Why Does Porosity Form in Aluminum Welds?

Porosity is one of the most common defects in aluminum and aluminum-alloy welds. Hydrogen is the principal gas involved. It can enter the arc from moisture in the atmosphere, shielding gas, oxide film, filler wire, or workpiece surface and then become trapped as the weld pool solidifies.

Why Hydrogen Becomes Trapped

The solubility of hydrogen in aluminum drops from about 0.69 mL/100 g in liquid aluminum to 0.036 mL/100 g in solid aluminum, a decrease of roughly twentyfold.

Pure aluminum is more sensitive to hydrogen porosity than Al-Mg alloys because its critical hydrogen partial pressure, p_H2, is lower. A gas content that forms pores in pure aluminum may therefore remain below the pore-formation threshold in an Al-Mg weld.

Where the Hydrogen Comes From

• Atmosphere and shielding gas: Water vapor can dissociate in the arc and supply hydrogen.
• Oxide films: Aluminum oxide readily adsorbs moisture. The loose oxide on Al-Mg alloys can retain even more water.
• Wire and workpiece contamination: Oil, dirt, and corrosion products all increase the available hydrogen.

MIG welding and TIG welding absorb hydrogen differently because their arc and droplet-transfer conditions differ. In TIG root welding, residual oxide at the root can produce concentrated pores. Pore distribution and changes in oxide color can help identify this cause. Pores immediately below the surface may later appear as blisters.

Why Bubbles Cannot Escape

Aluminum weld metal cools approximately four to seven times faster than carbon-steel weld metal. Although aluminum has a lower density, which assists bubble flotation, rapid solidification shortens the available escape time. Hydrogen bubbles that nucleate late are therefore frozen into the weld.

How to Prevent Aluminum Weld Porosity

1. Keep the workpiece, filler wire, and shielding system dry. The source recommends argon with a moisture content below 0.08%.
2. Remove oxide, oil, and other contamination immediately before welding. For manual TIG welding of 1.8 mm LF3 sheet, the source recommends scraping a 20 mm-wide area and using an inverted-V root preparation.
3. Choose parameters that allow gas to escape. TIG welding generally benefits from higher current, faster travel, and lower total heat input. MIG welding may require slower travel and greater heat input to extend pool life.
4. Use suitable preheating when necessary, particularly on thick or highly conductive workpieces.

Effective porosity control is a system: dry consumables, clean surfaces, stable shielding, and a thermal cycle that gives hydrogen time to leave the pool.