Page 227 - Welding of Aluminium and its Alloys

Weld defects and quality control 207

veyor line. The fluorescent dyes are better in this application than colour
contrast dyes because of their greater sensitivity.
Sensitivity of the process can be checked using standard test blocks. For
the examination of aluminium components, these are available in 2024 alloy
heat treated to give real cracks of a standard size. These blocks should be
scrupulously cleaned after each check to ensure that the cracks do not
become clogged with debris.
Although the technique is simple to use, interpretation of the results can
be difficult, particularly if the surface is ‘naturally’ rough or if the dye is
trapped in acceptable geometric features. Operatives should therefore be
trained and, for many tasks, a qualification in penetrant examination is
either an application standard or contract requirement. Both the British
Institute of NDT (BINDT) and the American Welding Society (AWS) run
accreditation schemes.
There are few health and safety risks involved in using the technique.The
cleaners and some of the solvents in which the dye and developer are dis-
solved will cause skin irritation if used with unprotected hands, and gloves
are strongly recommended.The cleaners and solvent vapours will also need
to be controlled if the process is used in confined spaces. Some of these
materials are also flammable, so there are fire and explosion risks.
Advantages:
• It can be used on both ferrous and non-ferrous metals.
• It is very portable.
• Large areas can be examined very quickly.
• It can be used on small parts with complex geometry.
• It is simple, cheap and easy to use and interpret.

Disadvantages:
• It will only detect defects open to the surface.
• Careful surface preparation and cleanliness are required.
• It is not possible to retest a component indefinitely.
• There may be health and safety problems with some of the chemicals.
• There are health and safety problems with fumes in confined spaces.

11.3.2 Eddy current examination

Eddy current examination is a process that may be used on any material
that will pass an electric current. A coil carrying an alternating current is
placed close to the item to be examined, inducing an eddy current in the
specimen. Defects in the specimen will interrupt this eddy current flow and
these perturbations can be detected by a second, search coil. The coils can