Instructions / Assembly

57
GMAW
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GMAW of Aluminum Alloys
K
eywords:
Thermal Conductivity
Axial Spray Transfer
Pulsed Spray Transfer
Properties of Aluminum
The engineering use of wrought and cast aluminum base materials
c
ontinues to increase, and it does so because of the basic
properties of this unique material. The more prominent features
of aluminum and its alloys are:
• Aluminum is lightweight – it weighs about one third that of
steel. A cubic inch of aluminum weighs 0.098 lbs./in.
3
com-
pared to steel, which weighs 0.283 lbs/in
3
.
• Aluminum has a wide range of strength properties that vary
from 13,000 tensile for pure aluminum up to 90,000 tensile for
the heat treatable aluminum alloys.
• Aluminum provides excellent corrosion resistance in many
environments. The thin refractory oxide that forms on the
surface of aluminum provides a protective barrier.
• Aluminum is an excellent conductor of heat. It is up to five
times more thermally conductive than steel.
• Aluminum is reflective of radiant heat, and the surface finish of
aluminum is frequently used to take advantage of this feature.
• Aluminum is widely available in either extruded shapes or
wrought sheet in an equally wide range of alloy compositions.
• Aluminum is widely available as a die cast base material.
For welding purposes, an important consideration for welding
aluminum is its thermal conductivity. This property has an
important facet:
• To compensate for the high rate of thermal conductivity,
aluminum requires the use of higher energy modes of metal
transfer. Axial Spray and Pulsed Spray are the two accepted
and recommended GMAW modes of metal transfer for
Aluminum. The use of the lower energy forms of metal
transfer will usually result in incomplete fusion defects.
Aluminum GMAW Modes of Metal Transfer
Keywords
Short-Circuit Transfer
Axial Spray Transfer
Pulsed Spray Transfer
Nearly all of the same modes of metal transfer that are
described earlier in this document for carbon steel, stainless
steel, and nickel alloys apply to the application of aluminum
solid wire electrode. What is important to note when welding
aluminum base material is that the thermal conductivity of the
aluminum base material is higher than it is for carbon steel, and
because of this the lower energy modes of metal transfer are
unable to provide sufficient melting of the base material to
ensure good fusion.
Axial spray and pulsed spray metal transfers are the preferred
metal transfer modes for aluminum, each of these are capable
of providing the required energy levels for base metal melting to
assure good fusion.
Table 22 supplies the typical axial spray transfer transition
currents related to specific aluminum electrode diameters (note
t
hat argon gas is the shielding gas associated with the transition
currents). In those cases where helium additions are made to
the argon, the required watt energy level (current x voltage) to
achieve the transition to axial spray will have to increase. Axial
s
pray is the higher energy transfer mode for GMAW, and
aluminum requires the use of higher energy modes of transfer to
compensate for the higher thermal conductivity. Because of
these two central facts, axial spray is generally applied to
aluminum base materials 0.125” (3.2 mm) or greater in material
thickness.
TABLE 22
Aluminum Electrode
Diameter Shielding Transition
Inches (mm) Gas Current
0.030 (0.8) 100% Argon 90 ± 5 Amps
0.035 (0.9) 100% Argon 110 ± 5 Amps
0.047 (1.2) 100% Argon 135 ± 5 Amps
0.062 (1.6) 100% Argon 180 ± 5 Amps
AXIAL SPRAY TRANSITION CURRENT
If available, GMAW-P is able to join thin and thick sections of
aluminum. For those materials that are less than or equal to
0.125” (3 mm), pulsed spray transfer is the preferred choice.
Pulsed spray transfer is more easily able to join materials less
than 0.125” (3.2 mm), and this is due to the fact that the aver
-
age current is lower in magnitude for GMAW-P than axial spray
transfer welding current. When compared to axial spray transfer
GMAW-P has the following advantages when used for welding
aluminum:
• Lower heat input – less distortion.
• Ability to handle poor fit-up.
• Ability to handle thinner materials.
• The lower heat input of GMAW-P reduces the size of the
heat affected zone.
• Out-of-position welding is greatly enhanced.
GMAW of Aluminum Alloys