Resistance Welding Metallurgy

Welding Metallurgy

One of the unique aspects of resistance welding is its ability to produce three different types of metallurgical bonds: solder/braze joint, solid-state bond, and fusion bond. Resistance welding is the only process than can produce all three types of bonds.

Solder/Braze Bond

In this type of bond, the resistance welding machine is being used as a soldering iron or a furnace; essentially a heat source to soften and melt the solder or braze alloy. In electronic/electrical connection application, the solder alloy is available as a plating on the parts to be joined. The parts are squeezed together and the flowing current then melts the solder which then wick along available surfaces to form a strong bond. In some cases, it may be necessary to apply flux to the surface of the parts prior to bonding. Similar process can be established for brazed connections commonly used for structural applications. The braze alloy is typically added in the form of a paste or a preform along with suitable flux. Resistance welding has the advantage of producing such bonds in a short time (compared to furnace heating) and localized to limit the heat exposure to the area being bonded. The section image on bottom left shows a solder bond

Solid-State Bond

The resistance welding process can be thought of as a forging operation where the parts to be joined are heated by the action of the welding current and forged together with the welding force. It is not necessary to melt the parts being welded but only to soften them enough so that they become pliant and can be forced into intimate contact under the action of the welding force. A metallographic section of the solid-state bond will show a distinct interface line between the two parts being welded. Even though there is no melting, the bond formed is very strong. Solid-state bonds are formed without melting and hence avoid formation of intermetallics between dissimilar metal. The image in the center below shows a solid-state bond.

Fusion Bond

As the name implies, the parts to be bonded melt (fuse) and mix at the interface during welding. As the molten mixture cools, it acts as a glue to hold the two parts together. Fusion bonds are commonly formed in welding of resistive materials such as stainless steels. One of the challenges in resistance welding is to keep good control over the molten metal zone to make sure that the metal does not squeeze out and produce weld splatter/splash/whiskers. Image on the right shows a fusion bond.