Successfully welding sheet metal with MIG, TIG

For fabricators and others with bottom line goals, welding sheet metal often means a constant battle between productivity and equipment investment vs. burn-through, warping, excessive heat affected zones (HAZ) and weld appearance. For the individual occasionally welding sheet metal, success can be as simple as learning the proper techniques.

Process selection
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), pulsed GMAW, GTAW and pulsed GTAW.

GMAW electrode, 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. For welding material 18 ga. and thicker, you may be able to use a .030 in. 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 fewer spatters.
The two most popular wires for aluminium are ER4043 and ER5356. While the latter feeds more easily, choose ER4043 in .030 in. 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 sheet metal applications. Always use 100 percent argon shielding gas.

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 “on” constantly 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 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.

GTAW electrode selection and preparation
Forget the ubiquitous 1/8-in. diameter tungsten electrode and use a smaller one. They come in diameters down to .020 in. Smaller electrodes carry less heat and enable you to better focus the arc in a smaller area. For steel and stainless steel applications, keep the tungsten pointed, and be sure to grind parallel with the length.

For best results on thin aluminium, use an inverter-based power source (see GTAW power source recommendations) and forget another popular practice: welding with a pure tungsten and balling the end. Instead, select a 3/32-in. diameter tungsten with 2 percent cerium (2 percent thorium as a second choice), grind it to a point and put a small land on the end. Compared to the balled tungsten used with conventional GTAW machines, a pointed electrode provides greater arc control and enables you to direct the arc precisely at the joint, minimising distortion.

Aluminium preparation
Clean all metals before welding, but especially aluminium. Remove oil and dirt with a degreaser/solvSugary Namwhan