Aluminum is well-suited for many industrial applications, because it's light, strong, conductive and inexpensive. It's not the easiest metal to seal, however. When it's welded, aluminum often loses strength and becomes susceptible to corrosion. To prevent these issues from arising, you can solder aluminum seals using ultrasonic welding and an inert gas.
Aluminum Can't Withstand High Temperatures
All aluminum can be categorized into two classifications: cold-worked and heat-treatable. Lincoln Electric explains that cold-worked aluminum is strengthened at cool temperatures and never exposed to heat, and heat-treatable aluminum is tempered at increasingly high temperatures, up to 400°F.
Even though heat-treatable aluminum is made to withstand higher temperatures than cold-worked aluminum, neither will stand up to welding temperatures. For instance, GoWelding.org notes that an arc can reach up to 11,000°F -- much hotter than 400°F. At temperatures above 400°F, aluminum will retain its form but quickly weaken.
You may be able to weld two pieces of aluminum together using heat and form a decent seal. The aluminum pieces that were bonded, though, will not be as strong as they were originally made to be, though.
Ultrasonic Welding Doesn't Use Heat
Unlike generic welding, ultrasonic welding doesn't use heat to bond two materials. Instead, the pieces are fused together using sound. HowStuffWorks details the process of converting electricity into high-frequency sound and then directing the sound waves at the materials being bonded. The sound energy seals materials together that can't withstand high temperatures.
Ultrasonic welding is often used to bond plastics. Since it doesn't require heat, plastics that would otherwise melt can be welded together using this technique. Sound waves can also be used to weld aluminum seals without damaging the aluminum pieces being fused together.
Inert Gasses Shield Aluminum from Oxides
Because ultrasonic welding doesn't heat up a gas, it's important to shield your aluminum pieces with an inert gas. It the aluminum's sealed in atmospheric conditions, oxygen in the atmosphere could cause the aluminum to corrode. Oxygen and carbon dioxide in the atmosphere can react and form oxides that can be trapped in the seal, itself, or on the aluminum's surface.
Aluminum's not known for corroding, even though it does. Aluminum oxide's not red like iron oxide, which is more commonly called rust. Instead, aluminum oxide looks much like aluminum -- grey or white. Upon close inspection, the corrosion will look like a powder or small crystals, but it requires a sharp eye to notice the corroded aluminum.
Inert gasses prevent aluminum oxide from forming, because they don't react under normal conditions. Also known as noble gasses, they have a complete set of electrons and are more stable than oxygen. The two most common inert gasses used for industrial purposes are helium and argon.
In the field, shielding aluminum with an inert gas may be impossible, but so is ultrasonic welding. In the controlled setting of an industrial plant, though, you should be able to adapt your ultrasonic welding equipment so that it also shields materials with an inert gas.
An Acid Bath Cleans Aluminum
Of course, shielding aluminum from oxides in the air only helps if the metal's first washed. All aluminum pieces, including sheets, bars and seals should be dipped in an acid bath immediately before welding. An acid bath will remove any oxides or other chemicals that are on the surface of the aluminum.
Aluminum has many uses, but it's not the easiest to weld. If your company needs to form strong aluminum seals, use ultrasonic welding and shield the aluminum with an inert gas. That's the only way to ensure the aluminum you use will be strong and not corrode. For more information on aluminum seals, check out http://www.phoenixspecialty.com/.