Page 228 - Welding of Aluminium and its Alloys

208 The welding of aluminium and its alloys

be placed either side of a thin plate-like sample or can be wound to give
side-by-side coils in a single probe. These may be shaped to fit in the bore
or around the outside of pipes and tubes and in these applications the process
lends itself to automation. The equipment is calibrated using a defect-free
specimen. The accuracy can be affected by metallurgical condition, stand-
off and coil dimensions. For these reasons eddy current testing is used only
rarely on welded components, although it is excellent in examining contin-
uously welded tube from pipe mills. The process has been developed over
recent years to make it more portable and simpler to use. Microprocessor-
based control and recording units, improved and more tolerant probes and
light-weight electronics have enabled the technique to be used on-site for
the examination of structures in service, where it is an effective tool for the
detection of cracking and corrosion problems. It is also possible for the
depth of surface cracks to be determined. It is of limited use for interro-
gating welds, however, being most commonly used in the examination of
continuously welded tube, and is not covered further in this chapter.

11.3.3 Ultrasonic examination

The ultrasonic examination of welds uses the same principles as when sonar
is used for the detection of submarines. A ‘sound’ wave emitted from a
transmitter is bounced off an object and this reflection captured by a
receiver. The direction and distance of the object can be determined by
measuring the elapsed time between transmission and detection of the
‘echo’. In welded components this is usually done by moving a small probe,
containing both transmitter and receiver, over the item to be examined and
displaying the echo on an oscilloscope screen. The probe transmits a beam
of ultrasound that passes through the metal and is reflected back from any
defects, much like shining a torch at a mirror, in principle with the same
rules applying to the reflection of the beam. This is illustrated in Fig. 11.11.
Deeply buried defects such as lack of fusion, lack of penetration and cracks
in addition to volumetric defects such as slag entrapment and porosity are
all easily detected.
The success of the technique depends upon the use of trained, experi-
enced operators who know precisely the characteristics of the metal being
examined, the beam direction, its amplitude and frequency and the weld
geometry. It is recommended that operators should be approved to one of
the certification schemes such as those operated by the BINDT or the
ASNT.
The frequency of the ultrasonic waves is generally in the range of 2–
5MHz, the lower frequencies being used for the examination of coarse-
grained material and on rough surfaces. The higher-frequency probes are
used for the detection of fine defects such as cracks, non-metallic inclusions,