8
The atmospheric gases that are primarily
responsible for porosity in steel are nitrogen and excessive
oxygen. However, considerable oxygen
can be tolerated without porosity in the absence of nitrogen. Oxygen
in the atmosphere can cause severe problems
with aluminum because of its rapid oxide formation. The gas
supply should be inspected at regular
intervals to insure freedom from leakage. In addition, excessive
moisture in the atmosphere can cause
porosity in steel and particularly aluminum. Care should be exercised
in humid climates. For example, a continuous
coolant flow in water cooled torches can cause condensation
during periods of high humidity and
consequent contamination of the shielding gas. Excessive
oxidation of the work pieces is an obvious source of oxygen as well as entrapped
moisture. Again,
this is particularly true for aluminum where a hydrated oxide may exist. Anodized
coatings on aluminum
must be removed prior to welding because they contain water as well as being an
insulator. Porosity
can be caused by inadequate wire deoxidation when welding semi-killed or rimmed
steels. The oxygen
in the steel can cause CO porosity if the proper deoxidizing elements are not
present. Foreign
matter can be a source of porosity. An example is excessive lubricant on the welding
wire. These hydrocarbons
are sources of hydrogen which is particularly harmful for aluminum.
Other causes of porosity may be extremely
fast weld solidification rates and erratic arc characteristics.
When solidification rates are extremely
rapid, any gas that would normally escape is trapped. Extremely
high travel speeds and low welding
current levels should be avoided. Erratic
arc characteristics can be caused by poor welding conditions (voltage too low
or high, poor metal transfer)
and fluctuation in the wire feed speed. All these occurrences cause severe weld
puddle turbulence. This
turbulence will tend to break up the shielding gas envelope and cause the molten
weld metal to be contaminated
by the atmosphere.
