can be welded with 4xxx series or 5xxx
series filler alloys depending on weld
performance requirements. It is very
important to dilute the base material
with sufficient filler alloy to reduce
weld metal crack sensitivity and hot
cracking problems (see Figure 2).
No. 2: Strength of the
Welded Joint
Groove Welds. Typically the heat-affected
zone (HAZ) of a groove weld dictates the
strength of the joint. Groove welds in non-heat-treatable alloys will result in complete
annealing of the HAZ area adjacent to the
weld. These alloys are annealed when they
are heated between 600 and 700 degrees F
for a short time. Therefore, the HAZ typically is the weakest area of the joint. The
WPS qualification requirement for these
alloys is based on the minimum tensile
strength of the base alloy in its annealed
condition.
Heat-treatable alloys must be heated for
an extended period to become fully annealed.
This does not occur during welding, and
therefore the strength of the HAZ will be
only partially annealed. However, even with
the best welding procedures and relatively
low heat input, these alloys experience a
significant loss of strength after arc welding
because of time and temperature. The faster
the welding process and heat dissipation, the
smaller the HAZ, and the higher the as-welded strength. Excessive preheating, lack of
interpass cooling, and excessive heat input all
increase peak temperature and time at that
FIGURE 2
Hot cracking of one form or another can be expected if the 6xxx series base alloys are welded without the
addition of filler material, as seen in the two GTAWs on 6061-T6 base plate. The top weld was deposited
with no filler alloy. Liquid penetrant testing revealed many fine linear indications (cracks) within the surface
of the weld. The lower weld, also without filler alloy, used a higher current and slower travel speed.
Excessive heat input during welding resulted in a more obvious cracking situation.
temperature, which result in lower strength
in the HAZ (see Figure 3).
Fillet Welds. Unlike groove welds, fillet
weld strength largely depends on the composition of the filler alloy. The joint
strength of fillet welds is based on shear
strength, which can be affected considerably
by the filler alloy. The 4xxx series filler
alloys have lower ductility and provide less
shear strength in fillet-welded joints. The
Base Alloy
& Temper
1100-H16
1350-H16
3003-H18
5005-H16
5052-H32
5083-H116
5086-H34
2219-T87
6061-T6
Tensile Strength
Before Welding (KSI)
19
16
27
24
31
44
44
64
42
Tensile Strength
After Welding (KSI)
11
8
14
15
25
40
35
35
24
5xxx series fillers typically have more ductility
and can provide close to twice the shear
strength of a 4xxx series filler alloy in some
circumstances.
Tests have shown that a required shear
strength value in a fillet weld in 6061 base
alloy needs a 1⁄4-inch fillet weld with 5556
filler, compared to a 7⁄16-in. fillet with 4043
filler, to meet the same required shear
strength. This can mean the difference
between a one-run and a three-run fillet to
achieve the same strength (see Figure 4).
FIGURE 3
These are the unwelded and welded minimum ultimate tensile strength values of some non-heat-treatable
and heat-treatable aluminum-base alloys.
No. 3: Ductility
Ductility is the ability of a material to flow
plastically before fracturing. Fracture characteristics are described in terms of ability
to undergo elastic stretching and plastic
deformation in the presence of stress raisers
(weld discontinuities).
Increased ductility ratings for a filler alloy
indicate greater ability to deform plastically
and to redistribute load, decreasing the crack
propagation sensitivity. Ductility may be a
consideration if forming is performed after
welding or if the weld is subjected to impact
loading. It is considered when conducting
