Page 169 - Welding of Aluminium and its Alloys

152 The welding of aluminium and its alloys

attainable.The main welding parameter is the laser power which determines
both the depth of penetration and the travel speed. Other variables are the
position of the focal point, generally at the upper surface, wire diameter
and feed speed and weld gap.
Defects in laser welded joints are similar to those encountered in welds
made by other fusion welding processes. Porosity is caused by hydrogen
from the environment, dissolved in the parent metal, contained in the oxide
film or from an unstable keyhole condition. The solution to this problem is
careful surface preparation including pickling and scraping, gas shielding
and the use of adequate power to ensure the creation of a stable keyhole.
Although most of the non-heat-treatable alloys are capable of being welded
successfully, hot cracking may be encountered, particularly in those alloys
that are sensitive. This can be reduced or eliminated by the addition of a
suitable filler wire. The last difficulty is caused by the low viscosity of the
molten weld metal. This causes the problem of ‘drop-through’, where the
weld metal falls out of the joint when welds are made in the flat position.
This problem can be overcome by welding in the horizontal–vertical (PC)
position.

8.3.1 CO 2 laser welding

As mentioned above improved focusing has enabled very concentrated
2
beams with energy densities above 40kJ/mm to be produced.This has been
achieved by using parabolic reflectors or transmissive systems with a focal
length of around 150mm. The alloy content affects the energy required to
achieve a keyhole with increasing levels of zinc or magnesium requiring less
energy.This is attributed to the low vaporisation temperature of these alloy-
ing elements assisting in the formation of the keyhole. One corollary of this
is that higher welding speeds are possible in those alloys with the higher
magnesium contents.
Helium gas shielding of both the root and face of the weld is re-
commended for the higher magnesium-containing alloys such as 5083
(Al4.5Mg). Over some 3mm in thickness a jet of helium, supplementing
the shielding gas, directed at the weld pool also gives improved weld
appearance. Helium–argon mixture and pure argon gas have also been
used with acceptable results although with a reduced parameter tolerance
box.
Wire additions may be used to increase the resistance to hot cracking in
those alloys that cannot be autogenously welded such as the 6XXX and
7XXX series of alloys.Wire additions are also beneficial in coping with gaps,
a 1.2mm wire can be used to fill gaps of up to 1.2mm. Wire diameters may
be between 0.8 and 1.2mm. Feeding the wire into the leading edge of the