1. Resistance Welding of Galvanized Steel
2. Flux Cored Arc Welding
3. Ain't no fire without a spark?
Resistance Welding of Galvanized Steel
Galvanized steel is a popular structural material for sheet metal fabrication where good corrosion resistance is required at a reasonable price. Galvanized steel is nothing but plain old mild steel with a surface coating of zinc; it is the zinc that provides corrosion resistance. Unfortunately, zinc does not have strong bonding capabilities, and hence when you are trying to resistance weld two sheets of galvanized steel to each other, the zinc coating comes in the way and it is much more difficult than welding two sheet of uncoated steels.
A sandwich of galvanized steel plates traps zinc in between and now the first thing a welding engineer has to do is to think of a strategy to get rid of the zinc so that the steel on either side can bond together. The obvious choice is to scrape off the zinc but that can work for making a few welds; not in mass production. In a manufacturing environment, the resistance welding process has to be setup such that the zinc is removed in the first half of the cycle followed by welding of steel in the second half. Fortunately, zinc has a lower melting point than steel and is easily vaporized and that fact can be used in the first half by providing lower energy, either in the form a low energy first pulse or an upslope at the beginning of the welding pulse. If there is an option, a lower welding force during the first pulse will also help in allowing the zinc vapors to escape. Another option, which makes the process easier but requires an extra step in manufacturing, is to provide a projection on one of the parts being welding. The projection design makes it much easier for the zinc to escape during the initial portion of the weld before the projection collapses and forms a weld.
Once majority of the zinc coating is removed, the second half of the weld can proceed as a normal steel weld. A slightly higher force can be useful to suppress any voids formed due vaporization of any residual zinc. Just because you think you got rid of the zinc, you are not out of the woods yet. There is the other issue of electrode wear. It so happens that zinc easily combines with copper to form a layer of brass on the surface of the copper electrode. Brass has higher resistance than copper and it generates unnecessary heat at the electrode/part interface which leads to more contamination on the electrode and the cycle continues with more buildup. Periodic cleaning of the electrode surface is an added wrinkle in the whole process. Keeping the electrode tips cool will help reduce the buildup.
Flux Cored Arc Welding (FCAW)
FCAW is an amalgam of many different fusion welding techniques. As the name implies, FCAW is an arc process where the flux is stored inside a tube-shaped electrode. In a sense it is the exact opposite of a stick welding electrode where the flux is on the outside and the wire electrode is on the inside. Having flux on the inside allows the electrode to be formed into a continuous spool, as opposed to the short segments in SMAW or stick welding. A continuously fed electrode allows FCAW to use the same equipment as used in MIG or GMAW welding. During the welding process, some of the flux ingredients form a protective layer of slag on the weld metal and in that sense FCAW is like submerged arc welding. Combination of continuous electrode and flux provides multiple benefits including high deposition rates, deeper penetration than SMAW, and a process that is more tolerant of rust and mill scale compared to GMAW. It is also simpler and more adaptable than SAW. On the downside, FCAW requires post-weld slag removal and fume extraction during welding. The welding process and equipment are practically identical to the GMAW process including polarity options, arc transfer characteristics, and shielding gas choices.
The flux inside FCAW is a motley crew of ingredients from gas formers (Lime, CaCO3) to slag formers such as Rutile (TiO2) and Fluorspar (CaF2) among others. Electrodes that are self-shielding include deoxidizers and denitrifiers such as Titanium and Aluminum in the flux. Synthetic frits are added to the flux to help stabilize the arc whereas Chromium, Nickel, and Molybdenum are added to tweak the weld metal composition.
Ain't no fire without a spark?
All TIG welding starts with initiating an arc between the electrode and the work piece. In this initial phase, the electrons have to be convinced to jump across the gap between the workpiece and the electrode. As they jump, the electrons have to ionize the gas which sets up an easy highway for more electrons to follow and ultimately have a stable arc. Now this is not as easy as it would seem; the arc can fail to initiate and consequently fail the start the welding process. Most power supplies have a high-frequency start option which helps but not always. Other options a user can try is to use a more pointed electrode tip and/or reduce the tip-to-work distance. If the tip is easily accessible, rubbing the electrode tip with a graphite pencil often does the trick. When all else fails, gently blowing on the electrode tip works like a charm. Blowing on the electrode tip displaces some of the shielding gas atoms that are difficult to ionize and introduces oxygen that is easier to ionize and helps initialize the arc.