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Eliminating Porosity on Galvanized Steel
Q: Our company manufactures storage rack systems made from mild steel and galvanized tubing. A majority of our parts are fixture-welded on a robot using 0.045-in.-diameter ER70S-6 wire with 90 percent argon/10 percent CO2 shielding gas. We have experienced problems with weld appearance quality and porosity when welding over the galvanized metal. As a result, we have resorted to grinding before welding, which has added considerable production time and cost. Any suggestions on how we can avoid grinding?
A: First, there are some things to understand to help you make the welding process more successful. Galvanized metal is simply steel or iron coated with zinc to help protect it from rusting. The layer of zinc is fairly thin, which makes welding over it possible—with some basic considerations.
It's important to note that zinc has a lower melting temperature than the filler metal or base metal. This means that as your weld puddle is progressing, the zinc is melted or vaporized by the welding arc, which is essential to acquiring a desirable weld bead. Unfortunately, for higher-speed robotic welding, this is difficult to do on thin material. You can avoid this by slightly lowering the amperage (wire feed speed) and travel speed while increasing the voltage. The result should provide more time for the arc to burn off the zinc and allow the weld puddle to flow more smoothly, producing a more desirable weld bead.
Additionally, be sure the gun angle relative to the direction of travel is as close to perpendicular as possible. The goal is to keep the welding arc slightly out in front of the weld puddle. This becomes more important for the vertical-down progression typically found in thin-walled tube welding. Also, have the gun angle favor the side of the joint that has the galvanized metal, as this can improve the overall results.
To address the porosity issue, make sure that your shielding gas flow rates are correct. The broad range is 30 to 50 cubic feet per hour (CFH), but you will need to consider weld nozzle diameter, nozzle-to-work distance, the general airflow through the work area, and potential drafts. Check to make sure there are no leaks in the gas supply line, which could aspirate atmospheric air into the shielding gas. If you are using bulk gas, have your supplier verify proper and consistent mixing, as well as limiting moisture.
Use the shielding gas pre-flow, post-flow and hot-start features of your welding machine to help eliminate any starting or stopping porosity issues.
Finally, make sure no other sources of contamination such as oil, grease, or paint, are present on the material that could be causing the porosity. Contending with anything more than the galvanized coating will only add to problems with the process.
This article originally appeared in The WELDER magazine.
It is reprinted here with permission of the Fabricators & Manufacturers Association, Intl.