Page 234 - Welding of Aluminium and its Alloys

214 The welding of aluminium and its alloys

Neutron and electron guns are also used to produce high-energy beams.
These can be used for interrogating materials in the same way as X- and
gamma radiation.This equipment is not as readily available but has its uses
in industry, particularly for very thick components where long exposure
times would be required using conventional lower energy sources.
The quality of the radiograph is affected by the source to film distance
– the greater this is the sharper the image; the size of the radiation source
– the smaller the source the sharper the image; the beam energy – the higher
the energy the less sharp the image; the film grain size and quality and the
correct film processing.To enable the radiographic quality to be determined
an image quality indicator (IQI) is used. This comprises a number of wires
of different diameters or a stepped wedge with varying diameters of holes
drilled in the steps. The IQI is placed on the source side of the test piece
and adjacent to or across the weld so that its image can be seen on the radi-
ograph after processing. The diameter of the thinnest wire or the smallest
diameter hole that can be seen is then expressed as a percentage of the
specimen thickness – the percentage sensitivity of the radiograph.The other
quality control measure is the density of the radiograph which may be mea-
sured easily with a densitometer. Ideally the density should be between 1.8
and 2.5. Radiographs produced using X-radiation are generally of better
quality than those produced using gamma radiation. Variations in sample
thickness will result in variations in density which may make parts of the
film either too dark or insufficiently dense for accurate defect detection.
Real time radiographic equipment is now being more widely used. This
uses a fluoroscopic screen and a video camera, enabling the image to be
stored, retrieved, and automatically judged almost instantly. This has
obvious benefits with respect to the speed of identifying and correcting
welding faults.
Radiographic interpretation should be entrusted to well-trained experi-
enced radiographers and should be performed in a darkened viewing room
on a viewer designed for the task.
Advantages:
• A permanent record is available.
• Both buried and surface defects can be detected and the technique
is particularly good for finding volumetric defects such as slag and
porosity.
• The equipment is portable, particularly the gamma ray sources.
• All materials can be examined.

Disadvantages:
• The capital cost of equipment, which will need to include the process-
ing and viewing facilities.