Aluminum and its alloys have a lower melting point compared to other metals, which makes the welding process more challenging. This requires skilled welders who are proficient in handling the material. Typically, arc welding is used only when TIG welding isn't feasible or when the welding torch cannot be easily inserted, especially for materials thicker than 2mm. It is mainly used for welding and repairing pure aluminum, cast aluminum, aluminum-manganese alloys, and some aluminum-magnesium alloys.
The key points of electrode arc welding for aluminum and its alloys include:
1. **Selection and Use of Welding Rods**
Pure aluminum rods are generally used for welding aluminum alloys that require lower joint performance. Aluminum-silicon rods offer good crack resistance, while aluminum-manganese rods provide better corrosion resistance. Aluminum-magnesium rods have higher strength. However, aluminum electrodes are hygroscopic and should be stored in a dry place. Before use, they should be baked at 150°C for 1–2 hours and kept in a sealed container. They are typically used with DC power, with the rod connected to the positive pole.
2. **Groove Preparation**
For plates thinner than 6mm, no groove is needed. For plates between 6–12mm thick, a V-groove is recommended. When the thickness exceeds 12mm, an X-shaped groove with a 70° angle is preferred.
3. **Pre-Weld Cleaning**
Proper cleaning before welding is crucial. The oxide layer and oil stains on the workpiece must be thoroughly removed to ensure proper fusion and prevent defects. Poor cleaning can lead to porosity and reduced mechanical properties.
4. **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. It helps form the reverse side of the weld properly. In field conditions, backing is usually avoided, and single-sided welding is performed from both sides, requiring skilled operation.
5. **Preheating**
Thin aluminum parts typically don’t need preheating. However, for parts over 5mm thick, preheating to 100–300°C is necessary to reduce distortion and minimize porosity. This can be done using oxyacetylene flame, electric stove, or torch.
6. **Welding Techniques**
Start the arc longer to allow droplet transfer, then gradually shorten it once the molten pool is formed. When ending the arc, fill the crater before removing it. Aluminum melts faster than steel, so rapid welding is essential. The electrode should move linearly perpendicular to the surface, and the arc should be kept short to prevent oxidation and spatter. Replace the electrode quickly. Thick joints may require preheating.
When repairing aluminum-magnesium castings, use thinner rods and lower current to avoid collapse. For aluminum castings, slightly higher current is acceptable. When repairing multiple cracks, start with small ones and then tackle larger cracks. Long cracks should be welded in segments (about 60–70mm), starting from the middle and working outward.
Multi-layer welding or repairs should be done layer by layer to avoid slag inclusion. After welding, remove all slag to prevent corrosion. Finally, perform full annealing at 300–350°C, followed by slow cooling to improve the weld structure and relieve residual stress.
Table 10-32 shows typical welding specifications for arc welding of aluminum and aluminum alloy electrodes.
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