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Title: Mechanism of electrode growth during spot welding of coated sheet steel
Author: Holliday, R. J.
Awarding Body: University College of Swansea
Current Institution: Swansea University
Date of Award: 1996
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It has been found that growth of the electrode tip is the dominant electrode degradation process controlling electrode life. Three distinct mechanisms then contribute to overall growth of the electrode tip. The relative contribution of these mechanisms is dependent on coating type and welding process details. In general, electrode tip growth rate is governed by the current and force density at the electrode contact face, but is independent of electrode material. Alloying between the electrode contact face and the coating is considered to be a secondary factor influencing electrode life. Techniques have been developed for measuring the change in resistance during weld formation at the electrode/sheet and sheet/sheet interfaces. The electrode/sheet interfacial resistance has been found to be significantly greater than that measured at the sheet/sheet interface. It has been established that the electrode degradation processes affect these resistance values. In particular, the resistance across the welding electrodes is inversely related to electrode tip contact area. Improvements in electrode life are thus possible through the use of current stepping techniques. In the present investigation, a number of criteria for establishing optimum rates of current increase have been evaluated. The most significant improvements in life were obtained using a rate of current increase based on the rate of electrode tip growth. An iterative technique has been derived for establishing an optimum non-linear current stepping programme. The implications of the results to manufacturing procedures have been considered, including the recommendation of preferred electrode geometry, practices for electrode dressing and current stepping. These practical considerations should lead to a more efficient mode of operation in modern manufacturing plants operating at high production rates.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (D.Eng.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available