AWS because carbon dioxide is used as a shielding
steels should be transferred directly to a stress-relief
gas when welding mild steel by the GMA process,
furnace without loss of preheat.
and carbon dioxide is not an inert gas.
The peening of weld metal helps minimize
A big increase has occurred in the use of the
cracking of weld metal and reduces distortion
GTA and GMA processes for all types of structural
because it distributes the residual stresses created by
and piping systems. This is especially true in
welding.
aluminum fabrication. For that reason, we have
given special attention to these two processes. Our
Various specifications and codes require that the
discussion includes basic information on the
first and last layer of weld metal not be peened.
c h a r a c t e r i s t i c s of aluminum that affect its
Peening of the first layer could pierce the weld or
weldability, as well as on the effect of heat on the
displace the members being welded. Peening of the
aluminum part being welded. The importance of
last layer can cause brittle fractures due to the cold
s u r f a c e preparation is explained. Detailed
working of the weld metal. Use peening on each
information is given on both the GTA and GMA
weld bead or layer except the first and last. The
processes. Practice exercises are provided to help
effectiveness decreases as the thickness of the bead
you develop techniques in various operations used
or layer increases. Peening becomes of doubtful
in each process.
value for deposits on one-fourth inch or thicker,
except in special instances where the rigidity or
BASIC THEORY OF
weight of the weldments permits the use of heavy
ALUMINUM WELDING
blows.
Selection of the arc-welding method to use on
Peening equipment should be selected with care.
aluminum depends largely upon the individual
The hammer, pneumatic tools, and so on, should be
application. You need to consider thickness; design
heavy enough in striking force to be effective
of the parts, components, or assemblies; and
without producing excessive work hardening, but not
available equipment. The best welding methods for
so heavy that bending moments are involved or
aluminum are the GTA and the GMA processes.
cracks produced in the weld.
Both use noncombustible gas (argon, helium, or a
The general causes of weld metal cracking, base
mixture of gases) to keep air away from the arc and
molten weld pool and to eliminate the need for a
metal cracking, porosity, and inclusions are outlined
welding flux. The gas shield is transparent; the
in table 10-4.
welder can see the fusion zone and make neater and
sounder welds. Aluminum can be welded in any
position by either method.
GAS SHIELDED-ARC WELDING
The GTA process is best for welding aluminum
This process uses a shielding gas to protect the
sections less than one-eighth inch in thickness. This
electrode, arc, molten weld metal, and weld area
method can also be used on heavier sections, but
from exposure to the atmosphere. The shielding gas
the GMA process is usually chosen for its higher
is noncombustible and may or may not be inert
welding speed and economy.
(chemically inactive).
The electrode may be
nonconsumable, or it may be a consumable wire
General Considerations
electrode that is fed automatically into the weld.
The factors that affect the welding of aluminum
There are two different types of gas shielded-arc
and the properties of aluminum weldments include
welding processes. One is the gas tungsten-arc
melting point, thermal conductivity, thermal
(GTA) process, which uses a nonconsumable
tungsten electrode. The other is the gas metal-arc
expansion and contraction, oxidation, gas porosity,
(GMA) process, which uses a consumable wire
and the effects of welding.
electrode that is fed automatically into the weld.
Pure aluminum melts at 1210F, and weldable
aluminum alloys start to melt at 1050F. This
These two processes were formerly known as
compares with steel, which melts at about 2800F
tungsten inert-gas (TIG) welding and metal inert-gas
and copper at about 1980F. Unlike these metals,
(MIG) welding. The names were changed by the
10-20
