When two metals are closely pressed together, and an electric current is passed through them – they offer an electric resistance to the flow of current. This resistance is greater than the one if the current was flowing through a continuous metal. The resistance to current causes heat to be generated between the two metals. <\/p>\n\n\n\n
If the metals are pressed to each other such that the pressure is applied at a concentrated area, then the heat gets generated only on that area. The heat causes the two pieces of metal to melt at the point of contact where pressure is applied.<\/p>\n\n\n\n
The melted metals when pressed against each other by application of pressure thus get joined together. This kind of welding can be done in a variety of ways. For example, when point of contact between the two metals is only a spot, the melting and fusion occurs only at that spot. The weld thus formed is called a weld nugget. This welding is called spot welding.<\/p>\n\n\n\n
When the welding electrodes that perform the function of applying pressure roll along the length of the metal such that the point of contact between the metals changes with time, the weld obtained is a line, or a seam between the two metals. Such welding is called seam welding.<\/p>\n\n\n\n