ssc-452 aluminum structure design and fabrication guide ship

Aluminum Marine Structure Guide
The requirements for welding consumables do not specify the chemical composition of
the filler metal. Rather, a filler material is deposited using the welding process for which it is
intended. The chemical composition is then to be in accordance with the manufacturer’s
specifications. Butt welds are made on plate 10 to 12 mm thick and 350 mm wide and of a
length greater than 350 mm using the same welding process. The welded plate is then sectioned
for transverse tensile tests and bend tests. A minimum tensile (but not yield) strength must be
achieved and a 180-degree bend test passed.
All classification societies that are members of IACS incorporate these requirements in
their rules, and in some cases amplify upon them. Therefore, the specific requirements of the
classification society being used should be consulted.
2.3 Welded Properties
Marine aluminum alloys receive their strength through either work hardening or heat
treatment. Exposure to temperatures of around 150 0 C (300 0 F) can begin the annealing process,
significantly reducing material properties. Welding will produce this reduction in properties,
particularly in the heat-affected zone of the weld. The O-tempers of the alloys are in the
annealed condition, so welding does not reduce their strength. The 5xxx-series alloys in the H
tempers are strengthened through work hardening, and the 6xxx-series alloys in the T tempers
are heat treated, so all of these alloys lose strength when welded, with the 6xxx-series having the
greatest reduction in strength. The effect of the annealing on the properties of the HAZ is
obvious when small tensile specimens are taken of weld metal or HAZ metal alone. However,
the effect on overall strength is less obvious.
The standard method of testing is to perform a butt weld to join two plates together, and
then cut tensile specimens that include the weld. The “dog bone” tensile specimen is cut
transverse to the weld, as shown in Figure 2-6. Standards call for the transverse specimens to be
either 50 mm (2 inches) or 250 mm (10 inches) in length. Much data on the strength of welded
aluminum was collected in the past using the 250-mm specimens. Because the weld metal and
HAZ constitutes a smaller percentage of the material in these specimens, the measured properties
are higher than when testing using 50-mm tensile specimens. Properties of the weld metal and
the HAZ are obtained by taking sections parallel to the direction of the weld, sections b and c in
Figure 2-6.
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