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Metal-Cored Wire for Welding Galvanized Steel Full Speed Ahead

By: Kevin Pagano and Deryck Hart

Kevin Pagano is segment engineering manager, Miller Electric Mfg. Co.; 920/734-9821, www.millerwelds.com. Deryck Hart is director, automotive segment North America, Hobart Brothers Co.; 937/332-4000, www.hobartbrothers.com.

Sunday, September 01, 2013
 

Due to the protective layer of zinc oxide on its surface, galvanized steel offers excellent corrosion resistance as well as high strength, even at thinner gauges. However, these same properties also challenge metalformers required to weld galvanized steel. For example, thinness increases the likelihood of burnthrough, while the zinc-oxide coating can cause weld defects when improper welding procedures are used.

Many automotive suppliers use galvanized steel in the 1.6- to 4-mm range, for components such as frames, engine cradles and suspension links, and use even thinner material for other areas of the vehicle. In fact, it’s not uncommon to find autobody skins on galvanized steel as thin as 0.7 mm.

Most automotive manufacturers use one of three types of galvanized steel: hot-dipped, galvanealled or electrogalvanized. Of these, hot-dipped galvanized steels tend to be the most challenging to weld, but also the most prevalent in the industry due to their lower cost.

Hot-dipped material tends to have an uneven surface thickness that makes the completion of consistent welds particularly challenging. To obtain its zinc-oxide finish, the material passes through molten zinc at high temperatures (in excess of 800 F), after which the zinc undergoes a series of chemical reactions. The result is the tough zinc-oxide surface that provides a protective barrier and acts as a sacrificial anode to provide corrosion resistance.

Welding Challenges

Most automotive manufacturers weld galvanized steel with the gas-metal-arc welding (GMAW) process—either pulsed or constant voltage (CV) —running solid weld wire. Pulsed and CV welding processes, however, have proven difficult to perfect when it comes to gaining consistent weld quality on galvanized steel when using similar travel speeds as those used to weld mild steel.

 Fig. 1—Penetration profile of a weld created with metal-cored wire and pulsed GMAW, optimized for galvanized steel.
Spatter is one troublesome issue that arises, and typically results from the shorter arc lengths associated with CV welding. Porosity is by far a greater problem, and the travel speeds used during welding directly impact this weld defect. The faster the travel speed on hot-dipped galvanized steel, the faster the weld pool tends to freeze. That is especially troublesome since zinc vaporizes at a much lower temperature than steel. The temperature differentiation can lead to gas pockets becoming entrapped, because the weld solidifies before the zinc gas can escape. Therefore, while the welds may look acceptable, they may contain significant subsurface porosity. Porosity can appear as small pockets, but also can span the entire length of a weld joint—referred to as “piping” or “worm tracking.” Or, porosity can appear in a linear path that can cause the weld to “unzip” during cyclic loading.

As a rule of thumb, to ensure quality welds, individual instances of porosity should be separated by at least their own diameter, and the total length of porosity (sum of diameters) should not exceed 6.4 mm in any 25 mm of weld length. The maximum diameter of any instance of porosity should not exceed 1.6 mm. Internal porosity is generally limited to less than 25 percent of the area being inspected.

Don’t Slow Down; Seek Technology Solutions

To avoid porosity when welding galvanized steels, metalformers often will simply reduce weld-travel speed—effective from a quality perspective, but a clear productivity-limiter. Instead, they should consider pairing their current pulsed-GMAW process with metal-cored wire, in place of solid welding wire.

Metal-cored weld wires are tubular —a metal sheath filled with metallic powders, alloys and arc stabilizers. As opposed to solid wire, metal-cored wires carry higher current densities (at equivalent amperage settings), allowing increased weld-deposition rates. Because of the fast travel speeds these wires allow, they often get the call for robotic-welding applications.

Recent advancements in metal-cored wires, specifically those carrying the AWS classification E70C-GS, provide significant advantages when welding hot-dipped galvanized steel. These wires feature formulations that allow them to weld with straight polarity (direct current/electrode negative). Operating in straight polarity offers two distinct advantages on galvanized steel:

• Softer arc penetration, which helps to prevent burnthrough on thin-gauge material and creates an improved penetration profile (Fig. 1); and

 
Fig. 2—Certain levels of porosity (circled) are acceptable in galvanized-steel applications, as depicted in these X-ray images. However, the ability to minimize this porosity while optimizing weld-travel speed will allow metalformers to achieve high quality and productivity.
• Sufficient arc energy to vaporize the galvanized zinc coating. This allows enough time for zinc vapors to outgas from the weld pool, minimizing the likelihood of porosity in the subsurface of the weld and on its surface.

Metal-cored wires also feature arc stabilizers that help improve weldmetal transfer from the wire to the weld, minimizing spatter to help avoid post-weld cleanup.

Controlled Freezing

Combining the use of a metal-cored wire with a pulsed-GMAW waveform also will help control the pace at which the weld pool freezes. This will allow the zinc vapors to escape more readily. In addition, the pulsed-GMAW process inputs less heat into a weldment than does a standard CV process with solid wire, helping to avoid burnthrough.

Other benefits of combining metal-cored wires for galvanized steels with the pulsed-GMAW process, when welding galvanized steel:

• Improved T-joint and downhill welding;

• A fine ball transfer of weldmetal, which creates a broad arc pattern and wide weld bead with good gap-bridging ability;

• The ability to weld a wide range of material thicknesses (1.2 to 4.0 mm); and

• The ability to weld in multiple positions. MF

 

See also: Hobart Brothers Co., Miller Electric Mfg. Co.

Related Enterprise Zones: Materials/Coatings, Welding


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