Aluminum and its alloys have a lower melting point compared to other metals, making the welding process more challenging. This requires skilled welders who are proficient in handling such materials. Typically, arc welding is used only when TIG welding is not feasible or when the welding torch cannot be easily inserted, especially for materials thicker than 2mm. It is mainly applied for welding and repairing pure aluminum, cast aluminum, aluminum-manganese alloys, and some aluminum-magnesium alloys.
The main points of electrode arc welding for aluminum and its alloys include:
1. **Selection and Use of Welding Rods**
Pure aluminum rods are primarily used for welding aluminum alloys with lower joint performance. Aluminum-silicon rods offer good crack resistance, while aluminum-manganese rods provide better corrosion resistance. Aluminum-magnesium rods have higher strength. These rods are often salty and prone to moisture, so they should be stored in a dry and sealed container. Before use, they should be baked at 150°C for 1–2 hours. They are typically used with DC power, with the rod connected to the positive terminal.
2. **Groove Preparation**
For plates thinner than 6mm, no groove is needed. For plates between 6–12mm thick, a V-shaped groove is recommended. When the thickness exceeds 12mm, an X-shaped groove with a 70° angle is used to ensure proper penetration.
3. **Pre-Weld Cleaning**
Cleaning is essential to ensure high-quality welds. The oxide layer and oil on the workpiece must be thoroughly removed before welding, as any residue can lead to poor fusion and defects. Proper cleaning significantly affects the weld's porosity and mechanical properties.
4. **Use of Backing Strip**
Due to aluminum’s low strength at high temperatures, molten metal can collapse during welding. A backing strip made of graphite, stainless steel, or carbon steel is often used to support the molten pool. A curved groove is formed on the backing surface to help shape the reverse side of the weld. On-site welding usually avoids backing strips, requiring one-sided welding from both sides, which demands advanced skills.
5. **Preheating Before Welding**
Thin aluminum parts generally don’t require preheating. However, for parts over 5mm thick, preheating is necessary to reach the desired temperature near the weld area, reduce distortion, and minimize defects like porosity. The workpiece is usually heated slowly to 100–300°C using an oxyacetylene flame, electric furnace, or torch.
6. **Welding Techniques**
When starting the arc, it should be longer initially to allow droplet transfer. Once the molten pool is filled, the arc length should be gradually reduced. At the end of the weld, the crater should be filled before extinguishing the arc. Aluminum electrodes melt faster than steel, so rapid welding is essential. The electrode should move back and forth perpendicular to the workpiece surface. Keeping the arc short helps prevent oxidation, reduce spatter, and increase penetration. Electrodes should be replaced quickly, and preheating is required for thick sections.
For repairing aluminum-magnesium castings, thinner rods and lower currents are preferred to avoid collapse. However, casting welding may require slightly higher current than plate welding. When repairing multiple cracks, smaller cracks should be fixed first, followed by larger ones. Long cracks should be welded in segments (about 60–70mm), starting from the middle and working outward.
Multi-layer welding or repair should be done layer by layer to avoid slag inclusion. After welding, all slag must be carefully removed to prevent corrosion. Finally, the repaired casting should undergo full annealing at 300–350°C, followed by slow cooling to improve the weld structure and relieve residual stresses.
Table 10-32 shows typical welding specifications for aluminum and aluminum alloy electrode arc welding.
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