Aluminum Welding: Filler Selection, Cleaning, and Common Problems
Aluminum is the second most common metal welded after steel, but it behaves nothing like steel under an arc. It conducts heat three times faster, melts at a lower temperature, oxidizes instantly when exposed to air, and gives no color change before it melts. These properties make aluminum welding more demanding in terms of technique, preparation, and equipment settings.
Filler Wire Selection
Aluminum filler selection is simpler than stainless but the consequences of getting it wrong are worse.
ER4043 is the general-purpose choice. It contains 5% silicon, produces a fluid puddle, and works with most 6xxx-series and casting alloys. It is the default recommendation when you are unsure.
ER5356 contains 5% magnesium, which produces a stronger, more ductile weld. It is required for 5xxx-series alloys (marine aluminum) and structural applications. ER5356 welds anodize better than ER4043.
ER4047 has 12% silicon for maximum fluidity and lowest melting point. It is a specialty wire for brazing, casting repair, and thin-material applications.
Never mix 4xxx and 5xxx filler families on the same joint. Using ER4043 on a 5xxx base metal (or vice versa when the combination creates a magnesium-silicon ratio in the crack-sensitive zone) can cause solidification cracking.
Surface Preparation
Aluminum oxide melts at 3700F but aluminum metal melts at 1220F. If you do not remove the oxide layer before welding, you will melt the base metal long before the oxide breaks down, trapping contamination in the weld.
Mechanical cleaning: Use a dedicated stainless steel wire brush (never carbon steel) to scrub the weld area and 1 inch beyond on both sides. Brush in one direction only.
Chemical cleaning: Wipe with acetone immediately before welding to remove oils and hydrocarbons. Aluminum absorbs shop dirt and fingerprint oils quickly.
Freshness matters: Oxide reforms within minutes. Clean immediately before welding. Material that was cleaned yesterday is not clean today.
TIG Welding Aluminum
TIG is the most common process for aluminum welding, especially on thinner material. Use AC polarity on a machine with adjustable AC balance and frequency.
AC balance controls how much of the cycle cleans oxide (EN) versus how much provides penetration (EP). Start at 65-70% EN and adjust from there.
AC frequency controls arc focus. Higher frequency (120-200 Hz) narrows the arc cone for more precise control. Lower frequency (60 Hz) widens the arc for more cleaning action.
Shielding gas: 100% argon for most work. Add helium (75Ar/25He or 50/50) for thicker material to increase heat input and penetration.
Preheat is common for aluminum thicker than 1/4 inch. Heat to 200-300F to reduce the thermal gradient and improve wetting.
MIG Welding Aluminum
MIG welding aluminum requires a spool gun or a push-pull system. Standard MIG guns cannot reliably feed soft aluminum wire through a long cable without bird-nesting.
Shielding gas: 100% argon. Do not use CO2 or Ar/CO2 mixes on aluminum.
Wire size: 0.035 inch is the most common. Use 0.030 for thin material. Thicker wire (0.045 and above) requires higher amperage machines.
Travel direction: Push the gun (forehand technique) when MIG welding aluminum. Dragging (backhand) causes porosity by allowing the shielding gas to trail behind the puddle.
Common problems: Burn-through on thin material (use pulse if available), porosity from contamination or insufficient gas coverage, and crater cracking at the end of beads (use crater fill function or add filler at the end).
Classification system defined by AWS A5.10, Aluminum Association welding guide, Lincoln Electric.