Skip-Welding
Unevenly distributed heat causes distortion and warping, which in turn wreaks havoc on parts that theoretically fit together. To minimize warping, distribute the heat as evenly as possible. You can accomplish this by using a skip-welding technique.
For example, let's weld a 2x2ft piece of 18-gauge stainless steel to repair the side of a tank. Start by making a 1-inch-long weld. Skip 6 inches and make another 1-inch-long weld. Continue to work your way around the plate's circumference, welding 1 inch out of every 6 inches. You may have heard of this as a 1-inch-on-6-inch weld. After you've traveled around once, make your next 1-inch-long weld 3 inches from the first weld. Continue to place the second set of welds between the ones you made on the first pass, and so on until you achieve the integrity desired.
The same technique holds true for welding linear parts. If the metal starts to warp or pull to one side, solve this by: increasing the distance skipped between welds; welding at the beginning, middle and end of the piece, then repeating the sequence; or welding on alternate sides of the joint.
Backing Bars
To dissipate heat from the weld area faster than atmospheric cooling alone, place the heat-affected zone (HAZ) in contact with a "backing bar" or "chill bar." A backing bar can be as simple as a metal bar (usually copper or aluminum because they dissipate heat best) clamped to the back of the weldment. This simple technique enabled one fabricator to use an all-in-one pulsed MIG power source to weld a continuous seam on .040-inch aluminum.
In higher-duty cycle applications, you may need to consider a water-cooled backing bar. Elaborate versions feature a water cooler that circulates chilled water or special coolant through holes drilled in the bar. Simple, homemade versions feature a water cooler circulating coolant through PVC pipe touching the back of the bar.
Fit-Up And Joint DesignWelding thin metal demands tight fit-up. Imagine a butt weld on 20-gauge metal. If the parts fail to touch for even 1/16-inch, you have just created a hole that begs for burn-through and left a gap that cannot absorb the heat. On thicker metal, the edges of the metal can support the arc, but not here. Gaps cause nothing but trouble. To avoid rework caused by burn-through, adhere to the old saying "measure twice, cut once."
If you can redesign the part with joints that can withstand more heat, do so. For example, instead of a butt weld, can you make a lap joint? If you can, you double the amount of metal available to absorb heat.
Don't Over-Weld
Most people, especially those without formal training, feel compelled to over-weld a joint to obtain greater strength. Assuming you have sufficient heat, the leg of the joint does not need to be any longer than the thinnest plate. For example, when welding a 1/16-inch plate to a 1/8-inch plate in a T or lap joint, the weld only needs to be 1/16-inch wide. Excessively wide welds reduce travel speed, waste time, waste filler metal and gas, may lead to unnecessary post-weld grinding, and may affect the temper of the metal.
GMAW Power Sources
When selecting a power source for short circuit GMAW, use one with good voltage control at the low end for good arc starts and arc stability.
If you plan to buy an all-in-one power source that uses 115V household current, go with one from a major manufacturer of industrial welding equipment. Machines with low-ball prices simply do not have the slope and inductance necessary for good control over the short circuit. Be sure the unit comes with a contactor and gas solenoid valve; some units designed only for flux-cored welding do not.