As a way to reduce overall vehicle weight in the automotive industry, thinner, high-strength galvanized steel parts are growing increasingly popular. The use of galvanized steel allows manufacturers to fabricate lighter weight vehicles without sacrificing corrosion resistance. In addition to auto bodies, use of galvanized steel has expanded into other components in recent years in chassis parts, such as frames, engine cradles, cross members and suspension parts.
“The automotive industry is very focused on producing high-quality parts while maintaining very high productivity,” says Francis Krivanka, product manager for cored wire consumables, The Lincoln Electric Co. “You’re talking about turning out hundreds of parts in one shift, so consistency and repeatability are crucial.”
The main types of galvanized zinc coatings used in the automotive industry are hot-dipped, galvannealed and electro-galvanized. Hot-dipped galvanized steel typically has a thicker coating, which creates more challenges and can be harder to weld.
Galvanized steel has numerous properties that make it a good choice for automotive applications. Its protective layer of zinc on the surface offers excellent corrosion resistance, which is important for thinner gauges.
However, welding galvanized steel has always been difficult due to the low boiling temperature (906 C) of zinc. Vaporized zinc becomes trapped in the molten weld puddle, and the weld solidifies before the zinc vapors can escape, generating internal and external porosity (on and below the surface of the weld).
The travel speed used during the welding process impacts the issue of porosity. The faster the travel speed, the faster the weld puddle tends to freeze, trapping those zinc vapors.
“Slowing the cooling enables zinc vapors to bubble out,” Krivanka says. “But if you travel too slowly, you could overheat and distort your part, or burn a hole through your part.”
Burn-through due to heat input, therefore, is a definite risk on thin galvanized steel automotive parts.
“We used to see 2-mm- to 3-mm-thick auto parts, but now it’s under 2 mm and going to 1 mm and even thinner in some cases,” says Vaidyanath Rajan, R&D group leader, Lincoln Electric. “When you weld thinner parts, you don’t want to cut through the galvanized steel plate. You want to manage your heat input carefully, but at the same time keep the puddle fluid enough so the zinc you’re burning bubbles out.”
Fortunately, there is a solution to the heat input associated with welding galvanized steel. Switching from solid wire and self-shielded flux-cored wire to metal-cored wire formulated for use with galvanized steel, paired with the pulsed MIG welding process, offers many advantages.
“Different approaches have been used,” Rajan says. “A few years ago, it was typical to use self-shielded flux-cored wire designed to flux with the zinc. The zinc would become trapped in the slag that would form, and that was one way to keep the zinc pores from forming in the weld. Within the last year or two, however, there is more of a preference to use metal-cored wire.”
In general, metal-cored wires can promote fast travel speeds and high deposition rates. Additionally, the penetration profile and resulting weld bead shape of metal-cored wire, compared to solid wire, can be beneficial in automotive applications. But these benefits alone do not always solve the problems that arise when welding galvanized steel automotive parts.
“You are welding over a layer of zinc, which is a volatile metal,” Rajan explains. “Welding is a fairly quick process. You don’t get enough time for the zinc to bubble out, so it gets trapped in the weld and manifests itself as porosity. In addition, increased arc turbulence due to the zinc vapors causes an abnormal increase in spatter leading to poor weld quality.”
The Z solution
A unique solution for welding thin galvanized steel auto parts combines specialized metal-cored wire, Metalshield Z, with a new AC waveform called Rapid Z. Metal-cored wire is often used with other waveforms, such as standard pulse. Lincoln Electric has made a custom waveform tailored to welding over galvanized steel parts. The specialized wire and waveform are part of Process Z, Lincoln Electric’s solution for welding galvanized material.
The tailored AC waveform takes a two-prong approach. “It takes the best parts of welding with positive polarity and combines that with the benefits of welding with negative polarity,” Krivanka says. “The arc is designed to be more focused to create less zinc vapors in the first place, which leads to less porosity in the weld. And it also provides a controlled, stable metal transfer and a good penetration profile.”
In addition, the Metalshield Z wire is designed to lower the freezing temperature, keeping the puddle molten longer to allow the zinc to bubble out.
The Metalshield and Rapid Z solution enables faster travel speeds without affecting critical weld attributes. “One of the ways to combat porosity is to slow the welding process,” Rajan says. “But automakers still want high productivity. So the custom AC waveform in conjunction with the Metalshield Z wire helps to use nominally fast travel speeds to obtain the same objective. With solid wire, it can slow down to 20 ipm to 30 ipm. With a metal-cored wire, it could change from 30 ipm to 40 ipm. The Process Z solution enables travel speeds in excess of 50 ipm.”
Krivanka adds that Process Z is able to successfully bridge gaps due to poor part fit up and provides consistency from part to part which otherwise is difficult to achieve. “With gaps or part thickness or coating thickness variations – this solution can accommodate those variabilities.”
Welding with the AC waveform can significantly reduce the amount of spatter. “By controlling how the polarity switches from positive to negative, the AC waveform creates an arc with minimal disruptions,” Rajan explains. “Galvanized steel by itself will always try to disrupt the arc. Being able to produce a weld with very low spatter and controlled arc allows faster travel.”
Reducing spatter, and the need for post-weld cleaning or slag removal, is important because in order to minimize corrosion, a post-weld protective coating, or e-coating, is usually required with galvanized steel. Any residues left after welding on the surface of the part can interfere with the coating process, affecting the corrosion resistance. “It is important to have a clean part before coating,” Krivanka says. “If you keep the spatter down that is less clean up you’re going to have to do before you go to e-coat.”