The two modes of resistance butt welding are flash welding and upset welding. In flash welding, pieces having essentially the same cross section are brought together slowly, and an electrical voltage is applied. On contact, heating and flashing occur and continue as the pieces move together, until the entire cross section is molten. At this time the pieces are forced together under pressure to squeeze out the molten metal.

In upset welding, pieces of essentially the same cross section are brought together under pressure; then voltage is applied, causing electric current to flow, heating the contact area. Current flow and pressure continue until the joint area is heated to the plastic range and a hot forge type weld is formed. No melting occurs in this type of weld. See RESISTANCE WELDING,

FLASH WELDING, and UPSET WELDING.

X