by Shaun Relyea
What is it and why use it?
The use of coated metals such as galvanized steel has increased over the years in the search for lighter, more durable fabrication solutions. Galvanized means the steel is coated with zinc, usually through either a continuous hot-dip process for sheets, or a literal dip in a zinc bath for smaller and formed parts. Zinc can also be applied through an electroplate process, which gives the metal a thinner but smoother coating. However, the thicker the coating, the more challenging it is to weld.
When welding any coated metals, pay attention to how the coating changes the properties of the application. Ideally, the coating would be removed before welding, but this is rarely an option in a production environment. When the welding arc touches the galvanized coating, the heat vaporizes the zinc causing a multitude of challenges such as high spatter, porosity and toxic fumes. Knowing how to minimize the impact of the zinc vapour will ensure a quality weld and a healthy welder.
Welding is dangerous enough without breathing in toxic fumes. Inhaling zinc fumes can cause “metal fume fever.” Symptoms are flu-like and last for about 24-48 hours. It can take up to three weeks to fully recover after exposure. A regimen of bed rest, proper hydration and preventing further exposure is the recommended treatment. The best practice is to prevent exposure by utilizing proper room ventilation and breathing respirators. Many options are available to remove fumes at the weld or even provide fresh air within the welding helmet.
Fumes harm the worker and the weld
As the arc hits the coated metal, the zinc volatilizes and the fumes travel the path of least resistance. A properly positioned and gapped weld will allow the gasses to more easily escape away from the molten puddle, rather than become trapped in it. If the zinc becomes trapped in the weldment, it can result in internal and external porosity and may cause cracking issues later due to stress corrosion. Depending on the application used, you may encounter issues with arc stability due to the vapour rising back into the nozzle. This instability of the arc will cause even more spatter. This is especially true with spray metal transfer and short circuit welding.
So what are the best practices when welding galvanized steel? The most common variables in a welding application— welding process, consumables and speed— can be impacted by the zinc coating. However, they can be fine tuned to get the best weld possible.
Welding galvanized can be done either manually or robotically. The most noticeable benefit to robotic welding is that it removes the operator from the fumes. Otherwise the same issues still apply— high levels of fume, spatter and porosity. Which process you use will ultimately depend on your application, but certain processes do work better on galvanized than others. The most commonly used process is GMAW because it provides the best combination of high quality weld joints and consumable life. Pulsed GMAW has proven to create the least amount of spatter and porosity with a clean, consistent bead. GTAW is rarely used because the vaporized zinc collects on the tungsten, requiring constant removal or replacement. However, plasma welding can be used since a copper nozzle protects the tungsten. Since the zinc coating impedes electrical conductivity, resistance welding is used only on thin joints of less than 1/4 in. total. This method is often used in the automotive industry paired with continuous-dip sheet metal due to its thinner coating. (The coating can also contaminate and destroy the electrodes.)
Consumables are key
Choosing the correct type of wire is important. Silicon reacts with oxygen to create silicon oxide, which has a higher melting point than steel. During welding, the silicon oxide rises up through the molten metal, preventing zinc gasses from escaping. The trapped zinc is one of the main causes of increased porosity. Using a solid wire with a low percentage of silicon, such as ER70S-3, will give a better weld with less porosity. It has also been noted that utilizing a .045 in. diameter wire removes less coating near the weld and increases travel speeds. Some wire manufacturers are developing special metal core wires for welding galvanized sheets that decrease spatter and porosity while maintaining relatively high travel speeds.
Since the silicon reacts with the oxygen in CO2, it’s best to choose a shielding gas mixture low in CO2. A mixture of Argon with just 8-10 per cent of CO2 provides a stable environment with minimized porosity and spatter. Adding helium to the gas mix will increase the arc voltage and create a more fluid puddle which helps to dissipate trapped zinc. The higher fluidity also helps to keep the weld bead more uniform so it is more visually attractive.
To get a properly seated weld, the zinc coating must burn off before the puddle is formed. This requires slower welding travel speeds than when welding uncoated steel. Hot-dipped parts have a thicker coating, requiring slower speeds than continuous-dip sheet parts or electroplated pieces. Production speeds can be significantly impacted by the thickness of the coating required.
In the automotive industry, galvanized sheet is often used for automotive body panels. This allows automakers to guarantee over 10 years of corrosion resistance without significantly adding to the cost of the vehicle. In order to preserve that resistance, brazing is the common choice for joining. Brazing uses lower temperatures, which preserves the zinc coating on the back of the weldment, maintaining the original corrosion resistance.
Welding galvanized steel can be challenging and potentially hazardous, but knowing the best practices will keep production moving right along. SMT
Shaun Reylea is manager of the North American technical support team for Fronius USA.