While aluminum offers many advantages for applications in the fabrication and manufacturing industry, using the gas metal arc welding (GMAW) process on this material comes with its challenges. Issues with cracking, porosity and more can offset the benefits of a high strength-to-weight ratio and good corrosion resistance. To take the trouble out of welding aluminum and get the best quality results, it’s critical to maintain exceptional cleanliness, to select the right filler metal and to employ the correct welding preparations to remove the oxide layer on the surface of the material. Here are some tips to help along the way.
GET RID OF DISCOLORATION
When completed properly, aluminum welds tend to be quite aesthetically pleasing. The introduction of oxygen into the weld pool, however, can lead to discoloration and smut. There are several key ways to protect against this issue.
Minimize the risk of oxygen entering the weld pool by decreasing the MIG gun angle and/or by increasing the nozzle size so the shielding gas envelope is larger. Holding the nozzle slightly closer to the base material when welding can also help. Be certain that there is no spatter buildup in the nozzle that could hinder the shielding gas flow, and set up barriers when welding in drafty areas to prevent the shielding gas from being disturbed or blowing away. Always use a push angle when welding aluminum, as this direction helps put the cleaning action from the arc in front of the weld and results in less discoloration.
In addition to the proper welding techniques, filler metals can also play a role in preventing discoloration and/or smut. In particular, filler metals with a 4XXX designation tend to create a cleaner aluminum weld than 5XXX series aluminum products, as they have little to no magnesium in them. This element vaporizes during the welding process and condenses in the form of black soot.
Porosity is an exceptionally common problem when welding aluminum and results from gas being trapped in the weld upon solidification. Hydrocarbons from paint, oil, grease and lubricants can lead to this issue, as can moisture from the atmosphere and/or contaminated shielding gas.
The best way to protect against porosity is to clean the aluminum thoroughly before welding. Using a dry, clean cloth, apply a solvent designated for aluminum cleaning (contact a local welding distributor for recommendations). Next, use a stainless steel wire brush to gently remove the aluminum oxides on the surface of the material — be certain to designate the brush for this sole purpose, as using it for other jobs allows the bristles to pick up contaminants that could be introduced into the aluminum weld pool.
Take care that the aluminum to be welded and the filler metals do not have any condensation on their surfaces. If these products are being stored in a cooler area than where the welding will take place, allow them to sit in the weld cell for 24 hours prior to welding, so the moisture can dissipate. Store unpackaged filler metals in a heated cabinet or room to keep them from cycling through dew points and minimize the risk of hydrated oxides accumulating on their surface.
When possible, purchase low dew point shielding gases (argon or argon/helium mixtures) to help reduce the risk of porosity. Always follow the recommended shielding gas flow rates and purge cycles found in the welding procedure for the given welding position.
As with many materials, aluminum is prone to cracking during the welding process. Hot cracking is the most prominent, but stress cracking can also occur. The tendency for aluminum to experience hot cracking depends on the particular alloy; some are more sensitive than others. It is important to select the right filler metal to match the alloy at hand. To identify the particular alloy chemistry, aluminum filler metals feature an American Welding Society (AWS; Miami, FL) classification that corresponds to the Aluminum Association registration number. Always reference a reputable filler metal selection guide to make the best selection.
Consider using beveled joints to prevent hot cracking. This wider surface area provides more space for the filler metal to be deposited, which helps dilute the base metal and makes solidified weld less crack sensitive.
To prevent stress cracking, which results when the weld shrinks or cools rapidly, choose a filler metal containing silicon (when appropriate to match the base material). Increasing travel speeds can also help by narrowing the heat affected zone (HAZ) and reducing the amount the base metal melts. Remember to fill in the “craters” at the end of welds, since this is the most common area for stress cracks to begin. Some welding equipment has an automated crater fill function that will do the job. Preheating the aluminum can also help reduce residual stress levels in the base material during and after welding, and lower the chance of stress cracking. Do not overheat the material — 150 deg F is safe for all aluminum alloys.
OTHER CONSIDERATIONS FOR ALUMINUM WELDING
Taking steps to prevent issues like cracking is important when welding aluminum, but it’s not the only defense against problems. It is also important to use the right equipment. For the best results, consider these tips:
- Use contact tips designed for aluminum welding and sized appropriately for the wire being used. Doing so can help prevent burnbacks.
- Recess contact tips 1/8 in to 1/4 in to minimize spatter and provide adequate cooling.
- Use U-groove drive rolls designed for aluminum for the best wire feeding results (a V-groove design can compress the wire, leading to an erratic arc), and set the drive roll at the lowest pressure capable of consistently feeding the wire.
- Use a push-pull gun or a spool gun for optimal wire feeding.
As with any welding process, following some best practices such as these can help reduce costly downtime and help improve quality results.
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