Meeting the many demands of customers is and should always be priority No. 1 – even when the material of choice is a challenge to work with. For customers in the automotive industry, galvanized steel is a go-to for many parts and components. It offers excellent corrosion resistance because of its zinc coating and offers high strength, even at thinner gauges. However, when welding galvanized steel, maximizing travel speeds and minimizing porosity issues is key to high quality and productivity.
The zinc coating and thinner gauges are two factors that make galvanized steel particularly difficult to weld. Weld porosity that can lead to weld defects is the biggest challenge.
“The zinc coating is very volatile,” says Tre’ Heflin-King, lead welding engineer, CWI – applications at Hobart. “It has a low melting point and once the weld arc is introduced, the coating starts to vaporize. If the weld pool solidifies before the zinc vapor can escape, it becomes trapped and creates porosity issues that basically reduce the strength of your weld. But with enough heat, you can vaporize that zinc more quickly and allow it to outgas and you have a way to get rid of porosity.”
The potential for burnthrough is increased due to the thinness of the material, which can lead to rework. “In the automotive industry, the thinnest galvanized steel I’ve dealt with is 0.8 mm, and the thickest is typically 1/4 in. thick, but burnthrough issues start around 2.2 mm and less,” Heflin-King notes.
In addition to porosity, the zinc vapor interaction with the weld arc contributes to excess spatter, requiring post-weld cleanup on the part itself and on the welding and robotic equipment.
Solid wire is the typical choice in the automotive industry for welding galvanized steel, but switching to metal-cored wire developed specifically for welding galvanized steel can be beneficial. It provides faster travel speeds and higher deposition rates with less heat input to produce high weld quality, especially when combined with pulsed MIG welding. Because of the faster travel speeds, it is a good fit for robotic welding applications, like those found in automotive manufacturing.
Compared to solid wire, metal-cored wire carries higher current densities at equivalent amperage settings. That is where that increased deposition comes from. To achieve the similar amperage, more wire feed speed is needed, making it possible to add more weld metal in less time.
Another advantage is the wider penetration profile provided by metal-cored wire. “The way the metal-cored wire melts and burns off in a traditional spray transfer mode produces a wider penetration profile,” Heflin-King says. “Solid wire tends to have more of a focused point – sort of like a nipple on a baby bottle. That’s traditional for solid wire. Metal-cored wire tends to have a more rounded design, which provides wider penetration. On thinner materials, that focused point also increases the chance of burnthrough.”
Metal-cored wire also reduces spatter. “Typically, metal-cored wire takes a little less energy to transfer droplets across the arc,” Heflin-King says. “Because of this, there is less agitation, so less disturbance of the weld pool, which, of course, reduces the amount of spatter.”
Metal-cored wire can also feature arc stabilizers that help improve metal transfer, which means less spatter and reduces the need for post-weld cleaning or rework.
Welding galvanized steel with metal-cored wire, or solid wire, combined with pulsed MIG process provides an even greater tool to address heat and spatter.
“Automotive companies use pulse with solid and metal-cored wire because it helps reduce that heat input and reduce burnthrough and spatter,” Heflin-King says. “You can manipulate the waveform to help affect heat and penetration and also set it up to control that droplet transfer. A control scheme pinches off the droplet as it transfers across the arc to reduce spatter and again reduce that heat input.
“In my experience,” he adds, “usually with galvanized steel about 2.5 mm and up, you can run with or without pulse and still have pretty good success but as soon as you start to get below that. 2.5 mm, then you need pulse. You need all the help you can get.”
Reasons to switch
The biggest reason automakers and other manufacturers hesitate to switch from solid to metal-cored wire is the cost, which can be twice as expensive or more.
“Most automakers today are not using metal-cored wire for their galvanized steel welding,” Heflin-King says. “The way industry works is it takes a very long time to get a new way of doing things accepted. Especially when you talk about pricing because everyone is always trying to do more with less. And, even if there is success with a conversion, anytime a new person is brought in, the first thing they try to do is lower costs, which typically means a cheaper wire.”
He stresses that the wire choice should be based on whether it’s the best fit for the application.
“For a narrow tight joint, like a small 20-degree bevel angle where you have to get down deep, solid wire would be a better fit because of the way the profile is,” he says. “For a square butt joint that doesn’t require full penetration, metal-cored wire would probably make sense.”
Where metal-cored wire is a fit, Hobart offers its FabCor F6 metal-cored wire for galvanized steel to help improve productivity and efficiency while producing high weld quality. This wire formulation results in high deposition rates, good gap-bridging capabilities and a very high-strength deposit. It is designed to produce limited subsurface porosity at travel speeds of 40 ipm, compared to 23 or 25 ipm for solid wire.
“This is probably the most unique product we have as far as metal-cored wire,” Heflin-King says. “It doesn’t weld like any other. We developed the formula until we could consistently get quality X-ray welds on 2.2-mm-thick galvanized material at 40 imp before we released it.”
Another unique FabCor F6 characteristic is that it is for use with direct current electrode negative polarity. Most other metal-core wire and solid wire run on direct current electrode positive.
“Electrode negative puts more heat on the wire and less into your base materials,” Heflin-King says. “So that also affects your penetration profile; it gives you a bit more crown to your weld and more penetration into the base material, which is important for thin-gauge materials and gap bridging.”
FabCor F6 is specifically developed for robotic and mechanized applications. “That’s important to mention,” Heflin-King concludes, “because you don’t get the full range of benefits in semi-auto. This product likes to go hot and fast. Fast travel speeds are how we make the quality welds.”