Page 32 - Welding of Aluminium and its Alloys

24 The welding of aluminium and its alloys

MIG
TIG TIG TIG
DC – ve DC + ve AC

1/3 2/3 1/2
Heat Heat Heat

IONS ELECTRONS IONS

2/3 Heat 1/3 Heat 1/2 Heat
+ve 1/2 Cycle
Electrode Oxide
overheating removal
–ve 1/2 Cycle
Oxide Electrode
removal cooling
2.10 Effect of polarity on cathodic cleaning and heat balance.

erratic metal transfer and poor weld quality. Oxide film removal is there-
fore an intrinsic part of the MIG process.
TIG welding, on the other hand, conventionally uses DCEN, which, if
used on aluminium, can result in poor weld quality. Using DCEP with TIG,
however, results in the tungsten electrode overheating as some 60–70% of
the heat generated in a TIG welding arc may be produced at the positive
pole. (Conventionally a rule of thumb for the heat balance in a TIG arc is
regarded as being two-thirds at the positive pole, one-third at the negative
pole. This, however, varies widely depending upon the shield gas, current,
arc length, etc.) This can cause melting of the electrode and bring the
welding operation to a premature end. A compromise is therefore reached
by using AC where oxide film removal takes place on the positive half cycle
and electrode cooling on the negative half cycle as illustrated in Fig. 2.10.
TIG welding of aluminium is therefore normally carried out with AC,
although there are a couple of techniques that use either DCEP or DCEN.
These will be discussed in Chapter 6 on TIG welding.

2.3.4 Hot cracking

Hot cracking is a welding problem that does not occur in pure metals but
may be found in certain alloy systems. It is not confined to the aluminium
alloys but is also encountered in steels, nickel and copper alloys.The funda-