Aluminum alloys possess a strong affinity for oxygen, readily forming a dense and robust alumina layer on their surface. This oxide film, typically around 0.1 micrometers thick, has a melting point of up to 2050°C—far higher than that of aluminum itself. Its density is approximately 1.4 times greater than that of pure aluminum. During welding, this oxide layer can hinder proper metal bonding, leading to slag inclusion and defects in the weld. Additionally, the oxide film absorbs moisture, which can cause porosity during the welding process. These issues significantly reduce the strength and quality of the welded joint.
To ensure high-quality welding, it's essential to clean the workpiece thoroughly before welding, removing any oxide layers using mechanical grinding or chemical methods like D40. Protective gas, such as 99.99% argon, should be used during the welding process to prevent re-oxidation and protect the molten metal from atmospheric contamination.
Another challenge in welding aluminum alloys is their high thermal conductivity and specific heat capacity. Although their melting point is lower than that of steel, they conduct heat much more efficiently, requiring concentrated energy sources and high-power heat inputs. For thicker sections (8 mm or more), preheating is often necessary to achieve proper fusion.
The linear expansion coefficient of aluminum alloys is about twice that of steel, and their solidification shrinkage rate reaches 6.5–6.6%, making them prone to warping and distortion. To minimize deformation, suitable welding fixtures, optimized process parameters, and proper welding sequences are crucial. Some aluminum alloys also have a tendency to develop hot cracks due to internal stress, especially in the brittle temperature range. Preventive measures include improving joint design, selecting appropriate filler materials, and controlling welding parameters.
Porosity is another common issue in aluminum welds, primarily caused by hydrogen. Moisture in the atmosphere, welding materials, and the oxide layer on the base metal are major sources of hydrogen. To reduce porosity, it's important to control moisture levels in all welding materials and ensure proper drying. Welding should be done within a short time after cleaning, and techniques like adjusting current and welding speed can help improve weld quality.
At elevated temperatures, aluminum alloys lose strength rapidly. At 370°C, their strength drops to just 10 MPa, which can lead to poor weld formation, collapse, or burn-through. Using backing plates or supports during welding helps prevent these issues.
Welding aluminum is particularly challenging because the metal doesn’t change color when it melts, making it hard for the welder to gauge the correct temperature. Skilled operators must rely on experience and use flat welding positions to better control the arc.
In summary, welding aluminum and its alloys requires careful preparation, proper equipment, and precise technique. Key steps include thorough cleaning, controlled environments, and the use of high-purity shielding gases. A reliable welding machine, skilled operation, and the right tools are essential for successful results. Whether at home or in a workshop, understanding these challenges and taking the necessary precautions will greatly improve the quality of the weld.
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