All right folks. Let’s cut to the chase. Successful resistance welding boils down to heat balance: getting both parts up to their bonding temperature at the same time. If too much heat goes into one part, and not enough into the other, the overheated part can become weak, and the weld won’t be strong.
We are often asked to weld materials of different sizes, with different hardness and melting points, which can make it difficult to get both parts hot enough to form the bond. Assuming we cannot change the materials, or the shape and size of the parts, we use these five techniques to balance the heat when developing the welding process:
- Electrode Force: Increase force to shift heat away from contact areas, and decrease force to shift heat to contact areas. This technique works because the heat generated in the work piece is distributed based on where the electrical resistance is. A higher electrode force will reduce contact resistance, which, in turn, will reduce heat at the contact points (electrode-to-part and part-to-part interfaces).
- Upslope: Increase upslope time to shift heat away from contact areas, and decrease upslope time to shift heat to contact areas. Allowing the weld current to gradually increase at the beginning of the weld pulse allows the electrodes and parts to seat together and reduces the contact resistance before peak current is reached. The reduced contact resistance will result in less heat at the electrode-to-part and part-to-part interfaces.
- Electrode Face Size: Increase electrode face size to shift heat away from electrode, decrease face size to shift heat toward electrode. This technique often has the greatest influence on heat balance because it affects three things at the same time. First of all, a larger electrode face will reduce the electrode-to-part contact resistance, which will reduce the heat generated at that interface. Second, a larger electrode face will reduce the current density; in other words, the current will be spread out, so it won’t generate as much heat. Lastly, a larger electrode will sink more heat away from the weld, resulting in less heat.
- Polarity: Depending on material combinations, heat may shift toward the positive or negative electrode. There is a well known scientific principle known as the Peltier Effect, which states that when electrical current flows through two dissimilar metals, the interface of the two materials can be hotter or colder depending on the direction the current flows. Since High Frequency Inverters, Linear DC and Capacitor Discharge Power Supplies all output DC (Direct Current), the Peltier Effect may come into play and can be used to help balance the heat when using these power supply technologies.
- Electrode Materials: Use more resistive electrode to shift heat toward electrode, use more conductive electrode to shift heat away from electrode. If you are familiar with the “rule of opposites”, you know that you typically use conductive electrodes to weld resistive parts, and more resistive electrodes to weld conductive parts. By following this rule, you are promoting heat balance between the two dissimilar materials. The conductive electrode will cool the resistive part, and the resistive electrode will heat the conductive part. When developing a resistance welding process, you may want to try several different combinations of electrode materials in an effort to optimize the heat balance. See our blog post on the Rule of Opposites for more information.
If you have a difficult application where you have tried all five of these techniques, but the heat still won't balance, you may want to try adding projections to one of the parts. I'll post a blog soon regarding the advantages of projection welding.
For more information, read our Fundamentals of Resistance Welding.