As one of the most abundant naturally occurring metals on the planet, aluminum is used everywhere. With its corrosion resistance and ability to absorb more impact energy than steel, aluminum also is a major part of the transportation industry. From pocket electronics to skyscrapers, aluminum is part of our everyday world—which means it’s also a major part of the welding industry.
This lightweight metal conducts heat exceptionally well, making it tricky to weld, especially at higher thicknesses. The best way to get the right weld is to make sure you know which series of aluminum you’re working with and to prepare the material’s surface properly for welding.
While a clean surface is important in every weld, it is especially important when welding aluminum. Failing to clean the surface properly can cause a lack of fusion, porosity, weld cracks, and other problems leading to rework or scrapping the piece altogether.
Pre-weld Aluminum Prep
Welders know part fit-up is important in achieving a good weld. Since aluminum tends to be fluid, you need to control gaps and keep them consistent. Be sure to preheat or bevel anytime you weld material more than 3/16 in. thick. Beveling allows for full penetration without overheating the person welding, burning through parts, or exceeding the duty cycle of the machine. Good joint design and planning go a long way to ensuring a quality weld.
Unless you’ve just removed the plastic from the workpiece, make sure to degrease and remove the oxide layer before you weld. Always degrease first. Use an industrial solvent such as acetone or a citrus-based degreaser to break down oils without leaving a residue. Be careful when using chemicals because they can harm your skin or be toxic when inhaled. Never use a wire brush before degreasing. It will simply push oils into small depressions and scratches and cause problems later.
Oxides form on the surface of aluminum when it is exposed to air and can cause porosity if not removed. Most often the oxides are removed with an alkaline solution or by grinding away a small amount of material. If you choose grinding, avoid consumables such as a grinding wheel or sandpaper that can embed foreign material into the base metal, and be sure to use consumables dedicated specifically for aluminum processes. If you use a brush that’s been used previously on steel, it can deposit tiny particles in the softer aluminum surface. Wipe down the area after grinding to remove any dust or contaminants left from grinding consumables.
The amount of material to remove while grinding depends on the application. In aerospace welding, the grinding can go all the way into the base material to ensure all contaminants are removed. In most other welding, the tolerances aren’t as strict, and an alkaline solution or small amount of grinding is all that’s needed. However, be careful when using a wire brush, as it may just polish the surface rather than remove oxides.
Before moving on to welding, be sure to rinse all parts and ensure they’re dry. Damp parts can introduce oxides and cause porosity. Another reason to rinse parts is to clear away any remaining chemicals from the cleaning process. When the metal heats during welding, those chemicals can vaporize and potentially become dangerous inhalants.
The Challenge of Repair Welding
Making new aluminum welds is tricky enough, but repair welding adds unique challenges. The clean area around the weld needs to be much larger in repair welding applications. Once the aluminum heats it can act as a sponge, pulling in any nearby contaminants. Knowing how large the heat-affected zone of your repair weld could be will help determine how large of an area you need to clean.
If your material or part is porous or heavily pitted, use a torch to help remove inaccessible contaminants. You may have to repeat the grinding and cleaning processes more than once to mitigate embedded contaminants.
Once the area is as clean as possible, plan the repair weld carefully. If you suspect that cracks are present within the weld, heat the area with a torch for a few seconds. The metal will expand as it heats and contract as it cools, possibly revealing cracks. If you’re using gas tungsten arc welding (GTAW), a positive AC balance can help break up surface contaminants on dirty parts. Low amperage and slow travel speed are important, but make sure that you reach proper heat input and penetration.
Adjusting the shielding gas also can help you achieve a clean repair weld. Using a helium mixture in conjunction with a fast travel speed to keep the arc length short will help reduce contamination. This also helps maintain penetration when using a positive AC balance for GTAW.
Sometimes getting the surface perfectly clean isn’t possible, especially with repair welding. In that case, having a power source with upgraded technology can be extremely beneficial. If the joint has contaminants deep in the weld, sometimes burst tacking or spot welding the seam is done to lay material over the problems, burying them where they won’t cause a problem with the repaired weld. A machine that has a hot-start feature can boost the amperage at the start of the weld to help tie in the toes. For GTAW, a machine that allows you to modify the waveform can help maintain penetration and the breakup of surface contaminants.
While new welding technology can make up for a lot of problems, it can never do its best work on a dirty surface. To get the best possible weld, start with a properly prepped surface every time.
By Rhonda Zatezalo & Sean McKeen, as published by thefabricator.com
Crearies Marketing Design
Fronius USA LLC