ã€China Aluminum Industry Network】 (1) Aluminum has a strong affinity with oxygen in the air, easily forming a dense and tough aluminum oxide layer on its surface. This oxide film is about 0.1μm thick, with a melting point as high as 2050°C—far exceeding that of aluminum itself. Its density is approximately 1.4 times that of aluminum. During welding, this oxide layer can hinder proper metal bonding, leading to slag inclusion. Additionally, the oxide film can absorb moisture, which contributes to porosity in the weld. These defects significantly reduce the performance of the welded joint. To ensure good welding quality, it's essential to thoroughly clean the surface of the workpiece before welding and protect it from re-oxidation during the process. This protection is crucial for achieving sound welds in aluminum and its alloys. Common protective measures include mechanical grinding or chemical cleaning (such as D40) to remove oxides from the weld area and surrounding regions. During welding, it's also important to use high-purity shielding gases like 99.99% argon. (2) Aluminum has high thermal conductivity and specific heat capacity. Although its melting point is much lower than that of steel, its thermal conductivity and specific heat are more than twice that of steel. This means a large amount of heat is quickly conducted into the base metal. To achieve high-quality welds, it's necessary to use energy-dense and powerful heat sources. For plates thicker than 8 mm, preheating and other techniques are required to ensure proper fusion. (3) The linear expansion coefficient of aluminum alloys is roughly twice that of steel, and the volumetric shrinkage during solidification is around 6.5% to 6.6%. This makes it prone to distortion during welding. Effective methods to prevent deformation include using appropriate welding tools, along with selecting suitable process parameters and welding sequences. This is especially important when working with thin plates. Some aluminum alloys tend to form hot cracks in the weld metal or liquefaction cracks in the heat-affected zone, often due to excessive internal stress within the brittle temperature range. Hot cracking is a common and serious issue, particularly in high-strength aluminum alloys. Preventive measures at the site usually involve improving joint design, choosing proper welding parameters, and using filler materials compatible with the parent material. (4) Porosity is a common defect in aluminum alloy welds, especially in pure aluminum and corrosion-resistant alloys. Hydrogen is the main cause of porosity, as proven by practical experience. Sources of hydrogen include moisture in the arc atmosphere, welding materials, and moisture absorbed by the base metal. Moisture trapped in the oxide layer on the welding wire and base metal often plays a significant role in pore formation. Aluminum melt pools can easily absorb gases, and as the metal cools and solidifies, the solubility of these gases drops rapidly. Instead of escaping, the gas accumulates in the weld, forming pores. To prevent porosity and achieve good welds, it's crucial to control the hydrogen source. Before welding, all materials—welding wire, rods, flux, and shielding gas—must be strictly dried and kept dry. Cleaned base metals and wires should ideally be welded within 2–3 hours, not more than 24 hours later. When TIG welding, a higher current and faster speed are recommended. For MIG welding, a larger current and slower speed help maintain a stable molten pool. (5) At high temperatures, aluminum loses strength. At 370°C, its strength drops to only 10 MPa. During welding, this weakens the ability to support liquid metal, potentially causing poor weld formation, collapse, or burn-through. To address this, backing plates are often used when welding aluminum and its alloys. (6) One challenge in welding aluminum is that it doesn't change color when melting, making it difficult for operators to judge the temperature. This requires welders to have excellent control over heating conditions. It’s also recommended to use flat welding positions and guide the arc onto a starting plate to improve weld quality.
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