Welding aluminum requires different welding techniques, different shielding gases, different specifications, and different pre-weld and post-weld processing than welding steel. The welding processes that are fit to weld both may require alterations so that they can be used to weld aluminum. Aluminum can be welded with relative ease, but first and foremost, the correct welding process must be selected.
Why Is Welding Aluminum Difficult?
Before highlighting different welding processes that are used for joining aluminum, it is important to understand some of the difficulties that are inherent to welding aluminum. One area of difficulty is filler metal. First, some aluminum alloys cannot be welded without filler materials. Alloys such as 6061 will undergo solidification cracking if welded without filler metal. Furthermore, the correct filler material must be selected. For instance, welding a 6061 alloy with a 6061 filler metal will result in weld failure. Instead, a 5356 or 4043 aluminum filler metal should be used when welding a 6061 base material. Another challenge with aluminum filler metal is feeding. If a mechanical wire feeding process is being used, special drive systems will most likely be needed. This is because aluminum has less column strength than steel, and will more than likely buckle and tangle if special wire drive systems, such as a push-pull gun, are not used. This is especially true for thinner aluminum filler metals (i.e. 0.8 mm or 1 mm diameter).
Aluminum also has a greater thermal conductivity than steel. The heat created when the welding process is initiated on aluminum is dispersed more rapidly than when welding an iron-based alloy. Therefore, full penetration may not occur until the weld has progressed quite far from the start. This is known as a cold start. Care must be taken so that cold starts do not occur when welding aluminum. Another result of the increased thermal conductivity is larger craters. By the time the end of the weld is reached, more heat is present than at the start. This heat disperses well in aluminum and can create a large crater. Aluminum is very susceptible to crater cracking, therefore, craters should be filled in so that failure does not occur at the end of a weld.
Aluminum also requires different pre-weld and post-weld processing. Aluminum forms an oxide layer that has a higher melting temperature than the actual aluminum itself. In order to avoid un-melted aluminum oxide particles in the weld, an oxide removal process, such as wire brushing or chemical cleaning, should be used prior to welding. Several aluminum alloys, such as 6061-T6, are artificially aged to increase their strength. The heat from welding ruins the benefits gained by artificial aging, and large reductions in strength will be found in the heat-affected zone. Therefore, post-weld artificial aging may be required for alloys such as these.
What Type of Welding Is Used for Aluminum?
The following are welding processes that can be used for aluminum:
- Laser Beam Welding and Electron Beam Welding
- Resistance Welding
One of the most popular welding processes for aluminum is gas tungsten arc welding (GTAW), otherwise known as tungsten inert gas (TIG) welding. GTAW is a great process for aluminum because it does not require mechanical wire feeding, which can create feedability issues. Instead, the filler material is fed into the puddle by the welder with his hand. Also, the GTAW process is extremely clean, which prevents aluminum from being contaminated by the atmosphere.
Gas metal arc welding (GMAW), or metal inert gas (MIG) welding is another great choice for welding aluminum. Gas metal arc welding generally has higher deposition rates and faster travel speeds than GTAW. However, GMAW uses a mechanical wire feeding system. Because of this, a push-pull gun or spool gun may be needed so that aluminum wire feeding is possible. Also, it is important to not use 100% CO2 or 75% Argon/25% C02 shielding gas. Such gas is a viable choice for steel, but aluminum cannot handle the reactive C02 gas. Follow the filler metal manufacturer recommendations for shielding gas type.
Laser Beam Welding and Electron Beam Welding
Beam welding processes are often quite capable of welding aluminum. Also, since the power density of beam welding processes are so high, cold starts are less of a concern. With laser welding, material light reflectivity can be a concern. Also, shielding gas optimization is key to avoid porosity. Electron beam welding generally does not have these problems because it does not use light as an energy medium and it is performed in a vacuum.
Resistance welding is a possibility when welding aluminum. However, difficulties arise from the electrical and thermal conductivity of aluminum. Parameter development time may be significant and special tips and resistance welding equipment may be required to overcome these challenges.
Processes That Are Not Recommended For Welding Aluminum
There are several processes that are not well suited for welding aluminum. Any welding process that uses a flux, such as stick welding, flux cored arc welding, and submerged arc welding, are generally not effective methods for welding aluminum. Oftentimes, the welds created by these processes result in large amounts of porosity.
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