Page 231 - Welding of Aluminium and its Alloys

Weld defects and quality control 211

To ensure that all of the defects in both the weld and the HAZ are
detected the probe must be scanned over the full cross-section and the full
length of the weld. Accurate sizing and positioning of any defects relies
upon accurate marking out of the weld. Flaws that lie parallel to the beam
may be missed and to ensure that this does not occur it is necessary to scan
in two directions at 90° to each other. Interpretation of the reflections from
regions such as root penetration beads, backing straps and fillet weld roots
can be very difficult, leading to incorrect defect sizing and sentencing. For
this reason the root area is frequently excluded from the area to be ultra-
sonically examined.
Advantages:
• It is very good for the detection of planar defects and cracks.
• It can easily determine defect depth.
• It is readily portable.
• Access is required to one side only.
• There are none of the health and safety problems associated with the
radiographic technique.

Disadvantages:
• Very skilled operators are required.
• Surface breaking defects are difficult to detect.
• Accurate sizing of small (<3mm) defects is difficult or impossible.
• The geometry of the joint can restrict the scanning pattern and prevent
accurate interpretation.
• No permanent objective record is available.
• The process can be slow and laborious.

11.3.4 Radiographic examination
Electromagnetic radiation has properties that are useful for industrial radi-
ography purposes. The rays travel in straight lines and cannot be deflected
or reflected by mirrors or lenses; they have wavelengths that enable the
radiation to penetrate many materials, including most metals. They will,
however, damage living tissue and therefore present some health and safety
problems.
The radiation, either X-rays from a suitable source or gamma rays from
a radioactive isotope, is absorbed as it passes through the material. This
absorption increases as the density of the material increases so that if a pho-
tographic film is placed on the side opposite the radiation source, any less
dense areas will appear as darker areas on the film (Figs. 11.14 and 11.15),
to give a shadow picture of the internal features of the test sample once the
film has been processed. Thus voids, porosity, slag, cracks and defects of