Electrode degradation is one of the common problems in the welding industry. It decreases the nugget size due to reduced current density at the weld interface and reduced conductivity of the electrode tip.
1. Mechanisms of electrode degradation
High current and pressure are key characteristics in the resistance welding process. Though they are necessary to generate heat, they expose the electrode tips at a high risk of degradation. With the increasing number of welds, there will be two major changes in the electrode tips:
- Geometric changes: Geometric changes happen typically in resistance welding for steel sheets. Due to the high melting temperature and higher strength of steel than the strength of electrode, the electrode tip diameter will increase due to deformation and wear, such as mushrooming, pitting, or local material removal by picking up.
- Metallurgical changes: The material properties near the tip surface will change during resistance welding such as alloying with a sheet and coating materials, and recrystallization and softening by overheating. Metallurgical changes in electrodes happen rapidly in spot welding of aluminium sheets due to sheet metal sticking, where more frequent electrode cleaning is needed to maintain stable production conditions.
2. Effects of electrode degradation
The increased tip diameter will make a larger contact area between electrode and sheet. It reduces the current density passing through the weld interface. Simultaneously, metallurgical changes at the tip surface will reduce the conductivity of the electrode tip. In that way, it drags heat concentration away from the weld interface. Both effects lead to progressively reducing weld nugget sizes. When an electrode cannot produce the required weld nugget size, the “electrode life” ends. This depends on the form and material of electrodes, the sheet materials, surface coatings, and the interactions of dynamic welding process parameters.
Apart from adopting new materials and new designs of electrodes, engineers use below two methods to maintain the weld quality and increase the electrode life:
Step current is a method to plan the spot welding process with a stepwise increasing weld current at each certain number of welds. The purpose is to compensate for the loss of current density due to increasing tip diameter as shown in the graph. You can optimize the current for the larger tip diameter through welding tests or by the support of numerical simulations. In this way, one electrode can produce more welds without replacement hence it has a prolonged electrode life.
Electrode tip-dressing is a method to mechanically re-shape or abrasively clean the electrode tip after a certain number of welds to preserve nearly the same initial tip diameter and surface conditions. In this way, engineers can use the same process parameters or slightly regulated parameters by an adaptive control system.
4. Developments on tip-dressing and optimization
SmartDress is a tip-dressing system developed by the European consortium of seven partners including SWANTEC in the EC financed research project on: “Adaptive Tip Dress Control for Automated Resistance Spot Welding”. The system has combined mechanical cutting and abrasive cleaning to monitor and maintain electrode tip quality. After producing a number of welds, the system optimizes the electrode tips. So it is possible to maintain weld quality and minimize production line stoppages.