Page 30 - Welding of Aluminium and its Alloys

22 The welding of aluminium and its alloys

Table 2.3 Summary of causes and prevention of porosity

Mechanism of Potential causes Remedial measures
porosity formation
Hydrogen Oxide film, grease, drawing soap Clean wire, use high-
entrapment on filler wire; oxides, grease, quality gas, change
dirt on parent plate; dirt/grease liner, protect wire
in liner; contaminated shield from contamination,
gas; water leaks in torch; spatter change torch, clean
on weld face. plate, minimise
spatter.
Gas/air (a) Weld pool turbulence due to (a) Use lower
entrapment high current. current, reduce
(b) Gas expanding from root of travel speed,
partial penetration/fillet change gun
welds. angle.
(b) Use full pen
weld, allow gap
in fillet weld
root, use high
heat input.
Rapid freezing Heat input too low, rapid heat Increase current,
trapping gas loss, viscous weld pool, cold slow travel speed,
backing bar. consider preheat,
heat backing bar,
replace argon shield
gas with helium.
Erratic wire feed Kinked, blocked or wrong size Straighten wire
liner, incorrect or badly adjusted conduit, replace
drive rolls, damaged contact tip, contact tip, adjust
unstable power supply. drive roll pressure,
fit correct liner, fit
grooved rolls.

A last source of porosity may be hydrogen dissolved within the alu-
minium.Although solubility of hydrogen is low in the solid phase there can
be sufficient in the parent metal to give a problem on welding. This is
unlikely in wrought products but may arise when welding castings or sin-
tered products. For this reason some purchasers specify in their purchase
orders a limit on hydrogen, typically 2ppm. Avoidance of porosity when
hydrogen is present in the parent metal is impossible to avoid.
Table 2.3 summarises the causes and prevention of porosity.

2.3.2 Oxide film removal during welding
The need to remove the oxide film prior to welding to reduce the risk of
porosity has been covered above. It is also necessary to disperse this film