Capacitive Discharge Welder Basics
Fundamentals of Capacitive Discharge, Fine-Spot Resistance Welding
During resistance welding a large electrical current is used to fuse weld metals in one location, or spot, hence the term "spot weld." This weld spot or "nugget" forms during the first few milli-seconds of the welding process. A CD welder performs superior spot welds for several reasons. It allows extremely fast energy release with large peak currents. The discharge speed of a CD welder allows more of the energy to go into weld formation and less into heating the surrounding material. CD welders also keep the heat affected zone - the area where the properties of the metal have been changed - localized to a small area around the weld spot. Fast energy discharge allows electrically and thermally conductive materials, such as copper or aluminum, to be welded. In addition to these features, capacitive welders deliver repeatable welds even during line voltage fluctuations because weld energy is stored before use. Figure 1 shows an example capacitive discharge curve.
Understanding Weld Resistance
Spot welding relies on metal resistivity (resistance) to heat and fuse metal. During the welding process an electrical current is passed through the weld materials. The metal's resistance causes it to heat and melt. There are two distinct phases in the melting process, namely: heating due to the weld materials' contact resistance and heating due to bulk material resistance.
Figure 2 shows an example of a micro-scale surface profile. On the micro-scale, surfaces are rough and mating surfaces only contact in a limited number of locations. Because the surfaces have limited contact area, this area has a higher electrical resistance than the metals' bulk resistance. The resistance is called contact resistance. In fine-spot welding applications contact resistance is the most important factor in weld formation. During the first few milli-seconds of weld formation the high-resistance metal bridges melt allowing other bridges to come into contact to continue the melting process. When all of the bridges have fused, the contact resistance is zero. The bulk resistance of the metal then completes the weld.
One way contact resistance can be controlled is through the pressure of the welding electrodes. High electrode pressure reduces contact resistance because the pressure creates more metal bridges or contact points (Figure 2). When contact resistance is reduced, less weld power is consumed at the material interface and therefore the weld is cooler. Conversely, less weld pressure translates to higher contact resistance and a hotter weld. Electrode pressure also contributes to weld strength. The applied pressure forces the liquid metal together during the welding process and allows the metal to mix and solidify. An appropriate amount of pressure should be used to ensure proper weld nugget formation. Table 1 demonstrates how electrode pressure affects weld formation.
We carry the following models of Capacitive Discharge spot welders: