Page 28 - Welding of Aluminium and its Alloys

20 The welding of aluminium and its alloys

hydrogen to bubble out of the weld. A similar effect can be achieved by
reducing the travel speed. Increasing arc voltage and/or arc length increases
the exposure of the molten metal to contamination, and porosity will
thereby increase. The alloy composition can also influence the amount of
porosity by changing the solubility of hydrogen – magnesium in particular
has a beneficial effect. It is thought that magnesium raises the solubility and
reduces absorption of hydrogen by as much as twice at 6% Mg. Copper and
silicon have the opposite effect. A conclusion that can be drawn from this
is that when porosity is encountered the use of Al-Mg filler can assist in
reducing the problem.This assumes of course that such filler metal is accept-
able in the specific application.
The sources of hydrogen are many and varied but one of the primary
sources is the welding consumables. Moisture is an intrinsic part of the flux
in any of the flux shielded processes such as manual metallic arc (MMA)
or SMA (shielded metal arc), and submerged arc (SA) welding. During
welding this moisture decomposes in the arc to give hydrogen, resulting in
a large amount of porosity. This is one reason why these processes are not
widely used to weld aluminium.
The gas used in the gas shielded processes is another source of moisture
which is easy to overlook.Ideally gas with a dew point of less than -50°C (39
ppm water) should be used. To achieve such a high purity it is essential to
purchase the gas with a guaranteed low dew point. It is also necessary
to ensure that when it is delivered to the weld pool it has maintained this
high degree of purity. This means that the gas supply system should be
checked at regular intervals for leaks, that damaged hoses are replaced
immediately and joints are sound. When faced with a porosity problem the
gas purity should be checked first of all at the torch nozzle before working
back along the gas delivery system in a logical manner to locate the source
of contamination.If the workshop layout permits it is recommended that the
gas is supplied from a bulk tank rather than from cylinders and distributed
around the workplace in copper or steel piping. Despite the best efforts of
the gas suppliers it is not always possible to guarantee completely the purity
of individual bottles except at great expense. Bulk supplies are generally of
superior quality.Screwed or bolted flanged connections are potential sources
of contamination and leaks and are best avoided by the use of a brazed or
welded system.
A further source of contamination may come from the gas hoses them-
selves. Many of the plastics used for gas hoses are porous to the water
present in the air. This results in moisture condensing on the inside of the
hose and being entrained in the shield gas. A number of reports published
recently have identified the permeability of hose compositions and a
summary of the results is presented in Table 2.2. From this it can be seen
that only a limited number of hose compositions will maintain gas purity.