Fundamentals and Principles of Resistance Welding
Resistance welding processes represent a family of industrial welding processes that produce the heat required for welding through what is known as joule (J = I Rt) heating. Much in the way a piece of wire will heat up when current is passed through it, a resistance weld is based on the heating that occurs due to the resistance of current passing through the parts being welded. Since steel is not a very good conductor of electricity, it is easily heated by the flow of current and is an ideal metal for resistance welding processes. There are many resistance welding processes, but the most common is Resistance Spot Welding (RSW) (Figure 1). All resistance welding processes use three primary process variables – current, time, and pressure (or force). The automotive industry makes extensive use of resistance welding, but it is also used in a variety of other industry sectors including aerospace, medical, light manufacturing, tubing, appliances, and electrical.
Types of Resistance Welding
In addition to RSW, three other common resistance welding processes are Resistance Seam (RSEW), Projection (RPW), and Flash Welding (RFW) (Figure 2). The RSEW process uses two rolling electrodes to produce a continuous-welded seam between two sheets. It is often the process of choice for welding leak tight seams needed for automotive fuel tanks. RPW relies on geometrical features machined or formed on the part known as projections to create the required weld current density. RFW is very different from the other processes in that it relies on a rapid succession of high-current-density short current pulses which create what is known as flashing. During flashing, molten metal is violently expelled as the parts are moved together. The flashing action heats the surrounding material which allows a weld to be created when the parts are later brought together with significant pressure. Other important resistance welding processes which are not shown include High-Frequency Resistance Welding (HFRW) (used for producing the seams in welded pipe), and Resistance Upset Welding (RUW).
In summary, most resistance welding processes offer the following advantages and limitations:
- Can weld most metals, but works best with steel
- Extremely fast welding speeds are possible (a typical spot weld is produced in 1/5 of a second)
- Very good for automation and production because of the “self-clamping” aspect of the electrodes
- No filler materials required
- RSW and RSEW are ideal for welding of thin sheets
- Equipment is much more expensive than arc welding equipment
- Welds cannot be visually inspected (except for RFW and RUW welds)
- The requirement for extremely high currents creates high power line demands
- Equipment is not portable
- Mechanical properties such as tensile and fatigue of welds made from processes such as spot welding can be poor due to the sharp geometrical features at the edge of the weld
- Electrode wear