The main purpose of resistance spot welding (RSW) is to join metal sheets together. This includes flat and formed pieces of bracing, stiffeners, brackets, hinges and other parts. Spot welding is also used in the Image Credit: Miller Electric Manufacturing Co.fabrication of entire sheet metal assembles such as enclosures and cabinets. RSW is used for joining sheet metal because it is fast, reliable and economical. Joint design considerations are an important aspect of RSW because it can affect the quality and overall cost of the weld. Size of the spot weld, accessibility, positioning, materials and thicknesses, and the number of spots needed are all important considerations when specifying resistance spot welding.

Key advantages of resistance spot welding:

• A strong reliable electro-mechanical joint
• Very short process time
• No consumables i.e. brazing or soldering materials

The RSW process joins metal surfaces by force and heat that comes from resistance to electric current. Spot welding occurs when current flows through opposing copper electrodes of a spot welder and through the pieces of metal to be joined. The resistance to the current flowing through the metal causes the localized material between the electrodes to heat up and flow. When the current is removed the material solidifies forming a cohesive connection from the material of each sheet. The material that flows is referred to as the weld nugget. RSW is unique because the weld nugget is formed internally with relation to the surface of the base metal. In comparison, a MIG or TIG spot weld starts at the surface of one piece and goes through the second piece to form the weld nugget.

RSW is a fairly simple heat generation process involving the passage of current through a resistance to generate heat. Resistances other than the material resistance that needs to be considered are called contact resistances. Contact resistances are those driven by the surface condition of the material such as surface roughness, cleanliness, oxidation and plating.

When all resistance values are factored in, a simple equation can be used to represent the weld process. Energy = I² x R x t. "Energy" represents weld energy, " I " represents current, " R " represents resistance, and the symbol " t " represents time. Energy increases exponentially as current increases. The higher the ampacity and duration of current, the higher the heat energy that will be produced.

To prevent excessive heat marks at the surface of the material and to concentrate the heat towards the center of the workpiece, an electrode cooling system is used to remove heat from the surfaces of the workpiece that make contact with the electrodes. An efficient cooling system also preserves the electrodes in order to control the current flow.

Thicknesses of most parts joined by resistance spot welding range from 1/8 in. (3 mm) to 1/4 in. (6 mm). Welding equal to the thickness of the material is ideal because it produces the best evenly distributed weld. When materials of different thicknesses need to be joined, it can be done by placing a larger electrode on the side of the thicker sheet to center the weld nugget. As a general rule a maximum ratio of about 3-to-1 (thickest to thinnest) is used for best results. Diameters of spot welds range from about 1/8 in. (3 mm) to 1/2 in. (13 mm) depending on the thicknesses of the workpieces and the material.

Like all other welding techniques the materials being welded is highly important in the welding process. Carbon steel, stainless steel and aluminum are the three material classes that are commonly resistance spot welded.

Low carbon steel is one of the most often spot welded materials. High carbon and low alloy steels can also be spot welded but doing so can be tricky. Because of its high carbon content it has a higher risk of forming harder welds that may be brittle, crack and cause eventual weld separation. High strength steels may require treatments like tempering to maintain weld integrity.

Stainless steels are spot weldable with some grades easier than others. 300 series stainless steels are the most commonly welded, followed by ferritic which is prone to sensitization of the heat and weld metal hot cracking. Martensitic stainlesses are the least common workpiece material because welded joints are much more brittle.

Aluminum alloys are routinely spot welded. Because of their high conductivity they require high power to form quality spot welds. For aluminum welding, surface cleanliness is a large concern because of its rapid oxidation characteristics. Extensive cleaning procedures may be needed to remove surface oxide to produce quality welds.

Materials with coatings such as chrome and nickel can typically be spot welded as easily as uncoated materials. Other materials such as zinc and terne-coated materials can also be spot welded but need additional process time and current to pulse and melt off the coating. Those coatings emit dangerous fumes, such as zinc oxide, that need special precautions, such as vapor evacuation system, to protect workers from galvanized poisoning or metal fume fever.