a precipitation
hardenable alloy to be precisely controlled to give the desired mechanical
properties. To precipitation or age harden an alloy the metal is first of all
heated to a sufficiently high temperature that the second phase goes into
solution.The metal is then ‘rapidly’ cooled,perhaps by quenching into water
or cooling in still air – the required cooling rate depends upon the alloy
system. Most aluminium alloys are quenched in water to give a very fast
cooling rate.This cooling rate must be sufficiently fast that the second phase
does not have time to precipitate. The second phase is retained in solution
at room temperature as a super-saturated solid solution which is metastable,