Review of resistance spot welding of steel sheets Part 2 - Factors influencing electrode life

The factors influencing the formation and growth of the weld nugget in resistance spot welding have been highlighted in Part 1 of this review. While various techniques based on theoretically derived models have been used for the control of weld growth, very few have found widespread application in any production control philosophy. The application of artificial intelligence techniques has proved only partially successful in overcoming the problems encountered. A limiting factor in manufacturing using spot welding is the life of the welding electrodes. A short life necessitates frequent electrode dressing or changing, resulting in lower production rates and higher costs. Also, weld quality is more variable towards the end of the life of an electrode, particularly when welding coated steels. Electrode deterioration causes growth of the electrode tip diameter, which has been shown to be dominant in determining the deterioration in weld quality in production operations. The process of electrode wear has been studied extensively, and metallurgical features such as recovery and recrystallisation of the electrode material, surface alloying, pitting and erosion have been highlighted as determining the rate of electrode growth. However, the relative contributions of these to the overall growth mechanism have not been fully evaluated. Further studies would be beneficial to deriving a solution to the problem. Production variables which influence electrode life include the start welding conditions and electrode tip diameter. The type and design of the welding machine also determine the electrode life, important parameters being (i) the mechanical characteristics of the electrode head assembly defined in terms of the force-time relationship developed on applying the welding force (frictional effects and machine rigidity are important in this context) and (ii) secondary transformer configuration, e.g. series, push-pull, indirect and parallel welding. It has been shown that electrode growth can be overcome by dressing/machining the electrode back to its original diameter at predetermined intervals. However, this procedure has been shown to be limited by the amount of available material which can be removed from the electrode tip before adversely influencing its strength. Alternative approaches have been investigated which indicate that electrode growth can be accommodated by increasing the welding current at predetermined intervals throughout the life. Optimisation of the current stepping program is essential if maximum benefits are to be achieved. This is difficult in practice, but a welding current program based on the gradient of the dynamic weldability lobe can give acceptable results. It is proposed that benefits could be realised by the development of an 'intelligent stepper' in which the current step would be triggered by a feedback signal. Preliminary work using voltage, energy or resistance based measurements has shown promise, although it is probable that a multiparameter based approach may be necessary.