by Noelle Stapinsky
Welding aluminum can be tricky business, even for the most skilled welders
In the constant quest to lightweight vehicles for better fuel economy and improved performance, aluminum is once again a hot topic with automotive manufacturers. Vehicles with aluminum component parts, engine cradles and even bumper systems are hitting the market. Even Ford launched its F-150 with a lightweight aluminum body. Regardless of the mode of transportation–trucks, cars, trailers or rail cars–there are still many challenges around working with and welding aluminum.
Some manufacturers are using a riveting process, which is what Ford uses on its F-150, while others are sticking to spot welding. Rivets are often supplemented with adhesives and Tyler Alexander, weld engineering supervisor for CenterLine (Windsor) Ltd., based in Windsor, ON, says, “one thing to consider when we’re talking riveting and adhesives versus spot welding is that you’re adding mass with each one of the rivets and there’s a penalty associated with that. There are issues with spot welding as well, but with resistance welding, we’re not adding anything, just dealing with two aluminum pieces.”
For resistance welding on aluminum, Alexander says that you’ll need a much higher current then what’s typically used for welding steel, which means investing in different transformers or inverters for your weld control. “You’ll likely have some different dimensional requirements because aluminum is going to move more with the heat. So you might have to look at your clamping and tooling consideration a bit differently.”
The current for spot welding aluminum is usually two to three times higher than that of steel as it is a much more conductive alloy. “From a resistance welding perspective, aluminum is less resistant to the flow of the current, so less heat is developed, but you also have the material conducting the heat away,” says Alexander. “With welding what we’re trying to do is develop that heat and keep it in that certain location. If the material is pulling heat away from the weld joint it becomes very difficult to create the weld.”
Through thick and thin
Aluminum is also a tricky alloy to deal with due to its thermal characteristics.
“If it’s too thin, you risk burn throughs and you need to control your heat input to reduce that risk. “Conversely, there are difficulties when you’re welding thick aluminum sections due to conductivity… you run the risk of lack of penetration and lack of fusion,” says Stephen Rudd, senior welding application specialist for KUKA Robotics Canada Ltd., Mississauga, ON. To weld thick sections, Rudd says a much higher heat input is needed. “I’ve used different gas mixtures like argon–typically used for aluminum welding applications–and helium, which provides a hotter arc. Aluminum has a higher ionization potential, which requires higher arc voltage, hence greater arc energy and the ability to weld those thicker materials.
well as using as much heat as needed to produce the weld, welders must also consider the cooling process. “This will add complexity regarding what electrodes are used, water flow and the length of those electrodes. All of those things come into play as far as the process,” says Marc Levesque, director of corporate marketing for CenterLine. “So instead of welding an entire shift before switching electrodes, you may want to get in there sooner to address wear.”
Lean and clean
Since aluminum is sensitive to the environment, it often forms an oxide layer, which if not attended to appropriately, can cause porosity issues.
Larry Koscielski, senior technical and strategic advisor for CenterLine advises that material flow has to be controlled very tightly when dealing with aluminum. “You don’t want to have coils of aluminum sitting around or processed parts sitting in bins for any length of time.”
Sure you can walk into a fabrication shop and see people welding steel parts with oil all over them, but with aluminum that just won’t fly. Material and weld preparation are paramount.
First it must be clean, dry, and degreased. And if the oxide layer is too thick, it can be removed with a wire brush. But if the material has been sitting outside, it can form a hydrated oxide layer that appears to be a milky stain if you attempt to weld it. In this case it needs to be cut off using a mechanical method. If a thicker than normal oxide layer is not dealt with appropriately the weld appearance will be affected as well as the porosity. “When you weld aluminum, it will absorb the hydrogen and as its cools and that weld solidifies it will reject the hydrogen and you get air bubbles that can’t escape,” says Thomas Pfaller, technical services engineer of AlcoTec Wire, an ESAB brand.
It’s also important to understand that, like steel, there are hundreds of aluminum alloys and each one is different and has a purpose. Knowing what you’re working with is crucial to selecting the appropriate filler alloy. Many may think working with aluminum is finicky and difficult now, but it’s really just a different school of thought that needs to be applied and a new process that will eventually get easier with practice. SMT