Projection welding is a variation of spot welding. Projections are formed on one of the pieces to be joined, usually by pressing the parts between flat copper electrodes. A current pulse makes the weld at the tip of the projection leaving clean surfaces without indentations. Schematic of the set-up is shown in Fig. 2.11.

2.3.2 Seam Welding

Seam welding is a continuous spot welding process where overlapped parts to be welded are fed between a pair of copper alloy (roller disc shaped) electrodes (Fig. 2.12).

2.3.3 Flash Welding

It is classified as a resistance welding process as the heat is generated at the faying surfaces of the joint by resistance to the flow of electric current, and by arcs across the interface. A thin layer of liquid metal forms at the faying surfaces. When the parts are forced together to form a joint, the layer of liquid metal on the faces alongwith the impurities is expelled, the hot metal upsets and forms a flash. No external filler metal is added during welding. Welds can be made in sheet and bar thicknesses ranging from 0.2 to 25 mm (sheets) and 1 to 76 mm (bars). Machines are available in capacities ranging from 10 kVA to 1500 kVA. The distance by which the pieces get shortened due to upsetting is called flashing allowance. The process is used for joining rails, steel strips, window frames, etc.