Abstract
In spot welding, if ordinary welding conditons are adopted, the nugget would begin to cool regardless of flowing the current after a certain elapse of time. Peripheral part of nugget will be columnar and inside of nugget will be cellular or cellualr dendrite structure. The area of columnar structure becomes larger as weld time elapses and at last the whole nugget will assume the columnar structure. Oval-ring-like figure appears in this columnar structure. We corroborated with the help of special control timer made by us that this oval-ring-like figure occurred as the result of cooling with repetition of heating and cooling by the alternating current.
