Page 32 - MetalForming March 2014

Troubleshooting Mig Welding of Aluminum
those in the 6xxx series, can unaccept- ably reduce base-material tensile strength.
Weld-Joint Porosity
While not as critical of an issue as cracking, porosity is perhaps the most common complaint when mig welding aluminum. Porosity refers to the cavi- ty-like discontinuities in the weld formed by gas entrapment during solidification.
Weld porosity results primarily from the absorption of hydrogen dur- ing melting and the expulsion of hydrogen during weld-pool solidifi- cation. Sources of hydrogen that cre- ate porosity:
• Hydrocarbons, in the form of paint, oil, grease and other lubricants and contaminants.
• Hydrated aluminum oxide—alu- minum oxide that has absorbed mois- ture can release hydrogen when sub- jected to heat during the welding operation.
• Moisture, which can come from the atmosphere (humidity) or from other sources such as compressed air, small leaks in water-cooled torches, contaminated shielding gas or pre- cleaning operations.
The first step in solving this issue: Identify the source of hydrogen respon- sible for producing the porosity. Weld shops also can purchase low-dew-point shielding gases (argon or argon/helium mixtures) to help reduce weld porosity. And, ensure welders closely follow the recommended shielding-gas flow rates and purge cycles for the welding pro- cedure and weld position being used.
Welding operators must thorough- ly clean base metals with a solvent and clean cloth or paper towel, followed by stainless-steel wire brushing prior to assembling the weld joint. Note: Shop rags typically are not clean enough for use on aluminum, as they can contain residual hydrocarbons that contribute to porosity.
Also, ensure that the base metal and
Forming Systems
Hydroforming Warm Forming Hot Gas Forming Hot Stamping
Pressure Systems
Leak Testing Burst Testing Impulse Testing Hydrostatic
filler metal are condensation-free. When bringing aluminum in from a cooler location (such as outdoors), allow it to sit in the welding area for 24 hr. before welding. Likewise, store unpackaged aluminum filler metals in a heated cabinet or room to help pre- vent them from cycling through dew points and avoid creating hydrated oxide on their surface.
Lastly, diligently purchase high- quality filler metals from reputable manufacturers. Such filler metals typ- ically have been diamond-shaved to eliminate harmful oxides; manufac- tured following procedures to produce low residual-hydrogen-containing compounds; and weld-tested to strin- gent AWS standards.
Prevent Erratic Wire Feeding and Unstable Arcs
Welders can follow several guide- lines to avoid erratic wire feeding and arc issues, starting with using a weld- ing power supply and consumables
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30 MetalForming/March 2014
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