of loss is a function of time and temperature. We
GTA process for joining such sections, to get better
mentioned earlier that the weld metal is over 1050F
when deposited; therefore, welding causes some
a n n e a l i n g of the parent metal. With the
Aluminum welds decrease about 6 percent in
heat-treatable alloys, welding also lowers the
volume when solidifying from the molten state. This
ductility of the joint.
contraction may cause excessive weld joint distortion
unless correct allowances are made before welding.
Preheating is necessary if the mass of the parent
metal causes heat to be conducted away from the
Thermal expansion of aluminum is
joint so fast that the welding arc cannot supply the
approximately twice that of steel and one-third
heat required to produce fusion. Insufficient heat
greater than copper. The surrounding surface
causes poor fusion of the weld bead and inadequate
expands due to the heat of welding. Thermal
melting of the parent metal. Preheating of the parts
expansion of the adjacent aluminum may reduce the
being joined helps to produce a satisfactory weld,
root opening on butt joints during the welding.
reduces distortion or cracking in the finished
Then, when the metal cools it contracts. This
product, and increases welding speed.
contraction, coupled with shrinkage of filler metal
on cooling, may put the weld in tension and cause
Preheating is necessary in GTA welding of heavy
cracking. Excessive restraint of the component
plate. For the heat-treatable alloys, such as 6061,
sections during cooling of the weld may also result
preheat should be used carefully. Too high a
in weld cracking.
temperature or too long a preheat period can
decrease the as-welded strength of the joint.
Speed is also a factor in preventing distortion.
Recommended preheat temperatures for various
Welding at a slow rate may cause greater area
thicknesses of aluminum plate and tube are shown
heating, t h u s creating more expansion and
in table 10-5.
In GMA welding, preheat is seldom required
Weldable aluminum alloys are of two types: the
regardless of plate thickness. This is one advantage
work-hardenable alloys, such as EC (electrical
of the GMA process over GTA. Another advantage
conductor grade), 1100, 3003, 5052, 5083, and 5086;
is the greater welding speed of GMA.
and the heat-treatable alloys, such as 6061, 6062,
6063, and 7039.
Residual stresses created in aluminum alloy by
the heat of welding may become excessive, due to
Although alloys in the 2000 and 7000 series are
the total amount of heat input, thickness of metal,
also heat-treatable, most of them are not
recommended for arc-welded fabrication because
and design of the weldment. In extreme cases, such
stresses may cause early failure of the weldment.
weldments are low in ductility. Better properties
are obtained with the resistance-welding method. A
One common method of modifying residual stresses
notable exception is alloy 7039, now employed for
is by peening (localized working of the metal by
hammering) to effect limited distribution of the
armor plate and other critical applications. Welding
qualities of alloys in the 2000 and 7000 series with
stresses. However, peening usually is not advisable
on thin sections. For these and certain other cases,
As-welded (GMA) strengths are upward of 48,000
stress r e l i e v i n g by thermal treatment is
recommended, where required.
psi, and ductility of these welds ranges from 8 to 12
percent elongation, in 2-inch increments.
All aluminum alloys can be completely annealed
by heating them to the proper temperatures for
Mechanical properties can be improved in
specified periods of time. Annealing of the metal
h e a t - t r e a t a b l e alloys by heat treatment at
relieves all residual stresses. The temperatures
t e m p e r a t u r e s above 900F, followed by a
required for substantial stress relief have an adverse
low-temperature aging treatment above 300F.
effect on the mechanical properties. This may lower
Aluminum alloys lose hardness and strength
when reheated to high temperatures. When heated
For aluminum-magnesium alloys (5000 series),
above 900F, the aluminum alloys revert to the
annealed condition almost immediately. The degree
high residual stresses may be reduced by heating the