Aluminum and its alloys can be joined by more methods than any other metal, but
aluminum has several chemical and physical properties that need to be understood
when using the various joining processes.
The specific properties that affect welding are its oxide characteristics, its
thermal, electrical, and nonmagnetic characteristics, lack of color change when
heated, and wide range of mechanical properties and melting temperatures that
result from alloying with other metals.
Oxide. Aluminum oxide melts at about 2050 oC which is much higher
than the melting point of the base alloy. If the oxide is not removed or displaced,
the result is incomplete fusion. In some joining processes, chlorides and fluorides
are used in order to remove the oxide contain. Chlorides and fluorides must be
removed after the joining operation to avoid a possible corrosion problem in
Hydrogen Solubility. Hydrogen dissolves very rapidly in molten aluminum.
However, hydrogen has almost no solubility in solid aluminum and it has been
determined to be the primary cause of porosity in aluminum welds. High temperatures
of the weld pool allow a large amount of hydrogen to be absorbed, and as the pool
solidifies, the solubility of hydrogen is greatly reduced. Hydrogen that exceeds
the effective solubility limit forms gas porosity, if it does not escape from the
Electrical Conductivity. For arc welding, it is important that aluminum
alloys possess high electrical conductivity -- pure aluminum has 62% that of pure
copper. High electrical conductivity permits the use of long contact tubes guns,
because resistance heating of the electrode does not occur, as is experienced with
Thermal Characteristics. The thermal conductivity of aluminum is about 6
times that of steel. Although the melting temperature of aluminum alloys is
substantially bellow that of ferrous alloys, higher heat inputs are required to
weld aluminum because of its high specific heat.
High thermal conductivity makes aluminum very sensitive to fluctuations in heat
input by the welding process.
Forms of Aluminum. Most forms of aluminum can be welded. All the wrought
forms (sheet, plate, extrusions, forgings, rod, bar and impact extrusions), as well
as sand and permanent mold castings, can be welded. Welding on conventional
die-castings produces excessive porosity in both the weld and the base metal
adjacent to the weld because of internal gas. Vacuum die-castings, however, have
been welded with excellent results. Powder metallurgy (P/M) parts also may suffer
from porosity during welding because of internal gas.
The alloy composition is a much more significant factor than the form in
determining the weldability of an aluminum alloy.
Filler Alloy Selection Criteria
When choosing the optimum filler alloy, the application (end use) of the welded
part and its desired performance must be prime considerations. Many alloys and
alloy combinations can be joined using any one of several filler alloys, but only
one filler may be optimal for a specific application.
The primary factors commonly considered when selecting a welding filler alloy are:
- Ease of welding
- Tensile or shear strength of the weld
- Weld ductility
- Service temperature
- Corrosion resistance
- Color match between the weld and the base alloy after anodizing
- Sensitivity to Weld Cracking.
Ease of welding is the first consideration for most welding applications. In
general, the non-heat-treatable aluminum alloys can be welded with a filler alloy
of the same basic composition as the base alloy.
The heat-treatable aluminum alloys are somewhat more metallurgically complex and
more sensitive to "hot short" cracking, which results from heat - affected zone
(HAZ) liquidation during the welding operation. Generally, a dissimilar alloy
filler having higher levels of solute (for example, copper or silicon) is used in
- The high-purity 1xxx series alloys and 3003 are easy to weld with a base alloy
filler, 1100 alloy, or an aluminum - silicon alloy filler, such as 4043.
- Alloy 2219 exhibits the best weldability of the 2xxx series base alloys and is
easily welded with 2319, 4043 and 4145 fillers.
- Aluminum-silicon-copper filler alloy 4145 provides the least susceptibility to
weld cracking with 2xxx series wrought copper bearing alloys, as well as
aluminum-copper and aluminum-silicon-copper aluminum alloy castings
- The cracking of aluminum-magnesium alloy welds decreases as the magnesium
content of the weld increases above 2%.
- The 6xxx series base alloys are most easily welded with the aluminum-silicon
type filler alloys, such as 4043 and 4047. However, the aluminum-magnesium type
filler alloys can also be employed satisfactorily with the low-copper bearing 6xxx
alloys when higher shear strength and weld metal ductility are required.
- The 7xxx series (aluminum-zinc-magnesium) alloys exhibit a wide range of crack
sensitivity during the welding. Alloys 7005 and 7039, with a low copper content
(<0.1%), have a narrow melting range and can be readily joined with the high
magnesium filler alloys 5356, 5183 and 5556. The 7xxx series alloys that possess a
substantial amount of copper, such as 7975 and 7178, have a very wide melting range
with a low solidus temperature and are extremely sensitive to weld cracking when
The GTAW (gas-metal arc welding) process has been used to weld thicknesses from
0,25 to 150 mm and can be used in all welding positions. Because it is relatively
slow, it is highly maneuverable for welding tubing, piping and variable shapes. It
permits excellent penetration control and can produce welds of excellent soundness.
Weld termination craters can be filled easily as the current is tapered down by a
foot pedal or electronic control.
The ac - GTAW process provides an arc cleaning action to remove the surface
oxide during the positive electrode half of the cycle and a penetrating arc when
the electrode is operated at negative polarity.
The dc - GTAW Process. Negative electrode polarity direct current can be
used to weld aluminum by manual and mechanized means.
Other arc welding processes include shielded metal arc welding (SMAW), as
well as electroslag and electrogas welding (ESW, EGW). SMAW with flux-coated rods
has been replaced to a very substantial degree by the GMAW process.
The oxyfuel gas welding (OFW) process uses a flux and either an oxyacetylene
or oxyhydrogen gas flame. When the oxyacetylene flame is used, a slightly reduced
flame is required, which causes a carbonaceous deposit that obscures the weld and
slows the travel speed.
Electron - beam welding (EBW) in a vacuum chamber produces a very deep,
narrow penetration at high welding speeds. The low overall heat input produces the
highest as-welded strengths in the heat treatable alloys. The high thermal gradient
from the weld into the base metal creates very limited metallurgical modifications
and is least likely to cause intergranular cracking in butt joints when no filler
Laser-beam welding (LBW) is now considered to be a viable fusion joining
process for aluminum with the advent of commercially available, stable, high-power
laser systems. Because of aluminum`s high reflectivity, effective coupling of the
laser beam and aluminum requires a relatively high power density.
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