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Metal Inert Gas (MIG) Welding: An arc is struck between a consumable electrode and the sheet metal to be welded. The consumable electrode is in the form of continuous filler metal. An inert gas surrounds the arc and shields it from the ambient to prevent oxidation.

Carbon steels, low alloy steels, stainless steels, most aluminum alloys, zinc based copper alloys can be welded using this process.

Tungsten Inert Gas (TIG) Welding: An arc is truck between a tungsten electrode (non-consumable) and the sheet metal to be welded. An inert gas shields the arc from the ambient to prevent oxidation. A filler material is optional

Carbon steels, low alloy steels, stainless steels, most aluminum alloys, zinc based copper alloys can be welded using this process. TIG is quite suitable for welding dissimilar materials, but usual cautions of galvanic corrosion still apply.

The TIG process is a slower process compared to the MIG process, but the quality of weld is cosmetically better. There is no weld spatter, and the quality of welds is higher than MIG welding.

Oxy Acetylene Gas Welding: Acetylene or some combustible gas is combined with Oxygen and the flame heats the sheet metal to be welded. A filler metal rod supplies the molten metal for the joint.

This method is readily available, but the heat can cause distortion in sheet metal. Due to this, this method is being displaced by other methods such as MIG and TIG welding.

Types of Weld Joints
Butt, T, corner, lap, and T joints are the common types of joints used in sheet metal welding. These can all be used with MIG and TIG welding.
Corner joints are used frequently in sheet metal cabinet construction.

Types of welds are often confused with the types of joints. The basic types of welds are fillet, square, and grooved.

Material Thickness and Weld Strength
MIG welding can be done for thicknesses ranging from 0.5 mm to 6.3 mm (0.020 to 0.250 in). TIG can be used for thicknesses as low as 0.125 mm (0.005 in).
Weld strength can be upto the strength of the underlying material. To improve overall system robustness it is better to increase material thickness rather than over specifying the weld.
Manufacturing Considerations
The surface to be welded needs to be clean and degreased and any foreign debris should be removed. In the case of hot-rolled steel or aluminum, wire brushing may need to be done to remove scales and oxides, for the highest quality welds.
Weld distortion occurs whenever a weld is done. This can be minimized by spacing the weld tacks at least 50 mm (2 in) apart. Welding distortion can also be minimized by fixturing (and clamping) while welding as well as heat sinking.
Parts should be designed with features that are self-locating with respect to mating parts. This removes the need for dedicated fixtures and its costs. Fixtures also introduce additional tolerances due to deflections imposed on the parts by the clamping pressure. Fixtures can also slow down production due to fact that parts have to be placed in the fixture and clamps (if any) that need to be activated.
Welds locations should be located with operator access in mind. If it is not accessible to a weld electrode, it is not weldable. Thus, in designs that involve sections such as channels, boxes access of electrodes needs to be considered.
Tolerancing of welded parts is usually generous. This allows the parts to be welded without too much elaborate fixturing or secondary processing such as grinding. If tight tolerancing is desired, it is best achieved by self-locating features on the mating parts.
Cosmetics Considerations
MIG welding causes a lot of spatter that needs to be sanded or filed, if cosmetically objectionable. Thus, it is best to avoid MIG welding on exterior surfaces if cosmetics are important. TIG welding is better suited for no spatter welding, even though it is more expensive.
The parts that are to be welded need to fit well with each other without too large a gap. This is particularly important in fusion welding, where no filler material is used. If too large a gap is used, then excessive shrinkage will take place.
Finish Considerations
Parts that need painting will require surface preparation in the form of sanding and grinding. The cost of this preparation needs to be considered in the total cost.
Parts that need to be electroplated should have seams that do not overlap, and not have corners or edges where the plating solutions can be trapped, leading to corrosion in the long term.