Page 121 - Welding of Aluminium and its Alloys

106 The welding of aluminium and its alloys

6.10 DC-TIG helium shielded, unbacked 6mm thick plate, single pass,
horizontal–vertical.

The power source controls should provide for both pre-flow and post-flow
of the shield gas.A pre-flow is used to purge the hoses and the torch and to
protect the electrode when the arc is established.Maintaining the flow of gas
when the weld is terminated is also necessary to protect both the weld pool
and the electrode from oxidation as they cool from welding temperature.
Gas flow rates are important in ensuring adequate gas coverage. ‘Bobbin’
type flow meters are often used attached to the regulator to control flow.Any
restriction between the bobbin meter and the torch means that the flow rate
will not be set accurately. It is a good idea to validate meter readings
by attaching a flow meter to the torch gas shroud and monitoring the flow.
Flow meters are also calibrated for a specific gas and will give inaccurate
readings if they are used to control the flow of other gases or gas mixtures.
This is particularly important when using helium or argon–helium mixtures.

6.2.3 Welding torches and cables
There is a wide variety of welding torches available with torch ratings
ranging from some tens of A to 450 A, the appropriate rating depending
essentially on the thickness of the metal to be welded. Most of the modern
torches (Fig. 6.11), are provided with current controls built into the torch
handle.All but the lightest torches, i.e those rated to operate below around
200 A, are water cooled and the same water may be used to cool the power
cables, enabling them to be lighter and more flexible.
Overheating of the torch can melt the brazed joints within the torch or
the plastic tube that sheaths the power cable and it is important that