Page 53 - MetalForming September 2013

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 qual- ity on galvanized steel when using sim- ilar travel speeds as those used to weld mild 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 tem- perature 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 sig- nificant 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 quali- ty welds, individual instances of poros- ity 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.
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To avoid porosity when welding gal- vanized 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 cur- rent 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 carry- ing 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 polar- ity (direct current/electrode negative). Operating in straight polarity offers two distinct advantages on galvanized steel:
• Softer arc penetration, which helps
MetalForming/September 2013 51
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