How to Weld Sheet Metal With MIG and TIG
For ages, fabricators have had a hard time welding sheet metal in an efficient and cost-productive way. They have to look at many factors such as equipment-investment vs. burn-through, weld appearance, warping, heat affected zones and more.
However, welding sheet metal can be very efficient if they adopt the proper techniques.
During Process Selection
You want to prevent warping, burn-through and excessive heat zones when you decide to weld metal. All this and ensuring that your welded metal contains adequate mechanical strength for the purpose it will serve. For this you need controlled processes such as short circuit transfer GMAW, pulsed GMAW, pulsed GTAW and GTAW.
GMAW Electrode and Shielding Gas Selection
For this process, you need wires with the smallest diameter as feasible. They require the minimum amount of heat to melt. This prevents burn-through to a larger extent. Plus it has a low deposition rate.
Professionals like I-CAR use .023 diameter wire for the process. If your welding material is thicker than 18 ga, it is recommended to use .030 wire.
Mild steel requires an AWS E70 wire with S-2, S-3 or S-6 classification. As for the shielding gas, a high-argon based gas is required. This is because Argon packs less heat than CO2 and its usage results in less spatter. It is recommended to go for 75% argon and 25% CO2 gas for the process.
For welding aluminum shields, use wires ER4043 and ER5356 for wires with high diameter. Use 100% argon gas for this process.
For welding 304 stainless steel, ER308, ER308L and ER308LSI wires are required. If its 316L stainless steel, then you need a compatible 316L wire for the process. For shielding gas you need 90% helium, 8% argon and 2% CO2. Don’t use flux cored wires with stainless steel.
If you use solid wires for welding, then make sure your electrode polarity is positive. This ensures the best results with the processes outlined above.
GTAW Electrode Selection and Preparation
Use a smaller than 1/8-in. diameter tungsten electrode for this process but no lesser than .020 in.
However for stainless steel, the tungsten should be kept pointed and its best to grind parallel with the length of your weld.
For thin aluminum, its best to use a inverter-based power source. Use a 3/32-in. diameter tungsten with 2 percent cerium (you can even substitute cerium for thorium). For aluminum welding, you need to grind it to a point and put some small land at the end. A pointed electrode offers greater control over the whole process and it also allows you to minimize distortions at the joints.
Preparation Stage for Aluminum
Before starting welding, wipe all metals clean. Aluminum, when exposed to air, forms an oxide layer at the surface. Therefore you need to clear this layer with a stainless steel wire brush, grinder or chemical oxide cleaner. This will help aluminum melt at a lower temperature.
Also bring it up to room temperature before welding it.
You should always direct the arc at the middle of the weld puddle, not the leading edge as is the normal practice. This helps the puddle stay insulated from the arc’s full force.
To avoid burn-through and warping, it is recommended not to weave or whip the torch. Use the arc in a straight line and maintain the fastest possible travel speed over the bead profile.
To minimize warping, you need to distribute the heat evenly during the welding process. To do this you need to adopt a skip welding technique.
Skip welding requires you to weld at even points in the material – the beginning, the middle and the end. This results in the optimum and even distribution of heat. Whether it’s a sheet or a linear object, the same theory applies.
Heat affected zones can be made to dissipate heat from a weld area at a faster rate. For that you need to expose them to contact with a ‘backing bar’ or ‘chill bar’. They can be a metal bar (aluminum or copper) that is clamped at the back of the weldment.
If your applications require higher-duty cycle, then it’s time to consider water-cooled backing bar. Cool water can be used with conjunction with PVC pipes to cool down the heat affected zones.
Fit-Up and Joint Design
You need tight fit-up when welding metals. It's better that you redesign any part with joints that can withstand more heat wherever possible. Otherwise all you will end up with is a waste of a weld.
People try to overweld a joint to obtain greater strength. Do not do that. Doing so will result in wide welds that travel speed, waste time, waste filler metal and gas. Even the temper of the metal is affected for the worse.
Power Sources - GMAW
For short circuit GMAW, always use a power source with good voltage control. Get an all-in-one power source from a major manufacturer of industrial equipment as opposed to other options. Ensure that they come with a contactor and gas solenoid valve.
If you are looking for a power source with 200-250 amp range, get one with a spool gun. It takes care of hook-up issues.
For metals that range in the 1/16-in. till 3/32, a proper pulsed GMAW system is recommended. They are spatter free and offer faster travel speeds than the rest.
Power Sources -GTAW
GTAW power sources can be put in two categories – A DC output one for ferrous metals and an AC/DC power source for non-ferrous metals. This is a good example of the Longevity TIGWELD 200 EX model.
GTAW inverters with pulsing controls and HF arc starts are best for welding thin steel or stainless steel (no aluminum).
Pulsed GTAW inverters are good for preventing warping and burn-through.
For aluminum welding, its best to use a GTAW machine with adjustable square wave output.