When welding structural parts of aluminum alloy casings, porosity significantly reduces weld density, thus affecting the mechanical properties and corrosion resistance of the structural parts. Porosity formation primarily stems from the high solubility of hydrogen in liquid aluminum and the difficulty of its escape during solidification. Additionally, the decomposition of moisture adsorbed by the oxide film on the base material surface, insufficient purity of the protective gas, or improper process parameters can exacerbate porosity. Therefore, a comprehensive approach involving material pretreatment, process optimization, process control, and post-treatment is necessary to ensure weld quality.
Cleanliness of the base material and welding wire is the first step in preventing porosity. Aluminum alloy surfaces readily form a dense alumina film with a melting point much higher than the aluminum substrate, and it adsorbs moisture from the environment. During welding, the high temperature causes the oxide film to decompose, releasing hydrogen gas into the molten pool, becoming a major source of porosity. Therefore, before welding, the oxide film must be thoroughly removed by mechanical grinding or chemical cleaning (such as alkaline soaking or acid pickling), and the surface should be wiped with dry compressed air or alcohol to avoid residual oil or moisture. For welding wire, high-purity products must be selected and dried before use to reduce the introduction of hydrogen.
The selection and control of the shielding gas are crucial for preventing air from entering the molten pool. Argon, due to its stable chemical properties and high density, is often used as a shielding gas for aluminum alloy welding. However, insufficient argon purity (such as the presence of air or moisture) can lead to oxidation of the molten pool or an increase in hydrogen content. Therefore, argon with a purity ≥99.99% must be used, and the gas supply system must be dry and leak-free. During welding, the gas flow rate must be adjusted according to the thickness of the workpiece; too high a flow rate will cause turbulence and air entrapment, while too low a flow rate will not effectively cover the molten pool. For thick plates or aluminum alloys with good thermal conductivity, the gas flow rate can be appropriately increased or a double-layer gas shielding can be used to enhance the isolation effect.
Optimization of welding process parameters is the core step in avoiding porosity. Welding current, arc voltage, and speed need to be adjusted comprehensively according to the material thickness, joint type, and welding wire diameter. Insufficient current leads to insufficient penetration, a short molten pool duration, and inadequate hydrogen bubble escape. Excessive current can overheat the molten pool, increasing hydrogen solubility and raising the risk of porosity during solidification. Excessive welding speed shortens the molten pool duration and hinders gas escape; excessive speed can result in coarse grains due to excessive heat input, reducing weld toughness. Generally, manual tungsten inert gas (TIG) welding requires a lower heat input to reduce hydrogen dissolution, while metal inert gas (MIG) welding requires a slightly higher heat input to promote bubble escape.
Welding techniques also significantly impact porosity control. During arc initiation and termination, arc-starting and termination plates should be placed at both ends of the weld to prevent sudden temperature changes in the molten pool due to arc instability, which can leave hydrogen bubbles. During welding, the torch angle must be kept stable to avoid arc blow leading to poor molten pool protection. For vertical or overhead welding positions, segmented back-welding or skip welding methods can be used to reduce the flow of the molten pool under gravity and decrease porosity. In addition, during multi-layer, multi-pass welding, it is essential to thoroughly remove slag and porosity from the previous weld layer to prevent the accumulation of interlayer defects.
Preheating and post-weld heat treatment are auxiliary methods to reduce porosity. For thick plates or aluminum alloys with good thermal conductivity, preheating before welding can reduce the cooling rate of the molten pool, prolong the hydrogen bubble escape time, and reduce porosity formation. The preheating temperature needs to be determined based on the material thickness and alloy type, and is usually controlled between 100-150℃. After welding, post-weld heat treatment (such as slow cooling) can further promote hydrogen diffusion and escape, reduce residual stress, and improve weld microstructure.
Environmental control during welding is also crucial. Aluminum alloy welding is sensitive to ambient humidity; excessive humidity will accelerate the absorption of moisture by the oxide film on the base material surface, increasing the risk of porosity. Therefore, the welding workshop must be kept dry, and the relative humidity should ideally be controlled below 60%. For products requiring high precision, dry argon gas can be filled into a local protective enclosure to create a "microenvironment" for protection.
To prevent porosity and ensure weld tightness during the welding of structural parts for aluminum alloy casings, a comprehensive approach is required, encompassing material pretreatment, process optimization, operational control, and environmental management. By rigorously cleaning the base material and welding wire, selecting high-purity shielding gas, optimizing process parameters, mastering operational techniques, assisting with preheating and post-heat treatment, and controlling environmental humidity, porosity defects can be significantly reduced, weld quality improved, and the strength and sealing requirements of structural parts met.
