Porosity formation within aluminium welding arises from the entrapment of gases in the molten weld pool, unable to escape before solidification. The chief concern is hydrogen, sourced from contaminants like hydrocarbons (e.g., grease, skin oils) or moisture, breaking down within the arc plasma to generate atomic hydrogen.
Hydrogen readily dissolves in molten aluminium but exhibits significantly lower solubility in solid aluminium (approximately 20 times lower). Consequently, as aluminium solidifies, the evolved hydrogen gas forms gas pores that become trapped within the weld bead.
Several pathways introduce these contaminants to the weld pool. Contaminants may directly originate from the parent material or the filler wire used for the joint. They might also be present on tools or the welders’ hands, transferred by contact. Notably, aluminium develops a thick oxide layer on its surface, which, if hydrated by absorbing moisture from the environment, can release hydrogen into the weld pool.
Moisture from the atmosphere, prevalent in damp or humid environments, can infiltrate the weld pool if shielding by an inert gas is inadequate or disrupted. Moreover, shielding gas flow itself might harbor moisture if the gas source or transmission lines contain leaks or allow condensation in cold conditions.
Hydrogen may even emanate from the original solid material, notably in cases of cast or sintered products. To prevent porosity, minimizing hydrogen absorption during welding is key, necessitating an understanding of various potential sources and adapting welding methods accordingly.
Emphasizing cleanliness is pivotal. Removing contaminants from parent and filler materials involves rigorous degreasing using solvents like acetone before welding. Mechanical abrasion, often via stainless steel brushing, is essential to eliminate potentially hydrated surface oxides. More aggressive chemical cleaners such as sodium hydroxide or nitric acid can directly remove oxide films but come with stricter safety measures.
Welding must commence swiftly after cleaning to prevent oxide reformation. Handling materials post-cleaning demands caution, with clean gloves and storage in uncontaminated conditions. Effective gas shielding entails minimizing the distance between the gas source and torch, using low-permeation piping, and conducting regular checks for leaks and gas quality.
Avoiding disruptions to arc or shielding gas flow reduces environmental moisture intake. Ensuring a steady wire feed without kinks, using correct contact tips and drive rolls, and adjusting torch cables effectively minimizes turbulence risks. Welding parameters, such as arc gap length and current, impact porosity, with higher currents increasing hydrogen solubility but also potentially aiding gas release.
Welding techniques like preheating, using specific gas mixtures, or modifying the welding current influence porosity. Moreover, AC currents in TIG welding help remove oxide film by alternating the electrode’s polarity, while MIG welding inherently cleans through the positive electrode.
It’s advisable to isolate aluminium welding from other material fabrication to prevent cross-contamination through fumes, metal powders, or tool usage.