When it comes to being able to produce safe, presentable welds there are a few important prerequisites, such as choosing the correct equipment, a bit of education be it professional instruction-book learnin'-or guidance by a skilled welder, and practice. The following info and illustrations are courtesy of Miller Electric (www.millerwelds.com) and will be a good bit of info to keep handy around the shop. Since my visit I've used this info quite a bit and it's definitely improved my welding results-and if it's helped me, I'm sure it'll be advantageous to you.
Successfully Welding Sheetmetal With GMAW
For fabricators, welding sheetmetal often means a constant battle between burn-through, warping, excessive heat-affected zones (HAZ) and weld appearance. For the individual occasionally welding sheetmetal, success can be as simple as learning the proper techniques.
When welding thin metal, the main objective is to avoid warping, burn-through and excessive heat-affected zones while still ensuring the weld has sufficient mechanical strength for the application. The welding processes that provide the most control over heat are short-circuit transfer GMAW ("short arc") and pulsed GMAW.
GMAW Electrode And Shielding Gas Selection
Use the smallest wire diameter feasible. A smaller wire takes less heat to melt, which in turn heats the metal less. A smaller wire also gives you more control over the weld bead and a better chance of recovering from mistakes because it has a lower deposition rate. That's why professional groups like I-CAR, the Inter-industry Conference on Auto Repair, recommend using .023-inch diameter wire for most collision repair work. For welding material 18-gauge and thicker, you may be able to use a .030-inch wire for higher deposition rates.
For welding mild steel, choose an AWS E70 wire in S-2, S-3 or S-6 classification. For shielding a shielding gas, always use a high Argon-based gas, such as 75 percent Argon/25 percent CO2 gas (commonly called 75/25 or C25). Argon carries less heat than pure CO2, and you'll get less spatter
The two most popular wires for aluminum are ER4043 and ER5356. While the latter feeds more easily, choose ER4043 in .030-inch diameter to solve heat-related problems. ER4043 melts at a lower temperature and uses slower wire feed speed, often making it the superior choice in sheetmetal applications. Always use 100 percent Argon shielding gas.
For welding 304 stainless steel, ER308, ER308L, and ER308LSI wires are compatible. For welding 316L stainless, you need a 316L wire. Use a "tri-mix" shielding gas consisting of 90 percent Helium/8 percent Argon/2 percent CO2. Note: Do not attempt to weld thin metal with flux-cored wires. These wires use more heat because they require globular transfer. Unlike short arc, where the weld puddle cools every time the wire touches the base metal, the arc remains constantly "on" with globular transfer.
Electrode Polarity
For welding with solid wires, use electrode positive or "reverse" polarity. While EP directs more heat into the base metal than electrode negative (EN or "straight" polarity), you will obtain the best results with EP and by following the guidelines provided here. If you've been using flux-cored wire, be sure to change your machine's polarity from EN to EP.
Weld Techniques
Direct the arc toward the middle of the weld puddle. Normally, you would keep the arc on the leading edge, where the weld puddle is thinnest, to drive the arc into the work for more penetration. However, staying back enables the puddle to insulate the base metal from the arc's full force.
To prevent burn-through and warping, do not whip or weave the torch, as the more time you keep the arc in an area, the hotter it becomes. Always travel in a straight line and use the fastest travel speed possible that still maintains a good bead profile.