Page 158 - Welding of Aluminium and its Alloys

MIG welding 141

pools the crater may extend down into the weld to form an elongated pore
– piping porosity. As the weld continues to cool and contract then the asso-
ciated shrinkage stresses may cause hot short or crater cracks to form. Any
form of cracking is unacceptable and is to be avoided. Methods of elimi-
nating this defect include the following:
• The use of run-off tabs on which the weld can be terminated, the tab
being subsequently removed.
• Increasing the travel speed just before releasing the trigger. This causes
the weld pool to tail out over a distance. It requires a high measure of
skill on the part of the welder to produce acceptable results.
• Making a small number of brief stops and starts into the crater as the
weld cools. This adds filler metal to the crater.
• As the trigger on the torch is released the wire feed speed and the
welding current are ramped down over a period of time. The crater is
fed with progressively smaller amounts of molten filler metal as it forms,
resulting in the filling and elimination of the crater. This crater filling
facility is standard on modern equipment and is the preferred method
for avoiding piping porosity and crater cracks.

7.5 Mechanised and robotic welding
As MIG welding is a continuously fed wire process it is very easily mech-
anised. The torch, having been taken out of the welder’s hand, can be used
at welding currents limited only by the torch or power source and at higher
travel speeds than can be achieved with manual welding. A typical robot
MIG welding cell where the robot is interfaced with a manipulator
for increased flexibility and a pulsed MIG power source is illustrated in
Fig. 7.19. Greater consistency in operation means that more consistent weld
quality can be achieved with fewer defects. The advantages may be sum-
marised as follows:
• More consistent quality.
• More consistent and aesthetically acceptable bead shape.
• More consistent torch height and angle mean that gas coverage can be
better and the number of defects reduced.
• Fewer stops and starts, hence fewer defects.
• Higher welding speeds means less heat input, narrower heat affected
zones and less distortion.
• Higher welding current means deeper penetration and less need for
large weld preparations with fewer weld passes and therefore fewer
defects.
• Higher weld currents mean a hotter weld and reduced porosity.