Resolving the challenges of welding coated steels
How wire and shielding gas choices affect quality and productivity
The increased use of coated steels has resulted in an intensified search for solutions to the problems posed by joining these materials. High levels of spatter and welding fume, weld porosity, and poor bead shape are common. These problems lead to increased post-weld cleaning costs, reduced quality, greater rework, and an overall reduction in productivity. The right wire size and type, matched with the most appropriate shielding gas, can substantially improve gas metal arc welding (GMAW) performance on galvanized and coated steels.
The increased use of coated (particularly galvanized) steels has resulted in an intensified search for solutions to the problems posed by joining these materials. High levels of spatter and welding fume, weld porosity, and poor bead shape are common. These problems lead to increased postweld cleaning costs, reduced quality, more rework, and an overall reduction in productivity.
The most frequently used coated materials include both hot-dipped galvanized and electrogalvanized carbon steel, zinc alloy-coated steel sheet (Galvanneal®), and aluminum-coated steel.
Electrolytically deposited coatings are thin, homogeneous, and provide the adherent coating needed in forming applications. Hot-dipped sheet, coated in a bath of either molten zinc or aluminum, has a less uniform coating but still provides excellent corrosion resistance. Galvannealed material, coated by either process, is heat-treated to increase coating adherence and improve its weldability and painting characteristics.
Why Do Welding Problems Arise?
When welding galvanized material, welders often encounter the problems of spatter, porosity, fume generation, and potential weld cracking as a result of the volatilization of zinc in the coating.
When short-circuiting or spray metal transfer is used to join this material, the volatilized zinc rising from the plate surface causes the arc to become unstable and generate considerable spatter. Zinc vapor sometimes can be trapped in the solidifying weld puddle, causing porosity.
The amount of welding fume generated during joining is a function of the coating composition and thickness and of the welding parameters used. A thicker coating increases the amount of fume generated. Cracking also may result from zinc entrapment in the weld.
Unlike a galvanized coating, an aluminum coating is not volatile but it does produce a high-melting-point oxide that can interfere with arc stability and cause spatter. This oxide also prevents good surface wetting, which can create poor bead shape.
Current Welding Methods Fall Short
Most galvanized steel is welded like its uncoated counterpart, with little modification to processes and parameters. Solid wire used with short-circuiting transfer and argon/25 percent CO2 shielding gas (C-25) is common.
Zinc sometimes is removed from the joint surfaces before welding to improve weld quality, or the joint is gapped to allow zinc vapor to escape from the joint area during welding, which reduces spatter and porosity. Voltage and current may be increased slightly to help improve arc stability and to increase the removal of the zinc coating before the puddle reaches the joint area. Quality may improve slightly, but lower productivity and poor weld appearance and soundness are constant problems.
Wire-Gas Combinations Improve Welding of Coated Steel
The right wire size and type, matched with the most appropriate shielding gas, can substantially improve gas metal arc welding (GMAW) performance on galvanized and coated steels.
An evaluation of different wire-gas combinations at Praxair's technology center in
Gas blend evaluation showed that argon/oxygen shielding produced a nonadherent oxide that reduced corrosion resistance in the area surrounding the weld joint. Argon/CO2 blends improved bead shape and weld quality as the CO2 content increased, but this typically increased weld spatter and fume generation.
Continued evaluation found that an experimental blend of argon, CO2, and a small amount of helium reduced spatter and weld fume generation, while also improving bead appearance. This gas blend with low-silicon solid wire produced optimized performance in short-circuiting transfer (see "Playground Equipmentmaker Spins Better Welds", below).
Pulsed Metal Transfer-Solid Wire
Pulsed metal transfer GMAW can help improve galvanized steel weld quality even more. By reducing spatter, it increases process efficiency and minimizes cleanup. The controlled fine droplet spray transfer produced by pulsed GMAW results in a more stable arc than with short-circuiting, so more joint types and a wider range of material thicknesses can be joined. Its lower average current levels and greater stability produce lower fume levels as less zinc is vaporized. Good results are obtained with a low-silicon wire and an argon/CO2 gas blend.
In a case example1, sheet material from 16 to 12 gauge (0.060 to 0.10 in.) was being joined using short-circuiting transfer. Substituting pulsed GMAW with an argon/low CO2 content gas blend reduced spatter and increased deposition efficiency from 85 percent to 98.5 percent. Little postweld recoating of the base material and weld joint was required.
Design Changes With New Welding Technology
Because of the extensive grinding and cleanup previously associated with welding coated steels, some fabricators are forced to produce components from uncoated steel and then clean and dip-galvanize before powder painting these parts to provide the needed corrosion resistance. These additional operations significantly increase production cost and time to complete the fabrication. Now designs that incorporate precoated or pregalvanized material and parts, such as tubing, can achieve corrosion resistance without postfabrication galvanizing and powder painting. This greatly increases productivity and reduces cost.
Joining Aluminized Sheet Steel
Aluminized steel presents different but more easily addressed welding problems. Here, too, control of bead shape and spatter levels are key issues. The aluminum coating forms a difficult-to-remove oxide that interferes with bead wetting and generates arc instability with spatter. Because this coating is not volatile like zinc-bearing coatings, weld soundness is not as much of a problem. Like galvanized material, short-circuiting transfer with C-25 is the most commonly used welding method.
Pulsed metal transfer significantly reduced spatter when joining aluminized sheet steel. A better overall bead shapeâ€"flatter, with less "humping"&â€"was obtained with the argon/ CO2 blends. Argon/8 percent CO2 was the best two-part gas mix. Of the three-part blends evaluated, an argon helium/CO2 blend produced better bead shape and further reduced spatter when compared with conventional two-part argon/CO2 mixes.
MIG Brazing of Coated Steels
An alternative to welding coated steel (particularly galvanized) is brazing using low-melting-point (1,500-1,600 degrees F) copper silicon (bronze) or aluminum-copper-silicon (aluminum bronze) alloys (1,000-1,100 degrees F). The lower operating temperatures for the process eliminate welded seam corrosion and reduce spatter and coating loss. The low heat input lessens distortion and lowers fume generation levels. The bond strength is equivalent to that of any brazing process.
Historically, brazing has been performed using a flame for the heat source. Recent equipment developments have resulted in a variation of pulsed GMAW known as MIG brazing.
In MIG brazing of galvanized sheet, a 3 percent silicon-bronze alloy is recommended for enhanced puddle fluidity. For aluminum-coated material, one of several aluminum-bronze alloys can be selected. Argon or argon with a small CO2 addition has been the shielding gases of choice with these alloys.
Critical to the success of this process is pulsed equipment that can regulate the transfer to one droplet of material per pulse. A short arc length with stable metal transfer is needed to minimize heat input. Optimal results can be achieved with an argon/ CO2/hydrogen blend, as the enhanced arc control and a slightly reducing atmosphere can promote even better bead surface appearance.
If the mechanical properties of the joint permit brazing to be used, and the cost for the consumable materials can be justified, this process can offer some considerable advantages over conventional arc welding.2
While there are many challenges to be faced in the joining of coated steels, new consumables and process technologies can improve weld quality and increase productivity. New gas blends and the right welding processes can save fabricators both time and money as they face the challenge of being competitive in a global economy.
Kevin A. Lyttle is manager of welding R&D, Praxair Inc., 175 E. Park Drive, Tonawanda, NY 14150, 716-879-7290, fax 716-879-7275, www.praxair.com.
Wednesday, December 24, 2008
Resolving the challenges of welding coated steels
By BR - December 24, 2008
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